JP7630559B2 - Lithium nickel-containing composite oxide, and positive electrode active material for lithium ion secondary battery using the lithium nickel-containing composite oxide as a base material and method for producing the same - Google Patents

Description

The present invention relates to a lithium nickel-containing composite oxide coated with a compound containing W and Li to be used as a positive electrode active material for lithium ion secondary batteries, more specifically, to a lithium nickel-containing composite oxide that serves as a base material for a positive electrode active material for lithium ion secondary batteries, the base material being a lithium nickel-containing composite oxide coated with a compound containing W and Li, in which fine particles and/or a coating of a compound containing W and Li, such as lithium tungstate, are present on the surfaces of the primary particles.

The present invention also relates to a positive electrode active material for lithium ion secondary batteries that uses the lithium nickel-containing composite oxide as a base material, and a method for producing the same.

In recent years, with the expanding use of small information terminals such as smartphones and tablet PCs, there is a strong demand for the development of small, lightweight secondary batteries with high energy density. There is also a strong demand for the development of large, high-output secondary batteries for use in hybrid and electric vehicles.

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, a separator, and a non-aqueous electrolyte, and the active materials that make up the negative and positive electrodes are materials that can extract and insert lithium during charging and discharging. The non-aqueous electrolyte may be a non-aqueous electrolyte solution made by dissolving a lithium salt, which serves as a supporting salt, in an organic solvent, or a solid electrolyte that is non-flammable and ionically conductive.

Research and development of lithium-ion secondary batteries is still being actively conducted, and lithium-ion secondary batteries that use lithium transition metal-containing composite oxides with layered rock salt or spinel structures as the positive electrode active material can achieve high voltages of around 4 V, and are therefore being put to practical use as secondary batteries with high energy density.

Examples of the lithium transition metal-containing composite oxides that have been proposed so far include 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 ), lithium manganese composite oxide (LiMn ₂ O ₄ ), which uses manganese, and lithium transition metal-containing composite oxides to which additive elements have been added depending on the characteristics required of these.

Of these, lithium nickel composite oxide has a larger charge/discharge capacity per weight than lithium cobalt composite oxide, and the main raw material, nickel, is cheaper than cobalt and has a stable supply. As a result, it is expected to be the next generation of positive electrode active material, and research and development into it is continuing vigorously.

Lithium nickel composite oxide is usually manufactured by mixing and sintering a lithium compound and a nickel compound, and is composed of a powder of monodispersed primary particles or a powder of secondary particles that are aggregates of primary particles. However, both have the disadvantage that their thermal stability in a charged state is inferior to that of lithium cobalt composite oxide.

In response to this, it is common to replace part of the nickel in the lithium-nickel composite oxide with other substances in order to stabilize the crystal structure when lithium is removed during the charging process and to obtain a lithium-nickel composite oxide that has good thermal stability and charge-discharge cycle characteristics as a positive electrode active material.

However, in lithium-nickel-containing composite oxides in which part of the nickel in the lithium-nickel composite oxide is replaced with other substances to reduce the nickel ratio, the thermal stability is high but the battery capacity decreases. On the other hand, if the nickel ratio in the lithium-nickel-containing composite oxide is increased in order to prevent the reduction in battery capacity, the thermal stability is not sufficiently improved. Furthermore, if the nickel ratio in the lithium-nickel-containing composite oxide is high, there is also the problem that cation mixing is likely to occur during firing, making synthesis difficult.

On the other hand, there is a strong demand for higher output (lower resistance) for lithium-ion secondary batteries, especially for battery applications in hybrid and electric vehicles. In response to this demand, a method of adding a tungsten compound to a lithium-nickel-containing composite oxide is being investigated to improve the output characteristics of the positive electrode active material.

For example, Japanese Patent Application Laid-Open No. 2012-079464 proposes a positive electrode active material for non-aqueous _{electrolyte secondary batteries, which comprises primary particles represented by the general formula LizNi1-x-yCoxMyO2 ( where 0.10} ≦x≦0.35, 0≦ _y ≦0.35, 0.97≦z≦1.20, and M is at least one element selected from Mn, V, Mg, Mo, Nb, Ti, and Al) and a base material of secondary particles formed by agglomeration of the primary particles, and which has fine particles of a compound containing W and Li, in particular fine particles made of lithium tungstate, on the surfaces of the primary particles, and which comprises a lithium nickel-containing composite oxide coated with a compound containing W and Li.

By having fine particles and/or a coating of a compound containing W and Li, such as lithium tungstate, present on the surface of the primary particles that make up the lithium-nickel-containing composite oxide base material, these fine particles or coatings prevent contact between the lithium-nickel-containing composite oxide and the electrolyte and function as a protective film that suppresses the formation of deposits such as phosphates, and because these fine particles or coatings have a crystal structure that has Li diffusion paths, it is believed that interface resistance can be reduced.

However, although this positive electrode active material has improved output characteristics, it is insufficient in terms of increasing capacity due to the low nickel ratio, and if the nickel ratio is increased, thermal stability must also be considered. For this reason, research is being conducted on lithium nickel-containing composite oxides coated with compounds containing W and Li, in which fine particles and/or coatings of compounds containing W and Li, such as lithium tungstate, are present on the surfaces of the primary particles, which achieve both high capacity and high output as a positive electrode active material and also have excellent thermal stability.

For example, in JP 2015-216105 A, a lithium mixture obtained by mixing a nickel compound and a lithium compound is fired in an oxidizing atmosphere at a temperature range of 700° C. to 780° C. for 1 hour to 6 hours to obtain a lithium oxide having a general formula: Li _b Ni _1-x-y Co _x M _y O ₂ (wherein M represents at least one element selected from Mg, Al, Ca, Ti, V, Cr, Mn, Nb, Zr, and Mo, and 0.95≦b≦1.03, 0<x≦0.15, 0<y≦0.07, and x+y≦0.16), a lithium-nickel-containing composite oxide composed of primary particles and secondary particles formed by agglomeration of the primary particles is obtained as a base material, a tungsten compound is added during or after a water-washing treatment of the base material, W is dispersed on the surfaces of the primary particles of the base material, and the resulting material is heat-treated at a temperature of 100° C. to 600° C. in an oxygen atmosphere or a vacuum atmosphere, the lithium-nickel-containing composite oxide having fine particles of a compound containing W and Li on the surfaces of the primary particles of the base material, and the lithium-nickel-containing composite oxide as the base material has a lattice constant length in the c-axis direction of 14.183 Å or more, as obtained by Rietveld analysis of X-ray diffraction.

JP 2017-084513 A proposes a positive electrode active material for non-aqueous electrolyte secondary batteries, which is obtained by adding a tungsten compound during or after the water washing treatment of a base material obtained in the same manner as in JP 2015-216105 A to control the moisture content of the base material to 6.5% by mass to 11.5% by mass, dispersing W on the surfaces of the primary particles of the base material, and heat treating the base material at a temperature of 100°C to 600°C in an oxygen atmosphere or a vacuum atmosphere, and which has a coating containing W and Li with a thickness of 1 nm to 200 nm on the surfaces of the primary particles of the base material, and further has a lattice constant length in the c-axis direction of 14.183 Å to 14.205 Å obtained by Rietveld analysis of X-ray diffraction.

These documents also state that it is preferable to perform firing in two stages, first at 400°C to 600°C for 1 to 5 hours, and then at 700°C to 780°C for 3 hours or more, in order to remove water of crystallization and the like in the lithium compound and to react the lithium compound and nickel compound uniformly in a temperature range in which crystal growth of the base material, the lithium-nickel-containing composite oxide, progresses.

JP 2012-079464 A JP 2015-216105 A JP 2017-084513 A

In the technology disclosed in JP 2015-216105 A and JP 2017-084513 A, a W- and Li-containing compound-coated lithium nickel-containing composite oxide is formed in which fine particles and/or a coating of a compound containing W and Li, such as lithium tungstate, are present on the surface of the primary particles. By devising a crystal structure of the lithium nickel-containing composite oxide and by having fine particles and/or a coating of a compound containing W and Li present on the surface of the primary particles of the lithium nickel-containing composite oxide, the insertion and extraction of lithium is improved, reaction resistance is reduced, and high capacity and high output are achieved. In addition, by increasing the crystallinity, high thermal stability is achieved even with a high nickel ratio.

However, for battery applications in hybrid and electric vehicles, there is a demand for even higher levels of capacity and power output for lithium-ion secondary batteries.

In view of these problems, the present invention aims to improve the battery characteristics, particularly the discharge capacity and charge/discharge characteristics, of lithium ion secondary batteries using a lithium nickel-containing composite oxide coated with a compound containing W and Li, the surface of which is provided with fine particles and/or a coating of a compound containing W and Li, such as lithium tungstate.

In order to solve the above problems, the present inventors focused on the lithium nickel-containing composite oxide itself, which is the base material of the lithium nickel-containing composite oxide coated with a compound containing W and Li, in which fine particles and/or a coating of a compound containing W and Li, such as lithium tungstate, are present on the surfaces of the primary particles, and they have conducted extensive research into the properties of the lithium nickel-containing composite oxide suitable for causing fine particles and/or a coating of a compound containing W and Li to be present on the surfaces of the primary particles, as well as the manufacturing conditions for obtaining a lithium nickel-containing composite oxide having such properties.

As a result, it was discovered that by appropriately controlling the temperature retention time within a specific temperature range during the firing step for producing the lithium nickel-containing composite oxide, it is possible to appropriately control the amount of lithium compound that exists on the surface of the primary particles of the obtained lithium nickel-containing composite oxide and corresponds to excess Li. In this way, it was discovered that by controlling the amount of excess Li existing on the surface of the primary particles within a predetermined range, it is possible to obtain a lithium nickel-containing composite oxide in which fine particles and/or coatings of compounds containing W and Li, such as lithium tungstate, which has excellent battery properties, exist on the surface of the primary particles by reacting the lithium compound with the tungsten compound without extracting Li from the crystals.

The present invention was completed based on these findings.

The first aspect of the present invention relates to a lithium-nickel-containing composite oxide, which is a base material for a positive electrode active material for a lithium-ion secondary battery, and is made of a lithium-nickel-containing composite oxide coated with a compound containing W and Li, and which is made of primary particles and/or secondary particles formed by agglomeration of the primary particles, and in which fine particles and/or a coating of a compound containing W and Li are present on the surfaces of the primary particles.

The lithium nickel-containing composite oxide of the present invention is characterized in that the filtrate obtained by adding 1.25 kg of the lithium nickel-containing composite oxide to 1 L of pure water, stirring, and filtering has an electrical conductivity in the range of 42.5 mS/cm to 62.5 mS/cm .

The lithium nickel-containing composite oxide is
General formula: Li _a Ni _1-x-y Co _x M _y O ₂ ... (1)
(In the formula, M is at least one element selected from Mg, Al, Ca, Ti, V, Cr, Mn, Nb, Zr, Mo, and W, and 0.98≦a≦1.11, 0<x≦0.15, 0<y≦0.07, and x+y≦0.16.)
It is preferable that the composition of the composition is:

The length of the lattice constant in the c-axis direction of the crystal of the lithium nickel-containing composite oxide obtained by Rietveld analysis of X-ray diffraction is preferably in the range of 14.183 Å to 14.205 Å.

The volume-based average particle size MV of the lithium nickel-containing composite oxide is preferably in the range of 5 μm to 30 μm.

The second aspect of the present invention relates to a method for producing a positive electrode active material for a lithium ion secondary battery, i.e., a lithium nickel-containing composite oxide coated with a compound containing W and Li, which is made up of primary particles and/or secondary particles formed by agglomeration of the primary particles, and in which fine particles and/or a coating of a compound containing W and Li are present on the surfaces of the primary particles.

