Method for producing positive electrode active material for non-aqueous electrolyte secondary battery, positive electrode active material for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

Abstract

This method for producing a positive electrode active material contained in a non-aqueous electrolyte secondary battery comprises a step for using sulfuric acid with a concentration of 0.0001-1 mol / L, inclusive, to wash a lithium transition metal composite oxide that has a composition represented by the general formula Li1+aNibMncMedOe (in the formula, 0≤a≤0.15, 0.4≤b≤0.8, 0.2≤c≤0.6, 0≤d≤0.1, b+c+d=1, Me is at least one element selected from elements other than Li, Ni, Mn, and O, and e is a value that satisfies electroneutrality).

Description

Method for producing positive electrode active material for non-aqueous electrolyte secondary batteries, positive electrode active material for non-aqueous electrolyte secondary batteries, and non-aqueous electrolyte secondary batteries

[0001] This disclosure relates to a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, a positive electrode active material for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery, and more particularly to a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, a positive electrode active material for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery containing Ni in a proportion of 40% to 80%.

[0002] In recent years, non-aqueous electrolyte secondary batteries, which consist of a positive electrode, a negative electrode, and a non-aqueous electrolyte, and charge and discharge by moving Li ions between the positive and negative electrodes, have been widely used as high-power, high-capacity secondary batteries. From the perspective of reducing battery resistance and increasing capacity, there is a need to improve the properties of the positive electrode active material contained in the positive electrode of the battery.

[0003] Lithium nickelate (LiNiO) has been used as a positive electrode active material. ２ Ni is known to have a high energy density, and by substituting some of the Ni with Co, Mn, etc., battery characteristics such as reliability can be improved.

[0004] However, lithium transition metal composite oxides with a high Ni content may have an unstable layered structure, leading to reduced charge-discharge cycle characteristics. Patent Document 1 discloses a technique for coating the surface of a lithium transition metal composite oxide with a Ni content of 80 mol% or more with a layer containing W and a layer containing Me (Me is at least one element selected from Nb, Sr, Mo, and Ca).

[0005] International Publication No. 2023 / 176503

[0006] By the way, in order to further popularize rechargeable batteries, it is important to reduce the cost of manufacturing them. The positive electrode active material accounts for a large proportion of the component cost of a rechargeable battery, and it is necessary to reduce the proportion of expensive elements such as Ni and Co in the positive electrode active material. However, LiNiO ２ Substituting Ni with Mn in this mixture tends to decrease the initial charge-discharge efficiency, and this tendency is particularly pronounced when the Ni content is 80 mol% or less.

[0007] An object of the present disclosure is to provide a positive electrode active material for a non-aqueous electrolyte secondary battery having a Ni content of 40 mol% or more and 80 mol% or less and excellent initial charge-discharge efficiency.

[0008] A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, which is one aspect of the present disclosure, is a lithium transition metal composite oxide having a composition represented by the general formula Li １＋ａ Ni ｂ Mn ｃ Me ｄ O ｅ (where 0 ≤ a ≤ 0.15, 0.4 ≤ b ≤ 0.8, 0.2 ≤ c ≤ 0.6, 0 ≤ d ≤ 0.1, b + c + d = 1, Me is at least one element selected from elements other than Li, Ni, Mn, and O, and e is a value that satisfies electrical neutrality), and the method includes a step of washing with sulfuric acid having a concentration of 0.0001 mol / L or more and 1 mol / L or less.

