Method for producing positive electrode active material for secondary battery

Abstract

This method for producing a positive electrode active material for secondary batteries is characterized by including: an oxidation step of adding at least one hydroxide selected from the group consisting of aluminum hydroxide, strontium hydroxide, titanium hydroxide, bismuth hydroxide, zirconium hydroxide, and barium hydroxide to an Ni-containing compound, and performing oxidation treatment by heating the mixture; a firing step of mixing an oxide obtained in the oxidation step with an Li-containing compound, and firing the mixture; a water-washing step of mixing a fired product obtained in the firing step with water or an aqueous solution, and washing the mixture with water; and a drying step of drying the wet powder obtained in the water-washing step to obtain a lithium transition metal composite oxide, wherein the heating temperature in the oxidation step is 250°C-650°C inclusive.

Description

Method for producing a positive electrode active material for a secondary battery

[0001] The present disclosure relates to a method for producing a positive electrode active material for a secondary battery.

[0002] In recent years, non-aqueous electrolyte secondary batteries have been increasingly used in power sources for electric vehicles and energy storage devices for utilizing natural energy. Since the positive electrode greatly affects battery characteristics including battery capacity, output characteristics, cycle characteristics, etc., many studies have been conducted on the positive electrode. For example, Patent Document 1 discloses a manufacturing method in which an Al-containing compound is added to a positive electrode active material after firing and then fired again. According to the manufacturing method of Patent Document 1, LiAlO ２ is formed on the surface of the positive electrode active material, and residual Li present on the surface of the positive electrode active material can be removed.

[0003] Japanese Patent Application Laid-Open No. 2018-506141

[0004] In recent years, lithium transition metal composite oxides with a high Ni content have attracted attention as positive electrode active materials that achieve high battery capacity. However, when a lithium transition metal composite oxide with a high Ni content is used as a positive electrode active material, for example, side reactions between the lithium transition metal composite oxide and the non-aqueous electrolyte tend to occur, and the cycle characteristics tend to deteriorate. In non-aqueous electrolyte secondary batteries, it is an important issue to improve the cycle characteristics while achieving high capacity. Conventional technologies including the technology described in Patent Document 1 cannot sufficiently address such issues, and there is still a large room for improvement.

[0005] The method for producing a positive electrode active material for a secondary battery according to the present disclosure includes an oxidation step of adding at least one hydroxide selected from the group consisting of aluminum hydroxide, strontium hydroxide, titanium hydroxide, bismuth hydroxide, zirconium hydroxide, and barium hydroxide to a Ni-containing compound and performing oxidation treatment by heating; a firing step of mixing the oxide obtained in the oxidation step and a Li-containing compound and firing; a water washing step of mixing the fired product obtained in the firing step with water or an aqueous solution and performing water washing; and a drying step of drying the wet powder obtained in the water washing step to obtain a lithium transition metal composite oxide. The heating temperature in the oxidation step is 250°C or higher and 650°C or lower.

[0006] According to the method for producing a positive electrode active material for secondary batteries of this disclosure, a positive electrode active material can be obtained that can achieve high capacity while improving cycle characteristics.

[0007] The present disclosure provides a method for producing a positive electrode active material for a secondary battery, comprising: a crystallization step to obtain a Ni-containing compound; an oxidation step to add at least one hydroxide selected from the group consisting of aluminum hydroxide, strontium hydroxide, titanium hydroxide, bismuth hydroxide, zirconium hydroxide, and barium hydroxide to the Ni-containing compound and oxidize it by heating; a calcination step to mix the oxide obtained in the oxidation step with a Li-containing compound and calcine it; a water washing step to wash the calcined product obtained in the calcination step with water; and a drying step to dry the wet powder obtained in the water washing step to obtain a lithium transition metal composite oxide.

[0008] As will be described in detail later, the positive electrode active material (lithium transition metal composite oxide) produced by the manufacturing method of this disclosure is composed of secondary particles formed by the aggregation of primary particles. At least one element selected from the group consisting of Al, Sr, Ti, Bi, Zr, and Ba is dissolved in the interior, and a compound containing this element (hereinafter sometimes referred to as a "surface modification compound") coats the surfaces of the primary and secondary particles.

[0009] The surface modification compound coats the particle surface, suppressing side reactions between the lithium transition metal composite oxide and the non-aqueous electrolyte on the surface of the lithium transition metal composite oxide. As a result, the formation of products from these side reactions on the surface of the lithium transition metal composite oxide and the elution of transition metals from the lithium transition metal composite oxide are suppressed, improving the cycle characteristics. Furthermore, the solid solution of at least one element selected from the group consisting of Al, Sr, Ti, Bi, Zr, and Ba into the lithium transition metal composite oxide stabilizes the crystal structure and improves the cycle characteristics. The method for producing the positive electrode active material of this disclosure will be described in detail step by step below.

