JP2024135143A - Positive electrode active material for lithium ion secondary battery, method for producing same, and lithium ion secondary battery - Google Patents

Abstract

To provide a positive electrode active material for a lithium ion secondary battery which has excellent cycle characteristics and can constitute a lithium ion secondary battery that generates less gas during charging and discharging.SOLUTION: A positive electrode active material for a lithium ion secondary battery has particles of a lithium transition metal composite oxide and a coating layer covering at least a part of the surfaces of the particles. The lithium transition metal composite oxide has a substance ratio of components other than oxygen of Li:Ni:Co:M which is represented by t:1-x-y:x:y (in the formula, M is at least one element selected from the group consisting of Mg, Al, Ca, Si, Mn, Ti, V, Fe, Cu, Cr, Zn, Zr, Nb, Mo and W, and 0.95≤t≤1.20, 0<x≤0.22, 0≤y≤0.15). The coating layer contains a lithium tungsten compound, and the amount of W in the coating layer per surface area of 1 m2 of the lithium transition metal composite oxide excluding the coating layer is 0.04 mmol or more and 0.12 mmol or less.SELECTED DRAWING: None

Description

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

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 and durability. There is also a strong demand for the development of high-output secondary batteries as power sources for electric vehicles, including hybrid vehicles, or large secondary batteries for power storage.

Lithium ion secondary batteries are a type of secondary battery that meets these requirements. Lithium ion secondary batteries are composed of a negative electrode, a positive electrode, an electrolyte, and the like, and the active materials used for the negative and positive electrodes are materials capable of releasing and absorbing lithium ions. Currently, research and development of such lithium ion secondary batteries is being actively conducted. Among these, lithium ion secondary batteries that use lithium transition metal composite oxides as the positive electrode active material can obtain a voltage of about 4V, and are therefore being put into practical use as batteries with high energy density.

Specifically, lithium transition metal composite oxides such as lithium cobalt composite oxide (LiCoO ₂ ), which is relatively easy to synthesize, lithium nickel composite oxide (LiNiO ₂ ), which uses nickel, which is cheaper than cobalt, lithium nickel cobalt manganese composite oxide (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ), and lithium manganese composite oxide (LiMn ₂ O ₄ ), which uses manganese, have been proposed. Among these lithium transition metal composite oxides, lithium nickel composite oxide, which can realize a secondary battery with a high battery capacity without using cobalt, which is scarce in reserve, has been attracting attention in recent years as a material with high energy density.

However, compared to other lithium transition metal composite oxides, lithium nickel composite oxides have the problem that the battery capacity decreases with repeated charging and discharging, i.e., they have inferior cycle characteristics, and the problem that the battery expands due to gas generation inside the battery when it is kept at high temperatures in a charged state or when it is repeatedly charged and discharged at high temperatures.

One of the causes of the decrease in battery capacity and gas generation is the presence of Li _1-x NiO ₂ in the charged state in which Li is extracted from the lithium nickel composite oxide crystal. Li _1-x NiO ₂ is prone to oxygen desorption from its crystal structure in terms of charge balance, and when oxygen is desorbed from Li _1-x NiO ₂ and an electrochemically inactive NiO phase is formed, the charge/discharge capacity decreases. In addition, it is believed that the desorbed oxygen oxidizes the electrolyte and generates CO ₂ gas, which causes the battery to expand. For this reason, in order to improve the cycle characteristics of a lithium ion secondary battery using a lithium nickel composite oxide as a positive electrode active material and to suppress gas generation, it is considered important to control the surface state of the lithium nickel composite oxide particles, which are the reaction field with the electrolyte.

As an example of a method for controlling the surface state of lithium-nickel composite oxide particles, Patent Documents 1 and 2 disclose that by coating layered rock-salt type lithium-nickel composite oxide particles with a high nickel composition, in which nickel accounts for 60% or more by mass of the transition metal, with lithium tungstate, improvements in initial discharge capacity and cycle characteristics can be observed.

JP 2020-102426 A JP 2017-063003 A

However, the above proposals do not consider preventing battery swelling due to gas generation in lithium-ion secondary batteries. Furthermore, Patent Document 1 mentions improving the capacity retention rate after cycling as a cycle characteristic, but does not mention suppressing resistance increase after cycling. Furthermore, Patent Document 2 mentions the effect of improving cycle characteristics, but does not mention the amount of tungsten present on the surface of lithium nickel composite oxide particles.

In view of the problems inherent in the above-mentioned conventional techniques, the present invention aims to provide a positive electrode active material for lithium ion secondary batteries that can be used to construct lithium ion secondary batteries that have excellent cycle characteristics and generate little gas during charging and discharging.

A positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention is a positive electrode active material for a lithium ion secondary battery having particles of a lithium transition metal composite oxide and a coating layer coating at least a part of the surface of the particles, wherein a substance amount ratio Li:Ni:Co:M of components other than oxygen in the lithium transition metal composite oxide is expressed as t:1-x-y:x:y (wherein M is at least one element selected from the group consisting of Mg, Al, Ca, Si, Mn, Ti, V, Fe, Cu, Cr, Zn, Zr, Nb, Mo, and W, and 0.95≦t≦1.20, 0<x≦0.22, 0≦y≦0.15), the coating layer contains a lithium tungsten compound, and an amount of W in the coating layer per ^m2 of surface area of the lithium transition metal composite oxide excluding the coating layer is 0.04 mmol or more and 0.12 mmol or less.

In this way, it is possible to provide a positive electrode active material for lithium ion secondary batteries that has excellent cycle characteristics and generates little gas during charging and discharging.

In this case, in one embodiment of the present invention, the ratio Ws/(Nis+Cos+Ws) of the amount of W present on the surface of the positive electrode active material for a lithium ion secondary battery to the sum of the amounts of Ni, Co, and W present on the surface of the positive electrode active material for a lithium ion secondary battery, Nis+Cos+Ws, may be 0.15 or more and 0.50 or less.

In this way, the side reaction between the positive electrode active material and the electrolyte can be suppressed, and excessive decomposition of the electrolyte can be suppressed. In addition, the lithium tungsten compound that forms the coating layer can be prevented from becoming a resistance layer.

In this case, in one embodiment of the present invention, the lithium transition metal composite oxide may have a crystal structure belonging to the space group R-3m.

In this way, the internal resistance of a lithium ion secondary battery (hereinafter also referred to as a "secondary battery") using the positive electrode active material for a lithium ion secondary battery (hereinafter also referred to as a "positive electrode active material") according to one embodiment of the present invention can be suppressed.

In this case, in one embodiment of the present invention, the carbon content of the positive electrode active material for the lithium ion secondary battery may be 0.05% by mass or more and 0.40% by mass or less.

In this way, the generation of carbon dioxide, which is caused by, for example, the decomposition of the organic components contained in the coating layer, can be sufficiently suppressed during charging and discharging of the secondary battery, thereby preventing the battery from expanding, etc.

In this case, in one embodiment of the present invention, the amount of moisture relative to the entire positive electrode active material for a lithium ion secondary battery, as measured by the Karl Fischer method when the positive electrode active material is heated to 300°C, may be 0.05% by mass or more and 0.10% by mass or less.

This can suppress the hydrolysis reaction of the electrolyte in the electrolyte solution and inhibit the deterioration of the positive electrode active material caused by acidic components such as hydrogen fluoride.

In this case, in one embodiment of the present invention, the specific surface area of the lithium transition metal composite oxide particles may be 0.1 m ² /g or more and 1.0 ^{m 2} /g or less.

In this way, the desorption/insertion of lithium ions can be sufficiently promoted during charging and discharging of the secondary battery, the internal resistance can be sufficiently suppressed, and excessive decomposition of the electrolyte can be suppressed.

In this case, in one embodiment of the present invention, the volume average particle size of the lithium transition metal composite oxide particles may be 2 μm or more and 20 μm or less.

In this way, it is possible to sufficiently increase the battery capacity per volume of the lithium-ion secondary battery, and to obtain excellent battery characteristics such as high safety and high output.

Another aspect of the present invention is a lithium ion secondary battery comprising a positive electrode using the above-mentioned positive electrode active material for lithium ion secondary batteries, a negative electrode, a separator, and a non-aqueous electrolyte.

In this way, it is possible to provide a lithium ion secondary battery that has excellent cycle characteristics and generates little gas during charging and discharging. It is also possible to provide a lithium ion secondary battery that has a high initial discharge capacity and low internal resistance.

In addition, one aspect of the present invention is a method for producing a positive electrode active material for a lithium ion secondary battery having particles of a lithium transition metal composite oxide and a coating layer that coats at least a part of the surface of the particles, the method comprising: a precursor crystallization step of preparing a transition metal composite hydroxide, which is a precursor of the lithium transition metal composite oxide, by a crystallization reaction; an oxidation roasting step of oxidizing and roasting the transition metal composite hydroxide obtained in the precursor crystallization step to obtain a transition metal composite oxide; a lithium transition metal composite oxide synthesis step of mixing the transition metal composite oxide obtained in the oxidation roasting step with a lithium compound and baking the mixture to obtain a lithium transition metal composite oxide; and a coating step of forming the coating layer containing a lithium tungsten compound on at least a part of the surface of the particles of the lithium transition metal composite oxide obtained in the lithium transition metal composite oxide synthesis step, the coating step being characterized in that the particles of the lithium transition metal composite oxide obtained in the lithium transition metal composite oxide synthesis step are mixed with a coating agent containing a lithium compound and a tungsten compound, dried, and heat-treated.

In this way, it is possible to provide a positive electrode active material for a lithium ion secondary battery that has excellent cycle characteristics and generates little gas during charging and discharging. It is also possible to provide a positive electrode active material for a lithium ion secondary battery that has a high initial discharge capacity and a low internal resistance.

In this case, in one embodiment of the present invention, the lithium transition metal composite oxide synthesis process may be performed by firing in an oxygen atmosphere at a temperature of 700°C or higher and 800°C or lower.

In this way, the crystal structure of the lithium transition metal composite oxide can be sufficiently grown, and cation mixing can be suppressed, preventing deterioration of the battery characteristics of the secondary battery.

In one embodiment of the present invention, the coating step is carried out such that the amount of W in the lithium tungsten compound is equal to or larger than 1 m of the surface area of the lithium transition metal composite oxide. The tungsten compound may be added so that the amount of tungsten compound is 0.04 mmol or more and 0.12 mmol or less per ² , and components other than oxygen of the lithium transition metal composite oxide obtained in the coating step may be expressed as Li:Ni:Co:M=t:1-x-y:x:y (wherein M is at least one element selected from the group consisting of Mg, Al, Ca, Si, Mn, Ti, V, Fe, Cu, Cr, Zn, Zr, Nb, Mo, and W, and 0.95≦t≦1.20, 0<x≦0.22, 0≦y≦0.15), and a ratio Ws/(Nis+Cos+Ws) of an amount of substance Ws of W present on the surface of the positive electrode active material for a lithium ion secondary battery to a sum of the amounts of substances Nis+Cos+Ws of Ni, Co, and W may be 0.15 or more and 0.50 or less.