[0009] A positive electrode active material for a non-aqueous electrolyte secondary battery, which is one aspect of the present disclosure, includes a lithium transition metal composite oxide in which secondary particles are formed by aggregation of primary particles, and an inorganic sulfur compound present on the surface of the primary particles or at the grain boundaries between the primary particles. The composition of the lithium transition metal composite oxide is represented by the general formula Li １＋ａ Ni ｂ Mn ｃ Me ｄ O ｅ (where 0 ≤ a ≤ 0.15, 0.4 ≤ b ≤ 0.8, 0.2 ≤ c ≤ 0.6, 0 ≤ d ≤ 0.1, b + c + d = 1, Me is at least one element selected from elements other than Li, Ni, Mn, and O, and e is a value that satisfies electrical neutrality). The lithium transition metal composite oxide has a layered structure, and the ratio of metal elements other than Li present in the Li layer of the layered structure is 3 mol% or more and 10 mol% or less with respect to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide. The amount of S contained in the inorganic sulfur compound is 0.01 mol% or more and 1 mol% or less with respect to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

[0010] A non-aqueous electrolyte secondary battery, which is one aspect of the present disclosure, includes a positive electrode containing the above positive electrode active material for a non-aqueous electrolyte secondary battery, a negative electrode, and a non-aqueous electrolyte.

[0011] According to a positive electrode active material for a non-aqueous electrolyte secondary battery, which is one aspect of this disclosure, the initial charge-discharge efficiency of the non-aqueous electrolyte secondary battery is improved.

[0012] This is a longitudinal cross-sectional view of a non-aqueous electrolyte secondary battery, which is an example of an embodiment.

[0013] Lithium transition metal composite oxides have a layered structure, and the charge and discharge reactions of batteries proceed as Li ions reversibly enter and exit this layered structure. Generally, lithium nickelate (LiNiO) ２ LiNiO is known as a high-capacity positive electrode active material, but when the amount of Li extracted during charging increases, the layered structure becomes unstable, leading to deformation and collapse of the layered structure, and the charge-discharge cycle characteristics tend to deteriorate. In addition, for the purpose of cost reduction, LiNiO ２ Substituting Ni with Mn in this mixture reduces the initial charge-discharge efficiency, and this tendency is particularly pronounced when the Ni content is 80 mol% or less.

[0014] The inventors have conducted extensive research and discovered that in a positive electrode active material containing a lithium transition metal composite oxide with a Ni content of 40 mol% to 80 mol%, the initial charge-discharge efficiency can be improved by having an inorganic sulfur compound present on the surface of the primary particles of the lithium transition metal composite oxide or at the grain boundaries between primary particles, while setting the proportion of metal elements other than Li in the Li layer of the layered structure of the lithium transition metal composite oxide to 3 mol% to 10 mol%, and setting the amount of S contained in the inorganic sulfur compound to 0.01 mol% to 1 mol%. The inventors have also discovered that the above positive electrode active material can be obtained by washing the lithium transition metal composite oxide with sulfuric acid at a concentration of 0.0001 mol / L to 1 mol / L.

[0015] Hereinafter, an example of an embodiment of a non-aqueous electrolyte secondary battery according to this disclosure will be described in detail. In the following, a cylindrical battery in which a wound electrode body is housed in a cylindrical outer casing will be given as an example, but the electrode body is not limited to the wound type and may be a laminated type in which multiple positive electrodes and multiple negative electrodes are stacked alternately one by one with separators in between. Furthermore, the outer casing is not limited to a cylindrical shape and may be, for example, rectangular, coin-shaped, etc., or may be a battery case made of a laminate sheet including a metal layer and a resin layer.

[0016] Figure 1 is an axial cross-sectional view of a cylindrical secondary battery 10, which is an example of an embodiment. As shown in Figure 1, the secondary battery 10 comprises a wound electrode body 14, an electrolyte, and an outer casing 16 that houses the electrode body 14 and the electrolyte. The electrode body 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape via the separator 13. The outer casing 16 is a bottomed cylindrical metal container with one side open in the axial direction, and the opening of the outer casing 16 is closed by a sealing body 17. In the following, for convenience of explanation, the side of the battery with the sealing body 17 will be referred to as the top, and the bottom side of the outer casing 16 as the bottom.