The method for producing a positive electrode active material for a lithium ion secondary battery of the present invention comprises the steps of:
In the step of preparing the lithium nickel-containing composite oxide of the first aspect of the present invention , a nickel compound and a lithium compound are mixed to obtain a lithium mixture, and then the lithium mixture is heated from room temperature to a firing temperature of 700° C. or higher in an oxidizing atmosphere, the lithium mixture is passed through and/or held in a temperature range of 600° C. to 650° C. for 20 minutes or more and 45 minutes or less;
Using the lithium nickel-containing composite oxide as a base material , a slurry is formed such that the amount of the lithium nickel-containing composite oxide is 700 g to 2000 g per 1 L of water, and the lithium nickel-containing composite oxide is washed with water;
During or after the water washing treatment, a tungsten compound is added to the lithium nickel-containing composite oxide to disperse W on the surfaces of the primary particles of the lithium nickel-containing composite oxide; and
The lithium nickel-containing composite oxide having W dispersed on the surfaces of the primary particles is heat-treated to form fine particles and/or a coating of a compound containing W and Li on the surfaces of the primary particles of the lithium nickel-containing composite oxide.
It is characterized by:

The positive electrode active material for a lithium ion secondary battery is
General formula: Li _a Ni _1-x-y Co _x M _y W _z O _2+α ...(2)
(In the formula, M is at least one element selected from Mg, Al, Ca, Ti, V, Cr, Mn, Nb, Zr, Mo, and W; 0.95≦a≦1.10, 0<x≦0.15, 0<y≦0.07, x+y≦0.16, 0.001≦z≦0.03, and 0≦α≦0.2.)
It is preferable that the lithium-nickel-containing composite oxide is coated with a compound containing W and Li, and has a composition consisting of the above.

It is preferable that the amount of W dispersed on the surface of the primary particles of the lithium nickel-containing composite oxide is in the range of 0.1 atomic % to 3.0 atomic % relative to the total number of Ni, Co, and M atoms contained in the lithium nickel-containing composite oxide.

The heat treatment is preferably carried out in an oxygen atmosphere or a vacuum atmosphere at a temperature in the range of 100°C to 600°C.

The third aspect of the present invention relates to a positive electrode active material for a lithium ion secondary battery, i.e., a lithium nickel-containing composite oxide coated with a compound containing W and Li, which is composed of primary particles and/or secondary particles formed by agglomeration of the primary particles, and in which fine particles and/or a coating of a compound containing W and Li are present on the surfaces of the primary particles.

The positive electrode active material for lithium ion secondary batteries of the present invention is characterized in that the lithium nickel-containing composite oxide according to the first aspect of the present invention, that is, the lithium nickel-containing composite oxide obtained by adding 1.25 kg of the lithium nickel-containing composite oxide to 1 L of pure water, stirring, and filtering the resulting filtrate has an electrical conductivity in the range of 42.5 mS/cm to 62.5 mS/cm, is used as a base material, and fine particles and/or a coating of a compound containing W and Li are present on the surface of the primary particles.

The positive electrode active material for a lithium ion secondary battery is
General formula: Li _a Ni _1-x-y Co _x M _y W _z O _2+α ...(2)
(In the formula, M is at least one element selected from Mg, Al, Ca, Ti, V, Cr, Mn, Nb, Zr, Mo, and W; 0.95≦a≦1.10, 0<x≦0.15, 0<y≦0.07, x+y≦0.16, 0.001≦z≦0.03, and 0≦α≦0.2.)
It is preferable that the lithium-nickel-containing composite oxide is coated with a compound containing W and Li, and has a composition consisting of the above.

The length of the lattice constant in the c-axis direction in the crystal of the lithium nickel-containing composite oxide coated with the compound containing W and Li, obtained from the Rietveld analysis of X-ray diffraction, is preferably in the range of 14.183 Å to 14.205 Å.

The volume-based average particle size MV of the lithium nickel-containing composite oxide coated with a compound containing W and Li is preferably in the range of 5 μm to 30 μm.

The present invention makes it possible to appropriately regulate the amount of unreacted lithium compounds, i.e., excess lithium compounds, present on the surface of primary particles in a lithium nickel-containing composite oxide, which is a base material for a lithium nickel-containing composite oxide coated with a compound containing W and Li, in which fine particles and/or a coating of a compound containing W and Li, such as lithium tungstate, are present on the surface of the primary particles. This prevents extraction of Li from the crystals when fine particles and/or a coating of a compound containing W and Li are formed on the surface of the primary particles of the lithium nickel-containing composite oxide. This makes it possible to provide a positive electrode active material for lithium ion secondary batteries with excellent battery characteristics without impairing crystallinity.

FIG. 1 is a perspective view and a cross-sectional view showing a 2032 type coin battery used in the battery evaluation. FIG. 2 is a schematic explanatory diagram of an equivalent circuit used for impedance measurement and analysis in battery evaluation.

As described above, the inventors have focused on the lithium nickel-containing composite oxide itself, which is the base material of a positive electrode active material for a lithium ion secondary battery consisting of a lithium nickel-containing composite oxide coated with a compound containing W and Li, in which fine particles and/or a coating of a compound containing W and Li, such as lithium tungstate, are present on the surfaces of the primary particles, and have conducted extensive research into the properties of the lithium nickel-containing composite oxide suitable for causing fine particles and/or a coating of a compound containing W and Li to be present on the surfaces of the primary particles, as well as the production conditions for obtaining a lithium nickel-containing composite oxide having such properties. As a result, it was found that by appropriately controlling the temperature retention time within a specific temperature range in the firing step for producing the lithium nickel-containing composite oxide, it is possible to appropriately control the amount of lithium compound present on the surface of the primary particles of the obtained lithium nickel-containing composite oxide and corresponding to excess Li, thereby preventing the extraction of Li from the crystals when forming fine particles and/or a coating of a compound containing W and Li on the surface of the primary particles of the lithium nickel-containing composite oxide, and it is possible to obtain a positive electrode active material for a lithium ion secondary battery having excellent battery characteristics, and that whether or not the amount of excess Li in the lithium nickel-containing composite oxide used as the base material is appropriate can be easily confirmed by adding 1.25 kg of the lithium nickel-containing composite oxide to 1 L of pure water, stirring, and filtering the resulting filtrate, and checking whether the electrical conductivity of the filtrate is in the range of 42.5 mS/cm to 62.5 mS/cm. The present invention was completed based on such findings.

Each aspect of the present invention will be described in detail below.

1. Lithium nickel-containing composite oxide A first aspect of the present invention relates to a lithium nickel-containing composite oxide, which is a base material of a lithium nickel-containing composite oxide coated with a compound containing W and Li, in which fine particles and/or a coating of a compound containing W and Li, such as lithium tungstate, are present on the surfaces of primary particles.

The lithium nickel-containing composite oxide of the present invention is composed of primary particles and secondary particles formed by aggregation of the primary particles, and is characterized in that the filtrate obtained by adding 1.25 kg of the lithium nickel-containing composite oxide to 1 L of pure water, stirring, and filtering has an electrical conductivity in the range of 42.5 mS/cm to 62.5 mS/cm.

[Conductivity of filtrate]
Usually, in the finally obtained positive electrode active material for lithium ion secondary batteries, the amount of excess Li, i.e., the amount of Li dissolved in water, is reduced, for example, to 0.2 mass% or less with respect to the entire lithium nickel-containing composite oxide. However, as described above, the inventors have focused on the reaction between Li of the lithium nickel-containing composite oxide and W of the tungsten compound when forming fine particles and/or a coating of a compound containing W and Li on the surface of the primary particles of the lithium nickel-containing composite oxide as the base material of the positive electrode active material for lithium ion secondary batteries, and have found that by appropriately controlling the amount of excess Li in the lithium nickel-containing composite oxide as the base material, it is possible to form fine particles and/or a coating of a compound containing W and Li without extracting Li present inside the primary particles of the lithium nickel-containing composite oxide in the reaction between Li of the lithium nickel-containing composite oxide and W of the tungsten compound.

In the present invention, the amount of excess Li in the lithium-nickel-containing composite oxide that is the base material for the positive electrode active material for lithium-ion secondary batteries can be measured as the electrical conductivity of the filtrate obtained by adding 1.25 kg of the lithium-nickel-containing composite oxide to 1 L of pure water, stirring, and filtering.

If the electrical conductivity of the filtrate is less than 42.5 mS/cm, there will be a shortage of excess Li on the surface of the primary particles of the lithium-nickel-containing composite oxide, and Li will be extracted from the crystals during the reaction between Li in the lithium-nickel-containing composite oxide and W in the tungsten compound.

If the conductivity of the filtrate is 42.5 mS/cm or more, the final lithium-ion secondary battery using the positive electrode active material for lithium-ion secondary batteries will achieve sufficient discharge capacity and charge/discharge efficiency.

On the other hand, if the conductivity of the filtrate exceeds 62.5 mS/cm, the amount of excess Li present on the surface of the primary particles of the lithium nickel-containing composite oxide will be excessive, and the Li site occupancy rate will decrease in the final positive electrode active material for lithium ion secondary batteries, resulting in poor crystallinity and poor battery characteristics.

The conductivity of the filtrate is preferably in the range of 45 mS/cm to 60 mS/cm, and more preferably 50 mS/cm to 60 mS/cm.

The electrical conductivity of the filtrate is measured using a conductivity meter by adding 1.25 kg of lithium nickel-containing composite oxide to 1 L of pure water (i.e., adding 125 parts by mass of lithium nickel-containing composite oxide to 100 parts by mass of pure water, stirring, and filtering the filtrate.

A stirrer or other agitator can be used for stirring. Any method can be used for filtration, including natural filtration, reduced pressure filtration, pressure filtration, and centrifugal filtration.

The lithium nickel-containing composite oxide of the present invention is a base material for a positive electrode active material for a lithium ion secondary battery, i.e., a base material for forming fine particles of a compound containing W and Li, or a coating consisting of a compound containing W and Li, on the surface of the primary particles, and the particle properties consisting of primary particles and/or secondary particles formed by agglomeration of primary particles can be obtained by the manufacturing method described in the first aspect of the present invention.

[composition]
The lithium nickel-containing composite oxide of the present invention is
General formula: Li _a Ni _1-x-y Co _x M _y O ₂ ... (1)
(In the formula, M is at least one element selected from Mg, Al, Ca, Ti, V, Cr, Mn, Nb, Zr, Mo, and W, and 0.98≦a≦1.11, 0<x≦0.15, 0<y≦0.07, and x+y≦0.16.)
It is preferable that the composition of the composition is:

The lithium nickel-containing composite oxide of the present invention is a hexagonal layered compound, and in the above general formula, (1-x-y), which indicates the Ni content, is 0.84 or more and less than 1.

In the lithium nickel-containing composite oxide of the present invention, the higher the Ni content, the higher the capacity can be when the final positive electrode active material is used. However, if the Ni content is too high, the thermal stability is not sufficient and cation mixing is likely to occur during firing. On the other hand, if the Ni content is too low, the capacity decreases, and problems such as insufficient capacity per battery volume can occur even if the filling property of the positive electrode is improved.

Therefore, the Ni content of the lithium nickel-containing composite oxide is preferably 0.84 to 0.98, more preferably 0.845 to 0.950, and even more preferably 0.85 to 0.95.

The Co content x is 0<x≦0.15, preferably 0.02≦x≦0.15, and more preferably 0.03≦x≦0.13.

By keeping the Co content within the above range, the final positive electrode active material has excellent cycle characteristics and thermal stability. By increasing the Co content, the cycle characteristics of the positive electrode active material can be improved, but if the Co content exceeds 0.15, it becomes difficult to increase the capacity of the positive electrode active material.

The content of at least one element M selected from Mg, Al, Ca, Ti, V, Cr, Mn, Nb, Zr, Mo, and W, y, is 0<y≦0.07, and preferably 0.01≦y≦0.05. By having the content of M in the above range, excellent cycle characteristics and thermal stability are obtained.

If y exceeds 0.07, it becomes difficult to increase the capacity of the positive electrode active material. If the additive element M is not added, the effect of improving the battery characteristics cannot be obtained, so in order to obtain a sufficient effect of improving the battery characteristics, it is preferable to set y to 0.01 or more.

