JP7793340B2 - Positive electrode active material for lithium secondary battery, method for producing same, and lithium secondary battery - Google Patents

Description

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

Traditionally, lithium cobalt oxide has been used as the positive electrode active material for lithium secondary batteries. However, because cobalt is a rare metal, lithium nickel manganese cobalt composite oxides with a low cobalt content have been developed (see, for example, Patent Documents 1 and 2).

Lithium secondary batteries that use lithium nickel manganese cobalt composite oxide as the positive electrode active material are known to be able to reduce costs by adjusting the atomic ratio of nickel, manganese, and cobalt contained in the composite oxide, and to have a higher capacity than lithium cobalt oxide (see, for example, Patent Document 3).

However, even with these conventional methods, lithium secondary batteries using lithium nickel manganese cobalt composite oxide as the positive electrode active material still had the problem of deterioration in cycle characteristics.

As a method for improving the cycle characteristics of lithium secondary batteries that use lithium nickel manganese cobalt composite oxide as the positive electrode active material, a method has been proposed in which the particle surface of the lithium nickel manganese cobalt composite oxide is coated with a Ti-containing compound (see, for example, Patent Document 4 and Patent Document 5).

Patent Documents 4 and 5 propose a method for coating the particle surfaces of lithium nickel manganese cobalt composite oxide with a Ti-containing compound, in which an alkoxide monomer or oligomer made of an organometallic compound such as Ti is mixed with an alcohol such as 2-propanol, a chelating agent such as acetylacetone is added, and water is then added to prepare a dispersion in which a precursor of Ti-containing fine particles with an average particle size of 1 to 20 nm is dispersed; the particle surfaces of the lithium nickel manganese cobalt composite oxide are coated with this dispersion, followed by heat treatment.

International Publication No. 2004/092073 Japanese Patent Application Laid-Open No. 2005-25975 JP 2011-23120 A JP 2016-24968 A JP 2016-72071 A

In recent years, lithium secondary batteries have been considered for use in the automotive field, including electric vehicles, hybrid vehicles, and plug-in hybrid vehicles. For this reason, there is a demand for further improvements in the cycle characteristics of lithium secondary batteries that use lithium-nickel-manganese-cobalt composite oxide as the positive electrode active material.

Accordingly, an object of the present invention is to provide a positive electrode active material for lithium secondary batteries that can impart excellent cycle characteristics to lithium secondary batteries that use lithium nickel manganese cobalt composite oxide as the positive electrode active material, and to provide a lithium secondary battery with excellent cycle characteristics.

In light of the above-mentioned circumstances, the inventors conducted extensive research and discovered that a lithium secondary battery using a lithium nickel manganese cobalt composite oxide particle represented by general formula (1) with a specific compound attached to at least a portion of the particle surface as a positive electrode active material exhibits excellent cycleability, leading to the completion of the present invention.

That is, the present invention (1) provides a granular material having a Ti-containing oxide and a Mg-containing oxide attached to at least a part of the particle surface, the granular material being represented by the following general formula (1):
Li _x Ni _y Mn _z Co _t M _p O _1+x (1)
(In the formula, M represents one or more metal elements selected from Mg, Al, Ti, Zr, Cu, Fe, Sr, Ca, V, Mo, Bi, Nb, Si, Zn, Ga, Ge, Sn, Ba, W, Na, and K. x represents 0.98≦x≦1.20, y represents 0.60≦y<1.00, z represents 0<z≦0.50, t represents 0<t≦0.50, p represents 0≦p≦0.05, and y+z+t+p=1.)
The present invention provides a positive electrode active material for a lithium secondary battery, which comprises lithium nickel manganese cobalt composite oxide particles represented by the formula (I), wherein the lithium nickel manganese cobalt composite oxide particles are agglomerated particles, the Ti-containing oxide is TiO2, and the Mg-containing oxide is MgO _.

The present invention (2) also provides a positive electrode active material for a lithium secondary battery according to (1), characterized in that the amount of the Ti-containing oxide attached is, in atomic terms, 0.01 to 5.00 mol % as Ti relative to the total amount of Ni, Mn, Co, and M in the lithium nickel manganese cobalt composite oxide particles.

The present invention (3) also provides a positive electrode active material for a lithium secondary battery according to (1) or (2), characterized in that the amount of the oxide containing Mg attached is, in atomic terms, 0.01 to 5.00 mol % as Mg relative to the total amount of Ni, Mn, Co, and M in the lithium nickel manganese cobalt composite oxide particles.

The present invention (4) also provides a positive electrode active material for a lithium secondary battery according to any one of (1) to (3), characterized in that the molar ratio (Ti:Mg) of Ti in the Ti-containing oxide to Mg in the Mg-containing oxide, in atomic terms, is 1.0:16.0 to 3.0:2.0.

The present invention (5) also provides a positive electrode active material for a lithium secondary battery according to any one of (1) to (4), characterized in that the residual alkali content is 1.20 mass% or less.

The present invention ( 6 ) also provides a positive electrode active material for a lithium secondary battery according to any one of (1) to ( 5 ), which is a mixture of large particles having an average particle diameter of 7.5 to 30.0 μm and small particles having an average particle diameter of 0.50 to 7.5 μm.

The present invention ( 7 ) also provides a positive electrode active material for a lithium secondary battery according to ( 6 ), characterized in that the mixing ratio of the large particles to the small particles is 7:13 to 19:1 by mass.

The present invention ( 8 ) also provides the positive electrode active material for a lithium secondary battery according to ( 6 ) or ( 7 ), characterized in that the mixture has a compressed density of 2.7 g/ ^cm3 or more when compressed ^at 0.65 tonf/cm2.

The present invention (9) also provides a compound represented by the following general formula (1):
Li _x Ni _y Mn _z Co _t M _p O _1+x (1)
(In the formula, M represents one or more metal elements selected from Mg, Al, Ti, Zr, Cu, Fe, Sr, Ca, V, Mo, Bi, Nb, Si, Zn, Ga, Ge, Sn, Ba, W, Na, and K. x represents 0.98≦x≦1.20, y represents 0.60≦y<1.00, z represents 0<z≦0.50, t represents 0<t≦0.50, p represents 0≦p≦0.05, and y+z+t+p=1.)
and a Ti-containing oxide and an Mg-containing oxide are attached to the surface of the lithium nickel manganese cobalt composite oxide particles to obtain Ti- and Mg-containing oxide-attached composite oxide particles, and then the Ti- and Mg-containing oxide-attached composite oxide particles are heat-treated at a temperature of 400°C or higher and lower than 750°C. The lithium nickel manganese cobalt composite oxide particles represented by general formula (1) are agglomerated particles obtained by a raw material mixing step of mixing a lithium source, a nickel source, a manganese source, a cobalt source, and an M source, which is added as needed, to prepare a raw material mixture, and then a firing step of firing the resulting raw material mixture at 700 to 950°C , wherein the Ti-containing oxide is TiO2 _and the Mg-containing oxide is MgO .

The present invention ( 10 ) also provides a lithium secondary battery characterized by using any one of the positive electrode active materials for lithium secondary batteries (1) to (8).

The positive electrode active material for lithium secondary batteries of the present invention can impart excellent cycle characteristics to lithium secondary batteries that use lithium nickel manganese cobalt composite oxide as the positive electrode active material. Furthermore, the use of the positive electrode active material for lithium secondary batteries of the present invention can result in lithium secondary batteries with excellent cycle characteristics.

The positive electrode active material for a lithium secondary battery of the present invention is characterized by comprising lithium nickel manganese cobalt composite oxide particles having a Ti-containing oxide and an Mg-containing oxide attached to at least a portion of the particle surface. In other words, the positive electrode active material for a lithium secondary battery of the present invention is an aggregate of lithium nickel manganese cobalt composite oxide particles having a Ti-containing oxide and an Mg-containing oxide present on at least a portion of the particle surface.

