JP7655686B2 - Positive electrode active material for lithium-ion batteries - Google Patents

Description

This disclosure relates to positive electrode active materials for lithium ion batteries.

Lithium-ion batteries are widely used as portable power sources for personal computers and mobile devices, and as vehicle power sources for electric vehicles, hybrid vehicles, and other vehicles. In particular, there is a demand for higher capacity batteries as vehicle power sources.

A lithium-ion battery includes a positive electrode, a negative electrode, an electrolyte, etc. The positive electrode includes a positive electrode active material layer and a positive electrode substrate. In order to increase the capacity of this lithium-ion battery, it has been proposed to use a lithium transition metal composite oxide as the positive electrode active material (for example, Patent Documents 1 and 2).

JP 2020-107536 A JP 2021-51880 A

However, lithium-ion batteries using the positive electrode active materials disclosed in Patent Documents 1 and 2 have the problem that they have high initial resistance and that the resistance increases when the batteries are repeatedly charged and discharged.

Therefore, the objective of this disclosure is to provide a positive electrode active material for lithium ion batteries that has low initial resistance and suppresses the increase in resistance during repeated charging and discharging.

[1] The present disclosure relates to a positive electrode active material for a lithium ion battery, comprising a lithium nickel cobalt manganese-containing composite oxide,
The positive electrode active material includes secondary particles formed by agglomeration of a plurality of primary particles,
The positive electrode active material has the formula (1):
(18.768-a)/b≦0.0024 (1)
and formula (2):
c/d≦2.66 (2)
Fulfilling the relationship,
In the formula (1),
a is the peak top of the (003) plane measured by X-ray diffraction of the positive electrode active material, and is in the range of a<18.768;
b is the number of moles of manganese relative to the total number of moles of nickel, cobalt, and manganese in the positive electrode active material,
In the formula (2),
c is 1/20th the width of a,
d is the half width of a;
Positive electrode active material.

According to the new findings of this disclosure, it is expected that by making the positive electrode active material satisfy the relationship between formulas (1) and (2) above, the initial resistance can be reduced and the increase in resistance during repeated charging and discharging can be suppressed.

That is, the closer the peak top value is to 18.768, the more uniformly the transition metal (nickel, cobalt, and manganese) is dispersed in the transition metal layer (nickel, cobalt, and manganese) in the crystal structure of the lithium nickel cobalt manganese-containing composite oxide, and the higher the lithium occupancy rate in the lithium layer. On the other hand, the peak top is affected by the composition ratio of the transition metal, particularly the composition ratio of manganese. Therefore, formula (1) is defined as an index that represents the value of the peak top that removes the influence of the composition ratio of manganese in the transition metal. Since the crystal structure of the lithium nickel cobalt manganese-containing composite oxide that satisfies formula (1) is stable, it is believed that durability is improved and an increase in resistance is suppressed even when charging and discharging are repeated. In addition, since the transition metal is uniformly dispersed in the transition metal layer of the positive electrode active material layer and a large amount of lithium is present in the lithium layer, it is believed that no uneven reaction occurs and the initial resistance does not become high.

The broadening of the base of the 1/20th width peak represents defects in the crystal structure of the lithium nickel cobalt manganese-containing composite oxide, and the broader the base, the greater the defects in the crystal structure. On the other hand, the 1/20th width is affected by the half-width. Therefore, formula (2) is defined as an index representing the value of the 1/20th width excluding the influence of the half-width. The crystal structure of the lithium nickel cobalt manganese-containing composite oxide that satisfies formula (2) has small defects, so it is believed that durability is improved and the increase in resistance is suppressed even when the charge and discharge are repeated. In addition, since the defects in the crystal structure are small, it is believed that no uneven reaction occurs and the initial resistance does not increase.

FIG. 1 is a schematic diagram of a lithium ion battery according to the present embodiment. FIG. 2 is a schematic diagram of an electrode assembly.

An embodiment of the present disclosure is described below. However, the present disclosure is not limited to this.

In this specification, the phrase "consists essentially of..." indicates that additional components may be included in addition to the essential components, to the extent that the purpose of this disclosure is not hindered. For example, components that are normally expected in the field of the technology (e.g., unavoidable impurities, etc.) may be included as additional components.

In this specification, when a compound is expressed by a stoichiometric composition formula such as "LiCoO ₂ ", the stoichiometric composition formula is merely a representative example. For example, when lithium cobalt oxide is expressed as "LiCoO ₂ ", unless otherwise specified, the lithium cobalt oxide is not limited to a composition ratio of "Li/Co/O = 1/1/2" and may contain Li, Co, and O in any composition ratio. The composition ratio may be non-stoichiometric.