M preferably contains at least Al, and in the final lithium-nickel-containing composite oxide, y, which indicates the Al content, is preferably 0.03 to 0.07.

The Li content, a, is 0.95≦a≦1.03. If a is less than 0.95, metal elements such as Ni are mixed into the Li layer in the layered compound, reducing the insertion and extraction of Li, resulting in a decrease in the Li site occupancy rate and a decrease in battery capacity, as well as a deterioration in output characteristics. On the other hand, if a exceeds 1.03, Li is mixed into the metal layer in the layered compound, reducing battery capacity.

Therefore, in order to obtain good battery capacity and output characteristics, the Li content in the lithium nickel-containing composite oxide is 0.95≦a≦1.03, and more preferably 0.95≦a≦1.01.

[Length of lattice constant in the c-axis direction]
Here, the Ni content in the lithium nickel-containing composite oxide of the present invention is 0.84 or more, preferably 0.98 or less, and has a very high Ni content. Since a high Ni content causes problems such as a decrease in thermal stability, the Ni content is usually adjusted to be lower than 0.84, generally about 0.80 to 0.83.

However, in the present invention, a high nickel content is made possible by appropriately controlling the length of the lattice constant in the c-axis direction (hereinafter referred to as the "c-axis length") obtained by performing Rietveld analysis of X-ray diffraction on the crystals of the lithium-nickel-containing composite oxide.

In other words, a high nickel ratio is made possible by making the length of the c-axis of the crystal of the lithium nickel-containing composite oxide 14.183 Å or more, preferably 14.185 Å or more.

In the case of the hexagonal lithium-nickel-containing composite oxide of the present invention, the length of the c-axis affects the insertion and extraction of Li from the crystal. In general, as the length of the c-axis increases, the interlayer distance of the lithium layers increases, improving the insertion and extraction of Li from the crystal. By using such a lithium-nickel-containing composite oxide as a base material, a positive electrode active material with high capacity and high output can be obtained.

On the other hand, when the length of the c-axis becomes shorter than 14.183 Å, the insertion and extraction of Li from the crystal decreases, so the capacity and output of the final positive electrode active material decrease. In addition, the crystallinity decreases due to cation mixing, so the cycle characteristics and thermal stability also deteriorate.

The upper limit of the length of the c-axis is not particularly limited, but the upper limit is about 14.205 Å, and in the present invention, it is preferable that the length of the c-axis is in the range of 14.183 Å to 14.205 Å.

[Average particle size]
The volume-based average particle size MV of the particles (primary particles and/or secondary particles) constituting the lithium nickel-containing composite oxide of the present invention is preferably in the range of 5 μm to 30 μm, more preferably in the range of 5 μm to 25 μm, even more preferably in the range of 8 μm to 20 μm, and even more preferably in the range of 8 μm to 17 μm.

If the volume-based average particle size MV is less than 5 μm, the filling property in the positive electrode when used as a positive electrode active material in a battery may decrease, resulting in a decrease in battery capacity per volume. On the other hand, if the volume-based average particle size MV exceeds 30 μm, the contact area between the positive electrode active material and the battery electrolyte may decrease, resulting in a decrease in battery capacity and output characteristics.

Therefore, by controlling the volume-based average particle diameter MV within the above-mentioned range, the battery capacity and output characteristics of the finally obtained positive electrode active material can be maintained while further improving the filling property of the positive electrode. Furthermore, the finally obtained positive electrode active material is composed of primary particles and/or secondary particles, and due to such a particle structure, contact with the electrolyte occurs not only on the outer surfaces of the secondary particles, but also near the surfaces of the secondary particles, in internal voids, and even incomplete grain boundaries. From the viewpoint of sufficient contact with the electrolyte, it is preferable to set the volume-based average particle diameter MV to the above-mentioned range.

The specific surface area of the lithium nickel-containing composite oxide of the present invention, as measured by the BET method, is preferably in the range of 0.20 m ^{2 /} g to 0.50 ^{m 2} /g, and more preferably in the range of 0.25 m ² /g to 0.45 m ² /g.

By having such a specific surface area, the contact with the electrolyte is in an appropriate range, and the battery capacity and output characteristics can be made higher. If the BET specific surface area is less than 0.20 m ² /g, the excess Li present on the surface of the primary particles of the lithium nickel-containing composite oxide is insufficient, so that Li is extracted from the crystals during the reaction between Li of the lithium nickel-containing composite oxide and W of the tungsten compound. On the other hand, if the BET specific surface area exceeds 0.50 m ² /g, the amount of excess Li present on the surface of the primary particles of the lithium nickel-containing composite oxide becomes excessive, that is, the reaction between the nickel compound and the lithium compound does not proceed sufficiently in the synthesis of the lithium nickel-containing composite oxide. Therefore, in the finally obtained positive electrode active material for lithium ion secondary batteries, the Li site occupancy rate decreases, the crystallinity deteriorates, and the battery characteristics deteriorate.

[Method of producing lithium nickel-containing composite oxide]
The lithium nickel-containing composite oxide of the first aspect of the present invention can be obtained by the following production method. Specifically, in the step of mixing a nickel compound and a lithium compound to obtain a lithium mixture, and then heating the lithium mixture in an oxidizing atmosphere from room temperature to a firing temperature of 700°C or higher to prepare a lithium nickel-containing composite oxide composed of primary particles and/or secondary particles formed by aggregation of the primary particles, the lithium mixture is passed through and/or held in a temperature range where the physical temperature is 600°C to 650°C for 20 minutes or more and 45 minutes or less, preferably 20 minutes to 40 minutes.

(Nickel compounds)
The nickel compound used in the firing step is a compound containing Ni and Co, and also containing at least one element selected from Mg, Al, Ca, Ti, V, Cr, Mn, Nb, Zr, Mo and W as an additional element M.

As the nickel compound, for example, any of nickel composite hydroxides, nickel oxycomposite hydroxides obtained by oxidizing the nickel composite hydroxide with an oxidizing agent, and nickel composite oxides obtained by oxidizing and roasting the nickel composite hydroxide and/or the nickel oxycomposite hydroxide at a temperature in the range of 500°C to 750°C can be used.

Nickel composite hydroxides that can be used include, but are not limited to, nickel composite hydroxides obtained by crystallization methods such as coprecipitation and homogeneous precipitation.

In the crystallization method, nickel composite hydroxide can be obtained under various conditions, and the crystallization conditions are not particularly limited, but the following conditions are preferred.

Specifically, a nickel composite hydroxide is preferred, which is obtained by dropping an aqueous solution of a metal compound containing Ni and Co and containing at least one element selected from Mg, Al, Ca, Ti, V, Cr, Mn, Nb, Zr, Mo, and W as an added element M, and an aqueous solution containing an ammonium ion donor into a reaction vessel heated to a temperature in the range of 40°C to 60°C.

During crystallization, it is preferable to add an aqueous solution of an alkali metal hydroxide dropwise as necessary so that the reaction solution is kept alkaline, preferably in the range of pH 10 to 14 at a liquid temperature of 25°C.

It is also possible to obtain a nickel composite hydroxide that contains Ni and Co and also contains the additive element M by coprecipitating the additive element M with Ni and Co, or by obtaining a nickel composite hydroxide by crystallization, and then coating the nickel composite hydroxide with a metal compound containing the additive element M, or by impregnating the nickel composite hydroxide with an aqueous solution containing a metal compound of the additive element M.

The nickel composite hydroxide obtained by such a crystallization method is composed of a powder with a high bulk density. Such a composite hydroxide with a high bulk density is easy to obtain a lithium-nickel-containing composite oxide consisting of primary particles and/or secondary particles formed by agglomeration of primary particles with a small specific surface area after washing with water after the firing process, and is therefore a nickel composite hydroxide suitable as a raw material for the positive electrode active material for lithium-ion secondary batteries.

When nickel hydroxide is crystallized at a temperature of over 60°C or a pH of over 14, nucleation takes precedence in the solution, and crystal growth does not proceed, resulting in only fine particles. On the other hand, when nickel composite hydroxide is crystallized at a temperature of less than 40°C or a pH of less than 10, nucleation is low in the solution, and crystal growth of particles takes precedence, resulting in the nickel composite hydroxide being contaminated with coarse particles. In addition, the amount of metal ions remaining in the reaction solution increases, and composition deviations may occur. If nickel composite hydroxide with such coarse particle contamination or composition deviations is used as a raw material, the battery characteristics of the resulting positive electrode active material are reduced.

Therefore, when using nickel composite hydroxide obtained by a crystallization method as the nickel compound, it is preferable to perform crystallization of the nickel composite hydroxide while maintaining the reaction solution at a temperature in the range of 40°C to 60°C and maintaining the pH value of the reaction solution at 10 to 14 based on a liquid temperature of 25°C.

When using nickel oxycomposite hydroxide as the nickel compound, the method of preparation is not particularly limited. However, when nickel composite hydroxide is prepared by oxidizing it with an oxidizing agent such as sodium hypochlorite or hydrogen peroxide, particles with high bulk density are obtained, so it is preferable to use nickel oxycomposite hydroxide obtained by this method.

When a nickel composite oxide is used as the nickel compound, the method of preparation is not particularly limited. However, it is preferable to obtain the nickel composite hydroxide and/or nickel oxycomposite hydroxide by oxidative roasting at a temperature in the range of 500°C to 750°C in an oxidative atmosphere, and it is more preferable to set the oxidative roasting temperature in the range of 550°C to 700°C.

When the nickel composite oxide obtained in this manner is used, it is possible to stabilize the composition ratio of Li to metals other than Li in the lithium nickel-containing composite oxide when the lithium mixture obtained by mixing it with a lithium compound and calcining it is obtained. This provides the advantage that when a positive electrode active material using the lithium nickel-containing composite oxide as a base material is used in a lithium ion secondary battery, it is possible to achieve high capacity and high output.

When nickel composite hydroxide and/or nickel oxycomposite hydroxide are oxidatively roasted, if the oxidative roasting temperature is less than 500°C, the conversion to oxide may be incomplete. If such a nickel composite oxide incompletely converted to oxide is used, it becomes difficult to stabilize the composition of the resulting lithium-nickel-containing composite oxide, and the composition is likely to become non-uniform during firing. In addition, nickel composite hydroxide and/or nickel oxycomposite hydroxide may remain in the nickel composite oxide after oxidative roasting, which may generate water vapor during firing, inhibiting the reaction between the lithium compound and the nickel composite oxide and reducing the crystallinity.

On the other hand, if the oxidation roasting temperature exceeds 750°C, the crystallinity of the resulting nickel composite oxide will be high, and the reactivity of the lithium compound and the nickel composite oxide in the subsequent firing process will decrease, which may result in a decrease in the crystallinity of the final lithium-nickel-containing composite oxide. Alternatively, the nickel composite oxide may undergo rapid grain growth, forming a nickel composite oxide made of coarse particles, which may result in the average particle size of the final lithium-nickel-containing composite oxide becoming too large.

The holding time at the oxidizing roasting temperature is preferably set in the range of 1 to 10 hours, and more preferably in the range of 2 to 6 hours. If it is less than 1 hour, the conversion to the oxide may be incomplete, and if it exceeds 10 hours, the crystallinity of the nickel composite oxide may become too high.

The atmosphere for the oxidizing roasting may be any oxidizing atmosphere, but considering ease of handling and cost, it is preferable to use an air atmosphere.

The nickel composite hydroxide used as the nickel compound preferably has a sulfate (SO ₄ ) content in the range of 0.1 mass % to 0.4 mass %, more preferably 0.1 mass % to 0.3 mass %, which makes it easier to control the crystallinity of the lithium-nickel-containing composite oxide in the subsequent firing step.

When obtaining nickel composite hydroxide by crystallization and using a sulfate such as nickel sulfate as the raw material, it is possible to regulate the sulfate content to a range of 0.1% to 0.4% by mass by thoroughly washing with an alkaline aqueous solution adjusted to a pH of 11 to 13 at a liquid temperature of 25°.