In this way, the coating layer can be uniformly arranged over the entire surface of the lithium transition metal composite oxide particles. In the secondary battery, the deterioration of the positive electrode active material and the decomposition of the electrolyte can be suppressed, and the internal resistance can be reduced. In addition, the lithium tungsten compound that forms the coating layer can be prevented from becoming a resistive layer.

In this case, in one embodiment of the present invention, the coating process may include adding the lithium compound and the tungsten compound so that the mass ratio of Li to W (Li/W) in the lithium tungsten compound of the coating layer is 1.8 or more and 2.2 or less.

In this way, it is possible to provide a positive electrode active material for lithium ion secondary batteries that has better cycle characteristics, produces less gas during charging and discharging, has a high initial discharge capacity, and has low internal resistance.

In this case, in one embodiment of the present invention, the heat treatment may be performed in a decarbonated air, inert gas or vacuum atmosphere at a temperature of 100°C or higher and 400°C or lower for 3 hours or higher and 15 hours or lower.

In this way, the coating layer can contain a lithium tungsten compound and the shape of the coating layer can be maintained. This makes it possible to provide a positive electrode active material for lithium ion secondary batteries that has better cycle characteristics, less gas generation during charging and discharging, a high initial discharge capacity, and low internal resistance. In addition, it is possible to prevent the reaction of the lithium on the surface of the lithium transition metal composite oxide particles with moisture and carbonic acid in the atmosphere.

In this case, in one aspect of the present invention, the coating agent may contain the lithium compound and the tungsten compound used in the coating process dissolved in a solvent, or a mixture of the lithium compound and the tungsten compound that is liquid at room temperature or has a low melting point that melts during the heat treatment in the coating process.

In this way, the coating layer can be more uniformly distributed over the entire surface of the lithium transition metal composite oxide particles.

In this case, in one embodiment of the present invention, the tungsten compound used in the coating process may be one or more of tungsten oxide, tungstic acid, ammonium paratungstate, and ammonium metatungstate.

This will help prevent impurities from getting into the coating layer.

According to the present invention, it is possible to provide a positive electrode active material for a lithium ion secondary battery that has excellent cycle characteristics and generates little gas during charging and discharging. It is also possible to provide a positive electrode active material for a lithium ion secondary battery that has a high initial discharge capacity and a small internal resistance.

FIG. 1 is a process diagram showing an outline of a method for producing a positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention. FIG. 2 is a schematic cross-sectional view of a coin-type secondary battery used for evaluating the battery characteristics. FIG. 3 shows a laminated battery used in the evaluation of battery characteristics, where (A) is a schematic diagram of the laminated battery, and (B) is a front view of the laminated battery. 1 shows an example of impedance evaluation measurement, and an equivalent circuit diagram and a Nyquist plot used in the analysis.

As a result of intensive research conducted by the inventors to solve the above problems, the inventors discovered that by coating at least a portion of a lithium transition metal composite oxide with a coating layer containing a lithium tungsten compound and controlling the ratio of tungsten to the transition metal element and the amount of tungsten per surface area, it is possible to improve cycle characteristics while suppressing gas generation during charging and discharging, and thus completed the present invention. The following describes preferred embodiments of the present invention.

The following describes the embodiments for carrying out the present invention. However, the present invention is not limited to the following embodiments, and various modifications and substitutions can be made to the following embodiments without departing from the scope of the present invention. Furthermore, not all of the configurations described in the present embodiments are necessarily essential as a means for solving the problem of the present invention.

A positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention, a method for producing the same, and a lithium ion secondary battery will be described in the following order.
1. Positive electrode active material for lithium ion secondary batteries 1-1. Particles of lithium transition metal composite oxide 1-2. Coating layer 1-3. Characteristics of positive electrode active material 2. Manufacturing method of positive electrode active material for lithium ion secondary batteries 2-1. Precursor crystallization process 2-2. Oxidation roasting process 2-3. Lithium transition metal composite oxide synthesis process 2-4. Coating process 3. Lithium ion secondary battery 3-1. Positive electrode 3-2. Negative electrode 3-3. Separator 3-4. Non-aqueous electrolyte 3-5. Shape and configuration of secondary battery 3-6. Characteristics of secondary battery

<1. Positive electrode active material for lithium ion secondary batteries>
First, a configuration example of the positive electrode active material for a lithium ion secondary battery according to this embodiment will be described.

The positive electrode active material of this embodiment has a lithium transition metal composite oxide particle and a coating layer that coats at least a part of the particle surface. The lithium transition metal composite oxide particle of the positive electrode active material for lithium ion secondary batteries of this embodiment can contain lithium (Li), nickel (Ni), cobalt (Co), and element M (M) in a substance amount ratio of Li:Ni:Co:M = t:1-x-y:x:y. However, it is preferable to satisfy 0.95≦t≦1.20, 0<x≦0.22, and 0≦y≦0.15. Furthermore, the element M can be at least one element selected from magnesium (Mg), aluminum (Al), calcium (Ca), silicon (Si), manganese (Mn), titanium (Ti), vanadium (V), iron (Fe), copper (Cu), chromium (Cr), zinc (Zn), zirconium (Zr), niobium (Nb), molybdenum (Mo), and tungsten (W).

The coating layer contains a lithium tungsten compound and can contain tungsten in a ratio of 0.04 mmol to 0.12 mmol per m ² of the surface area of the lithium transition metal composite oxide before coating, i.e., excluding the coating layer.

The positive electrode active material for lithium ion secondary batteries of this embodiment can have a ratio of the amount of W present on the surface to the sum of the amounts of W, Ni, and Co (Ws/(Nis+Cos+Ws)) of 0.15 or more and 0.50 or less.

Hereinafter, the positive electrode active material for lithium ion secondary batteries of this embodiment (hereinafter, also simply referred to as "positive electrode active material") will be specifically described.

The positive electrode active material of this embodiment can have lithium transition metal composite oxide particles and a coating layer that coats at least a portion of the lithium transition metal composite oxide particles. The lithium transition metal composite oxide particles and the coating layer are described below.

<1-1. Particles of lithium transition metal composite oxide>
The lithium transition metal composite oxide particles can contain lithium (Li), nickel (Ni), cobalt (Co), and element M (M) in a substance amount ratio of Li:Ni:Co:M=t:1-xy:x:y.

In the formula expressing the mass ratio of each element in the lithium transition metal composite oxide described above, the value of t indicating the content of lithium (Li) can be 0.95 or more and 1.20 or less, preferably 0.98 or more and 1.10 or less, and more preferably 1.00 or more and 1.10 or less.

By setting the value of t to 0.95 or more, the internal resistance of a secondary battery using a positive electrode active material containing the lithium transition metal complex oxide can be suppressed, and the output characteristics can be improved. Furthermore, by setting the value of t to 1.20 or less, the initial discharge capacity of a secondary battery using a positive electrode active material containing the lithium transition metal complex oxide can be maintained high. In other words, by setting the value of t to the above range, the output characteristics and capacity characteristics of a secondary battery using a positive electrode active material containing the lithium transition metal complex oxide can be improved.

The nickel (Ni) in the above-mentioned lithium transition metal composite oxide is an element that contributes to increasing the capacity of secondary batteries that use positive electrode active materials containing lithium transition metal composite oxides.

The cobalt (Co) in the lithium transition metal composite oxide described above is an element that contributes to reducing the irreversible capacity of a secondary battery using a positive electrode active material containing the lithium transition metal composite oxide. The value of x, which indicates the cobalt content, can be greater than 0 and less than or equal to 0.22, preferably greater than or equal to 0.10 and less than or equal to 0.22, and more preferably greater than or equal to 0.10 and less than or equal to 0.20.

By making the value of x greater than 0, it is possible to reduce the irreversible capacity, which is the difference between the charge capacity and the discharge capacity, in a secondary battery using a positive electrode active material containing the lithium transition metal composite oxide. In addition, by making the value of x 0.22 or less, it is possible to obtain a high battery capacity.

The lithium transition metal composite oxide may contain, in addition to the above metal elements, an additive element, element M. As the above-mentioned element M, at least one element selected from magnesium (Mg), aluminum (Al), calcium (Ca), silicon (Si), manganese (Mn), titanium (Ti), vanadium (V), iron (Fe), copper (Cu), chromium (Cr), zinc (Zn), zirconium (Zr), niobium (Nb), molybdenum (Mo), and tungsten (W) can be used. Element M is appropriately selected depending on the application and required performance of the secondary battery constructed using the positive electrode active material.

Since the element M itself does not contribute to the redox reaction, the value of y indicating the content of element M can be 0.15 or less, preferably 0.10 or less, and more preferably 0.05 or less. Since the lithium transition metal composite oxide does not need to contain element M, the lower limit of y indicating the content of element M can be 0.

The lithium transition metal composite oxide particles are preferably such that a peak attributable to the layered rock salt crystal structure of the "R-3m" structure is detected from the diffraction pattern obtained by X-ray diffraction (XRD) measurement. In particular, it is more preferable that only peaks attributable to the layered rock salt crystal structure of the "R-3m" structure are detected from the diffraction pattern. This is because the layered rock salt oxide of the "R-3m" structure is preferable because it can suppress the internal resistance in particular when a positive electrode active material containing the lithium transition metal composite oxide particles is used as a secondary battery.

However, lithium transition metal composite oxides having a layered rock-salt crystal structure cannot be obtained in a single phase, and impurities may be mixed in. Even in such cases where impurities are mixed in, it is preferable that the intensity of the peaks of phases other than the layered rock-salt structure of the "R-3m" structure does not exceed the peak intensity attributable to the layered rock-salt structure of the "R-3m" structure.

When observed with an electron microscope such as SEM or TEM, the particles of the lithium transition metal composite oxide are preferably secondary particles having a particle size of 3.0 μm to 15.0 μm formed by the aggregation of a large number of primary particles having a particle size of 0.1 μm to 2.0 μm, single primary particles having a particle size of 1.0 μm to 7.0 μm, or a mixture thereof. Each particle may have a space or void surrounded by one or more primary particles inside.

The lithium transition metal composite oxide particles preferably have a specific surface area of 0.1 m ² /g or more and 1.0 m ² /g or less, and more preferably 0.1 m ² /g or more and 0.8 m ² /g or less. The desorption/insertion of lithium ions occurring in the lithium transition metal composite oxide particles during charging/discharging of the secondary battery occurs through the interface between the lithium transition metal composite oxide particles and the electrolyte, that is, the surface of the lithium transition metal composite oxide particles. And, by making the specific surface area of the lithium transition metal composite oxide particles contained in the positive electrode active material 0.1 m ² /g or more, the desorption/insertion of the lithium ions can be sufficiently promoted, and the internal resistance can be sufficiently suppressed, which is preferable. In addition, as described above, the electrolyte may be decomposed due to a side reaction occurring at the interface between the positive electrode active material particles and the electrolyte, but by making the specific surface area of the lithium transition metal composite oxide particles 1.0 m ² /g or less, excessive decomposition of the electrolyte can be suppressed. Regarding the initial discharge capacity of a secondary battery, a larger BET specific surface area is preferable because it increases the number of reaction sites.

