Positive electrode active material for non-aqueous electrolyte secondary batteries and non-aqueous electrolyte secondary batteries

Abstract

To enhance the thermal stability of a positive electrode active substance including lithium transition metal oxide of which the proportion of Ni is in a range of 75-95 mol% to a total amount of metal elements excluding Li.SOLUTION: A positive electrode active substance for a nonaqueous electrolyte secondary battery, which is an embodiment disclosed herein comprises a lithium transition metal oxide having a layered structure and containing Ni, Mn and Co of an optional element. With the positive electrode active substance, a proportion of Ni in the lithium transition metal oxide is in a range of 75-95 mol%; a proportion of Mn in the lithium transition metal oxide is equal to or larger than a proportion of Co in the lithium transition metal oxide; a proportion of Co in the lithium transition metal oxide is in a range of 0 -2 mol%; and the proportion of metal elements which is present in a Li layer of the layered structure, excluding Li is in a range of 1-2.5 mol%. On the lithium transition metal oxide, a half-value width n of a diffraction peak of (208) plane of an X-ray diffraction pattern on X-ray diffraction satisfies: 0.30°≤n≤0.50°.SELECTED DRAWING: None

Description

Technical Field

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

Background Art

[0002] In recent years, as a secondary battery with high output and high energy density, a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, which performs charge and discharge by moving lithium ions or the like between the positive electrode and the negative electrode, has been widely used.

[0003] As the positive electrode active material used for the positive electrode of the non-aqueous electrolyte secondary battery, for example, the following are known.

[0004] For example, in Patent Document 1, a positive electrode active material composed of a composite oxide represented by the composition formula Li 3b Ni b Co c Mn d O2 (where 0.1 ≦ a ≦ 1.2, 0.40 ≦ b < 1.15, 0 < c < 0.60, 0 < d < 0.60, and 1.00 ≦ b + c + d ≦ 1.15, 0 < c + d ≦ 0.60), and the transition metal occupancy e of the Li layer is in the range of 0.006 ≦ e ≦ 0.150, is disclosed.

[0005] Also, for example, in Patent Document 2, [Li] 3a [Ni 1-x-y Co x Al y 3b [O2] 6c (However, the subscript in [ ] represents a site, and x and y satisfy the conditions of 0 < x ≦ 0.20 and 0 < y ≦ 0.15), and in a hexagonal lithium nickel composite oxide having a layered structure, the site occupancy of metal ions other than lithium at the 3a site obtained from the Rietveld analysis of the X-ray diffraction pattern is 3% or less, and the average particle size of the primary particles is 0.1 μm or more, and the primary particles are aggregated to form secondary particles, a positive electrode active material is disclosed.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] Japanese Patent Publication No. 2000-133262 [Patent Document 2] Japanese Patent Publication No. 2000-30693 [Overview of the project] [Problems that the invention aims to solve]

[0007] Incidentally, positive electrode active materials containing lithium transition metal oxides, in which the proportion of Ni is in the range of 75 mol% to 95 mol% relative to the total amount of metal elements excluding Li, exhibit high charge and discharge capacity, but have the problem of low thermal stability. If the thermal stability of the positive electrode is low, the self-heating start temperature of the battery will be low, and if the temperature inside the battery rises due to overcharging or a short circuit, further chemical reactions (self-heating reactions) that generate heat will proceed inside the battery as the temperature rises, which may cause the battery temperature to rise even further.

[0008] Therefore, this disclosure aims to improve the thermal stability of a cathode active material containing a lithium transition metal oxide in which the proportion of Ni is in the range of 75 mol% to 95 mol% relative to the total amount of metal elements excluding Li. [Means for solving the problem]

[0009] A positive electrode active material for a non-aqueous electrolyte secondary battery, according to one aspect of the present disclosure, comprises a layered lithium transition metal oxide containing Ni, Mn, and an arbitrary element Co, wherein the ratio of Ni to the total amount of metal elements excluding Li in the lithium transition metal oxide is in the range of 75 mol% to 95 mol%, the ratio of Mn to the total amount of metal elements excluding Li in the lithium transition metal oxide is greater than the ratio of Co to the total amount of metal elements excluding Li in the lithium transition metal oxide, and the lithium transition metal oxide is characterized in that the full width at half maximum n of the diffraction peak of the (208) plane in the X-ray diffraction pattern obtained by X-ray diffraction is 0.30° ≤ n ≤ 0.50°.

[0010] One embodiment of the present disclosure of a non-aqueous electrolyte secondary battery is characterized by comprising a positive electrode having the above-mentioned positive electrode active material for a non-aqueous electrolyte secondary battery. [Effects of the Invention]

[0011] According to one aspect of this disclosure, it is possible to improve the thermal stability of a cathode active material containing a lithium transition metal oxide in which the proportion of Ni is in the range of 75 mol% to 95 mol% relative to the total amount of metal elements excluding Li. [Modes for carrying out the invention]

[0012] A positive electrode active material for a non-aqueous electrolyte secondary battery, according to one aspect of the present disclosure, comprises a layered lithium transition metal oxide containing Ni, Mn, and an arbitrary element Co, wherein the ratio of Ni to the total amount of metal elements excluding Li in the lithium transition metal oxide is in the range of 75 mol% to 95 mol%, the ratio of Mn to the total amount of metal elements excluding Li in the lithium transition metal oxide is greater than or equal to the ratio of Co to the total amount of metal elements excluding Li in the lithium transition metal oxide, the ratio of Co to the total amount of metal elements excluding Li in the lithium transition metal oxide is in the range of 0 mol% to 2 mol%, the ratio of metal elements other than Li present in the Li layer of the layered structure is in the range of 1 mol% to 2.5 mol% of the total amount of metal elements excluding Li in the lithium transition metal oxide, and the lithium transition metal oxide is characterized in that the full width at half maximum n of the diffraction peak of the (208) plane in the X-ray diffraction pattern obtained by X-ray diffraction is 0.30° ≤ n ≤ 0.50°.

[0013] Lithium transition metal oxides, where the proportion of Ni is typically in the range of 75 mol% to 95 mol% relative to the total amount of metal elements excluding Li, have low thermal stability. This is thought to be because lithium transition metal oxides with a Ni proportion of 75 mol% or more have low structural stability.

[0014] However, according to one aspect of this disclosure, it is presumed that the release of oxygen in the layered structure is suppressed by the Mn contained in the lithium transition metal oxide, and that the repulsion between O in the layered structure is suppressed during charging by the presence of a predetermined amount of metal elements other than Li in the Li layer of the layered structure, thereby stabilizing the layered structure. Furthermore, it is presumed that by limiting the proportion of Ni to 95 mol% or less, the reactivity of Ni is suppressed, so that the effects of Mn and the effects of metal elements other than Li present in a predetermined amount in the Li layer of the layered structure are sufficiently obtained, thereby stabilizing the layered structure. Moreover, it is presumed that by limiting the amount of Co contained in the lithium transition metal oxide and making the Mn content greater than the Co content, the effects of Mn are sufficiently obtained, thereby stabilizing the layered structure. Furthermore, the full width at half maximum of the diffraction peak of the (208) plane in the X-ray diffraction pattern obtained by X-ray diffraction is an indicator of the fluctuation in the arrangement between the layered Li layer and the transition metal layer. However, as in one aspect of this disclosure, when it falls within the predetermined range, it is thought that a moderate fluctuation occurs in the arrangement between the layered Li layer and the transition metal layer, leading to the stabilization of the layered structure. Thus, each of the above configurations in one aspect of this disclosure contributes to the stabilization of the layered structure of the lithium transition metal oxide, and the effect of improving thermal stability is brought about only through the combination of these configurations.

