Positive electrode active material for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

Abstract

This positive electrode active material for nonaqueous electrolyte secondary batteries contains: a lithium transition metal composite oxide having secondary particles, each of which is formed of aggregated primary particles; and a surface modification layer which is formed on the surface of each primary particle of the lithium transition metal composite oxide. The lithium transition metal composite oxide contains at least Al and 80% by mole or more of Ni relative to the total number of moles of the metal elements excluding Li; and the surface modification layer contains at least Ba, and at least one of Sr and Ca.

Description

[Technical Field]

[0001] The present disclosure relates to a positive electrode active material for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery. [Background technology]

[0002] In recent years, non-aqueous electrolyte secondary batteries have been widely used as high-power, high-capacity secondary batteries. These batteries include a positive electrode, a negative electrode, and a non-aqueous electrolyte, and are charged and discharged by transferring lithium ions between the positive electrode and the negative electrode. From the viewpoint of reducing the resistance and increasing the capacity of batteries, there is a demand for improving the properties of the positive electrode active material contained in the positive electrode of the battery.

[0003] For example, Patent Document 1 discloses an NCA (nickel-cobalt-aluminum) positive electrode active material having a layered structure and containing a small amount of alkaline earth metal as a solid solution, as a positive electrode active material for high-power applications. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5245210 Summary of the Invention

[0005] One possible design for the lithium transition metal composite oxide contained in the positive electrode active material is to increase the Ni content to obtain a high discharge capacity. However, if the ratio of Ni to the total number of moles of metal elements excluding Li is 80 mol % or more, the layered structure of the lithium transition metal composite oxide becomes unstable, and the battery capacity may decrease with charge / discharge cycles. The technology in Patent Document 1 does not take into account the decrease in battery capacity with charge / discharge cycles, and there is still room for improvement.

[0006] A positive electrode active material for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a lithium transition metal composite oxide having secondary particles formed by aggregation of primary particles, and a surface modification layer formed on the surfaces of the primary particles of the lithium transition metal composite oxide, wherein the lithium transition metal composite oxide contains at least 80 mol % or more of Ni and Al relative to the total number of moles of metal elements excluding Li, and the surface modification layer contains at least one of Sr and Ca, and at least Ba.

[0007] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is characterized by including a positive electrode containing the above-described positive electrode active material for a non-aqueous electrolyte secondary battery, a negative electrode, and a non-aqueous electrolyte.

[0008] According to the positive electrode active material for a nonaqueous electrolyte secondary battery according to one aspect of the present disclosure, a nonaqueous electrolyte secondary battery having high output and improved charge / discharge cycle characteristics can be provided. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] The layered structure of a lithium transition metal composite oxide includes a transition metal layer (e.g., Ni), a Li layer, and an oxygen layer. The reversible movement of Li ions from the Li layer promotes the charge / discharge reaction of the battery. When the ratio of Ni to the total moles of metal elements excluding Li in the lithium transition metal composite oxide contained in the positive electrode active material is 80 mol % or more, many Li ions are extracted from the Li layer during battery charging, which can destabilize the layered structure. A modified layer forms on the surface of the lithium transition metal composite oxide with an unstable layered structure due to reaction with the electrolyte. Further structural changes of the lithium transition metal composite oxide occur from the modified layer, resulting in a gradual decrease in battery capacity with charge / discharge. However, by incorporating a predetermined amount of at least one of Sr and Ca into the surface modification layer formed on the surface of the lithium transition metal composite oxide, as in the positive electrode active material for a nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure, the synergistic effect with Al suppresses the reaction with the electrolyte at the surface and stabilizes the surface structure, thereby suppressing the decrease in battery capacity with charge / discharge cycles. Since Al does not change its oxidation state during charge and discharge, its inclusion in the transition metal layer is thought to stabilize the structure of the transition metal layer. Sr and Ca are thought to be able to change the surface state of the lithium transition metal composite oxide through electronic interactions. Furthermore, by adding Ba, which has a lower electronegativity than Sr and Ca, to the surface modification layer, not only the discharge cycle characteristics but also the output characteristics are improved.

[0011] An example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail below. Hereinafter, a cylindrical battery in which a wound electrode assembly is housed in a cylindrical battery case will be exemplified. However, the electrode assembly is not limited to the wound type and may be a laminated type in which multiple positive electrodes and multiple negative electrodes are alternately stacked one by one with separators interposed therebetween. Furthermore, the battery case is not limited to a cylindrical shape and may be, for example, a prismatic or coin-shaped battery case, or may be a battery case made of a laminate sheet including a metal layer and a resin layer.

[0012] Fig. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As illustrated in Fig. 1, the nonaqueous electrolyte secondary battery 10 includes an electrode assembly 14, a nonaqueous electrolyte (not shown), and a battery case 15 that accommodates the electrode assembly 14 and the nonaqueous electrolyte. The electrode assembly 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween. The battery case 15 includes a cylindrical outer can 16 with a bottom, and a sealing member 17 that closes the opening of the outer can 16.

[0013] The electrode assembly 14 is composed of a long positive electrode 11, a long negative electrode 12, two long separators 13, a positive electrode tab 20 joined to the positive electrode 11, and a negative electrode tab 21 joined to the negative electrode 12. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the longitudinal direction and width direction (short direction). The two separators 13 are formed to be at least slightly larger than the positive electrode 11 and are arranged to sandwich the positive electrode 11, for example.