In addition, by using a nickel compound containing Ni, Co, and at least one additive element selected from Mg, Al, Ca, Ti, V, Cr, Mn, Nb, Zr, Mo and W as the nickel compound, the lithium nickel-containing composite oxide of the first embodiment, that is,
General formula: Li _a Ni _1-x-y Co _x M _y O ₂ ... (1)
(In the formula, M is at least one element selected from Mg, Al, Ca, Ti, V, Cr, Mn, Nb, Zr, Mo, and W, and 0.98≦a≦1.11, 0<x≦0.15, 0<y≦0.07, and x+y≦0.16.)
It is preferable to obtain a lithium nickel-containing composite oxide having a composition consisting of:

(Mixing with lithium compounds)
The lithium compound to be mixed with the nickel compound is not particularly limited, but it is preferable to use at least one selected from the group consisting of hydroxides, oxyhydroxides, oxides, carbonates, nitrates, and halides of lithium. By using these, it is possible to obtain an advantage that no impurities remain after firing. Among these, it is more preferable to use lithium hydroxide, which has good reactivity with the nickel compound.

The mixing ratio of the nickel compound and the lithium compound is not particularly limited, but the composition of Li and metal elements other than Li in the lithium nickel-containing composite oxide after firing is almost maintained as the composition in the mixture obtained by mixing the nickel compound and the lithium compound. Therefore, it is preferable to adjust the amount of Li in the lithium compound to a molar ratio of 0.98 to 1.11 relative to the total amount of Ni, Co, and other additive elements M in the nickel compound. If this molar ratio is less than 0.98, the crystallinity of the resulting fired powder may be very poor. In addition, the lithium content in the resulting fired powder may be less than 0.98. On the other hand, if this molar ratio exceeds 1.11, firing is likely to proceed and over-firing is likely to occur, and the Li content of the resulting fired powder may also exceed 1.11.

The device and method for mixing the nickel compound and the lithium compound are not particularly limited as long as they can mix the two uniformly. For example, a dry mixer such as a V blender or a mixing granulator can be used.

(Firing)
The lithium mixture obtained by mixing the nickel compound and the lithium compound is fired at a firing temperature of 700° C. or higher in an oxidizing atmosphere.

When fired at a temperature above 500°C, a lithium-nickel-containing composite oxide is produced, but below 700°C, the crystals are underdeveloped and structurally unstable. If such a lithium-nickel-containing composite oxide is used as the base material for the positive electrode active material, the crystal structure of the positive electrode active material may be destroyed due to phase transitions caused by charging and discharging, or the primary particles may not grow sufficiently.

In the present invention, the firing temperature is preferably set to 780°C or less. If the firing temperature exceeds 780°C, cation mixing is likely to occur, the layered structure in the crystal of the lithium nickel-containing composite oxide is destroyed, and the insertion and desorption of lithium ions may become difficult. In addition, as described below, it is preferable that the length of the lattice constant in the c-axis direction of the crystal of the lithium nickel-containing composite oxide obtained from the Rietveld analysis of X-ray diffraction is in the range of 14.183 Å to 14.205 Å. However, if the firing temperature exceeds 780°C, the length of the lattice constant in the c-axis direction will not be 14.183 Å or more. Furthermore, the crystal of the lithium nickel-containing composite oxide may decompose, and nickel oxide or the like may be generated. In addition, the particles constituting the lithium nickel-containing composite oxide may sinter together to form coarse particles, and the average particle size of the lithium nickel-containing composite oxide may become too large.

From the above perspective, it is preferable to set the firing temperature in the range of 730°C to 760°C.

The holding time at such a firing temperature is preferably set in the range of 1 to 6 hours, and more preferably in the range of 2 to 4 hours. If the holding time is less than 1 hour, crystallization may be insufficient, and if it exceeds 6 hours, firing may proceed too far, resulting in cationic mixing.

In addition, in order to remove water of crystallization and the like in the lithium compound and to react the lithium compound and nickel compound uniformly in the temperature range in which the crystal growth of the lithium-nickel-containing composite oxide progresses, it is preferable to perform a two-stage firing process in which the passage and/or holding time in the temperature range of 400°C to 600°C is set to 1 to 5 hours, followed by a holding time at a temperature of 700°C to 780°C for 3 hours or more.

In addition to these conditions, the present invention is characterized in that, in the process of increasing the temperature of the lithium mixture from room temperature to a firing temperature of 700° C. or higher in an oxidizing atmosphere, the passage time and/or retention time in a temperature region where the material temperature of the lithium mixture is 600° C. to 650° C. is set to a range of 20 minutes to 45 minutes, preferably 20 minutes to 40 minutes.

As mentioned above, in the calcination process in which a lithium mixture is calcined to obtain a lithium-nickel-containing composite oxide, the calcination temperature of the lithium mixture, i.e., the maximum temperature at which the lithium mixture is held and the holding time thereof, have been investigated. In addition, from the viewpoint of causing a uniform reaction between the lithium compound and the nickel compound, the passage time and/or holding time in the temperature range of 400°C to 600°C have also been investigated.

In response to this, the present inventors focused on the reaction between Li of the lithium nickel-containing composite oxide and W of the tungsten compound when forming fine particles and/or a coating of a compound containing W and Li on the surface of the primary particles of the lithium nickel-containing composite oxide. From this perspective, the inventors conducted extensive research into the amount of excess Li in the lithium nickel-containing composite oxide that serves as the base material and the firing conditions of this lithium nickel-containing composite oxide. As a result, the present inventors discovered that in the process of raising the temperature of the lithium mixture from room temperature to a firing temperature of 700°C or higher, in the temperature range of 600°C to 650°C where the formation of the lithium nickel-containing composite oxide progresses, there is a correlation between the passing time and/or holding time and the amount of excess Li present on the surface of the primary particles of the lithium nickel-containing composite oxide finally obtained, and that by appropriately controlling the amount of excess Li, it is possible to form fine particles and/or a coating of a compound containing W and Li without extracting Li present inside the primary particles of the lithium nickel-containing composite oxide in the reaction between Li of the lithium nickel-containing composite oxide and W of the tungsten compound.

Previously, in this temperature range, the heating rate was set to about 1°C/min or less from the viewpoint of improving the crystallinity of the resulting lithium nickel-containing composite oxide, and the time for passing the lithium mixture in the temperature range of 600°C to 650°C was more than 45 minutes, for example, 50 minutes or more. However, if the time for passing through the temperature range of 600°C to 650°C exceeds 45 minutes, Li will sufficiently penetrate into the crystals of the primary particles in the process of generating the lithium nickel-containing composite oxide, improving the crystallinity, but the amount of excess Li present on the surface of the primary particles will be insufficient. For this reason, when Li of the lithium nickel-containing composite oxide reacts with W of the tungsten compound, Li present inside the primary particles of the lithium nickel-containing composite oxide is extracted, and the amount of Li inside the primary particles of the resulting positive electrode active material will be insufficient, which may deteriorate the battery characteristics.

The shorter the passage time of the lithium mixture in the temperature range of 600°C to 650°C, the more the amount of excess Li in the lithium nickel-containing composite oxide that can react with W tends to increase. For this reason, the passage time in the temperature range of 600°C to 650°C is preferably 40 minutes or less, and more preferably 35 minutes or less. However, from the viewpoint of the fact that the amount of excess Li becomes excessive by shortening the time for passing the lithium compound in this temperature range and the increase in the energy and equipment costs required for passing the lithium compound in a shorter time, the passage time in the temperature range of 600°C to 650°C is preferably 20 minutes or more.

In the present invention, the temperature range is not regulated by the temperature of the calcination furnace, but by the material temperature, because this temperature is an important temperature range in which the production of lithium-nickel-containing composite oxide progresses, and the range and transit time of the temperature range are short, so it is preferable to regulate it by the temperature of the lithium mixture itself. Any device capable of measuring the temperature of the lithium mixture itself in the calcination furnace can be used to measure the material temperature, but it is also possible to use a non-contact thermometer such as a thermocouple or a radiation thermometer.

In addition, since this temperature region is usually in the process of heating, the passage time in the temperature region of 600°C to 650°C can be reduced to 45 minutes or less by raising the temperature at a rate of 1.11°C/min or more. However, if the retention time in this temperature region is 45 minutes or less, the amount of excess Li present on the surface of the primary particles can be appropriately regulated. For this reason, it is within the scope of the present invention not only to pass through the temperature region at a constant heating rate, but also to raise the temperature stepwise or to hold at any temperature within the temperature region for a certain period of time.

2. Method for Producing Positive Electrode Active Material for Lithium Ion Secondary Battery A second aspect of the present invention relates to a method for producing a positive electrode active material for a lithium ion secondary battery, i.e., a lithium nickel-containing composite oxide coated with a compound containing W and Li, in which fine particles and/or a coating of a compound containing W and Li are present on the surfaces of primary particles.

The method for producing a positive electrode active material for a lithium ion secondary battery of the present invention comprises the steps of:
A slurry is formed using the lithium nickel-containing composite oxide according to the first aspect of the present invention such that the amount of the lithium nickel-containing composite oxide is 700 g to 2000 g per 1 L of water, and the lithium nickel-containing composite oxide is washed with water;
During or after the water washing treatment, a tungsten compound is added to the lithium nickel-containing composite oxide to disperse W on the surfaces of the primary particles of the lithium nickel-containing composite oxide; and
The lithium nickel-containing composite oxide having W dispersed on the surfaces of the primary particles is heat-treated to form fine particles and/or a coating of a compound containing W and Li on the surfaces of the primary particles of the lithium nickel-containing composite oxide.
It is characterized by:

[Water washing process]
The water-washing step is a step of washing the calcined powder of the lithium-nickel-containing composite oxide obtained in the calcination step with water. Specifically, a slurry is formed so that the calcined powder is 700 g to 2000 g per 1 L of water, and the slurry is washed with water, filtered, and dried to obtain a lithium-nickel-containing composite oxide powder (water-washed powder).

In the water washing step, the water washing temperature during the water washing process is adjusted to preferably be in the range of 10°C to 40°C, more preferably in the range of 10°C to 30°C. This removes impurities present on the surface of the calcined powder of lithium nickel-containing composite oxide. The water washing process removes excess Li such as lithium carbonate and lithium hydroxide present on the surface along with the impurities. By performing the water washing under the above water washing conditions, even after water washing, the amount of excess Li exceeds 0.10 mass%, preferably 0.15 mass% or more, more preferably 0.20 mass% or more, based on the entire lithium nickel-containing composite oxide.

As a result, when forming fine particles and/or a coating of a compound containing W and Li on the surface of the primary particles of the lithium nickel-containing composite oxide, sufficient Li is present on the surface of the primary particles to react with the W of the tungsten compound, and Li is not extracted from the inside by reaction with W. On the other hand, by appropriately regulating the amount of excess Li, no excess Li is present on the surface of the finally obtained positive electrode active material. Therefore, when this positive electrode active material is used in the positive electrode of a lithium ion secondary battery, gas generation during high temperature storage can be suppressed, and high capacity and high output can be achieved at the same time as high safety.

On the other hand, if the water washing temperature is less than 10°C, the washing may be insufficient, and impurities adhering to the surface of the fired powder may not be removed and may remain. The presence of such remaining impurities increases the resistance of the surface of the positive electrode active material, and the resistance value of the positive electrode of the secondary battery increases. In addition, the specific surface area of the positive electrode active material becomes too small, and the reactivity with the electrolyte decreases, making it difficult to achieve high capacity and high output of the secondary battery.

On the other hand, if the water washing temperature exceeds 40°C, the amount of Li eluted from the fired powder increases, and nickel oxide (NiO) from which Li has been removed or nickel oxyhydroxide (NiOOH) from which Li has been replaced with H may be produced in the surface layer of the lithium-nickel-containing composite oxide. Both nickel oxide and nickel oxyhydroxide have high electrical resistance, which increases the resistance of the surface of the positive electrode active material and reduces Li in the positive electrode active material, reducing the capacity of the secondary battery.