The specific surface area of the lithium transition metal composite oxide particles of this embodiment can be measured, for example, by the BET method using nitrogen adsorption.

The volume average particle diameter of the lithium transition metal composite oxide particles of this embodiment, when measured with a laser diffraction scattering type particle size distribution analyzer, is preferably 2 μm or more and 20 μm or less, more preferably 2 μm or more and 15 μm or less, and even more preferably 3 μm or more and 15 μm or less. This is because, when the volume average particle diameter of the lithium transition metal composite oxide particles is 2 μm or more and 20 μm or less, a secondary battery using a positive electrode active material containing the lithium transition metal composite oxide particles in the positive electrode can have a sufficiently large battery capacity per volume, and excellent battery characteristics such as high safety and high output can be obtained.

<1-2. Covering layer>
The coating layer contains a compound containing tungsten, i.e., a tungsten compound, and coats at least a part of the particles of the lithium transition metal composite oxide. The coating layer may be composed of a lithium tungsten compound. By disposing the coating layer, it is possible to suppress deterioration of the positive electrode active material and decomposition of the electrolyte in a secondary battery having a positive electrode containing the positive electrode active material of this embodiment.

There is no need for a clear boundary between the coating layer and the lithium transition metal composite oxide particles. Therefore, the coating layer refers to a portion or region on the surface side of the positive electrode active material of this embodiment that has a higher tungsten concentration than the lithium transition metal composite oxide particles, i.e., the central region, which are the material to be coated. The coating layer may be partially dissolved in the lithium transition metal composite oxide.

The content of tungsten in the coating layer is not particularly limited, but it is preferable to adjust the content according to the specific surface area of the lithium transition metal composite oxide particles to be coated.Specifically, for example, the coating layer preferably contains tungsten in a ratio of 0.04 mmol to 0.12 mmol per 1 ^m2 of the surface area (hereinafter also simply referred to as "surface area") of the lithium transition metal composite oxide particles before coating, i.e., excluding the coating layer, and more preferably contains tungsten in a ratio of 0.06 mmol to 0.10 mmol per 1 m2.

This is because, by setting the tungsten content per ^m2 of the surface area (specific surface area) of the lithium transition metal composite oxide particle to 0.04 mmol or more, the coating layer can be uniformly disposed over the entire surface of the lithium transition metal composite oxide particle, and the effect of the coating layer in suppressing deterioration of the surface of the lithium transition metal composite oxide particle appears during evaluation of the cycle characteristics.

In addition, by providing a coating layer, the effects of suppressing the reduction of internal resistance, suppressing the decrease in discharge capacity, improving cycle characteristics, and suppressing battery swelling can be obtained, but depending on the tungsten content, there is a risk of increasing internal resistance, decreasing discharge capacity, decreasing cycle characteristics, and increasing battery swelling. And, by making the tungsten content per 1 ^m2 of the surface area (specific surface area) of the lithium transition metal composite oxide particles 0.12 mmol or less, it is possible to suppress the coating layer from becoming an obstacle to the intercalation/deintercalation reaction of lithium into the lithium transition metal composite oxide, reduce the internal resistance, and suppress the decrease in discharge capacity, which is preferable. In addition, it is possible to prevent the lithium tungsten compound forming the coating layer from becoming a resistive layer.

The evaluation and calculation method of the tungsten content of the coating layer is not particularly limited, but an example of the method is shown below. First, the tungsten content (mmol/g) in 1 g of the positive electrode active material after the coating treatment with the lithium tungsten compound is measured by a method such as chemical analysis. For example, the measurement can be performed by ICP (Inductively Coupled Plasma) emission spectrometry. In addition, the specific surface area (m ² /g) of the lithium transition metal composite oxide particles before the coating treatment with the lithium tungsten compound is measured by the BET method using nitrogen adsorption. Then, the tungsten content (mmol/g) in 1 g of the positive electrode active material after the coating treatment with the lithium tungsten compound is divided by the specific surface area (m ² /g) of the lithium transition metal composite oxide particles before the coating treatment, so that the tungsten content (mmol/m ² ) per m ² of the surface area of the lithium transition metal composite oxide particles can be calculated.

The tungsten content in 1 g of the positive electrode active material after coating with a lithium tungsten compound and the specific surface area of the lithium transition metal composite oxide particles before coating and without a coating layer are different denominators, so this is not an exact value; however, since the amount of tungsten used for coating is small, it can be used approximately as the amount of tungsten supported per ^m2 of the surface area of the lithium transition metal composite oxide.

When the lithium transition metal composite oxide particles before the coating process contain tungsten, it is preferable to use the difference in the tungsten content before and after the coating process as the amount of tungsten used in the coating.

The extent of the coverage area of the lithium transition metal composite oxide particles in the coating layer can be determined from the ratio of the amount of W present on the surface, known from semi-quantitative analysis using X-ray photoelectron spectroscopy (XPS), to the sum of the amounts of W, Ni, and Co (Ws/(Nis+Cos+Ws)) (hereinafter also referred to as the "ratio of the amount of coated metal surface area").

Due to its characteristics, XPS can selectively obtain information on the surface of the measurement target from 1 nm to 5 nm, so the composition ratio of the surface layer of the material can be known. The ratio of the coated metal surface amount is preferably 0.15 or more. By setting the ratio of the coated metal surface amount to 0.15 or more, it means that the tungsten that exists as a coating layer, that is, that does not diffuse to the side of the lithium transition metal composite oxide particles and exists as a coating layer, is sufficiently secured, and a coating layer that is sufficiently uniform to suppress side reactions between the active material and the electrolyte is formed. And, by suppressing side reactions between the positive electrode active material and the electrolyte, excessive decomposition of the electrolyte can be suppressed. In addition, the coating layer suppresses deterioration due to the reaction between the positive electrode active material surface and the non-aqueous electrolyte during charging and discharging, so that the deterioration of the cycle characteristics can be suppressed. When the ratio of the coated metal surface amount is 0.15 or more, particularly high cycle characteristics can be obtained, so it is more preferable. By setting the ratio of the coated metal surface amount to 0.50 or less, it is possible to prevent the lithium tungsten compound that forms the coating layer from becoming a resistance layer. This can prevent an increase in internal resistance, a decrease in discharge capacity, and a decrease in cycle characteristics, and also suppress battery swelling. Also, if the ratio of the amount of coated metal surface exceeds 0.50, there is a risk that the lithium tungsten compound formed under the excess tungsten will become a resistive layer.

By introducing lithium into the coating layer to form a lithium tungsten compound such as lithium tungstate, the lithium ion conductivity can be improved, the internal resistance can be reduced, and the decrease in discharge capacity can be suppressed.

The substance ratio (Li/W) of the lithium tungsten compound of the coating layer is preferably 1.8 or more and 2.2 or less. By setting the substance ratio of Li to W of the lithium tungsten compound of the coating layer to 1.8 or more, a lithium tungsten compound with high lithium ion conductivity is generated, which is preferable. In addition, by setting the substance ratio of Li to W of the lithium tungsten compound of the coating layer to 2.2 or less, the amount of unreacted lithium compound can be reduced, and the deterioration of battery characteristics such as a decrease in initial discharge capacity and an increase in positive electrode resistance can be suppressed. In addition, by setting the substance ratio (Li/W) of Li to W of the lithium tungsten compound to 1.8 or more and 2.2 or less, the cycle characteristics can be further improved, the swelling of the battery can be further reduced, the initial discharge capacity can be further increased, and the internal resistance can be further reduced. In addition, the lithium tungsten compound preferably contains Li ₂ WO ₄ , and more preferably is Li ₂ WO ₄ .

1-3. Characteristics of positive electrode active material
Up to this point, the lithium transition metal composite oxide particles and the coating layer have been described. It is preferable that the positive electrode active material of this embodiment has the following properties.

The positive electrode active material of this embodiment is preferably composed only of the lithium transition metal composite oxide particles and the coating layer described above, but impurities may be mixed in during production. In particular, moisture and carbon are impurities that may increase due to the coating process. Since moisture and carbon may affect cycle characteristics, it is preferable that they are controlled within a specified range.

The positive electrode active material of this embodiment preferably has a carbon content of 0.05% by mass or more and 0.40% by mass or less, and more preferably 0.05% by mass or more and 0.2% by mass or less. By setting the carbon content to 0.40% by mass or less, the generation of carbon dioxide due to, for example, decomposition of an organic component contained in the coating layer during charging and discharging of a secondary battery using the positive electrode active material can be sufficiently suppressed, and the occurrence of battery expansion, etc. can be prevented. However, since carbon is mixed into the positive electrode active material of this embodiment due to carbon dioxide in the air, etc., it is difficult to set the carbon content to less than 0.01 mass. For this reason, the lower limit of the carbon content is preferably 0.05% by mass. Here, the carbon content of the positive electrode active material is the carbon content relative to the entire positive electrode active material, expressed in mass%. The carbon content of the positive electrode active material of this embodiment can be evaluated, for example, by infrared absorption method, etc.

In addition, the positive electrode active material of this embodiment preferably has a moisture content (moisture content) of 0.05% by mass or more and 0.10% by mass or less. By setting the moisture content to 0.10% by mass or less, the hydrolysis reaction of the electrolyte in the electrolyte solution can be more reliably suppressed in a secondary battery using the positive electrode active material.

When the electrolyte of the electrolytic solution is hydrolyzed, acidic components such as hydrogen fluoride are generated, and the acidic components react with the positive electrode active material, causing degradation. However, as described above, by setting the moisture content of the positive electrode active material of this embodiment to 0.10 mass% or less, the hydrolysis reaction of the electrolyte of the electrolytic solution can be more reliably suppressed, and such degradation can be suppressed, which is preferable. The moisture content of the positive electrode active material of this embodiment can be evaluated, for example, by the Karl Fischer method at a heating temperature of 300°C.

According to one embodiment of the present invention, a positive electrode active material for a lithium ion secondary battery having excellent cycle characteristics and generating little gas during charging and discharging can be provided.

2. Method for producing positive electrode active material for lithium ion secondary battery
Next, a method for producing a positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a process diagram showing an outline of a method for producing a positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention. The method for producing a positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention is a method for producing a positive electrode active material for a lithium ion secondary battery having particles of a lithium transition metal composite oxide and a coating layer that coats at least a part of the surface of the particles.

As shown in FIG. 1, the method for producing a positive electrode active material according to one embodiment of the present invention can include the following four steps.

First step: [Precursor crystallization step S1]
In the precursor crystallization step S1, a transition metal composite hydroxide, which is a precursor of a lithium transition metal composite oxide, is prepared by a crystallization reaction. A mixed aqueous solution is prepared using water-soluble raw materials so that M=1-x-y:x:y, and reacted in a reaction vessel with an alkali metal aqueous solution or the like to obtain a transition metal composite hydroxide. In addition, x and y in the above formula can be set to the same preferred ranges as x and y explained in the lithium transition metal composite oxide particles.