Thus, in the lithium nickel manganese cobalt composite oxide particles constituting the positive electrode active material for a lithium secondary battery of the present invention, a Ti-containing oxide and an Mg-containing oxide are attached to a portion of the particle surface, or the entire particle surface is covered with a Ti-containing oxide and an Mg-containing oxide. "A Ti-containing compound is attached to a portion of the particle surface" refers to a state in which the particle surface has a portion where the surface of the object to be coated is exposed in addition to the Ti-containing oxide. "A Mg-containing oxide is attached to a portion of the particle surface" refers to a state in which the particle surface has a portion where the surface of the object to be coated is exposed in addition to the Mg-containing oxide.

In the positive electrode active material for a lithium secondary battery of the present invention, it is observed that the Ti-containing oxide and the Mg-containing oxide are not solid-dissolved in the lithium nickel manganese cobalt composite oxide particles, but are present in the lithium nickel manganese cobalt composite oxide particles in the form of the Ti-containing oxide and the Mg-containing oxide.
The lithium nickel manganese cobalt composite oxide particles having an oxide containing Ti and an oxide containing Mg attached to at least a portion of the particle surface can be confirmed by observing Ti and Mg being unevenly distributed, such as unevenly distributed, on the sample particle surface when the particle surface of the sample particle to be analyzed is analyzed by elemental mapping analysis of Ti and Mg using SEM-EDX at a magnification of 10,000 to 30,000 times.

The lithium nickel manganese cobalt composite oxide forming the lithium nickel manganese cobalt composite oxide particles according to the positive electrode active material for a lithium secondary battery of the present invention is a composite oxide containing lithium, nickel, manganese, and cobalt, and is represented by the following general formula (1):
Li _x Ni _y Mn _z Co _t M _p O _1+x (1)
(In the formula, M represents one or more metal elements selected from Mg, Al, Ti, Zr, Cu, Fe, Sr, Ca, V, Mo, Bi, Nb, Si, Zn, Ga, Ge, Sn, Ba, W, Na, and K. x represents 0.98≦x≦1.20, y represents 0.30≦y<1.00, z represents 0<z≦0.50, t represents 0<t≦0.50, p represents 0≦p≦0.05, and y+z+t+p=1.)

In general formula (1), x is in the range 0.98≦x≦1.20. In terms of achieving a high initial capacity, x is preferably in the range 1.00≦x≦1.10. In terms of achieving both initial capacity and cycle characteristics, y is preferably in the range 0.30≦y<1.00. In terms of achieving both initial capacity and cycle characteristics, y is preferably in the range 0.50≦y≦0.95, and particularly preferably in the range 0.60≦y≦0.90. In terms of achieving excellent safety, z is preferably in the range 0<z≦0.50. In terms of achieving excellent safety, z is preferably in the range 0.025≦z≦0.45. In terms of achieving excellent safety, t is preferably in the range 0<t≦0.50. t is preferably in the range 0.025≦t≦0.45. y+z+t+p=1. y/z is preferably (y/z)>1, particularly preferably (y/z)≧1.5, and more preferably 3≦(y/z)≦38.

In addition, M in the formula is a metal element that is optionally contained in the lithium nickel manganese cobalt composite oxide represented by general formula (1) for the purpose of improving battery performance such as cycle characteristics and safety. Examples of M include one or more metal elements selected from Mg, Al, Ti, Zr, Cu, Fe, Sr, Ca, V, Mo, Bi, Nb, Si, Zn, Ga, Ge, Sn, Ba, W, Na, and K. In general formula (1), p satisfies 0≦p≦0.05, and preferably 0.0001≦p≦0.045.

The lithium nickel manganese cobalt composite oxide particles are granular lithium nickel manganese cobalt composite oxide represented by general formula (1). The lithium nickel manganese cobalt composite oxide particles may be monodispersed primary particles or agglomerated secondary particles formed by aggregation of primary particles. The lithium nickel manganese cobalt composite oxide particles have an average particle size of 0.50 to 30.0 μm, preferably 1.0 to 25.0 μm, and particularly preferably 1.5 to 20.0 μm, as determined by a laser diffraction/scattering method, as the 50% volume particle size (D50). The lithium nickel manganese cobalt composite oxide particles also have a BET specific surface area of preferably 0.05 to 2.00 m ² /g, particularly preferably 0.15 to 1.00 m ² /g. When the average particle size or BET specific surface area of the lithium nickel manganese cobalt composite oxide particles is within the above range, the preparation and coating properties of the positive electrode mixture become easy, and furthermore, an electrode with high packing properties can be obtained.

Furthermore, the content of residual alkali in the lithium nickel manganese cobalt composite oxide particles is preferably 1.20% by mass or less, and particularly preferably 1.00% by mass or less. By keeping the content of residual alkali in the lithium nickel manganese cobalt composite oxide particles within this range, it is possible to suppress battery expansion and deterioration caused by gas generation due to residual alkali.

In the present invention, the residual alkali refers to the alkali component eluted into water when lithium nickel manganese cobalt composite oxide particles are stirred and dispersed in water at 25°C. The amount of residual alkali is determined by weighing 5 g of lithium nickel manganese cobalt composite oxide particles and 100 g of pure water into a beaker, dispersing them at 25°C for 5 minutes with a magnetic stirrer, filtering the dispersion, and subjecting the amount of alkali present in the resulting filtrate to neutralization titration. The amount of residual alkali is determined by measuring the amount of lithium by titration and converting it to lithium carbonate.

The lithium nickel manganese cobalt composite oxide particles represented by general formula (1) can be produced, for example, by carrying out a raw material mixing step in which a lithium source, a nickel source, a manganese source, a cobalt source, and an optionally added M source are mixed to prepare a raw material mixture, and then a calcination step in which the resulting raw material mixture is calcined.

The lithium source, nickel source, manganese source, cobalt source, and optionally added M source used in the raw material mixing process may be, for example, their hydroxides, oxides, carbonates, nitrates, sulfates, or organic acid salts. The average particle size (D50) of the lithium source, nickel source, manganese source, cobalt source, and M source, as determined by a laser scattering method, is preferably 0.50 to 30.0 μm, and more preferably 1.0 to 25.0 μm.

The nickel source, manganese source, and cobalt source used in the raw material mixing step may be compounds containing nickel atoms, manganese atoms, and cobalt atoms. Examples of compounds containing nickel atoms, manganese atoms, and cobalt atoms include composite oxides, composite hydroxides, composite oxyhydroxides, and composite carbonates containing these atoms.

Note that known methods can be used to prepare compounds containing nickel, manganese, and cobalt atoms. For example, composite hydroxides can be prepared by coprecipitation. Specifically, composite hydroxides can be coprecipitated by mixing an aqueous solution containing predetermined amounts of nickel, cobalt, and manganese atoms with an aqueous solution of a complexing agent and an aqueous alkali solution (see JP-A-10-81521, JP-A-10-81520, JP-A-10-29820, JP-A-2002-201028, etc.). In the case of composite carbonates, examples include a method in which a solution (Liquid A) containing nickel ions, manganese ions, and cobalt ions and a solution (Liquid B) containing carbonate ions or bicarbonate ions are added to a reaction vessel to carry out the reaction (Japanese Patent Laid-Open No. 2009-179545), or a method in which a solution (Liquid A) containing a nickel salt, manganese salt, and cobalt salt and a solution (Liquid B) containing a metal carbonate or metal bicarbonate are added to a solution (Liquid C) containing the same anions as the anions of the nickel salt, manganese salt, and cobalt salt in Liquid A and the same anions as the anions of the metal carbonate or metal bicarbonate in Liquid B to carry out the reaction (Japanese Patent Laid-Open No. 2009-179544). The compound containing nickel atoms, manganese atoms, and cobalt atoms may be commercially available.

The average particle size of the compound containing nickel atoms, cobalt atoms, and manganese atoms, as determined by a laser scattering method, is preferably 0.50 to 100 μm, and more preferably 1.0 to 80.0 μm.

In producing lithium nickel manganese cobalt composite oxide particles represented by general formula (1), it is preferable to use a composite hydroxide containing nickel atoms, cobalt atoms, and manganese atoms as the nickel source, manganese source, and cobalt source, as this will result in better reactivity.

