JP7521179B2 - Method for producing positive electrode active material - Google Patents

Description

The present invention relates to a method for producing a positive electrode active material for a lithium ion secondary battery, and a method for producing a lithium ion secondary battery using the positive electrode active material produced by the method .

In recent years, with the widespread use of small information terminals such as smartphones and tablet PCs, there is an increasing demand for small, lightweight secondary batteries with high energy density. In addition, there is a strong demand for the development of high-output secondary batteries as power sources for electric vehicles such as hybrid electric vehicles, plug-in hybrid electric vehicles, and battery-powered electric vehicles. As a secondary battery for the above-mentioned applications, a non-aqueous electrolyte secondary battery that is mainly composed of a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator is known. In addition, all-solid-state secondary batteries using solid electrolytes are expected to be the next-generation energy storage device.

Among the secondary batteries mentioned above, lithium-ion secondary batteries have already been put to practical use, which use an active material made of a layered or spinel-type lithium transition metal composite oxide as the positive electrode material, and release and insert lithium by charging and discharging. Lithium-ion secondary batteries can obtain a voltage of about 4V and have excellent properties such as energy density, but research and development is still being actively conducted to further improve their properties. Composite oxides of various compositions have been proposed for the lithium transition metal composite oxide used as the positive electrode active material in these lithium-ion secondary batteries.

Examples of such oxides include lithium cobalt composite oxide (LiCoO ₂ ), which is relatively easy to synthesize, lithium nickel composite oxide (LiNiO ₂ ), which uses nickel, which is less expensive than cobalt, lithium manganese composite oxide (LiMn ₂ O ₄ ) and lithium nickel manganese composite oxide (LiNi _0.5 Mn _0.5 O ₂ ), which use manganese, and ternary lithium nickel cobalt manganese composite oxide (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ), in which part of the cobalt is replaced with nickel and manganese.

In addition, various forms of positive electrode active materials have been proposed. For example, Patent Document 1 discloses a positive electrode active material made of a hexagonal lithium-containing composite oxide with a layered structure, which has the form of secondary particles with voids. It describes that by setting the average particle size to 3 to 12 μm and narrowing the particle size distribution, a secondary battery with large discharge capacity and high output can be obtained.

JP 2013-229339 A

As disclosed in the above-mentioned Patent Document 1, by using a positive electrode active material with a relatively small particle size and narrow particle size distribution, i.e., a uniform particle size, it is possible to obtain a lithium ion secondary battery with excellent cycle characteristics and output characteristics. However, in recent years, small information terminals and electric vehicles have been increasingly equipped with high functionality and performance, and there is a tendency for even higher stability during charging to be required for the lithium ion secondary batteries installed in these devices. In order to meet such demands, it is considered effective to increase the crystallinity of the positive electrode active material by increasing the heat treatment temperature or lengthening the heat treatment time in the manufacturing process of the positive electrode active material.

However, under the above heat treatment conditions, the aggregation and sintering of the particles of the positive electrode active material are promoted, which makes it easier for agglomerates and sintered bodies to form, resulting in problems in handling such as a decrease in the fluidity of the positive electrode active material in the form of powder obtained after the heat treatment, and in some cases causing variations in the battery characteristics of lithium-ion secondary batteries using this positive electrode active material. The present invention has been made in view of the above situation, and aims to provide a method for stably producing a positive electrode active material with excellent battery characteristics without causing handling problems.

In order to achieve the above object, the manufacturing method of the positive electrode active material according to the present invention is a manufacturing method of a positive electrode active material made of a lithium transition metal composite oxide produced by heat-treating a mixture of a transition metal composite hydroxide and a lithium compound as a precursor, and the heat treatment is performed in an air atmosphere in a low temperature range below a predetermined threshold temperature in the range of 700 ° C. to 900 ° C., and in a high temperature range of 850 ° C. to 1050 ° C. (excluding 850 ° C.) that exceeds the predetermined threshold temperature , the heat treatment is performed in an oxygen atmosphere, and the heating rate in the heat treatment is set to 5 to 10 ° C. / min and the lithium compound is held near the melting point for 1 to 5 hours, and the positive electrode active material has a ratio of D50 obtained by dividing its median diameter D50 by the median diameter D50 of the transition metal composite hydroxide as the precursor of less than 1.05, and the crystallite diameter obtained by substituting the (003) plane diffraction peak width measured by X-ray diffraction method into the Scherrer formula is 1000 Å or more .

According to the present invention, it is possible to stably produce a positive electrode active material that is easy to handle and has excellent battery characteristics on an industrial scale.

4 is a graph showing a furnace temperature profile in a firing step employed in a method for producing a positive electrode active material according to an embodiment of the present invention. 4 is a graph showing a furnace temperature profile in a firing step employed in a method for producing a positive electrode active material according to a comparative example of the present invention.

The inventors have conducted extensive research into a method for producing a positive electrode active material for lithium ion secondary batteries that is easy to handle and that provides excellent output characteristics when used as a positive electrode material for lithium ion secondary batteries. As a result, the inventors have discovered that by performing a heat treatment of a mixture of a precursor transition metal composite hydroxide and a lithium raw material under specific atmospheric conditions, it is possible to obtain a positive electrode active material that exhibits excellent output characteristics when used as a positive electrode material for lithium ion secondary batteries while suppressing sintering of particles, and have thus completed the present invention. Hereinafter, an embodiment of the method for producing the positive electrode active material of the present invention will be described. First, a method for producing transition metal composite hydroxide particles as a precursor that serves as an intermediate raw material in the method for producing the positive electrode active material will be described.