<Positive electrode active material>
The positive electrode active material of this embodiment is a positive electrode active material for a lithium ion battery made of a lithium nickel cobalt manganese-containing composite oxide (hereinafter also referred to as "lithium composite oxide"). The lithium ion battery will be described in detail later. The positive electrode active material is a particle. The positive electrode active material may have any size. The positive electrode active material may have a D50 of, for example, 1 μm or more and 30 μm or less, or may have a D50 of 5 μm or more and 20 μm or less. In this specification, "D50" is defined as a particle size at which the cumulative frequency from the smallest particle size becomes 50% in a volume-based particle size distribution. The volume-based particle size distribution can be measured by a laser diffraction particle size distribution measuring device.

<Particle Structure>
The positive electrode active material includes secondary particles formed by agglomeration of a plurality of primary particles. The positive electrode active material may be, for example, substantially composed of secondary particles. The secondary particles are formed by agglomeration of 50 or more primary particles. The number of primary particles contained in the secondary particles is measured in a scanning electron microscope (SEM) image of the secondary particles. The magnification of the SEM image may be, for example, 10,000 to 30,000 times. The secondary particles may be formed by agglomeration of 100 or more primary particles. There is no upper limit to the number of primary particles in the secondary particles. The secondary particles may be formed by agglomeration of 10,000 or less primary particles. The secondary particles may be formed by agglomeration of 1,000 or less primary particles. The primary particles may have any shape. The primary particles may be, for example, spherical, columnar, or lumpy.

"composition"
The positive electrode active material can reversibly absorb and release lithium ions. The positive electrode active material can have any crystal structure. The positive electrode active material can have, for example, a layered rock salt structure, a spinel structure, an olivine structure, or the like. The positive electrode active material can have any chemical composition. The chemical composition of the positive electrode active material can be measured, for example, by inductively coupled plasma atomic emission spectrometry (ICP-AES) or the like.

The positive electrode active material is made of a lithium nickel cobalt manganese-containing composite oxide. In other words, it is widely applicable to ternary positive electrode active materials that contain at least nickel, cobalt, and manganese as transition metals, and there are no particular limitations on the composition.

However, the lithium nickel cobalt manganese-containing composite oxide disclosed herein preferably has a composition represented by the following formula (i):

Li _z Ni _1−x−y Co _x Mny _{O 2} (i)
In the above formula (1), "x" satisfies the relationship "0.1≦x≦0.4", "y" satisfies the relationship "0.1≦y≦0.5", and "z" satisfies the relationship "0.95≦z≦1.2".

X-ray diffraction
The positive electrode active material satisfies the relationships of the following formulas (1) and (2).

(18.768-a)/b≦0.0024 (1)
c/d≦2.66 (2)
In the above formula (1), "a" is the peak top of the (003) plane measured by X-ray diffraction of the positive electrode active material and is in the range of a<18.768. "b" is the number of moles of manganese relative to the total number of moles of nickel, cobalt, and manganese in the positive electrode active material, and is represented by the following formula (3).

b = 100 × moles of manganese [mol] / (moles of nickel [mol] + moles of cobalt [mol] + moles of manganese [mol]) (3)
In the above formula (2), "c" is 1/20th the width of the above "a", and "d" is the half width of the above "a".

When the positive electrode active material satisfies the relationship between formulas (1) and (2) above, the initial resistance can be reduced and the increase in resistance during repeated charging and discharging can be suppressed.

Here, the X-ray diffraction measurement may be, for example, powder X-ray diffraction measurement using CuKα rays. This may be performed by irradiating X-rays (CuKα rays) irradiated from an X-ray generating source onto the sample surface of the sample. The sample surface may be a surface made of a positive electrode active material (typically a powder), or may be a surface (surface of a positive electrode active material layer) on which a positive electrode is actually formed by binding the positive electrode active material with a binder. Then, the angle difference (diffraction angle 2θ) between the diffraction direction and the incident direction of the X-rays and the diffracted X-ray intensity are measured, and the half-width and 1/20 width of the diffraction peak derived from the (003) plane can be calculated from the obtained diffraction pattern. Such X-ray diffraction measurement and calculation of the half-width and 1/20 width of the (003) plane can be performed using an X-ray diffraction measurement device and attached analysis software commercially available from various measurement device manufacturers.

(Formula (1))
With regard to the value of a, the closer the value of a is to 18.768, the more uniformly the transition metal (nickel, cobalt and manganese) is dispersed in the transition metal layer in the crystal structure of the lithium nickel cobalt manganese-containing composite oxide, and the higher the occupancy rate of lithium in the lithium layer, so that the durability is improved and the increase in resistance is suppressed even when charging and discharging is repeated. The value of a is in the range of a<18.768, preferably in the range of 18.68<a<18.768, and more preferably in the range of 18.7<a<18.768.