2nd step: [Oxidation roasting step S2]
In the oxidation roasting step S2, a transition metal composite oxide is prepared. Specifically, for example, the transition metal composite hydroxide obtained in the precursor crystallization step S1 is heated to 500° C. or higher and 700° C. in an oxidizing atmosphere. The transition metal composite oxide is obtained by firing at the following temperature.

Third step: [Lithium transition metal composite oxide synthesis step S3]
In the lithium transition metal composite oxide synthesis step S3, the transition metal composite oxide obtained in the oxidation roasting step S2 is mixed with a lithium compound and fired at a temperature of 700° C. or more and 800° C. or less in an oxygen atmosphere to obtain a lithium transition metal composite oxide.

Fourth step: [Coating step S4]
In the coating step S4, a coating layer is formed on the surface of the lithium transition metal composite oxide particles. Specifically, for example, the lithium transition metal composite oxide particles obtained in the lithium transition metal composite oxide synthesis step S3 and A liquid coating agent containing a lithium compound and a tungsten compound is mixed, dried, and then heat-treated at a temperature of 100° C. to 400° C. in a decarbonated air, inert gas, or vacuum atmosphere to form a lithium transition metal composite oxide. A coating layer is provided on the surface of the particles of the object.

Below, a more specific example of a configuration of the method for producing the positive electrode active material of this embodiment will be described. Note that the following description is an example of the production method and is not intended to limit the production method.

<2-1. Precursor crystallization step S1>
In the precursor crystallization step S1, first, a metal compound containing nickel, a metal compound containing cobalt, and optionally an element M (M is Mg, Al, Ca, Si, Mn, Ti, V, Fe, A metal compound containing at least one element selected from the group consisting of Cu, Cr, Zn, Zr, Nb, Mo, and W) is dissolved in water at a predetermined ratio to prepare a mixed aqueous solution (mixed The composition ratio of each metal in the mixed aqueous solution is the same as the composition ratio of the transition metal composite hydroxide to be finally obtained. Therefore, the composition ratio of each metal in the mixed aqueous solution is the same as the composition ratio of the transition metal composite hydroxide to be finally obtained. It is preferable to prepare a mixed aqueous solution by adjusting the ratio of the metal compounds dissolved in water so that the composition ratio of each metal is the same as that of the metal composite hydroxide particles. Although sulfates, chlorides, nitrates, etc. can be used if available, sulfates are preferred from the viewpoint of cost. If no suitable water-soluble raw material can be found for element M, etc., the mixed aqueous solution should contain Alternatively, the lithium-transition metal composite oxide may be added in the oxidation roasting step S2 or the lithium-transition metal composite oxide synthesis step S3 described below.

Next, water is poured into the reaction tank, and an appropriate amount of an alkaline substance and an ammonium ion donor are added to prepare an initial aqueous solution (initial aqueous solution preparation step). At this time, it is preferable to prepare the initial aqueous solution so that the pH value is 11.2 or more and 12.2 or less at a liquid temperature of 25°C, and the ammonia concentration is 2 g/L or more and 15 g/L or less.

When the precursor crystallization step S1 is carried out to prepare the transition metal composite hydroxide, impurities resulting from the anions constituting the metal compounds contained in the mixed aqueous solution used may be mixed into the transition metal composite hydroxide. However, by setting the pH value of the initial aqueous solution to 11.2 or more, it is possible to suppress the mixing of impurities resulting from the anions constituting the metal compounds of the raw materials, which is preferable. In addition, by setting the pH value of the initial aqueous solution to 12.2 or less, it is possible to suppress the resulting transition metal composite hydroxide particles from becoming fine particles, and to obtain an optimal size, which is preferable.

In addition, by setting the ammonia concentration of the initial aqueous solution to 2 g/L or more, it is possible to easily obtain particles of the transition metal complex hydroxide having a spherical shape, which is preferable. Furthermore, by setting the ammonia concentration of the initial aqueous solution to 15 g/L or less, it is possible to prevent the solubility of the transition metal that forms the ammonia complex from increasing excessively, and it is possible to more reliably obtain the composition of the transition metal complex hydroxide obtained as the target composition, which is preferable.

The alkaline substance used in preparing the initial aqueous solution is not particularly limited, but is preferably one or more selected from sodium carbonate, sodium bicarbonate, potassium carbonate, sodium hydroxide, and potassium hydroxide. It is preferable to add it in the form of an aqueous solution, since the amount added can be easily adjusted. The ammonium ion donor is not particularly limited, but is preferably one or more selected from an aqueous ammonium carbonate solution, an aqueous ammonia solution, an aqueous ammonium chloride solution, and an aqueous ammonium sulfate solution.

In the precursor crystallization step S1, the mixed aqueous solution described above can be added dropwise to the initial aqueous solution to form a reaction aqueous solution, but it is preferable to maintain the pH value and ammonia concentration of the reaction aqueous solution within the preferred ranges described above.

It is preferable that the atmosphere in the reaction vessel is a non-oxidizing atmosphere, for example, an atmosphere with an oxygen concentration of 1% by volume or less. This is because a non-oxidizing atmosphere, for example, an atmosphere with an oxygen concentration of 1% by volume or less, can suppress the oxidation of the raw materials, etc., which is preferable. This makes it possible to prevent, for example, oxidized cobalt from precipitating as fine particles.

The temperature of the reaction tank in the precursor crystallization step S1 is preferably maintained between 40°C and 60°C, and more preferably between 45°C and 55°C. In order to maintain the reaction tank in this temperature range, it is preferable that the initial aqueous solution and the reaction aqueous solution placed in the reaction tank are also maintained within the same temperature range.

The temperature of the reaction tank naturally rises due to the heat of reaction and the energy of stirring, so a temperature of 40°C or higher is preferable because it avoids the consumption of extra energy for cooling. In addition, a temperature of 60°C or lower in the reaction tank is preferable because it is possible to suppress the amount of ammonia evaporation from the initial aqueous solution and the reaction aqueous solution, making it easy to maintain the target ammonia concentration.

In the precursor crystallization step S1, the initial aqueous solution is placed in a reaction tank, and after adjusting the temperature, etc., the mixed aqueous solution is dripped into the reaction tank at a constant rate to form a reaction aqueous solution, thereby crystallizing the precursor transition metal composite hydroxide particles (crystallization step).

As mentioned above, it is preferable that the pH value and ammonia concentration of the reaction aqueous solution are in the same suitable range as described for the initial aqueous solution. For this reason, when dripping the mixed aqueous solution into the initial aqueous solution or the reaction aqueous solution, it is preferable to drip the ammonium ion donor and alkaline substance into the initial aqueous solution or the reaction aqueous solution at a constant rate. It is preferable to control the pH value of the reaction aqueous solution to be 11.2 or more and 12.2 or less at a liquid temperature of 25°C, and the ammonia concentration to be maintained at 2 g/L or more and 15 g/L or less.

Then, the slurry containing the transition metal composite hydroxide particles is collected from an overflow port provided in the reaction tank, filtered, and dried to obtain the precursor powdered transition metal composite hydroxide particles.

<2-2. Oxidation roasting step S2>
Next, the oxidizing roasting step S2 will be described. In the oxidizing roasting step S2, the transition metal composite hydroxide obtained in the precursor crystallization step S1 is oxidizing roasted to obtain a transition metal composite oxide. In the oxidizing roasting step S2, the transition metal composite hydroxide produced in the precursor crystallization step S1 is fired in an oxidizing atmosphere and then cooled to room temperature to obtain a transition metal composite oxide.

The roasting conditions in the oxidation roasting step S2 are not particularly limited, but it is preferable to roast in an oxidizing atmosphere, for example, in an air atmosphere, at a temperature of 500°C to 700°C for 1 hour to 12 hours. This is because a roasting temperature of 500°C or higher is preferable because the transition metal composite hydroxide particles can be completely converted to transition metal composite oxide. In addition, a roasting temperature of 700°C or lower is preferable because it is possible to prevent the specific surface area of the transition metal composite oxide from becoming excessively small.

By setting the firing time to 1 hour or more, the temperature inside the firing vessel can be made particularly uniform, and the reaction can proceed uniformly, which is preferable. Furthermore, even if firing is performed for a time longer than 12 hours, no significant changes are observed in the physical properties of the resulting transition metal composite oxide, so from the viewpoint of energy efficiency, it is preferable to set the firing time to 12 hours or less.

It is preferable that the oxygen concentration in the oxidizing atmosphere during firing is equal to or greater than the oxygen concentration in the air atmosphere, i.e., the oxygen concentration is equal to or greater than 20% by volume. Since an oxygen atmosphere can also be used, the upper limit of the oxygen concentration in the oxidizing atmosphere can be set to 100% by volume.

For example, if the compound containing element M cannot be coprecipitated in the precursor crystallization step S1, the compound containing element M may be added to the transition metal composite hydroxide to be subjected to the oxidation roasting step S2 so as to have the same composition ratio as the intended composition, and then calcined. The compound containing element M to be added is not particularly limited, and may be, for example, an oxide, hydroxide, carbonate, or a mixture thereof.

After the oxidation roasting step S2 is completed, if slight sintering is observed in the transition metal composite oxide particles, a crushing process may be added.

<2-3. Lithium transition metal composite oxide synthesis step S3>
In the lithium transition metal composite oxide synthesis step S3, the transition metal composite oxide obtained in the oxidizing roasting step S2 is mixed with a lithium compound and then calcined to obtain a lithium transition metal composite oxide.

In the lithium transition metal composite oxide synthesis step S3, a lithium compound is first added to and mixed with the transition metal composite oxide particles obtained in the oxidation roasting step S2 so that the amount of lithium is 95% or more and 120% or less of the total amount of the component metal elements contained in the particles, thereby obtaining a lithium mixture (lithium mixture preparation step).

The lithium compound to be added is not particularly limited, and for example, lithium hydroxide, lithium nitrate, lithium carbonate, or a mixture thereof can be used. As the lithium compound, it is preferable to use lithium hydroxide, which has a particularly low melting point and high reactivity.

Next, the resulting lithium mixture is calcined in an oxygen atmosphere and then cooled to room temperature to obtain a lithium-transition metal composite oxide containing lithium (calcination step). The calcination conditions are not particularly limited, but it is preferable to calcinate at a temperature of 700°C or higher and 800°C or lower for 1 hour or longer and 24 hours or shorter.

The oxygen atmosphere is preferably an atmosphere containing 80% or more by volume of oxygen. This is because by setting the oxygen concentration in the atmosphere to 80% or more by volume, it is possible to particularly suppress cation mixing, in which Ni atoms are mixed into the Li sites in the resulting lithium transition metal composite oxide, and to prevent deterioration of the battery characteristics of the secondary battery, which is preferable. Since an oxygen atmosphere can also be used, the upper limit of the oxygen concentration of the oxygen atmosphere can be set to 100% by volume.

The firing temperature is preferably 700°C or higher, since it stabilizes the crystal structure of the lithium transition metal composite oxide and allows the lithium transition metal composite oxide particles to grow sufficiently. The firing temperature is preferably 800°C or lower, since it suppresses cation mixing, in which Ni atoms are mixed into the Li sites in the resulting lithium transition metal composite oxide, and prevents deterioration of the battery characteristics of the secondary battery.