1. Method for Producing Transition Metal Composite Hydroxide Particles The transition metal composite hydroxide particles, which are precursors of the positive electrode active material according to an embodiment of the present invention, have a composition formula A represented by, for example, Ni _x Mn _y Co _z M _t (OH) _{2 + α} (wherein x + y + z + t = 1, 0.3 ≤ x ≤ 0.95, 0.05 ≤ y ≤ 0.55, 0 ≤ z ≤ 0.4, 0 ≤ t ≤ 0.1, -0.20 ≤ α ≤ 0.20, and M is one or more additive elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W).

These transition metal composite hydroxide particles are preferably produced by supplying a raw material aqueous solution containing the transition metal prepared in the raw material preparation step and an aqueous solution containing an ammonium ion donor to a reaction tank and carrying out a crystallization reaction in the reaction tank. This crystallization reaction is preferably carried out in two steps, a nucleation step and a particle growth step. Specifically, first, in the nucleation step, the pH value of the reaction aqueous solution in the reaction tank is adjusted to about 12.0 to 14.0 at a liquid temperature of 25°C to produce nuclei, and then, in the particle growth step, the pH value of the reaction aqueous solution containing the nuclei produced in the nucleation step is adjusted to, for example, about 10.5 to 12.0, preferably 0.5 or more lower than the pH value in the nucleation step at a liquid temperature of 25°C, to grow the nuclei.

In the initial stages of the nucleation and particle growth processes, it is preferable to make the atmosphere in the reaction tank an oxidizing atmosphere with an oxygen concentration of more than 5% by volume, and in stages after the initial stage of the particle growth process, to make the atmosphere in the reaction tank a non-oxidizing atmosphere with an oxygen concentration of 5% by volume or less. This makes it possible to efficiently produce transition metal composite hydroxide particles with a narrow particle size distribution.

It is preferable to control the temperature of the reaction aqueous solution within the range of 20 to 60°C throughout the formation process and particle growth process. If the temperature is below 20°C, nucleation is likely to occur due to the low solubility of the reaction aqueous solution, making it difficult to control the average particle size and particle size distribution of the final transition metal composite hydroxide particles. Conversely, if the temperature exceeds 60°C, the volatilization of ammonia is promoted, and to compensate for this, the supply amount of the aqueous solution containing the ammonium ion donor is increased, resulting in increased production costs. Each of the above steps constituting the method for producing transition metal composite hydroxide particles will be specifically described below.

1.1 Raw Material Preparation Step First, a raw material aqueous solution containing a transition metal compound that serves as a raw material for the reaction aqueous solution in which the crystallization reaction will be carried out, an aqueous solution containing an ammonium ion donor that serves as a complexing agent in the reaction aqueous solution, and an alkaline aqueous solution that serves to adjust the pH value of the reaction aqueous solution are each prepared by the methods described below.
(a) Raw material aqueous solution In preparing the raw material aqueous solution, the transition metal elements are mixed in such a molar ratio that the raw material aqueous solution contains matches the composition of the intended transition metal composite hydroxide particles. For example, when producing transition metal composite hydroxide particles represented by the above-mentioned general formula A, the metal elements are mixed in such a way that the molar ratio in the raw material aqueous solution is Ni:Mn:Co:M=x:y:z:t (where x+y+z+t=1, 0.3≦x≦0.95, 0.05≦y≦0.55, 0≦z≦0.4, 0≦t≦0.1).

The above transition metals are each added to water in the form of a compound to prepare the raw material aqueous solution. There are no limitations on the specific type of compound, but from the viewpoint of ease of handling, water-soluble compounds such as nitrates, sulfates, or chlorides are preferred, and among these, sulfates are particularly preferred from the viewpoints of cost and preventing the inclusion of halogens. Similarly, water-soluble compounds are preferred for the additive element M (M is one or more additive elements selected from the group consisting of Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) that is added to the transition metal composite hydroxide particles as needed.

The above transition metal compounds and the compound of the additive element M, which is added as necessary, preferably have a total molar concentration in the raw material aqueous solution of 1.0 to 2.6 mol/L, more preferably 1.5 to 2.2 mol/L. If the molar concentration is less than 1.0 mol/L, the amount of precipitation per unit volume of the reaction aqueous solution will be small, resulting in reduced productivity. Conversely, if the molar concentration exceeds 2.6 mol/L, the concentration may exceed the saturation concentration at room temperature, and crystals of the metal compound may be reprecipitated, causing clogging of piping, etc. Note that in order to prevent the production of compounds other than the target compound, an aqueous solution may be prepared for each metal compound and introduced into the reaction tank.

(b) Aqueous solution containing an ammonium donor There is no particular limitation on the type of aqueous solution containing an ammonium ion donor, and for example, an aqueous solution of ammonia water, ammonium sulfate, ammonium chloride, ammonium carbonate, ammonium fluoride, or the like can be used. Among these, aqueous ammonia is preferred. When aqueous ammonia is used, its concentration is preferably 20 to 30 mass%, more preferably 22 to 28 mass%. By adjusting the concentration of aqueous ammonia to within the range of 20 to 30 mass%, it is possible to suppress the loss of ammonia due to volatilization, etc., and therefore to reduce production costs.

(c) Alkaline aqueous solution The type of the alkaline aqueous solution is not particularly limited, and a general aqueous solution of an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide can be used. The alkali metal hydroxide is preferably added in the form of an aqueous solution in order to facilitate pH control. In this case, the concentration of the aqueous solution of the alkali metal hydroxide is preferably 20 to 50 mass%, more preferably 26 to 30 mass%. By adjusting the concentration of the aqueous solution of the alkali metal hydroxide to within the range of 20 to 50 mass% as described above, it is possible to prevent the pH value from becoming locally high at the addition position of the alkali metal hydroxide while suppressing the amount of water as a solvent introduced into the crystallization reaction system. As a result, it is possible to efficiently obtain composite hydroxide particles with a narrow particle size distribution.