[0010] [Crystallization Process] In the crystallization process, for example, while stirring a solution of a metal salt containing Ni and an arbitrary metal element (Co, Mn, Al, etc.), an alkaline solution such as sodium hydroxide is added dropwise to adjust the pH to the alkaline side (for example, 8.5 or higher and 12.5 or lower), thereby precipitating (coprecipitation) a composite hydroxide (Ni-containing compound) containing Ni and an arbitrary metal element. However, the method for producing the Ni-containing compound is not limited to this.

[0011] [Oxidation Process] In the oxidation process, at least one hydroxide selected from the group consisting of aluminum hydroxide, strontium hydroxide, titanium hydroxide, bismuth hydroxide, zirconium hydroxide, and barium hydroxide (hereinafter sometimes referred to as "hydroxide X") is added to the Ni-containing compound obtained in the crystallization process, and the compound is heated and oxidized.

[0012] When hydroxide X is added, the number of moles of Al, Sr, Ti, Bi, Zr, and Ba contained in hydroxide X relative to the number of moles of metal elements (Ni, Co, Mn, Al, etc.) contained in the Ni-containing compound is preferably 0.3 mol% or less, and more preferably 0.25 mol% or less. In this case, it becomes easier to achieve higher battery capacity.

[0013] Here, the heating temperature in the oxidation process is between 250°C and 650°C. By setting the heating temperature to 250°C or higher, hydroxide X is melted, and at least one element selected from the group consisting of Al, Sr, Ti, Bi, Zr, and Ba is diffused into the interior of the particles, and a compound containing the element is uniformly formed on the particle surface. As a result, the cycle characteristics can be improved. In other words, if the heating temperature is less than 250°C, hydroxide X may not melt sufficiently, and the above element may not diffuse sufficiently into the interior of the particles. Also, if the heating temperature is less than 250°C, hydroxide X may not melt sufficiently, and a compound containing the element may be formed non-uniformly on the particle surface.

[0014] Furthermore, by setting the heating temperature to 650°C or lower, the Ni-containing oxide during oxidation treatment does not transition to the spinel phase, and the crystal structure can be stabilized. As a result, it is possible to achieve high capacity while improving cycle characteristics.

[0015] The heating temperature in the oxidation process may be 250 °C or higher and 650 °C or lower, but may also be 300 °C or higher and 600 °C or lower, or 350 °C or higher and 550 °C or lower. When the heating temperature in the oxidation process is 300 °C or higher and 600 °C or lower, or 350 °C or higher and 550 °C or lower, the crystal structure can be more stabilized while the hydroxide X can be sufficiently melted. As a result, high capacity can be achieved while the cycle characteristics can be further improved.

[0016] The heating rate during heating is, for example, 0.2 °C / min or higher and 4.5 °C / min or lower. When the heating rate is within the above range, the crystal structure can be stabilized while the hydroxide X can be sufficiently melted. As a result, high capacity can be achieved while the cycle characteristics can be further improved. The heating rate may be set in plural for each temperature region, or there may be one or more soaking zones within each temperature region.

[0017] The heating time (holding time) at the maximum temperature is, for example, 1 hour or longer and 15 hours or shorter, and may also be 2 hours or longer and 12 hours or shorter. When the heating time at the maximum temperature is within the above range, the crystal structure can be stabilized while the hydroxide X can be sufficiently melted. As a result, high capacity can be achieved while the cycle characteristics can be further improved.

[0018] In addition, in the oxidation process, a compound other than the hydroxide X may be added to the Ni-containing compound. Examples of the compound include a compound containing at least one of phosphates, sulfates, oxides, hydroxides, and chlorides containing at least one element selected from the group consisting of Sr, Ti, Bi, Zr, Ba, Mg, Ca, Ti, V, Al, Zr, Mo, and W. An example of the compound is SrO, SrCl ２ , Nb ２ O ５ , MgO, MgCl ２ , CaO, CaCl ２ , Ca(OH) ２ , TiO ２ , VO, V ２ O ５ , Al ２ O ３Al(OH) ３ Al ２ (SO) ４、 NaAl(OH) ４ AlPO ４ , ZrSO ４ , ZrO ２ MoO ２ WO ３ WS ２ These are some examples. The compounds may be used individually or in combination of two or more. Furthermore, these compounds may be added before the calcination process (after the oxidation process) described later.

[0019] [Casturing Process] In the calcination process, the Ni-containing oxide obtained in the oxidation process and the Li-containing compound are mixed and calcined. Examples of Li-containing compounds include Li ２ CO ３ LiOH, Li ２ O ２ Li ２ O, LiNO ３ LiNO ２ Li ２ SO ４ LiOH H ２ Examples include O, LiH, and LiF. The mixing ratio of the metal compound and the Li-containing compound is preferably such that the molar ratio of the total amount of metal elements in the Ni-containing oxide to Li is in the range of 1:0.8 to 1.2, and more preferably 1:1.0 to 1.1. In addition, hydroxide X may be further added during the calcination process.