A firing time of 1 hour or more is preferable because it makes it possible to make the temperature inside the firing vessel uniform and allows the reaction to proceed uniformly. Furthermore, even if firing is performed for a time longer than 24 hours, no significant changes are observed in the resulting lithium transition metal composite oxide, so from the viewpoint of energy efficiency, it is preferable to set the firing time to 24 hours or less.

If slight sintering is observed in the lithium transition metal composite oxide obtained after the lithium transition metal composite oxide synthesis step S3, a crushing process may be added. Crushing allows the average particle size and particle size distribution of the obtained positive electrode active material to be adjusted to a suitable range. Crushing refers to the operation of applying mechanical energy to an aggregate of multiple secondary particles generated by sintering necking between secondary particles during firing, separating the secondary particles with almost no destruction of the secondary particles themselves, and loosening the aggregates.

<2-4. Coating step S4>
In the coating step S4, a coating layer containing a lithium tungsten compound is formed on at least a part of the surface of the particles of the lithium transition metal composite oxide obtained in the lithium transition metal composite oxide synthesis step S3.

In the coating step S4, first, the specific surface area of the lithium transition metal composite oxide obtained in the lithium transition metal composite oxide synthesis step S3 is measured, and a liquid coating agent can be prepared according to the target amount of tungsten carried in the coating layer (coating agent preparation step).

In the coating agent preparation step, a coating agent containing a tungsten compound and a lithium compound is prepared. By using a coating agent containing a tungsten compound and a lithium compound, a coating layer containing a lithium tungsten compound can be generated in the heat treatment step described below. The surface of the lithium transition metal composite oxide particles can be uniformly coated with the coating layer. Note that it is also possible to use a tungsten compound as a coating agent, add a lithium compound after the drying step described below, and perform the heat treatment step described below, but there is a possibility that the lithium compound may peel off during the heat treatment step.

The coating agent is not particularly limited as long as it contains tungsten, a tungsten compound, and a lithium compound. For uniform coating, it is preferable to use a coating agent in which a tungsten compound and a lithium compound are dissolved in a solvent, or a low-melting tungsten compound and a lithium compound that are liquid at room temperature or melt by low-temperature heat treatment.

Examples of tungsten compounds include one or more selected from tungsten oxide, tungstic acid, ammonium paratungstate, ammonium metatungstate, and a tungsten oxide sol aqueous solution in which fine particles of tungsten oxide are uniformly dispersed in an aqueous solution. The lithium compound is not particularly limited, and examples thereof include lithium hydroxide, lithium oxide, lithium nitrate, lithium ethoxide, lithium acetate, lithium formate, lithium chloride, lithium sulfate, lithium carbonate, and mixtures thereof. As a coating agent, a solution of tungsten oxide in a lithium hydroxide aqueous solution is particularly preferred, as it can be prepared more easily and the inclusion of impurities can be more effectively suppressed.

In the coating agent preparation step, it is preferable to adjust the amount of tungsten compound and lithium compound added so that the mass ratio (Li/W) of Li to W in the lithium tungsten compound of the coating layer is 1.8 or more and 2.2 or less. By setting the mass ratio of Li to W in the lithium tungsten compound of the coating layer to 1.8 or more, a lithium tungsten compound with high lithium ion conductivity is generated, which is preferable. In addition, by setting the mass ratio of Li to W in the lithium tungsten compound of the coating layer to 2.2 or less, it is possible to reduce unreacted lithium compounds and suppress deterioration of battery characteristics such as a decrease in initial discharge capacity and an increase in positive electrode resistance. In addition, by setting the mass ratio of Li to W in the lithium tungsten compound to 1.8 or more and 2.2 or less, it is possible to further improve cycle characteristics, further reduce battery swelling, further increase initial discharge capacity, and further reduce internal resistance.

In the coating step S4, the lithium transition metal composite oxide particles can then be mixed with the coating agent. A general mixer can be used for mixing (mixture preparation step). After mixing, the mixture is dried (drying step), and then heat treated to fix the lithium tungsten compound as a coating layer (heat treatment step).

In the drying step, drying can be performed at a temperature sufficient to remove the solvent from the coating agent. For example, drying can be performed at a temperature of 80°C or higher and lower than 300°C.

The heat treatment conditions of the heat treatment step are not particularly limited, but it is preferable to perform the heat treatment in decarbonated air, inert gas, or vacuum atmosphere at a temperature of 100°C to 400°C, more preferably 150°C to 350°C, for 3 hours to 15 hours. After the heat treatment, the product is cooled to room temperature, and the positive electrode active material, which is a particle of lithium transition metal composite oxide having a coating layer as the final product, can be obtained. Here, the decarbonated air will be explained. Normal air contains a small amount of carbon dioxide gas (CO ₂ ), but the decarbonated air of the present invention is decarbonated to reduce the content of this carbon dioxide gas to 10 ppm or less, which is lower than that of normal air. Inert gas means an atmosphere of one or more gases selected from rare gases and nitrogen gas.

The heat treatment is preferably performed in a decarbonated air, inert gas, or vacuum atmosphere to avoid reactions between moisture and carbonic acid in the atmosphere and the lithium on the surface of the lithium transition metal composite oxide particles.

The baking temperature during the heat treatment is preferably 100°C or higher, which can particularly prevent impurities contained in the coating agent from remaining in the positive electrode active material, and allows the tungsten compound in the coating layer to react with the lithium compound and exist as a lithium tungsten compound. In addition, a baking temperature of 400°C or lower is preferable, because it can prevent the components of the coating layer from being excessively diffused and maintain the shape of the coating layer. The baking temperature during the heat treatment is preferably selected so that the coating layer can sufficiently maintain its thickness, depending on the target amount of tungsten carried in the coating layer, etc.

It is preferable to set the calcination time for the heat treatment to 3 hours or more, since this can particularly prevent impurities contained in the coating agent from remaining in the positive electrode active material. Furthermore, since no significant changes are observed in the resulting positive electrode active material even if calcination is performed for a time longer than 15 hours, from the viewpoint of energy efficiency, it is preferable to set the calcination time to 15 hours or less.

If slight sintering is observed in the positive electrode active material obtained after the coating step S4, a crushing process may be added.

As described above, the method for producing a positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention can provide a positive electrode active material for a lithium ion secondary battery that has excellent cycle characteristics and generates little gas during charging and discharging.

3. Lithium-ion secondary battery
Next, a configuration example of the lithium ion secondary battery of this embodiment will be described. The lithium ion secondary battery of this embodiment (hereinafter, also simply described as "secondary battery") can have a configuration including a positive electrode using the above-mentioned positive electrode active material, a negative electrode, a separator, and a non-aqueous electrolyte. Each member of the secondary battery of this embodiment will be described below. Note that the embodiment described below is merely an example, and the lithium ion secondary battery of the present invention can be implemented in a form in which various changes and improvements are made based on the embodiment described in this specification and on the knowledge of a person skilled in the art. In addition, the lithium ion secondary battery of the present invention is not particularly limited in its use.

<3-1. Positive electrode>
The positive electrode is a sheet-like member, and can be formed, for example, by applying a positive electrode mixture paste containing the above-mentioned positive electrode active material to the surface of a current collector made of, for example, aluminum foil, and drying it. The positive electrode can also be formed by molding the positive electrode mixture. The positive electrode is appropriately processed according to the battery to be used. For example, cutting to form the electrode into an appropriate size according to the target battery, or pressure compression using a roll press or the like to increase the electrode density can be performed.

The above-mentioned positive electrode mixture paste can be formed by adding a solvent to the positive electrode mixture as necessary and kneading it. The positive electrode mixture can be formed by mixing the above-mentioned positive electrode active material in powder form, a conductive agent, and a binder.

The conductive agent is added to give the electrode appropriate conductivity. There are no particular limitations on the material of the conductive agent, but examples that can be used include graphite such as natural graphite, artificial graphite, and expanded graphite, as well as carbon black materials such as acetylene black and Ketjen Black (registered trademark).

The binder serves to hold the positive electrode active material together. There are no particular limitations on the binder used in the positive electrode mixture, but it is possible to use, for example, one or more selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose-based resin, polyacrylic acid, etc.

Activated carbon can also be added to the positive electrode mixture. By adding activated carbon to the positive electrode mixture, the electric double layer capacity of the positive electrode can be increased.

The solvent dissolves the binder and disperses the positive electrode active material, conductive agent, activated carbon, etc. in the binder. The solvent is not particularly limited, but an organic solvent such as N-methyl-2-pyrrolidone can be used.

The mixing ratio of each material in the positive electrode mixture paste is not particularly limited, and can be the same as that of the positive electrode of a general lithium ion secondary battery. For example, if the solid content of the positive electrode mixture excluding the solvent is 100 parts by mass, the content of the positive electrode active material can be 60 parts by mass or more and 95 parts by mass or less, the content of the conductive agent can be 1 part by mass or more and 20 parts by mass or less, and the content of the binder can be 1 part by mass or more and 20 parts by mass or less.

However, the method for producing the positive electrode is not limited to the above-mentioned example, and other methods may also be used.

<3-2. Negative electrode>
The negative electrode is a sheet-like member formed by applying a negative electrode mixture paste to the surface of a metal foil current collector such as copper and drying it. The negative electrode can also be a sheet-like member made of a material containing lithium, such as metallic lithium.

Although the components and their composition that make up the negative electrode mixture paste, and the material of the current collector, etc., are different, the negative electrode is formed in substantially the same manner as the positive electrode described above, and various treatments are performed as necessary in the same manner as the positive electrode.

The negative electrode mixture paste can be prepared by adding a suitable solvent to a negative electrode mixture made by mixing a negative electrode active material and a binder to form a paste. For example, the negative electrode active material can be a material containing lithium, such as metallic lithium or a lithium alloy, or an occlusion material that can absorb and release lithium ions.

The storage material is not particularly limited, but may be one or more selected from, for example, natural graphite, artificial graphite, baked organic compounds such as phenolic resin, and powdered carbon materials such as coke. When such a storage material is used as the negative electrode active material, a fluorine-containing resin such as PVDF may be used as the binder, as in the case of the positive electrode, and an organic solvent such as N-methyl-2-pyrrolidone may be used as the solvent for dispersing the negative electrode active material in the binder.

<3-3. Separator>
The separator is sandwiched between the positive electrode and the negative electrode, and has the function of separating the positive electrode and the negative electrode and retaining the electrolyte. The material of the separator may be, for example, a thin membrane such as polyethylene or polypropylene having a large number of fine pores, but is not particularly limited as long as it has the above function.

<3-4. Non-aqueous electrolyte>
As the non-aqueous electrolyte, a non-aqueous electrolyte solution can be used. For example, the non-aqueous electrolyte solution may be one in which a lithium salt as a supporting salt is dissolved in an organic solvent. Alternatively, an ionic liquid in which a lithium salt is dissolved may be used. Note that the ionic liquid refers to a salt that is composed of cations and anions other than lithium ions and that is in a liquid state even at room temperature.