1.2 Nucleation Step In the nucleation step, first, the alkaline aqueous solution prepared in the raw material preparation step and an aqueous solution containing an ammonium ion donor are introduced into a reaction tank to prepare a pre-reaction aqueous solution having a pH value of 12.0 to 14.0 at a liquid temperature of 25°C as measured with a pH meter and an ammonium ion concentration of 3 to 25 g/L as measured with an ion meter.

Next, while stirring this pre-reaction aqueous solution, the raw material aqueous solution prepared in the raw material preparation process is supplied. This generates nuclei that will become the cores of the transition metal composite hydroxide particles in the reaction tank. In this nucleation process, it is preferable to control the pH value of the reaction aqueous solution in which nuclei are generated within the range of 12.0 to 14.0 at a liquid temperature of 25°C. This makes it possible to prioritize the generation of new nuclei while suppressing the growth of nuclei, and therefore the nuclei generated in the nucleation process can be homogeneous and have a narrow particle size distribution.

If the pH value of the reaction solution is less than 12.0, nuclei will tend to grow as they are generated, which may result in non-uniform particle size of the transition metal composite hydroxide particles finally obtained in the crystallization process, and the particle size distribution may become wider. Conversely, if the pH value exceeds 14.0, the nuclei generated will be too fine, and the reaction solution may gel. In this nucleation process, it is further preferable to control the fluctuation range of the pH value to within ±0.2. This makes it possible to further narrow the particle size distribution of the transition metal composite hydroxide particles.

In the nucleation step, since the pH value and ammonium ion concentration of the reaction aqueous solution change with nucleation, it is preferable to supply an alkaline aqueous solution and an aqueous solution containing an ammonium donor appropriately so that the above pH range and ammonium ion concentration range are maintained. There are no particular limitations on the method of supplying the alkaline aqueous solution, but it is preferable to supply the aqueous solution using a pump capable of flow control, such as a metering pump, while thoroughly stirring the reaction aqueous solution so that the pH value of the reaction aqueous solution in the reaction tank does not become locally high. Similarly, it is preferable to supply the aqueous solution containing an ammonium ion donor using a pump capable of flow control.

This nucleation process is terminated when a predetermined amount of nuclei are generated in the reaction aqueous solution. The time when this predetermined amount of nuclei are generated can be determined from the amount of metal compounds contained in the raw aqueous solution supplied to the reaction tank. Specifically, it is preferable to terminate the nucleation process when 0.1 to 2 atomic %, and more preferably 0.2 to 1.5 atomic %, of all metal elements in the metal compounds contained in all raw aqueous solutions supplied through the nucleation process and particle growth process have been supplied. This makes it possible to generate transition metal composite hydroxide particles with a narrow particle size distribution.

1.3 Particle Growth Step In the particle growth step following the nucleation step, the pH value of the reaction aqueous solution in the reaction tank is adjusted to 10.5 to 12.0 at a liquid temperature of 25°C, and the ammonium ion concentration is adjusted to 3 to 25 g/L. This makes it possible to grow the nuclei generated in the nucleation step contained in the reaction aqueous solution while suppressing the generation of new nuclei. As a result, the transition metal composite hydroxide particles that are finally generated can be made more homogeneous and have a narrow particle size distribution.

If the pH value is less than 10.5, the ammonium ion concentration increases and the solubility of the metal ions increases, which slows down the crystallization reaction and may reduce productivity due to an increase in the amount of metal ions remaining in the reaction aqueous solution. Conversely, if the pH value exceeds 12.0, the amount of nucleation during the particle growth process increases, and the particle size of the resulting composite hydroxide particles may become non-uniform. In this particle growth process, it is preferable to further control the pH value fluctuation range to within ±0.2. This makes it possible to produce transition metal composite hydroxide particles with a narrower particle size distribution. In this particle growth process, the pH value and ammonium ion concentration of the reaction aqueous solution change with particle growth, so it is preferable to appropriately supply an alkaline aqueous solution and an ammonia aqueous solution so that the above pH value and ammonium ion concentration ranges are maintained.

In both the nucleation process and the particle growth process, if the ammonium ion concentration is less than 3 g/L, it becomes difficult to maintain a constant solubility of the metal ions, the reaction aqueous solution becomes more likely to gel, and it becomes difficult to obtain transition metal composite hydroxide particles with a regular shape and particle size. Conversely, if the ammonium ion concentration exceeds 25 g/L, the solubility of the metal ions becomes too high, so the amount of metal ions remaining in the reaction aqueous solution increases, and problems such as composition deviation may occur. It is preferable to limit the fluctuation range of the ammonium ion concentration to a constant fluctuation range of about ±5 g/L.

At the end of the particle growth process, the slurry in the reaction tank preferably has a solids concentration of transition metal composite hydroxide particles in the range of 30 to 200 g/L, more preferably in the range of 80 to 150 g/L. If the solids concentration is less than 30 g/L, the primary particles may not aggregate sufficiently. Conversely, if it exceeds 200 g/L, the transition metal composite hydroxide particles may not diffuse sufficiently in the reaction tank, which may cause uneven particle growth.