[0014] The nonaqueous electrolyte secondary battery 10 includes insulating plates 18 and 19 disposed above and below the electrode assembly 14. In the example shown in Fig. 1 , a positive electrode tab 20 attached to the positive electrode 11 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and a negative electrode tab 21 attached to the negative electrode 12 passes outside the insulating plate 19 and extends toward the bottom of the outer can 16. The positive electrode tab 20 is connected to the underside of a bottom plate 23 of the sealing body 17 by welding or the like, and a cap 27 of the sealing body 17 electrically connected to the bottom plate 23 serves as the positive electrode terminal. The negative electrode tab 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0015] The outer can 16 is, for example, a cylindrical metal container with a bottom. A gasket 28 is provided between the outer can 16 and the sealing body 17, sealing the internal space of the battery case 15. The outer can 16 has a grooved portion 22 that supports the sealing body 17, formed, for example, by pressing the side surface from the outside. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its upper surface.

[0016] The sealing body 17 has a structure in which, in order from the electrode body 14 side, a bottom plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0017] The positive electrode 11, negative electrode 12, separator 13, and nonaqueous electrolyte that constitute the nonaqueous electrolyte secondary battery 10 will be described in detail below, particularly the positive electrode active material contained in the positive electrode active material layer 31 that constitutes the positive electrode 11.

[0018] [Positive electrode] The positive electrode 11 has a positive electrode current collector 30 and a positive electrode active material layer 31 formed on both sides of the positive electrode current collector 30. The positive electrode current collector 30 can be a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode 11, or a film with such a metal disposed on its surface. The positive electrode active material layer 31 contains a positive electrode active material, a conductive material, and a binder. The thickness of the positive electrode active material layer 31 is, for example, 10 μm to 150 μm on one side of the positive electrode current collector 30. The positive electrode 11 can be produced by applying a positive electrode slurry containing a positive electrode active material, a conductive material, a binder, etc. to the surface of the positive electrode current collector 30, drying the coating, and then compressing it to form the positive electrode active material layer 31 on both sides of the positive electrode current collector 30.

[0019] Examples of the conductive material contained in the positive electrode active material layer 31 include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of the binder contained in the positive electrode active material layer 31 include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. These resins may be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like.

[0020] The positive electrode active material contained in the positive electrode active material layer 31 includes a lithium transition metal composite oxide having secondary particles formed by aggregation of primary particles, and a surface modification layer formed on the surfaces of the primary particles of the lithium transition metal composite oxide. This can suppress reaction with the electrolyte. Here, "the surface modification layer formed on the surfaces of the primary particles" means that the surface modification layer is present on the surfaces of the secondary particles or at the interface where the primary particles contact each other.

[0021] The secondary particles of the lithium transition metal composite oxide preferably have a volume-based median diameter (D50) of 3 μm to 30 μm, more preferably 5 μm to 25 μm, and particularly preferably 7 μm to 15 μm. D50 refers to the particle size at which the cumulative frequency of particles in the volume-based particle size distribution is 50% from the smallest particle size, and is also called the median diameter. The particle size distribution of the secondary particles of the lithium transition metal composite oxide can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II, manufactured by Microtrack Bell Corporation) using water as the dispersion medium.

[0022] The particle size of the primary particles that make up the secondary particles is, for example, 0.05 μm to 1 μm. The particle size of the primary particles is measured as the diameter of the circumscribed circle in a particle image observed with a scanning electron microscope (SEM).

[0023] The lithium transition metal composite oxide may have, for example, a layered structure belonging to the space group R-3m, a layered structure belonging to the space group C2 / m, etc. Among these, in terms of high capacity, stability of the crystal structure, etc., it is preferable that it has a layered structure belonging to the space group R-3m. The layered structure of the lithium transition metal composite oxide includes a transition metal layer, a Li layer, and an oxygen layer.

[0024] The lithium transition metal composite oxide contains at least Ni and Al at 80 mol% or more with respect to the total molar number of metal elements excluding Li. By setting the ratio of Ni to the total molar number of metal elements excluding Li in the lithium transition metal composite oxide to 80 mol% or more, a high-capacity battery can be obtained.

[0025] The ratio of Ni to the total molar number of metal elements excluding Li in the lithium transition metal composite oxide is preferably 90 mol% or more. Thereby, a higher-capacity battery can be obtained.

[0026] The lithium transition metal composite oxide has the general formula Li a Ni x Al y Co z M w O 2-b (where 0.95 < a < 1.05, 0.8 ≤ x ≤ 0.96, 0 < y ≤ 0.10, 0 ≤ z ≤ 0.15, 0 ≤ w ≤ 0.1, 0 ≤ b < 0.05, x + y + z + w = 1, and M is at least one element selected from Mn, Fe, Ti, Si, Nb, Zr, Mo, and Zn). The positive electrode active material may contain, within a range not impairing the object of the present disclosure, a lithium transition metal composite oxide other than that represented by the above general formula, or other compounds. The molar fraction of the metal elements contained in the entire particles of the lithium transition metal composite oxide is measured by inductively coupled plasma (ICP) emission spectrometry.

[0027] a, which represents the ratio of Li in the lithium transition metal composite oxide, preferably satisfies 0.95 ≦ a < 1.05, and more preferably satisfies 0.97 ≦ a ≦ 1.03. When a is less than 0.95, the battery capacity may decrease as compared with the case where a satisfies the above range. When a is 1.05 or more, it may lead to a decrease in charge-discharge cycle characteristics as compared with the case where a satisfies the above range.

[0028] y, which represents the ratio of Al to the total molar number of metal elements excluding Li in the lithium transition metal composite oxide, preferably satisfies 0 < y ≦ 0.10, and more preferably satisfies 0.03 ≦ y ≦ 0.07. Since the oxidation number of Al does not change during charge and discharge, it is considered that the structure of the transition metal layer is stabilized by being contained in the transition metal layer. On the other hand, when y exceeds 0.10, Al impurities may be generated and the battery capacity may decrease. Also, when y is 0.07 or less, the layered structure of the lithium transition metal composite oxide tends to be unstable, so the effect of structure stabilization caused by containing either Sr or Ca is remarkable. Al may be uniformly dispersed in the layered structure of the lithium transition metal composite oxide, for example, or may be present in a part of the layered structure.