[0015] The following describes an example of a non-aqueous electrolyte secondary battery using a positive electrode active material for non-aqueous electrolyte secondary batteries, which is one aspect of this disclosure.

[0016] An example of an embodiment of a non-aqueous electrolyte secondary battery comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte. A separator is preferably provided between the positive and negative electrodes. Specifically, it has a structure in which a wound-type electrode body, in which the positive and negative electrodes are wound around a separator, and the non-aqueous electrolyte are housed in an outer casing. The electrode body is not limited to a wound-type electrode body; other forms of electrode bodies may be used, such as a laminated electrode body in which the positive and negative electrodes are laminated with a separator in between. Furthermore, the form of the non-aqueous electrolyte secondary battery is not particularly limited, and examples include cylindrical, prismatic, coin-type, button-type, and laminate-type batteries.

[0017] The following describes in detail the positive electrode, negative electrode, non-aqueous electrolyte, and separator used in a non-aqueous electrolyte secondary battery, which is an example of an embodiment.

[0018] <Positive electrode> The positive electrode is composed of a positive electrode current collector, such as a metal foil, and a positive electrode active material layer formed on the positive electrode current collector. The positive electrode current collector can be made of a metal foil that is stable in the positive electrode potential range, such as aluminum, or a film with the metal arranged on its surface. The positive electrode active material layer includes, for example, a positive electrode active material, a binder, a conductive material, etc.

[0019] The positive electrode is obtained, for example, by applying and drying a positive electrode composite slurry containing positive electrode active material, binder, conductive material, etc., onto a positive electrode current collector to form a positive electrode active material layer on the positive electrode current collector, and then rolling the positive electrode active material layer.

[0020] The positive electrode active material includes a layered lithium transition metal oxide containing Ni, Mn, and an optional Co element. Hereinafter, the layered lithium transition metal oxide containing Ni, Mn, and an optional Co element will be referred to as "the lithium transition metal oxide of this embodiment."

[0021] The layered structure of the lithium transition metal oxide in this embodiment can be, for example, a layered structure belonging to space group R-3m or a layered structure belonging to space group C2 / m. Among these, a layered structure belonging to space group R-3m is preferred in terms of high capacity and stability of the layered structure.

[0022] In this embodiment, the ratio of Ni to the total amount of metal elements excluding Li in the lithium transition metal oxide is in the range of 75 mol% to 95 mol%, preferably in the range of 85 mol% to 95 mol%, in order to increase the battery capacity and improve thermal stability. If the ratio of Ni exceeds 95 mol%, the reactivity of Ni increases, leading to a decrease in thermal stability due to a decrease in the stability of the layered structure. Furthermore, if the ratio of Ni is less than 75 mol%, it becomes difficult to increase the battery capacity in the first place.

[0023] In this embodiment, the ratio of Mn to the total amount of metal elements excluding Li in the lithium transition metal oxide is sufficient to enhance thermal stability, etc., as long as it is greater than or equal to the ratio of Co to the total amount of metal elements excluding Li in the lithium transition metal oxide of this embodiment, but is preferably in the range of 1 mol% to 10 mol%, and more preferably in the range of 2 mol% to 8 mol%. If the ratio of Co is greater than the ratio of Mn, the release of oxygen in the layered structure is not suppressed, and the stability of the layered structure decreases, leading to a decrease in thermal stability.

[0024] In this embodiment, the ratio of Co to the total amount of metal elements excluding Li in the lithium transition metal oxide is in the range of 0 mol% to 2 mol%, preferably in the range of 0.5 to 2, in order to enhance thermal stability. If the ratio of Co exceeds 2 mol%, for example, the above effect by Mn is reduced, and the stability of the layered structure decreases, leading to a decrease in thermal stability.

[0025] The lithium transition metal oxide of this embodiment may contain metal elements other than Li, Ni, Mn, and Co. Examples include at least one metal element selected from Al, Fe, Mg, Si, Ti, Cr, Cu, Sn, Zr, Nb, Mo, Ta, W, Na, K, Ba, Sr, Bi, Be, Zn, Ca, and B. Among these, at least one metal element selected from Al, Fe, Nb, Si, Mo, and Ti is preferred in terms of suppressing the deterioration of charge-discharge cycle characteristics, and Al is further preferred among these. Other metal elements may be uniformly dispersed within the layered structure of the lithium transition metal oxide of this embodiment, or they may be present in a part of the layered structure. Furthermore, during the manufacturing stage of the lithium transition metal oxide of this embodiment, some of the other metal elements contained within the layered structure may precipitate on the particle surface of the lithium transition metal oxide of this embodiment, and these precipitated metal elements are also metal elements constituting the lithium transition metal oxide of this embodiment.

[0026] The content of the elements constituting the lithium transition metal oxide in this embodiment can be measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron beam microanalyzer (EPMA), an energy dispersive X-ray spectrometer (EDX), or the like.

[0027] In this embodiment, the lithium transition metal oxide has metal elements other than Li in its layered Li layer. The proportion of metal elements other than Li present in the layered Li layer is 1 mol% to 2.5 mol%, preferably 1 mol% to 2 mol%, relative to the total amount of metal elements other than Li in the lithium transition metal oxide, in order to raise the self-heating onset temperature. If the proportion of metal elements other than Li present in the layered Li layer is outside the above range, the stability of the layered structure decreases, leading to a decrease in thermal stability. The metal elements other than Li present in the layered Li layer are mainly Ni, based on the proportion of elements constituting the lithium transition metal oxide in this embodiment, but other metal elements may also be present.

[0028] The proportion of metal elements other than Li present in the layered Li layer is obtained from the Rietveld analysis results of the X-ray diffraction pattern obtained by X-ray diffraction measurement of the lithium transition metal oxide in this embodiment.

[0029] The X-ray diffraction pattern was obtained by powder X-ray diffraction using a powder X-ray diffractometer (manufactured by Rigaku Corporation, product name "RINT-TTR", source Cu-Kα) under the following conditions. Measurement range: 15-120° Scan speed: 4° / min Analysis range: 30-120° Background; B-spline Profile function; Split type pseudo-Voigt function Constraint condition; Li(3a) + Ni(3a)=1 Ni(3a) + Ni(3b) = y ICSD No.; 98-009-4814

[0030] Furthermore, PDXL2 (Rigaku Corporation), a Rietveld analysis software, is used for Rietveld analysis of X-ray diffraction patterns.