1.4 Coating step The additive element M, which is added as necessary, may be prepared as a raw material aqueous solution together with the essential transition metal as described above, but in this coating step, it may also be added by coating the surface of the transition metal composite hydroxide particles obtained in the particle growth step with a compound containing the additive element M. Specifically, water is added to the transition metal composite hydroxide particles produced in the particle growth step to form a slurry, and then a coating aqueous solution in which the compound containing the additive element M is dissolved is added while controlling the pH value of the slurry to a predetermined range. As a result, the compound containing the additive element M is precipitated on the particle surface of the transition metal composite hydroxide, thereby obtaining coated transition metal composite hydroxide particles.

Instead of using the above-mentioned coating aqueous solution, the transition metal composite hydroxide particles may be coated by adding an alkoxide solution of the additive element M to the slurried transition metal composite hydroxide particles, or the transition metal composite hydroxide particles may be coated by spraying an aqueous solution or slurry in which a compound containing the additive element M is dissolved directly onto the transition metal composite hydroxide particles and drying the solution. As another coating method, the transition metal composite hydroxide particles may be coated by spray drying a slurry prepared by mixing the transition metal composite hydroxide particles and the compound containing the additive element M, or the transition metal composite hydroxide particles may be coated by mixing the transition metal composite hydroxide particles and the compound containing the additive element M by a solid phase method.

When coating the surface of the transition metal composite hydroxide particles with the additive element M in this way, it is necessary to appropriately adjust the composition of each of the raw material aqueous solution and the coating aqueous solution and their mixing ratios so that the overall composition of the coated transition metal composite hydroxide particles matches the desired composition. In addition, this coating process may be performed after the drying process in the manufacturing method for the positive electrode active material described below.

2. Manufacturing method of the positive electrode active material Next, a manufacturing method of the positive electrode active material according to an embodiment of the present invention using the above-mentioned transition metal composite hydroxide particles as an intermediate raw material will be described. The manufacturing method of the positive electrode active material according to the embodiment of the present invention includes a drying step S1 in which the above-mentioned transition metal composite hydroxide particles are heated and dried, a mixing step S2 in which a lithium compound is added to the heated and dried transition metal composite hydroxide particles and mixed, a firing step S3 in which the lithium mixture obtained in the mixing step S2 is fired preferably at 850 to 1050°C, and a crushing step S4 in which the aggregates and sintered bodies generated in the firing step S3 are crushed as necessary. Each step will be described in detail below.

2.1 Drying Step In the drying step S1, the transition metal composite hydroxide particles are dried by heating, preferably, to 105 to 150° C. This makes it possible to remove excess moisture contained in the transition metal composite hydroxide particles to a certain extent, and therefore makes it possible to effectively reduce the moisture remaining in the positive electrode active material particles after the firing treatment in the subsequent firing step S3, and as a result, makes it possible to suppress variation in the composition of the positive electrode active material.

If the heating temperature is less than 105°C, the excess moisture contained in the transition metal composite hydroxide particles will not be sufficiently removed, and the composition of the final positive electrode active material may vary greatly. Conversely, if the heating temperature exceeds 150°C, not only will no further effect be expected, but production costs will increase, which is not preferable. The atmosphere during the heat treatment is preferably a non-reducing atmosphere, and more preferably an air stream, which is easy to perform. In addition, the drying process time is preferably at least 1 hour, and more preferably 5 to 15 hours, from the viewpoint of sufficiently removing excess moisture from the transition metal composite hydroxide particles.

2.2 Mixing Step In the mixing step S2, a lithium compound is added to the transition metal composite hydroxide particles dried by heating in the drying step S1 and mixed thoroughly to obtain a lithium mixture. In mixing the transition metal composite hydroxide particles and the lithium compound, the ratio (Li/Me) of the number of lithium atoms (Li) to the total number of atoms (Me) of the metal atoms other than lithium in the lithium mixture, nickel, cobalt, manganese, and the additive element M, is mixed so that it matches the desired Li/Me of the positive electrode active material to be finally produced. This is because Li/Me does not change before and after the subsequent baking step S3. Specifically, in this mixing step S2, Li/Me is preferably mixed to be 0.95 to 1.5, more preferably 1.0 to 1.5, even more preferably 1.0 to 1.35, and most preferably 1.0 to 1.2.

In terms of ease of availability, it is preferable to use lithium hydroxide, lithium nitrate, lithium carbonate, or a mixture of these as the lithium compound to be added to the transition metal composite hydroxide in this mixing step S2. Of these, lithium hydroxide or lithium carbonate is more preferable in terms of ease of handling and quality stability, and lithium carbonate is most preferable.

A general mixer such as a shaker mixer, a Loedige mixer, a Julia mixer, or a V blender can be used to mix the transition metal composite hydroxide particles and the lithium compound. In this case, it is preferable to mix thoroughly enough to avoid the generation of fine powder. If the mixing is insufficient, there may be some areas that deviate significantly from the desired Li/Me value, making it difficult to obtain good battery characteristics.

2.3 Firing step In the firing step S3, the lithium mixture obtained in the mixing step S2 is loaded into a firing furnace and fired under a predetermined condition. As a result, the transition metal composite hydroxide is decomposed and oxidized, and lithium is diffused into the particles of the transition metal composite hydroxide to produce a lithium transition metal composite oxide, and further defects are reduced by oxidation and atomic diffusion, and lithium transition metal composite oxide particles with improved crystallinity of each particle are produced. There is no particular limitation on the firing furnace used in this firing step S3, and it may be a batch type or a continuous type, but an electric furnace that does not generate gas is preferable so that the atmosphere inside the furnace can be easily adjusted as described below. The treatment conditions for this firing treatment are specifically described below.