In the raw material mixing process, the lithium source, nickel source, manganese source, cobalt source, and optionally added M source are mixed in such a ratio that the molar ratio of Li atoms (Li/(Ni+Mn+Co+M)) to the total number of moles of Ni atoms, Mn atoms, Co atoms, and M atoms (Ni+Mn+Co+M) in the nickel source, manganese source, and cobalt source is 0.98 to 1.20, and a ratio of 1.00 to 1.10 is particularly preferred, in terms of increasing discharge capacity.

Furthermore, in the raw material mixing step, the mixing ratio of the nickel source, manganese source, cobalt source, and optionally added M source may be adjusted so as to achieve the atomic molar ratio of nickel, manganese, cobalt, and M represented by the general formula (1) above.

The manufacturing history of the raw materials, lithium source, nickel source, manganese source, cobalt source, and M source, is not important, but in order to produce high-purity lithium-nickel-manganese-cobalt composite oxide particles, it is preferable that the impurity content be as low as possible.

In the raw material mixing process, the lithium source, nickel source, manganese source, cobalt source, and optionally the M source can be mixed by either a dry or wet method, but dry mixing is preferred due to ease of production.

When dry mixing, it is preferable to use mechanical means to ensure uniform mixing of the raw materials. Examples of mixing equipment include high-speed mixers, super mixers, turbosphere mixers, Eirich mixers, Henschel mixers, Nauta mixers, ribbon blenders, V-type mixers, conical blenders, jet mills, cosmomizers, paint shakers, bead mills, and ball mills. At the laboratory level, a household mixer is sufficient.

When wet mixing, it is preferable to use a media mill as the mixing device, as this allows for the preparation of a slurry in which the raw materials are uniformly dispersed. Furthermore, it is preferable to spray-dry the slurry after the mixing process, as this provides a highly reactive raw material mixture in which the raw materials are uniformly dispersed.

The calcination process is a process in which the raw material mixture obtained in the raw material mixing process is calcined to obtain lithium nickel manganese cobalt composite oxide.

In the calcination step, the raw material mixture is calcined to react the raw materials at a calcination temperature of 600 to 1000°C, preferably 700 to 950°C. The reason for this is that if the calcination temperature is below 600°C, the reaction tends to be insufficient, leaving a large amount of unreacted lithium, while if the temperature exceeds 1000°C, the lithium nickel manganese cobalt composite oxide that has been produced tends to decompose.

The firing time in the firing step is 3 hours or more, preferably 5 to 30 hours. The firing atmosphere in the firing step is an oxidizing atmosphere of air or oxygen gas.

Furthermore, in the calcination step, calcination may be performed in multiple stages. By performing calcination in multiple stages, lithium nickel manganese cobalt composite oxide particles with even better cycle characteristics can be obtained. When performing calcination in multiple stages, it is preferable to calcinate at a temperature in the range of 650 to 800°C for 1 to 10 hours, then further raise the temperature to 800 to 950°C, which is higher than the calcination temperature, and continue calcining at that temperature for 5 to 30 hours.

The lithium nickel manganese cobalt composite oxide obtained in this manner may be subjected to multiple firing processes as necessary.

A lithium nickel manganese composite oxide having a residual alkali content within the above range can be produced by mixing the lithium source, nickel source, manganese source, cobalt source, and optionally added M source in a raw material mixing step such that the molar ratio of Li atoms to the total number of moles of Ni atoms, Mn atoms, Co atoms, and M atoms (Ni + Mn + Co + M) in the nickel source, manganese source, cobalt source, and M source (Li/(Ni + Mn + Co + M)) is 0.98 to 1.20, and then subjecting the mixture to a calcination reaction at 700°C or higher, preferably 750 to 1000°C, for 3 hours or longer, preferably 5 to 30 hours, to thoroughly react the lithium source, nickel source, manganese source, cobalt source, and optionally added M source. In this production method, by performing the calcination in the multistage manner described above, a lithium nickel manganese cobalt composite oxide with an even reduced residual alkali content can be produced.

The positive electrode active material for a lithium secondary battery of the present invention has a Ti-containing oxide and an Mg-containing oxide attached to at least a portion of the particle surface of the lithium nickel manganese cobalt composite oxide particles represented by general formula (1) that constitute the positive electrode active material for a lithium secondary battery.

In the positive electrode active material for a lithium secondary battery of the present invention, the Ti-containing oxide may be attached to a portion of the particle surface of the lithium nickel manganese cobalt composite oxide particles, or may be attached to cover the entire particle surface of the lithium nickel manganese cobalt composite oxide particles. In the positive electrode active material for a lithium secondary battery of the present invention, the Mg-containing oxide may be attached to a portion of the particle surface of the lithium nickel manganese cobalt composite oxide particles, or may be attached to cover the entire particle surface of the lithium nickel manganese cobalt composite oxide particles. In the positive electrode active material for a lithium secondary battery of the present invention, the Ti-containing oxide is attached to a portion of the particle surface of the lithium nickel manganese cobalt composite oxide particles, and the Mg-containing oxide is attached to a portion of the particle surface of the lithium nickel manganese cobalt composite oxide particles. This results in a positive electrode active material that exhibits little cycle degradation and high energy density retention even when repeatedly charged and discharged at high voltages in a lithium secondary battery.

Examples of the oxide containing titanium in the positive electrode active material for a lithium secondary battery of the present invention include oxides of Ti such as TiO ₂ , Ti ₂ O ₃ , and Ti _n O _2n-1 (n = 3 to 9), and composite oxides containing Ti and one or more elements selected from Mg, Li, Ni, Mn, Co, and M.

In the positive electrode active material for lithium secondary batteries of the present invention, the amount of Ti-containing oxide attached is, in atomic terms, preferably 0.01 to 5.00 mol %, and particularly preferably 0.02 to 4.50 mol %, relative to the total amount (moles) of Ni, Mn, Co, and M in the lithium nickel manganese cobalt composite oxide. By having the amount of Ti-containing oxide attached within this range, both initial capacity and cycle characteristics can be achieved.

In the positive electrode active material for a lithium secondary battery of the present invention, examples of the oxide containing Mg attached to the surface of the lithium nickel manganese cobalt composite oxide particles include oxides of Mg such as MgO and _MgO2 , and composite oxides containing Mg and one or more elements selected from Ti, Li, Ni, Mn, Co, and M.

In the positive electrode active material for lithium secondary batteries of the present invention, the amount of Mg-containing oxide attached is, in atomic terms, preferably 0.01 to 5.00 mol %, and particularly preferably 0.02 to 4.50 mol %, relative to the total amount (moles) of Ni, Mn, Co, and M in the lithium nickel manganese cobalt composite oxide. By having the amount of Mg-containing oxide attached within the above range, it is possible to achieve both desirable initial capacity and cycle characteristics.

In the positive electrode active material for lithium secondary batteries of the present invention, the molar ratio of Ti in the Ti-containing oxide to Mg in the Mg-containing oxide (Ti:Mg), in atomic terms, is preferably 1.0:16.0 to 3.0:2.0, and particularly preferably 1.0:12.0 to 5.0:4.0. By having the molar ratio of Ti in the Ti-containing oxide to Mg in the Mg-containing oxide within the above range, it is possible to achieve both desirable initial capacity and cycle characteristics.

The average particle size of the positive electrode active material for a lithium secondary battery of the present invention is 0.50 to 30.0 μm, preferably 1.0 to 25.0 μm, and particularly preferably 1.5 to 20.0 μm, as the particle size at 50% volume (D50) in the particle size distribution determined by a laser diffraction/scattering method. The BET specific surface area of the positive electrode active material for a lithium secondary battery of the present invention is preferably 0.05 to 2.00 ^{m 2} /g, and particularly preferably 0.15 to 1.00 m ² /g. When the average particle size or BET specific surface area of the positive electrode active material for a lithium secondary battery of the present invention is within the above ranges, the preparation and coating properties of the positive electrode mixture are facilitated, and an electrode with high packing properties can be obtained.