On the other hand, the inventors have found that since the value of a is affected by the composition ratio of the transition metal, particularly the composition ratio of manganese, it is necessary for the values of a and b to satisfy the relationship of formula (1) above. In formula (1) above, it is preferable that the value of (18.768-a)/b is 0.0022 or less.

(Formula (2))
Regarding the value of c, the broadening of the base of the peak having a width 1/20 of the value of a represents defects in the crystal structure of the lithium composite oxide, and the larger the value of c, the larger the defects in the crystal structure. The value of c is preferably 0.32 or less, and more preferably 0.3 or less.

On the other hand, the inventors have found that since the value of c is affected by the value of d, i.e., the half-width of the value of a, it is necessary for the values of c and d to satisfy the relationship of the above formula (2). In the above formula (2), it is preferable that the value of c/d is 2.60 or less.

《Vacation》
In the positive electrode active material, the secondary particles may have a core portion having a void. The void is preferably large due to the gap between the primary particles. Since the secondary particles having a large void have a large outer surface area and there are few primary particles present inside the secondary particles, the proportion of primary particles that can contact the conductive material increases, and as a result, the initial resistance can be further reduced. In the present disclosure, the maximum diameter of the void is preferably, for example, 200 nm or more. In addition, in the positive electrode active material, it is more preferable that the proportion of secondary particles having such voids with a maximum diameter of 200 nm or more inside is high, and for example, it is more preferable that the proportion is 50% or more.

The maximum diameter of the voids in the secondary particles and the proportion of secondary particles having voids with a maximum diameter of 200 nm or more can be calculated, for example, by the following method. First, multiple positive electrode active materials are embedded in a resin or the like, cut using a cross-section polisher (CP) to enable cross-sectional observation of the secondary particles, and then images are obtained using a SEM. The observation magnification is, for example, 8000 times. From the obtained images, 100 secondary particles that show the entire particle are selected, and the maximum diameter of the voids in each particle is measured. The secondary particles whose maximum diameter is 200 nm or more are counted, and the proportion is calculated.

Shell section
In the positive electrode active material, the secondary particles may further include a shell portion electrically connected to the core portion on the outside of the core portion. By providing the secondary particles with the shell portion, the electronic conductivity is improved and the initial resistance can be further reduced. From the viewpoint of electronic conductivity, the thickness of the shell portion is preferably small. In the present disclosure, the thickness of the shell portion is, for example, preferably 0.9 μm or less, more preferably 0.6 μm or less.

The shell portion and the thickness of the shell portion can be confirmed and calculated, for example, by the same method as that for the maximum diameter of the voids in the secondary particles and the proportion of secondary particles having voids with a maximum diameter of 200 nm or more. However, in this disclosure, the thickness of the shell portion is determined by measuring the thickness of each shell portion at two points for secondary particles whose maximum diameter of voids in the secondary particles measured by the above-mentioned method is 200 nm or more, and averaging all of the values.

<Method of manufacturing positive electrode active material>
The method for producing a positive electrode active material according to the present embodiment includes at least a precursor preparation step (S10), a mixing step (S20), and a firing step (S30). The method may further include a precursor growth step between the precursor preparation step (S10) and the mixing step (S20), and a crushing step after the firing step (S30).

<<Precursor Preparation Step (S10)>>
In the precursor preparation step, a precursor (composite hydroxide particles) containing nickel salt, cobalt salt, and manganese salt is prepared. The precursor is prepared, for example, by dissolving nickel salt, cobalt salt, and manganese salt in water in a predetermined ratio to prepare a raw material aqueous solution, adjusting the pH of the raw material aqueous solution, and then drying it.

Nickel salts are not particularly limited, but for example, nickel sulfate, nickel nitrate, nickel chloride, etc. can be used. Cobalt salts are not particularly limited, but for example, cobalt sulfate, cobalt nitrate, cobalt chloride, etc. can be used. Manganese salts are not particularly limited, but for example, manganese sulfate, manganese nitrate, manganese chloride, etc. can be used.

The nickel salt, cobalt salt, and manganese salt may be mixed, for example, so that the molar ratio of nickel, cobalt, and manganese is "(1-x-y):x:y". "x" and "y" may satisfy, for example, the relationship of "0.1≦x≦0.4" and "0.1≦y≦0.5". By adjusting the concentration of the raw material aqueous solution containing the nickel salt, cobalt salt, and manganese salt, secondary particles having the desired values of the above formulas (1) and (2) can be produced.

A reaction tank is prepared. The reaction tank may include an agitator, a heater, a temperature controller, and the like. In the reaction tank, water, an ammonium ion donor, and a pH adjuster are mixed to prepare an alkaline aqueous solution. The temperature of the alkaline aqueous solution is adjusted to, for example, 20°C or higher and 30°C or lower. The ammonia concentration of the alkaline aqueous solution is adjusted to, for example, 10 g/L or higher and 20 g/L or lower. The pH of the alkaline aqueous solution (measured at 25°C) is adjusted to, for example, within a range of 11 to 13.