[0029] Co and M (M is at least one element selected from Mn, Fe, Ti, Si, Nb, Zr, Mo, and Zn) are optional components. z and w, which represent the ratios of Co and M to the total molar number of metal elements excluding Li in the lithium transition metal composite oxide, preferably satisfy 0 ≦ z ≦ 0.15 and 0 ≦ w ≦ 0.1, respectively. Since Co is expensive, it is preferable to suppress the Co content from the viewpoint of manufacturing cost.

[0030] The surface modification layer contains at least Ba and at least one of Sr and Ca. The surface modification layer may contain, for example, Sr or a compound containing Sr, or Ca or a compound containing Ca. An example of a compound containing Sr is SrO. An example of a compound containing Ca is CaO. Furthermore, the surface modification layer may contain, for example, Ba or a compound containing Ba. An example of a compound containing Ba is BaO.

[0031] The surface modification layer may further contain Al. The Al contained in the surface modification layer may be, for example, Al or a compound containing Al. An example of an Al-containing compound is Al2O3. Furthermore, an example of an Al-containing compound may be a compound containing Al and Sr, Ca, or Ba, such as SrAlO4, CaAlO4, or BaAlO4. The surface modification layer may further contain Li.

[0032] The proportion of Sr and Ca in the surface modification layer can be, for example, 0.05 mol % to 0.25 mol % relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide. Within this range, the surface state of the lithium transition metal composite oxide can be changed by electronic interaction.

[0033] The proportion of Ba in the surface modification layer can be, for example, 0.000001 mol % to 0.00001 mol % in the lithium transition metal composite oxide. Within this range, the reaction resistance of the battery can be reduced, improving the output characteristics. Note that, although the presence of Ba at this level can be confirmed by ICP, the amount is so small that it may not be possible to quantitatively measure the content.

[0034] The thickness of the surface modification layer is, for example, 0.1 nm to 2 nm, which can suppress the reaction between the surface of the lithium transition metal composite oxide and the electrolyte.

[0035] The content of the lithium transition metal composite oxide in the positive electrode active material is preferably 90 mass% or more, and more preferably 99 mass% or more, relative to the total mass of the positive electrode active material, for example, from the viewpoint of improving the battery capacity and effectively suppressing deterioration of the charge-discharge cycle characteristics.

[0036] The positive electrode active material of this embodiment may contain, in addition to the lithium transition metal composite oxide of this embodiment, other lithium transition metal composite oxides, such as lithium transition metal composite oxides having a Ni content of 0 mol % or more but less than 85 mol %.

[0037] Next, an example of a method for producing a positive electrode active material including a lithium transition metal composite oxide and a surface modification layer will be described.

[0038] The method for producing a positive electrode active material includes, for example, a first step of obtaining a composite oxide containing Ni, Al, and an arbitrary metal element, a second step of mixing the composite oxide obtained in the first step with a Li compound or the like to obtain a mixture, and a third step of firing the mixture. Parameters such as the composition of the surface modification layer in the finally obtained positive electrode active material can be adjusted by controlling, for example, the mixing ratio of the raw materials in the second step and the firing temperature and time in the third step.

[0039] In the first step, for example, an alkaline solution such as sodium hydroxide is added dropwise to a stirred solution of a metal salt containing Ni, Al, and an optional metal element (Co, Mn, Fe, etc.) to adjust the pH to the alkaline side (e.g., 8.5 to 12.5), thereby precipitating (co-precipitating) a composite hydroxide containing Ni, Al, and the optional metal element, and the composite hydroxide is calcined to obtain a composite oxide containing Ni, Al, and the optional metal element. The calcination temperature is not particularly limited, but is, for example, in the range of 300°C to 600°C.

[0040] In the second step, the composite oxide obtained in the first step, a Li compound, a Sr compound or a Ca compound, and a Ba compound are mixed to obtain a mixture. Examples of Li compounds include Li2CO3, LiOH, Li2O2, Li2O, LiNO3, LiNO2, Li2SO4, LiOH·H2O, LiH, and LiF. Examples of Sr compounds include Sr(OH)2, Sr(OH)2·8H2O, SrO, SrCo3, SrSO4, and Sr(NO3)2. Examples of Ca compounds include Ca(OH)2, CaO, CaCO3, CaSO4, and Ca(NO3)2. Examples of Ba compounds include Ba(OH)2·H2O, BaO, BaCO3, and BaSO4. The mixing ratio of the composite oxide obtained in the first step with the Li compound is preferably, for example, such that the molar ratio of metal elements excluding Li to Li is in the range of 1:0.98 to 1:1.1, in order to facilitate adjustment of each of the above parameters within the specified ranges. Furthermore, the mixing ratio of the composite oxide obtained in the first step with at least one of the Sr compound and the Ca compound is preferably, for example, such that the molar ratio of metal elements excluding Li to (Sr+Ca) is in the range of 1:0.0005 to 1:0.0025, in order to facilitate adjustment of each of the above parameters within the specified ranges. Furthermore, the mixing ratio of the composite oxide obtained in the first step with the Ba compound is preferably, for example, such that the molar ratio of metal elements excluding Li to Ba is in the range of 1:0.000001 to 1:0.00001, in order to facilitate adjustment of each of the above parameters within the specified ranges. In the second step, when mixing the composite oxide obtained in the first step, the Li compound, the Sr compound or the Ca compound, and the Ba compound, other metal raw materials may be added as necessary. The other metal raw materials are oxides containing metal elements other than the metal elements constituting the composite oxide obtained in the first step.