[0020] In the firing process, for example, firing is carried out under an oxygen stream. The flow rate of the oxygen stream during firing is, for example, 20 mL / min or more per liter of firing furnace or 0.3 L or more per 1 kg of mixture.

[0021] The maximum temperature during firing is preferably between 650°C and 900°C, and more preferably between 700°C and 900°C. In this case, the crystallinity of the final lithium transition metal composite oxide is further improved. As a result, it becomes easier to achieve higher battery capacity.

[0022] The heating rate during firing may be, for example, 0.2°C / min or more and 10.0°C / min or less, or 0.3°C / min or more and 4.5°C / min or less. The firing time (holding time) at the maximum temperature may be, for example, 1 hour or more and 10 hours or less, or 2 hours or more and 8 hours or less.

[0023] The firing conditions in the firing process may be a multi-stage firing process that includes, for example, a first firing process in which the product is fired at a temperature of 300°C or higher and 680°C or lower, and a second firing process in which the product obtained in the first firing process is fired at a maximum temperature exceeding 680°C. In the first firing process, the temperature is raised to a first set temperature of 680°C or lower at a first heating rate of 0.2°C / min or higher and 10°C / min or lower. In the second firing process, the temperature is raised to a second set temperature (maximum temperature) of 900°C or lower at a rate of 0.5°C / min or higher and 10°C / min or lower. The first heating rate and the second heating rate may be set multiple times for each temperature range, as long as they are within the ranges specified above, and there may be one or more uniform temperatures within each temperature range.

[0024] The holding time at the first set temperature in the first firing process is preferably 5 hours or less, and more preferably 3 hours or less. The holding time at the second set temperature (maximum temperature) in the second firing process is preferably 1 hour or more and 10 hours or less, and more preferably 1 hour or more and 8 hours or less.

[0025] As described above, by adding hydroxide X in the oxidation process and heating it, the hydroxide X melts, and at least one element selected from the group consisting of Al, Sr, Ti, Bi, Zr, and Ba diffuses into the interior of the particles, and a compound containing that element is uniformly formed on the particle surface. Then, by firing at a higher temperature in the subsequent firing process, at least one element selected from the group consisting of Al, Sr, Ti, Bi, Zr, and Ba diffuses further into the interior of the particles, and a compound containing that element is formed more uniformly on the particle surface. As a result, the cycle characteristics can be greatly improved. In other words, if hydroxide X is not added in the oxidation process and is only added in the firing process, at least one element selected from the group consisting of Al, Sr, Ti, Bi, Zr, and Ba will not diffuse easily into the interior of the particles, and a compound containing that element will not be uniformly formed on the particle surface. As a result, the cycle characteristics cannot be sufficiently improved.

[0026] [Water Washing Process] In the water washing process, the slurry obtained by mixing the calcined product obtained in the calcination process with water or an aqueous solution is stirred and washed with water. Before the water washing process, unreacted Li compounds (e.g., lithium carbonate, etc.) used during mixing may remain on the particle surface of the calcined product. By performing the water washing process, unreacted Li compounds remaining on the particle surface of the lithium transition metal composite oxide can be removed.

[0027] Washing is carried out by known methods. For example, the calcined material and water or an aqueous solution are placed in a reaction vessel equipped with a stirring device and stirred. In the washing process, the slurry produced in the washing process is separated into solid and liquid to obtain a cake-like wet powder. The method of solid-liquid separation is not particularly limited and is carried out by known methods. For example, a suction filter, centrifuge, or filter press can be used for solid-liquid separation.

[0028] [Drying Process] In the drying process, the wet powder obtained in the washing process is dried to obtain dry powder (lithium transition metal composite oxide). In the drying process, for example, from the viewpoint of suppressing deterioration of battery characteristics when used as a positive electrode active material, it is preferable to dry until the moisture content is 1.0% by mass or less. The drying conditions are preferably such that the drying is performed at a temperature of 100°C or higher and 300°C or lower. The drying time is preferably 0.5 hours or more.

[0029] The positive electrode active material obtained by this manufacturing method is preferably subjected to a sieving process to remove coarse particles, if necessary. This results in positive electrode active material adjusted to a predetermined particle size. Examples of equipment used for the sieving process include vibrating screens and centrifugal classifiers. Furthermore, the positive electrode active material before the sieving process may be crushed using a jet mill, roll mill, muscoloider, or the like, if necessary.

[0030] A non-aqueous electrolyte secondary battery to which the positive electrode active material produced by the above manufacturing method is applied can be obtained, for example, by housing an electrode body, in which electrodes (positive electrode, negative electrode) and a separator are stacked or wound together, with a non-aqueous electrolyte in a container such as an outer casing or laminate. The positive electrode, negative electrode, separator, and non-aqueous electrolyte will be described below.