As the organic solvent, one selected from 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 may be used alone or in combination of two or more.

Examples of the supporting salt that can be used include _LiPF6 , _LiBF4 , _LiClO4 , _LiAsF6 , LiN( _CF3SO2 ) ₂ , and composite salts thereof. The non _- aqueous electrolyte may contain a radical scavenger, a surfactant, a flame retardant, etc., in order to improve the battery characteristics.

A solid electrolyte may also be used as the non-aqueous electrolyte. A solid electrolyte has the property of being able to withstand high voltages. Examples of solid electrolytes include inorganic solid electrolytes and organic solid electrolytes.

As inorganic solid electrolytes, oxide-based solid electrolytes, sulfide-based solid electrolytes, etc. are used.

The oxide-based solid electrolyte is not particularly limited, and any oxide-based solid electrolyte that contains oxygen (O) and has lithium ion conductivity and electronic insulation properties can be used. Examples of oxide-based solid electrolytes include 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, Li6BaLa2Ta2 Examples include O 12 , Li 3.6} Si _0.6 P _0.4 O ₄ and the _like .

The sulfide-based solid electrolyte is not particularly limited, and any electrolyte that contains sulfur (S) and has lithium ion conductivity and electronic insulation can be used. Examples of sulfide-based solid electrolytes 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 .

As the inorganic solid electrolyte, materials other than those mentioned above may be used, for example, Li ₃ N, LiI, Li ₃ N-LiI-LiOH, etc. may be used.

The organic solid electrolyte is not particularly limited as long as it is a polymeric compound that exhibits ion conductivity, and examples of the organic solid electrolyte that can be used include polyethylene oxide, polypropylene oxide, and copolymers thereof. The organic solid electrolyte may also contain a supporting salt (lithium salt).

<3-5. Shape and configuration of secondary battery>
Next, an example of the arrangement and configuration of the components of the secondary battery of this embodiment will be described. The secondary battery of this embodiment, which is composed of the positive electrode, negative electrode, separator, and non-aqueous electrolyte described above, can be made into various shapes, such as a cylindrical shape or a laminated shape. In any shape, the positive electrode and the negative electrode can be laminated via a separator to form an electrode body, and the obtained electrode body can be impregnated with a non-aqueous electrolyte. Then, the positive electrode current collector and the positive electrode terminal leading to the outside, and the negative electrode current collector and the negative electrode terminal leading to the outside can be connected using a current collecting lead or the like, and the battery can be sealed in a battery case to form a secondary battery.

The secondary battery of this embodiment, which uses the positive electrode active material described above, can be a secondary battery with excellent cycle characteristics and little battery swelling.

Specifically, a 2032-type coin battery is formed using the positive electrode active material of this embodiment for the positive electrode, and the battery is charged to a cutoff voltage of 4.3 V at a current density of 0.13 mA/ ^cm2 , and then discharged to a cutoff voltage of 3.0 V after a one-hour pause. The initial discharge capacity is preferably 200 mAh/g or more, and more preferably 202 mAh/g or more.

The positive electrode resistance of a battery can be evaluated by measuring the positive electrode resistance using a coin-type battery. An example of a method for measuring the positive electrode resistance is as follows. When the frequency dependence of the battery reaction is measured using the AC impedance method, which is a common electrochemical evaluation method, a Nyquist diagram based on the solution resistance, the negative electrode resistance and negative electrode capacity, and the positive electrode resistance and positive electrode capacity is obtained. The battery reaction in the electrode consists of a resistance component associated with charge transfer and a capacity component due to the electric double layer, and when these are represented as an electric circuit, they become a parallel circuit of resistance and capacity, and the entire battery is represented by an equivalent circuit in which the solution resistance and the parallel circuits of the negative electrode and positive electrode are connected in series. Each resistance component and capacity component can be estimated by performing a fitting calculation on the Nyquist diagram measured using this equivalent circuit. The positive electrode resistance is equal to the diameter of the semicircle on the low-frequency side of the obtained Nyquist diagram. Therefore, the positive electrode resistance can be estimated by performing an AC impedance measurement on the positive electrode and performing a fitting calculation on the obtained Nyquist diagram using an equivalent circuit.

The positive electrode resistance is shown as a positive electrode resistance ratio relative to the positive electrode resistance in Example 1, which is set to 1.

The cycle characteristics are evaluated by the capacity retention rate and the rate of increase in the positive electrode resistance. The capacity retention rate can be evaluated, for example, by the following procedure. For example, a laminated battery is constructed using a sheet formed by applying a negative electrode mixture paste made by mixing graphite powder and polyvinylidene fluoride to a copper foil as the negative electrode. Then, the laminated battery is conditioned by repeating five cycles of charging to a cutoff voltage of 4.3 V at a current density of 0.33 mA/cm ² in a thermostatic chamber held at 25 ° C., resting for one hour, and discharging to a cutoff voltage of 3.0 V. Next, the cycle of charging to a cutoff voltage of 4.3 V at a current density of 2.7 mA/cm ² in a thermostatic chamber held at 60 ° C., resting for one hour, and discharging to a cutoff voltage of 3.0 V is repeated 500 cycles.

The capacity retention rate can be calculated by dividing the discharge capacity obtained in the 500th cycle by the discharge capacity obtained in the first cycle after conditioning.

Furthermore, the secondary battery of this embodiment using the above-described positive electrode active material preferably has a capacity retention rate of 80% or more, and more preferably 83% or more.

The increase rate of the positive electrode resistance can be calculated by dividing the positive electrode resistance after 500 cycles by the positive electrode resistance after conditioning.

In addition, in the secondary battery of this embodiment using the above-described positive electrode active material, the increase rate of the positive electrode resistance is preferably less than 15 times, and more preferably less than 13 times.

The swelling of the battery can be evaluated by the thickness of the battery after the high voltage retention test of the laminated battery. A laminated battery having the same structure as that of the cycle test is conditioned, charged to 4.3 V at 0.27 mA/ ^cm2 , and continued to be charged until the current value becomes 1/10 while maintaining 4.3 V. After the battery is charged, it is kept at 60°C for 3 days, and then returned to room temperature. The battery is sandwiched between SUS plates so that 10 mm of the short side of the battery protrudes, and the SUS plates are pressurized with 3.9 kN. The thickness of the center of the part of the battery protruding from the SUS plates can be regarded as the degree of the swelling of the battery.

In addition, in the secondary battery of this embodiment using the above-mentioned positive electrode active material, the battery swelling is preferably 0.18 mm or less, and more preferably 0.15 mm or less.

The use of the secondary battery of this embodiment is not particularly limited, and it can be suitably used in applications requiring various power sources. Since the secondary battery of this embodiment has the above characteristics, it can be suitably used as a power source for small portable electronic devices (such as notebook personal computers and mobile phone terminals) that always require high cycle characteristics and high capacity. In addition, since the secondary battery of this embodiment has excellent cycle characteristics and can be miniaturized, it is also suitable as a power source for electric vehicles that are repeatedly charged and discharged and are subject to mounting space restrictions.

Next, the positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention, the manufacturing method thereof, and the lithium ion secondary battery will be described in detail with reference to examples. Note that the present invention is not limited to these examples. Note that the analysis method of the metals contained in the positive electrode active material and the various evaluation methods of the positive electrode active material in the examples and comparative examples are as follows.

[Example 1]
1. Production of Lithium Transition Metal Composite Oxide Particles A positive electrode active material was produced by carrying out the following steps.

(a) Precursor crystallization step First, 2.5 L of pure water was put into a reaction tank (5 L) and the temperature in the tank was set to 40° C. while stirring. At this time, the reaction tank was filled with a nitrogen atmosphere with an oxygen concentration of 1% by volume or less. An initial aqueous solution was prepared by adding an appropriate amount of 25% by weight sodium hydroxide aqueous solution and 25% by weight ammonia water to the water in the reaction tank so that the pH value was 11.5 based on a liquid temperature of 25° C. and the ammonia concentration was 5 g/L (initial aqueous solution preparation step).

At the same time, nickel sulfate, cobalt sulfate, and aluminum sulfate were dissolved in pure water so that the mass ratio of nickel, cobalt, and aluminum was Ni:Co:Al = 0.82:0.15:0.03, to prepare a 2.0 mol/L mixed aqueous solution (mixed aqueous solution preparation step).

This mixed aqueous solution was added dropwise at a constant rate to the initial aqueous solution in the reaction tank to obtain a reaction aqueous solution. At this time, 25% by mass ammonia water and 25% by mass sodium hydroxide aqueous solution were also added dropwise at a constant rate to the initial aqueous solution to control the pH value of the reaction aqueous solution to 11.5 at a liquid temperature of 25°C and the ammonia concentration to 5 g/L. Through this operation, transition metal composite hydroxide particles (hereinafter referred to as "composite hydroxide particles"), which are precursors of lithium transition metal composite oxide, were crystallized (crystallization step).

Then, the slurry containing the composite hydroxide particles was collected from the overflow port installed in the reaction tank, filtered, water-soluble impurities were washed away with ion-exchanged water, and the slurry was dried to obtain powdered composite hydroxide particles.

(b) Oxidation Roasting Step Using an atmospheric firing furnace (BM-50100M, manufactured by Siliconit Corporation), the prepared composite hydroxide particles were fired at 600° C. for 2 hours in an air atmosphere with an oxygen concentration of 20% by volume, and then cooled to room temperature to obtain transition metal composite oxide particles.

(c) Lithium transition metal composite oxide synthesis step Lithium hydroxide monohydrate was weighed out so that the ratio of the substance amount of lithium to the total substance amounts of nickel, cobalt, and aluminum contained in the composite oxide particles was 1.02, and mixed using a Turbula shaker mixer (T2F, manufactured by Dalton Co., Ltd.) to obtain a lithium mixture (lithium mixture preparation step).

The lithium mixture was placed in an alumina sagger and calcined in an atmospheric calcination furnace (BM-50100M, manufactured by Siliconit Co., Ltd.) at 750°C for 10 hours in an oxygen atmosphere with an oxygen concentration of 90% by volume or more, and then cooled to room temperature (calcination step). This resulted in the production of lithium transition metal composite oxide particles.

2. Evaluation of Lithium Transition Metal Composite Oxide Particles The obtained lithium transition metal composite oxide particles were subjected to the following evaluations.

(a) Composition Quantitative analysis using an ICP optical emission spectrometer (725ES, manufactured by VARIAN) confirmed that the lithium transition metal composite oxide had a substance ratio of Li, Ni, Co, and Al expressed as Li:Ni:Co:Al=1.02:0.82:0.15:0.03.

(b) Crystal structure The crystal structure of the particles of this lithium transition metal composite oxide was measured using XRD (X'Pert, PROMR D, manufactured by PANARYTICAL), and it was confirmed that the crystal structure was a layered rock salt type crystal structure, with peaks belonging to the R-3m structure being detected in the diffraction pattern.