(a) Maximum temperature and holding time at that temperature In the calcination step S3, the lithium mixture is charged into a calcination furnace, the temperature inside the furnace (also referred to as the atmospheric temperature) is gradually increased, and when the temperature inside the furnace reaches a maximum temperature (also referred to as the calcination temperature) of preferably 850°C to 1050°C, more preferably 900°C to 1000°C, the maximum temperature is held for a predetermined time to perform the calcination process. If the maximum temperature is less than 850°C, lithium may not be sufficiently diffused into the transition metal composite hydroxide particles, and excess lithium or unreacted transition metal composite hydroxide particles may remain, or the crystallinity of the finally obtained lithium transition metal composite oxide particles may be insufficient. Conversely, if the maximum temperature exceeds 1050°C, sintering between the particles of the lithium transition metal composite oxide may be promoted, and the content ratio of amorphous coarse particles may increase.

The holding time at the maximum temperature is preferably 2 hours or more, and more preferably 4 hours or more and 24 hours or less. If the holding time is less than 2 hours, as in the above case, lithium may not be sufficiently diffused into the transition metal composite hydroxide particles, and excess lithium or unreacted transition metal composite hydroxide particles may remain, or the crystallinity of the resulting lithium transition metal composite oxide particles may be insufficient. Conversely, if the holding time exceeds 24 hours, no further effect can be expected, and this is not preferred from the viewpoint of productivity.

(b) Heating rate and temperature decreasing rate The heating rate up to the maximum temperature is preferably 2 to 10 ° C./min, more preferably 5 to 10 ° C./min. This can suppress the generation of thermal stress in the transition metal composite hydroxide particles, and can prevent quality problems such as cracks in the transition metal composite hydroxide particles. In addition, during the heating, it is preferable to hold the temperature near the melting point of the lithium compound for preferably about 1 to 5 hours, more preferably about 2 to 5 hours. This allows the transition metal composite hydroxide particles and the lithium compound to react more uniformly. After the predetermined holding time at the maximum temperature has elapsed, it is preferable to cool the temperature from the maximum temperature to 200 ° C. at a temperature decreasing rate of preferably 2 to 10 ° C./min, more preferably 3 to 7 ° C./min. This can ensure productivity while preventing equipment such as saggers from being damaged by rapid cooling.

(c) Sintering atmosphere In the above-mentioned sintering treatment, the atmosphere in the sintering furnace is switched between a low-temperature region from when the temperature in the furnace starts to rise until it reaches a predetermined threshold temperature, and a high-temperature region from when the temperature in the furnace starts to rise until it reaches a predetermined threshold temperature until it reaches a predetermined holding time at the maximum temperature. Specifically, as described above, in the sintering treatment in which a lithium mixture is charged into a sintering furnace and heat-treated according to a predetermined temperature profile, the heat treatment is performed in an air atmosphere in the low-temperature region from when the temperature in the furnace starts to rise until it reaches a predetermined threshold temperature that is equal to or lower than the maximum temperature, and the heat treatment is performed in an oxygen atmosphere in the high-temperature region from when the temperature in the furnace starts to rise until it reaches a predetermined threshold temperature that is equal to or lower than the maximum temperature until it reaches a predetermined holding time at the maximum temperature. The threshold temperature is preferably in the range of 700°C to 900°C.

By switching the firing atmosphere around the threshold temperature as described above, the crystallinity of the transition metal composite oxide particles can be increased without increasing the maximum temperature or extending the holding time at the maximum temperature, so that a highly crystalline positive electrode active material can be produced without increasing the content of aggregates or sintered bodies. In particular, when the precursor transition metal composite hydroxide contains tungsten (W), a high heat treatment temperature has been used in the past to increase crystallinity, but the method of the embodiment of the present invention is effective because it allows the heat treatment temperature to be lowered compared to the past.

In other words, in the past, in order to increase the crystallinity of the positive electrode active material, the heat treatment temperature was increased or the heat treatment time was extended during the calcination of the precursor, but all of these treatment methods made the transition metal composite hydroxide particles and their oxide particles prone to sintering with each other, which resulted in handling problems such as reduced fluidity for the resulting positive electrode active material, and problems with large variations in quality when used as a positive electrode material. In this case, it was thought that calcination in an oxygen atmosphere would reduce oxygen vacancies and slow the diffusion rate of the transition metal, thereby suppressing the sintering of the particles, but the crystallinity could not be sufficiently increased under these treatment conditions.

In contrast, in the manufacturing method according to the embodiment of the present invention, in the low temperature range below the threshold temperature, the lithium mixture is heat-treated in an air atmosphere, which introduces an appropriate amount of oxygen vacancies into the crystal structure of the transition metal composite hydroxide particles and the lithium transition metal composite oxide particles before the crystallinity is increased, thereby promoting crystal growth. On the other hand, in the high temperature range after exceeding the threshold temperature at which the diffusion rate of the transition metal is significantly increased, the lithium mixture is heat-treated in an oxygen atmosphere, which reduces the oxygen vacancies on the surface of the transition metal composite hydroxide particles and/or the oxide particles, thereby suppressing the diffusion of the transition metal, and as a result, it is possible to suppress sintering of the lithium transition metal composite oxide particles.

2.4 Crushing step As described above, in the calcination step S3, heat treatment is performed under conditions that make sintering difficult, but due to sintering necking between secondary particles, the lithium transition metal composite oxide particles after the calcination treatment may contain aggregates or lightly sintered sintered bodies. Therefore, it is preferable to apply mechanical energy to the aggregates and sintered bodies of the lithium transition metal composite oxide particles through the crushing step S6 as necessary to crush them. This makes it possible to adjust the average particle size and particle size distribution of the finally obtained positive electrode active material within a suitable range.