[0031] [Positive Electrode] The positive electrode comprises, for example, a positive electrode core and a positive electrode mixture layer formed on the surface of the positive electrode core. Preferably, the positive electrode mixture layer is formed on both sides of the positive electrode core. The positive electrode core can be made of a metal foil that is stable in the positive electrode potential range, such as aluminum or an aluminum alloy, or a film with the metal arranged on its surface. The thickness of the positive electrode core is, for example, 10 μm or more and 30 μm or less.

[0032] The positive electrode mixture layer includes, for example, a positive electrode active material, a conductive agent, and a binder. The thickness of the positive electrode mixture layer is, for example, 10 μm or more and 150 μm or less on one side of the positive electrode core. The positive electrode can be manufactured, for example, by applying a positive electrode mixture slurry containing the positive electrode active material, conductive agent, etc., to the surface of the positive electrode core, drying the coating film, and then rolling it to form the positive electrode mixture layer on both sides of the positive electrode core.

[0033] Examples of conductive agents included in the positive electrode mixture layer include acetylene black (AB), carbon black (CB) such as Ketjenblack, carbon nanotubes (CNT), graphene, and carbon-based particles such as graphite. These may be used individually or in combination of two or more types.

[0034] Examples of binders included in the positive electrode mixture layer include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyimide resins, acrylic resins, polyolefin resins, and polyacrylonitrile (PAN). These may be used individually or in combination of two or more types.

[0035] The positive electrode mixture layer contains a lithium transition metal composite oxide (hereinafter sometimes referred to as "lithium transition metal composite oxide Z") manufactured by the above-described manufacturing method as the positive electrode active material. The positive electrode mixture layer may also contain composite oxides other than lithium transition metal composite oxide Z as the positive electrode active material, but it is preferable that it contains lithium transition metal composite oxide Z as the main component. Here, the main component refers to the component with the highest mass ratio among the constituent components of the positive electrode active material. The content of lithium transition metal composite oxide Z is preferably 80% by mass or more, and may be substantially 100% by mass, based on the total mass of the positive electrode active material.

[0036] Lithium transition metal composite oxide Z is composed of secondary particles formed by the aggregation of primary particles. The particle size of the primary particles constituting the secondary particles of lithium transition metal composite oxide Z is, for example, between 0.02 μm and 2 μm. The particle size of the primary particles is measured as the diameter of the circumscribed circle in the particle image observed by a scanning electron microscope (SEM). The average particle diameter of the secondary particles of lithium transition metal composite oxide Z is, for example, between 2 μm and 30 μm. Here, the average particle diameter refers to the volume-based median diameter (D50). D50 refers to the particle size at which the cumulative frequency of the smallest particle size accounts for 50% in the volume-based particle size distribution, and is also called the median diameter. The particle size distribution of the secondary particles of lithium transition metal composite oxide Z can be measured using a laser diffraction particle size distribution analyzer (for example, MT3000II manufactured by Microtrac-Bell Co., Ltd.) with water as the dispersion medium.

[0037] Lithium transition metal composite oxide Z has, for example, a layered structure. Examples of the layered structure of lithium transition metal composite oxide Z include a layered structure belonging to space group R-3m and a layered structure belonging to space group C2 / m. From the viewpoint of increasing capacity and stabilizing the crystal structure, it is preferable that lithium transition metal composite oxide Z has a layered structure belonging to space group R-3m.

[0038] Lilithium transition metal composite oxides Z are, for example, those with the general formula Li ａ Ni １－ｘ－ｙ Co ｘ M1 ｙ M2 ｚ O ２＋α (In the formula, M1 is at least one element selected from the group consisting of Mn and Al, M2 is at least one element selected from the group consisting of Sr, BiBa, Mg, Ca, Ti, V, Cr, Zr, Mo and W, with 0.95 ≤ a ≤ 1.20, 0 ≤ x ≤ 0.15, 0 ≤ y ≤ 0.20, 0 ≤ z ≤ 0.05 and x + y + z ≤ 0.3, and α is a value that satisfies the total valency.) The content of the elements constituting the lithium transition metal composite oxide Z can be measured by an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron beam microanalyzer (EPMA), or an energy dispersive X-ray spectrometer (EDX), etc.

[0039] The Ni content in the lithium transition metal composite oxide Z may be 80 mol% or more, or 85 mol% or more. By setting the Ni content in the lithium transition metal composite oxide Z to 80 mol% or more, the battery capacity can be increased. Furthermore, the higher the Ni content, the more likely side reactions will occur between the lithium transition metal composite oxide and the non-aqueous electrolyte on the surface of the lithium transition metal composite oxide, thus exhibiting the effects of this disclosure more significantly.