The ammonium ion donor is not particularly limited, but for example, ammonia water, an aqueous solution of ammonium sulfate, etc. can be used. The pH adjuster is not particularly limited, but for example, sodium hydroxide, potassium hydroxide, etc. can be used.

While the alkaline aqueous solution in the reaction tank is stirred by a stirrer, the raw aqueous solution is dropped into the alkaline aqueous solution. During the dropping of the raw aqueous solution, heating and addition of an ammonium ion donor and a pH adjuster are appropriately performed so that the temperature of the reaction solution, the ammonia concentration of the reaction solution, and the pH of the reaction solution do not change significantly. The temperature of the reaction solution is adjusted to, for example, 20°C or higher and 30°C or lower. The ammonia concentration of the reaction solution is adjusted to, for example, 10 g/L or higher and 20 g/L or lower. The pH of the reaction solution (measured at 25°C) is adjusted to, for example, within a range of 11 to 13. When the pH of the reaction solution is within the above range, secondary particles having the desired voids can be produced.

The precursor-containing aqueous solution after the above-mentioned reaction is dried to prepare the precursor.
《Mixing step (S20)》
In the mixing step, the precursor obtained in the precursor preparation step is mixed with a lithium salt to prepare a lithium mixture. The lithium salt is not particularly limited, but for example, lithium carbonate, lithium hydroxide, lithium nitrate, etc. can be used.

In the mixing step, the precursor and the lithium salt may be mixed so that the ratio of the number of lithium atoms in the lithium mixture to the sum of the numbers of nickel, cobalt and manganese atoms is, for example, in the range of 1.1 to 1.5. When the ratio of the number of lithium atoms in the lithium mixture to the sum of the numbers of nickel, cobalt and manganese atoms is in the range of 1.1 to 1.5, secondary particles having the desired values for the above formulas (1) and (2) can be produced. It is preferable to adjust the ratio of the number of lithium atoms in the lithium mixture to the sum of the numbers of nickel, cobalt and manganese atoms to be in the range of 1.1 to 1.3. A general mixer can be used for mixing.

<<Firing step (S30)>>
In the firing step, the lithium mixture obtained in the mixing step is fired to produce a lithium composite oxide.

The firing temperature is 750°C or higher and 950°C or lower. If the firing temperature is lower than 750°C, the lithium and the precursor may not react sufficiently, resulting in excess lithium or unreacted precursor, or the crystallinity of the resulting lithium composite oxide may be insufficient. If the firing temperature exceeds 950°C, the external surface area of the resulting lithium composite oxide may decrease, or the secondary particles of the lithium composite oxide may sinter abnormally, resulting in an increase in amorphous secondary particles. The firing temperature is preferably 800°C or higher and 900°C or lower.

The heating rate is 0.3°C/min or more and less than 5°C/min. If the heating rate is within the above range, the reaction between lithium and the precursor does not become uneven, and the crystallinity of the resulting lithium composite oxide is stable. It is preferable that the heating rate is 0.5°C/min or more and less than 2°C/min.

The firing time is 2 hours or more and 24 hours or less. If the firing time is less than 2 hours, the lithium and the precursor may not react sufficiently, and excess lithium or unreacted precursor may remain, or the crystallinity of the resulting lithium composite oxide may be insufficient. If the firing temperature exceeds 24 hours, the outer surface area of the resulting lithium composite oxide may decrease, or the secondary particles of the lithium composite oxide may sinter abnormally with each other, resulting in an increase in amorphous secondary particles. The firing time is preferably 5 hours or more and 18 hours or less. In this disclosure, the firing time refers to the time from the start of heating until the maximum temperature is reached to the time during which heating is performed at the maximum temperature, excluding the time until the maximum temperature is reached (heating time) and the time until cooling to room temperature (30°C) after heating at the maximum temperature is completed (cooling time).

For example, dry air, an oxygen atmosphere, an inert atmosphere, etc. can be used for the calcination, but an oxygen atmosphere is preferable. The oxygen concentration of the oxygen atmosphere is preferably 20% by volume or more and 100% by volume or less, and more preferably 50% by volume or more and 100% by volume or less. If the oxygen concentration of the oxygen atmosphere is less than 20% by volume, the crystallinity of the resulting lithium composite oxide may be insufficient.

Precursor Growth Process
In this embodiment, a precursor growing step may be further provided between the precursor producing step and the mixing step. In the precursor growing step, the precursor obtained in the precursor producing step is grown.