There are no particular limitations on the equipment used for this disintegration, so long as it can break down the aggregates and sintered bodies without destroying the secondary particles themselves, and for example, a pin mill or a hammer mill can be suitably used. When using these equipment to carry out the disintegration process, it is preferable to use lithium transition metal composite oxide particles sampled in advance to investigate the disintegration state when the conditions, such as the rotation speed of the disintegration device, are changed in various ways, and thereby to determine the conditions, such as the rotation speed, under which an appropriate disintegration force acts without destroying the secondary particles, and to carry out the disintegration process under those conditions.

3. Positive electrode active material for lithium ion secondary battery The positive electrode active material produced by the manufacturing method of the lithium transition metal composite oxide according to the embodiment of the present invention is, for example, a lithium nickel manganese composite oxide containing lithium, _{nickel and} manganese as essential elements, and cobalt and an additive element M as optional elements, and its composition formula B is Li1 _{+ uNixMnyCozMtO2 +β} (-0.05≦u≦0.50, x+y+z+t=1, 0.3≦x≦0.95, 0.05≦y≦0.55 _, 0≦z≦0.4, 0≦t≦0.1, -0.2≦β≦0.2, M is one or more additive elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W). This composite oxide has a hexagonal layered crystal structure.

(a) Volume Average Particle Size Ratio The cathode active material produced by the manufacturing method according to the embodiment of the present invention described above can have a ratio of D50 (hereinafter also referred to as D50 (cathode material)/D50 (precursor)) calculated by dividing the median diameter D50 of the cathode active material by the median diameter D50 of the transition metal composite hydroxide particles that are the precursor of the cathode active material, preferably less than 1.05, more preferably 1.0 or less. In other words, the particle size does not increase significantly before and after the firing process in the firing step S3, so that the variation in quality can be suppressed.

When the ratio of the median diameters D50 of the particles before and after the above-mentioned calcination treatment, "D50 (cathode material)/D50 (precursor)", is 1.05 or more, the proportion of aggregates and sintered bodies contained in the cathode active material increases, and the battery characteristics of a lithium-ion secondary battery using this cathode active material as the cathode material may be degraded. The median diameter D50 of the cathode active material and transition metal composite hydroxide particles can be determined, for example, from the integrated volume value measured with a laser light diffraction/scattering particle size analyzer.

(b) Crystallite diameter The positive electrode active material produced by the manufacturing method of the embodiment of the present invention described above can be preferably 1000 Å (100 nm) or more, more preferably 1200 Å (120 nm) or more, by measuring the (003) plane diffraction peak by X-ray diffraction (XRD) and substituting the peak width into the Scherrer formula. The crystallite diameter (hereinafter, (003) plane crystallite diameter) can be preferably 1000 Å (100 nm) or more, more preferably 1200 Å (120 nm) or more. A secondary battery using a positive electrode active material having high crystallinity with a (003) plane crystallite diameter of 1000 Å or more as a positive electrode material has a low positive electrode resistance, so that the output characteristics are improved and the thermal stability is also improved. On the other hand, if the (003) plane crystallite diameter is less than 1000 Å, the crystallinity may be insufficient or the thermal stability of the secondary battery may be reduced. The (003) plane crystallite diameter can be adjusted by appropriately changing the firing temperature and holding time in the firing step S3 described above.

4. Lithium-ion secondary battery 4.1 Non-aqueous electrolyte secondary battery The positive electrode active material produced by the manufacturing method of the embodiment of the present invention described above can be suitably used as a material for the positive electrode of a non-aqueous electrolyte secondary battery, which is a general lithium-ion secondary battery mainly composed of a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte solution. The shape of this non-aqueous electrolyte secondary battery is not particularly limited, and it can be made into various shapes such as a cylindrical shape or a laminated shape. Regardless of the shape, a non-aqueous electrolyte secondary battery can be produced by impregnating an electrode body consisting of a positive electrode and a negative electrode arranged through a separator with a non-aqueous electrolyte solution, connecting the current collector of the positive electrode to the positive electrode terminal leading to the outside, and connecting the current collector of the negative electrode to the negative electrode terminal leading to the outside using a current collector lead or the like, and housing and sealing the battery case. Each component will be described below.

(a) Positive electrode First, a conductive material and a binder are mixed with the powdered positive electrode active material produced by the manufacturing method of the embodiment of the present invention described above, and activated carbon for increasing the electric double layer capacity and a solvent for viscosity adjustment, etc. are added as necessary, and these are kneaded to produce a positive electrode mixture paste. The mixing ratio of the materials constituting this positive electrode mixture paste is set to an appropriate mixing ratio because it affects the performance of the non-aqueous electrolyte secondary battery. For example, when the solid content of the positive electrode mixture excluding the solvent is 100 parts by mass, it is preferable to set the content of the positive electrode active material to 60 to 95 parts by mass, the content of the conductive material to 1 to 20 parts by mass, and the content of the binder to 1 to 20 parts by mass, similar to the positive electrode of a general non-aqueous electrolyte secondary battery.

The obtained positive electrode mixture paste is applied to the surface of a current collector made of, for example, aluminum foil, and the solvent is evaporated by drying. To increase the electrode density, pressure may be applied using a roll press or the like, as necessary. This produces a sheet-shaped positive electrode, which is then cut to an appropriate size according to the desired battery shape, thereby producing the positive electrode. Note that the method for producing the positive electrode is not limited to this, and other methods may also be used.