[0031] In the lithium transition metal oxide of this embodiment, the full width at half maximum (FWHM) n of the diffraction peak of the (208) plane in the X-ray diffraction pattern obtained by the above-mentioned X-ray diffraction is 0.30° ≤ n ≤ 0.50°, preferably 0.30° ≤ n ≤ 0.45°, in terms of improving thermal stability. If the FWHM n of the diffraction peak of the (208) plane is outside the above range, the fluctuations in the arrangement between the Li layer and the transition metal layer in the layered structure become too small or too large, which reduces the stability of the layered structure and leads to a decrease in thermal stability.

[0032] In this embodiment, the lithium transition metal oxide preferably has a lattice constant a, which indicates the a-axis length of the crystal structure obtained from the X-ray diffraction pattern obtained by the above-mentioned X-ray diffraction, in the range of 2.867 Å ≤ a ≤ 2.877 Å, and a lattice constant c, which indicates the c-axis length, in the range of 14.18 Å ≤ c ≤ 14.21 Å. If the lattice constant a is smaller than 2.867 Å, ​​the interatomic distance in the crystal structure becomes narrower and more unstable compared to when the above range is satisfied, which may lead to a decrease in thermal stability. Also, if the lattice constant a is larger than 2.877 Å, the interatomic distance in the crystal structure becomes wider and more unstable, which may lead to a decrease in thermal stability compared to when the above range is satisfied. Also, if the lattice constant c is smaller than 14.18 Å, the interatomic distance in the crystal structure becomes narrow and more unstable, which may lead to a decrease in thermal stability compared to when the above range is satisfied. Furthermore, if the lattice constant c is greater than 14.21 Å, the interatomic distances in the crystal structure become wide, resulting in an unstable structure, which may lead to a decrease in thermal stability compared to when the above range is satisfied.

[0033] In this embodiment, the lithium transition metal oxide preferably has a crystallite size s calculated from the full width at half maximum of the (10⁴) plane diffraction peak of the X-ray diffraction pattern obtained by the above-mentioned X-ray diffraction, using Scherrer's equation, such that 400 Å ≤ s ≤ 650 Å. If the crystallite size s of the lithium transition metal oxide in this embodiment is outside the above range, the stability of the layered structure may decrease, leading to a decrease in thermal stability. Scherrer's equation is expressed as follows.

[0034] s = Kλ / Bcosθ In the formula, s is the crystallite size, λ is the wavelength of the X-ray, B is the full width at half maximum of the diffraction peak of the (10⁴) plane, θ is the diffraction angle (rad), and K is the Scherrer constant. In this embodiment, K is set to 0.9.

[0035] In this embodiment, the lithium transition metal oxide content is preferably 90% by mass or more, and more preferably 99% by mass or more, relative to the total mass of the positive electrode active material, for example, in terms of effectively improving the thermal stability of the positive electrode active material.

[0036] Furthermore, the positive electrode active material of this embodiment may contain other lithium transition metal oxides in addition to the lithium transition metal oxide of this embodiment. Examples of other lithium transition metal oxides include lithium transition metal oxides with a Ni content of 0 mol% to less than 75 mol%.

[0037] An example of a method for producing lithium transition metal oxides according to this embodiment will be described.

[0038] The lithium transition metal oxide manufacturing method of this embodiment comprises, for example, a first step of obtaining a composite oxide containing Ni, Mn, and an arbitrary metal element; a second step of mixing the composite oxide obtained in the first step with a Li compound; and a third step of calcining the mixture. In the lithium transition metal oxide finally obtained according to this embodiment, parameters such as the proportion of metal elements other than Li present in the Li layer of its layered structure, the full width at half maximum n of the (208) plane diffraction peak, lattice constant a, lattice constant c, and crystallite size s are adjusted by controlling the mixing ratio of raw materials in the second step and the calcination conditions in the third step.

[0039] In the first step, for example, while stirring a solution of a metal salt containing Ni, Mn, and an arbitrary metal element (Co, Al, etc.), an alkaline solution such as sodium hydroxide is added dropwise to adjust the pH to the alkaline side (e.g., 8.5 to 14.0), thereby precipitating (coprecipitation) a composite hydroxide containing Ni, Mn, and the arbitrary metal element. By calcining this composite hydroxide, a composite oxide containing Ni, Mn, and the arbitrary metal element is obtained. The mixing ratio of Ni to the arbitrary metal element can be appropriately determined so that the proportions of Ni, Mn, etc. are within the above range. The calcination temperature is not particularly limited, but for example, it is in the range of 500°C to 600°C.

[0040] In the second step, the composite oxide obtained in the first step is mixed with a Li compound to obtain a mixture. The mixing ratio of the composite oxide obtained in the first step and the Li compound is such that the molar ratio of metal elements other than Li to Li is in the range of 1:0.98 to 1:1.08, while adjusting each of the above parameters to the range specified above. In the second step, when mixing the composite oxide obtained in the first step with the Li compound, other metal raw materials may be added as needed. Other metal raw materials include metal elements that constitute the composite oxide obtained in the first step and oxides containing metal elements other than Li.

[0041] In the third step, the mixture obtained in the second step is calcined at a predetermined temperature and time to obtain the lithium transition metal oxide according to this embodiment. The calcination of the mixture in the third step employs the following two-stage calcination, in which the above parameters are adjusted to the range specified above. The first-stage calcination conditions are, for example, calcining to the range of 450°C to 650°C at a heating rate in the range of 5.5°C / min to 1.5°C / min, with a holding time of 0 to 5 hours. The second-stage calcination conditions are, for example, calcining to the range of the first-stage calcination temperature to 800°C at a heating rate in the range of 3.5°C / min to 0.1°C / min, with a holding time of 1 to 10 hours. When calcining the mixture, in which the above parameters are adjusted to the range specified above, for example, it 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 in the calcination furnace. 3 The flow rate should be in the range of 0.2 mL / min to 4 mL / min per unit area, and at least 0.3 L / min per 1 kg of mixture.

[0042] The following describes the other materials included in the positive electrode active material layer.

[0043] Examples of conductive materials included in the positive electrode active material layer include carbon black, acetylene black, Ketjen black, and carbon powder such as graphite. These may be used individually or in combination of two or more types.

[0044] Examples of binders included in the positive electrode active material layer include fluorine-based polymers and rubber-based polymers. Examples of fluorine-based polymers include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), or modified versions thereof, while examples of rubber-based polymers include ethylene-propylene-isoprene copolymers and ethylene-propylene-butadiene copolymers. These may be used individually or in combination of two or more.

[0045] <Negative electrode> The negative electrode comprises a negative electrode current collector, such as a metal foil, and a negative electrode active material layer formed on the negative electrode current collector. The negative electrode current collector can be a metal foil that is stable in the negative electrode potential range, such as copper, or a film with the metal arranged on its surface. The negative electrode active material layer includes, for example, a negative electrode active material, a binder, a thickener, etc.

[0046] The negative electrode can be obtained, for example, by applying and drying a negative electrode mixture slurry containing a negative electrode active material, a thickener, and a binder onto a negative electrode current collector to form a negative electrode active material layer on the negative electrode current collector, and then rolling the negative electrode active material layer.