[0040] The lithium transition metal composite oxide Z contains at least one element selected from the group consisting of Al, Sr, Ti, Bi, Zr, and Ba as a solid solution internally, while a compound containing this element (surface modification compound) coats the surfaces of primary and secondary particles. By coating the particle surfaces with the surface modification compound, side reactions between the lithium transition metal composite oxide Z and the non-aqueous electrolyte on the surface of the lithium transition metal composite oxide Z are suppressed. As a result, the formation of products from these side reactions on the surface of the lithium transition metal composite oxide Z and the elution of transition metals from the lithium transition metal composite oxide Z are suppressed, improving the cycle characteristics. Furthermore, the solid solution of at least one element selected from the group consisting of Al, Sr, Ti, Bi, Zr, and Ba internally in the lithium transition metal composite oxide Z stabilizes the crystal structure and improves the cycle characteristics.

[0041] [Negative Electrode] The negative electrode may, for example, have a negative electrode core and a negative electrode mixture layer formed on the surface of the negative electrode core, or a metallic Li foil may be used as the negative electrode. Alternatively, the negative electrode may have a negative electrode core, and lithium metal may be deposited on the surface of the negative electrode core by charging. When the negative electrode has a negative electrode mixture layer, it is preferable that the negative electrode mixture layer is formed on both sides of the negative electrode core. The negative electrode core can be made of a metal foil that is stable in the negative electrode potential range, such as copper or a copper alloy, or a film with the metal arranged on its surface. The thickness of the negative electrode core is, for example, 5 μm or more and 30 μm or less. The negative electrode mixture layer includes, for example, a negative electrode active material and a binder. The thickness of the negative electrode mixture layer is, for example, 10 μm or more and 200 μm or less on one side of the negative electrode core. The negative electrode can be manufactured, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc., to the surface of the negative electrode core, drying the coating, and then rolling it to form a negative electrode mixture layer on both sides of the negative electrode core.

[0042] The negative electrode active material contained in the negative electrode mixture layer is not particularly limited as long as it can reversibly intercept and release lithium ions, and generally carbon materials such as graphite are used. The graphite may be any of the following: natural graphite such as flake graphite, lump graphite, or clay graphite; lump artificial graphite; or artificial graphite such as graphitized mesophase carbon microbeads. In addition, metals that alloy with Li such as Si and Sn, metal compounds containing Si and Sn, or lithium titanium composite oxides may be used as the negative electrode active material. Furthermore, materials with a carbon coating may also be used. For example, SiO ｘ Si-containing compounds represented by (0.5 ≤ x ≤ 1.6), or Li ２ｙ SiO （２＋ｙ） A Si-containing compound in which fine Si particles are dispersed in a lithium silicate phase represented by (0 < y < 2) may be used in combination with graphite.

[0043] Examples of binders included in the negative electrode mixture layer include styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethylcellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts (PAA-Na, PAA-K, etc., or partially neutralized salts), and polyvinyl alcohol (PVA). These may be used individually or in combination of two or more types.

[0044] [Separator] A porous sheet having ion permeability and insulating properties is 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, polyolefins such as polypropylene, and cellulose. The separator may have a single-layer structure or a multi-layer structure. In addition, a heat-resistant resin layer, such as aramid resin, may be formed on the surface of the separator.

[0045] 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 metal elements such as Ti, Al, Si, and Mg, and phosphoric acid compounds. The filler layer can be formed by coating the surface of the positive electrode, the negative electrode, or the separator with a slurry containing the filler.

[0046] [Non-aqueous electrolytes] Non-aqueous electrolytes are ionic conductive (for example, lithium ion conductive). Non-aqueous electrolytes may be liquid electrolytes (electrolytes) or solid electrolytes.

[0047] A liquid electrolyte (electrolyte solution) includes, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixtures 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. Examples of halogen-substituted solvents include fluorinated cyclic carbonate esters such as fluoroethylene carbonate (FEC), fluorinated linear carbonate esters, and fluorinated linear carboxylic acid esters such as methyl fluoropropionate (FMP).

[0048] 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), and ethyl propionate (EP).

[0049] 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.

[0050] The electrolyte salt is preferably a lithium salt. A suitable lithium salt is LiClO ４ LiBF ４ LiPF ６ LiAlCl ４ LiSbF ６ , LiSCN, LiCF ３ SO ３ LiCF ３ CO ２ LiAsF ６ LiB １０ Cl １０ Examples include lithium lower aliphatic carboxylates, LiCl, LiBr, LiI, phosphates, borates, and imide salts. Examples of phosphates include lithium difluorophosphate (LiPO4). ２ F ２Examples include lithium difluorobis(oxalato)phosphate (LiDFOBP), lithium tetrafluoro(oxalato)phosphate, etc. Examples of borates include lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), etc. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO)). ２ ) ２ ), bistrifluoromethanesulfonate lithium (LiN(CF ３ SO ２ ) ２ ), trifluoromethanesulfonic acid nonafluorobutanesulfonic acid lithium (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), bispentafluoroethanesulfonate lithium (LiN(C) ２ F ５ SO ２ ) ２ ) etc. are used. Of these, LiPF is used from the viewpoint of ionic conductivity, electrochemical stability, etc. ６ It is preferable to use the following. The concentration of the lithium salt may be, for example, 4 moles or less per liter of non-aqueous solvent, or 3 moles or less, preferably 1.8 moles or less, and more preferably 0.8 moles or more and 1.8 moles or less.

