JP7621066B2 - Positive electrode active material for lithium ion secondary battery and method for producing same - Google Patents

Description

The present invention relates to a positive electrode active material for lithium ion secondary batteries and a manufacturing method thereof. More specifically, the present invention relates to a positive electrode active material for lithium ion secondary batteries that has low positive electrode resistance and high discharge capacity retention rate when used in a lithium ion secondary battery, a manufacturing method thereof, and a lithium ion secondary battery having a positive electrode containing the positive electrode active material for lithium ion secondary batteries.

In recent years, environmentally friendly vehicles such as hybrid vehicles and plug-in hybrid vehicles that emit less carbon dioxide have become more common in order to protect the global environment. Among the batteries used in environmentally friendly vehicles, there is a demand for the development of lithium-ion secondary batteries, which have excellent output characteristics and charge/discharge cycle characteristics.

Lithium ion secondary batteries are generally classified into non-aqueous electrolyte secondary batteries that use an organic solvent electrolyte (electrolyte solution) and all-solid-state batteries that use a non-flammable solid electrolyte. However, non-aqueous electrolyte secondary batteries that use an organic solvent electrolyte (electrolyte solution) are flammable and therefore require safety measures. In contrast, all-solid-state batteries that use a non-flammable solid electrolyte have the advantage that they do not require safety measures like organic solvent electrolytes (electrolyte solutions) because the solid electrolyte is non-flammable.

An all-solid-state battery using a non-flammable solid electrolyte is composed of a positive electrode, a negative electrode, and a lithium ion conductive solid electrolyte. The active materials of the positive and negative electrodes are generally made of materials capable of desorption and insertion of lithium. Among all-solid-state batteries using such materials, all-solid-state batteries using layered or spinel-type lithium metal composite oxides as the positive electrode active material are expected to have high energy density because they can obtain an electromotive force of 4.5 V or more. However, the all-solid-state battery has the disadvantage that a heterogeneous interface is formed by the reaction between the lithium ion conductive solid electrolyte and the positive electrode active material at the interface between the two, and the diffusion of lithium ions is hindered at the heterogeneous interface, which increases the electrical resistance and reduces the output characteristics of the battery.

As a coated active material capable of suppressing an increase in the interface resistance between an active material and a solid electrolyte material, a coated active material has been proposed that has an active material and a coating layer that coats the active material, the coating layer being composed of a material containing elemental tungsten (see, for example, Patent Document 1). The coated active material uses LiNi1 _{/3Co1 / 3Mn1/ 3O2} as a base powder, and a rolling flow coating method using simple tungsten and a tungsten compound as a coating raw material is adopted as a means for forming a resistance reducing layer on the surface of the base powder.

In addition, a positive electrode active material for a non-aqueous electrolyte secondary battery, which can suppress gelation of a positive electrode composite paste, can maintain battery capacity when used in a secondary battery, has high output characteristics, and is excellent in charge-discharge cycle characteristics, is composed of a lithium metal composite oxide powder having a layered crystal structure, and the lithium metal composite oxide powder has a formula: Li _s Ni _1-xyz Co _x Mn _y M _z O _2+α (wherein 0.05≦x≦0.35, 0≦y≦0.35, 0≦z≦0.10, 1.00<s<1.30, 0≦α≦0.2, and M represents at least one element selected from the group consisting of V, Mg, Mo, Nb, Ti, W, and Al), the positive electrode active material contains secondary particles formed by aggregation of primary particles and a second compound containing lithium, the second compound being present on the surface of the primary particles, and the at least one first compound selected from the group consisting of an oxide not containing lithium, a hydrate of the oxide, and an inorganic acid salt not containing lithium being reacted with lithium ions in the presence of water, and the positive electrode active material is dispersed in 5 g of 100 mL of pure water, and the pH of the supernatant liquid after leaving the dispersion for 10 minutes is 11 to 11.9 at a temperature of 25° C. (see, for example, Patent Document 2).

In addition, in all-solid-state batteries, as a means for suppressing the reaction between the sulfide-based solid electrolyte and _LiCoO2 and reducing the interface resistance, a method has been proposed in which a metal alkoxide is used as a coating raw material for coating the surfaces of the _LiCoO2 particles of the base powder with compounds such as _Li2SiO3 , _LiNbO3 , and _LiTaO3 using a sol-gel method (see, for example, Non-Patent Documents 1 to 3 ₎ .

However, in the inventions described in Patent Documents 1-2 and Non-Patent Documents 1-3, excess lithium compounds such as lithium hydroxide are contained on the particle surfaces of the lithium metal composite oxide that constitutes the base powder. When the lithium metal composite oxide is used to prepare a positive electrode composite paste, the lithium compounds may dissolve into the solvent contained in the positive electrode composite paste, causing the positive electrode composite paste to gel.

In addition, as described in Patent Document 1 and Non-Patent Documents 1 to 3, when the rolling flow coating method using tungsten or a tungsten compound as the coating raw material or the sol-gel method using a metal alkoxide as the coating raw material is used as the method for forming a resistance-reducing layer on the particle surface of the base powder, the raw material costs and the costs required for the coating process are high, making it impossible to manufacture lithium-ion secondary batteries industrially.

In the inventions described in Patent Documents 1 and 2, when excess lithium compound is used as a lithium supply source for the coating compound, if the amount of coating raw material such as tungsten compound or molybdenum compound added is relatively small compared to the amount of the lithium compound, the resulting positive electrode composite paste will gel, and if the amount of coating raw material added is relatively large compared to the amount of the lithium compound, even the lithium necessary for the base powder itself, which constitutes the composite oxide together with nickel, cobalt, etc., will be taken away, and there is a risk that the performance as a positive electrode active material will not be fully expressed.

In addition, in the invention described in Patent Document 2, when determining the amount of the coating raw material, such as a tungsten compound or a molybdenum compound, to be added, a test positive electrode active material is first prepared using a small amount of base material powder and coating raw material, and a complicated preliminary experiment is required to confirm the amount of the compound required to make the pH of the slurry of the positive electrode active material fall within a predetermined range. This makes it impossible to efficiently produce lithium ion secondary batteries on an industrial scale.

International Publication No. WO 2012/105048 International Publication No. 2017/073238

Chemistry of Materials, Vol. 22, 2010, pp. 949-956 Electrochemistry Communications, Vol. 9, 2007, pp. 1486-1490 Solid State Ionics, Vol. 179, 2008, pp. 1333-1337

The present invention has been made in consideration of the above-mentioned conventional technology, and aims to provide a positive electrode active material for lithium ion secondary batteries that has low positive electrode resistance and a high retention rate of discharge capacity when used in lithium ion secondary batteries, a method for producing the same, and a lithium ion secondary battery having a positive electrode containing the positive electrode active material for lithium ion secondary batteries.

The present invention provides a method for producing a positive electrode active material for a lithium ion secondary battery using a base powder containing secondary particles formed by agglomeration of primary particles of a lithium metal composite oxide containing lithium, nickel, and cobalt as a raw material , comprising the steps of:
The lithium metal composite oxide has the formula (I):
Li _s Ni _1-xy C _x M _y O _2+α (I)
(In the formula, s, x, y, and α are numbers satisfying 1.00≦s≦1.30, 0.05≦x≦0.35, 0≦y≦0.45, and −0.1≦α≦0.2, respectively, and M represents Al.)
a coating layer containing at least one lithium salt selected from the group consisting of lithium tungstate and lithium molybdate is formed on a surface of the secondary particles, the coating layer has an average thickness of 3 to 150 nm, and a coefficient of variation (CV%) indicating the variation in thickness of the coating layer is 15% or less;
preparing a first mixture by mixing a base material powder and water such that the mixture of the base material powder and water has a water content of 4 to 7 mass %;
The temperature of the first mixture obtained above is adjusted to 25 to 50°C, and the first mixture is mixed with a coating material to prepare a second mixture;
The second mixture is dried at a temperature of 150 to 250° C. to form a coating layer on the surface of the secondary particles contained in the base powder, thereby producing a positive electrode active material;
A coating raw material containing at least one compound selected from the group consisting of tungsten compounds and molybdenum compounds is used as the coating raw material,
At least one tungsten compound selected from the group consisting of tungsten oxide, tungstic acid, and ammonium tungstate is used as the tungsten compound, and at least one molybdenum compound selected from the group consisting of molybdenum oxide, molybdic acid, and ammonium molybdate is used as the molybdenum compound,
The present invention relates to a method for producing a positive electrode active material for a lithium ion secondary battery, characterized in that the total amount of tungsten and molybdenum per 100 parts by mass of the base powder is adjusted to 0.1 to 6 parts by mass.

The present invention provides a positive electrode active material for lithium ion secondary batteries that has low positive electrode resistance and high discharge capacity retention when used in lithium ion secondary batteries, a method for producing the same, and a lithium ion secondary battery having a positive electrode containing the positive electrode active material for lithium ion secondary batteries.

FIG. 1 is a schematic diagram illustrating one embodiment of a positive electrode active material for a lithium ion secondary battery according to the present invention.

The following provides a detailed description of the positive electrode active material for lithium ion secondary batteries of the present invention, its manufacturing method, and a lithium ion secondary battery having a positive electrode containing the positive electrode active material for lithium ion secondary batteries. The present invention is not limited to the embodiments described below, and the embodiments can be modified based on the knowledge of those skilled in the art without departing from the spirit of the present invention.

1. Positive electrode active material for lithium ion secondary batteries The positive electrode active material for lithium ion secondary batteries of the present invention is a positive electrode active material for lithium ion secondary batteries in which, as described above, a base powder containing secondary particles formed by agglomeration of primary particles of a lithium metal composite oxide containing lithium, nickel, and cobalt is used as a raw material.

The positive electrode active material for lithium ion secondary batteries of the present invention has a feature in that a coating layer containing at least one lithium salt selected from the group consisting of lithium tungstate and lithium molybdate is formed on the surface of the secondary particles, the average thickness of the coating layer is 3 to 150 μm, and the coefficient of variation (CV%) indicating the variation in the thickness of the coating layer is 15% or less. Since the positive electrode active material for lithium ion secondary batteries of the present invention has the above-mentioned constituent requirements, it has the excellent effects of low positive electrode resistance and high retention rate of discharge capacity when used in a lithium ion secondary battery.

As a raw material for the positive electrode active material for lithium ion secondary batteries, a base powder containing secondary particles formed by agglomeration of primary particles of lithium metal composite oxide containing lithium, nickel, and cobalt can be used.

(1) Base material powder A method for preparing the base material powder is described below, but the present invention is not limited to this preparation method. The base material powder contains secondary particles formed by aggregation of primary particles of lithium metal composite oxide. The base material powder may inevitably contain a small amount of primary particles of lithium metal composite oxide.

Lithium compound particles, nickel compound particles, cobalt compound particles, and, if necessary, additive metal M particles can be used as raw materials for the secondary particles of lithium metal composite oxide that constitute the base powder.

(a) Preparation of Metal Composite Hydroxide (Precursor) Particles [Crystallization]
First, an aqueous solution of a metal compound containing nickel, cobalt, and, if necessary, an additive metal M such as manganese or aluminum is prepared as a raw material solution. The aqueous solution of the metal compound can be easily prepared, for example, by dissolving the metal compound in water having a temperature of about 30°C. The water is preferably pure water such as ion-exchanged water. The temperature of the aqueous solution of the metal compound is preferably about 30°C.

Examples of the metal compounds include metal sulfates, metal aluminates, and metal hydrates, but the present invention is not limited to these examples. These metal compounds may be used alone or in combination of two or more types. Any of these compounds can be suitably used in the present invention.

The concentration of the metal compound in the aqueous solution of the metal compound is not particularly limited, but is usually 1.0 to 2.6 mol/L.

Next, the aqueous solution of the metal compound is added to water at 40 to 60° C., and the pH and ammonium ion (NH ₄ ⁺ ) concentration are adjusted as described below to crystallize the particles. When the particles are crystallized, the formation of secondary particles by aggregation of the primary particles proceeds in parallel.

The temperature of the water is preferably 40°C or higher in order to prevent the metal composite hydroxide particles from becoming coarse, and is preferably 60°C or lower in order to prevent the metal composite hydroxide particles from becoming finer. The water is preferably pure water such as ion-exchanged water.

When adding the aqueous solution of the metal compounds to water at 40 to 60°C, it is preferable to use a reaction tank. When using a reaction tank, the aqueous solution of the metal compounds is added to water at 40 to 60°C that has been poured into the reaction tank. The following describes the case where the reaction tank is used.

When the aqueous solution of the metal compound is added to water at 40 to 60°C, it is preferable to stir the water in order to efficiently crystallize the particles.

When crystallizing particles, if the particles are to have a "solid" particle structure, it is preferable to introduce an inert gas such as nitrogen gas from the top of the reaction tank and adjust the atmosphere in the reaction tank to a non-oxidizing atmosphere with an oxygen concentration of 1% by volume or less.

In addition, when the particles are to have a "hollow" or "porous" particle structure, it is preferable to add the aqueous solution of the metal compound and ammonia water to adjust the ammonium ion (NH ₄ ⁺ ) concentration to the reaction tank while adjusting the pH of the water in the reaction tank to 11.0 to 12.5 with a pH adjuster such as an aqueous sodium hydroxide solution. The pH of the water is preferably 11.0 or more from the viewpoint of reducing the concentration of sulfate remaining in the positive electrode active material and improving the output characteristics of the battery, and is preferably 12.5 or less from the viewpoint of preventing the metal composite hydroxide particles from becoming too small. The concentration of ammonium ion (NH ₄ ⁺ ) in the content of the reaction tank is preferably 5 to 30 g/L, more preferably 10 to 20 g/L, from the viewpoint of stabilizing the crystallization treatment.

After the particles are crystallized, they are subsequently processed under the following conditions to ensure that they have one of the following particle structures: solid, hollow, or porous.

When the particles are to have a "solid" particle structure, it is preferable to continue the crystallization process for growing the particles while maintaining the reaction tank in a non-oxidizing atmosphere. When the particles are to have a "hollow" particle structure, it is preferable to stop the addition of the aqueous solution of the metal compound and the ammonia water, change the atmosphere in the reaction tank from an oxidizing atmosphere to a non-oxidizing atmosphere, and then resume the addition of the aqueous solution of the metal compound and the ammonia water to continue the crystallization process for growing the particles. When the particles are to have a "porous" particle structure, it is preferable to continue the crystallization process for growing the particles by performing a series of operations multiple times, for example, stopping the addition of the aqueous solution of the metal compound and the ammonia water, changing the atmosphere in the reaction tank from an oxidizing atmosphere to a non-oxidizing atmosphere, resuming the addition of the aqueous solution of the metal compound and the ammonia water, changing the atmosphere from a non-oxidizing atmosphere to an oxidizing atmosphere, and changing the atmosphere from an oxidizing atmosphere to a non-oxidizing atmosphere.