[0047] The negative electrode active material contained in the negative electrode active material layer is not particularly limited as long as it is a material capable of intercalating and releasing lithium ions. Examples include carbon materials, metals capable of forming alloys with lithium, or alloy compounds containing such metals. As carbon materials, graphites such as natural graphite, non-graphitizable carbon, and artificial graphite, as well as cokes, can be used. As alloy compounds, those containing at least one metal capable of forming alloys with lithium can be used. Silicon and tin are preferred elements capable of forming alloys with lithium, and silicon oxide and tin oxide, which are formed by the combination of these elements with oxygen, can also be used. Furthermore, a mixture of the above carbon material and a compound of silicon or tin can be used. In addition to the above, materials such as lithium titanate, which have a charge-discharge potential higher than that of carbon materials, can also be used.

[0048] As the binder included in the negative electrode active material layer, for example, fluorine-based polymers, rubber-based polymers, etc., can be used, as in the case of the positive electrode, but styrene-butadiene copolymer (SBR) or a modified version thereof may also be used. As the binder included in the negative electrode active material layer, fluorine-based resins, PAN, polyimide-based resins, acrylic-based resins, polyolefin-based resins, etc., can be used, as in the case of the positive electrode. When preparing the negative electrode mixture slurry using an aqueous solvent, it is preferable to use styrene-butadiene rubber (SBR), CMC or its salts, polyacrylic acid (PAA) or its salts (PAA-Na, PAA-K, etc., or partially neutralized salts), polyvinyl alcohol (PVA), etc.

[0049] Examples of thickening agents included in the negative electrode active material layer include carboxymethylcellulose (CMC) and polyethylene oxide (PEO). These may be used individually or in combination of two or more.

[0050] <Non-aqueous electrolytes> A non-aqueous electrolyte comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The non-aqueous electrolyte is not limited to a liquid electrolyte (non-aqueous electrolyte solution), but may also be a solid electrolyte using a gel-like polymer or the like. Examples of non-aqueous solvents include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain halogen-substituted solvents in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine.

[0051] Examples of the above esters include cyclic carbonate esters such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; linear carbonate esters 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 linear carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate, and γ-butyrolactone.

[0052] Examples of the above 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 methylphenyl ether. Examples include chain ethers such as ethylphenyl ether, butylphenyl ether, pentylphenyl 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.

[0053] As the halogen-substituted product, it is preferable to use fluorinated cyclic carbonate esters such as fluoroethylene carbonate (FEC), fluorinated linear carbonate esters, or fluorinated linear carboxylic acid esters such as methyl fluoropropionate (FMP).

[0054] The electrolyte salt is preferably a lithium salt. Examples of lithium salts include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), 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, borate salts such as Li2B4O7, Li(B(C2O4)F2), imide salts such as LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 SO2){l, m are integers of 0 or more}, and the like. The lithium salt may be used alone or in combination of a plurality of kinds. Among these, from the viewpoints of ion conductivity, electrochemical stability, etc., it is preferable to use LiPF6. The concentration of the lithium salt is preferably 0.8 to 1.8 mol per 1 L of the non-aqueous solvent.

[0055] <Separator> For example, a porous sheet having ion permeability and insulating properties can be used as the separator. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator include polyethylene, olefin resins such as polypropylene, and cellulose. The separator may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin resin, or a separator with an aramid resin or the like coated on its surface may be used. A filler layer containing an inorganic filler may be formed at the interface between the separator and at least one of the positive electrode and the negative electrode. Examples of inorganic fillers include oxides containing at least one of titanium (Ti), aluminum (Al), silicon (Si), and magnesium (Mg), phosphate compounds, or those whose surfaces are treated with hydroxides or the like. The filler layer can be formed, for example, by coating a slurry containing the filler onto the surface of the positive electrode, the negative electrode, or the separator. [Examples]

[0056] The present invention will be further described below with reference to examples, but the present invention is not limited to these examples.

[0057] <Example 1> [Fabrication of positive electrode active material] A composite oxide containing Ni, Mn, Co, and Al (Ni 0.90 Mn 0.04 Co 0.02 Al 0.04 O2 and LiOH were mixed so that the molar ratio of Li to the total amount of Ni, Mn, Co, and Al was 1:1.03. The flow rate was 10 cm³. 3The mixture was calcined in an oxygen stream with an oxygen concentration of 95% at a heating rate of 3.5°C / min to 650°C, and then calcined from 650°C to 730°C at a heating rate of 0.5°C / min. The calcined product was washed with water to obtain lithium transition metal oxide. The proportions of Ni, Mn, Co, and Al in the obtained lithium transition metal oxide were measured, and the proportions were found to be 90 mol% for Ni, 4 mol% for Mn, 2 mol% for Co, and 4 mol% for Al.

[0058] Furthermore, powder X-ray diffraction measurements were performed on the lithium transition metal oxide of Example 1 under the conditions described above to obtain an X-ray diffraction pattern. As a result, diffraction lines indicating a layered structure were confirmed, the proportion of metal elements other than Li present in the Li layer was 1.7 mol%, the full width at half maximum of the (208) plane diffraction peak was 0.43°, the lattice constant a was 2.873 Å, the lattice constant c was 14.20 Å, and the crystallite size s was 548 Å. This was used as the positive electrode active material for Example 1.

[0059] <Comparative Example 1-1> A composite oxide containing Ni, Mn, Co, and Al (Ni 0.93 Co 0.02 Al 0.05 O2), Ti(OH)4, and LiOH were mixed so that the total amount of Ni, Mn, Co, Al, and Ti was mixed with Li in a molar ratio of 1:1.05, and the flow rate was 10 cm³. 3 Lithium transition metal oxide was prepared in the same manner as in Example 1, except that the flow rate was 0.1 mL / min per unit area and 0.25 L / min per 1 kg of transition metal oxide. The proportions of Ni, Co, Al, and Ti in the obtained lithium transition metal oxide were measured, and the proportions were found to be 90 mol% for Ni, 2 mol% for Co, 5 mol% for Al, and 3 mol% for Ti.

[0060] Furthermore, powder X-ray diffraction measurements were performed on the lithium transition metal oxide of Comparative Example 1-1 in the same manner as in Example 1. The results showed diffraction lines indicating a layered structure, with the proportion of metal elements other than Li present in the Li layer being 1.4 mol%, and the full width at half maximum of the diffraction peak of the (208) plane being 0.37°. This was used as the positive electrode active material for Comparative Example 1-1.

[0061] <Comparative Example 1-2> A lithium transition metal oxide was prepared in the same manner as in Example 1, except that the total amount of Ni, Mn, Co, and Al was mixed to have a molar ratio of 1:0.97 with Li. The proportions of Ni, Mn, Co, and Al in the obtained lithium transition metal oxide were measured, and the proportions were found to be 90 mol% for Ni, 4 mol% for Mn, 2 mol% for Co, and 4 mol% for Al.