[0041] In the third step, the mixture obtained in the second step is fired at a predetermined temperature and time to obtain a cathode active material according to this embodiment. The firing of the mixture in the third step includes a multi-stage firing process, for example, a first firing step in which the mixture is fired in a firing furnace under an oxygen stream at a first heating rate to a first set temperature of 450°C to 680°C, and a second firing step in which the fired product obtained in the first firing step is fired in a firing furnace under an oxygen stream at a second heating rate to a second set temperature of more than 680°C to 800°C. Here, the first heating rate is in the range of 1.5°C / min to 5.5°C / min, and the second heating rate is slower than the first heating rate and is in the range of 0.1°C / min to 3.5°C / min. By performing such multi-stage firing, parameters such as the composition of the surface modification layer in the finally obtained cathode active material according to this embodiment can be adjusted to the above-specified ranges. The first temperature increase rate and the second temperature increase rate may be set in plural for each temperature range as long as they are within the above-specified range. The holding time of the first set temperature in the first firing step is preferably 0 to 5 hours, more preferably 0 to 3 hours, in order to adjust each of the above parameters of the lithium transition metal composite oxide to the above-specified range. The holding time of the first set temperature is the time for which the first set temperature is maintained after the first set temperature is reached. The holding time of the second set temperature in the second firing step is preferably 1 to 10 hours, more preferably 1 to 5 hours, in order to adjust each of the above parameters of the lithium transition metal composite oxide to the above-specified range. The holding time of the second set temperature is the time for which the second set temperature is maintained after the second set temperature is reached. When firing the mixture, in order to adjust each of the above parameters to the above-specified range, for example, the firing is carried out in an oxygen stream with an oxygen concentration of 60% or more, and the flow rate of the oxygen stream is set to 10 cm / min. 3 The flow rate can be in the range of 0.2 mL / min to 4 mL / min per kg of the mixture and 0.3 L / min or more per kg of the mixture.

[0042] The method for producing a positive electrode active material may include a washing step of washing the positive electrode active material with water after the third step. This allows for the removal of unreacted components such as Li compounds and impurities from the positive electrode active material obtained in the third step. The washing step may involve, for example, mixing the positive electrode active material with water to a slurry concentration in the range of 500 g / L to 2000 g / L, stirring for 3 minutes to 1 hour, and then filtering. A W compound or a W-containing solution may be added to the positive electrode active material after the washing step. This further suppresses the formation and erosion of a structurally deteriorated layer on the surface of the lithium transition metal composite oxide due to reaction with the electrolyte, thereby improving charge-discharge cycle characteristics. Li compounds remain in the positive electrode active material after the washing step, and the remaining Li compounds dissolve in the water contained in the positive electrode active material to produce an alkaline aqueous solution. When a W compound is added to the positive electrode active material, the W compound dissolves in the alkaline aqueous solution and spreads over the entire surface of the positive electrode active material. Examples of W compounds include tungsten oxide (WO3) and lithium tungstate (Li2WO4, Li4WO5, Li6W2O9). The amount of W added may be 0.5 mol% or less, and preferably 0.3 mol% or less, relative to the total number of moles of metal elements excluding Li in the positive electrode active material. When a W-containing solution is added to the positive electrode active material, the W concentration in the W-containing solution is, for example, 0.05 mol / L or more, and preferably 0.1 mol / L to 1 mol / L. The W-containing solution is not particularly limited as long as it contains W, but is preferably one in which a W compound that is easily soluble in alkaline solution, such as tungsten oxide, lithium tungstate, or ammonium tungstate, is dissolved in an aqueous solution of lithium hydroxide.

[0043] The molar fraction of the metal element contained in the positive electrode active material obtained above was measured by inductively coupled plasma (ICP) emission spectroscopy, and the molar fraction of the metal element contained in the positive electrode active material was determined by the general formula Li a Ni x Al y Co z M1 w M2 α Ba β O 2-b(where 0.95 < a < 1.05, 0.8 ≤ x ≤ 0.96, 0 < y ≤ 0.10, 0 ≤ z ≤ 0.15, 0 ≤ w ≤ 0.1, 0.05 ≤ α ≤ 0.25, 0.0001 ≤ β ≤ 0.001, 0 ≤ b < 0.05, x + y + z + w = 1, M1 is at least one element selected from Mn, Fe, Ti, Si, Nb, Zr, Mo, and Zn, and M2 is at least one element selected from Sr and Ca). Note that Sr is not dissolved in the lithium transition metal composite oxide but is contained in the surface modification layer present on the surface of the lithium transition metal composite oxide. Also, a part of Al may be contained in the surface modification layer.)

[0044] [Negative electrode] The negative electrode 12 has a negative electrode current collector 40 and negative electrode active material layers 41 formed on both surfaces of the negative electrode current collector 40. As the negative electrode current collector 40, a foil of a metal stable within the potential range of the negative electrode 12 such as copper or a copper alloy, or a film having such a metal disposed on the surface layer can be used. The negative electrode active material layer 41 contains a negative electrode active material and a binder. The thickness of the negative electrode active material layer 41 is, for example, 10 μm to 150 μm on one side of the negative electrode current collector 40. The negative electrode 12 can be produced by applying a negative electrode slurry containing a negative electrode active material, a binder, etc. to the surface of the negative electrode current collector 40, drying the coating film, and then rolling to form the negative electrode active material layer 41 on both surfaces of the negative electrode current collector 40.