As explained above, by controlling the atmosphere in the reaction tank to a non-oxidizing atmosphere or an oxidizing atmosphere, the metal composite hydroxide can be adjusted to have a predetermined particle structure.

Here, by controlling the temperature in the reaction tank to 40 to 60°C, the particle size of the metal composite hydroxide can be adjusted to a preferred size. In addition, by controlling the pH of the water to 11.0 to 12.5, it is possible to reduce sulfates in the subsequent process and adjust the particle size. In addition, by controlling the ammonium ion ( _NH4 ⁺ ) concentration to preferably 5 to 30 g/L, more preferably 10 to 20 g/L, it is possible to stabilize the crystallization treatment.

By carrying out the above-mentioned operation, metal composite hydroxide (precursor) particles are obtained. From the viewpoint of improving the particle strength and increasing the contact area with the electrolyte, the porosity of the metal composite hydroxide particles is preferably 20% or less, more preferably 5% or less, when the particle structure of the metal composite hydroxide particles is "solid", is preferably 10 to 90%, more preferably 20 to 50%, when the particle structure of the metal composite hydroxide particles is "hollow", and is preferably 40 to 90%, more preferably 50 to 80%, when the particle structure of the metal composite hydroxide particles is "porous".

[Filtration and drying]
The crystallized product of the metal composite hydroxide particles obtained above is subjected to solid-liquid separation using a filtering device such as a filter press filter, and the recovered solid metal composite hydroxide particles are washed with washing water, whereby impurities can be removed from the metal composite hydroxide particles.

Since moisture is attached to the surface of the washed metal composite hydroxide particles, it is preferable to dry the metal composite hydroxide particles in a dryer. Examples of dryers include a stationary dryer, a fluidized dryer, and an airflow dryer, but the present invention is not limited to these examples. When a heating dryer is used as the dryer, it is preferable that the heating dryer is an electric heating dryer that does not generate carbon gas in a dry atmosphere.

The drying temperature for the metal composite hydroxide particles is preferably 100°C or higher, more preferably 120°C or higher, from the viewpoint of increasing the drying efficiency, and is preferably 200°C or lower, more preferably 180°C or lower, from the viewpoint of suppressing deterioration of the metal composite hydroxide particles. The drying time for the metal composite hydroxide particles varies depending on the drying temperature of the metal composite hydroxide particles, and therefore cannot be determined in general terms. Therefore, it is preferable to determine the drying time appropriately depending on the drying temperature of the metal composite hydroxide particles, but it is usually about 1 to 5 hours.

The average particle diameter of the metal composite hydroxide particles is preferably 3 μm or more, more preferably 4 μm or more, from the viewpoint of increasing the packing density of the metal composite hydroxide particles to increase the battery capacity, and is preferably 20 μm or less, more preferably 15 μm or less, from the viewpoint of increasing the specific surface area of the positive electrode active material and improving the output characteristics of the battery. The average particle diameter of the metal composite hydroxide particles is determined from the volume-based distribution measured using a laser diffraction/scattering type particle size distribution measuring device (manufactured by Microtrack Bell Co., Ltd., product name: Microtrack MT3300EXII).

The composition of the metal composite hydroxide particles can be analyzed by chemical analysis methods such as acid decomposition-ICP (inductively coupled plasma) emission spectrometry.

(b) Preparation of metal composite oxide (intermediate) particles [oxidizing roasting]
The metal composite hydroxide particles obtained above are oxidatively roasted in an oxygen-containing atmosphere such as air, whereby metal composite oxide particles, which are oxidatively roasted products, can be obtained.

When the metal composite hydroxide particles are oxidized and roasted, a heating furnace such as a high-temperature heating furnace can be used. The heating furnace may be either a batch type heating furnace or a continuous type heating furnace. Among heating furnaces, an electric furnace is preferred because there is no risk of combustion gas being generated.

The temperature at which the metal composite hydroxide particles are oxidatively roasted is preferably 800 to 1000°C, more preferably 800 to 900°C, from the viewpoint of improving the mechanical strength of the metal composite oxide particles and preventing sintering of the metal composite oxide particles. The time required to oxidatively roast the metal composite hydroxide particles varies depending on the temperature at which the metal composite hydroxide particles are oxidatively roasted and cannot be determined in general terms. Therefore, it is preferable to determine the time appropriately depending on the temperature at which the metal composite hydroxide particles are oxidatively roasted, but it is usually about 1 to 5 hours.

Before subjecting the metal composite hydroxide particles to oxidative roasting, the metal composite hydroxide particles may be calcined in order to sufficiently remove moisture from the metal composite hydroxide particles and improve the mechanical strength of the metal composite hydroxide particles. Calcination of the metal composite hydroxide particles can be carried out, for example, by heating the metal composite hydroxide particles at a temperature of 400 to 550°C, preferably 450 to 500°C, for about 1 to 3 hours.

The average particle diameter of the metal composite oxide particles is preferably 3 μm or more, more preferably 4 μm or more, from the viewpoint of increasing the packing density of the metal composite oxide particles to increase the battery capacity, and is preferably 20 μm or less, more preferably 15 μm or less, from the viewpoint of increasing the specific surface area of the positive electrode active material and improving the output characteristics of the battery. The average particle diameter of the metal composite oxide particles is determined from the volume-based distribution measured using a laser diffraction/scattering type particle size distribution measuring device (manufactured by Microtrack Bell Co., Ltd., product name: Microtrack MT3300EXII).

The composition of the metal composite oxide particles can be analyzed by chemical analysis methods such as acid decomposition-ICP (inductively coupled plasma) emission spectrometry.

(c) Preparation of Base Powder The metal composite oxide particles and lithium compound particles are used as raw materials for the base powder.

[Lithium compound particles]
Examples of the lithium compounds constituting the lithium compound particles include lithium _carbonate ( _Li2CO3 , melting point: 723°C), lithium hydroxide (LiOH, melting point: 462°C), lithium nitrate ( _LiNO3 , melting point: 261°C), lithium chloride (LiCl, melting point: 613°C), and lithium _sulfate ( _Li2SO4 , melting point: 859°C), but the present invention is not limited to these examples. Each of these lithium compounds may be used alone, or two or more types may be used in combination. Among these lithium compounds, lithium hydroxide and lithium carbonate are preferred because they are easy to handle and have stable quality.

The maximum particle size of the lithium compound particles is preferably 10 μm or less, and the average particle size of the lithium compound particles is preferably 5 μm or less. The maximum particle size and the average particle size of the lithium compound particles are measured using a laser diffraction/scattering type particle size distribution measuring device (manufactured by Microtrack Bell Co., Ltd., product name: Microtrack MT3300EXII), and the average particle size of the lithium compound particles is determined from the volume-based distribution measured using the measuring device.

[Preparation of lithium mixture]
The metal composite oxide particles and lithium compound particles are mixed to obtain a lithium mixture.

The metal composite oxide particles and the lithium compound particles are preferably mixed so that the ratio value between the total number of atoms of the metal elements contained in the lithium metal composite oxide and the number of lithium atoms (hereinafter simply referred to as the "ratio value") is 1.0 to 1.2. The upper limit of the ratio value is preferably 1.2 or less from the viewpoint of suppressing the coarsening of the particle size and the crystallite size and improving the cycle characteristics. When the metal composite oxide and the lithium compound are mixed at the ratio value, lithium atoms are taken into the 3a site, which is a lithium site, so that the battery characteristics can be improved. The site means a crystallographically equivalent lattice position. The presence of an atom at a lattice position is said to be "occupied by the site", and the occupied site is called an "occupied site". Taking LiCoO ₂ as an example, there are three occupied sites in LiCoO _2. The three occupied sites are called lithium sites, cobalt sites, and oxygen sites, or 3a sites, 3b sites, and 6c sites, respectively.

[Preparation of base powder]
The lithium mixture is fired in an oxidizing atmosphere such as air to obtain a base powder containing secondary particles formed by aggregation of primary particles of the lithium metal composite oxide. The oxygen concentration in the oxidizing atmosphere is preferably 18 to 100% by volume.

The calcination temperature of the lithium mixture is preferably 800°C or higher, more preferably 900°C or higher, from the viewpoint of reducing the amount of unreacted lithium compounds and improving crystallinity, and is preferably 1000°C or lower, from the viewpoint of suppressing excessive sintering between the particles of the lithium metal composite oxide. The calcination time of the lithium mixture cannot be determined in general because it varies depending on the calcination temperature of the lithium mixture, and is preferably determined appropriately depending on the calcination temperature of the lithium mixture, but is usually preferably about 1 to 20 hours, more preferably about 10 to 15 hours.

Before calcining the lithium mixture, the lithium mixture may be calcined in an oxygen-containing atmosphere such as air in order to improve the mechanical strength of the calcined product by gently proceeding the reaction during calcination of the lithium mixture and to reduce the amount of unreacted lithium compounds and improve crystallinity. Calcination of the lithium mixture can be performed, for example, by heating the lithium mixture at a temperature of 400 to 550°C, preferably 450 to 500°C, for about 1 to 3 hours. The oxygen concentration in the oxygen-containing atmosphere is preferably 18 to 100% by volume.

By firing the lithium mixture in the above manner, primary particles of lithium metal composite oxide are formed from the metal composite oxide particles and the lithium compound particles, and a base powder containing secondary particles formed by agglomeration of the primary particles of the lithium metal composite oxide is obtained. The secondary particles are usually spherical or elliptical, but may also contain particles of irregular shape.

The particle size of the primary particles of the lithium metal composite oxide is not particularly limited, but is usually about 0.2 to 1 μm. The particle size of the secondary particles is usually about 3 to 30 μm. The particle sizes of the primary particles and the secondary particles are determined from the volume-based distribution measured using a laser diffraction/scattering type particle size distribution measuring device (manufactured by Microtrac-Bell, product name: Microtrac MT3300EXII).

The base powder is essentially composed of the secondary particles, but may unavoidably contain trace amounts of the primary particles in addition to the secondary particles.

In the present invention, the base material powder obtained as described above can be used as the raw material for the positive electrode active material for lithium ion secondary batteries, but commercially available base material powders may also be used. Among commercially available base material powders, those that produce a slurry having a predetermined pH using the base material powder can be used as the base material powder. The pH of the base material powder slurry will be described later.

The coating layer formed on the surface of the positive electrode active material of the present invention has an extremely thin average thickness of 3 to 150 nm, as described below, so the positive electrode active material of the present invention inherits the powder characteristics and particle structure of the base powder, such as the average particle diameter MV, particle size distribution, and tap density (bulk density). Therefore, by selecting the powder characteristics and particle structure of the base powder, it is possible to impart the properties of the base powder to the positive electrode active material.

[Composition of lithium metal composite oxide]
As described above, the base powder is substantially composed of the secondary particles, but may inevitably contain a small amount of the primary particles in addition to the secondary particles. Both the primary particles and the secondary particles are composed of a lithium metal composite oxide.

The lithium metal composite oxide is represented by the formula (I):
Li _s Ni _1-xy C _x M _y O _2+α (I)
(In the formula, s, x, y, and α are numbers satisfying 1.00≦s≦1.30, 0.05≦x≦0.35, 0≦y≦0.45, and −0.1≦α≦0.2, respectively, and M represents at least one metal element selected from the group consisting of Mn, V, Mg, Mo, Nb, Ti, W, and Al.)
It is preferable that the lithium metal composite oxide is represented by the following formula:

The composition of the base powder can be determined, for example, by mixing the base powder and inorganic acid (35% hydrochloric acid) in appropriate amounts, decomposing the mixture by heating, adding an appropriate amount of pure water to the decomposition liquid to prepare an analytical specimen liquid, and subjecting the analytical specimen liquid to spectroscopic analysis using a multi-type ICP optical emission spectrometer (Shimadzu Corporation, product number: ICPE-9000).

[pH of base powder slurry]
Since the excess lithium compound contained in the base material powder is mainly composed of lithium hydroxide, there is a strong correlation between the content of the excess lithium compound in the base material powder and the pH of the base material powder slurry. Therefore, by measuring the pH of the base material powder slurry, the content of the excess lithium compound in the base material powder can be determined more quickly and simply than by conventional titration methods.

The base material powder slurry can be prepared by mixing the base material powder with pure water in a ratio of 5 g of base material powder to 95 mL of pure water, and dispersing the base material powder in the pure water. The pH of the base material powder slurry is preferably 10.5 to 12.5, more preferably 11.0 to 12.5, even more preferably 11.0 to 12.2, and even more preferably 11.6 to 12.2, from the viewpoint of controlling the content of excess lithium compounds contained in the base material powder to the optimal content of 0.045 to 1.6 mass %.

The pH of the base powder slurry is the value measured when the base powder slurry is left to stand for 10 minutes and the pH of the supernatant is measured with a pH meter (manufactured by Thermo Fisher Scientific Co., Ltd., product name: Orion-star-A214 (tabletop type) or product name: Orion-star-A221 (handheld type)).

[Average particle size MV and particle size distribution of base powder]
The particle size distribution [(d90-d10)/average particle diameter MV], which is an index showing the spread of the particle size distribution of the base powder, is preferably 1.0 or less from the viewpoints of improving the packing property of the positive electrode active material and the battery capacity per battery volume, uniforming the applied voltage between the secondary particles constituting the positive electrode active material, and uniforming the load between the secondary particles to improve cycle characteristics.

In the particle size distribution [(d90-d10)/average particle size MV], which is an index showing the spread of the particle size distribution, d10 means the particle size when the number of particles having each particle size is accumulated from the smallest particle size side and the accumulated volume is 10% of the total volume of all particles. d90, like d10, means the particle size when the number of particles having each particle size is accumulated from the smallest particle size side and the accumulated volume is 90% of the total volume of all particles.

The average particle size MV and particle size distribution [(d90-d10)/average particle size MV] of the base powder are determined from the volume-based distribution of the base powder measured using a laser diffraction/scattering type particle size distribution measuring device [Microtrac-Bell, product name: Microtrac MT3300EXII].

As with d10 and d90, the particle diameter where the cumulative volume is 50% of the total particle volume, in other words the median diameter d50, is sometimes used instead of the average particle diameter MV.

[Tap density (bulk density) of base powder]
The tap density of the base material powder is preferably 1.0/cm ³ or more, more preferably 1.8/cm 3 or more, and even more preferably 2.0/cm ³ or more, from the viewpoint of reducing voids when an electrode for a lithium ion secondary battery is produced and improving battery performance. The tap density of the base material powder is preferably 3.0/cm 3 or less, more preferably 2.4/cm 3 or less, and even more preferably 2.2/cm ³ or less, from the viewpoint of improving the contact area between the positive electrode active material and the electrolyte when the positive electrode active material is used in a non-aqueous electrolyte lithium ^ion secondary battery, and introducing a solid electrolyte between the particles of the positive electrode active material when the positive electrode active material is used in an all-solid- ^state lithium ion secondary battery, thereby reducing the electrical resistance of the lithium ion ^secondary battery and increasing the battery capacity.