The amount of calcined powder per 1 L of water in the slurry is adjusted to 700 g to 2000 g, preferably 700 g to 1500 g.

That is, if the slurry concentration exceeds 2000 g/L, the viscosity of the slurry becomes high, making it difficult to stir. Furthermore, the alkali concentration of the liquid in the slurry becomes high, so that due to equilibrium, the dissolution rate of the deposits adhering to the fired powder slows down, and even if the deposits peel off from the powder, they may reattach, making it difficult to remove impurities.

On the other hand, if the slurry concentration is less than 700 g/L, it is too dilute, and a large amount of Li will dissolve from the surface of each particle into the slurry. In particular, the higher the nickel ratio, the greater the amount of Li that dissolves, and the amount of Li on the surface becomes too small. This causes Li to be removed from the crystal lattice of the lithium-nickel-containing composite oxide, making the crystals more likely to collapse. This reduces the capacity of the secondary battery.

The time for rinsing the sintered powder with water is not particularly limited, but it is preferably about 5 to 60 minutes. If the rinsing time is too short, impurities on the powder surface may not be sufficiently removed and may remain. On the other hand, extending the rinsing time will not improve the cleaning effect and will reduce productivity.

The water used to form the slurry is not particularly limited, but in order to prevent a decrease in battery performance due to the adhesion of impurities to the positive electrode active material, water with an electrical conductivity measurement of less than 10 μS/cm is preferred, and water with an electrical conductivity of 1 μS/cm or less is more preferred.

[Drying process]
The temperature and method for drying the lithium-nickel-containing composite oxide after washing are not particularly limited, but the drying temperature is preferably set in the range of 80° C. to 500° C., and more preferably in the range of 120° C. to 250° C. By setting the drying temperature at 80° C. or higher, the powder after washing can be dried in a short time, and the occurrence of a gradient in lithium concentration between the surface and the interior of the particles can be suppressed, thereby further improving the characteristics of the positive electrode active material.

On the other hand, it is expected that near the surface of the sintered powder after washing with water, the ratio is very close to the stoichiometric ratio, or that some Li has been removed, resulting in a state close to the charged state. For this reason, temperatures above 500°C may cause the crystal structure of the powder close to the charged state to collapse, leading to a deterioration in electrical properties.

From these perspectives, the drying temperature is preferably in the range of 80°C to 500°C, and taking into account productivity and thermal energy costs, it is even more preferable for the temperature to be in the range of 120°C to 250°C.

As for the drying method, it is preferable to dry the filtered powder at a specified temperature using a dryer that can control the atmosphere to a gas or vacuum that does not contain compounds containing carbon and sulfur.

[Tungsten addition process]
The tungsten addition step is a step in which a tungsten compound is added to the powder during or after the water washing treatment, thereby dispersing W on the surfaces of the primary particles.

That is, W can be added to the slurry during the water washing process, to the powder before drying after the water washing process, or to the powder dried after the water washing process. In any case, by controlling the moisture content of the powder after dehydration, it is possible to increase the uniformity of the dispersion of W and suppress the elution of Li, and form fine particles and/or a coating of a compound containing W and Li on the surface of the primary particles of the positive electrode active material.

The amount of W dispersed on the surface of the primary particles of the sintered powder is preferably in the range of 0.1 atomic % to 3.0 atomic %, and more preferably in the range of 0.1 atomic % to 1.0 atomic %, relative to the total number of Ni, Co, and M atoms contained in the sintered powder. This allows fine particles and/or coatings of compounds containing W and Li to be more uniformly formed on the surface of the primary particles of the positive electrode active material, and when used in the positive electrode of a secondary battery, a conductive path for Li is formed at the interface with the electrolyte, reducing the reaction resistance of the positive electrode active material and further improving the output characteristics.

Whether fine particles of a compound containing W and Li, a coating of a compound containing W and Li, or both are formed on the surface of the primary particles is determined by the moisture content of the powder after dehydration. When added to powder before drying after water washing, the moisture content of the powder after dehydration is controlled to be in the range of 3.0% to 11.5% by mass. When the moisture content of the powder is 3.0% by mass or more and less than 6.5% by mass, fine particles of a compound containing W and Li are formed on the surface of the primary particles. By making the moisture content of the powder 6.5% by mass or more, a coating of a compound containing W and Li tends to be formed on the surface of the primary particles, and the coating and fine particles are mixed. By making the moisture content of the powder 9.0% by mass or more, only a coating of a compound containing W and Li can be formed on the surface of the primary particles. The moisture content after dehydration is calculated from the mass of the hydrated powder after dehydration and the mass of the powder after drying it at 180°C for 3 hours using the formula: "moisture content = (mass of hydrated powder before drying - mass of powder after drying) / (mass of hydrated powder before drying)".

W can be added to the slurry during the water washing process, to the calcined powder after the water washing process and before drying, or to the calcined powder after the water washing process and drying.

W may be added either in the form of an alkaline solution in which a tungsten compound is dissolved (hereinafter referred to as "alkaline solution (W)") or in the form of a tungsten compound.

When added as an alkaline solution (W), the tungsten compound may be one that is soluble in an alkaline solution, and it is preferable to use a tungsten compound that is easily soluble in alkali, such as tungsten oxide, lithium tungstate, or ammonium tungstate.

As the alkali used in the alkaline solution (W), in order to obtain a high charge/discharge capacity, a general alkaline solution that does not contain impurities that are harmful to the positive electrode active material, such as ammonia or lithium hydroxide, can be used. It is preferable to use lithium hydroxide in order to make it possible to supply a sufficient amount of Li from the surplus Li and the alkaline solution (W) to form a compound containing W and Li, and from the viewpoint of not inhibiting the intercalation of Li.

On the other hand, when it is added in the form of a tungsten compound, it is preferable to use a tungsten compound that is soluble in alkali as the tungsten compound, it is _more preferable to use _a tungsten compound containing lithium, and it is even more preferable to use lithium tungstate. As the lithium _tungstate , at least one selected from _Li2WO4 , _Li4WO5 , _{and Li6W2O9} can be used.

In either case, the W addition process conditions and the means of addition must be selected so that the amount of W in the fine particles and/or coating of the compound containing W and Li, such as lithium tungstate, formed on the surface of the particles constituting the lithium nickel-containing composite oxide by solid-liquid separation or crushing after heat treatment, falls within the range of the present invention. Note that these process conditions and means of addition are publicly known, so a detailed description thereof will be omitted here.

[Heat treatment process]
The heat treatment step is a step of forming fine particles and/or a coating of a compound containing W and Li on the surface of the primary particles of the lithium nickel-containing composite oxide by heat treating the lithium nickel-containing composite oxide having W dispersed on the surface of the primary particles. As a result, fine particles and/or a coating of a compound containing W and Li are formed from the W and Li supplied in the tungsten addition step, and a positive electrode active material for a lithium ion secondary battery is obtained, which is made of a lithium nickel-containing composite oxide coated with a compound containing W and Li, which has fine particles and/or a coating of a compound containing W and Li on the surface of the primary particles of the lithium nickel-containing composite oxide. Note that when W is added to the slurry during the water washing process or the calcined powder before drying after the water washing process, the drying process can be replaced by a heat treatment process.

The heat treatment method is not particularly limited, but in order to prevent deterioration of the electrical characteristics when used as a positive electrode active material for a lithium ion secondary battery, it is preferable to perform the heat treatment in an oxygen atmosphere or a vacuum atmosphere at a temperature in the range of 100°C to 600°C.

If the heat treatment temperature is less than 100°C, evaporation of water is insufficient, and fine particles and/or a coating of the compound containing W and Li may not be formed sufficiently. On the other hand, if the heat treatment temperature exceeds 600°C, the primary particles of the lithium-nickel-containing composite oxide may be sintered, and some W may be dissolved in the layered structure of the lithium-nickel-containing composite oxide, resulting in a decrease in the charge/discharge capacity of the battery. From this perspective, it is more preferable to set the heat treatment temperature to 550°C or less, and even more preferable to set it to 500°C or less.

The heat treatment is preferably performed in an oxidizing atmosphere such as an oxygen atmosphere or a vacuum atmosphere to avoid reactions with moisture and carbon dioxide in the atmosphere.

The heat treatment time is not particularly limited, but it is preferable to set it to a range of 5 to 15 hours in order to sufficiently evaporate the water in the composite oxide particles and form a coating.

The moisture content of the composite oxide particles after drying is not particularly limited, but is preferably 0.2% by mass or less, and more preferably 0.1% by mass or less. If the moisture content of the powder exceeds 0.2% by mass, it may absorb gas components containing carbon and sulfur in the air and form lithium compounds on the surface.

If the particles aggregate after the heat treatment, they are crushed to a degree that does not destroy the secondary particle structure, and a positive electrode active material is produced consisting of particles with a volume-based average particle size MV in the range of 5 μm to 30 μm.

3. Positive Electrode Active Material for Lithium Ion Secondary Battery A third aspect of the present invention relates to a positive electrode active material for a lithium ion secondary battery, that is, a lithium nickel-containing composite oxide coated with a compound containing W and Li, the compound having fine particles and/or a coating of a compound containing W and Li, such as lithium tungstate, present on its surface.

More specifically, the positive electrode active material for lithium ion secondary batteries of the present invention has fine particles of a compound containing W and Li or a coating of a compound containing W and Li formed on the surfaces of the primary particles constituting the lithium nickel-containing composite oxide as the base material and/or on the surfaces of the primary particles constituting the secondary particles formed by agglomeration of the primary particles. In particular, the positive electrode active material for lithium ion secondary batteries of the present invention is characterized in that the lithium nickel-containing composite oxide is the lithium nickel-containing composite oxide of the first aspect of the present invention, that is, a lithium nickel-containing composite oxide obtained by adding 1.25 kg of the lithium nickel-containing composite oxide to 1 L of pure water, stirring, and filtering the resulting filtrate, the electrical conductivity of which is in the range of 42.5 mS/cm to 62.5 mS/cm.

The characteristics of the base material have already been described, so a duplicated description will be omitted, and the following describes the fine particles and/or coating of the compound containing W and Li.

[Fine particles and/or coating of compound containing W and Li]
Generally, when the surface of the positive electrode active material is completely covered with a different compound, the movement (intercalation) of lithium ions is greatly restricted, and as a result, the advantage of the lithium nickel-containing composite oxide, that is, high capacity, is eliminated.

In contrast, in the present invention, fine particles of a compound containing W and Li are formed on the surface of the primary particles of the lithium nickel-containing composite oxide, or a coating consisting of a compound containing W and Li is formed on the surface of the primary particles, with a thickness in the range of 1 nm to 200 nm, preferably in the range of 1 nm to 150 nm, and more preferably in the range of 2 nm to 100 nm.

Fine particles of a compound containing W and Li have high lithium ion conductivity and have the effect of promoting the movement of lithium ions. Therefore, by forming fine particles of a compound containing W and Li on the surface of the primary particles, a conductive path for Li is formed at the interface with the electrolyte, making it possible to reduce the reaction resistance of the positive electrode active material (also called "positive electrode resistance") and improve its output characteristics. By restricting the thickness of the coating of the compound containing W and Li to the range of 1 nm to 200 nm, the same effect as the fine particles of a compound containing W and Li can be achieved.

In other words, by reducing the positive electrode resistance, the voltage loss in the secondary battery is reduced, and the voltage actually applied to the load side is relatively high, resulting in high output. In addition, by increasing the voltage applied to the load side, lithium is sufficiently inserted and removed from the positive electrode, improving the battery capacity. Furthermore, by reducing the positive electrode resistance, the load on the positive electrode active material during charging and discharging is also reduced, improving the cycle characteristics.