[0062] Furthermore, powder X-ray diffraction measurements were performed on the lithium transition metal oxides of Comparative Examples 1-2 in the same manner as in Example 1. The results showed diffraction lines indicating a layered structure, with the proportion of metal elements other than Li present in the Li layer being 3 mol%, and the full width at half maximum of the diffraction peak of the (208) plane being 0.48°. This was used as the positive electrode active material for Comparative Examples 1-2.

[0063] <Comparative Example 1-3> Lithium transition metal oxides were prepared in the same manner as in Example 1, except that the total amount of Ni, Mn, Co, and Al was mixed to have a molar ratio of 1:1.1 with Li. The proportions of Ni, Mn, Co, and Al in the obtained lithium transition metal oxide were measured, and the proportions were found to be 90 mol% for Ni, 4 mol% for Mn, 2 mol% for Co, and 4 mol% for Al.

[0064] Furthermore, powder X-ray diffraction measurements were performed on the lithium transition metal oxides of Comparative Examples 1-3 in the same manner as in Example 1. The results showed diffraction lines indicating a layered structure, with the proportion of metal elements other than Li present in the Li layer being 0.5 mol%, and the full width at half maximum of the diffraction peak of the (208) plane being 0.31°. This was used as the positive electrode active material for Comparative Examples 1-3.

[0065] <Comparative Example 1-4> A composite oxide containing Ni, Mn, Co, and Al (Ni 0.92 Co 0.02 Al 0.05 Lithium transition metal oxides were prepared in the same manner as in Example 1, except that O2), Nb2O5, and LiOH were mixed so that the molar ratio of the total amount of Ni, Mn, Co, Al, and Nb to Li was 1:1.1, and the mixture was calcined at a heating rate of 1°C / min to 650°C, and then calcined from 650°C to 730°C at a heating rate of 0.05°C / min. The proportions of Ni, Mn, Co, Al, and Nb in the obtained lithium transition metal oxide were measured, and the proportions were 90 mol% for Ni, 4 mol% for Mn, 2 mol% for Co, 2 mol% for Al, and 2 mol% for Nb.

[0066] Furthermore, powder X-ray diffraction measurements were performed on the lithium transition metal oxides of Comparative Examples 1-4 in the same manner as in Example 1. The results showed diffraction lines indicating a layered structure, with the proportion of metal elements other than Li present in the Li layer being 3 mol%, and the full width at half maximum of the diffraction peak of the (208) plane being 0.23°. This was used as the positive electrode active material for Comparative Examples 1-4.

[0067] <Comparative Example 1-5> Lithium transition metal oxides were prepared in the same manner as in Example 1, except that the molar ratio of the total amount of Ni, Mn, Co, and Al to Li was changed to 1:1.1, and the maximum temperature reached during firing was changed to 680°C. The proportions of Ni, Mn, Co, and Al in the obtained lithium transition metal oxide were measured, and the proportions of Ni were 90 mol%, Mn was 4 mol%, Co was 2 mol%, and Al was 4 mol%.

[0068] Furthermore, powder X-ray diffraction measurements were performed on the lithium transition metal oxides of Comparative Examples 1-5 in the same manner as in Example 1. The results showed diffraction lines indicating a layered structure, with the proportion of metal elements other than Li present in the Li layer being 0.5 mol%, and the full width at half maximum of the diffraction peak of the (208) plane being 0.64°. This was used as the positive electrode active material for Comparative Examples 1-5.

[0069] [Fabrication of the positive electrode] 95 parts by mass of the positive electrode active material from Example 1, 3 parts by mass of acetylene black as a conductive material, and 2 parts by mass of polyvinylidene fluoride as a binder were mixed. The mixture was kneaded using a kneader (TK Hibismix, manufactured by Primix Corporation) to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to a 15 μm thick aluminum foil, and the coating was dried to form a positive electrode active material layer on the aluminum foil. This was used as the positive electrode for Example 1. The positive electrodes for Comparative Examples 1-1 to 1-5 were prepared in the same manner.

[0070] [Preparation of non-aqueous electrolytes] Ethylene carbonate (EC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 3:3:4. Lithium hexafluoride phosphate (LiPF6) was dissolved in this mixed solvent to a concentration of 1.2 mol / liter to prepare a non-aqueous electrolyte.

[0071] [Preparation of test cells] The positive electrode of Example 1 and the negative electrode, which uses graphite as the negative electrode active material, were stacked facing each other with a separator in between, and this was wound to create an electrode body. Next, the electrode body and the non-aqueous electrolyte were inserted into an aluminum casing to create a test cell. Comparative Examples 1-1 to 1-5 were prepared in the same manner.

[0072] [Runaway reaction measurement test] The self-heating start temperature of the battery was measured using an Accelerated Rate Calorimeter (ARC, manufactured by Thermal Hazard Technology) under the following conditions. Measurement start temperature: 100℃ Holding temperature: 20min Fever detection temperature: 0.02℃ / min Temperature increase range: 5℃ Battery voltage: 4.2V (charged) If no temperature increase above the fever detection temperature is detected for 20 minutes from the measurement start temperature, the temperature is increased and measurement is performed again in the next step. If a temperature increase above the fever detection temperature is detected, it is determined that self-heating has started, and the temperature at that time is set as the self-heating start temperature.

[0073] Table 1 summarizes the results for the self-heating onset temperature of the batteries in Example 1 and Comparative Examples 1-1 to 1-5. In Table 1, the self-heating onset temperature of Comparative Example 1-1 is used as the baseline, and the difference between this temperature and the self-heating onset temperatures of Example 1 and the other comparative examples is calculated and shown as the change in self-heating onset temperature. Therefore, a positive value indicates that the self-heating onset temperature was increased, and the thermal stability of the positive electrode active material was improved.

[0074] [Table 1]

[0075] As can be seen from the results in Table 1, only in Example 1 was the self-heating start temperature increased, resulting in improved thermal stability of the positive electrode active material.

[0076] <Example 2> [Fabrication of positive electrode active material] A composite oxide containing Ni, Mn, and Co (Ni 0.94 Mn 0.04 Co 0.02 O2 and LiOH were mixed so that the molar ratio of Li to the total amount of Ni, Mn, and Co was 1:1.03. The flow rate was 10 cm³. 3 The mixture was calcined in an oxygen stream with an oxygen concentration of 95% at a heating rate of 3.5°C / min to 650°C, and then calcined from 650°C to 700°C at a heating rate of 0.5°C / min. The calcined product was washed with water to obtain lithium transition metal oxide. The proportions of Ni, Mn, and Co in the obtained lithium transition metal oxide were measured, and the proportions of Ni were 94 mol%, Mn were 4 mol%, and Co were 2 mol%.

[0077] Furthermore, powder X-ray diffraction measurements were performed on the lithium transition metal oxide of Example 2 in the same manner as in Example 1. The results showed diffraction lines indicating a layered structure, with the proportion of metal elements other than Li present in the Li layer being 2.1 mol%, the full width at half maximum of the (208) plane diffraction peak being 0.44°, the lattice constant a being 2.875 Å, the lattice constant c being 14.21 Å, and the crystallite size s being 607 Å. This was used as the positive electrode active material for Example 2.