The tap density of the base powder is the density of the base powder measured after filling a 20 mL measuring cylinder with 12 g of base powder, attaching the measuring cylinder to a shaking specific gravity measuring device (Kuramochi Scientific Instruments Manufacturing Co., Ltd., product number: KRS-409), and allowing the measuring cylinder to freely fall from a height of 2 cm 500 times to densely pack the base powder.

(2) First Mixture The first mixture can be obtained by mixing the base material powder and water so that the water content in the mixture of the base material powder and water is 4 to 7 mass %.

The water content in the mixture of the base material powder and water is 4% by mass or more, preferably 5% by mass or more, from the viewpoint of allowing water to fully penetrate into the voids and grain boundaries between the primary particles that make up the secondary particles contained in the base material powder, and is 7% by mass or less, preferably 6% by mass or less, from the viewpoint of increasing the drying efficiency in the subsequent drying step, reducing the amount of lithium eluted from the base material powder, and improving the battery characteristics of a battery in which the positive electrode active material is used in the positive electrode.

The atmosphere and temperature when mixing the base material powder and water are not particularly limited. The atmosphere when mixing the base material powder and water may be air or an inert gas such as nitrogen gas or argon gas. The temperature when mixing the base material powder and water can be appropriately determined usually from a temperature range of 5 to 95°C, preferably a temperature range of 10 to 80°C, but from the viewpoint of improving energy efficiency, it is usually preferable to use room temperature.

The base material powder and water can be mixed using a mixer such as a shaker mixer, stirring mixer, or rocking mixer. It is preferable to mix the base material powder and water until both are uniformly dispersed and a mixture with a uniform composition is obtained.

(3) Second Mixture The second mixture can be prepared by adjusting the temperature of the first mixture obtained above to 25 to 50°C and mixing the first mixture with a coating raw material.

In the present invention, when the first mixture is mixed with the coating raw material, a coating raw material containing at least one compound selected from the group consisting of tungsten compounds and molybdenum compounds is used as the coating raw material, the total amount of tungsten and molybdenum contained in the coating raw material relative to the lithium of the excess lithium compound contained in the base powder is adjusted to 0.8 to 1.2 equivalents, and the total amount of tungsten and molybdenum per 100 parts by mass of the base powder is adjusted to 0.1 to 6 parts by mass.

In the present invention, the above-mentioned operation is adopted, so that the lithium compound remaining on the surface of the base powder, etc. can be dissolved, and the solubility in water of the added coating raw material can be improved, and thus a positive electrode active material for lithium ion secondary batteries can be obtained that has low positive electrode resistance and a high retention rate of discharge capacity when used in lithium ion secondary batteries.

As the coating raw material, a coating raw material containing at least one compound selected from the group consisting of tungsten compounds and molybdenum compounds is used. The tungsten compounds and molybdenum compounds may be used alone or in combination. Among the tungsten compounds and molybdenum compounds, tungsten compounds are preferred because they are less likely to produce water as a by-product when reacted with excess lithium compounds.

Examples of tungsten compounds include tungsten oxide, tungstic acid, and ammonium tungstate, but the present invention is not limited to these examples. These tungsten compounds may be used alone or in combination of two or more. Among tungsten compounds, tungsten oxide and tungstic acid are more preferred, and tungsten oxide is even more preferred, because they are less likely to produce water as a by-product when reacted with excess lithium compounds.

Examples of molybdenum compounds include molybdenum oxide, molybdic acid, and ammonium molybdate, but the present invention is not limited to these examples. These molybdenum compounds may be used alone or in combination of two or more. Among the molybdenum compounds, molybdenum oxide and molybdic acid are preferred, and molybdenum oxide is more preferred, because they are less likely to produce water as a by-product when reacted with excess lithium compound.

Tungsten oxide and molybdenum oxide are preferred from the viewpoint of improving the output characteristics of lithium ion secondary batteries and increasing the battery capacity. Among tungsten oxide and molybdenum oxide, tungsten oxide is preferred from the viewpoint of improving the lithium ion conductivity of the coating compound and promoting the movement of lithium ions.

The coating raw material may contain coating raw materials other than tungsten compounds and molybdenum compounds within the scope of the present invention. Examples of coating raw materials other than tungsten compounds and molybdenum compounds include vanadium pentoxide, niobium oxide, tin dioxide, manganese oxide, ruthenium oxide, rhenium oxide, tantalum oxide, phosphorus oxide, and boron oxide, but the present invention is not limited to these examples. Each of these coating raw materials may be used alone, or two or more types may be used in combination.

When oxides such as tungsten oxide and molybdenum oxide, or hydrates of these oxides are used as the coating raw material, the coating raw material forms oxoanions in the presence of water, and the oxoanions react with the lithium of the excess lithium compound to produce lithium salts of the oxoanions such as lithium tungstate and lithium molybdate. When the positive electrode active material containing the coating raw material is used in a lithium ion secondary battery, the positive electrode resistance of the lithium ion secondary battery can be reduced and the retention rate of the discharge capacity can be increased.

In addition, in the present invention, the total amount of tungsten and molybdenum contained in the coating raw material relative to the lithium of the excess lithium compound contained in the base powder is 0.8 to 1.2 equivalents, so by using the positive electrode active material in which the coating raw material is used in a lithium ion secondary battery, the positive electrode resistance of the lithium ion secondary battery can be reduced and the retention rate of the discharge capacity can be increased.

If the total amount of tungsten and molybdenum contained in the coating raw material is small relative to the amount of excess lithium compound contained in the base material powder, the mixture of the first mixture and the coating raw material tends to gel, and if the total amount is large relative to the amount of excess lithium compound contained in the base material powder, even the lithium necessary for intercalation and deintercalation is taken from the crystal lattice of the base material powder, which constitutes a composite oxide together with nickel and cobalt, and the performance as a positive electrode active material tends to be unable to be fully exhibited. Therefore, the total amount of tungsten and molybdenum contained in the coating raw material relative to the lithium of the excess lithium compound contained in the base material powder is 0.8 to 1.2 equivalents, preferably 0.9 to 1.1 equivalents, and more preferably 0.95 to 1.05 equivalents, from the viewpoint of suppressing gelation of the mixture of the first mixture and the coating raw material and fully exhibiting the performance as a positive electrode active material.

In addition, the total amount of tungsten and molybdenum contained in the coating raw material relative to the lithium of the excess lithium compound contained in the base powder is 1 equivalent, which means, for example, when the coating compound described later is dilithium ditungstate (Li ₂ WO ₄ ), 1 mole of tungsten atom (atomic weight: 183.85) reacts with 2 moles of lithium atom (atomic weight: 6.941), and when the coating compound is dilithium dimolybdate (Li ₂ MoO ₄ ), it means the amount of 1 mole of molybdenum atom (atomic weight: 95.94) reacts with 2 moles of lithium atom (atomic weight: 6.941). For example, in pure lithium tungstate (Li ₂ WO ₄ ) and lithium molybdate (Li ₂ MoO ₄ ), the composition ratio of lithium to tungsten (Li/W) and the composition ratio of lithium to molybdenum (Li/Mo) are both 2, respectively.

For example, as described above, when the coating compounds are _{Li2WO4 and Li2MoO4 ,} the equivalents of the coating compounds are represented by formula (II):
[Equivalents of Coating Compound]
= [tungsten or molybdenum content (%) / atomic weight of tungsten or molybdenum] / [lithium content (%) / (atomic weight of lithium × 2)] (II)
It is determined based on the following.

In addition _, when the coating compound is a compound such as _Li4WO5 , _{Li6W2O9 , Li4MoO5} , _{Li6Mo2O9 , etc.} , the equivalent weight of the coating compound can be determined by taking into _account the moles of lithium atoms, tungsten atoms, _and molybdenum atoms, respectively, and modifying formula (II).

The total amount of tungsten and molybdenum contained in the coating raw material per 100 parts by mass of the base powder is 0.1 to 6 parts by mass from the viewpoint of obtaining a positive electrode active material for lithium ion secondary batteries that has low positive electrode resistance and a high retention rate of discharge capacity when used in lithium ion secondary batteries.

The temperature of the first mixture when mixing the first mixture with the coating raw material is 25°C or higher, preferably 30°C or higher, and more preferably 33°C or higher, from the viewpoint of obtaining a positive electrode active material for lithium ion secondary batteries that has low positive electrode resistance and high discharge capacity retention rate when used in lithium ion secondary batteries, and is 50°C or lower, preferably 45°C or lower, more preferably 40°C or lower, and even more preferably 37°C or lower, from the viewpoint of obtaining a positive electrode active material for lithium ion secondary batteries that has low positive electrode resistance and high discharge capacity retention rate when used in lithium ion secondary batteries.

The atmosphere and temperature when the first mixture and the coating raw material are mixed are not particularly limited. The atmosphere when the first mixture and the coating raw material are mixed may be air, or may be, for example, an inert gas such as nitrogen gas or argon gas. The temperature when the first mixture and the coating raw material are mixed can be appropriately determined preferably from the range of 5 to 95°C, more preferably from the range of 10 to 80°C, but is usually preferably room temperature.

The first mixture and the coating raw material can be mixed using a mixer such as a shaker mixer, a stirring mixer, or a rocking mixer. It is preferable to mix the first mixture and the coating raw material until both are uniformly dispersed and have a uniform composition.

(4) Positive Electrode Active Material The positive electrode active material for lithium ion secondary batteries of the present invention is characterized in that a base powder containing secondary particles formed by agglomeration of primary particles of a lithium metal composite oxide containing lithium, nickel, and cobalt is used as a raw material, a coating layer containing at least one lithium salt selected from the group consisting of lithium tungstate and lithium molybdate is formed on the surface of the secondary particles, the coating layer has an average thickness of 3 to 150 μm, and a coefficient of variation (CV%) indicating the variation in thickness of the coating layer is 15% or less. Since the positive electrode active material for lithium ion secondary batteries of the present invention has the above-mentioned configuration, by using the positive electrode active material, a lithium ion secondary battery having low positive electrode resistance and high retention rate of discharge capacity can be obtained.

The positive electrode active material can be prepared by drying the second mixture obtained above at a temperature of 150 to 250°C to form a coating layer on the surfaces of the secondary particles contained in the base powder.

The drying temperature of the second mixture is 150 to 250°C, preferably 160 to 240°C, and more preferably 170 to 230°C, from the viewpoint of efficiently drying the second mixture and forming a coating layer having a uniform thickness with little variation in thickness.

The second mixture is preferably dried in an atmosphere of air from which carbon dioxide has been removed, nitrogen gas, argon gas, or other inert gas, in order to avoid a reaction between the carbon dioxide contained in the air and the coating layer. The pressure of the atmosphere in which the second mixture is dried is preferably reduced from atmospheric pressure in order to increase the drying efficiency of the second mixture. The second mixture is preferably dried until there is almost no noticeable weight loss of the second mixture. The drying time of the second mixture varies depending on the amount of the second mixture, the drying temperature, etc., and therefore cannot be determined in general. It is preferable to determine the drying time appropriately depending on the amount of the second mixture, the drying temperature, etc., but it is usually about 1 to 24 hours.

When drying the second mixture, a dryer such as a stationary dryer, a vibration dryer, a drum dryer, or a spray dryer can be used.

In the present invention, the above-mentioned operation is adopted, and thus a positive electrode active material containing secondary particles having a coating layer with a preferred thickness on the surface of the base powder, with small variation in thickness, and containing at least one lithium salt selected from the group consisting of lithium tungstate and lithium molybdate at a high content is obtained. In addition, since the secondary particles have a coating layer of lithium salt on their surfaces, the decrease in the specific surface area of the positive electrode active material is suppressed, and the conduction path of lithium ions at the interface with the electrolyte and solid electrolyte can be secured, so that the battery capacity of the lithium ion secondary battery can be increased and the positive electrode resistance can be reduced.

In this manner, a positive electrode active material containing secondary particles having a coating layer on the surface thereof containing at least one lithium salt selected from the group consisting of lithium tungstate and lithium molybdate can be obtained.

If the coating compound constituting the coating layer contains excess lithium compounds such as lithium hydroxide as impurities, when a solvent is used to prepare a positive electrode composite paste for a non-aqueous electrolyte lithium-ion secondary battery, the lithium compounds may dissolve into the solvent, and as the pH of the solvent increases, the concentration of the lithium salt in the solvent increases, which may cause gelation or lead to the formation of a heterogeneous interface in an all-solid-state lithium-ion secondary battery. In addition, when a positive electrode active material containing the lithium compounds is used for the positive electrode of a lithium-ion secondary battery, the battery capacity and output characteristics are reduced. For these reasons, at least one lithium salt selected from the group consisting of lithium tungstate and lithium molybdate, which have excellent resistance reducing functions, is used for the coating compound.

The average particle diameter MV of the positive electrode active material is preferably 5 to 15 μm from the viewpoint of improving the packing property of the positive electrode active material and the battery capacity per battery volume of the lithium ion secondary battery, uniforming the applied voltage between the secondary particles constituting the positive electrode active material, and uniforming the load between the secondary particles to improve cycle characteristics. The average particle diameter MV of the positive electrode active material is determined from the volume-based distribution measured by the laser diffraction/scattering method.

The tap density (bulk density) of the positive electrode active material is preferably 1.0 g/cm3 or more from the viewpoint of reducing voids when an electrode for a lithium ion secondary battery is produced and increasing the number of contact points between the positive electrode active material and the electrolyte and the solid electrolyte, thereby enhancing battery performance, and is preferably 3.0 g/ ^cm3 or less from the viewpoints of improving the contact area between the positive electrode active material and the electrolyte when the positive electrode active material is used in a non-aqueous electrolyte lithium ion secondary battery, and of sufficiently introducing the solid electrolyte between the particles of the positive electrode active material when the positive electrode active material is used in an all-solid-state lithium ion secondary battery, thereby reducing the electrical resistance of the lithium ion ^secondary battery and enhancing the battery capacity.

When the positive electrode active material of the present invention is used as the positive electrode active material of a non-aqueous electrolyte lithium ion secondary battery, the intercalation and deintercalation of lithium ions in the electrolyte and the positive electrode active material becomes active, and in an all-solid-state lithium ion secondary battery, the lithium ions diffuse without stagnation at the interface between the solid electrolyte and the positive electrode active material, so that the positive electrode resistance can be reduced and the output characteristics can be improved while maintaining the durability of the lithium ion secondary battery.

If the secondary particles are aggregated, they may be deflocculated, if necessary, using a deflocculating machine such as a pin mill, hammer mill, or pulverizer. The secondary particles may also be deflocculated using a drum dryer that can simultaneously dry the second mixture and deflocculate the aggregated secondary particles.