Furthermore, the residual alkali content of the positive electrode active material for lithium secondary batteries of the present invention is preferably 1.20% by mass or less, and particularly preferably 1.00% by mass or less. By ensuring that the residual alkali content of the positive electrode active material for lithium secondary batteries is within this range, it is possible to suppress battery expansion and deterioration caused by gas generation due to the residual alkali.

The residual alkali in the positive electrode active material for a lithium secondary battery of the present invention and the method for measuring it are as described above for the lithium nickel manganese cobalt composite oxide particles. The residual alkali refers to the alkaline component eluted into water when the positive electrode active material for a lithium secondary battery is stirred and dispersed in water at 25°C. The amount of residual alkali is determined by weighing 5 g of the positive electrode active material for a lithium secondary battery and 100 g of pure water into a beaker, dispersing the mixture with a magnetic stirrer at 25°C for 5 minutes, filtering the dispersion, and subjecting the amount of alkali present in the resulting filtrate to neutralization titration. The amount of residual alkali is determined by measuring the amount of lithium by titration and converting it to lithium carbonate.

The positive electrode active material for a lithium secondary battery according to the present invention is preferably a mixture of large particles having an average particle diameter of 7.5 to 30.0 μm and small particles having an average particle diameter of 0.50 to 7.5 μm, in terms of increasing capacity per volume. The average particle diameter of the large particles is 7.5 to 30.0 μm, preferably 8.0 to 25.0 μm, and particularly preferably 8.5 to 20.0 μm. The average particle diameter of the small particles is 0.5 to 7.5 μm, preferably 1.0 to 7.0 μm, and particularly preferably 1.5 to 6.5 μm. The mixing ratio of the large particles to the small particles, by mass, is preferably 7:13 to 19:1, and particularly preferably 1:1 to 9:1. Furthermore, it is preferable that the mixture of large and small particles has a compressed density of 2.7 g/ ^cm3 or more, preferably 2.8 to 3.3 g/ ^cm3 when compressed at 0.65 tonf/ ^cm2 , in terms of increasing the capacity per volume. When the positive electrode active material for a lithium secondary battery according to the present invention is a mixture of large particles and small particles, the mixture can be obtained by separately producing lithium nickel manganese cobalt composite oxide particles (large particles) represented by the general formula (1) having an average particle size of 7.5 to 30.0 μm, preferably 8.0 to 25.0 μm, particularly preferably 8.5 to 20.0 μm, and having an oxide containing Ti and an oxide containing Mg attached to at least a part of the particle surface, and lithium nickel manganese cobalt composite oxide particles (small particles) represented by the general formula (1) having an average particle size of 0.5 to 7.5 μm, preferably 1.0 to 7.0 μm, particularly preferably 1.5 to 6.5 μm, and having an oxide containing Ti and an oxide containing Mg attached to at least a part of the particle surface, and then mixing the resulting large particles and small particles at a predetermined mixing ratio.

The positive electrode active material for a lithium secondary battery according to the present invention may be produced by any production method. However, a preferred method for producing the positive electrode active material for a lithium secondary battery according to the present invention involves dry-mixing lithium nickel manganese cobalt composite oxide particles represented by general formula (1) with a Ti-containing oxide and an Mg-containing oxide, thereby adhering the Ti-containing oxide and Mg-containing oxide to the surfaces of the lithium nickel manganese cobalt composite oxide particles to obtain Ti- and Mg-containing oxide-attached composite oxide particles, and then heat-treating the Ti- and Mg-containing oxide-attached composite oxide particles at a temperature of 400°C or higher but lower than 750°C, preferably 500°C or higher but lower than 700°C, and particularly preferably 550°C or higher but lower than 700°C, to obtain the positive electrode active material for a lithium secondary battery according to the present invention (hereinafter also referred to as the method for producing the positive electrode active material for a lithium secondary battery according to the present invention). This method results in a lithium secondary battery with minimal cycle degradation and a high energy density retention rate.

The lithium-cobalt composite oxide particles represented by general formula (1) used in the method for producing a positive electrode active material for a lithium secondary battery of the present invention are the lithium-nickel-manganese-cobalt composite oxide particles represented by general formula (1) in the positive electrode active material for a lithium secondary battery of the present invention described above before the Ti-containing oxide and the Mg-containing oxide are attached.

Examples of the oxide containing Ti in the method for producing a positive electrode active material for a lithium secondary battery of the present invention include oxides of Ti such as TiO ₂ , Ti ₂ O ₃ , and Ti _n O _2n-1 (n = 3 to 9), and composite oxides containing Ti and one or more elements selected from Mg, Li, Ni, Mn, Co, and M. Of these, oxides of Ti, particularly TiO ₂ , are preferred because of their enhanced effect in improving cycle characteristics.

The average particle size of the Ti-containing oxide, as determined by laser diffraction/scattering, is 100 μm or less, preferably 0.01 to 10.0 μm, as the particle size at 50% of the volume (D50). This allows the Ti-containing oxide to be efficiently attached to the particle surface of the lithium nickel manganese cobalt composite oxide particles.

The Ti-containing oxide may be in the form of aggregates, in which primary particles aggregate to form secondary particles. In the method for producing a positive electrode active material for a lithium secondary battery of the present invention, lithium nickel manganese cobalt composite oxide particles, a Ti-containing oxide, and an Mg-containing oxide are mixed together in a dry process. Therefore, the aggregated Ti-containing oxide is finely broken down during mixing, allowing the finely divided Ti-containing oxide to adhere to the particle surfaces of the lithium nickel manganese cobalt composite oxide particles.

When using aggregated Ti-containing oxides, it is preferable that the primary particle diameter of the Ti-containing oxides be 2.0 μm or less, preferably 0.001 to 1.0 μm, as the average particle diameter of the primary particles determined from scanning electron micrographs, since this allows the Ti-containing oxides to be efficiently attached to the particle surfaces of the lithium nickel manganese cobalt composite oxide particles.

Examples of oxides containing Mg in the method for producing a positive electrode active material for a lithium secondary battery of the present invention include oxides of Mg such as MgO and _MgO2 , and composite oxides containing Mg and one or more elements selected from Ti, Li, Ni, Mn, Co, and M.

The average particle size of the Mg-containing oxide, as determined by laser diffraction/scattering, is 100 μm or less, preferably 0.01 to 10.0 μm, as the particle size at 50% of the volume (D50). This allows the Mg-containing oxide to be efficiently attached to the particle surface of the lithium nickel manganese cobalt composite oxide particles.

The Mg-containing oxide may be in the form of aggregates, where primary particles aggregate to form secondary particles. In the method for producing a positive electrode active material for a lithium secondary battery of the present invention, lithium nickel manganese cobalt composite oxide particles, an Mg-containing oxide, and a Ti-containing oxide are mixed together in a dry process. Therefore, the aggregated Mg-containing oxide is finely broken down during mixing, allowing the finely divided Mg-containing oxide to adhere to the particle surfaces of the lithium nickel manganese cobalt composite oxide particles.

When using an aggregated Mg-containing oxide, the primary particle size of the Mg-containing oxide, as determined from a scanning electron microscope photograph, is preferably 2.0 μm or less, and preferably 0.001 to 1.0 μm, in order to efficiently adhere the Mg-containing oxide to the particle surface of the lithium nickel manganese cobalt composite oxide particles.

In the method for producing a positive electrode active material for a lithium secondary battery of the present invention, the amount of Ti-containing oxide mixed into the lithium nickel manganese cobalt composite oxide particles is preferably 0.01 to 5.00 mol %, and more preferably 0.02 to 4.50 mol %, in terms of atomic conversion, relative to the total amount (moles) of Ni, Mn, Co, and M in the lithium nickel manganese cobalt composite oxide particles, as this allows for achieving both desirable initial capacity and cycle characteristics.