[0078] <Comparative Example 2-1> A composite oxide containing Ni, Mn, and Co is a composite oxide containing Ni, Mn, Co, and Al (Ni 0.94 Mn 0.02 Co 0.03 Al 0.01 Lithium transition metal oxides were prepared in the same manner as in Example 2, except that O2 was used instead. The proportions of Ni, Mn, Co, and Al in the obtained lithium transition metal oxide were measured, and the proportions were found to be 94 mol% for Ni, 2 mol% for Mn, 3 mol% for Co, and 1 mol% for Al.

[0079] Furthermore, powder X-ray diffraction measurements were performed on the lithium transition metal oxide of Comparative Example 2-1 in the same manner as in Example 1. The results showed diffraction lines indicating a layered structure, with the proportion of metal elements other than Li present in the Li layer being 1.4 mol%, and the full width at half maximum of the diffraction peak of the (208) plane being 0.32°. This was used as the positive electrode active material for Comparative Example 2-1.

[0080] <Comparative Example 2-2> A composite oxide containing Ni, Mn, and Co (Ni 0.94 Mn 0.03 Co 0.03 Lithium transition metal oxides were prepared in the same manner as in Example 2, except that O2 was used instead. The proportions of Ni, Mn, and Co in the obtained lithium transition metal oxide were measured, and the proportions of Ni were 94 mol%, Mn was 3 mol%, and Co was 3 mol%.

[0081] Furthermore, powder X-ray diffraction measurements were performed on the lithium transition metal oxide of Comparative Example 2-2 in the same manner as in Example 1. The results showed diffraction lines indicating a layered structure, with the proportion of metal elements other than Li present in the Li layer being 1.9 mol%, and the full width at half maximum of the diffraction peak of the (208) plane being 0.49°. This was used as the positive electrode active material for Comparative Example 2-2.

[0082] <Comparative Example 2-3> A composite oxide containing Ni, Mn, and Co is a composite oxide containing Ni, Mn, Co, and Al (Ni 0.94 Mn 0.01 Co 0.02 Al 0.03 Lithium transition metal oxides were prepared in the same manner as in Example 2, except that O2 was used instead. The proportions of Ni, Mn, Co, and Al in the obtained lithium transition metal oxide were measured, and the proportions were found to be 94 mol% for Ni, 1 mol% for Mn, 2 mol% for Co, and 3 mol% for Al.

[0083] Furthermore, powder X-ray diffraction measurements were performed on the lithium transition metal oxides of Comparative Examples 2-3 in the same manner as in Example 1. The results showed diffraction lines indicating a layered structure, with the proportion of metal elements other than Li present in the Li layer being 1.6 mol%, and the full width at half maximum of the diffraction peak of the (208) plane being 0.32°. This was used as the positive electrode active material for Comparative Example 2-3.

[0084] <Comparative Example 2-4> A composite oxide containing Ni, Mn, and Co is a composite oxide containing Ni, Co, and Al (Ni 0.94 Co 0.015 Al 0.045 Lithium transition metal oxides were prepared in the same manner as in Example 2, except that O2 was used instead. The proportions of Ni, Co, and Al in the obtained lithium transition metal oxide were measured, and the proportions of Ni were 94 mol%, Co was 1.5 mol%, and Al was 4.5 mol%.

[0085] Furthermore, powder X-ray diffraction measurements were performed on the lithium transition metal oxides of Comparative Examples 2-4 in the same manner as in Example 1. The results showed diffraction lines indicating a layered structure, with the proportion of metal elements other than Li present in the Li layer being 1.1 mol%, and the full width at half maximum of the diffraction peak of the (208) plane being 0.4°. This was used as the positive electrode active material for Comparative Example 2-4.

[0086] Using the positive electrode active materials of Example 2 and Comparative Examples 2-1 to 2-4, test cells were prepared in the same manner as in Example 1, and the self-heating start temperature of the battery was measured under the same conditions as described above.

[0087] Table 2 summarizes the results for the self-heating onset temperature of Example 2 and Comparative Examples 2-1 to 2-4. In Table 2, the difference between the self-heating onset temperature of Comparative Example 2-4 and the self-heating onset temperatures of Example 2 and the other comparative examples is calculated and shown as the change in self-heating onset temperature. Therefore, a positive value indicates that the self-heating onset temperature was increased and the thermal stability of the positive electrode active material was improved.

[0088] [Table 2]

[0089] As can be seen from the results in Table 2, only in Example 2 was the self-heating start temperature increased, resulting in improved thermal stability of the positive electrode active material.

[0090] <Reference example 3-1> A composite oxide containing Ni, Mn, Co, and Al (Ni 0.77 Mn 0.16 Co 0.02 Al 0.05 O2, Ti(OH)4, and LiOH were mixed so that the molar ratio of Ni, Mn, Co, Al, and Ti to Li was 1:1.05. The flow rate was 10 cm³. 3The mixture was calcined in an oxygen stream with an oxygen concentration of 95% at a heating rate of 3.5°C / min to 650°C, and then calcined from 650°C to 780°C at a heating rate of 0.5°C / min. The calcined product was washed with water to obtain lithium transition metal oxide. The proportions of Ni, Mn, and Co in the obtained lithium transition metal oxide were measured and found to be 73 mol% for Ni, 15 mol% for Mn, 2 mol% for Co, 5 mol% for Al, and 5 mol% for Ti.

[0091] Furthermore, powder X-ray diffraction measurements were performed on the lithium transition metal oxide of Reference Example 3-1 in the same manner as in Example 1. The results showed diffraction lines indicating a layered structure, with the proportion of metal elements other than Li present in the Li layer being 2.4 mol%, and the full width at half maximum of the diffraction peak of the (208) plane being 0.46°. This was used as the positive electrode active material for Reference Example 3-1. Using the positive electrode active material of Reference Example 3-1, a test cell was prepared in the same manner as in Example 1, and the self-heating start temperature of the battery was measured under the same conditions as above. In addition, a test cell was prepared using a lithium transition metal oxide as the positive electrode active material, which was Mn-free and contained 73 mol% Ni, 15 mol% Co, 7 mol% Al, and 5 mol% Ti. The self-heating start temperature (reference value) was measured under the same conditions as above, and the change in the self-heating start temperature of Reference Example 3-1 was determined. The results are shown in Table 3.

[0092] <Example 3-2> A composite oxide containing Ni, Mn, Co, and Al (Ni 0.82 Mn 0.11 Co 0.02 Al 0.05 Lithium transition metal oxides were prepared in the same manner as in Reference Example 3-1, except that O2), Fe2O3, and LiOH were mixed so that the molar ratio of the total amount of Ni, Mn, Co, Al, and Fe to Li was 1:1.05. The proportions of Ni, Mn, Co, Al, and Fe in the obtained lithium transition metal oxide were measured, and the proportions were found to be 77 mol% for Ni, 10 mol% for Mn, 2 mol% for Co, 5 mol% for Al, and 6 mol% for Fe.