[0045] The negative electrode active material contained in the negative electrode active material layer 41 is not particularly limited as long as it can reversibly occlude and release lithium ions. Generally, a carbon material such as graphite is used. The graphite may be any of natural graphite such as flaky graphite, massive graphite, and earthy graphite, artificial massive graphite, and artificial graphite such as graphitized mesophase carbon microbeads. Also, as the negative electrode active material, a metal that alloys with Li such as Si and Sn, a metal compound containing Si, Sn, etc., a lithium titanium composite oxide, etc. may be used. Also, those provided with a carbon coating may be used. For example, SiO x (0.5 ≤ x ≤ 1.6) represented by the Si-containing compound, or Li 2y SiO (2+y)Si-containing compounds in which fine particles of Si are dispersed in a lithium silicate phase represented by (0 < y < 2) may be used in combination with graphite.

[0046] As the binder contained in the negative electrode active material layer 41, fluorine-containing resins such as PTFE and PVdF, PAN, polyimide, acrylic resin, polyolefin, etc. may be used as in the case of the positive electrode 11, but preferably styrene-butadiene rubber (SBR) is used. Further, the negative electrode active material layer 41 may contain CMC or its salt, polyacrylic acid (PAA) or its salt, polyvinyl alcohol (PVA), etc.

[0047] [Separator] For the separator 13, for example, a porous sheet having ion permeability and insulation is used. Specific examples of the porous sheet include microporous thin films, woven fabrics, non-woven fabrics, etc. As the material of the separator, polyolefins such as polyethylene and polypropylene, cellulose, etc. are suitable. The separator 13 may have a single-layer structure or a laminated structure. Further, a resin layer with high heat resistance such as an aramid resin or a filler layer containing an inorganic compound filler may be provided on the surface of the separator 13.

[0048] [Non-aqueous electrolyte] The non-aqueous electrolyte contains, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. For the non-aqueous solvent, for example, esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these can be used. The non-aqueous solvent may contain a halogen-substituted product in which at least a part of the hydrogen of these solvents is substituted with a halogen atom such as fluorine. Examples of the halogen-substituted product include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP), etc.

[0049] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0050] Examples of the ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methyl phenyl ether. and chain ethers such as ethyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0051] The electrolyte salt is preferably a lithium salt. Examples of lithium salts include LiBF, LiClO, LiPF, LiAsF, LiSbF, LiAlCl, LiSCN, LiCF, SO, LiCF, CO, Li(P(C), O)F, and LiPF.6-x (C n F 2n+1 ) x (1 < x < 6, n is 1 or 2), LiB 10 Cl 10 、LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylate, borates such as Li2B4O7, Li(B(C2O4)F2), LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 SO2) {l, m are integers of 0 or more}, and imide salts such as these can be mentioned. The lithium salt may be used individually by these 1 type, or may be used in mixture of multiple types. Among these, from viewpoints, such as ionic conductivity and electrochemical stability, it is preferable to use LiPF6. The concentration of the lithium salt is, for example, 0.8 mol to 1.8 mol per 1 L of non-aqueous solvent. Also, vinylene carbonate or a propane sultone-based additive may be further added.

[0052] [Preparation of Positive Electrode Active Material] Hereinafter, the present disclosure will be further described by way of Examples and Comparative Examples, but the present disclosure is not limited to the following Examples.

[0053] [Preparation of Positive Electrode Active Material] [Example 1] The composite hydroxide represented by [Ni 0.82 Al 0.05 Co 0.13 (OH)2 obtained by the coprecipitation method was calcined at 500 °C for 8 hours to obtain a composite oxide (Ni 0.82 Al 0.05 Co 0.13 O2). Next, LiOH, the said composite oxide, Sr(OH)2, and Ba(OH)2 were mixed so that the molar ratio of Li to the total amount of Ni, Al, and Co, Sr, and Ba would become 1.03:1:0.0005:0.000001 to obtain a mixture. This mixture was under an oxygen stream with an oxygen concentration of 95% (10 cm 3The mixture was fired at a flow rate of 2 mL / min per kg of mixture and 5 L / min per kg of mixture at a temperature increase rate of 2.0°C / min from room temperature to 650°C, and then at a temperature increase rate of 0.5°C / min from 650°C to 780°C. Water was added to the fired product so that the slurry concentration was 1500 g / L, and the mixture was stirred for 15 minutes and then filtered to obtain a positive electrode active material. Furthermore, the composition of the obtained positive electrode active material was measured using an ICP optical emission spectrometer (manufactured by Thermo Fisher Scientific, product name "iCAP6300") and found to be LiNi 0.82 Al 0.05 Co 0.13 Sr 0.0005 Ba β O2. This was used as the positive electrode active material of Example 1. Here, it was confirmed that Ba was contained in the positive electrode active material, but the Ba content β was too small to be quantified. From the amount added, β is estimated to be about 0.000001.

[0054] [Preparation of positive electrode] A positive electrode slurry was prepared by mixing 91 parts by mass of the positive electrode active material of Example 1, 7 parts by mass of acetylene black as a conductive material, and 2 parts by mass of polyvinylidene fluoride as a binder, and then mixing this with N-methyl-2-pyrrolidone (NMP). The slurry was then applied to a positive electrode current collector made of aluminum foil with a thickness of 15 μm. The coating was dried, and then rolled with a rolling roller and cut to a predetermined electrode size, resulting in a positive electrode with a positive electrode active material layer formed on both sides of the positive electrode core. An exposed portion was provided in part of the positive electrode, exposing the surface of the positive electrode core.

[0055] [Preparation of negative electrode] Natural graphite was used as the negative electrode active material. The negative electrode active material, carboxymethyl cellulose sodium (CMC-Na), and styrene-butadiene rubber (SBR) were mixed in an aqueous solution at a solids mass ratio of 100:1:1 to prepare a negative electrode slurry. The negative electrode slurry was applied to both sides of a negative electrode core made of copper foil, and the coating was dried. The coating was then rolled using a rolling roller and cut to a predetermined electrode size to obtain a negative electrode with a negative electrode active material layer formed on both sides of the negative electrode core. An exposed portion was provided in part of the negative electrode, exposing the surface of the negative electrode core.