When the coating layer formed on the surface of the secondary particles is lithium tungstate, examples of the lithium _tungstate include _Li2WO4 , _Li4WO5 , _{Li6W2O9 ,} and mixtures thereof. When the coating layer formed on the surface of the secondary particles is lithium molybdate, _examples of the lithium _molybdate include _{Li2MoO4 , Li4MoO5 , Li6Mo2O9 ,} and mixtures thereof _.

The coating layer may contain lithium salts other than lithium tungstate and lithium molybdate, provided that the object of the present invention is not impaired. Examples of lithium salts other than lithium tungstate and lithium molybdate include lithium vanadate, lithium niobate, lithium stannate, lithium manganate, lithium ruthenate, lithium rhenate, lithium tantalate, lithium phosphate, and lithium borate, but the present invention is not limited to these examples. These lithium salts may be used alone or in combination of two or more.

[Analysis of Positive Electrode Active Material]
The positive electrode active material can be analyzed by the following method.
(a) Lithium hydroxide (LiOH) content and lithium carbonate ( _Li2CO3 ) content in the positive electrode active material _The lithium hydroxide (LiOH) content and lithium carbonate ( _Li2CO3 ) content in the positive electrode active material are preferably less than 0.003 mass% from the viewpoint of obtaining _a lithium ion secondary battery having a low positive electrode resistance and a high discharge capacity retention rate by using the positive electrode active material. The lithium hydroxide (LiOH) and lithium _carbonate ( _Li2CO3 ) contents in the positive electrode active material are values measured based on the following methods.

[Method of measuring the content of lithium hydroxide (LiOH) and lithium carbonate (Li ₂ CO ₃ ) in the positive electrode active material]
The lithium hydroxide (LiOH) content and the lithium carbonate (Li ₂ CO ₃ ) content in the positive electrode active material can each be determined by neutralization titration.

More specifically, lithium hydroxide and lithium carbonate present on the surface of the positive electrode active material particles dissolve in water and become hydroxide ions and carbonate ions, respectively, which are ionized from lithium ions. The amount of lithium hydroxide and lithium carbonate can be determined by titrating these ionized ions with an inorganic acid or the like. The neutralization titration method is a sequential titration method based on the so-called R. B. Warder method. In the neutralization titration method, the first end point (pH: about 8.3) is the inflection point of pH when the entire amount of lithium hydroxide reacts with half of the lithium carbonate, and the second end point (pH: about 3.8) is the inflection point of pH when half of the remaining lithium carbonate reacts. The inflection point (end point) of pH can be confirmed by visually observing the color change of an indicator such as phenolphthalein or methyl orange, or by instrumentally reading the change in potential difference using a pH composite electrode.

More specifically, for example, 2 to 20 g of the positive electrode active material is taken, the positive electrode active material is mixed with 120 mL of pure water, and 1 mol/L of hydrochloric acid is gradually added to the mixture while stirring the mixture, and the lithium hydroxide (LiOH) content and the lithium carbonate (Li ₂ CO ₃ ) content in the positive electrode active material can be determined based on the two pH inflection points that appear. In addition, when the lithium hydroxide (LiOH) content and the lithium carbonate (Li ₂ CO ₃ ) content in the positive electrode active material are determined using a pH composite electrode, these contents can be determined based on the end point (potential difference) using, for example, an automatic titrator (manufactured by Hiranuma Sangyo Co., Ltd., product number: COM-1750) using a pH composite electrode.

Furthermore, the lower the lithium hydroxide (LiOH) content and lithium _carbonate ( _Li2CO3 ) content in the positive electrode active material, the more evidence there is that the boron contained in the coating raw material and the lithium of the lithium compound contained in the base material powder have reacted at a value close to one equivalent.

(b) pH of the positive electrode active material slurry
The positive electrode active material slurry can be prepared by mixing the positive electrode active material and pure water in a ratio of 5 g of the positive electrode active material to 95 mL of pure water, and dispersing the positive electrode active material in the pure water. When the pH of the slurry is measured, it can be confirmed that no excess lithium compound remains by finding that the pH is 10 or less. The pH of the positive electrode active material slurry is a value measured based on the following method.

[Method for measuring pH of positive electrode active material slurry]
The pH of the positive electrode active material slurry is determined by allowing the positive electrode active material slurry to stand for 10 minutes and then measuring the pH of the supernatant liquid with a pH meter (manufactured by Thermo Fisher Scientific K.K., trade name: Orion-star-A214 (tabletop type)).

(c) Identification of the coating compound The coating compound constituting the coating layer present on the surface of the secondary particles used in the positive electrode active material can be identified by synchrotron radiation powder X-ray diffraction method (SXRD) described later, but it is preferable to identify it by synchrotron radiation powder X-ray diffraction method (SXRD) using "Spring-8" of the Synchrotron Radiation Science Research Center (National Research and Development Agency RIKEN). The principle and phenomenon of the synchrotron radiation powder X-ray diffraction method (SXRD) are almost the same as general X-ray diffraction method (XRD), and atomic arrangement and structure analysis can be performed from information such as the wavelength of X-rays, the spacing of crystal planes, and the relationship between the angle between the crystal planes and X-rays based on Bragg's law.

By using synchrotron radiation powder X-ray diffraction method (SXRD), it is possible to analyze the type, structure, and properties of trace substances in detail and accurately by taking advantage of the higher X-ray intensity and short pulse characteristics than X-ray diffraction method (XRD). Therefore, by using synchrotron radiation powder X-ray diffraction method (SXRD), it is possible to perform detailed structural analysis of samples in which the background noise is high and it is difficult to identify the target compound by X-ray diffraction method (XRD). Therefore, synchrotron radiation powder X-ray diffraction method (SXRD) can be suitably used to confirm that the coating layer having a thickness of nanometers present on the surface of the secondary particles used in the positive electrode active material of the present invention is composed of pure lithium salt.

(d) Average thickness and coefficient of variation CV% of the coating layer
The average thickness of the coating layer formed on the surface of the secondary particles is 3 to 150 nm, preferably 4 to 120 nm, from the viewpoint of obtaining a positive electrode active material for a lithium ion secondary battery that has low positive electrode resistance and high retention rate of discharge capacity when used in a lithium ion secondary battery. The average thickness of the coating layer is a value measured based on the following method.

The coefficient of variation CV% indicates the variation in the thickness of the coating layer. The coefficient of variation CV% is 15% or less from the viewpoint of stabilizing the intercalation and deintercalation of lithium ions and improving lithium ion conductivity. The coefficient of variation CV% can be adjusted to 15% or less, for example, by controlling the thickness of the coating layer to a predetermined value. The coefficient of variation CV% is a value measured based on the following method.

[Method of measuring average thickness and coefficient of variation CV% of coating layer]
When a positive electrode active material is observed using a transmission electron microscope (TEM), an image of the diffraction contrast (scattering contrast) of the interface between primary particles can be obtained, and therefore the TEM is suitable for analyzing the crystallinity of the positive electrode active material. In contrast, a scanning transmission electron microscope (STEM) is suitable for analyzing the composition of the layered structure in a compound and a semiconductor, since a HAADF-STEM (High-angle-Annular-Dark-Field-Scanning-TEM) image by Z (atomic number) contrast such as confirmation of the layered structure in a compound and a semiconductor can be obtained using the scanning transmission electron microscope. According to the scanning transmission electron microscope (STEM), an image reflecting the composition information can be obtained, and therefore the thickness of the coating layer in nanometers formed on the surface of the base material powder containing nickel and cobalt, such as the positive electrode active material of the present invention, can be accurately confirmed.

After drying the positive electrode active material, a cross-section polisher (CP) [manufactured by JEOL Ltd., product number: SM-09010] is used to process the secondary particles contained in the positive electrode active material to prepare a cross-sectional sample of the secondary particles, and the cross-section of the cross-sectional sample is observed with a scanning transmission electron microscope (STEM) [manufactured by JEOL Ltd., product number: ARM200F]. Two straight lines that intersect at right angles at approximately the center of the observed image of the cross-section of the cross-sectional sample are drawn, and the thickness of the coating layer at four points that intersect with these two straight lines is measured, and the average thickness of the coating layer and the coefficient of variation CV%, which indicates the variation in the thickness of the coating layer, are calculated from the numerical values of 10 secondary particles (n=40).

(e) Constituent elements of the coating compound (equivalent and composition ratio)
In order to accurately specify the constituent elements and their morphology of the coating compound identified by synchrotron radiation powder X-ray diffraction (SXRD), it is preferable to conduct a chemical analysis in addition to an analysis of the physical properties of the coating compound.

The coating compound contains a water-soluble compound such as at least one lithium salt selected from the group consisting of lithium tungstate and lithium molybdate. The water solubility of the coating compound can be utilized to analyze the constituent elements of the coating compound, and the total amount (equivalent weight) of tungsten and molybdenum can be determined based on formula (II). In addition, the type and form of each constituent element in the coating compound identified by synchrotron radiation powder X-ray diffraction (SXRD) can be confirmed to be appropriate by the following method for analyzing the constituent elements of the coating compound.

[Method of Analyzing Constituent Elements of Coating Compound]
A slurry was prepared by dispersing 5 g of the positive electrode active material in 95 mL of pure water at 25° C., and the slurry was stirred for 10 minutes and then filtered. Pure water was added to the filtrate so that the total volume was 200 mL to prepare an analysis specimen liquid. The analysis specimen liquid was appropriately diluted, and the contents of lithium, tungsten, and molybdenum contained in the analysis specimen liquid were measured using a multi-type ICP optical emission spectrometer (manufactured by Shimadzu Corporation, product number: ICPE-9000).

The composition ratio of lithium, tungsten, and molybdenum can be determined from the content of each element measured above and the total amount (equivalent) of tungsten and molybdenum determined based on formula (II), and it can be confirmed whether the composition ratio matches the coating compound identified by synchrotron radiation powder X-ray diffraction (SXRD).

For example, as described in Example 1 below, when the amount of tungsten oxide ( _WO3 ) per 100 g of base powder is adjusted to 0.23 g (tungsten (W) equivalent amount: 0.18 g) and tungsten oxide is added to the base powder as a coating raw material, when the lithium content in the entire cathode active material (dry material) contained in the coating compound (coating layer) present on the surface of the obtained cathode active material (dry material) is 0.014 mass% and the tungsten content in the coating compound (coating layer) is 0.20 mass% in the cathode active material (dry material), 0.1 mmol (0.7 mg) of lithium and 0.05 mmol (10 mg) of tungsten are contained per 5 g of cathode active material, and the composition ratio of lithium to tungsten (lithium/tungsten) is 2.

Furthermore, for example, as described in Example 1 below, when the coating compound identified by synchrotron radiation powder X-ray diffraction (SXRD) is only dilithium ditungstate (Li ₂ WO ₄ ), the Li ₂ WO ₄ is composed of 2 moles of lithium, 1 mole of tungsten, and 4 moles of oxygen, and therefore the composition ratio of lithium to tungsten (lithium/tungsten) is 2, which matches the composition ratio of lithium to tungsten (lithium/tungsten) of 2 determined by chemical analysis using a multi-type ICP optical emission spectrometer (manufactured by Shimadzu Corporation, product number: ICPE-9000).

In this way, whether the coating compound identified by synchrotron radiation powder X-ray diffraction (SXRD) is coated on the particle surface of the base powder can be evaluated by confirming that the analytical results by the physical analysis method and the analytical results by the chemical analysis method match. The reliability of the analytical results can be increased by performing both the physical analysis method and the chemical analysis method rather than performing only one of the two methods.

As shown in Comparative Example 9 below, when 100 g of base powder is mixed with 1.04 g of dilithium ditungstate ( _{Li2WO4 )} (tungsten equivalent: 0.73 g) as a coating compound, when the lithium content in the positive electrode active material (dry material) is 0.13 mass% and the tungsten content is 0.73 mass%, the amount of lithium per 5 g of positive electrode active material is 0.9 mmol (6.5 mg) and the amount of tungsten is 0.2 mmol (36 mg), and the composition ratio of lithium to tungsten (ratio of the number of lithium/tungsten atoms) is 5.

However, when the coating compound identified by synchrotron radiation powder X-ray diffraction (SXRD) is only dilithium ditungstate ( _Li2WO4 ), as described above, the composition ratio of lithium to tungsten (lithium/ _tungsten ) should be 2, but in Comparative Example 9, this does not match the composition ratio of lithium to tungsten determined by the ICP method, which is a chemical analysis method, being 5. From this, it is recognized that the coating compound contains lithium hydroxide (LiOH) and lithium _carbonate ( _Li2CO3 ) in addition to dilithium ditungstate ( _{Li2WO4 )} .

(f) Lithium Content in Base Powder after Production of Positive Electrode Active Material When preparing a positive electrode active material, if the amount of coating raw material is greater than the amount of excess lithium compound present on the surfaces of the secondary particles of the base powder, etc. (if lithium is insufficient), as described above, lithium will be taken away by the base powder, and the obtained positive electrode active material may not fully exhibit its performance as a positive electrode active material.

The lithium salt that constitutes the coating layer of the positive electrode active material is insoluble in organic solvents and easily soluble in water. Therefore, by immersing the positive electrode active material in water, dissolving the coating layer to isolate the base material powder, and completely dissolving the base material powder with an inorganic acid or the like, an analytical sample liquid for measuring the lithium content in the base material powder can be prepared. More specifically, the lithium content in the base material powder can be measured by the following method.

[Method for measuring lithium content in base powder after preparation of positive electrode active material]
A slurry is prepared by mixing 5 g of the positive electrode active material with 100 mL of pure water at room temperature, and the slurry is stirred for 10 minutes to dissolve the coating layer present on the surface of the base material powder in water. The slurry is then filtered, and the resulting cake-like solid content is collected.

The collected solids are added to an inorganic acid such as 35% hydrochloric acid and heated to a temperature of about 100-200°C to decompose, and purified water is added to the resulting decomposition liquid so that the total volume of the decomposition liquid is 200 mL to prepare the analytical sample liquid.

The analytical sample liquid obtained above is appropriately diluted, and the amount of lithium in the analytical sample liquid is measured using a multi-type ICP optical emission spectrometer (Shimadzu Corporation, product number: ICPE-9000), thereby determining the lithium content in the base powder after the positive electrode active material is produced.

As described above, according to the method for producing a positive electrode active material for a lithium ion secondary battery of the present invention, the coating compound constituting the coating layer having a resistance reducing function is coated on the particle surface of the base powder, the thickness of the coating layer is optimized, and a positive electrode active material for a lithium ion secondary battery in which the variation in the thickness of the coating layer (coefficient of variation CV%) is suppressed can be efficiently obtained.

Therefore, when the positive electrode active material for lithium ion secondary batteries of the present invention is used in a non-aqueous electrolyte lithium ion secondary battery, gelation during the preparation of the positive electrode composite paste can be suppressed, improving the quality and safety of the secondary battery. When the positive electrode active material is used in an all-solid-state lithium ion secondary battery, the formation of a heterogeneous interface is suppressed and the thickness of the coating layer becomes uniform and homogeneous, improving the battery characteristics after preparation of the lithium ion secondary battery.