In the method for producing a positive electrode active material for a lithium secondary battery of the present invention, the amount of oxide containing Mg mixed into the lithium nickel manganese cobalt composite oxide particles is preferably 0.01 to 5.00 mol %, and more preferably 0.02 to 4.50 mol %, in terms of atomic conversion, relative to the total amount (moles) of Ni, Mn, Co, and M in the lithium nickel manganese cobalt composite oxide particles, as this allows for achieving both desirable initial capacity and cycle characteristics.

In the method for producing a positive electrode active material for a lithium secondary battery of the present invention, the mixing ratio of the Ti-containing oxide and the Mg-containing oxide is such that the molar ratio of Ti in the Ti-containing oxide to Mg in the Mg-containing oxide (Ti:Mg) is, in atomic terms, 1.0:16.0 to 3.0:2.0, preferably 1.0:12.0 to 5.0:4.0, which is preferable in terms of achieving a balance between initial capacity and cycle characteristics within preferred ranges.

Then, by dry mixing the lithium nickel manganese cobalt composite oxide particles represented by general formula (1), an oxide containing Ti, and an oxide containing Mg, the oxide containing Ti and the oxide containing Mg are adhered to the particle surfaces of the lithium nickel manganese cobalt composite oxide particles represented by general formula (1), thereby obtaining composite oxide particles with an oxide containing Ti and an oxide containing Mg adhered thereto, i.e., lithium nickel manganese cobalt composite oxide particles represented by general formula (1) in which at least a portion of the particle surface is coated with an oxide containing Ti and an oxide containing Mg.

Equipment used in the mixing process includes, for example, a high-speed mixer, super mixer, turbosphere mixer, Henschel mixer, Nauta mixer, ribbon blender, V-type mixer, etc. However, these mixing operations are not limited to the mechanical means exemplified above. Furthermore, at the laboratory level, a household mixer or laboratory mill is also sufficient.

In the method for producing a positive electrode active material for a lithium secondary battery of the present invention, the composite oxide particles having an oxide containing Ti and an oxide containing Mg attached thereto are then heat-treated at a temperature of 400°C or higher but lower than 750°C, preferably at a temperature of 500°C or higher but lower than 700°C, and particularly preferably at a temperature of 550°C or higher but lower than 700°C. This heat treatment allows the oxide containing Ti and the oxide containing Mg to be firmly attached to the particle surfaces of the lithium nickel manganese cobalt composite oxide particles.

In the method for producing a positive electrode active material for a lithium secondary battery of the present invention, the heat treatment time is not critical, and a heat treatment time of at least 1 hour, preferably 2 to 10 hours, will generally yield a positive electrode active material for a lithium secondary battery with satisfactory performance. The heat treatment atmosphere is preferably an oxidizing atmosphere such as air or oxygen gas.

The lithium secondary battery of the present invention uses the positive electrode active material for lithium secondary batteries of the present invention as the positive electrode active material. The lithium secondary battery of the present invention comprises a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte containing a lithium salt.

The positive electrode of the lithium secondary battery of the present invention is formed, for example, by applying a positive electrode mixture to a positive electrode current collector and drying the mixture. The positive electrode mixture comprises a positive electrode active material, a conductive agent, a binder, and, if necessary, a filler. The lithium secondary battery of the present invention has the positive electrode uniformly coated with the positive electrode active material for lithium secondary batteries of the present invention. As a result, the lithium secondary battery of the present invention has high battery performance, and in particular, excellent cycle characteristics.

The content of the positive electrode active material contained in the positive electrode mixture for the lithium secondary battery of the present invention is preferably 70 to 100 mass%, and particularly preferably 90 to 98 mass%.

The positive electrode current collector for the lithium secondary battery of the present invention is not particularly limited as long as it is an electronic conductor that does not undergo chemical changes in the constructed battery. Examples include stainless steel, nickel, aluminum, titanium, calcined carbon, and aluminum or stainless steel surfaces that have been surface-treated with carbon, nickel, titanium, or silver. These materials may be used with their surfaces oxidized, or the current collector surface may be roughened by surface treatment. The current collector may be in the form of, for example, foil, film, sheet, net, punched, lath, porous, foam, fiber group, or nonwoven fabric. The thickness of the current collector is not particularly limited, but a thickness of 1 to 500 μm is preferred.

The conductive agent for the lithium secondary battery of the present invention is not particularly limited as long as it is an electronically conductive material that does not undergo chemical changes in the constructed battery. Examples include graphite such as natural graphite and artificial graphite; carbon blacks such as carbon black, 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 carbon fluoride, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. Examples of natural graphite include scaly graphite, flake graphite, and amorphous graphite. These can be used alone or in combination. The conductive agent is blended in an amount of 1 to 50% by mass, preferably 2 to 30% by mass, of the positive electrode mixture.

Examples of binders for the lithium secondary battery of the present invention include starch, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, regenerated cellulose, diacetyl cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber, fluororubber, tetrafluoroethylene-hexafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-perfluoroalkyl Examples of the polymerizable compound include vinyl ether copolymers, vinylidene fluoride-hexafluoropropylene copolymers, vinylidene fluoride-chlorotrifluoroethylene copolymers, ethylene-tetrafluoroethylene copolymers, polychlorotrifluoroethylene, vinylidene fluoride-pentafluoropropylene copolymers, propylene-tetrafluoroethylene copolymers, ethylene-chlorotrifluoroethylene copolymers, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymers, vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene copolymers, ethylene-acrylic acid copolymers or (Na + ) ion crosslinked products thereof, ethylene-methacrylic acid copolymers or (Na ⁺ ) ion crosslinked products thereof, ethylene-methyl acrylate copolymers or (Na ⁺ ) ion crosslinked products thereof, ethylene-methyl methacrylate copolymers or ( ^{Na + )} ion crosslinked products thereof, polysaccharides such as polyethylene oxide, thermoplastic resins, and polymers having rubber elasticity, and these can be used alone or in combination of two or more. When a compound containing a functional group that reacts with lithium, such as a polysaccharide, is used, it is preferable to deactivate the functional group by adding a compound such as an isocyanate group. The blending ratio of the binder in the positive electrode mixture is 1 to 50 mass %, preferably 5 to 15 mass %.

The filler in the lithium secondary battery of the present invention suppresses volume expansion of the positive electrode in the positive electrode mixture and is added as needed. Any fibrous material that does not undergo chemical changes in the constructed battery can be used as the filler, including fibers of olefin polymers such as polypropylene and polyethylene, glass, and carbon. The amount of filler added is not particularly limited, but is preferably 0 to 30% by mass in the positive electrode mixture.

The negative electrode of the lithium secondary battery of the present invention is formed by applying a negative electrode material to a negative electrode current collector and drying the applied material. The negative electrode current collector of the lithium secondary battery of the present invention is not particularly limited as long as it is an electronic conductor that does not undergo chemical changes in the constructed battery. Examples include stainless steel, nickel, copper, titanium, aluminum, calcined carbon, copper or stainless steel surfaces surface-treated with carbon, nickel, titanium, or silver, and aluminum-cadmium alloys. These materials may also be used with their surfaces oxidized, or the current collector surface may be roughened by surface treatment. Examples of the current collector form include foil, film, sheet, net, punched, lath, porous, foam, fiber clusters, and nonwoven fabric molded bodies. The thickness of the current collector is not particularly limited, but a thickness of 1 to 500 μm is preferred.

The negative electrode material for the lithium secondary battery of the present invention is not particularly limited, and examples thereof include carbonaceous materials, metal composite oxides, lithium metal, lithium alloys, silicon-based alloys, tin-based alloys, metal oxides, conductive polymers, chalcogen compounds, Li—Co—Ni-based materials, Li ₄ Ti ₅ O ₁₂ , lithium niobate, silicon oxide (SiOx: 0.5≦x≦1.6), etc. Examples of carbonaceous materials include non-graphitizable carbon materials and graphite-based carbon materials. Examples of metal composite oxides include compounds such as Sn _p (M ¹ ) _1-p (M ² ) _q O _r (wherein M ¹ represents one or more elements selected from Mn, Fe, Pb, and Ge, and M ² represents one or more elements selected from Al, B, P, Si, Group 1, Group 2, and Group 3 of the periodic table, and halogen elements, and 0<p≦1, 1≦q≦3, and 1≦r≦8), Li _t Fe ₂ O ₃ (0≦t≦1), and Li _t WO ₂ (0≦t≦1). Examples of metal oxides include GeO, GeO ₂ , SnO, SnO ₂ , PbO, PbO ₂ , Pb ₂ O ₃ , Pb ₃ O ₄ , Sb ₂ O ₃ , Sb ₂ O ₄ , Sb ₂ O ₅ , Bi ₂ O ₃ , Bi ₂ O ₄ , and Bi ₂ O _5. Examples of conductive polymers include polyacetylene and poly-p-phenylene.