[0056] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 3:3:4. Lithium hexafluorophosphate (LiPF6) was dissolved in the mixed solvent to a concentration of 1.2 mol / L to prepare a nonaqueous electrolyte.

[0057] [Test cell construction] An aluminum lead was attached to the exposed portion of the positive electrode containing the positive electrode active material of Example 1, and a nickel lead was attached to the exposed portion of the negative electrode. The positive and negative electrodes were spirally wound with a polyolefin separator interposed therebetween, and then pressed in the radial direction to produce a flat wound electrode assembly. This electrode assembly was housed in an outer casing, and the nonaqueous electrolyte was poured into it. The opening of the outer casing was then sealed to obtain a test cell.

[0058] [Measurement of reaction resistance] The test cell was subjected to constant current charging at 0.7 mA at 25°C until the cell voltage reached 4.3 V, followed by constant voltage charging at 4.3 V until the current reached 0.07 mA. Subsequently, a constant current discharge was performed at 0.7 mA until the cell voltage reached 2.5 V. Thereafter, again at 25°C, a constant current charge was performed at 0.7 mA until the cell voltage reached 4.3 V, followed by constant voltage charging at 4.3 V until the current reached 0.07 mA. The AC impedance of the test cell was then measured at 20 kHz to 0.01 Hz using an AC impedance meter. A Cole-Cole plot was drawn from the measured data, and the reaction resistance was calculated from the size of the arc between 10 Hz and 0.1 Hz. The reaction resistances shown in Table 1 are expressed relative to the reaction resistance of the test cell containing the positive electrode active material of Comparative Example 1, which is set to 100.

[0059] [Capacity retention rate evaluation] The test cell was subjected to the following cycle test. The discharge capacity at the first cycle and the discharge capacity at the 100th cycle of the cycle test were determined, and the capacity retention rate was calculated using the following formula.

[0060] Capacity retention rate (%) = (100th cycle discharge capacity ÷ 1st cycle discharge capacity) × 100 <Cycle test> The test cell was charged at a constant current of 0.5 It in a 45°C environment until the battery voltage reached 4.2 V, and then charged at a constant voltage until the current value reached 1 / 50 It at 4.2 V. It was then discharged at a constant current of 0.5 It until the battery voltage reached 2.5 V. This charge / discharge cycle was repeated 100 times.

[0061] <Comparative Example 1> A positive electrode active material was obtained in the same manner as in Example 1, except that Sr(OH)2 and Ba(OH)2 were not added in the step of obtaining a mixture (second step).

[0062] <Comparative Example 2> A positive electrode active material was obtained in the same manner as in Example 1, except that Sr(OH)2 was not added in the step of obtaining a mixture (second step).

[0063] <Comparative Example 3> A positive electrode active material was obtained in the same manner as in Example 1, except that Ba(OH)2 was not added in the step of obtaining a mixture (second step).

[0064] <Example 2> [Ni 0.88 Al 0.03 Co 0.09 ](OH)2 was used to obtain the composite oxide (Ni 0.88 Al 0.03 Co 0.09 A positive electrode active material was obtained in the same manner as in Example 1, except that LiOH, the above composite oxide, Sr(OH)2, and Ba(OH)2 were mixed so that the molar ratio of the total amount of Li, Ni, Al, and Co to Sr and Ba was 1.03:1:0.001:0.000002. The composition of the obtained positive electrode active material was LiNi 0.88 Al 0.03 Co 0.09 Sr 0.001 Ba β O2. This was used as the positive electrode active material of Example 2. As in Example 1, it was confirmed that Ba was contained in the positive electrode active material, but the Ba content β was too small to be quantified. From the amount added, β is estimated to be about 0.000002.

[0065] Example 3 A positive electrode active material was obtained in the same manner as in Example 2, except that Ca(OH)2 was added instead of Sr(OH)2 in the step of obtaining a mixture (second step).

[0066] <Comparative Example 4> A positive electrode active material was obtained in the same manner as in Example 2, except that Sr(OH)2 and Ba(OH)2 were not added in the step of obtaining a mixture (second step).

[0067] <Comparative Example 5> A positive electrode active material was obtained in the same manner as in Example 2, except that Sr(OH)2 was not added in the step of obtaining a mixture (second step).

[0068] <Comparative Example 6> A positive electrode active material was obtained in the same manner as in Example 2, except that Ba(OH)2 was not added in the step of obtaining a mixture (second step).

[0069] <Comparative Example 7> A positive electrode active material was obtained in the same manner as in Example 2, except that in the step of obtaining a mixture (second step), Ca(OH)2 was added instead of Sr(OH)2, and further, Ba(OH)2 was not added.

[0070] Example 4 [Ni 0.91 Al 0.04 Co 0.05 ](OH)2 was used to obtain the composite oxide (Ni 0.91 Al 0.04 Co 0.05 A positive electrode active material was obtained in the same manner as in Example 1, except that LiOH, the above composite oxide, Sr(OH)2, and Ba(OH)2 were mixed so that the molar ratio of the total amount of Li, Ni, Al, and Co to Sr and Ba was 1.03:1:0.0007:0.000004. The composition of the obtained positive electrode active material was LiNi 0.91 Al 0.04 Co 0.05 Sr 0.0007 Ba β O2. This was used as the positive electrode active material of Example 4. As in Example 1, it was confirmed that Ba was contained in the positive electrode active material, but the Ba content β was too small to be quantified. From the amount added, β is estimated to be about 0.000004.