[0051] Non-aqueous electrolytes may contain additives. Examples of additives include unsaturated carbonate esters, acid anhydrides, phenol compounds, benzene compounds, nitrile compounds, isocyanate compounds, sultone compounds, sulfuric acid compounds, borate ester compounds, phosphate ester compounds, and phosphite ester compounds.

[0052] Examples of unsaturated cyclic carbonate esters include vinylene carbonate, 4-methylvinylene carbonate, 4,5-dimethylvinylene carbonate, 4-ethylvinylene carbonate, 4,5-diethylvinylene carbonate, 4-propylvinylene carbonate, 4,5-dipropylvinylene carbonate, 4-phenylvinylene carbonate, 4,5-diphenylvinylene carbonate, vinylethylene carbonate, and divinylethylene carbonate. Unsaturated cyclic carbonate esters may be used individually or in combination of two or more. Some hydrogen atoms in the unsaturated cyclic carbonate esters may be substituted with fluorine atoms. The acid anhydride may be an anhydride formed by the intermolecular condensation of multiple carboxylic acid molecules, but it is preferable that it be an acid anhydride of a polycarboxylic acid. Examples of polycarboxylic acid acid anhydrides include succinic anhydride, maleic anhydride, and phthalic anhydride.

[0053] Examples of phenolic compounds include phenol and hydroxytoluene. Examples of benzene compounds include fluorobenzene, hexafluorobenzene, and cyclohexylbenzene (CHB).

[0054] Examples of nitrile compounds include adiponitrile, pimelonitrile, propionitrile, and succinonitrile. Examples of isocyanate compounds include methyl isocyanate (MIC), diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), and bisisocyanate methylcyclohexane (BIMCH). Examples of sultone compounds include propanesultone and propensultone. Examples of sulfate compounds include ethylene sulfate, ethylene sulfite, dimethyl sulfate, and lithium fluorosulfate. Examples of borate ester compounds include trimethylborate and tris(trimethylsilyl)borate. Examples of phosphate ester compounds include trimethylphosphate and tris(trimethylsilyl)phosphate. Examples of phosphite ester compounds include trimethylphosphite and tris(trimethylsilyl)phosphite.

[0055] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc., can be used. As the inorganic solid electrolyte, materials known for all-solid-state lithium-ion secondary batteries, etc. (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, etc.

[0056] The present disclosure will be further explained below with reference to examples and comparative examples, but the present disclosure is not limited to the following examples. <Example 1> [Preparation of positive electrode active material (lithium transition metal composite oxide Z)] [Ni obtained by coprecipitation method ０．９０ Co ０．０５ Mn ０．０５ ] (OH) ２Aluminum hydroxide was added to the composite hydroxide (Ni-containing compound) represented by [formula], and the mixture was heated at 350°C for 10 hours under an oxygen stream to obtain a Ni-containing oxide, which is a cathode active material precursor containing Ni, Co, and Mn (oxidation step). The aluminum hydroxide was added so that the ratio of the number of moles of Al contained in the aluminum hydroxide to the number of moles of the metal element (Me) contained in the Ni-containing compound (Al / Me) was 0.05 mol%.

[0057] Subsequently, the Ni-containing oxide obtained in the oxidation process was mixed with lithium hydroxide (LiOH) as a Li-containing compound so that the molar ratio of Li to the total amount of metal elements contained in the Ni-containing oxide was 1:1.03, thereby obtaining a mixture. This mixture was then heated under an oxygen stream at a heating rate of 2.5°C / min from room temperature to 650°C, and then heated at a heating rate of 1°C / min from 650°C to 760°C, and held at 760°C (maximum temperature) for 5 hours to perform calcination (calcination process).

[0058] Subsequently, the calcined product obtained was added to water and washed with water at a stirring speed of 300 rpm for 10 minutes, and dewatered using a filter press to obtain wet powder (washing step). Then, the obtained wet powder was dried under a vacuum atmosphere at 180°C for 2 hours to obtain the positive electrode active material (lithium transition metal composite oxide Z) of Example 1 (drying step).

[0059] [Preparation of the positive electrode] The above positive electrode active material, acetylene black (AB), and polyvinylidene fluoride were mixed in a mass ratio of 86:10:4, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode slurry. Next, the positive electrode slurry was applied to a positive electrode core made of aluminum foil, the coating was dried and compressed, and then the positive electrode core was cut to a predetermined electrode size to obtain a positive electrode in which positive electrode slurry layers were arranged on both sides of the positive electrode core.