Here, if the surface of the primary particles constituting the positive electrode active material is coated too thickly with a compound containing W and Li, the specific surface area decreases, and therefore even if the compound containing W and Li has high lithium ion conductivity, the contact area with the electrolyte becomes small, which is likely to lead to a decrease in charge/discharge capacity and an increase in reaction resistance. In addition, although the presence of a compound containing W and Li reduces reaction resistance, the compound containing W and Li itself has low electronic conductivity, and therefore reduces the electronic conductivity of the electrode, leading to a decrease in the output characteristics of the secondary battery.

In the present invention, by having fine particles made of a compound containing W and Li and having a predetermined particle size, or a coating made of a compound containing W and Li and having a thickness in a predetermined range, present on the surface of the primary particles, it is possible to effectively improve lithium ion conduction, suppress the decrease in charge/discharge capacity, and reduce reaction resistance without excessively reducing the electrode surface or significantly increasing the bulk resistance of the electrode.

The particle diameter of the fine particles of the compound containing W and Li is preferably in the range of 1 nm to 100 nm. If the particle diameter is less than 1 nm, the fine particles may not have sufficient lithium ion conductivity. If the particle diameter exceeds 100 nm, the coating formed by the fine particles may become non-uniform, and the effect of reducing reaction resistance may not be sufficiently obtained. It is not necessary for all the fine particles to have a particle diameter in the range of 1 nm to 100 nm, and it is preferable that 50% or more of the total number of fine particles formed on the surface of the primary particle have a particle diameter in the range of 1 nm to 100 nm, in order to obtain a higher effect of improving the battery characteristics. In this case, the maximum particle diameter of the fine particles is about 200 nm.

The coating of the compound containing W and Li preferably has a thickness in the range of 1 nm to 200 nm. If the thickness is less than 1 nm, the coating may not have sufficient lithium ion conductivity. If the thickness exceeds 200 nm, the reaction area may decrease and the bulk resistance of the electrode may increase, and the effect of reducing the reaction resistance may not be sufficient.

Since contact with the electrolyte occurs on the surface of the primary particles, it is important that fine particles and/or a coating of a compound containing W and Li are formed on the surface of the primary particles. Here, the surface of the primary particles in the present invention refers to the surface of the primary particles when the lithium nickel-containing composite oxide is composed of primary particles. When the lithium nickel-containing composite oxide is composed of secondary particles, the surface of the primary particles includes both the surface of the primary particles exposed on the outer surface of the secondary particles and the surface vicinity and internal voids of the secondary particles through which the electrolyte can penetrate via the outside of the secondary particles. Furthermore, even if the grain boundary between primary particles is incompletely bonded and the electrolyte can penetrate, such a place is also included in the surface of the primary particles.

Therefore, by forming fine particles and/or a coating of a compound containing W and Li over the entire surface of the primary particles, it is possible to further promote the movement of lithium ions and further reduce the reaction resistance of the particles that make up the positive electrode active material.

The fine particles and/or coating of the compound containing W and Li do not need to be formed on the entire surface of the primary particles that can come into contact with the electrolyte. The fine particles and/or coating of the compound containing W and Li may be in a scattered state. Even in a scattered state, the effect of reducing reaction resistance can be obtained as long as the fine particles and/or coating of the compound containing W and Li are formed on the surfaces of the primary particles that constitute the positive electrode active material, or the primary particles that are exposed to the outer surfaces and internal voids of the secondary particles that constitute the positive electrode active material. However, the higher the proportion of these on the surface of the primary particles, the easier it is to obtain the effect of reducing reaction resistance.

On the other hand, if fine particles are formed unevenly between the particles that make up the lithium nickel-containing composite oxide, the movement of lithium ions between the particles will be uneven, placing a load on certain particles and easily leading to deterioration of cycle characteristics and an increase in reaction resistance. Therefore, it is preferable that fine particles are formed uniformly even between the particles that make up the lithium nickel-containing composite oxide.

A high effect on battery characteristics can also be obtained when fine particles of a compound containing W and Li are mixed with a coating and formed on the surface of a primary particle. In this case, in order to obtain a higher effect in improving battery characteristics, it is preferable that the fine particles of the compound containing W and Li have a particle diameter in the range of 1 nm to 200 nm. It is not necessary for all of the fine particles to have a particle diameter in the range of 1 nm to 200 nm, and preferably, if 50% or more of the total number of fine particles formed on the surface of the primary particle have a particle diameter in the range of 1 nm to 200 nm, a higher effect in improving battery characteristics can be obtained through interaction with the coating.

By adopting this type of configuration, the contact area with the electrolyte can be made sufficient, effectively improving lithium ion conduction, thereby improving charge/discharge capacity and more effectively reducing reaction resistance.

The surface properties of such primary particles can be determined, for example, by cross-sectional observation with a field emission scanning electron microscope, cross-sectional element mapping by EDX analysis with a scanning transmission electron microscope (STEM), cross-sectional observation with a transmission ionization microscope, etc. By these means, it is possible to confirm that fine particles and/or a coating of a compound containing W and Li are formed on the surfaces of the primary particles constituting the lithium nickel-containing composite oxide of the positive electrode active material for lithium ion secondary batteries of the present invention.

In the present invention, the compound containing W and Li constituting the fine particles and _/ or coating of the compound containing W _{and Li} is _preferably lithium _{tungstate.The} lithium tungstate _is preferably in the form of _{at least} one selected from Li2WO4, _{Li4WO5 , Li6WO6 , Li2W4O13 , Li2W2O7 , Li6W2O9 , Li2W2O7 ,} Li2W5O16, _Li9W19O55 , _Li3W10O30 , _Li18W5O15 , and hydrates thereof.By forming such _{lithium tungstate} , _{the lithium ion} conductivity is further _increased , and _{the effect} of reducing the reaction resistance is greater.

The number of W atoms contained in the compound containing W and Li is preferably in the range of 0.1 atomic % to 3.0 atomic %, more preferably in the range of 0.1 atomic % to 1.0 atomic %, and even more preferably in the range of 0.1 atomic % to 0.6 atomic %, relative to the total number of Ni, Co, and M atoms contained in the particles that make up the positive electrode active material. This makes it possible to achieve both high charge/discharge capacity and output characteristics.

If the W content is less than 0.1 atomic %, the effect of improving the output characteristics may not be sufficient. If the W content exceeds 3.0 atomic %, the surface of the primary particles is covered too thickly with fine particles and/or coatings of compounds containing W and Li, reducing the specific surface area and increasing the bulk resistance of the electrode, so that a sufficient effect of reducing reaction resistance may not be obtained.

The amount of Li contained in the fine particles and/or coating of the compound containing W and Li is not particularly limited, and as long as Li is contained, the effect of improving lithium ion conductivity can be obtained. However, it is preferable that this amount of Li is sufficient to form lithium tungstate.

When the amount of W is in the range of 0.1 atomic % to 3.0 atomic %, the positive electrode active material is
General formula: Li _a Ni _1-x-y Co _x M _y W _z O _2+α ...(2)
(In the formula, M is at least one element selected from Mg, Al, Ca, Ti, V, Cr, Mn, Nb, Zr, Mo, and W; 0.95≦a≦1.10, 0<x≦0.15, 0<y≦0.07, x+y≦0.16, 0.001≦z≦0.03, and 0≦α≦0.2.)
It is preferable that the lithium nickel-containing composite oxide has a composition consisting of the following: Note that W can be contained inside the lithium nickel-containing composite oxide as an additive element M, but in this case, the amount of W (z in the composition formula) does not include the amount of W contained as the additive element M.

The value of a, which indicates the Li content, is 0.95≦a≦1.10. If a exceeds 1.10, the Li content in the lithium nickel-containing composite oxide becomes too high, and Li may be mixed into the metal layer in the layered compound. On the other hand, if a is 0.95 or less, metal elements such as Ni may be mixed into the lithium layer. For this reason, in order to improve the battery capacity and output characteristics, it is more preferable that 0.95<a≦1.08. The value of z, which indicates the W content, is preferably 0.001≦z≦0.01.

A lithium ion secondary battery obtained by using the positive electrode active material for lithium ion secondary batteries of the present invention is composed of a positive electrode, a negative electrode, a separator, a non-aqueous electrolyte, etc., like a general lithium ion battery. For example, a 2032 type coin battery 1 (hereinafter referred to as a "coin battery") shown in FIG. 1 is composed of a case 2 and an electrode 3 housed in this case 2.

The case 2 has a hollow positive electrode can 2a with one open end, and a negative electrode can 2b that is placed in the opening of the positive electrode can 2a. When the negative electrode can 2b is placed in the opening of the positive electrode can 2a, a space is formed between the negative electrode can 2b and the positive electrode can 2a to accommodate the electrode 3.

The electrode 3 is composed of a positive electrode 3a, a separator 3c, and a negative electrode 3b, which are stacked in this order. The positive electrode 3a is in contact with the inner surface of the positive electrode can 2a, and the negative electrode 3b is in contact with the inner surface of the negative electrode can 2b, and each is housed in the case 2.

The case 2 is equipped with a gasket 2c, which prevents the positive electrode can 2a and the negative electrode can 2b from moving relative to each other so that they are kept out of contact. The gasket 2c also seals the gap between the positive electrode can 2a and the negative electrode can 2b, providing an airtight and liquid-tight barrier between the inside of the case 2 and the outside.

Note that the embodiments described below are merely examples, and the lithium ion secondary battery of the present invention can be implemented in various forms with modifications and improvements based on the embodiments described in this specification and the knowledge of those skilled in the art. Furthermore, the lithium ion secondary battery of the present invention is not particularly limited in its use.

[Positive electrode]
The lithium nickel-containing composite oxide of the present invention can be used as a positive electrode active material constituting a positive electrode, and a positive electrode of a lithium ion secondary battery can be produced, for example, as follows.

First, the powdered positive electrode active material, conductive agent, and binder are mixed, and then a slurry solvent and, if necessary, activated carbon and viscosity adjusting additives are added, and the mixture is kneaded to produce a positive electrode mixture paste.

The mixing ratio of each component in the positive electrode mixture paste is also an important factor in determining the performance of a lithium-ion secondary battery. If the total mass of the solid content of the positive electrode mixture excluding the solvent is 100 parts by mass, it is preferable that the content of the positive electrode active material is 60 parts by mass to 95 parts by mass, the content of the conductive agent is 1 part by mass to 20 parts by mass, and the content of the binder is 1 part by mass to 20 parts by mass, similar to the positive electrodes of general lithium-ion secondary batteries.

The obtained positive electrode mixture paste is applied, for example, to the surface of a current collector and dried to evaporate the slurry solvent. If necessary, pressure can be applied using a roll press or the like to increase the electrode density. In this way, a sheet-shaped positive electrode is produced.

The sheet-like positive electrode is cut to an appropriate size depending on the desired battery, and used to make the battery. However, the method of making the positive electrode is not limited to the example, and other methods can also be used.

The conductive agent is not particularly limited as long as it is an electronically conductive material that is chemically stable in the battery. For example, graphites such as natural graphite (e.g., flake graphite) and artificial graphite, carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black, conductive fibers such as carbon fiber and metal fiber, metal powders such as aluminum, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, organic conductive materials such as polyphenylene derivatives, and carbon fluoride can be used. These may be used alone or in combination of two or more.

The amount of conductive agent added to the positive electrode mixture is not particularly limited, but is preferably in the range of 0.5% to 50% by mass, more preferably in the range of 0.5% to 30% by mass, and even more preferably in the range of 0.5% to 15% by mass, relative to the positive electrode active material contained in the positive electrode mixture.

The binder serves to hold the active material particles together. Examples of binders include polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene butadiene rubber, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), vinylidene fluoride-pentafluoropropylene copolymer, propylene-tetrafluoroethylene copolymer, ethylene-chlorotrifluoroethylene copolymer (ECTFE), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene copolymer, ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, ethylene-methyl acrylate copolymer, ethylene-methyl methacrylate copolymer, and the like.

The above resins may be used alone or in combination of two or more kinds. These may be crosslinked with Na ⁺ ions or the like.

Specific examples of the paste solvent include organic solvents such as N-methyl-2-pyrrolidone. Activated carbon can also be added to the positive electrode mixture to increase the electric double layer capacity.