As the conductive material used as the raw material of the positive electrode mixture paste, for example, graphite such as natural graphite, artificial graphite, and expanded graphite, and carbon black materials such as acetylene black and ketjen black can be used. The binder plays a role of binding the active material particles, and for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose-based resin, polyacrylic acid, etc. can be used. In addition, a solvent that disperses the positive electrode active material, conductive material, and activated carbon added as needed, and dissolves the binder, may be added to the positive electrode mixture. For this solvent, for example, an organic solvent such as N-methyl-2-pyrrolidone can be used.

(b) Negative electrode For the negative electrode, metallic lithium, lithium alloy, or a negative electrode active material capable of absorbing and desorbing lithium ions is prepared, and a binder and a suitable solvent are added to the negative electrode and kneaded to prepare a paste-like negative electrode mixture. This negative electrode mixture paste is applied to the surface of a metal foil current collector such as copper, dried, and compressed as necessary to increase the electrode density. This allows the negative electrode to be produced.

As described above, the negative electrode may be made of a material containing lithium, such as metallic lithium or a lithium alloy, or may be made of a fired organic compound such as natural graphite, artificial graphite, or phenolic resin, which can occlude and desorb lithium ions, or a powder of a carbon material such as coke. As with the positive electrode, the binder for the negative electrode may be a fluorine-containing resin such as PVDF, and the solvent for dispersing these active materials and binders may be an organic solvent such as N-methyl-2-pyrrolidone.

(c) Separator The separator is interposed between the positive electrode and the negative electrode to separate them and to hold the electrolyte. For this reason, the material of the separator is not limited, but a thin film having a countless number of fine pores, such as polyethylene or polypropylene, is preferably used.

(d) Non-aqueous electrolyte For the non-aqueous electrolyte, a solution in which a lithium salt as a supporting salt is dissolved in an organic solvent is preferably used, but a solution in which a lithium salt is dissolved in an ionic liquid may also be used. Note that the ionic liquid refers to a salt that is composed of cations and anions other than lithium ions and is liquid even at room temperature. In addition, the non-aqueous electrolyte may contain a radical scavenger, a surfactant, a flame retardant, etc., in order to improve the battery characteristics.

The supporting salt may be LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiN(CF ₃ SO ₂ ) ₂ , or a composite salt thereof. On the other hand, the organic solvent may be one selected from the group consisting of cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate, chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate, ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane, sulfur compounds such as ethyl methyl sulfone and butane sultone, and phosphorus compounds such as triethyl phosphate and trioctyl phosphate, or may be a mixture of two or more selected from the group.

4.2 All-solid-state secondary battery The positive electrode active material produced by the manufacturing method according to the embodiment of the present invention described above can be suitably used as a positive electrode material for all-solid-state secondary batteries, which are expected to be the next generation of lithium ion secondary batteries. It is preferable to use a solid electrolyte for use in this all-solid-state secondary battery that has the property of being able to withstand high voltages. Examples of such solid electrolytes include inorganic solid electrolytes and organic solid electrolytes. As the former inorganic solid electrolyte, oxide-based solid electrolytes and sulfide-based solid electrolytes are suitably used.

The oxide-based solid electrolyte is not limited, but is preferably one that contains oxygen (O) and has lithium ion conductivity and electronic insulation properties. Specifically, lithium phosphate (Li ₃ PO ₄ ), Li ₃ PO ₄ N _x , LiBO ₂ N _x , LiNbO _{3 , LiTaO 3 ,} Li ₂ SiO ₃ , Li ₄ SiO ₄ -Li ₃ PO ₄ , Li ₄ SiO ₄ -Li ₃ VO ₄ , Li ₂ O-B ₂ O ₃ -P ₂ O ₅ , Li ₂ O-SiO ₂ , Li ₂ O-B ₂ O ₃ -ZnO, Li _1+x Al _x Ti _2-x (PO ₄ ) ₃ (0≦x≦1), Li _1+x Al _x Ge _2-x (PO ₄ ) ₃ (0≦x≦1), _LiTi2 ( _PO4 ) _{3 , Li3xLa2 /3- xTiO3 (} 0 _≦ x≦2/ _{3 ) , Li5La3Ta2O12 , Li7La3Zr2O12 , Li6BaLa2Ta2O12} , _{Li3.6Si0.6P0.4O4 , and the like} .

The sulfide-based solid electrolyte is preferably one that contains sulfur (S) and has lithium ion conductivity and electronic insulation, but is not limited thereto. Specific examples include Li ₂ S-P ₂ S ₅ , Li 2 S-SiS 2 , LiI-Li ₂ S-SiS ₂ , LiI-Li ₂ S-P ₂ S _{5 ,} LiI-Li ₂ S- _{B 2 S 3} , Li ₃ PO ₄ -Li ₂ S-Si ₂ S, Li ₃ PO ₄ -Li ₂ S-SiS ₂ , LiPO ₄ -Li ₂ S-SiS, LiI-Li ₂ S-P ₂ O ₅ , and LiI-Li ₃ PO ₄ -P ₂ S ₅ .

Furthermore, inorganic solid electrolytes other than those mentioned above may be used, for example, Li ₃ N, LiI, Li ₃ N-LiI-LiOH, etc. On the other hand, the latter organic solid electrolyte is not particularly limited as long as it is a polymer compound exhibiting ion conductivity, and for example, polyethylene oxide, polypropylene oxide, copolymers thereof, etc. may be used. The organic solid electrolyte may contain a supporting salt (lithium salt).