[0060] [Preparation of Non-Aqueous Electrolyte] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 3:3:4. Lithium hexafluoride phosphate (LiPF) was added to this mixed solvent. ６ A non-aqueous electrolyte was prepared by dissolving the substance to a concentration of 1.2 mol / liter.

[0061] [Preparation of Test Cell] A lithium metal foil was used as the negative electrode, and the positive and negative electrodes were arranged facing each other via a separator to form an electrode body. This electrode body and the non-aqueous electrolyte were placed in a coin-shaped outer casing, and the opening of the outer casing was sealed with a gasket and a sealing body to produce a test cell (non-aqueous electrolyte secondary battery).

[0062] [Evaluation of Initial Discharge Capacity and Capacity Retention Rate] The fabricated test cell was charged at 0.3C with a constant current to 4.2V at an ambient temperature of 25°C, and then charged at 4.2V with a constant voltage to 0.01C. After that, it was discharged at 0.2C with a constant current to 2.5V, and the initial discharge capacity was measured. This charge-discharge cycle was considered one cycle, and 50 cycles were performed. The capacity retention rate of the test cell in the charge-discharge cycle was calculated using the following formula: Capacity Retention Rate = (Discharge Capacity at Cycle 300 / Initial Discharge Capacity)

[0063] <Example 2> In preparing the positive electrode active material, a test cell was prepared and evaluated in the same manner as in Example 1, except that aluminum hydroxide was added so that the ratio of the number of moles of Al contained in aluminum hydroxide to the number of moles of the metal element (Me) contained in the Ni-containing compound (Al / Me) was 0.10 mol%.

[0064] <Example 3> In the preparation of the positive electrode active material, a test cell was prepared and evaluated in the same manner as in Example 1, except that aluminum hydroxide was added so that the ratio of the number of moles of Al contained in aluminum hydroxide to the number of moles of the metal element (Me) contained in the Ni-containing compound (Al / Me) was 0.15 mol%.

[0065] <Example 4> In the preparation of the positive electrode active material, a test cell was prepared and evaluated in the same manner as in Example 1, except that aluminum hydroxide was added so that the ratio of the number of moles of Al contained in aluminum hydroxide to the number of moles of the metal element (Me) contained in the Ni-containing compound (Al / Me) was 0.20 mol%.

[0066] <Example 5> In the preparation of the positive electrode active material, the test cell was prepared and evaluated in the same manner as in Example 1, except that the heating temperature was changed from 350°C to 500°C.

[0067] <Example 6> In the preparation of the positive electrode active material, the test cell was prepared and evaluated in the same manner as in Example 2, except that the heating temperature was changed from 350°C to 500°C.

[0068] <Example 7> In the preparation of the positive electrode active material, the test cell was prepared and evaluated in the same manner as in Example 3, except that the heating temperature was changed from 350°C to 500°C.

[0069] <Example 8> In the preparation of the positive electrode active material, the test cell was prepared and evaluated in the same manner as in Example 4, except that the heating temperature was changed from 350°C to 500°C.

[0070] <Example 9> In the preparation of the positive electrode active material, [Ni obtained by coprecipitation method ０．９０ Co ０．０５ Mn ０．０４ Al ０．０１ ] (OH) ２ A test cell was prepared and evaluated in the same manner as in Example 2, except that aluminum hydroxide was added to the composite hydroxide (Ni-containing compound) represented by [formula].

[0071] <Comparative Example 1> In the preparation of the positive electrode active material, aluminum hydroxide was not added in the oxidation step, but was added when mixing the Li-containing compound in the calcination step. Except for this, a test cell was prepared and evaluated in the same manner as in Example 2.

[0072] <Comparative Example 2> In the preparation of the positive electrode active material, a test cell was prepared and evaluated in the same manner as in Example 1, except that aluminum hydroxide was not added in the oxidation process.

[0073] <Comparative Example 3> In the preparation of the positive electrode active material, the test cell was prepared and evaluated in the same manner as in Example 2, except that the heating temperature was changed from 350°C to 100°C.

[0074] <Comparative Example 4> In the preparation of the positive electrode active material, the test cell was prepared and evaluated in the same manner as in Example 2, except that the heating temperature was changed from 350°C to 700°C.

[0075] Table 1 shows the capacity retention rate and initial discharge capacity of the test cells for the examples and comparative examples. Note that the capacity retention rate and initial discharge capacity shown in Table 1 are expressed relatively, with the capacity retention rate and initial discharge capacity of the test cell for Comparative Example 1 set to 100. A higher capacity retention rate indicates better cycle characteristics, and a higher initial discharge capacity indicates higher capacity.