The separator used in the lithium secondary battery of the present invention is an insulating thin film with high ion permeability and a predetermined mechanical strength. Sheets or nonwoven fabrics made from olefin polymers such as polypropylene, glass fibers, or polyethylene are used because of their resistance to organic solvents and hydrophobicity. The pore size of the separator may be within a range generally useful for batteries, for example, 0.01 to 10 μm. The thickness of the separator may be within a range generally used for batteries, for example, 5 to 300 μm. Note that when a solid electrolyte such as a polymer is used as the electrolyte described below, the solid electrolyte may also serve as the separator.

The lithium salt-containing non-aqueous electrolyte for the lithium secondary battery of the present invention comprises a non-aqueous electrolyte and a lithium salt. The non-aqueous electrolyte for the lithium secondary battery of the present invention may be a non-aqueous electrolytic solution, an organic solid electrolyte, or an inorganic solid electrolyte. Examples of non-aqueous electrolytes include aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 3-methyl-2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, diethyl ether, 1,3-propane sultone, methyl propionate, and ethyl propionate, either alone or in combination.

Examples of organic solid electrolytes for the lithium secondary battery of the present invention include polyethylene derivatives, polyethylene oxide derivatives or polymers containing same, polypropylene oxide derivatives or polymers containing same, phosphate ester polymers, polymers containing ionically dissociable groups such as polyphosphazene, polyaziridine, polyethylene sulfide, polyvinyl alcohol, polyvinylidene fluoride, and polyhexafluoropropylene, and mixtures of polymers containing ionically dissociable groups with the above-mentioned nonaqueous electrolyte solutions.

As the inorganic solid electrolyte for the lithium secondary battery of the present invention, a nitride, halide, oxyacid salt, sulfide, etc. of Li can be used, and examples thereof include Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N—LiI—LiOH, LiSiO ₄ , LiSiO ₄ —LiI—LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Li ₄ SiO ₄ —LiI—LiOH, P ₂ S ₅ , Li ₂ S or Li ₂ S—P ₂ S ₅ , Li ₂ S—SiS ₂ , Li ₂ S—GeS ₂ , Li ₂ S—Ga ₂ S ₃ , Li ₂ S—B ₂ S ₃ , Li ₂ S—P ₂ S ₅ -X, Li _2S —SiS ₂ —X, Li _2S —GeS ₂ —X, Li _2S —Ga _{2S 3} —X, Li _2S —B _{2S 3} —X (wherein X is at least one selected from LiI, B _{2S 3} , and Al _{2S 3} ), and the like.

Furthermore, when the inorganic solid electrolyte is amorphous (glass), the inorganic solid electrolyte can contain oxygen-containing compounds such as lithium phosphate (Li ₃ PO ₄ ), lithium oxide (Li ₂ O), lithium sulfate (Li ₂ SO ₄ ), phosphorus oxide (P ₂ O ₅ ), and lithium borate (Li ₃ BO ₃ ), and nitrogen-containing compounds such as Li ₃ PO _{4 -u N 2u /3} (u is 0<u<4), Li ₄ SiO _4-u N _2u/3 (u is 0<u<4), Li 4 GeO _4-u N _2u/3 (u is 0<u<4), and Li ₃ BO 3-u N 2u/3 (u is 0<u<3). The addition of the oxygen-containing compound or nitrogen-containing compound widens the gaps in the amorphous framework that is formed, reducing the hindrance to the movement of lithium ions, and further improving ion conductivity.

The lithium salt used in the lithium secondary battery of the present invention is one that dissolves in the non-aqueous electrolyte, and examples thereof include LiCl, LiBr, LiI, _LiClO4 , _LiBF4 , _LiB10Cl10 , _LiPF6 , LiCF3SO3, _LiCF3CO2 , _LiAsF6 , _LiSbF6 , _{LiB10Cl10 , LiAlCl4} , _CH3SO3Li , _CF3SO3Li , ( _{CF3SO2 ) 2NLi} , lithium chloroborane, _lithium lower aliphatic carboxylate, lithium tetraphenylborate, imides _, and the like _, or _a mixture of two _or more of _these .

The following compounds can be added to the non-aqueous electrolyte to improve discharge and charge characteristics and flame retardancy. Examples include pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones and N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, polyethylene glycol, pyrrole, 2-methoxyethanol, aluminum trichloride, conductive polymer electrode active material monomers, triethylenephosphonamide, trialkylphosphine, morpholine, aryl compounds with carbonyl groups, hexamethylphosphoric triamide and 4-alkylmorpholine, bicyclic tertiary amines, oils, phosphonium salts and tertiary sulfonium salts, phosphazene, and carbonate esters. Furthermore, halogen-containing solvents, such as carbon tetrachloride and ethylene trifluoride, can be added to the electrolyte to make it nonflammable. Carbon dioxide can also be added to the electrolyte to improve its suitability for high-temperature storage.

The lithium secondary battery of the present invention is a lithium secondary battery that exhibits little cycle deterioration even when repeatedly charged and discharged, particularly at high voltages, and has a high energy density retention rate.The battery may be in any shape, such as button, sheet, cylinder, prism, or coin shape.

The uses of the lithium secondary battery of the present invention are not particularly limited, but examples include electronic devices such as notebook computers, laptop computers, pocket word processors, mobile phones, cordless handsets, portable CD players, radios, LCD televisions, backup power supplies, electric shavers, memory cards, and video camcorders, as well as consumer electronic devices such as automobiles, electric vehicles, drones, game devices, and power tools.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.
<Preparation of Lithium Nickel Manganese Cobalt Composite Oxide Particles (LNMC) Sample>
<LNMC Sample 1>
Lithium carbonate (average particle size 5.7 μm) and nickel manganese cobalt composite hydroxide (Ni:Mn:Co=6:2:2 (molar ratio), average particle size 9.8 μm) were weighed and thoroughly mixed in a household mixer to obtain a raw material mixture with a Li/(Ni+Mn+Co) molar ratio of 1.01. Note that a commercially available nickel manganese cobalt composite hydroxide was used.
The resulting raw material mixture was then fired in an alumina pot at 700°C for 2 hours, followed by 10 hours at 850°C in an air atmosphere. After firing, the fired product was crushed and classified. XRD analysis of the resulting fired product confirmed that it was a single-phase lithium nickel manganese cobalt composite oxide. The resulting product had an average particle size of 10.2 μm, a BET specific surface area of 0.21 ^m2 /g, and was agglomerated spherical lithium _nickel manganese cobalt composite oxide particles _{( LiNi0.6Mn0.2Co0.2O2 )} .

<LNMC Sample 2>
Lithium carbonate (average particle size 5.7 μm) and nickel manganese cobalt composite hydroxide (Ni:Mn:Co=6:2:2 (molar ratio), average particle size 3.7 μm) were weighed and thoroughly mixed in a household mixer to obtain a raw material mixture with a Li/(Ni+Mn+Co) molar ratio of 1.01. Note that a commercially available nickel manganese cobalt composite hydroxide was used.
The resulting raw material mixture was then fired in an alumina pot at 700°C for 2 hours, followed by 10 hours at 850°C in an air atmosphere. After firing, the fired product was crushed and classified. XRD analysis of the resulting fired product confirmed that it was a single-phase lithium nickel manganese cobalt composite oxide. The resulting product had an average particle size of 5.4 μm, a BET specific surface area of 0.69 ^m2 /g, and was agglomerated spherical lithium _nickel manganese cobalt composite oxide particles _{( LiNi0.6Mn0.2Co0.2O2 )} .