The above describes the positive electrode active material and the manufacturing method thereof according to the embodiment of the present invention, as well as the manufacturing method of a lithium ion secondary battery using the positive electrode active material. However, the present invention is not limited to the above embodiment, and includes various modifications and alternative examples. In other words, the scope of the rights of the present invention extends to the scope of the claims and their equivalents. Next, the present invention will be described using examples, but the present invention is not limited in any way to the following examples.

First, to prepare a precursor of the positive electrode active material by a general wet method, a raw material aqueous solution containing a transition metal compound, an aqueous solution containing an ammonium ion donor acting as a complexing agent, and an alkaline aqueous solution were prepared, and these were fed into a reaction tank and crystallized to produce a nickel manganese cobalt composite hydroxide as a precursor. When preparing the raw material aqueous solution, the compounding ratio of the metal element compounds was adjusted to nickel:cobalt:manganese:tungsten = 0.375:0.319:0.299:0.007 on a molar basis.

This composite hydroxide was placed in an alumina sagger and loaded into a roller hearth kiln simulator furnace (manufactured by Noritake Co., Ltd.) and dried for 3 hours in an air atmosphere at 105°C. After that, lithium hydroxide prepared separately was added, and the mixture was loaded into a shaker mixer (TURBULA Type T2C manufactured by Willy & Bachofen (WAB)) and mixed to obtain a lithium mixture. The lithium hydroxide was blended so that the Li/Me ratio of the lithium mixture was 1.1.

The lithium mixture obtained above was placed in an alumina sagger and loaded into a roller hearth kiln simulator furnace (manufactured by Noritake Company), and sintered according to the temperature profile shown in Figure 1. During this sintering, the atmosphere in the furnace was air in the low temperature range from room temperature to 850°C, and oxygen in the high temperature range exceeding 850° _C . In _this manner, lithium _nickel manganese cobalt composite oxide of sample 1 having _the composition formula _{LiNi0.375Co0.319Mn0.299W0.007O2} was produced.

The lithium nickel manganese cobalt composite oxides of samples 2 to 8 were prepared in the same manner as sample 1, except that the treatment conditions during the above-mentioned calcination treatment were changed in various ways. In particular, for samples 4 and 5, the calcination treatment was performed based on the temperature profile and furnace atmosphere shown in Figure 2. The D50 (cathode material) of the positive electrode active material of these lithium nickel manganese cobalt composite oxides of samples 1 to 8 was obtained from the volume distribution measured with a laser light diffraction scattering type particle size distribution analyzer, and this was divided by the D50 (precursor) measured in the same manner for the precursor transition metal composite hydroxide particles to examine the change in particle size due to the calcination treatment. In addition, the crystallite diameter [Å] of the positive electrode active material was obtained by substituting the (003) plane diffraction peak width measured by the XRD method into the Scherrer formula. The results are shown in Table 1 below, along with the treatment conditions for the calcination treatment.

From the results in Table 1 above, the lithium nickel manganese cobalt composite oxides of samples 1 to 3, which were produced using a manufacturing method that satisfied the requirements of the present invention, all had the above-mentioned "D50 (cathode material)/D50 (precursor)" value of 1.0 or less, and a crystallite diameter of more than 1200 Å. In contrast, the lithium nickel manganese cobalt composite oxides of samples 4 to 8, which were produced using a manufacturing method that did not satisfy the requirements of the present invention, had a "D50 (cathode material)/D50 (precursor)" value of more than 1.0, or a crystallite diameter of less than 1200 Å.

Claims (5)

A method for producing a positive electrode active material made of a lithium transition metal composite oxide produced by heat-treating a mixture of a transition metal composite hydroxide and a lithium compound as a precursor, the heat treatment being carried out in an air atmosphere in a low temperature range below a predetermined threshold temperature in the range of 700°C to 900°C , and being carried out in an oxygen atmosphere in a high temperature range above the predetermined threshold temperature of 850°C to 1050°C (excluding 850°C), the heat treatment is carried out at a temperature increase rate of 5 to 10°C/min and is maintained at about the melting point of the lithium compound for 1 to 5 hours, the positive electrode active material having a D50 ratio obtained by dividing its median diameter D50 by the median diameter D50 of the transition metal composite hydroxide as the precursor of less than 1.05, and a crystallite diameter obtained by substituting the (003) plane diffraction peak width measured by X-ray diffraction method into the Scherrer formula of 1000 Å or more . 2. The method for producing a positive electrode active material according to claim 1, wherein the transition metal composite hydroxide has a composition formula represented by Ni _x Mn _y Co _z M _t (OH) _{2 + α} (x + y + z + t = 1, 0.3 ≤ x ≤ 0.95, 0.05 ≤ y ≤ 0.55, 0 ≤ z ≤ 0.4, 0 ≤ t ≤ 0.1, -0.20 ≤ α ≤ 0.20, and M is one or more additive elements selected from the group consisting of Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W). The lithium _transition metal _composite oxide has a composition formula of Li1 _{+uNixMnyCozMtO2 + β} (-0.05≦u≦0.50, x+y+z+t= ₁ , 0.3≦x≦0.95, 0.05≦y≦0.55, 0≦z≦0.4, 0≦t≦0.1, -0.20≦β≦0.20, M is one or more additive elements selected from the group consisting of Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W). The method for producing a positive electrode active material according to any one of claims 1 to 3, characterized in that the lithium compound is lithium carbonate, lithium hydroxide, or a mixture of both. A method for producing a lithium ion secondary battery comprising at least a positive electrode, a negative electrode, and an electrolyte, the method being characterized in that the positive electrode material of the positive electrode is a positive electrode active material produced by the method according to any one of claims 1 to 4.

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