After the precursor preparation process is completed, the pH of the precursor-containing aqueous solution in the reaction tank (measured at 25°C) is adjusted to be lower than the pH in the precursor preparation process to prepare an aqueous solution for precursor growth. The pH of the aqueous solution for precursor growth is adjusted, for example, by changing the amount of pH adjuster added.

After adjusting the pH of the precursor growth aqueous solution, the precursor growth aqueous solution is stirred while heating and adding an ammonium ion donor and a pH adjuster as appropriate so that the temperature, ammonia concentration, and pH of the precursor growth aqueous solution do not change significantly. The temperature of the precursor growth aqueous solution is adjusted to, for example, 20°C or higher and 30°C or lower. The ammonia concentration of the precursor growth aqueous solution is adjusted to, for example, 10 g/L or higher and 20 g/L or lower. The pH of the precursor growth aqueous solution (measured at 25°C) is adjusted to, for example, 10.5 or higher and 12 or lower. When the pH of the precursor growth aqueous solution is within the above range, secondary particles with larger voids can be produced.

<Crushing process>
In this embodiment, a crushing step may be further provided after the firing step, because the lithium composite oxide obtained in the firing step is in the form of secondary particles, and may be aggregated or sintered.

The crushing method can be any conventional method known in the art, for example, a mortar, ball mill, hammer mill, etc. can be used.

<Lithium-ion battery>
The lithium ion battery of the present embodiment will be described below. The lithium ion battery of the present embodiment may be a secondary battery. The lithium ion battery of the present embodiment may be a primary battery. Hereinafter, the lithium ion battery may be abbreviated to "battery".

<Lithium-ion battery>
FIG. 1 is a schematic diagram of a lithium ion battery in this embodiment. Hereinafter, the "lithium ion battery in this embodiment" may be abbreviated as "this battery". The battery 100 includes a case 90. The case 90 may be made of metal, for example. The case 90 may have any shape. The case 90 may be, for example, rectangular (flattened rectangular parallelepiped) or cylindrical. The case 90 may be, for example, a pouch made of an Al laminate film. The case 90 may be provided with a positive terminal 91 and a negative terminal 92.

The case 90 houses the electrode body 50 and an electrolyte. The electrolyte is impregnated into the electrode body 50. A portion of the electrolyte may be stored in the bottom of the case 90. The electrode body 50 is connected to a positive terminal 91 and a negative terminal 92.

Figure 2 is a schematic diagram of an electrode body. The electrode body 50 includes a positive electrode 10, a separator 30, and a negative electrode 20. The electrode body 50 may have any structure. The electrode body 50 may be, for example, a wound type. The electrode body 50 may include, for example, a laminate 40. The laminate 40 is formed by laminating a positive electrode 10, a separator 30 (first sheet), a negative electrode 20, and a separator 30 (second sheet) in this order. The laminate 40 is wound in a spiral shape to form the electrode body 50. After winding, the electrode body 50 may be formed into a flat shape.

Positive pole
The positive electrode 10 may be, for example, a strip-shaped sheet. The positive electrode 10 may include a positive electrode current collector and a positive electrode active material layer. The positive electrode current collector may include, for example, Al foil or the like. The positive electrode active material layer may be disposed on the surface of the positive electrode current collector. The positive electrode active material layer may be disposed on only one side of the positive electrode current collector, or may be disposed on both the front and back sides. The positive electrode active material layer includes a positive electrode active material. The positive electrode active material layer may further include, for example, a conductive material, a binder, and the like.

The conductive material may include, for example, carbon black (e.g., acetylene black), carbon fiber, etc. The amount of the conductive material may be, for example, 0.1 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the positive electrode active material. The binder may include, for example, polyvinylidene fluoride (PVDF), etc. The amount of the binder may be, for example, 0.1 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the positive electrode active material.

Negative pole
The negative electrode 20 may be, for example, a strip-shaped sheet. The negative electrode 20 may include a negative electrode current collector and a negative electrode active material layer. The negative electrode current collector may include, for example, Cu foil or the like. The negative electrode active material layer may be disposed on the surface of the negative electrode current collector. The negative electrode active material layer may be disposed on only one side of the negative electrode current collector, or may be disposed on both the front and back sides. The negative electrode active material layer includes a negative electrode active material. The negative electrode active material layer may further include, for example, a conductive material, a binder, and the like.

The negative electrode active material is typically a particle group. The negative electrode active material may have a D50 of, for example, 1 μm or more and 30 μm or less. The negative electrode active material absorbs and releases lithium ions. The negative electrode active material should not be particularly limited. The negative electrode active material may be, for example, graphite (e.g., amorphous coated graphite, etc.), graphitizable carbon, non-graphitizable carbon, silicon, silicon oxide, silicon-based alloy, tin, tin oxide, tin-based alloy, lithium (pure metal), lithium alloy (e.g., Li-Al alloy, etc.), lithium titanate, etc. The negative electrode 20 may contain one type of negative electrode active material alone. The negative electrode 20 may contain two or more types of negative electrode active materials.