(g) Schematic diagram of the positive electrode active material for lithium ion secondary batteries The positive electrode active material for lithium ion secondary batteries of the present invention will be described below with reference to Fig. 1. Fig. 1 is a schematic diagram showing one embodiment of the positive electrode active material for lithium ion secondary batteries of the present invention.

Figure 1 (a) shows a state in which primary particles 1 of the base powder are aggregated to form secondary particles 2. Since secondary particles 2 of the base powder do not have a coating layer with high lithium ion conductivity such as lithium tungstate and lithium molybdate, the intercalation of lithium ions into the crystal lattice shown by arrow A and the deintercalation into the crystal lattice shown by arrow B are not actively performed. Therefore, when secondary particles 2 are used in a lithium ion secondary battery, it is thought that the positive electrode resistance will be high and the retention rate of discharge capacity will be low.

In contrast, in the positive electrode active material for lithium ion secondary batteries of the present invention, as shown in FIG. 1(b), primary particles 1 are aggregated to form secondary particles 2, and further, a coating layer 3 is formed on the surface of the secondary particles 2. Therefore, due to the high lithium ion conductivity of the coating layer 3, the intercalation of lithium ions into the crystal lattice shown by arrow C and the deintercalation into the crystal lattice shown by arrow D are actively carried out. Therefore, when the secondary particles 2 are used in a lithium ion secondary battery, it is believed that the positive electrode resistance is reduced and the retention rate of the discharge capacity is improved.

2. Lithium-ion secondary battery The lithium-ion secondary battery of the present invention is characterized by having a positive electrode containing a positive electrode active material for lithium-ion secondary batteries.

The components of the lithium ion secondary battery of the present invention are not particularly limited as long as they are similar to the components of commonly used lithium ion secondary batteries. For example, when the lithium ion secondary battery of the present invention is a non-aqueous electrolyte lithium ion secondary battery, it includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. When the lithium ion secondary battery of the present invention is an all-solid-state lithium ion secondary battery, it includes a positive electrode, a negative electrode, and a solid electrolyte.

The embodiments of the lithium ion secondary battery of the present invention described below are merely examples, and embodiments in which various modifications or improvements have been made to the above embodiments based on the knowledge of those skilled in the art are included within the scope of the present invention.

(1) Non-aqueous electrolyte lithium ion secondary battery [positive electrode]
First, the positive electrode active material of the present invention, a conductive material, and a binder are mixed to prepare a positive electrode mixture. Components such as activated carbon and an organic solvent used to adjust the viscosity are added to the positive electrode mixture as necessary, and the resulting mixture is further kneaded to obtain a positive electrode mixture paste.

The ratio of each component in the positive electrode mixture is an important factor in determining the performance of a lithium-ion secondary battery. The ratio of each component in the positive electrode mixture is not particularly limited. The solid content of the positive electrode mixture from which the organic solvent has been removed preferably contains 60 to 95 mass% positive electrode active material, 1 to 20 mass% conductive material, and 1 to 20 mass% binding agent (binder), similar to the positive electrode of a typical lithium secondary battery.

Next, the positive electrode composite paste is applied to the surface of a current collector made of, for example, aluminum foil, and then dried to volatilize and remove the organic solvent, thereby obtaining a positive electrode. If necessary, the positive electrode may be pressurized using a roll press or the like to increase the electrode density.

The positive electrode is obtained in the manner described above, and is usually in the form of a sheet. The positive electrode is cut to an appropriate size as necessary and then used in manufacturing a battery.

The present invention is not limited to the above-mentioned method for producing the positive electrode, and the positive electrode may be produced by other methods.

The conductive material may be, for example, graphite such as natural graphite, artificial graphite, or expanded graphite, or carbon black such as acetylene black or ketjen black. The binder may be, for example, fluorine-containing resins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene, ethylene-propylene-diene rubber, or fluorine rubber, styrene-butadiene rubber, cellulose-based resins, acrylic resins, polypropylene, or olefin-based resins such as polyethylene, which serve to bind the particles of the positive electrode active material together.

After preparing the positive electrode mixture by mixing the positive electrode active material, conductive material, and binder, an organic solvent may be added to the positive electrode mixture to dissolve the binder, and the mixture may be kneaded to prepare a positive electrode mixture paste. For example, N-methyl-2-pyrrolidone (NMP) may be used as the organic solvent. Activated carbon may be added to the positive electrode mixture as necessary to increase the capacity of the electric double layer.

[Negative Electrode]
The negative electrode can be produced by mixing a negative electrode active material that absorbs and releases lithium metal, a lithium alloy, or lithium ions with a binder, adding an organic solvent to the resulting mixture to prepare a negative electrode mixture paste, applying the negative electrode mixture paste onto the surface of a current collector made of a metal foil such as copper foil, and drying the paste to volatilize and remove the organic solvent. If necessary, the negative electrode may be pressurized using a roll press or the like to increase the electrode density.

For example, the negative electrode active material may be natural graphite, artificial graphite, a sintered body of an organic compound such as phenolic resin, or a powder of a carbon material such as coke. For the negative electrode binder, similar to the positive electrode, for example, a fluorine-containing resin binder such as polyvinylidene fluoride (PVDF), a water-based binder such as styrene-butadiene rubber, or an organic solvent such as N-methyl-2-pyrrolidone (NMP) used to disperse the negative electrode active material and the binder may be used. In addition, a viscosity modifier such as carboxymethyl cellulose (CMC) may be used in an appropriate amount in the negative electrode active material to improve the binding force.

[Separator]
A separator is disposed between the positive electrode and the negative electrode. The separator is used to separate the positive electrode and the negative electrode and to hold the electrolyte. For example, a membrane made of a resin such as polyethylene or polypropylene with many fine through holes formed therein can be used as the separator.

[Non-aqueous electrolyte]
The non-aqueous electrolyte is a solution in which a lithium salt as a supporting salt is dissolved in an organic solvent. Examples of the organic solvent include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and trifluoropropylene carbonate (TFPC); chain carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dipropyl carbonate (DPC); ether compounds such as tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), and dimethoxyethane (DME); sulfur compounds such as ethyl methyl sulfone and 1,4-butane sultone, and phosphate ester compounds such as triethyl phosphate and trioctyl phosphate, but the present invention is not limited to only these examples. These organic solvents may be used alone or in combination of two or more types.

Examples of supporting salts include lithium hexafluorophosphate ( _LiPF6 ), lithium tetrafluoroborate ( _LiBF4 ), lithium perchlorate ( _LiClO4 ), lithium hexafluoroarsenate _{(LiAsF6), lithium bis(trifluoromethanesulfonyl)imide [LiN(CF3SO2)2 ] , and} composite salts thereof, but the present invention is not limited to these examples. These supporting salts may be used alone or in combination of two or more kinds.

The non-aqueous electrolyte may contain additives such as radical scavengers, surfactants, and flame retardants, as long as they do not impair the objectives of the present invention.

[Shape and configuration of lithium-ion secondary battery]
The lithium ion secondary battery, which is composed of a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte solution, may have various shapes such as a cylindrical type, a laminated type, etc. Regardless of the shape of the lithium ion secondary battery, the positive electrode and the negative electrode are laminated with a separator interposed therebetween to form an electrode body, and the electrode body is impregnated with a non-aqueous electrolyte solution. A current collecting lead or the like is connected between the positive electrode and a positive electrode terminal connected to the outside, and between the negative electrode and a negative electrode terminal connected to the outside, respectively.

The lithium-ion secondary battery can be completed by housing the electrode body having the above configuration in a battery case and sealing it.

[Characteristics]
A. Positive Electrode Resistance A 2032-type coin battery CBA is prepared, charged at a charging potential of 4.1 V, and measured by an AC impedance method using a frequency response analyzer and a potentiogalvanostat to obtain a Nyquist plot. This Nyquist plot is expressed as the sum of characteristic curves showing the solution resistance, the negative electrode resistance and its capacity, and the positive electrode resistance and its capacity, so that the positive electrode resistance can be calculated by performing a fitting calculation using an equivalent circuit based on the Nyquist plot.

B. Initial Discharge Capacity and Retention Rate of Discharge Capacity The initial discharge capacity is the discharge capacity when the 2032 type coin battery CBA is left for about 24 hours after production, and after the open circuit voltage OCV (Open Circuit Voltage) is stabilized, the battery is charged to a cutoff voltage of 4.3 V with a current density of 0.1 mA/ ^cm2 to the positive electrode, and then discharged to a cutoff voltage of 3.0 V after a 1-hour pause.

The discharge capacity retention rate was determined by holding the temperature of the coin battery CBA at 60° C., measuring the discharge capacity at the first cycle (initial discharge capacity), pausing for 10 minutes, repeating the charge/discharge cycle for 500 cycles (charge/discharge), including the measurement of the initial discharge capacity, in the same manner as in the measurement of the initial discharge capacity, measuring the discharge capacity at the 500th cycle, and calculating the retention rate according to the formula:
[Discharge capacity retention rate (%)]
= [(discharge capacity at 500th cycle)÷(initial discharge capacity)]×100
This value was calculated based on the above.

(2) All-solid-state lithium-ion secondary battery [cathode]
For the positive electrode, a mixture obtained by mixing a positive electrode active material powder with a sulfide-based lithium ion conductive _{solid electrolyte powder such as Li2S-P2S5-based glass or Li10GeP2S12 in an appropriate ratio (} for example, a mass ratio of 7:3) can be used.

[Negative Electrode]
The negative electrode may be a mixture obtained by mixing a negative electrode active material capable of absorbing and desorbing lithium ions, such as metallic lithium, a lithium-indium alloy, or a sulfide-based lithium ion conductive solid electrolyte powder in an appropriate ratio (for example, a mass ratio of 7:3). For example, the negative electrode active material may be natural graphite, artificial graphite, a fired body of an organic compound such as phenolic resin, or a powder of a carbon material such as coke.

[Solid electrolyte]
The solid electrolyte is preferably a lithium ion conductor having an ion conductivity of ^10-4 S/cm or more, such as a sulfide-based lithium ion conductive solid electrolyte. Examples of the sulfide-based lithium ion conductive solid electrolyte include _Li2S - _{P2S5 -} based glass, _{Li10GeP2S12 -} based glass, and _{Li3PO4 - Li2S} - _{SiS2 -} based glass, but the present invention is not limited to these examples.

[Shape and configuration of all-solid-state lithium-ion secondary battery]
As described above, the all-solid-state lithium ion secondary battery has a positive electrode, a negative electrode, and a solid electrolyte. The shape of the all-solid-state lithium ion secondary battery can be, for example, a circular type or a sheet type. Regardless of the shape of the all-solid-state lithium ion secondary battery, the positive electrode and the negative electrode are used as electrode bodies, a solid electrolyte is present between the positive electrode and the negative electrode, and a positive electrode current collector provided on the positive electrode and a positive electrode terminal connected to the outside, and a negative electrode current collector provided on the negative electrode and a negative electrode terminal connected to the outside are connected by current collecting leads or the like, and these are housed in a battery case and sealed to obtain an all-solid-state lithium ion secondary battery.

[Characteristics]
A. Positive Electrode Resistance The positive electrode resistance is a value calculated by charging an all-solid-state battery to a charging potential of 4.0 V, measuring the AC impedance using a frequency response analyzer and a potentiogalvanostat, and calculating the positive electrode resistance from the obtained impedance spectrum using an equivalent circuit.

B. Initial Discharge Capacity and Discharge Capacity Retention Rate The initial discharge capacity is the capacity when the all-solid-state battery is produced, charged to a charging potential of 4.2 V at a constant current density, and discharged to 3.0 V. The discharge capacity retention rate is measured by holding the all-solid-state battery at 25° C. and measuring the initial discharge capacity, and then repeating the charge/discharge cycle for 50 cycles (charge/discharge) including the measurement of the initial discharge capacity in the same manner as in the measurement of the initial discharge capacity, and measuring the discharge capacity at the 50th cycle, and calculating the discharge capacity retention rate from the formula:
[Discharge capacity retention rate (%)]
= [(discharge capacity at 50th cycle)÷(initial discharge capacity)]×100
This value was calculated based on the above.

Next, the present invention will be described in more detail based on examples, but the present invention is not limited to these examples.

In each example, the base material powder, the positive electrode active material, and the reagents used in manufacturing the lithium ion secondary battery were all special grade reagents manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. The base material powder and the positive electrode active material were analyzed and identified, and the results are shown in Tables 1 and 2 .

[Example 1]
As the base powder, a lithium metal _composite oxide powder was used having a _solid particle structure, a _composition of _{Li1.02Ni0.82Co0.15Al0.03O2} , an average particle diameter MV of _11.8 μm, a particle size distribution [(d90-d10)/average particle diameter] of 0.81, a tap density of 2.6 g/ ^cm3 , and a pH of the base powder slurry of 11.0.

The first mixture was obtained by mixing 100 g of the lithium metal composite oxide powder with 7 g of water.

Tungsten trioxide (WO ₃ ) was used as a coating raw material. The amount of tungsten contained in the tungsten trioxide (WO 3 ) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the “Total Amount of W and Mo” column in Table 1 so that the amount of tungsten contained in the coating compound produced would be 1 equivalent relative to the _{lithium of the excess lithium compound contained in the lithium metal composite oxide powder. The first mixture was mixed with tungsten trioxide (WO 3 )} while the temperature of the first mixture obtained above was adjusted to 40° C., thereby obtaining a second mixture.

Next, the second mixture obtained above was dried at a temperature of 230°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain a positive electrode active material.

The coating compound on the particle surfaces of the positive electrode active material obtained above was identified, and it was confirmed that the coating compound was lithium tungstate and had a composition of dilithium ditungstate (Li ₂ WO ₄ ).

The coating layer of the positive electrode active material had an average thickness of 4 nm, and the coefficient of variation CV% indicating the thickness variation was 15%. It was also confirmed that the pH of the slurry of the positive electrode active material was 9.5, and that the content of lithium hydroxide (LiOH) and the content of lithium _carbonate ( _Li2CO3 ) in the positive electrode active material were each less than 0.003 mass%.

The lithium content in the base powder after preparation of the positive electrode active material (listed in the "Li in base powder" column in Table 1) was 7.1% by mass.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.27. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 90%.

The positive electrode resistance and discharge capacity retention rate of the lithium ion secondary battery were measured based on the following method.

[Method of measuring positive electrode resistance value and discharge capacity retention rate]
The positive electrode used 15 mg of a mixture obtained by mixing the positive electrode active material and _75Li2S - _{25P2S5 glass} in a mass ratio of 7:3. The negative electrode used 6 mg of a mixture obtained by mixing artificial graphite and _75Li2S - _25P2S5 glass in a mass ratio of 7:3. The solid electrolyte used 150 _mg of _75Li2S - _{25P2S5 glass} .