[0076]

[0077] As shown in Table 1, the test cell of the example maintains battery capacity while improving capacity retention compared to the test cell of the comparative example. This is presumed to be because the positive electrode active material of the example has Al solid-solution throughout the particle interior, and the particle surface is uniformly coated with an Al-containing compound. As a result, the crystal structure is stabilized, side reactions with non-aqueous electrolytes on the particle surface are suppressed, and the cycle characteristics are improved. In addition, the test cells of Comparative Example 3, where the heating temperature in the oxidation process was less than 250°C, and Comparative Example 4, where it exceeded 650°C, showed decreased capacity retention and battery capacity.

[0078] This disclosure is further illustrated by the following embodiments. Embodiment 1: A method for producing a positive electrode active material for a secondary battery, comprising: an oxidation step of adding at least one hydroxide selected from the group consisting of aluminum hydroxide, strontium hydroxide, titanium hydroxide, bismuth hydroxide, zirconium hydroxide, and barium hydroxide to a Ni-containing compound and oxidizing it by heating; a calcination step of mixing the oxide obtained in the oxidation step with a Li-containing compound and calcining it; a water washing step of mixing the calcined product obtained in the calcination step with water or an aqueous solution and washing it with water; and a drying step of drying the wet powder obtained in the water washing step to obtain a lithium transition metal composite oxide, wherein the heating temperature in the oxidation step is 250°C or higher and 650°C or lower. Configuration 2: A method for producing a positive electrode active material for a secondary battery according to Configuration 1, wherein in the oxidation step, the number of moles of Al, Sr, Ti, Bi, Zr, and Ba contained in the hydroxide relative to the number of moles of metal elements contained in the Ni-containing compound is 0.3 mol% or less. Configuration 3: A method for producing a positive electrode active material for a secondary battery according to Configuration 1 or 2, wherein aluminum hydroxide is added in the oxidation step. Configuration 4: The lithium transition metal composite oxide has the general formula Li ａ Ni １－ｘ－ｙ Co ｘ M1 ｙ M2 ｚ O ２＋α A method for producing a positive electrode active material for a secondary battery according to any one of the configurations 1 to 3, represented by the formula (wherein M1 is at least one element selected from the group consisting of Mn and Al, M2 is at least one element selected from the group consisting of Sr, Bi, Nb, Ba, Mg, Ca, Ti, V, Cr, Zr, Mo and W, with 0.95 ≤ a ≤ 1.20, 0 ≤ x ≤ 0.15, 0 ≤ y ≤ 0.20, 0 ≤ z ≤ 0.05 and x + y + z ≤ 0.3, and α is a value that satisfies the overall valency). Configuration 5: A method for producing a positive electrode active material for a secondary battery according to any one of the configurations 1 to 4, wherein the content of Ni in the lithium transition metal composite oxide is 80 mol% or more with respect to the total number of moles of metal elements excluding Li.

Claims

1. A method for producing a positive electrode active material for a secondary battery, comprising: an oxidation step of adding at least one hydroxide selected from the group consisting of aluminum hydroxide, strontium hydroxide, titanium hydroxide, bismuth hydroxide, zirconium hydroxide, and barium hydroxide to a Ni-containing compound and oxidizing it by heating; a calcination step of mixing the oxide obtained in the oxidation step with a Li-containing compound and calcining it; a water washing step of mixing the calcined product obtained in the calcination step with water or an aqueous solution and washing it with water; and a drying step of drying the wet powder obtained in the water washing step to obtain a lithium transition metal composite oxide, wherein the heating temperature in the oxidation step is 250°C or higher and 650°C or lower.

2. The method for producing a positive electrode active material for a secondary battery according to claim 1, wherein in the oxidation step, the number of moles of Al, Sr, Nb, and Ba contained in the hydroxide relative to the number of moles of metal elements contained in the Ni-containing compound is 0.3 mol% or less.

3. The method for producing a positive electrode active material for a secondary battery according to claim 1, wherein aluminum hydroxide is added in the oxidation step.

4. The lithium transition metal composite oxide has the general formula Li ａ Ni １－ｘ－ｙ Co ｘ M1 ｙ M2 ｚ O ２＋α A method for producing a positive electrode active material for a secondary battery according to claim 1, represented by the formula (wherein M1 is at least one element selected from the group consisting of Mn and Al, M2 is at least one element selected from the group consisting of Sr, Bi, Nb, Ba, Mg, Ca, Ti, V, Cr, Zr, Mo and W, with 0.95 ≤ a ≤ 1.20, 0 ≤ x ≤ 0.15, 0 ≤ y ≤ 0.20, 0 ≤ z ≤ 0.05 and x + y + z ≤ 0.3, and α is a value that satisfies the overall valency).

5. The method for producing a positive electrode active material for a secondary battery according to claim 1, wherein the content of Ni in the lithium transition metal composite oxide is 80 mol% or more relative to the total number of moles of metal elements excluding Li.

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