The conductive material may include, for example, carbon nanotubes (CNTs) and the like. The amount of the conductive material may be, for example, 0.1 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the negative electrode active material. The binder may include, for example, carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), and the like. The amount of the binder may be, for example, 0.1 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the negative electrode active material.

<Separator>
The separator 30 may be, for example, a strip-shaped film. The separator 30 is porous. The separator 30 is permeable to the electrolyte. The separator 30 separates the positive electrode 10 and the negative electrode 20. The separator 30 is electrically insulating. The separator 30 may contain, for example, a polyolefin resin. The polyolefin resin may contain, for example, at least one selected from the group consisting of polyethylene (PE) and polypropylene (PP). The separator 30 may have, for example, a single-layer structure. The separator 30 may be, for example, substantially composed of a PE layer. The separator 30 may have, for example, a multi-layer structure. The separator may be formed by, for example, laminating a PP layer, a PE layer, and a PP layer in this order. For example, a heat-resistant layer (ceramic particle layer) or the like may be formed on the surface of the separator 30.

《Electrolyte》
The electrolyte includes a solvent and a lithium salt. The solvent is aprotic. The solvent may include any component. The solvent may include at least one selected from the group consisting of, for example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), 1,2-dimethoxyethane (DME), methyl formate (MF), methyl acetate (MA), methyl propionate (MP), and γ-butyrolactone (GBL).

The lithium salt is a supporting electrolyte. The lithium salt is dissolved in a solvent. The lithium salt may include at least one selected from the group consisting of LiPF ₆ , LiBF ₄ , and LiN(FSO ₂ ) ₂ . The lithium salt may have a molar concentration of, for example, 0.5 mol/L or more and 2.0 mol/L or less, or may have a molar concentration of 0.8 mol/L or more and 1.2 mol/L or less.

In addition to the solvent and the lithium salt, the electrolyte may further contain any additive. For example, the electrolyte may contain an additive in a mass fraction of 0.01% to 5%. The additive may include at least one selected from the group consisting of vinylene carbonate (VC), vinyl ethylene carbonate (VEC), and the like.

The present embodiment will be described below using examples, but the present embodiment is not limited to these.

Example 1
<Production of Positive Electrode Active Material>
(Precursor Preparation Step)
Nickel sulfate, cobalt sulfate and manganese sulfate were dissolved in water to obtain a raw aqueous solution. The molar ratios of nickel, cobalt and manganese in the raw aqueous solution and the total molar concentration of nickel, cobalt and manganese in the raw aqueous solution are as shown in Table 1.

A reaction tank was prepared. The reaction tank was equipped with an agitator, a heater, and a temperature controller. In the reaction tank, an alkaline aqueous solution was prepared by supplying appropriate amounts of 25% by mass ammonia water and 25% by mass sodium hydroxide aqueous solution to 0.8 L of water. The temperature of the alkaline aqueous solution was adjusted to 25°C. The ammonia concentration of the alkaline aqueous solution was 12.5 g/L. The pH of the alkaline aqueous solution was adjusted to 11.4.

The raw material aqueous solution was added dropwise to the alkaline aqueous solution while the alkaline aqueous solution in the reaction tank was stirred by a stirrer. During the dropwise addition of the raw material aqueous solution, the temperature of the reaction solution was adjusted to 25°C. During the dropwise addition of the raw material aqueous solution, ammonia water was added appropriately to maintain the ammonia concentration of the reaction solution at 12.5 g/L. Sodium hydroxide aqueous solution was added appropriately to maintain the pH of the reaction solution at 11.4. After the reaction, the precursor-containing aqueous solution was dried to obtain the precursor.

(Mixing process)
The precursor was mixed with lithium carbonate to obtain a lithium mixture, in which the ratio of the number of lithium atoms to the sum of the numbers of nickel, cobalt, and manganese atoms was as shown in Table 1.

(Firing process)
The lithium mixture was fired at the firing temperature, heating rate, firing time and oxygen concentration shown in Table 1 to obtain a lithium composite oxide.

(Crushing process)
The lithium composite oxide was crushed in a mortar to obtain a lithium composite oxide (positive electrode active material) of Example 1.

<Comparative Example 1>
(Precursor Preparation Step)
A raw material aqueous solution was obtained in the same manner as in Example 1. The molar ratios of nickel, cobalt, and manganese in the raw material aqueous solution and the total molar concentration of nickel, cobalt, and manganese in the raw material aqueous solution are as shown in Table 1. Next, a precursor was obtained in the same manner as in Example 1.