[0093] Furthermore, powder X-ray diffraction measurements were performed on the lithium transition metal oxide of Example 3-2 in the same manner as in Example 1. The results showed diffraction lines indicating a layered structure, with the proportion of metal elements other than Li in the Li layer being 2.5 mol%, and the full width at half maximum of the (208) plane diffraction peak being 0.49°. This was used as the positive electrode active material for Example 3-2. A test cell was prepared using the positive electrode active material of Example 3-2 in the same manner as in Example 1, and the self-heating onset temperature was measured under the same conditions as above. Additionally, a test cell was prepared using a lithium transition metal oxide without Mn, with a Ni content of 77 mol%, Co content of 10 mol%, Al content of 7 mol%, and Fe content of 6 mol% as the positive electrode active material. The self-heating onset temperature (reference value) was measured under the same conditions as above, and the change in the self-heating onset temperature of Example 3-2 was determined. The results are shown in Table 3.

[0094] <Example 3-3> A composite oxide containing Ni, Mn, Co, and Al (Ni 0.88 Mn 0.05 Co 0.02 Al 0.05 Lithium transition metal oxides were prepared in the same manner as in Reference Example 3-1, except that O2), Fe2O3, and LiOH were mixed so that the molar ratio of the total amount of Ni, Mn, Co, Al, and Fe to Li was 1:1.05, and the maximum temperature reached during firing was changed to 750°C. The proportions of Ni, Mn, Co, Al, and Fe in the obtained lithium transition metal oxide were measured, and the proportions were found to be 85 mol% for Ni, 5 mol% for Mn, 2 mol% for Co, 5 mol% for Al, and 3 mol% for Fe.

[0095] Furthermore, powder X-ray diffraction measurements were performed on the lithium transition metal oxide of Example 3-3 in the same manner as in Example 1. The results showed diffraction lines indicating a layered structure, with the proportion of metal elements other than Li present in the Li layer being 1.7 mol%, the full width at half maximum of the (208) plane diffraction peak being 0.45°, the lattice constant a being 2.870 Å, the lattice constant c being 14.19 Å, and the crystallite size s being 454 Å. This was used as the positive electrode active material for Example 3-3. Using the positive electrode active material of Example 3-3, a test cell was prepared in the same manner as in Example 1, and the self-heating start temperature was measured under the same conditions as described above. Furthermore, a test cell was prepared using a lithium transition metal oxide as the positive electrode active material, which was Mn-free, contained 85 mol% Ni, 12 mol% Co, 3 mol% Al, 1.0 mol% of metal elements other than Li in the Li layer, and had a (208) plane diffraction peak with a full width at half maximum of 0.41°. Under the same conditions as above, the self-heating onset temperature (reference value) was measured, and the change in self-heating onset temperature for Example 3-3 was determined. The results are shown in Table 3.

[0096] <Example 3-4> A composite oxide containing Ni, Mn, Co, and Al (Ni 0.89 Mn 0.06 Co 0.02 Al 0.03 Lithium transition metal oxides were prepared in the same manner as in Reference Example 3-1, except that O2), Nb2O5, and LiOH were mixed so that the molar ratio of the total amount of Ni, Mn, Co, Al, and Nb to Li was 1:1.05, and the maximum temperature reached during firing was changed to 750°C. The proportions of Ni, Mn, Co, Al, and Nb in the obtained lithium transition metal oxide were measured, and the proportions were found to be 88.5 mol% for Ni, 6 mol% for Mn, 2 mol% for Co, 3 mol% for Al, and 0.5 mol% for Nb.

[0097] Furthermore, powder X-ray diffraction measurements were performed on the lithium transition metal oxide of Example 3-4 in the same manner as in Example 1. The results showed diffraction lines indicating a layered structure, with the proportion of metal elements other than Li present in the Li layer being 2.1 mol%, the full width at half maximum of the (208) plane diffraction peak being 0.39°, the lattice constant a being 2.873 Å, the lattice constant c being 14.21 Å, and the crystallite size s being 521 Å. This was used as the positive electrode active material for Example 3-4. Using the positive electrode active material of Example 3-4, a test cell was prepared in the same manner as in Example 1, and the self-heating onset temperature was measured under the same conditions as above. In addition, a test cell was prepared using a lithium transition metal oxide without Mn, with a proportion of Ni 88.5 mol%, Co 8 mol%, Al 3 mol%, and Nb 0.5 mol% as the positive electrode active material. The self-heating onset temperature (reference value) was measured under the same conditions as above, and the change in the self-heating onset temperature of Example 3-4 was determined. The results are shown in Table 3.

[0098] <Example 3-5> A composite oxide containing Ni, Mn, Co, and Al (Ni 0.9 Mn 0.04 Co 0.02 Al 0.04 Lithium transition metal oxides were prepared in the same manner as in Reference Example 3-1, except that O2) and LiOH were mixed so that the molar ratio of Ni, Mn, Co, and Al to Li was 1:1.05, and the maximum temperature reached during firing was changed to 730°C. The proportions of Ni, Mn, Co, and Al in the obtained lithium transition metal oxide were measured, and the proportions of Ni were 90 mol%, Mn 4 mol%, Co 2 mol%, and Al 4 mol%.

[0099] Furthermore, powder X-ray diffraction measurements were performed on the lithium transition metal oxide of Example 3-5 in the same manner as in Example 1. The results showed diffraction lines indicating a layered structure, with the proportion of metal elements other than Li present in the Li layer being 1.7 mol%, the full width at half maximum of the (208) plane diffraction peak being 0.43°, the lattice constant a being 2.873 Å, the lattice constant c being 14.20 Å, and the crystallite size s being 548 Å. This was used as the positive electrode active material for Example 3-5. Using the positive electrode active material of Example 3-5, a test cell was prepared in the same manner as in Example 1, and the self-heating start temperature was measured under the same conditions as described above. Furthermore, a test cell was prepared using a lithium transition metal oxide as the positive electrode active material, which was Mn-free, contained 90 mol% Ni, 6 mol% Co, 4 mol% Al, 1.2 mol% of metal elements other than Li in the Li layer, and had a (208) plane diffraction peak with a full width at half maximum of 0.42°. Under the same conditions as above, the self-heating onset temperature (reference value) was measured, and the change in self-heating onset temperature for Examples 3-5 was determined. The results are shown in Table 3.

[0100] <Example 3-6> A composite oxide containing Ni, Mn, Co, and Al (Ni 0.92 Mn 0.015 Co 0.015 Al 0.04 Lithium transition metal oxides were prepared in the same manner as in Reference Example 3-1, except that O2), SiO, Fe2O3, and LiOH were mixed so that the molar ratio of Ni, Mn, Co, Al, Si, and Fe to Li was 1:1.05, and the maximum temperature reached during firing was changed to 700°C. The proportions of Ni, Mn, Co, Al, Si, and Fe in the obtained lithium transition metal oxide were measured, and the proportions were found to be 90.5 mol% for Ni, 1.5 mol% for Mn, 1.5 mol% for Co, 5 mol% for Al, 0.5 mol% for Si, and 1 mol% for Fe.