[0071] <Example 5> In the step of obtaining a mixture (second step), LiOH, a composite oxide (Ni 0.91 Al 0.04 Co 0.05A positive electrode active material was obtained in the same manner as in Example 4, except that a mixture was obtained by mixing O2), Sr(OH)2, and Ba(OH)2 so that the molar ratio of the total amount of Li, Ni, Al, and Co to Sr and Ba was 1.03:1:0.0015:0.000009.

[0072] <Comparative Example 8> A positive electrode active material was obtained in the same manner as in Example 4, except that Sr(OH)2 and Ba(OH)2 were not added in the step of obtaining a mixture (second step).

[0073] <Comparative Example 9> A positive electrode active material was obtained in the same manner as in Example 4, except that Sr(OH)2 was not added in the step of obtaining a mixture (second step).

[0074] <Comparative Example 10> A positive electrode active material was obtained in the same manner as in Example 4, except that Ba(OH)2 was not added in the step of obtaining a mixture (second step).

[0075] <Comparative Example 11> A positive electrode active material was obtained in the same manner as in Example 5, except that Ba(OH)2 was not added in the step of obtaining a mixture (second step).

[0076] Example 6 [Ni 0.91 Al 0.055 Co 0.035 ](OH)2 was used to obtain the composite oxide (Ni 0.91 Al 0.055 Co 0.035 A positive electrode active material was obtained in the same manner as in Example 1, except that LiOH, the above composite oxide, Sr(OH)2, and Ba(OH)2 were mixed so that the molar ratio of the total amount of Li, Ni, Al, and Co to Sr and Ba was 1.03:1:0.0007:0.00001. The composition of the obtained positive electrode active material was LiNi 0.91 Al 0.055 Co 0.035 Sr 0.0007 Ba βO2. This was used as the positive electrode active material of Example 6. As in Example 1, it was confirmed that Ba was contained in the positive electrode active material, but the Ba content β was too small to be quantified. From the amount added, β is estimated to be about 0.00001.

[0077] Example 7 A positive electrode active material was obtained in the same manner as in Example 6, except that Ca(OH)2 was added instead of Sr(OH)2 in the step of obtaining a mixture (second step).

[0078] <Comparative Example 12> A positive electrode active material was obtained in the same manner as in Example 6, except that Sr(OH)2 and Ba(OH)2 were not added in the step of obtaining a mixture (second step).

[0079] <Comparative Example 13> A positive electrode active material was obtained in the same manner as in Example 6, except that Sr(OH)2 was not added in the step of obtaining a mixture (second step).

[0080] <Comparative Example 14> A positive electrode active material was obtained in the same manner as in Example 6, except that Ba(OH)2 was not added in the step of obtaining a mixture (second step).

[0081] <Comparative Example 15> A positive electrode active material was obtained in the same manner as in Example 6, except that in the step of obtaining a mixture (second step), Ca(OH)2 was added instead of Sr(OH)2, and further, Ba(OH)2 was not added.

[0082] Example 8 [Ni 0.91 Al 0.05 Mn 0.04 ](OH)2 was used to obtain the composite oxide (Ni 0.91 Al 0.05 Mn 0.04A positive electrode active material was obtained in the same manner as in Example 1, except that LiOH, the above composite oxide, Sr(OH)2, and Ba(OH)2 were mixed so that the molar ratio of the total amount of Li, Ni, Al, and Co to Sr and Ba was 1.03:1:0.0015:0.000004. The composition of the obtained positive electrode active material was LiNi 0.91 Al 0.05 Mn 0.04 Sr 0.0015 Ba β O2. This was used as the positive electrode active material of Example 8. As in Example 1, it was confirmed that Ba was contained in the positive electrode active material, but the Ba content β was too small to be quantified. From the amount added, β is estimated to be about 0.000004.

[0083] <Comparative Example 16> A positive electrode active material was obtained in the same manner as in Example 8, except that Sr(OH)2 and Ba(OH)2 were not added in the step of obtaining a mixture (second step).

[0084] <Comparative Example 17> A positive electrode active material was obtained in the same manner as in Example 8, except that Sr(OH)2 was not added in the step of obtaining a mixture (second step).

[0085] <Comparative Example 18> A positive electrode active material was obtained in the same manner as in Example 8, except that Ba(OH)2 was not added in the step of obtaining a mixture (second step).

[0086] Example 9 [Ni 0.925 Al 0.05 Mn 0.025 ](OH)2 was used to obtain the composite oxide (Ni 0.925 Al 0.05 Mn 0.025A positive electrode active material was obtained in the same manner as in Example 1, except that LiOH, the above composite oxide, Sr(OH)2, and Ba(OH)2 were mixed so that the molar ratio of the total amount of Li, Ni, Al, and Co to Sr and Ba was 1.03:1:0.0018:0.000003. The composition of the obtained positive electrode active material was LiNi 0.925 Al 0.05 Mn 0.025 Sr 0.0018 Ba β O2. This was used as the positive electrode active material of Example 9. As in Example 1, it was confirmed that Ba was contained in the positive electrode active material, but the Ba content β was too small to be quantified. From the amount added, β is estimated to be about 0.000003.

[0087] <Comparative Example 19> A positive electrode active material was obtained in the same manner as in Example 9, except that Sr(OH)2 and Ba(OH)2 were not added in the step of obtaining a mixture (second step).

[0088] <Comparative Example 20> A positive electrode active material was obtained in the same manner as in Example 9, except that Sr(OH)2 was not added in the step of obtaining a mixture (second step).

[0089] <Comparative Example 21> A positive electrode active material was obtained in the same manner as in Example 9, except that Ba(OH)2 was not added in the step of obtaining a mixture (second step).