(c) Specific Surface Area The BET specific surface area of the lithium transition metal composite oxide particles was measured using a fully automatic BET specific surface area measuring device (Maxsorb, manufactured by Mountec Co., Ltd.) and was found to be 0.67 m ² /g.

(d) Volume average particle diameter The volume average particle diameter of the lithium transition metal composite oxide particles was measured using a laser diffraction scattering type particle size distribution measuring device (Microtrack HRA, manufactured by Nikkiso Co., Ltd.) and was found to be 6.5 μm.

3. Manufacturing of Positive Electrode Active Material The positive electrode active material was manufactured by carrying out the following coating process on the lithium transition metal composite oxide particles. The total surface area of the total weight of the lithium transition metal composite oxide particles, which are the particles to be coated, was calculated from the specific surface area of the lithium transition metal composite oxide particles. Then, tungsten oxide weighed so that the amount of tungsten per 1 ^m2 was 0.08 mmol relative to the total surface area was added to 0.4 mL of pure water per 1 g of tungsten oxide and stirred. Next, lithium hydroxide weighed so that the material ratio (Li/W) of Li and W in the lithium tungsten compound of the coating layer was 2 was added to the tungsten oxide slurry and stirred to dissolve the lithium hydroxide and tungsten oxide. Furthermore, pure water was added to dilute the coating agent so that the molar concentration of W in the coating agent was 0.2 mol/L (coating agent preparation step).

500 g of lithium transition metal composite oxide particles were placed in a rolling fluidized coating device (Powrex Corporation, model: FP-MP-01D), the supply air temperature was set to 120°C, and the coating agent was added at a rate of 2 μL/min per 1 g of lithium transition metal composite oxide particles for 90 minutes, followed by drying while mixing. (Mixture preparation step and drying step).

The particles were heat-treated in a vacuum atmosphere at 190°C for 10 hours using a vacuum dryer (Tokyo Rikakikai Co., Ltd., model: VOS-451SD) (heat treatment step). After that, the particles were cooled to room temperature to obtain lithium transition metal composite oxide particles having a coating layer, which is the positive electrode active material.

4. Evaluation of Positive Electrode Active Material The positive electrode active materials thus obtained were evaluated as follows.

(a) Composition Analysis using an ICP optical emission spectrometer (Varian, 725ES) revealed that this positive electrode active material contained 0.95 mass% W. Comparison with the specific surface area before and after the coating treatment revealed that the amount of tungsten supported in the coating layer was 0.08 mmol/ ^m2 .

(b) Surface analysis The obtained positive electrode active material was measured using XPS (Versa Probe II, manufactured by ULVAC-PHI). The amounts of substances on the surface of the positive electrode active material were obtained from semi-quantitative values calculated from the peak areas of the obtained ^Ni2P3 / ₂ spectrum, ^Co2P3 / ₂ spectrum, and W4f spectrum. Together with the results of the composition analysis described above, it was found that the ratio of the amount of W present on the surface to the sum of the amounts of Ni, Co, and W (Ws/Nis+Cos+Ws) was 0.21.

(c) Carbon Content The carbon content of the obtained positive electrode active material was measured by a high-frequency combustion infrared absorption method using a carbon analyzer (manufactured by LECO, Model: CS-600) and found to be 0.17% by mass.

(d) Moisture Content The moisture content of the obtained positive electrode active material was measured using a Karl Fischer moisture meter (manufactured by Kyoto Electronics Manufacturing Co., Ltd., model: MKC210) at a heating temperature of 300° C. and found to be 0.07% by mass.

5. Preparation of Secondary Battery To evaluate the capacity of the obtained positive electrode active material, a 2032 type coin battery 10 (hereinafter referred to as a "coin battery") shown in Fig. 2 was used. The coin battery 10 is composed of a case 11 and an electrode 12 housed in the case 11.

The case 11 has a hollow positive electrode can 111 with one end open, and a negative electrode can 112 that is placed in the opening of the positive electrode can 111. When the negative electrode can 112 is placed in the opening of the positive electrode can 111, a space is formed between the negative electrode can 112 and the positive electrode can 111 to accommodate the electrode 12.

The electrode 12 is composed of a positive electrode 121, a separator 122, and a negative electrode 123, which are stacked in this order and housed in the case 11 so that the positive electrode 121 contacts the inner surface of the positive electrode can 111 and the negative electrode 123 contacts the inner surface of the negative electrode can 112. The case 11 is provided with a gasket 113, which prevents relative movement between the positive electrode can 111 and the negative electrode can 112 so that they are kept out of contact.

The gasket 113 also has the function of sealing the gap between the positive electrode can 111 and the negative electrode can 112 to provide an airtight and liquid-tight barrier between the inside of the case 11 and the outside. Such a coin-type battery 10 was produced as follows.

First, 20.0 g of the obtained positive electrode active material, 2.35 g of acetylene black, and 1.18 g of polyvinylidene fluoride were dispersed in N-methyl-2-pyrrolidone (NMP) to form a slurry, which was applied to an Al foil so that 5.0 mg of the positive electrode active material was present per ^cm2. The slurry was then dried in the air at 120°C for 30 minutes to remove the NMP. The Al foil to which the positive electrode active material was applied was cut into a strip shape with a width of 66 mm, and roll-pressed with a load of 1.2 t to prepare a positive electrode 121. The positive electrode 121 was punched into a circle with a diameter of 13 mm, and dried in a vacuum dryer at 120°C for 12 hours.

Next, a coin-type battery 10 was fabricated using the positive electrode 121 in a glove box with an Ar atmosphere whose dew point was controlled at -80°C. At this time, a lithium foil punched into a disk shape with a diameter of 14 mm was used as the negative electrode 123.

To evaluate cycle characteristics and battery expansion, a laminated battery 100 shown in Figure 3 was used. Such a laminated battery 100 was fabricated as follows.

The positive electrode was prepared in the same manner as the coin-type battery 10, cut into a rectangle of 50 mm x 30 mm, and dried in a vacuum dryer at 120°C for 12 hours. Next, the laminated battery 100 was prepared using the positive electrode 110 in a dry room with a dew point controlled at -80°C. In this case, the negative electrode 120 was a negative electrode sheet in which a negative electrode mixture paste, a mixture of graphite powder with an average particle size of about 20 μm and polyvinylidene fluoride, was applied to copper foil.

The separator 130 was a polyethylene porous film having a thickness of 25 μm, and the electrolyte was a 3:7 mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with 1 M LiPF ₆ as the supporting electrolyte (manufactured by Toyama Chemical Industry Co., Ltd.). The laminated battery 100 was then sealed with a laminate film 140 to complete the battery.

6. Evaluation of Secondary Battery The performance of the produced secondary battery was evaluated in terms of initial discharge capacity, positive electrode resistance, cycle characteristics (capacity retention rate and rate of increase in positive electrode resistance), and battery swelling, as described below.

(a) Initial Discharge Capacity The initial discharge capacity was evaluated by measuring the discharge capacity (initial discharge capacity) when a coin-type battery using lithium foil for the negative electrode was left for about 24 hours after production, the open circuit voltage OCV (Open Circuit Voltage) was stabilized, the current density to the positive electrode was set to 0.13 mA/ ^cm2 , the battery was charged to a cutoff voltage of 4.3 V, and the battery was allowed to rest for 1 hour and then discharged to a cutoff voltage of 3.0 V. The measurement result was 204 mAh/g.

(b) Positive Electrode Resistance When the positive electrode resistance is measured by an AC impedance method using a frequency response analyzer and a potentiogalvanostat (Solartron, 1255B) while charging the coin battery 10 at a charging potential of 4.1 V, the Nyquist plot shown in Fig. 4 is obtained. This Nyquist plot is expressed as the sum of characteristic curves showing the solution resistance, the negative electrode resistance and its capacity, and the positive electrode resistance and its capacity, so that a fitting calculation was performed using an equivalent circuit based on this Nyquist plot to calculate the value of the positive electrode resistance.

(c) Capacity Retention The capacity retention was evaluated by the following procedure. A laminated battery using a negative electrode made of a sheet in which a mixture of graphite powder and polyvinylidene fluoride was applied to copper foil was first conditioned in a thermostatic chamber held at 25° C. by charging the battery to a cutoff voltage of 4.3 V at a current density of 0.33 mA/cm ² , pausing for 1 hour, and discharging the battery to a cutoff voltage of 3.0 V for 5 cycles. Next, the battery was charged to a cutoff voltage of 4.3 V at a current density of 2.7 mA/cm ² in a thermostatic chamber held at 60° C., pausing for 1 hour, and discharging the battery to a cutoff voltage of 3.0 V for 500 cycles. The capacity retention was evaluated by measuring the discharge capacity of each cycle.

The capacity retention rate, which is the ratio of the discharge capacity obtained in the first cycle after conditioning of the laminated battery of Example 1 divided by the discharge capacity obtained in the 500th cycle after conditioning, was 82%.

(d) Increase rate of positive electrode resistance The increase rate of positive electrode resistance was evaluated by the following procedure. First, after conditioning a laminated battery having the same structure as the cycle test, it was charged at a charging potential of 4.1 V, and measured by an AC impedance method using a frequency response analyzer and a potentiogalvanostat (manufactured by Solartron, 1255B). The obtained Nyquist plot was subjected to fitting calculations using an equivalent circuit to calculate the value of the positive electrode resistance. Next, after discharging to 3.0 V, the battery was charged to a cutoff voltage of 4.3 V at a current density of 2.7 mA/cm ² in a thermostatic chamber maintained at 60 ° C., and after a pause of 1 hour, the battery was discharged to a cutoff voltage of 3.0 V for 500 cycles. After 500 cycles, the battery was charged at a charging potential of 4.1 V, and measured by an AC impedance method using a frequency response analyzer and a potentiogalvanostat (manufactured by Solartron, 1255B). The obtained Nyquist plot was subjected to fitting calculations using an equivalent circuit to calculate the value of the positive electrode resistance.

The increase rate of the positive electrode resistance, which is the ratio of the positive electrode resistance after conditioning of the laminated battery of Example 1 divided by the positive electrode resistance after 500 cycles, was 12 times.

(e) Battery swelling The swelling of the battery was evaluated by measuring the thickness of the battery after the high voltage retention test of the laminated battery. Specifically, after conditioning the laminated battery having the same structure as the cycle test, it was charged at 0.27 mA/ ^cm2 up to 4.3 V, and continued charging until the current value became 1/10 while maintaining it at 4.3 V. The charged battery was kept at 60°C for 3 days. Thereafter, the battery was returned to room temperature, and the battery was sandwiched between SUS plates so that 10 mm of the short side of the battery protruded, and the SUS plates were pressurized with 3.9 kN. The thickness of the center of the part of the battery protruding from the SUS plate was measured with a dial gauge, and was taken as the degree of expansion of the battery. The swelling of the battery after the retention test of the laminated battery of Example 1 was 0.18 mm.

[Example 2]
When carrying out the coating process of "3. Production of positive electrode active material", a lithium transition metal composite oxide particle, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 1, except that in the coating agent preparation step, a target tungsten loading amount in the coating layer was set to 0.04 ^mmol /m2 and a coating agent was prepared.