[0101] Furthermore, powder X-ray diffraction measurements were performed on the lithium transition metal oxide of Example 3-6 in the same manner as in Example 1. The results showed diffraction lines indicating a layered structure, with the proportion of metal elements other than Li present in the Li layer being 1.4 mol%, the full width at half maximum of the (208) plane diffraction peak being 0.33°, the lattice constant a being 2.873 Å, the lattice constant c being 14.20 Å, and the crystallite size s being 587 Å. This was used as the positive electrode active material for Example 3-6. Using the positive electrode active material of Example 3-6, a test cell was prepared in the same manner as in Example 1, and the self-heating onset temperature was measured under the same conditions as above. In addition, a test cell was prepared using a lithium transition metal oxide without Mn, with a proportion of Ni of 90.5 mol%, Co of 4.5 mol%, and Al of 5 mol%, as the positive electrode active material. The self-heating onset temperature (reference value) was measured under the same conditions as above, and the change in the self-heating onset temperature of Example 3-6 was determined. The results are shown in Table 3.

[0102] <Example 3-7> A composite oxide containing Ni, Mn, and Co (Ni 0.94 Mn 0.04 Co 0.02 Lithium transition metal oxides were prepared in the same manner as in Reference Example 3-1, except that O2) and LiOH were mixed so that the molar ratio of Ni, Mn, and Co to Li was 1:1.05, and the maximum temperature reached during firing was changed to 700°C. The proportions of Ni, Mn, and Co in the obtained lithium transition metal oxide were measured, and the proportions of Ni were 94 mol%, Mn was 4 mol%, and Co was 2 mol%.

[0103] Furthermore, powder X-ray diffraction measurements were performed on the lithium transition metal oxide of Example 3-7 in the same manner as in Example 1. The results showed diffraction lines indicating a layered structure, with the proportion of metal elements other than Li present in the Li layer being 2.1 mol%, the full width at half maximum of the (208) plane diffraction peak being 0.44°, the lattice constant a being 2.875 Å, the lattice constant c being 14.21 Å, and the crystallite size s being 607 Å. This was used as the positive electrode active material for Example 3-7. Using the positive electrode active material of Example 3-7, a test cell was prepared in the same manner as in Example 1, and the self-heating start temperature was measured under the same conditions as described above. Furthermore, a test cell was prepared using a lithium transition metal oxide as the positive electrode active material, which was Mn-free, contained 94 mol% Ni, 1.5 mol% Co, 4.5 mol% Al, 1.7 mol% of metal elements other than Li in the Li layer, and had a (208) plane diffraction peak width at half maximum of 0.47. Under the same conditions as above, the self-heating onset temperature (reference value) was measured, and the change in self-heating onset temperature for Example 3-7 was determined. The results are shown in Table 3.

[0104] <Reference example 3-8> A composite oxide containing Ni, Mn, Co, and Al (Ni 0.955 Mn 0.02 Co 0.02 Al 0.05 Lithium transition metal oxides were prepared in the same manner as in Reference Example 3-1, except that the mixture of O2) and LiOH was changed to a total amount of Ni, Mn, Co, and Al with a molar ratio of 1:1.05 to Li, and the maximum temperature reached during firing was changed to 700°C. The proportions of Ni, Mn, Co, and Al in the obtained lithium transition metal oxide were measured, and the proportions of Ni were 95.5 mol%, Mn was 2 mol%, Co was 2 mol%, and Al was 0.5 mol%.

[0105] Furthermore, powder X-ray diffraction measurements were performed on the lithium transition metal oxide of Reference Example 3-8 in the same manner as in Example 1. The results showed diffraction lines indicating a layered structure, with the proportion of metal elements other than Li present in the Li layer being 2.1 mol%, the full width at half maximum of the (208) plane diffraction peak being 0.42°, the lattice constant a being 2.875 Å, the lattice constant c being 14.21 Å, and the crystallite size s being 425 Å. This was used as the positive electrode active material for Reference Example 3-7. Using the positive electrode active material of Reference Example 3-7, a test cell was prepared in the same manner as in Example 1, and the self-heating onset temperature was measured under the same conditions as above. In addition, a test cell was prepared using a lithium transition metal oxide without Mn, with a Ni content of 95.5 mol% and an Al content of 4.5 mol%, as the positive electrode active material. The self-heating onset temperature (reference value) was measured under the same conditions as above, and the change in the self-heating onset temperature of Reference Example 3-8 was determined. The results are shown in Table 3.

[0106] [Table 3]

[0107] Table 4 summarizes the lattice constants a, c, and crystallite size s for Examples 3-3 to 3-6 and Reference Example 3-8. [Table 4]

[0108] As can be seen from the results in Table 3, in all of Examples 3-2 to 3-7, the self-heating start temperature of the battery was increased, and the thermal stability of the positive electrode active material was improved.

[0109] From the above results, it can be said that thermal stability can be enhanced by using a positive electrode active material having a layered structure, containing Ni, Mn, and an arbitrary element Co, wherein the ratio of Ni to the total amount of metal elements excluding Li in the lithium transition metal oxide is in the range of 75 mol% to 95 mol%, the ratio of Mn to the total amount of metal elements excluding Li in the lithium transition metal oxide is greater than or equal to the ratio of Co to the total amount of metal elements excluding Li in the lithium transition metal oxide, the ratio of Co to the total amount of metal elements excluding Li in the lithium transition metal oxide is in the range of 0 mol% to 2 mol%, the ratio of metal elements other than Li present in the Li layer of the layered structure is in the range of 1 mol% to 2.5 mol%, and the lithium transition metal oxide having a full width at half maximum n of the diffraction peak of the (208) plane in the X-ray diffraction pattern obtained by X-ray diffraction is 0.30° ≤ n ≤ 0.50°.

Claims

1. It has a layered structure and contains lithium transition metal oxides containing Ni, Mn, Al, and Co of any element, The ratio of Ni to the total amount of metal elements excluding Li in the lithium transition metal oxide is in the range of 75 mol% to 95 mol%, The ratio of Mn to the total amount of metal elements excluding Li in the lithium transition metal oxide is greater than the ratio of Co to the total amount of metal elements excluding Li in the lithium transition metal oxide. The proportion of metal elements other than Li present in the Li layer of the layered structure is in the range of 1 mol% to 2.5 mol% relative to the total amount of metal elements other than Li in the lithium transition metal oxide, wherein this is a positive electrode active material for a non-aqueous electrolyte secondary battery.

2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the ratio of Co to the total amount of metal elements excluding Li in the lithium transition metal oxide is in the range of 0 mol% to 2 mol%.

3. The lithium transition metal oxide is a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the lattice constant a representing the a-axis length and the lattice constant c representing the c-axis length of the crystal structure obtained from the analysis of the X-ray diffraction pattern by X-ray diffraction are in the ranges of 2.867 Å ≤ a ≤ 2.877 Å and 14.18 Å ≤ c ≤ 14.21 Å.

4. The lithium transition metal oxide is a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the crystallite size s calculated by Scherrer's formula from the full width at half maximum of the diffraction peak of the (10⁴) plane of the X-ray diffraction pattern obtained by X-ray diffraction is in the range of 400 Å ≤ s ≤ 600 Å.

5. A non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material for a non-aqueous electrolyte secondary battery described in any one of claims 1 to 4.

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