[0090] <Reference example 1> [Ni 0.595 Co 0.21 Mn 0.195 ](OH)2 was used to obtain the composite oxide (Ni 0.595 Co 0.21 Mn 0.195A positive electrode active material was obtained in the same manner as in Example 1, except that LiOH, the above composite oxide, Sr(OH)2, and Ba(OH)2 were mixed so that the molar ratio of the total amount of Li, Ni, Al, and Co to Sr and Ba was 1.03:1:0.0007:0.00001. The composition of the obtained positive electrode active material was LiNi 0.595 Co 0.21 Mn 0.195 Sr 0.0007 Ba β O2. This was used as the positive electrode active material of Reference Example 1. As in Example 1, it was confirmed that Ba was contained in the positive electrode active material, but the Ba content β was too small to be quantified. From the amount added, β is estimated to be about 0.00001.

[0091] <Reference example 2> A positive electrode active material was obtained in the same manner as in Reference Example 1, except that Sr(OH)2 and Ba(OH)2 were not added in the step of obtaining a mixture (second step).

[0092] <Reference example 3> A positive electrode active material was obtained in the same manner as in Reference Example 1, except that Sr(OH)2 was not added in the step of obtaining a mixture (second step).

[0093] <Reference example 4> A positive electrode active material was obtained in the same manner as in Reference Example 1, except that Ba(OH)2 was not added in the step of obtaining a mixture (second step).

[0094] <Reference example 5> A positive electrode active material was obtained in the same manner as in Reference Example 1, except that in the step of obtaining a mixture (second step), Ca(OH)2 was added instead of Sr(OH)2.

[0095] The reaction resistances and capacity retention rates of the Examples, Comparative Examples, and Reference Examples are shown in Tables 1 to 7. Tables 1 to 7 also show the results of ICP analysis of the obtained positive electrode active materials. The reaction resistances and capacity retention rates of the test cells of Example 1 and Comparative Examples 2 and 3 shown in Table 1 are expressed relative to the reaction resistances and capacity retention rates of the test cell of Comparative Example 1, which are set to 100.

[0096] The reaction resistances and capacity retention rates of the test cells of Examples 2 and 3 and Comparative Examples 5 to 7 shown in Table 2 are expressed relative to the reaction resistances and capacity retention rates of the test cell of Comparative Example 4, which are set at 100.

[0097] The reaction resistances and capacity retention rates of the test cells of Examples 4 and 5 and Comparative Examples 9 to 11 shown in Table 3 are expressed relative to the reaction resistances and capacity retention rates of the test cell of Comparative Example 8, which are set at 100.

[0098] The reaction resistances and capacity retention rates of the test cells of Examples 6 and 7 and Comparative Examples 13 to 15 shown in Table 4 are expressed relative to the reaction resistances and capacity retention rates of the test cell of Comparative Example 12, which are set at 100.

[0099] The reaction resistance and capacity retention rate of the test cells of Example 8 and Comparative Examples 17 and 18 shown in Table 5 are expressed relative to the reaction resistance and capacity retention rate of the test cell of Comparative Example 16, which is set to 100.

[0100] The reaction resistance and capacity retention rate of the test cells of Example 9 and Comparative Examples 20 and 21 shown in Table 6 are expressed relative to the reaction resistance and capacity retention rate of the test cell of Comparative Example 19, which is set to 100.

[0101] The reaction resistance and capacity retention rate of the test cells of Reference Examples 1 and 3 to 5 shown in Table 7 are expressed relative to the reaction resistance and capacity retention rate of the test cell of Reference Example 2, which is set to 100.

[0102] [Table 1]

[0103] [Table 2]

[0104] [Table 3]

[0105] [Table 4]

[0106] [Table 5]

[0107] [Table 6]

[0108] [Table 7]

[0109] In all of Tables 1 to 6, the examples in which the surface modification layer contained Sr and Ba had lower reaction resistance and higher capacity retention than the comparative examples in which the surface modification layer did not contain at least one of Sr and Ba. In Table 7, since the lithium transition metal composite oxides in all of Reference Examples 1 to 5 did not contain Al, there was no change in reaction resistance and capacity retention whether Sr and Ba were added or not. [Explanation of symbols]

[0110] 10 Nonaqueous electrolyte secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode body 15 Battery case 16 outer can 17 Sealing body 18,19 Insulating plate 20 Positive electrode tab 21 Negative electrode tab 22 Grooved part 23 Bottom plate 24 Lower valve body 25 Insulating material 26 Superior valve 27 Cap 28 Gasket 30 Positive electrode current collector 31 Positive electrode active material layer 40 Negative electrode current collector 41 Negative electrode active material layer

Claims

1. a lithium transition metal composite oxide having secondary particles formed by aggregation of primary particles; a surface modification layer formed on the surface of the primary particles of the lithium transition metal composite oxide, The lithium transition metal composite oxide contains at least 80 mol % or more of Ni and Al relative to the total number of moles of metal elements excluding Li, The surface modification layer contains at least one of Sr and Ca, Ba, and Al.

2. The lithium transition metal composite oxide has the general formula Li ａ Ni ｘ Al ｙ Co ｚ M ｗ O ２－ｂ 2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the positive electrode active material is represented by the formula (wherein 0.95<a<1.05, 0.8≦x≦0.96, 0<y≦0.10, 0≦z≦0.15, 0≦w≦0.1, 0≦b<0.05, x+y+z+w=1, and M is at least one element selected from Mn, Fe, Ti, Si, Nb, Zr, Mo, and Zn).

3. 3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein a ratio of Sr and Ca in the surface modification layer is 0.05 mol % to 0.25 mol % with respect to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

4. 4. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the ratio of Ni to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide is 90 mol % or more.

5. 5. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the surface modification layer has a thickness of 0.1 nm to 2 nm.

6. A non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, a negative electrode, and a non-aqueous electrolyte.

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