The positive electrode, the solid electrolyte, and the negative electrode were stacked to produce a three-layer laminate, which was then pressure-molded to a diameter of 10 mm at a pressure of 300 MPa. Stainless steel plates were pressed onto both sides of the resulting molded body to form current collectors, producing a circular all-solid-state lithium-ion secondary battery.

(1) Positive Electrode Resistance Value The all-solid-state lithium ion secondary battery obtained above was charged at a charging potential of 4.0 V, and an impedance spectrum was obtained by measuring by an AC impedance method using a frequency response analyzer and a potentiogalvanostat 1255B (manufactured by Solartron, UK, product name). Two semicircles were observed in the high frequency region and the intermediate frequency region in the impedance spectrum obtained above, and the positive electrode resistance value was calculated by analyzing the semicircle in the intermediate frequency region using an equivalent circuit.

(2) Retention Rate of Discharge Capacity Using the all-solid-state lithium ion secondary battery obtained above, the battery was charged in air at 25° C. at a constant current density until the potential reached 4.2 V, and then discharged until the potential reached 3.0 V, and the initial discharge capacity was measured.

After measuring the initial discharge capacity, charge and discharge were repeated 50 cycles including the measurement of the initial discharge capacity in the same manner as in the measurement of the initial discharge capacity, and then the discharge capacity at the 50th cycle was measured to obtain the formula (III):
[Discharge capacity retention rate (%)]
= {[discharge capacity at a given time (mAh or Ah)]}
÷ [Initial discharge capacity (mAh or Ah)] × 100 (III)
The discharge capacity retention rate was calculated based on the above.

[Example 2]
As the base powder, a lithium metal _composite oxide powder was used having a _solid particle structure, a _composition of _{Li1.02Ni0.82Co0.15Al0.03O2} , an average particle diameter MV of _11.8 μm, a particle size distribution [(d90-d10)/average particle diameter] of 0.82, a tap density of 2.6 g/ ^cm3 , and a pH of the base powder slurry of 11.3.

The first mixture was obtained by mixing 100 g of the lithium metal composite oxide powder with 7 g of water.

Tungsten trioxide (WO ₃ ) was used as a coating raw material. The amount of tungsten contained in the tungsten trioxide (WO 3 ) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the “Total Amount of W and Mo” column in Table 1 so that the amount of tungsten contained in the coating compound produced would be 1 equivalent relative to the _{lithium of the excess lithium compound contained in the lithium metal composite oxide powder. The first mixture was mixed with tungsten trioxide (WO 3 )} while the temperature of the first mixture obtained above was adjusted to 40° C., thereby obtaining a second mixture.

Next, the second mixture obtained above was dried at a temperature of 230°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain a positive electrode active material.

The coating compound on the particle surfaces of the positive electrode active material obtained above was identified, and it was confirmed that the coating compound was lithium tungstate and had a composition of dilithium ditungstate (Li ₂ WO ₄ ).

The coating layer of the positive electrode active material had an average thickness of 7 nm, and the coefficient of variation CV% indicating the thickness variation was 12%. It was also confirmed that the pH of the slurry of the positive electrode active material was 9.7, and that the content of lithium hydroxide (LiOH) and the content of lithium _carbonate ( _Li2CO3 ) in the positive electrode active material were each less than 0.003 mass%.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.24. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 91%.

[Example 3]
As the base powder, a lithium metal _composite oxide powder was used having a _solid particle structure, a _composition of _{Li1.02Ni0.82Co0.15Al0.03O2} , an average particle diameter MV of _11.7 μm, a particle size distribution [(d90-d10)/average particle diameter] of 0.82, a tap density of 2.7 g/ ^cm3 , and a pH of the base powder slurry of 11.6.

The first mixture was obtained by mixing 100 g of the lithium metal composite oxide powder with 7 g of water.

Tungsten trioxide (WO ₃ ) was used as a coating raw material. The amount of tungsten contained in the tungsten trioxide (WO 3 ) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the "Total amount of W and Mo" column in Table 1 so that the amount of tungsten contained in the coating compound produced would be 1 equivalent relative to the amount of lithium in the excess lithium compound contained in the lithium metal composite oxide powder _. The first mixture was mixed with tungsten trioxide (WO ₃ ) while the temperature of the first mixture obtained above was adjusted to 40° C., thereby obtaining a second mixture.

Next, the second mixture obtained above was dried at a temperature of 230°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain a positive electrode active material.

The coating compound on the particle surfaces of the positive electrode active material obtained above was identified, and it was confirmed that the coating compound was lithium tungstate and had a composition of dilithium ditungstate (Li ₂ WO ₄ ).

The coating layer of the positive electrode active material had an average thickness of 14 nm, and the coefficient of variation CV% indicating the thickness variation was 8.6%. It was also confirmed that the pH of the slurry of the positive electrode active material was 9.7, and that the lithium hydroxide (LiOH) content and the lithium _carbonate ( _Li2CO3 ) content in the positive electrode active material were each less than 0.003 mass%.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.18. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 92%.

[Example 4]
As the base powder, a lithium metal _composite oxide powder was used having a _solid particle structure, a _composition of _{Li1.02Ni0.82Co0.15Al0.03O2} , an average particle diameter MV of _11.8 μm, a particle size distribution [(d90-d10)/average particle diameter] of 0.81, a tap density of 2.6 g/ ^cm3 , and a pH of the base powder slurry of 11.9.

The first mixture was obtained by mixing 100 g of the lithium metal composite oxide powder with 7 g of water.

Tungsten trioxide (WO ₃ ) was used as a coating raw material. The amount of tungsten contained in the tungsten trioxide (WO 3 ) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the "Total amount of W and Mo" column in Table 1 so that the amount of tungsten contained in the coating compound produced would be 1 equivalent relative to the amount of lithium in the excess lithium compound contained in the lithium metal composite oxide powder _. The first mixture was mixed with tungsten trioxide (WO ₃ ) while the temperature of the first mixture obtained above was adjusted to 40° C., thereby obtaining a second mixture.

Next, the second mixture obtained above was dried at a temperature of 230°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain a positive electrode active material.

The coating compound on the particle surfaces of the positive electrode active material obtained above was identified, and it was confirmed that the coating compound was lithium tungstate and had a composition of dilithium ditungstate (Li ₂ WO ₄ ).

The coating layer of the positive electrode active material had an average thickness of 29 nm, and the coefficient of variation CV% indicating the thickness variation was 5.2%. It was also confirmed that the pH of the slurry of the positive electrode active material was 9.6, and that the lithium hydroxide (LiOH) content and the lithium _carbonate ( _Li2CO3 ) content in the positive electrode active material were each less than 0.003 mass%.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.11. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 93%.

[Example 5]
As the base powder, a lithium metal _composite oxide powder was used having a _solid particle structure, a _composition of _{Li1.02Ni0.82Co0.15Al0.03O2} , an average particle diameter MV of _11.8 μm, a particle size distribution [(d90-d10)/average particle diameter] of 0.80, a tap density of 2.6 g/ ^cm3 , and a pH of the base powder slurry of 12.2.

The first mixture was obtained by mixing 100 g of the lithium metal composite oxide powder with 7 g of water.

Tungsten trioxide (WO ₃ ) was used as a coating raw material. The amount of tungsten contained in the tungsten trioxide (WO 3 ) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the "Total amount of W and Mo" column in Table 1 so that the amount of tungsten contained in the coating compound produced would be 1 equivalent relative to the amount of lithium in the excess lithium compound contained in the lithium metal composite oxide powder _. The first mixture was mixed with tungsten trioxide (WO ₃ ) while the temperature of the first mixture obtained above was adjusted to 40° C., thereby obtaining a second mixture.

Next, the second mixture obtained above was dried at a temperature of 230°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain a positive electrode active material.

The coating compound on the particle surfaces of the positive electrode active material obtained above was identified, and it was confirmed that the coating compound was lithium tungstate and had a composition of dilithium ditungstate (Li ₂ WO ₄ ).

The coating layer of the positive electrode active material had an average thickness of 57 nm, and the coefficient of variation CV% indicating the thickness variation was 3.4%. It was also confirmed that the pH of the slurry of the positive electrode active material was 9.5, and that the lithium hydroxide (LiOH) content and the lithium _carbonate ( _Li2CO3 ) content in the positive electrode active material were each less than 0.003 mass%.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.15. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 92%.

[Example 6]
As the base powder, a lithium metal _composite oxide powder was used having a _solid particle structure, a _composition of _{Li1.02Ni0.82Co0.15Al0.03O2} , an average particle diameter MV of _11.7 μm, a particle size distribution [(d90-d10)/average particle diameter] of 0.82, a tap density of 2.7 g/ ^cm3 , and a pH of the base powder slurry of 12.5.

The first mixture was obtained by mixing 100 g of the lithium metal composite oxide powder with 7 g of water.

Tungsten trioxide (WO ₃ ) was used as a coating raw material. The amount of tungsten contained in the tungsten trioxide (WO 3 ) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the "Total amount of W and Mo" column in Table 1 so that the amount of tungsten contained in the coating compound produced would be 1 equivalent relative to the amount of lithium in the excess lithium compound contained in the lithium metal composite oxide powder _. The first mixture was mixed with tungsten trioxide (WO ₃ ) while the temperature of the first mixture obtained above was adjusted to 40° C., thereby obtaining a second mixture.

Next, the second mixture obtained above was dried at a temperature of 230°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain a positive electrode active material.

The coating compound on the particle surfaces of the positive electrode active material obtained above was identified, and it was confirmed that the coating compound was lithium tungstate and had a composition of dilithium ditungstate (Li ₂ WO ₄ ).

The coating layer of the positive electrode active material had an average thickness of 116 nm, and the coefficient of variation CV% indicating the thickness variation was 5.1%. It was also confirmed that the pH of the slurry of the positive electrode active material was 9.6, and that the content of lithium hydroxide (LiOH) and the content of lithium _carbonate ( _Li2CO3 ) in the positive electrode active material were each less than 0.003 mass%.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.22. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 90%.

[Example 7]
As the base powder, a lithium metal _composite oxide powder was used having a _solid particle structure, a _composition of _{Li1.02Ni0.82Co0.15Al0.03O2} , an average particle diameter MV of _12.0 μm, a particle size distribution [(d90-d10)/average particle diameter] of 0.81, a tap density of 2.5 g/ ^cm3 , and a pH of the base powder slurry of 11.5.

The first mixture was obtained by mixing 100 g of the lithium metal composite oxide powder with 4 g of water.

Molybdenum trioxide (MoO ₃ ) was used as a coating raw material. The amount of molybdenum contained in the molybdenum trioxide (MoO 3 ) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the “Total Amount of W and Mo” column in Table 1 so that the amount of molybdenum contained in the coating compound produced would be 1 equivalent relative to the amount of lithium in the excess lithium compound contained in the lithium metal composite oxide powder _{. The first mixture was mixed with molybdenum trioxide (MoO 3 )} while the temperature of the first mixture obtained above was adjusted to 30° C., thereby obtaining a second mixture.

Next, the second mixture obtained above was dried at a temperature of 170°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain a positive electrode active material.

The coating compound on the particle surfaces of the positive electrode active material obtained above was identified, and it was confirmed that the coating compound was lithium molybdate and had a composition of dilithium dimolybdate (Li ₂ MoO ₄ ).

The coating layer of the positive electrode active material had an average thickness of 10 nm, and the coefficient of variation CV% indicating the thickness variation was 11%. It was also confirmed that the pH of the slurry of the positive electrode active material was 9.6, and that the content of lithium hydroxide (LiOH) and the content of lithium _carbonate ( _Li2CO3 ) in the positive electrode active material were each less than 0.003 mass%.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.20. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 90%.

[Example 8]
As the base powder, a lithium metal _composite oxide powder was used having a _solid particle structure, a _composition of _{Li1.02Ni0.82Co0.15Al0.03O2} , an average particle diameter MV of _12.1 μm, a particle size distribution [(d90-d10)/average particle diameter] of 0.80, a tap density of 2.5 g/ ^cm3 , and a pH of the base powder slurry of 12.0.

The first mixture was obtained by mixing 100 g of the lithium metal composite oxide powder with 4 g of water.

Molybdenum trioxide (MoO ₃ ) was used as a coating raw material. The amount of molybdenum contained in the molybdenum trioxide (MoO 3 ) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the “Total Amount of W and Mo” column in Table 1 so that the amount of molybdenum contained in the coating compound produced would be 1 equivalent relative to the amount of lithium in the excess lithium compound contained in the lithium metal composite oxide powder _{. The first mixture was mixed with molybdenum trioxide (MoO 3 )} while the temperature of the first mixture obtained above was adjusted to 30° C., thereby obtaining a second mixture.

Next, the second mixture obtained above was dried at a temperature of 170°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain a positive electrode active material.

The coating compound on the particle surfaces of the positive electrode active material obtained above was identified, and it was confirmed that the coating compound was lithium molybdate and had a composition of dilithium dimolybdate (Li ₂ MoO ₄ ).

The coating layer of the positive electrode active material had an average thickness of 33 nm, and the coefficient of variation CV% indicating the thickness variation was 6.2%. It was also confirmed that the pH of the slurry of the positive electrode active material was 9.6, and that the lithium hydroxide (LiOH) content and the lithium _carbonate ( _Li2CO3 ) content in the positive electrode active material were each less than 0.003 mass%.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.13. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 93%.

[Example 9]
As the base powder, a lithium metal _composite oxide powder was used having a _solid particle structure, a _composition of _{Li1.02Ni0.82Co0.15Al0.03O2} , an average particle diameter MV of _12.1 μm, a particle size distribution [(d90-d10)/average particle diameter] of 0.82, a tap density of 2.5 g/ ^cm3 , and a pH of the base powder slurry of 12.4.

The first mixture was obtained by mixing 100 g of the lithium metal composite oxide powder with 4 g of water.

Molybdenum trioxide (MoO ₃ ) was used as a coating raw material. The amount of molybdenum contained in the molybdenum trioxide (MoO 3 ) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the “Total Amount of W and Mo” column in Table 1 so that the amount of molybdenum contained in the coating compound produced would be 1 equivalent relative to the amount of lithium in the excess lithium compound contained in the lithium metal composite oxide powder _{. The first mixture was mixed with molybdenum trioxide (MoO 3 )} while the temperature of the first mixture obtained above was adjusted to 30° C., thereby obtaining a second mixture.

Next, the second mixture obtained above was dried at a temperature of 170°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain a positive electrode active material.

The coating compound on the particle surfaces of the positive electrode active material obtained above was identified, and it was confirmed that the coating compound was lithium molybdate and had a composition of dilithium dimolybdate (Li ₂ MoO ₄ ).