The physical properties of the lithium nickel manganese cobalt composite oxide sample (LNMC sample) obtained above are shown in Table 1.
The average particle size, residual alkali content and pressed density of the LMNC samples were measured as follows.
<Average particle diameter>
It was determined by laser diffraction and scattering method.
<Measurement of remaining alkali amount>
5 g of the sample and 100 g of ultrapure water were weighed into a beaker and dispersed using a magnetic stirrer for 5 minutes at 25° C. Next, this dispersion was filtered, and 70 ml of the filtrate was titrated with 0.1 N-HCl using an automatic titrator (model COMTITE-2500), and the amount of alkali remaining in the sample (a value obtained by measuring the amount of lithium and converting it into lithium carbonate) was calculated.
<Compression density>
2.25 g of the sample was weighed and placed in a biaxial molding machine with a diameter of 1.5 cm, and a pressure of 0.65 tonf/ ^cm2 was applied for 1 minute using a press. The height of the compressed product was measured, and the compressed density of the sample was calculated from the apparent volume of the compressed product calculated from the height and the mass of the sample.

Example 1
29.8 g of LNMC sample 1 was taken, to which 0.126 g of magnesium oxide (MgO) and 0.0622 g of titanium oxide (TiO ₂ ) were added, and the mixture was thoroughly mixed in a laboratory mill to obtain TiO ₂ and MgO-attached composite oxide particles. The resulting TiO ₂ and MgO-attached composite oxide particles were then fired at 600°C for 5 hours, followed by heat treatment, pulverization, and classification to obtain a cathode active material sample (modified LNMC sample) with magnesium oxide and titanium oxide attached. The resulting cathode active material had an average particle diameter (D50) of 10.2 μm and a BET specific surface area of 0.30 m ² /g.
The residual alkali content and pressed density were also measured in the same manner as for the LNMC sample, and the results are shown in Table 2.
Furthermore, the particle surfaces of the obtained positive electrode active material sample were subjected to elemental mapping analysis of Ti and Mg using SEM-EDX (a field emission scanning electron microscope SU-8220 manufactured by Hitachi High-Technologies Corporation and an energy dispersive X-ray analyzer XFlash5060FlatQUAD manufactured by BRUKER) at a magnification of 20,000 times, and it was confirmed that Ti and Mg were unevenly distributed on the particle surfaces of LNMC sample 1, and that Ti and Mg were present only on part of the particle surfaces.
The measurement conditions for SEM-EDX are as follows:
Acceleration voltage: 15 kV, magnification: 20,000x, working distance: 9.5 to 11.5 mm, measurement time: 6 minutes. The magnesium oxide used was an aggregate consisting of secondary particles formed by aggregation of primary particles. The particle diameter (D50) of the aggregate, converted into 50% by volume, was measured using a laser diffraction/scattering method and found to be 3.3 μm. The average particle diameter of the primary particles determined from SEM photographs was 0.22 μm.
The titanium oxide used was an aggregate of secondary particles formed by aggregation of primary particles. The particle size (D50) calculated as 50% by volume as determined by a laser diffraction/scattering method was 0.38 μm, and the average particle size of the primary particles as determined by SEM photography was 0.035 μm.
The average particle diameter of the primary particles was determined by randomly selecting 30 particles from the scanning electron microscope, measuring the minor axis and major axis of each particle, calculating half of the sum of the two, and averaging the values for the 30 particles to obtain the average particle diameter.

Example 2
29.7 g of LNMC sample 2 was collected, to which 0.126 g of magnesium oxide (MgO) and 0.144 g of titanium oxide (TiO ₂ ) were added, and the mixture was thoroughly mixed in a laboratory mill to obtain TiO ₂ and MgO-attached composite oxide particles. The resulting TiO ₂ and MgO-attached composite oxide particles were then fired at 600°C for 5 hours, heat-treated, pulverized, and classified to obtain a cathode active material sample (modified LNMC sample) with magnesium oxide and titanium oxide attached. The average particle diameter (D50) of the resulting cathode active material was 3.8 μm, and the BET specific surface area was 0.87 m ² /g. The residual alkali content and pressed density were also measured using the same method as for the LNMC sample. The results are shown in Table 2.
Furthermore, the particle surfaces of the sample particles of the positive electrode active material sample obtained in the same manner as in Example 1 were subjected to elemental mapping analysis of Ti and Mg using SEM-EDX (a field emission scanning electron microscope SU-8220 manufactured by Hitachi High-Technologies Corporation and an energy dispersive X-ray analyzer XFlash5060FlatQUAD manufactured by BRUKER) at a magnification of 20,000 times, and it was confirmed that Ti and Mg were unevenly distributed on the particle surfaces of LNMC sample 2, and that Ti and Mg were present only on part of the particle surfaces.

(Reference example 1)
29.8 g of commercially available lithium cobalt oxide ( _LiCoO2 : average particle size 9.1 μm, BET specific surface area 0.38 ^m2 /g) was collected, 0.0616 g of magnesium oxide (MgO) and 0.124 g of titanium oxide ( _TiO2 ) were added, and the mixture was thoroughly mixed in a laboratory mill to obtain _TiO2 and MgO-attached composite oxide particles. The obtained _TiO2 and MgO-attached composite oxide particles were then calcined at 600°C for 5 hours, followed by heat treatment, pulverization, and classification to obtain a modified LCO sample with attached magnesium oxide and titanium oxide.
The amount of residual alkali in the modified LCO was also measured in the same manner as in the LNMC sample. The results are shown in Table 2.
Furthermore, the modified LCO sample was subjected to SEM-EDX analysis to map Ti atoms and Mg atoms on the particle surfaces, and it was confirmed that Ti and Mg were present on part of the particle surfaces of the modified LCO sample.

1) The deposition weights in Examples 1 and 2 were calculated as the atomic amounts of Mg and Ti relative to the total amount of Ni, _Mn , Co, and M in the LNMC sample, which was determined from the amounts of TiO2 and MgO added.
2) The deposition amount in Reference Example 1 was calculated as the amount of Mg and Ti in atomic terms relative to the amount of Co in atomic terms in the LCO sample, which was determined from the amounts of TiO ₂ and MgO added.
3) "-" indicates not measured.

The battery performance test was carried out as follows.
<Preparation of Lithium Secondary Battery 1>
A positive electrode agent was prepared by mixing 95% by mass of the modified LNMC sample obtained in the example, 2.5% by mass of graphite powder, and 2.5% by mass of polyvinylidene fluoride, and this was dispersed in N-methyl-2-pyrrolidinone to prepare a kneaded paste. The kneaded paste was applied to aluminum foil, dried, pressed, and punched into a disk with a diameter of 15 mm to obtain a positive electrode plate.

A coin-type lithium secondary battery was fabricated using this positive electrode plate, a separator, a negative electrode, a positive electrode, a current collector, mounting hardware, external terminals, an electrolyte, etc. Metallic lithium foil was used for the negative electrode, and 1 mol of _LiPF6 dissolved in 1 liter of a 1:1 mixture of ethylene carbonate and methyl ethyl carbonate was used as the electrolyte.
Next, the performance of the obtained lithium secondary battery was evaluated. The results are shown in Table 3. Lithium secondary batteries were fabricated in the same manner using LNMC sample 1 (Comparative Example 1), LNMC sample 2 (Comparative Example 2), and modified LCO (Reference Example 1) as the positive electrode active material, and similar evaluations were performed. The results are shown in Table 3.