(Mixing process)
A lithium mixture was obtained in the same manner as in Example 1. In the lithium mixture, the ratio of the number of lithium atoms to the sum of the numbers of nickel, cobalt and manganese atoms is as shown in Table 1.

(Firing process)
The lithium mixture was fired at the firing temperature, heating rate, firing time and oxygen concentration shown in Table 1 to obtain a lithium composite oxide.

(Crushing process)
The lithium composite oxide was crushed in the same manner as in Example 1 to obtain a lithium composite oxide (positive electrode active material) of Comparative Example 1.

<Examples 2 to 6, Comparative Examples 2 to 3>
(Precursor Preparation Step)
A raw material aqueous solution was obtained in the same manner as in Example 1. The molar ratios of nickel, cobalt, and manganese in each raw material aqueous solution and the total molar concentration of nickel, cobalt, and manganese in each raw material aqueous solution are as shown in Table 1. Next, each precursor-containing aqueous solution was obtained in the same manner as in Example 1. The pH of each precursor-containing aqueous solution was adjusted to 12.5.

(Precursor Growth Step)
In the precursor-containing aqueous solution, the pH was adjusted by adjusting the amount of sodium hydroxide aqueous solution to be supplied, and an aqueous solution for precursor growth was obtained. The pH of each aqueous solution for precursor growth is as shown in Table 1. After adjusting the pH of the aqueous solution for precursor growth, the aqueous solution for precursor growth was stirred. During stirring of the aqueous solution for precursor growth, the temperature of the aqueous solution for precursor growth was adjusted to 25° C. During stirring of the aqueous solution for precursor growth, ammonia water was appropriately added so that the ammonia concentration of the aqueous solution for precursor growth was maintained at 12.5 g/L. During stirring of the aqueous solution for precursor growth, sodium hydroxide aqueous solution was appropriately added so that the pH of the aqueous solution for precursor growth was each pH shown in Table 1. After the reaction, the aqueous solution for precursor growth was dried to obtain each precursor.

(Mixing process)
The precursor and lithium carbonate were mixed to obtain lithium mixtures, in which the ratio of the number of lithium atoms to the sum of the numbers of nickel, cobalt, and manganese atoms in each lithium mixture was as shown in Table 1.

(Firing process)
Each lithium mixture was fired at the firing temperature, heating rate, firing time and oxygen concentration shown in Table 1 to obtain each lithium composite oxide.

(Crushing process)
Each lithium composite oxide was crushed in the same manner as in Example 1 to obtain lithium composite oxides (positive electrode active materials) of Examples 2 to 6 and Comparative Examples 2 to 3.

<Production of lithium-ion batteries>
As the materials for the positive electrode, the positive electrode active materials of Examples 1 to 6 and Comparative Examples 1 to 3, acetylene black (Denka Co., Ltd.) as a conductive material, PVDF (Kureha Co., Ltd.) as a binder, and Al foil (thickness: 15 μm) as a positive electrode current collector were prepared. A positive electrode was produced from the above materials.

The materials used for the negative electrode were natural graphite (Hitachi Chemical Co., Ltd.) as the negative electrode active material, CMC (Nippon Paper Industries Co., Ltd.) and SBR (JSR Corporation) as the binder, and Cu foil (thickness: 10 μm) as the negative electrode current collector. The negative electrode was made from the above materials.

A porous resin (PP/PE/PP) (thickness: 24 μm) was prepared as a separator, in which a PP layer was laminated on both sides of a PE layer. The positive electrode, separator, and negative electrode were laminated so that the separator separated the positive electrode and the negative electrode. This formed an electrode body.

A pouch made of a laminate film was prepared as the exterior body. The electrode body was housed in the exterior body. As the non-aqueous electrolyte, a supporting salt (LiPF ₆ ) was dissolved at a concentration of 1 mol/L in a mixed solvent containing EC, DMC, and EMC. The electrolyte was injected into the exterior body. After the electrolyte was injected, the exterior body was sealed. As described above, the test batteries (pouch-type lithium ion batteries) of Examples 1 to 6 and Comparative Examples 1 to 3 were manufactured.

Analysis
"composition"
The compositions of the positive electrode active materials of Examples 1 to 6 and Comparative Examples 1 to 3 were confirmed by an ICP optical emission spectrometer (PS3520UVDD, manufactured by Hitachi High-Tech Science Corporation). The compositions of the positive electrode active materials are shown in Table 2.

X-ray diffraction
The positive electrode active materials of Examples 1 to 6 and Comparative Examples 1 to 3 were irradiated with X-rays using an X-ray diffractometer (Smartlab, manufactured by Rigaku Corporation) under the following measurement conditions to obtain X-ray diffraction patterns. From the obtained X-ray diffraction patterns, the peak top (a) of the (003) plane, the 1/20th width of the peak top (b), and the half width of the peak top (c) were calculated to obtain the values of formulas (1) and (2). The results are shown in Table 2.