The current collector is not particularly limited as long as it is an electronic conductor that is chemically stable inside the battery. For example, a foil or sheet made of aluminum, stainless steel, nickel, titanium, carbon, conductive resin, etc. can be used, and among these, aluminum foil, aluminum alloy foil, etc. are more preferable. A carbon or titanium layer can be applied to the surface of the foil or sheet, or an oxide layer can be formed. In addition, the surface of the foil or sheet can be made uneven, and a net, punched sheet, lath body, porous body, foam body, fiber group molded body, etc. can be used. The thickness of the current collector is not particularly limited, but is preferably in the range of 1 μm to 500 μm.

(2) Negative electrode For the negative electrode, metallic lithium, lithium alloys, etc. can be used. For example, the negative electrode is a sheet-shaped electrode that is formed by mixing a binder with a negative electrode active material capable of absorbing and desorbing lithium ions, adding an appropriate solvent to make a paste-like negative electrode mixture, applying the mixture to the surface of a metal foil collector such as copper, drying it, and compressing it to increase the electrode density as necessary.

As the negative electrode active material, for example, natural graphite, artificial graphite, baked organic compounds such as phenolic resin, and powdered carbon materials such as coke can be used. In this case, as with the positive electrode, fluorine-containing resins such as PVDF can be used as the negative electrode binder, and an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent for dispersing these active materials and binders.

(3) Separator A separator is sandwiched between the positive electrode and the negative electrode. The separator electrically separates the positive electrode and the negative electrode, holds the electrolyte, and serves as a path for lithium ions to move between the positive electrode and the negative electrode. A typical separator is a thin membrane made of polyethylene, polypropylene, or the like, and has many minute pores. A solid electrolyte can also be used.

The separator can also be made of a microporous thin film. In this case, the pore size of the pores formed in the separator is not particularly limited, but is preferably in the range of 0.01 μm to 1 μm, for example. The porosity of the separator is also not particularly limited, but is generally about 30% to 80%. The thickness of the separator is also not particularly limited, but is generally about 10 μm to 300 μm.

The separator may be separate from the positive and negative electrodes, but a polymer electrolyte consisting of a non-aqueous electrolyte and a polymer material that holds it can also be integrated with the positive or negative electrode and used as a separator. This polymer material can be anything that can hold a non-aqueous electrolyte, and is not particularly limited, but is preferably a copolymer of vinylidene fluoride and hexafluoropropylene.

(4) Nonaqueous Electrolyte As the nonaqueous electrolyte, in addition to a nonaqueous electrolytic solution obtained by dissolving a lithium salt, which is a supporting salt, in an organic solvent, a nonflammable solid electrolyte having ion conductivity, or the like, can be used.
A non-aqueous electrolyte solution that is widely used as a non-aqueous electrolyte for lithium ion secondary batteries is prepared by dissolving a lithium salt as a supporting salt in an organic solvent. The organic solvent used is, for example,
Cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, and trifluoropropylene carbonate;
Chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate;
Aliphatic carboxylates such as methyl formate, methyl acetate, methyl propionate, and ethyl propionate;
Lactones such as γ-butyrolactone and γ-valerolactone;
Chain ethers such as 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), and ethoxymethoxyethane (EME);
Ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane;
Sulfur compounds such as ethyl methyl sulfone and butane sultone;
Phosphorus compounds such as triethyl phosphate and trioctyl phosphate; and
Dimethyl sulfoxide, 1,3-dioxolane, formamide, acetamide, dimethylformamide, dioxolane, acetonitrile, propylnitrile, nitromethane, ethyl monoglyme, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethyl ether, 1,3-propane sultone, anisole, dimethyl sulfoxide, N-methyl-2-pyrrolidone, and the like;
These may be used alone or in combination of two or more.

Examples of the supporting salt include _LiPF6 , _LiBF4 , LiClO4, _LiAsF6 , _{LiN ( CF3SO2 ) 2} , _LiAlCl4 , _LiSbF6 , LiSCN, LiCl, _{LiCF3SO3 , LiCF3CO2} , Li( _CF3SO2 ) ₂ , _LiB10Cl10 , lower aliphatic lithium carboxylate, LiCl, LiBr, LiI, lithium chloroborane _, lithium tetraphenylborate, and lithium imide salt. These may be used alone or in combination of _two or more. It is preferable to use at least _LiPF6 .

The lithium salt concentration in the non-aqueous solvent is not particularly limited, but is preferably in the range of 0.2 mol/L to 2 mol/L, and more preferably in the range of 0.5 mol/L to 1.5 mol/L.

Furthermore, the non-aqueous electrolyte may contain a radical scavenger, a surfactant, a flame retardant, etc.

On the other hand, as the solid electrolyte, Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ , Li ₂ S—SiS ₂ , or the like can be used.

[Shape and configuration of lithium-ion secondary battery]
The lithium ion secondary battery of the present invention, which is composed of the positive electrode, negative electrode, separator, and non-aqueous electrolyte described above, can have various shapes such as a cylindrical type or a laminated type.

Regardless of the shape, for example, when a non-aqueous electrolyte solution is used as the non-aqueous electrolyte, the positive and negative electrodes are laminated with a separator between them to form an electrode body, the resulting electrode body is impregnated with a non-aqueous electrolyte solution, and the positive electrode current collector and the positive electrode terminal connected to the outside, and the negative electrode current collector and the negative electrode terminal connected to the outside are connected using current collecting leads or the like, and the battery is sealed in a battery case to complete the lithium-ion secondary battery.

[Characteristics of lithium-ion secondary batteries]
A lithium ion secondary battery using the W- and Li-containing compound-coated lithium nickel-containing composite oxide of the present invention, in which fine particles and/or a coating of a compound containing W and Li are present on the surfaces of the primary particles, as a positive electrode active material has high capacity and high output.

In particular, a lithium ion secondary battery using the positive electrode active material for lithium ion secondary batteries of the present invention obtained in a more preferred form has a high initial discharge capacity of 200 mAh/g or more, preferably 205 mAh/g or more, and a high initial charge/discharge efficiency of 88% or more, resulting in high capacity and high output, when used, for example, in the positive electrode of a coin-type battery as shown in FIG. 1.

The present invention will be specifically explained below using examples, but the present invention is not limited to these examples. In the examples, special-grade reagent samples manufactured by Wako Pure Chemical Industries, Ltd. were used for the precursor nickel compound, cobalt compound, compound of added element M, lithium compound, and materials used in the manufacture of the lithium-nickel-containing composite oxide and lithium-ion secondary battery.

Example 1
[Production and evaluation of lithium nickel-containing composite oxide]
The temperature in the reaction tank was set to 49.5° C., and while the reaction solution in the reaction tank was maintained at pH 13.0 at a liquid temperature of 25° C. using a 20% by mass sodium hydroxide solution, a mixed aqueous solution of nickel sulfate and cobalt sulfate, an aqueous sodium aluminate solution, and 25% by mass aqueous ammonia were added to the reaction solution and recovered by overflow. The solution was further washed with a 45 g/L aqueous sodium hydroxide solution having a pH of 12.5 at a liquid temperature of 25° C., then washed with water and dried to obtain a nickel composite hydroxide (neutralization crystallization method).

This nickel composite hydroxide is composed of secondary particles formed by the aggregation of multiple primary particles of 1 μm or less into a spherical shape, and analysis by ICP emission spectrometry using an ICP emission spectrometer (Shimadzu Corporation, ICPE-9000) confirmed that it is a nickel composite hydroxide with a Ni:Co:Al molar ratio of 90.5:4.8:4.7.

The volume-based average particle size MV of this nickel composite hydroxide was measured using a laser light diffraction scattering particle size analyzer (Microtrack HRA, manufactured by Nikkiso Co., Ltd.) and was found to be 12 μm.

In addition, sulfur was quantitatively analyzed by ICP atomic emission spectrometry, and the sulfate ion content was calculated by multiplying the content by a coefficient assuming that all sulfur is oxidized to sulfate ion (SO ₄ ), and was found to be 0.28 mass %.

Next, this nickel composite hydroxide was oxidized and roasted at 600°C in an air atmosphere to obtain a nickel composite oxide, after which the nickel composite oxide and lithium hydroxide monohydrate were weighed out so that the molar ratio was Ni:Co:Al:Li = 90.5:4.8:4.7:1.02, and mixed using a shaker mixer (TURBULA Type T2C manufactured by WAB) to obtain a lithium mixture.

The obtained lithium mixture was calcined in an oxygen atmosphere in an electric furnace at 500°C for 3 hours, then heated from 500°C to 745°C at a heating rate of 2.5°C/min so that the time it took to pass through the temperature range of 600°C to 650°C was 20 minutes, and then held at 745°C for 3 hours, for a total time of 20 hours from the start of heating to the end of holding. It was then cooled to room temperature in the furnace and crushed to obtain a sintered powder (hereinafter referred to as the "base material"). During the sintering process, a thermocouple was inserted into the lithium mixture to measure the temperature of the mixture itself.

The obtained base material was analyzed by ICP method, and it was confirmed that the molar ratio of Ni:Co:Al:Li was 90.5:4.8:4.7:1.02. In addition, 1.25 g of the base material was taken and placed in 1 L of pure water maintained at 25°C, and stirred with a magnetic stirrer and a stirrer for 3 minutes, and then vacuum filtered. The conductivity of the obtained filtrate was measured using a conductivity meter (ES-71, manufactured by Horiba, Ltd.). As a result, the conductivity of the filtrate of this base material was 45 mS/cm.

Next, 20°C pure water was added to the obtained base material to make a slurry containing 750 g of base material per 1 L of water. After stirring this slurry for 20 minutes, it was passed through a filter press and dehydrated to produce a base material cake.

W was added to this base material by passing an alkaline solution (W) containing a tungsten compound through the base material cake after dehydration in a filter press, dehydrating it again, and dispersing W on the surface of the primary particles of the base material.

Here, the amount of tungsten compound to be added is determined by the tungsten concentration in the alkaline solution (W) being passed through and the moisture content of the base cake after dehydration. In other words, the amount of tungsten contained in the moisture after dehydration is the amount to be added.

In this embodiment, an alkaline solution (W) with a tungsten concentration of 0.58 mol/L was used, which was prepared by adding 131.8 g of tungsten oxide (WO ₃ ) to an aqueous solution in which 50.6 g of lithium hydroxide (LiOH.H ₂ O) was dissolved in 1 L of pure water. The moisture content of the base cake after dehydration was 6.1%.

The lithium-nickel-containing composite oxide soaked in the obtained alkaline solution (W) was left to dry for 10 hours in a vacuum dryer heated to 190°C.

Finally, the mixture was sieved through a 38 μm mesh sieve and crushed to obtain a positive electrode active material having fine particles and a coating of a compound containing W and Li on the primary particle surface.

The composition of the obtained positive electrode active material was analyzed by ICP emission spectrometry, and it was confirmed that the molar ratio of Ni:Co:Al:Li was 90.5:4.8:4.7:0.98, and the W content was 0.35 atomic % relative to the total number of atoms of Ni, Co, and Al. It was also confirmed that the fine particles and coating of the compound containing W and Li on the surface of the primary particles were composed of lithium tungstate (Li ₂ WO ₄ ).

The specific surface area of the resulting positive electrode active material, as measured by the BET method, was 0.93 m ² /g.

This positive electrode active material was embedded in resin and processed with a cross-section polisher, and cross-section observation was performed with a SEM at a magnification of 5000 times, confirming that it was made up of primary particles and secondary particles formed by agglomeration of the primary particles. The porosity of the secondary particles determined from this observation was 2.1%. In addition, this positive electrode active material was made in a state where cross-section observation was possible with a scanning transmission electron microscope (STEM: S-4700, manufactured by Hitachi High-Technologies Corporation), and the vicinity of the surface of the primary particles was observed with the STEM, confirming that fine particles with particle diameters in the range of 10 nm to 200 nm and a coating with a thickness in the range of 2 nm to 115 nm were formed on the surface of the primary particles. Furthermore, the results of powder XRD analysis using an X-ray diffraction (XRD) device (X'Pert PRO, manufactured by PANalytical) confirmed that these fine particles and coating were made of lithium tungstate.