The coating layer of the positive electrode active material had an average thickness of 82 nm, and the coefficient of variation CV% indicating the thickness variation was 4.3%. It was also confirmed that the pH of the slurry of the positive electrode active material was 9.5, and that the lithium hydroxide (LiOH) content and the lithium _carbonate ( _Li2CO3 ) content in the positive electrode active material were each less than 0.003 mass%.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.19. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 91%.

[Example 10]
As the base powder, a lithium metal _composite oxide powder was used having a _solid particle structure, a _composition of _{Li1.02Ni0.82Co0.15Al0.03O2} , an average particle diameter MV of _11.9 μm, a particle size distribution [(d90-d10)/average particle diameter] of 0.82, a tap density of 2.5 g/ ^cm3 , and a pH of the base powder slurry of 12.1.

The first mixture was obtained by mixing 100 g of the lithium metal composite oxide powder with 6 g of water.

As the coating raw material, a mixture of coating raw materials obtained by mixing tungsten trioxide ( _WO3 ) and molybdenum trioxide ( _MoO3 ) in a molar ratio of 1:1 was used. The total amount of tungsten and molybdenum contained in the coating compound produced was adjusted to be the amount shown in the "Total amount of W and Mo" column in Table 1 per 100 g of the lithium metal composite oxide powder so that the total amount of tungsten and molybdenum contained in the coating raw material mixture per 100 g of the lithium metal composite oxide powder was 1 equivalent to the amount of lithium in the excess lithium compound contained in the lithium metal composite oxide powder. The first mixture and the coating raw material mixture were mixed while adjusting the temperature of the first mixture obtained above to 35°C, thereby obtaining a second mixture.

Next, the second mixture obtained above was dried at a temperature of 200°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain a positive electrode active material.

The coating compounds on the particle surface of the positive electrode active material obtained above were identified, and it was confirmed that the coating compounds were lithium tungstate and lithium molybdate, and that their compositions were _dilithium ditungstate ( _Li2WO4 ) and dilithium dimolybdate ( _{Li2MoO4 )} .

The coating layer of the positive electrode active material had an average thickness of 45 nm, and the coefficient of variation CV% indicating the thickness variation was 4.4%. It was also confirmed that the pH of the slurry of the positive electrode active material was 9.7, and that the lithium hydroxide (LiOH) content and the lithium _carbonate ( _Li2CO3 ) content in the positive electrode active material were each less than 0.003 mass%.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described later was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.16. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 92%.

The positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.16, assuming that the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) was 1.00, and it was confirmed that the discharge capacity retention rate of the lithium ion secondary battery was 92%.

[Comparative Example 1]
As the base powder, a lithium metal _composite oxide powder was used having a _solid particle structure, a _composition of _{Li1.02Ni0.82Co0.15Al0.03O2} , an average particle diameter MV of _11.8 μm, a particle size distribution [(d90-d10)/average particle diameter] of 0.80, a tap density of 2.6 g/ ^cm3 , and a pH of the base powder slurry of 10.5.

The first mixture was obtained by mixing 100 g of the lithium metal composite oxide powder with 7 g of water.

Tungsten trioxide (WO ₃ ) was used as a coating raw material. The amount of tungsten contained in the tungsten trioxide (WO 3 ) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the "Total amount of W and Mo" column in Table 1 so that the amount of tungsten contained in the coating compound produced would be 1 equivalent relative to the amount of lithium in the excess lithium compound contained in the lithium metal composite oxide powder _. The first mixture was mixed with tungsten trioxide (WO ₃ ) while the temperature of the first mixture obtained above was adjusted to 40° C., thereby obtaining a second mixture.

Next, the second mixture obtained above was dried at a temperature of 230°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain a positive electrode active material.

The coating compound on the particle surfaces of the positive electrode active material obtained above was identified, and it was confirmed that the coating compound was lithium tungstate and had a composition of dilithium ditungstate (Li ₂ WO ₄ ).

The coating layer of the positive electrode active material had an average thickness of 1 nm, and the coefficient of variation CV% indicating the thickness variation was 82%. It was also confirmed that the pH of the slurry of the positive electrode active material was 9.6, and that the content of lithium hydroxide (LiOH) and the content of lithium _carbonate ( _Li2CO3 ) in the positive electrode active material were each less than 0.003 mass%.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.58. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 85%.

[Comparative Example 2]
As the base powder, a lithium metal _composite oxide powder was used having a _solid particle structure, a _composition of _{Li1.02Ni0.82Co0.15Al0.03O2} , an average particle diameter MV of _11.7 μm, a particle size distribution [(d90-d10)/average particle diameter] of 0.82, a tap density of 2.6 g/ ^cm3 , and a pH of the base powder slurry of 13.0.

The first mixture was obtained by mixing 100 g of the lithium metal composite oxide powder with 7 g of water.

Tungsten trioxide (WO ₃ ) was used as a coating raw material. The amount of tungsten contained in the tungsten trioxide (WO 3 ) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the "Total amount of W and Mo" column in Table 1 so that the amount of tungsten contained in the coating compound produced would be 1 equivalent relative to the amount of lithium in the excess lithium compound contained in the lithium metal composite oxide powder _. The first mixture was mixed with tungsten trioxide (WO ₃ ) while the temperature of the first mixture obtained above was adjusted to 40° C., thereby obtaining a second mixture.

Next, the second mixture obtained above was dried at a temperature of 230°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain a positive electrode active material.

The coating compound on the particle surfaces of the positive electrode active material obtained above was identified, and it was confirmed that the coating compound was lithium tungstate and had a composition of dilithium ditungstate (Li ₂ WO ₄ ).

The coating layer of the positive electrode active material had an average thickness of 360 nm, and the coefficient of variation CV% indicating the thickness variation was 18%. It was also confirmed that the pH of the slurry of the positive electrode active material was 9.6, and that the lithium hydroxide (LiOH) content and the lithium _carbonate ( _Li2CO3 ) content in the positive electrode active material were each less than 0.003 mass%.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.40. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 86%.

[Comparative Example 3]
As the base powder, a lithium metal _composite oxide powder was used having a _solid particle structure, a _composition of _{Li1.02Ni0.82Co0.15Al0.03O2} , an average particle diameter MV of _12.0 μm, a particle size distribution [(d90-d10)/average particle diameter] of 0.81, a tap density of 2.5 g/ ^cm3 , and a pH of the base powder slurry of 10.8.

The first mixture was obtained by mixing 100 g of the lithium metal composite oxide powder with 4 g of water.

Molybdenum trioxide (MoO ₃ ) was used as a coating raw material. The amount of molybdenum contained in the molybdenum trioxide (MoO 3 ) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the “Total Amount of W and Mo” column in Table 1 so that the amount of molybdenum contained in the coating compound produced would be 1 equivalent relative to the _{lithium of the excess lithium compound contained in the lithium metal composite oxide powder. The first mixture was mixed with molybdenum trioxide (MoO 3 )} while the temperature of the first mixture obtained above was adjusted to 30° C., thereby obtaining a second mixture.

Next, the second mixture obtained above was dried at a temperature of 170°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain a positive electrode active material.

The coating compound on the particle surfaces of the positive electrode active material obtained above was identified, and it was confirmed that the coating compound was lithium molybdate and had a composition of dilithium dimolybdate (Li ₂ MoO ₄ ).

The coating layer of the positive electrode active material had an average thickness of 2 nm, and the coefficient of variation CV% indicating the thickness variation was 53%. It was also confirmed that the slurry of the positive electrode active material had a pH of 9.6, and that the lithium hydroxide (LiOH) content and the lithium carbonate ( _Li2CO3 ) content in the positive electrode active _material were each less than 0.003 mass%.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.61. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 85%.

[Comparative Example 4]
As the base powder, a lithium metal _composite oxide powder was used having a _solid particle structure, a _composition of _{Li1.02Ni0.82Co0.15Al0.03O2} , an average particle diameter MV of _12.1 μm, a particle size distribution [(d90-d10)/average particle diameter] of 0.82, a tap density of 2.5 g/ ^cm3 , and a pH of the base powder slurry of 12.8.

The first mixture was obtained by mixing 100 g of the lithium metal composite oxide powder with 4 g of water.

Molybdenum trioxide (MoO ₃ ) was used as a coating raw material. The amount of molybdenum contained in the molybdenum trioxide (MoO 3 ) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the “Total Amount of W and Mo” column in Table 1 so that the amount of molybdenum contained in the coating compound produced would be 1 equivalent relative to the amount of lithium in the excess lithium compound contained in the lithium metal composite oxide powder _{. The first mixture was mixed with molybdenum trioxide (MoO 3 )} while the temperature of the first mixture obtained above was adjusted to 30° C., thereby obtaining a second mixture.

Next, the second mixture obtained above was dried at a temperature of 170°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain a positive electrode active material.

The coating compound on the particle surfaces of the positive electrode active material obtained above was identified, and it was confirmed that the coating compound was lithium molybdate and had a composition of dilithium dimolybdate (Li ₂ MoO ₄ ).

The positive electrode active material had an average thickness of 206 nm and a coefficient of variation (CV%) of 19%. It was also confirmed that the slurry of the positive electrode active material had a pH of 9.7 and that the lithium hydroxide (LiOH) content and the lithium carbonate ( _{Li2CO3 )} content in the positive electrode active material were each less than 0.003% by mass.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.44. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 86%.

[Comparative Example 5]
As the base powder, a lithium metal _composite oxide was used having a _solid particle structure, a composition _{of Li1.02Ni0.82Co0.15Al0.03O2} , an average particle diameter MV of _11.8 μm, a particle size distribution [(d90-d10)/average particle diameter] of 0.81, a tap density of 2.6 g/ ^cm3 , and a pH of the base powder slurry of 11.8.

The first mixture was obtained by mixing 100 g of the lithium metal composite oxide powder with 7 g of water.

Tungsten trioxide ( _WO3 ) was used as a coating raw material. The amount of tungsten contained in the tungsten trioxide (WO3) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the "Total amount of W and Mo" column in Table 1 so that the amount of tungsten contained in the coating compound produced would be 1 equivalent relative to the amount of lithium in the excess lithium compound contained in the lithium metal composite oxide powder _. The first mixture was mixed with tungsten trioxide ( _WO3 ) at room temperature to obtain a second mixture.

Next, the second mixture obtained above was dried at a temperature of 230°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain a positive electrode active material.

The coating compound on the particle surfaces of the positive electrode active material obtained above was identified, and it was confirmed that the coating compound was lithium tungstate and had a composition of dilithium ditungstate (Li ₂ WO ₄ ).

The positive electrode active material had an average thickness of 23 nm, a coefficient of variation CV% indicating the thickness variation was 17%, and the positive electrode active material slurry had a pH of 9.5, and the positive electrode active material contained less than 0.003 mass% of lithium hydroxide (LiOH) and lithium _carbonate ( _Li2CO3 ).

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.32. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 87%.

[Comparative Example 6]
As the base powder, a lithium metal _composite oxide was used having a _solid particle structure, a composition _{of Li1.02Ni0.82Co0.15Al0.03O2} , an average particle diameter MV of _12.0 μm, a particle size distribution [(d90-d10)/average particle diameter] of 0.80, a tap density of 2.5 g/ ^cm3 , and a pH of the base powder slurry of 11.7.

The first mixture was obtained by mixing 100 g of the lithium metal composite oxide powder with 4 g of water.

Molybdenum trioxide ( _MoO3 ) was used as a coating raw material. The amount of molybdenum contained in the molybdenum trioxide (MoO3) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the "Total amount of W and Mo" column in Table 1 so that the amount of molybdenum contained in the coating compound produced would be 1 equivalent relative to the amount of lithium in the excess lithium compound contained in the lithium metal composite oxide powder _. The first mixture was mixed with molybdenum trioxide ( _MoO3 ) at room temperature to obtain a second mixture.

Next, the second mixture obtained above was dried at 170°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain the positive electrode active material.

The coating compound on the particle surfaces of the positive electrode active material obtained above was identified, and it was confirmed that the coating compound was lithium molybdate and had a composition of dilithium dimolybdate (Li ₂ MoO ₄ ).

The positive electrode active material had an average thickness of 16 nm, a coefficient of variation CV% indicating the thickness variation was 16%, and a pH of the positive electrode active material slurry was 9.5. _It was also confirmed that the lithium hydroxide (LiOH) content and the lithium carbonate ( _Li2CO3 ) content in the positive electrode active material were each less than 0.003 mass%.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.38. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 87%.

[Comparative Example 7]
As the base powder, a lithium metal _composite oxide was used having a solid particle structure, a composition _{of Li1.02Ni0.82Co0.15Al0.03O2} , an average particle diameter MV of _11.8 μm, a particle size distribution [(d90- _d10 )/average particle diameter] of 0.82, a tap density of 2.6 g/ ^cm3 , and a pH of the base powder slurry of 12.1.

Tungsten trioxide (WO ₃ ) was used as the coating raw material. The amount of tungsten contained in the tungsten trioxide (WO 3 ) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the “Total Amount of W and Mo” column in Table 1 so that the amount of tungsten contained in the coating compound produced would be 1 equivalent relative to the amount of lithium in the excess lithium compound _{contained in the lithium metal composite oxide powder. Mixture A was obtained by mixing the first mixture and tungsten trioxide (WO 3 )} at room temperature.

Next, 100 g of the mixture A obtained above was mixed with 7 g of water to obtain mixture B. The obtained mixture B was dried at 230°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain a positive electrode active material.

The coating compound on the particle surfaces of the positive electrode active material obtained above was identified, and it was confirmed that the coating compound was lithium tungstate and had a composition of dilithium ditungstate (Li ₂ WO ₄ ).

The positive electrode active material had an average thickness of 45 nm and a coefficient of variation (CV%) of 23%. It was also confirmed that the slurry of the positive electrode active material had a pH of 9.6, and that the lithium hydroxide (LiOH) content and the lithium carbonate ( _{Li2CO3 )} content in the positive electrode active material were each less than 0.003 mass%.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.65. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 85%.

[Comparative Example 8]
As the base powder, a lithium metal composite _oxide was used having a _solid particle structure, a composition _{of Li1.02Ni0.82Co0.15Al0.03O2} , an average particle diameter MV of _12.0 μm, a particle size distribution [(d90-d10)/average particle diameter] of 0.82, a tap density of 2.5 g/ ^cm3 , and a slurry pH of 12.2.

Molybdenum trioxide (MoO ₃ ) was used as the coating raw material. The amount of molybdenum contained in the molybdenum trioxide (MoO 3 ) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the “Total Amount of W and Mo” column in Table 1 so that the amount of molybdenum contained in the coating compound produced would be 1 equivalent relative to the amount of lithium in the excess lithium compound _{contained in the lithium metal composite oxide powder. Mixture A was obtained by mixing the first mixture and molybdenum trioxide (MoO 3 )} at room temperature.