[Measurement conditions]
X-ray output: 45kV, 200mA
X-ray source: CuKα radiation (wavelength: 1.54051 Å), single crystal monochromator Diffraction angle: 10 to 120°
Measurement temperature: room temperature Scanning speed: 1 second/step 《Gap》
A part of the positive electrode active material of Examples 1 to 6 and Comparative Examples 1 to 3 was embedded in a resin, cut using a CP, and made into a state in which the cross section of the secondary particles could be observed, and then images were obtained using an SEM at an observation magnification of 8000 times. From the obtained images, 100 pieces in which the entire secondary particles were visible were selected, the maximum diameter of the voids in each particle was measured, and the number of secondary particles whose maximum diameter of the voids in the secondary particles was 200 nm or more was counted, and the proportion was calculated. The results are shown in Table 2.

<Shell thickness>
The presence or absence of a shell portion was confirmed for secondary particles whose maximum void diameter in the secondary particles measured by the above-mentioned method was 200 nm or more. 50 secondary particles in which a shell portion was confirmed were selected, and the thickness of the shell portion of each secondary particle was measured at two points, and the average value of the 50 secondary particles was taken as the shell thickness. The results are shown in Table 2.

<Evaluation>
Initial Resistance
The SOC (State of Charge) of the test battery was adjusted to 50% by constant current-constant voltage (CC-CV) charging in a temperature environment of 25°C. The current during constant current (CC) charging was 1 It. "1 It" is defined as the current that drains the rated capacity of the battery in 1 hour. At 50% SOC, the battery voltage was 3.7 V. After adjusting the SOC, the battery was discharged for 10 seconds at a current of 10 It with a 30-minute break in between. The initial discharge resistance (initial resistance) was calculated by the following formula (4). The results are shown in Table 2. The initial resistance in Table 2 is a relative value. The initial resistance of Comparative Example 1 is defined as 1.

r=(V ₀ - V ₁₀ )/current Formula (4)
In the above formula (4), r represents the discharge resistance, _V0 represents the voltage at the start of discharge, and _V10 represents the voltage 10 seconds after the start of discharge.

Resistance Increase Rate
After measuring the initial resistance, the SOC of the battery was adjusted to 80% by CC-CV charging in a temperature environment of 25° C. The current during CC charging was 1 It. After adjusting the SOC, a cycle test was performed in a temperature environment of 25° C. That is, the following discharge and charge were alternately repeated for 15 days.

Discharge: current = 1 It, discharge capacity = capacity equivalent to 20% SOC Charge: current = 1 It, charge capacity = capacity equivalent to 20% SOC After the cycle test, the discharge resistance after the cycle (post-cycle resistance) was measured as well as the initial resistance. The resistance increase rate (percentage) was calculated by dividing the post-cycle resistance by the initial resistance. The results are shown in Table 2.

<Results>
As shown in Table 2, in this embodiment, when formulas (1) and (2) are satisfied, the initial resistance and the resistance increase rate are low. In addition, it is found that the initial resistance is lowered when the ratio of the secondary particles having a maximum void diameter of 200 nm or more in the positive electrode active material is high. It is also found that the initial resistance is lowered when the thickness of the shell portion is reduced.

The embodiments and examples disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present disclosure is indicated by the claims, not the above description, and is intended to include all modifications within the meaning and scope of the claims.

10 positive electrode, 20 negative electrode, 30 separator, 40 laminate, 50 electrode body, 90 case, 91 positive electrode terminal, 92 negative electrode terminal, 100 lithium ion battery.

Claims (2)

A positive electrode active material for a lithium ion battery, comprising a lithium nickel cobalt manganese-containing composite oxide,
The positive electrode active material includes secondary particles formed by agglomeration of a plurality of primary particles,
The positive electrode active material has the formula (1):
(18.768-a) /b≦0.0022 (1)
and formula (2):
c/d≦2.66 (2)
Fulfilling the relationship,
In the formula (1),
a is the peak top of the (003) plane measured by X-ray diffraction of the positive electrode active material, and is in the range of 18.7< a<18.768;
b is the percentage of the number of moles of manganese relative to the total number of moles of nickel, cobalt, and manganese in the positive electrode active material,
In the formula (2),
c is 1/20th the width of a,
d is the half width of the a,
The secondary particles have a core portion having a void,
The ratio of the secondary particles having the voids with a maximum diameter of 200 nm or more is 50% or more.
Positive electrode active material. The secondary particles have a shell portion on the outside of the core portion,
The thickness of the shell portion is 0.9 μm or less.
The positive electrode active material according to claim 1 .

Images