<Battery performance evaluation 1>
The produced coin-type lithium secondary battery was operated at room temperature under the following test conditions, and the following battery performance was evaluated.
(1) Test Conditions for Cycle Performance Evaluation First, the battery was charged to 4.3 V at 0.5 C over 2 hours, and then constant current/constant voltage charging (CCCV charging) was performed, in which the voltage was maintained at 4.3 V for 3 hours. Subsequently, the battery was charged and discharged at a constant current (CC discharge) at 0.2 C to 2.7 V. This cycle constituted one cycle, and the discharge capacity was measured for each cycle. This cycle was repeated 20 times. Charge and discharge characteristics of lithium secondary batteries using the positive electrode active material samples obtained in Examples 1 and 2 and Comparative Examples 1 and 2 are shown in Figures 1 to 4, respectively.
(2) Initial charge capacity, initial discharge capacity (per weight of active material)
The charge capacity up to 4.3 V and the discharge capacity up to 2.7 V in the first cycle were defined as the initial charge capacity and the initial discharge capacity.
(3) Discharge capacity at 20th cycle (per weight of active material)
The discharge capacity up to 2.7 V at the 20th cycle was defined as the discharge capacity at the 20th cycle.
(4) Capacity Retention Rate The capacity retention rate was calculated from the discharge capacity (per weight of active material) up to 3.5 V in the first and 20th cycles in the cycle characteristic evaluation using the following formula.
Capacity retention rate (%) = (discharge capacity up to 3.5 V at 20th cycle/discharge capacity up to 3.5 V at 1st cycle) × 100
(5) Energy Density Retention Rate The energy density retention rate was calculated from the Wh capacity (per active material weight) during discharge to 3.5 V in the first and 20th cycles in the cycle characteristic evaluation, using the following formula.
Energy density maintenance rate (%) = (discharge Wh capacity up to 3.5 V in the 20th cycle/discharge Wh capacity up to 3.5 V in the 1st cycle) × 100

<Preparation of Lithium Secondary Battery 2>
The modified LNMC samples obtained in Examples 1 and 2 and the LNMC sample before modification were thoroughly mixed in a household mixer to prepare mixtures having the compositions shown in Table 4, which were used as positive electrode active material samples. The pressed densities of the positive electrode active material samples were measured in the same manner as for the LNMC samples, and the results are shown in Table 4.

95% by mass of the positive electrode active material sample, 2.5% by mass of graphite powder, and 2.5% by mass of polyvinylidene fluoride were mixed to form a positive electrode agent, which was then dispersed in N-methyl-2-pyrrolidinone to prepare a kneaded paste. The kneaded paste was applied to aluminum foil, dried, pressed, and punched into a 15 mm diameter disk to obtain a positive electrode plate.

A coin-type lithium secondary battery was fabricated using this positive electrode plate, a separator, a negative electrode, a positive electrode, a current collector, mounting hardware, external terminals, an electrolyte, etc. Metallic lithium foil was used for the negative electrode, and 1 mol of _LiPF6 dissolved in 1 liter of a 1:1 mixture of ethylene carbonate and methyl ethyl carbonate was used as the electrolyte.
The performance of the resulting lithium secondary battery was then evaluated, and the results are shown in Table 5.

<Battery performance evaluation 2>
The prepared coin-type lithium secondary batteries were operated at room temperature under the following test conditions, and the cycle characteristics were evaluated, including the initial charge capacity, initial discharge capacity (per weight of active material), charge capacity at the 20th cycle, discharge capacity (per weight of active material) at the 20th cycle, capacity retention rate, and energy density retention rate, in the same manner as in Battery Performance Evaluation 1. Furthermore, the discharge capacity per volume was also evaluated, and the results are shown in Table 5. The modified LNMC samples of Examples 1 and 2 were used as positive electrode active material samples, and evaluations were carried out in the same manner. The results are shown in Table 5.
(6) Discharge Capacity per Volume The discharge capacity per volume was calculated from the initial discharge capacity and the electrode density using the following formula.
Discharge capacity per volume (mAh/cm ³ )=Discharge capacity at first cycle (mAh/g)×Electrode density (g/cm ³ )×0.95 (proportion of active material amount in coating agent)
The electrode density was calculated by measuring the mass and thickness of the electrode prepared from the sample to be measured, and subtracting the thickness and mass of the current collector from this to obtain the density of the positive electrode material.
The positive electrode material was a mixture of 95 mass % of the positive electrode active material sample, 2.5 mass % of graphite powder, and 2.5 mass % of polyvinylidene fluoride, and the pressing pressure during electrode preparation was 0.38 ton/cm in linear pressure.

Claims (10)

The particle has a Ti-containing oxide and a Mg-containing oxide attached to at least a part of its surface, and the particle has a structure represented by the following general formula (1):
Li _x Ni _y Mn _z Co _t M _p O _1+x (1)
(In the formula, M represents one or more metal elements selected from Mg, Al, Ti, Zr, Cu, Fe, Sr, Ca, V, Mo, Bi, Nb, Si, Zn, Ga, Ge, Sn, Ba, W, Na, and K. x represents 0.98≦x≦1.20, y represents 0.60≦y<1.00, z represents 0<z≦0.50, t represents 0<t≦0.50, p represents 0≦p≦0.05, and y+z+t+p=1.)
a lithium-nickel-manganese-cobalt composite oxide particle represented by the formula (I) above, wherein the lithium-nickel-manganese -cobalt composite oxide particle is an agglomerated particle, the Ti-containing oxide is TiO2, and the Mg-containing oxide is MgO _. The positive electrode active material for a lithium secondary battery according to claim 1, characterized in that the amount of the Ti-containing oxide attached is, in atomic terms, 0.01 to 5.00 mol % as Ti relative to the total amount of Ni, Mn, Co, and M in the lithium nickel manganese cobalt composite oxide particles. The positive electrode active material for a lithium secondary battery according to claim 1 or 2, characterized in that the amount of the oxide containing Mg attached is, in atomic terms, 0.01 to 5.00 mol % as Mg relative to the total amount of Ni, Mn, Co, and M in the lithium nickel manganese cobalt composite oxide particles. The positive electrode active material for a lithium secondary battery according to any one of claims 1 to 3, characterized in that the molar ratio (Ti:Mg) of Ti in the Ti-containing oxide to Mg in the Mg-containing oxide, in atomic terms, is 1.0:16.0 to 3.0:2.0. The positive electrode active material for a lithium secondary battery according to any one of claims 1 to 4, characterized in that the residual alkali content is 1.20 mass% or less. The positive electrode active material for a lithium secondary battery according to any one of claims 1 to 5, characterized in that it is a mixture of large particles having an average particle diameter of 7.5 to 30.0 μm and small particles having an average particle diameter of 0.50 to 7.5 μm. The positive electrode active material for a lithium secondary battery according to claim 6, characterized in that the mixing ratio of the large particles to the small particles is 7:13 to 19:1 by mass. 8. The positive electrode active material for a lithium secondary battery according to claim 6, wherein the mixture has a compressed density of 2.7 g/cm ^<3> or more when compressed at 0.65 tonf/cm ^<2> . The following general formula (1):
Li _x Ni _y Mn _z Co _t M _p O _1+x (1)
(In the formula, M represents one or more metal elements selected from Mg, Al, Ti, Zr, Cu, Fe, Sr, Ca, V, Mo, Bi, Nb, Si, Zn, Ga, Ge, Sn, Ba, W, Na, and K. x represents 0.98≦x≦1.20, y represents 0.60≦y<1.00, z represents 0<z≦0.50, t represents 0<t≦0.50, p represents 0≦p≦0.05, and y+z+t+p=1.)
and a Ti-containing oxide and an Mg-containing oxide are attached to the surface of the lithium nickel manganese cobalt composite oxide particles to obtain Ti- and Mg-containing oxide-attached composite oxide particles, and then the Ti- and Mg-containing oxide-attached composite oxide particles are heat-treated at a temperature of 400°C or higher and lower than 750°C. The lithium nickel manganese cobalt composite oxide particles represented by general formula (1) are agglomerated particles obtained by a raw material mixing step of mixing a lithium source, a nickel source, a manganese source, a cobalt source, and an M source, which is added as needed, to prepare a raw material mixture, and then a firing step of firing the resulting raw material mixture at 700 to 950°C. The Ti-containing oxide is TiO2 _, and the Mg-containing oxide is MgO . A lithium secondary battery characterized by using the positive electrode active material for lithium secondary batteries described in any one of claims 1 to 8.