[Production and evaluation of lithium-ion batteries]
The coin-type battery 1 shown in FIG. 1 was produced as follows. First, 90 parts by mass of a positive electrode active material for lithium ion secondary batteries, 5 parts by mass of acetylene black, and 5 parts by mass of polyvinylidene fluoride were mixed, and n-methylpyrrolidone was added to form a paste. This paste was applied to an aluminum foil having a thickness of 20 μm. The paste was applied so that the mass of the positive electrode active material after drying was 0.05 g/cm ^2. The aluminum foil to which the paste was applied was then vacuum-dried at 120° C. for 12 hours, and then punched into a disk shape with a diameter of 1 cm to form a positive electrode 3a.

The coin-type battery 1 shown in FIG. 1 was fabricated using this positive electrode 3a, negative electrode 3b, separator 3c, and nonaqueous electrolyte in a glove box with an Ar atmosphere whose dew point was controlled at -80°C.

The negative electrode 3b was made of lithium metal punched into a disk shape with a diameter of 15 mm.

A polyethylene porous film with a thickness of 20 μm was used for the separator 3c.

The electrolyte used was a mixture of equal amounts of ethylene carbonate (EC) and diethyl carbonate (DEC) (manufactured by Ube Industries, Ltd.) with 1M _LiClO4 as the supporting electrolyte.

The initial discharge capacity, initial charge/discharge efficiency, and positive electrode resistance, which indicate the performance of the manufactured coin-type battery 1, were evaluated as follows.

The initial charge capacity and initial discharge capacity were determined by leaving the coin battery 1 for about 24 hours after production, and after the open circuit voltage OCV (Open Circuit Voltage) was stabilized, charging the battery to a cutoff voltage of 4.3 V with a current density of 0.1 mA/ ^cm2 to the positive electrode, and then discharging the battery to a cutoff voltage of 3.0 V after a 1-hour pause. [(Initial discharge capacity)/(Initial charge capacity)]×100 was calculated as the initial charge/discharge efficiency.

The positive electrode resistance is measured by the AC impedance method using a frequency response analyzer and a potentiogalvanostat (Solartron, 1255B) while charging the coin cell 1 at a charging potential of 4.1 V, and the Nyquist plot shown in Figure 2 is obtained.

This Nyquist plot is expressed as the sum of characteristic curves showing the solution resistance, negative electrode resistance and its capacity, and positive electrode resistance and its capacity, so a fitting calculation was performed using an equivalent circuit based on this Nyquist plot to calculate the value of the positive electrode resistance.

The coin-type battery 1 using a positive electrode made from the obtained positive electrode active material for lithium-ion secondary batteries had an initial discharge capacity of 205 mAh/g, an initial charge/discharge efficiency of 89.0%, and a positive electrode resistance of 4.8 Ω.

The calcination conditions for the lithium nickel-containing composite oxide, the properties of the obtained lithium nickel-containing composite oxide, and the battery properties of the lithium ion secondary battery are shown in Table 1. The results for Example 2 and Comparative Examples 1 and 2 below are also shown in Table 1.

Example 2
A lithium nickel-containing composite oxide, a positive electrode active material for a lithium ion secondary battery, and a lithium ion secondary battery were manufactured and evaluated in the same manner as in Example 1, except that the temperature was raised from 500 ° C to 745 ° C at a temperature rise rate of 1.43 ° C / min so that the passage time in the temperature range of 600 ° C to 650 ° C was 35 minutes. The composition of the obtained lithium nickel-containing composite oxide was the general formula: Li _1.02 Ni _0.905 Co _0.048 Al _0.047 O _2. The conductivity of the filtrate of the base material was 60 mS / cm. The composition of the obtained positive electrode active material for a lithium ion secondary battery was the general formula: Li _0.99 Ni _0.905 Co _0.048 Al _0.047 W _0.0035 O ₂ . The coin-type battery 1 using a positive electrode produced using the obtained positive electrode active material for lithium ion secondary batteries had an initial discharge capacity of 208 mAh/g, an initial charge/discharge efficiency of 90.0%, and a positive electrode resistance of 4.6 Ω.

(Comparative Example 1)
A lithium nickel-containing composite oxide, a positive electrode active material for a lithium ion secondary battery, and a lithium ion secondary battery were manufactured in the same manner as in Example 1, except that the temperature was raised from 500 ° C. to 745 ° C. at a temperature rise rate of 0.91 ° C. / min so that the passage time in the temperature range of 600 ° C. to 650 ° C. was 55 minutes, and each was evaluated. The composition of the obtained lithium nickel-containing composite oxide was the general formula: Li _1.02 Ni _0.905 Co _0.048 Al _0.047 O _2. The conductivity of the filtrate of the base material was 40 mS / cm. The composition of the obtained positive electrode active material for a lithium ion secondary battery was the general formula: Li _0.99 Ni _0.905 Co _0.048 Al _0.047 W _0.0035 O ₂ . The coin-type battery 1 using a positive electrode produced using the obtained positive electrode active material for lithium ion secondary batteries had an initial discharge capacity of 198 mAh/g, an initial charge/discharge efficiency of 85.0%, and a positive electrode resistance of 5.7Ω.

(Comparative Example 2)
A lithium nickel-containing composite oxide, a positive electrode active material for a lithium ion secondary battery, and a lithium ion secondary battery were manufactured and evaluated in the same manner as in Example 1, except that the temperature was raised from 500 ° C. to 745 ° C. at a heating rate of 5.0 ° C. / min so that the passage time in the temperature range of 600 ° C. to 650 ° C. was 10 minutes. The composition of the obtained lithium nickel-containing composite oxide was the general formula: Li _1.02 Ni _0.905 Co _0.048 Al _0.047 O _2. The conductivity of the filtrate of the base material was 65 mS / cm. The composition of the obtained positive electrode active material for a lithium ion secondary battery was the general formula: Li _0.99 Ni _0.905 Co _0.048 Al _0.047 W _0.0035 O ₂ . The coin-type battery 1 using a positive electrode made from the obtained positive electrode active material for lithium ion secondary batteries had an initial discharge capacity of 197 mAh/g, an initial charge/discharge efficiency of 84.0%, and a positive electrode resistance of 6.0Ω.

By obtaining a positive electrode active material for lithium ion secondary batteries using the lithium nickel-containing composite oxide of the present invention as a base material, in which fine particles and/or a coating of a compound containing W and Li are present on the surfaces of the primary particles, a positive electrode active material with high capacity and high output is provided without compromising crystallinity. Such a high-quality positive electrode active material with excellent electrical properties is suitable as a positive electrode material for lithium ion secondary batteries used as power sources for electric vehicles, hybrid vehicles, etc.

REFERENCE SIGNS LIST 1 coin battery 2 case 2a positive electrode can 2b negative electrode can 2c gasket 3 electrode 3a positive electrode 3b negative electrode 3c separator

Claims (11)

A lithium-nickel-containing composite oxide which is a base material for a positive electrode active material for a lithium ion secondary battery and is made of a lithium-nickel-containing composite oxide coated with a compound containing W and Li, the lithium-nickel-containing composite oxide being composed of primary particles and/or secondary particles formed by agglomeration of the primary particles, and in which fine particles and/or coatings of a compound containing W and Li are present on the surfaces of the primary particles,
1.25 kg of the lithium nickel-containing composite oxide is added to 1 L of pure water, stirred, and filtered to obtain a filtrate having an electrical conductivity in the range of 42.5 mS/cm to 62.5 mS/cm.
Lithium nickel-containing composite oxide. The lithium nickel-containing composite oxide according to claim 1, wherein the conductivity of the filtrate is in the range of 45 mS/cm to 60 mS/cm. The length of the lattice constant in the c-axis direction in the crystal of the lithium nickel-containing composite oxide obtained by Rietveld analysis of X-ray diffraction is in the range of 14.183 Å to 14.205 Å. The lithium nickel-containing composite oxide according to claim 1 or 2 . The lithium nickel-containing composite oxide according to any one of claims 1 to 3, having a volume-based average particle size MV in the range of 5 µm to 30 µm. A method for producing a positive electrode active material for a lithium ion secondary battery, comprising a lithium nickel-containing composite oxide coated with a compound containing W and Li, the lithium nickel-containing composite oxide comprising primary particles and/or secondary particles formed by agglomeration of the primary particles, and fine particles and/or a coating of a compound containing W and Li being present on the surfaces of the primary particles, the method comprising the steps of:
The lithium nickel-containing composite oxide according to any one of claims 1 to 4 is prepared by mixing a nickel compound and a lithium compound to obtain a lithium mixture, and then heating the lithium mixture in an oxidizing atmosphere from room temperature to a firing temperature of 700°C or higher, wherein the lithium mixture is passed through and/or held in a temperature range of 600°C to 650°C for 20 minutes or more and 45 minutes or less;
Using the lithium nickel-containing composite oxide as a base material, a slurry is formed such that the amount of the lithium nickel-containing composite oxide is 700 g to 2000 g per 1 L of water, and the lithium nickel-containing composite oxide is washed with water;
During or after the water washing treatment, a tungsten compound is added to the lithium nickel-containing composite oxide to disperse W on the surfaces of the primary particles of the lithium nickel-containing composite oxide; and
The lithium nickel-containing composite oxide having W dispersed on the surfaces of the primary particles is heat-treated to form fine particles of a compound containing W and Li, or a coating of a compound containing W and Li, on the surfaces of the primary particles of the lithium nickel-containing composite oxide.
Characterized in that
A method for producing a positive electrode active material for a lithium ion secondary battery. The positive electrode active material for a lithium ion secondary battery is
General formula: Li _a Ni _1-x-y Co _x M _y W _Z O _2+α ...(2)
(In the formula, M is at least one element selected from Mg, Al, Ca, Ti, V, Cr, Mn, Nb, Zr, Mo, and W; 0.95≦a≦1.10, 0<x≦0.15, 0<y≦0.07, x+y≦0.16, 0.001≦z≦0.03, and 0≦α≦0.2.)
The composition is a lithium nickel-containing composite oxide coated with a compound containing W and Li,
The method for producing the positive electrode active material for a lithium ion secondary battery according to claim 5 . 7. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 6, wherein the amount of W dispersed on the surface of the primary particles of the lithium nickel-containing composite oxide is in the range of 0.1 atomic % to 3.0 atomic % relative to the total number of Ni, Co and M atoms contained in the lithium nickel-containing composite oxide. The method for producing a positive electrode active material for a lithium ion secondary battery according to any one of claims 5 to 7, wherein the heat treatment is carried out in an oxygen atmosphere or a vacuum atmosphere at a temperature in the range of 100°C to 600°C. A positive electrode active material for a lithium ion secondary battery, comprising a lithium nickel-containing composite oxide coated with a compound containing W and Li, the lithium nickel-containing composite oxide comprising primary particles and/or secondary particles formed by agglomeration of the primary particles, and fine particles and/or a coating of a compound containing W and Li present on the surfaces of the primary particles,
The lithium nickel-containing composite oxide coated with a compound containing W and Li is characterized in that fine particles and/or a coating of a compound containing W and Li are present on the surfaces of primary particles of the lithium nickel-containing composite oxide according to any one of claims 1 to 4.
Positive electrode active material for lithium-ion secondary batteries. The length of the lattice constant in the c-axis direction in the crystal of the compound-coated lithium nickel-containing composite oxide containing W and Li obtained by Rietveld analysis of X-ray diffraction is in the range of 14.183 Å to 14.205 Å. The positive electrode active material for lithium ion secondary batteries according to claim 9. The positive electrode active material for lithium ion secondary batteries according to claim 9 or 10, wherein the volume-based average particle size MV of the lithium nickel-containing composite oxide coated with the compound containing W and Li is in the range of 5 μm to 30 μm.

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