[Example 3]
When carrying out the coating process of "3. Production of positive electrode active material", a lithium transition metal composite oxide particle, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 1, except that in the coating agent preparation step, a target tungsten loading amount in the coating layer was set to 0.12 ^mmol /m2 and a coating agent was prepared.

[Example 4]
When carrying out the coating process of "3. Production of positive electrode active material", the lithium transition metal composite oxide particles, the positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 1, except that the heat treatment temperature was 100° C. in the heat treatment step.

[Example 5]
When carrying out the coating process of "3. Production of positive electrode active material", the heat treatment temperature was set to 400° C. in the heat treatment step, and other conditions were the same as those in Example 1 to obtain lithium transition metal composite oxide particles, a positive electrode active material, and a secondary battery using the positive electrode active material.

[Comparative Example 1]
A lithium transition metal composite oxide particle, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 1, except that the coating step in Example 1 was not performed. Since the coating step was not performed, the lithium transition metal composite oxide particle obtained in "1. Production of lithium transition metal composite oxide particles" serves as the positive electrode active material.

[Comparative Example 2]
When carrying out the coating process of "3. Production of positive electrode active material", a lithium transition metal composite oxide particle, a positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 1, except that in the coating agent preparation step, a target tungsten loading amount in the coating layer was set to 0.24 ^mmol /m2 and a coating agent was prepared.

[Comparative Example 3]
When carrying out the coating process of "3. Production of positive electrode active material", the heat treatment temperature was set to 500° C. in the heat treatment step, and other conditions were the same as those in Example 1 to obtain lithium transition metal composite oxide particles, a positive electrode active material, and a secondary battery using the positive electrode active material.

[Comparative Example 4]
When carrying out the coating process of "3. Production of positive electrode active material", the lithium transition metal composite oxide particles, the positive electrode active material, and a secondary battery using the positive electrode active material were obtained under the same conditions as in Example 1, except that the heat treatment temperature was 80° C. in the heat treatment step.

Table 1 shows the BET specific surface area and volume average particle size of the lithium transition metal composite oxide particles before the coating process, the amount of tungsten carried in the coating layer, the ratio of the amount of W present on the surface of the positive electrode active material to the sum of the amounts of Ni, Co, and W (Ws/Nis+Cos+Ws), the carbon content of the positive electrode active material, the water content of the positive electrode active material, and the heat treatment temperature conditions in the heat treatment step in the examples and comparative examples.

The amount of tungsten supported in the coating layer was calculated from the tungsten content (mass %) in the positive electrode active material, the atomic weight of tungsten, and the BET specific surface area (0.67 m ² /g) of the above-mentioned lithium transition metal composite oxide particles.

The measurement results of the lithium to tungsten mass ratio (Li/W) in the coating layer, the initial discharge capacity of the secondary battery, the positive electrode resistance ratio, the capacity retention rate, the battery swelling, and the positive electrode resistance increase rate in the examples and comparative examples are shown in Table 2.

From Table 1, Comparative Example 1 is a condition where no coating layer exists on the lithium transition metal composite oxide particles, Comparative Example 2 is a condition where the amount of tungsten supported in the coating layer is increased, and Comparative Examples 3 and 4 are conditions where the temperature range of the heat treatment temperature in the heat treatment step is wider than in the examples.

Here, the mass ratio of lithium to tungsten in the coating layer (Li/W) was calculated from the amount of tungsten and the amount of lithium added in the coating agent preparation step of the coating process.

As is clear from Table 1, the positive electrode active materials of Examples 1 to 5 were produced according to the present invention, and therefore had higher initial discharge capacity, lower positive electrode resistance, better cycle characteristics, and less battery swelling than the comparative examples, resulting in batteries with superior characteristics.

In contrast, in Comparative Example 1, the lithium tungsten compound according to the present invention is not formed on the primary particle surface, so the positive electrode resistance is significantly high, the capacity retention rate is low, the battery swelling is large, and the positive electrode resistance increase rate is large. In Comparative Example 2, the amount of tungsten added is too much, so the lithium tungsten compound formed as a coating layer becomes a resistance layer, the initial characteristics (initial discharge capacity, positive electrode resistance) and cycle characteristics (capacity retention rate and positive electrode resistance increase rate) are reduced, and the battery swelling is also large. In Comparative Example 3, the heat treatment temperature is too high, so the lithium tungsten compound present on the surface of the positive electrode active material is dissolved in the positive electrode active material, and does not play the role of a coating layer, so the initial characteristics and cycle characteristics are reduced, and the battery swelling is also large. In Comparative Example 4, the heat treatment temperature is too low, so the reaction between the lithium compound and the tungsten compound in the coating layer does not proceed sufficiently, and the unreacted tungsten compound becomes a resistance layer, the initial characteristics and cycle characteristics are reduced, and the battery swelling is also large.

Although each embodiment and each example of the present invention has been described in detail above, it will be readily apparent to those skilled in the art that many modifications are possible that do not substantially deviate from the novel features and effects of the present invention. Therefore, all such modifications are intended to be included within the scope of the present invention.

For example, a term described at least once in the specification or drawings together with a different term having a broader or synonymous meaning may be replaced with that different term anywhere in the specification or drawings. Furthermore, the positive electrode active material for lithium ion secondary batteries, the manufacturing method thereof, and the configuration and operation of lithium ion secondary batteries are not limited to those described in the embodiments and examples of the present invention, and various modifications are possible.

S1: precursor crystallization process, S2: oxidation roasting process, S3: lithium transition metal composite oxide synthesis process, S4: coating process, 10: coin-type battery, 11: case, 12: electrode, 111: positive electrode can, 112: negative electrode can, 113: gasket, 121: positive electrode, 122: separator, 123: negative electrode, 100: laminated battery, 110: positive electrode, 120: negative electrode, 130: separator, 140: laminated film

Claims (15)

A positive electrode active material for a lithium ion secondary battery comprising particles of a lithium transition metal composite oxide and a coating layer that coats at least a portion of the surface of the particles,
the ratio of amounts of substances Li:Ni:Co:M of components other than oxygen in the lithium transition metal composite oxide is expressed by t:1-x-y:x:y (wherein M is at least one element selected from the group consisting of Mg, Al, Ca, Si, Mn, Ti, V, Fe, Cu, Cr, Zn, Zr, Nb, Mo, and W, and 0.95≦t≦1.20, 0<x≦0.22, and 0≦y≦0.15);
the coating layer comprises a lithium tungsten compound;
a W amount of the coating layer per ^m2 of the surface area of the lithium transition metal composite oxide excluding the coating layer being 0.04 mmol or more and 0.12 mmol or less; The positive electrode active material for lithium ion secondary batteries according to claim 1, wherein the ratio Ws/(Nis+Cos+Ws) of the amount of W present on the surface of the positive electrode active material for lithium ion secondary batteries to the sum of the amounts of Ni, Co, and W present on the surface of the positive electrode active material for lithium ion secondary batteries, Nis+Cos+Ws, is 0.15 or more and 0.50 or less. The positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, wherein the lithium transition metal composite oxide has a crystal structure belonging to the space group R-3m. The positive electrode active material for lithium ion secondary batteries according to claim 1 or 2, wherein the carbon content of the positive electrode active material for lithium ion secondary batteries is 0.05% by mass or more and 0.40% by mass or less. The positive electrode active material for lithium ion secondary batteries according to claim 1 or 2, wherein the amount of moisture relative to the entire positive electrode active material for lithium ion secondary batteries, as measured by the Karl Fischer method when the positive electrode active material is heated to 300°C, is 0.05% by mass or more and 0.10% by mass or less. 3. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the specific surface area of the particles of the lithium transition metal composite oxide is 0.1 m ² /g or more and 1.0 ^{m 2} /g or less. The positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, wherein the volume average particle diameter of the lithium transition metal composite oxide particles is 2 μm or more and 20 μm or less. A lithium ion secondary battery comprising at least a positive electrode using the positive electrode active material for lithium ion secondary batteries according to claim 1 or 2, a negative electrode, a separator, and a non-aqueous electrolyte. A method for producing a positive electrode active material for a lithium ion secondary battery, the positive electrode active material having a lithium transition metal composite oxide particle and a coating layer that coats at least a part of the surface of the particle, comprising:
a precursor crystallization step of preparing a transition metal composite hydroxide, which is a precursor of the lithium transition metal composite oxide, by a crystallization reaction;
an oxidizing roasting step of oxidizing and roasting the transition metal composite hydroxide obtained in the precursor crystallization step to obtain a transition metal composite oxide;
a lithium transition metal composite oxide synthesis step of mixing the transition metal composite oxide obtained in the oxidation roasting step with a lithium compound and calcining the mixture to obtain a lithium transition metal composite oxide;
a coating step of forming the coating layer containing a lithium tungsten compound on at least a part of the surface of the lithium transition metal composite oxide particles obtained in the lithium transition metal composite oxide synthesis step,
The method for producing a positive electrode active material for a lithium ion secondary battery is characterized in that, in the coating step, the particles of the lithium transition metal composite oxide obtained in the lithium transition metal composite oxide synthesis step are mixed with a coating agent containing a lithium compound and a tungsten compound, followed by drying and heat treatment. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 9, wherein the lithium transition metal composite oxide synthesis process is performed by firing in an oxygen atmosphere at a temperature of 700°C or higher and 800°C or lower. In the coating step, the tungsten compound is added so that the amount of W in the lithium tungsten compound is 0.04 mmol or more and 0.12 mmol or less per ^m2 of the surface area of the lithium transition metal composite oxide,
11. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 9 or 10, wherein components other than oxygen in the lithium transition metal composite oxide obtained in the coating step are represented by Li:Ni:Co:M=t:1-x-y:x:y (wherein M is at least one element selected from the group consisting of Mg, Al, Ca, Si, Mn, Ti, V, Fe, Cu, Cr, Zn, Zr, Nb, Mo, and W, and 0.95≦t≦1.20, 0<x≦0.22, 0≦y≦0.15), and a ratio Ws/(Nis+Cos+Ws) of an amount of substance Ws of W present on the surface of the positive electrode active material for a lithium ion secondary battery to a sum of amounts of substances Nis+Cos+Ws of substances Ni, Co, and W is 0.15 or more and 0.50 or less. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 9 or 10, characterized in that in the coating process, the lithium compound and the tungsten compound are added so that the mass ratio of Li to W (Li/W) in the lithium tungsten compound of the coating layer is 1.8 or more and 2.2 or less. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 9 or 10, wherein the heat treatment is carried out in a decarbonated air, inert gas or vacuum atmosphere at a temperature of 100°C to 400°C for 3 hours to 15 hours. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 9 or 10, characterized in that the coating agent contains the lithium compound and the tungsten compound used in the coating process dissolved in a solvent, or a mixture of the lithium compound and the tungsten compound that is liquid at room temperature or has a low melting point that melts during the heat treatment in the coating process. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 9 or 10, characterized in that the tungsten compound used in the coating step is one or more of tungsten oxide, tungstic acid, ammonium paratungstate, and ammonium metatungstate.