Next, 100 g of the mixture A obtained above was mixed with 4 g of water to obtain mixture B. The obtained mixture B was dried at 170°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain a positive electrode active material.

The coating compound on the particle surfaces of the positive electrode active material obtained above was identified, and it was confirmed that the coating compound was lithium molybdate and had a composition of dilithium dimolybdate (Li ₂ MoO ₄ ).

The positive electrode active material had an average thickness of 51 nm and a coefficient of variation (CV%) of 33%. It was also confirmed that the slurry of the positive electrode active material had a pH of 9.6 and that the lithium hydroxide (LiOH) content and the lithium carbonate ( _{Li2CO3 )} content in the positive electrode active material were each less than 0.003 mass%.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.77. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 85%.

[Comparative Example 9]
As the base powder, a lithium metal _composite oxide was used having a _solid particle structure, a composition _{of Li1.02Ni0.82Co0.15Al0.03O2} , an average particle diameter MV of _11.7 μm, a particle size distribution [(d90-d10)/average particle diameter] of 0.81, a tap density of 2.6 g/ ^cm3 , and a slurry pH of 11.6.

The first mixture was obtained by mixing 100 g of the lithium metal composite oxide powder with 7 g of water.

Dilithium ditungstate ( _Li2WO4 ) was used as the coating raw material. The amount of tungsten contained in the dilithium ditungstate ( _Li2WO4 ) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the "Total amount of W and Mo" column in Table 1 so that the amount of tungsten contained in the coating compound produced was 1 equivalent relative to the amount of lithium in the excess lithium compound contained in the lithium metal composite oxide powder _. The first mixture was mixed with dilithium ditungstate ( _{Li2WO4 )} while the temperature of the first mixture obtained above was adjusted to ₄₀ °C, thereby obtaining a second mixture.

Next, the second mixture obtained above was dried at 230°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain the positive electrode active material.

The coating compound on the particle surface of the positive electrode active material obtained above was identified. In addition to identifying lithium tungstate as the coating compound, lithium hydroxide and lithium _carbonate ( _Li2CO3 ) were detected as impurities, and the composition of the lithium tungstate was confirmed to be dilithium _ditungstate ( _Li2WO4 ).

The positive electrode active material had an average thickness of 21 nm, a coefficient of variation CV% indicating the thickness variation was 22%, and the positive electrode active material slurry had a pH of 11.2, a lithium hydroxide content of 0.076 mass%, and a lithium carbonate ( _{Li2CO3 )} content of 0.006 mass%.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.84. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 83%.

[Comparative Example 10]
As the base powder, a lithium metal _composite oxide powder was used having a _solid particle structure, _a composition of _{Li1.02Ni0.82Co0.15Al0.03O2} , an average particle diameter MV of _12.0 μm, a particle size distribution [(d90-d10)/average particle diameter] of 0.81, a tap density of 2.5 g/ ^cm3 , and a slurry pH of 11.5.

The first mixture was obtained by mixing 100 g of the lithium metal composite oxide powder with 4 g of water.

Dilithium dimolybdate ( _Li2MoO4 ) was used as a coating raw material. The amount of molybdenum contained in the dilithium dimolybdate ( _Li2MoO4 ) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the "Total amount of W and Mo" column in Table 1 so that the amount of molybdenum contained in the coating compound produced was 1 equivalent relative to the amount of lithium in the excess lithium compound contained in the lithium metal composite oxide powder _. The first mixture was mixed with _{dilithium dimolybdate (Li2MoO4 )} while the temperature of the first mixture obtained above was adjusted to ₃₀ °C, thereby obtaining a second mixture.

Next, the second mixture obtained above was dried at 170°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain the positive electrode active material.

The coating compound on the particle surface of the positive electrode active material obtained above was identified. In addition to being identified as lithium molybdate, lithium hydroxide and lithium _carbonate ( _Li2CO3 ) were detected as impurities, and the composition of the lithium molybdate was confirmed to be dilithium dimolybdate ( _{Li2MoO4 )} .

The positive electrode active material had an average thickness of 15 nm, a coefficient of variation CV% indicating the thickness variation was 26%, and the positive electrode active material slurry had a pH of 11.3, a lithium hydroxide content of 0.096 mass%, and a lithium carbonate ( _{Li2CO3 )} content of 0.008 mass%.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.89. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 82%.

[Comparative Example 11]
As the base powder, the same lithium metal composite oxide powder as that used in Example 6 was used.

The first mixture was obtained by mixing 100 g of the lithium metal composite oxide powder with 7 g of water.

Tungsten trioxide (WO ₃ ) was used as the coating raw material. The amount of tungsten contained in the tungsten trioxide (WO 3 ) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the "Total Amount of W and Mo" column in Table 1 so that the amount of tungsten contained in the coating compound produced would be significantly less than 1 equivalent relative to the amount of lithium in the excess lithium compound contained in the lithium metal composite oxide powder. The first mixture was mixed with tungsten trioxide (WO ₃ ) while the temperature of the first mixture obtained above was adjusted to 40° C., thereby obtaining a second mixture _.

Next, the second mixture obtained above was dried at 230°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain the positive electrode active material.

The coating compound on the particle surface of the positive electrode active material obtained above was identified. In addition to lithium tungstate, lithium hydroxide and lithium _carbonate ( _Li2CO3 ) were detected as impurities, and the composition of the lithium tungstate was confirmed to be dilithium ditungstate ( _{Li2WO4 )} .

The positive electrode active material had an average thickness of 54 nm, a coefficient of variation CV% indicating the thickness variation was 23%, and the positive electrode active material slurry had a pH of 12.3, a lithium hydroxide content of 0.95 mass%, and a lithium carbonate ( _{Li2CO3 )} content of 0.065 mass%.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.96. In addition, the retention rate of the discharge capacity of the lithium ion secondary battery obtained above was 74%.

[Comparative Example 12]
As the base powder, the same lithium metal composite oxide powder as that used in Example 1 was used.

The first mixture was obtained by mixing 100 g of the lithium metal composite oxide powder with 7 g of water.

Tungsten trioxide (WO ₃ ) was used as the coating raw material. The amount of tungsten contained in the tungsten trioxide (WO 3 ) per 100 g of the lithium metal composite oxide powder was adjusted to the amount shown in the “Total Amount of W and Mo” column in Table 1 so that the amount of tungsten contained in the coating compound produced would be significantly more than 1 equivalent relative to the amount of lithium in the excess lithium compound contained in the lithium metal composite oxide powder _{. The first mixture was mixed with tungsten trioxide (WO 3 )} while the temperature of the first mixture obtained above was adjusted to 40° C., thereby obtaining a second mixture.

Next, the second mixture obtained above was dried at 230°C for 12 hours, and some of the aggregates were deflocculated using a deflocculator to obtain the positive electrode active material.

The coating compound on the particle surfaces of the positive electrode active material obtained above was identified, and it was confirmed that the coating compound was lithium tungstate and had a composition of dilithium ditungstate (Li ₂ WO ₄ ).

The positive electrode active material had an average thickness of 36 nm and a coefficient of variation (CV%) of 17%. It was also confirmed that the pH of the positive electrode active material slurry was 9.5, and that the lithium hydroxide (LiOH) content and the lithium carbonate ( _{Li2CO3 )} content in the positive electrode active material were each less than 0.003 mass%.

The lithium content in the base powder after preparation of the positive electrode active material was 6.9% by mass.

When the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained in Comparative Example 13 (without coating) described below was taken as 1.00, the positive electrode resistance value of the lithium ion secondary battery produced using the positive electrode active material obtained above was 0.91. In addition, the discharge capacity retention rate of the lithium ion secondary battery obtained above was 78%.

[Comparative Example 13]
As the base powder, a lithium _{metal composite oxide} powder was used having a solid particle structure, a composition of _{Li1.02Ni0.82Co0.15Al0.03O2 ,} an average particle diameter MV of _11.9 μm, a particle size distribution [(d90-d10)/average particle diameter] of 0.82, a tap density of 2.5 g/ ^cm3 , and a slurry pH of 12.1.

The lithium metal composite oxide was not coated, and the lithium metal composite oxide was used as the positive electrode active material to evaluate the characteristics of a lithium ion secondary battery. As a result, the positive electrode resistance value of the lithium ion secondary battery was 1.00, and the discharge capacity retention rate was 67%.

[comprehensive evaluation]
From the results shown in Table 2 , it can be seen that the positive electrode active materials obtained in each Example have an average thickness of the coating layer in the range of 3 to 150 nm, and the coefficient of variation (CV%) indicating the thickness variation is 15% or less. It can also be seen that the positive electrode active materials obtained in each Example have a lithium hydroxide (LiOH) content and a lithium carbonate (Li ₂ CO ₃ ) content that are both less than 0.003 mass%, which is the detection limit. It can also be seen that the positive electrode active materials obtained in each Example have a total amount of tungsten and molybdenum contained in the coating compound produced that is 0.8 to 1.2 equivalents relative to the amount of lithium in the excess lithium compound contained in the lithium metal composite oxide.

Furthermore, the coating compound was identified as dilithium ditungstate ( _Li2WO4 ), dilithium dimolybdate ( _{Li2MoO4 )} , or a mixture thereof by synchrotron radiation powder X-ray diffraction (SXRD), which is a physical property analysis method. The composition ratios of lithium, tungsten, and molybdenum [lithium/tungsten, lithium/molybdenum, or lithium/(tungsten+ _molybdenum )] based on the results of the identification by the synchrotron radiation powder X-ray diffraction ( SXRD ) matched with the composition ratio of lithium [lithium/tungsten, lithium/molybdenum, or lithium/(tungsten+molybdenum)] determined based on the respective contents of lithium, tungsten, and molybdenum by a chemical analysis method performed in parallel with the results of the identification by the synchrotron radiation powder X-ray diffraction (SXRD), and therefore it was found that the coating compound formed was as identified by the results of the identification by the synchrotron radiation powder X-ray diffraction (SXRD).

From the above results, it can be seen that in each example, the particle surface of the base powder is purely coated with a coating compound having a resistance reducing function in an approximately equivalent amount, the thickness of the coating layer is optimized, and a positive electrode active material for a lithium ion secondary battery is obtained in which the variation in the thickness of the coating layer is minimized.

Therefore, it can be seen that the positive electrode resistance values of the lithium ion secondary batteries produced using the positive electrode active materials obtained in each Example were all less than 0.30, and the discharge capacity retention rates were all 90% or more, which is an extremely excellent result.

In contrast, the positive electrode active materials obtained in each comparative example, including those without a coating layer, had average coating layer thicknesses and coefficients of variation (CV%) indicating thickness variance that were both outside the optimal range, indicating inferior battery characteristics.

In addition, in Comparative Examples 9 and 10, the coating compounds dilithium ditungstate (Li ₂ WO ₄ ) or dilithium dimolybdate (Li ₂ MoO ₄ ) are used as they are, instead of tungstic acid and molybdic acid, so that the lithium of the excess lithium compound contained in the base material powder is mixed into the coating layer as an impurity in an unreacted state, and lithium hydroxide (LiOH) and lithium carbonate (Li ₂ CO ₃ ) are identified along with the coating compound, and it can be seen that the pH of the slurry of the positive electrode active material exceeds 11. From this, it can be seen that the content of lithium hydroxide is high, and the total amount of tungsten and molybdenum contained in the coating compound is not equivalent to the amount of lithium of the excess lithium compound contained in the lithium metal composite oxide. This tendency is also observed when the amount of coating raw material is insufficient relative to the amount of lithium of the excess lithium compound contained in the base material powder, as in the case of the positive electrode active material obtained in Comparative Example 11.

On the other hand, in the positive electrode active material obtained in Comparative Example 12, the amount of tungsten trioxide (WO ₃ ), which is the coating raw material, added was excessive, and it is possible that tungsten trioxide (WO ₃ ) remains in the coating compound. However, only dilithium ditungstate (Li ₂ WO ₄ ) was identified as being pure, and the lithium hydroxide (LiOH) content and lithium carbonate (Li ₂ CO ₃ ) content were both less than 0.003 mass%, and the coating compound was in the range of 0.8 to 1.2 equivalents.

However, the lithium content in the base powder of the positive electrode active material obtained in Comparative Example 12 was 6.9% by mass, whereas the lithium content in the positive electrode active material obtained in Example 1 was 7.1% by mass. This shows that in the positive electrode active material obtained in Comparative Example 12, lithium is removed not only from nickel and cobalt but also from the base powder itself that constitutes the composite oxide.

In addition, the positive electrode resistance value and discharge capacity retention rate of the lithium ion secondary battery were inferior in the positive electrode active material obtained in Comparative Example 12 to those in Example 1. This suggests that the battery characteristics of the positive electrode active material obtained in Comparative Example 12 were deteriorated because the excess amount of coating raw material removed lithium from the base powder itself.

As described above, the positive electrode active material for lithium ion secondary batteries of the present invention has low positive electrode resistance and high discharge capacity retention rate when used in lithium ion secondary batteries, and therefore can be suitably used in lithium ion secondary batteries.

1 Primary particles 2 Secondary particles 3 Coating layer

Claims (1)

A method for producing a positive electrode active material for a lithium ion secondary battery using a base powder containing secondary particles formed by agglomeration of primary particles of a lithium metal composite oxide containing lithium, nickel, and cobalt as a raw material, comprising the steps of:
The lithium metal composite oxide has the formula (I):
Li _s Ni _1-xy C _x M _y O _2+α (I)
(In the formula, s, x, y, and α are numbers satisfying 1.00≦s≦1.30, 0.05≦x≦0.35, 0≦y≦0.45, and −0.1≦α≦0.2, respectively, and M represents Al.)
a coating layer containing at least one lithium salt selected from the group consisting of lithium tungstate and lithium molybdate is formed on a surface of the secondary particles, the coating layer has an average thickness of 3 to 150 nm, and a coefficient of variation (CV%) indicating the variation in thickness of the coating layer is 15% or less;
preparing a first mixture by mixing the base material powder and water such that the water content in the mixture of the base material powder and water is 4 to 7 mass %;
The temperature of the first mixture obtained above is adjusted to 25 to 50°C, and the first mixture is mixed with a coating material to prepare a second mixture;
The second mixture is dried at a temperature of 150 to 250° C. to form a coating layer on the surface of the secondary particles contained in the base powder, thereby producing a positive electrode active material;
A coating raw material containing at least one compound selected from the group consisting of tungsten compounds and molybdenum compounds is used as the coating raw material,
At least one tungsten compound selected from the group consisting of tungsten oxide, tungstic acid, and ammonium tungstate is used as the tungsten compound, and at least one molybdenum compound selected from the group consisting of molybdenum oxide, molybdic acid, and ammonium molybdate is used as the molybdenum compound,
The method for producing a positive electrode active material for a lithium ion secondary battery comprises adjusting a total amount of tungsten and molybdenum to 0.1 to 6 parts by mass per 100 parts by mass of the base powder.

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