[0017] The positive electrode 11, negative electrode 12, and separator 13 constituting the electrode body 14 are all rectangular elongated bodies that are alternately stacked in the radial direction of the electrode body 14 by being wound in a spiral shape in the longitudinal direction. The separator 13 separates the positive electrode 11 and the negative electrode 12 from each other. The negative electrode 12 is formed to be slightly larger in dimensions than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer in both the longitudinal and transverse directions than the positive electrode 11. The two separators 13 are formed to be at least slightly larger in dimensions than the positive electrode 11 and are arranged, for example, to sandwich the positive electrode 11. The electrode body 14 includes a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like. In the electrode body 14, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is the winding direction, and the transverse direction of the positive electrode 11 and the negative electrode 12 is the axial direction. In other words, the end faces in the short direction of the positive electrode 11 and the negative electrode 12 form the axial end faces of the electrode body 14.

[0018] Insulating plates 18 and 19 are positioned above and below the electrode body 14, respectively. In the example shown in Figure 1, the positive electrode lead 20 extends through a through-hole in the insulating plate 18 towards the sealing body 17, and the negative electrode lead 21 extends outside the insulating plate 19 towards the bottom of the outer casing 16. The positive electrode lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, becomes the positive electrode terminal. The negative electrode lead 21 is connected to the bottom inner surface of the outer casing 16 by welding or the like, and the outer casing 16 becomes the negative electrode terminal.

[0019] A gasket 28 is provided between the outer casing 16 and the sealing body 17 to ensure airtightness inside the battery. The outer casing 16 has a grooved portion 22 formed on its side surface, which protrudes inward to support the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer casing 16, and its upper surface supports the sealing body 17. The sealing body 17 is fixed to the upper part of the outer casing 16 by the grooved portion 22 and the open end of the outer casing 16 which is crimped to the sealing body 17.

[0020] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in order from the electrode body 14 side, and functions as a safety valve. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, with the insulating member 25 interposed between their respective peripheral edges. When the internal pressure of the battery rises due to abnormal heat generation, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 towards the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0021] The following describes in detail the positive electrode 11, negative electrode 12, separator 13, and non-aqueous electrolyte that constitute the secondary battery 10, with particular emphasis on the positive electrode 11.

[0022] [Positive Electrode] The positive electrode 11 includes, for example, a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. Preferably, the positive electrode mixture layer is formed on both sides of the positive electrode current collector. The positive electrode current collector can be made of a metal foil that is stable in the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a film with the metal arranged on its surface. The thickness of the positive electrode current collector is, for example, 10 μm or more and 30 μm or less.

[0023] 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 to 150 μm on one side of the positive electrode current collector. The positive electrode 11 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 current collector, drying the coating film, and then rolling it to form the positive electrode mixture layer on both sides of the positive electrode current collector.

[0024] 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. The content of the conductive agent in the positive electrode mixture layer is, for example, 0.1% by mass or more and 10% by mass or less, relative to the total mass of the positive electrode mixture layer.

[0025] 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. The binder content in the positive electrode mixture layer is, for example, 0.1% by mass or more and 10% by mass or less, relative to the total mass of the positive electrode mixture layer.

[0026] The positive electrode mixture layer contains a positive electrode active material (hereinafter sometimes referred to as lithium composite oxide (Z)) comprising a lithium transition metal composite oxide and an inorganic sulfur compound. The positive electrode mixture layer may also contain positive electrode active materials other than lithium composite oxide (Z). In the positive electrode mixture layer, the content of lithium composite oxide (Z) is, for example, 90% by mass or more of the total mass of the positive electrode active material. Furthermore, the positive electrode mixture layer may contain only lithium composite oxide (Z) as the positive electrode active material.

[0027] The amount of Ni contained in the lithium transition metal composite oxide satisfies 40 mol% ≤ Ni amount ≤ 80 mol% relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide. If the Ni content is within this range, the initial charge and discharge efficiency can be improved while reducing costs through the synergistic effect with the inorganic sulfur compound described later. Preferably, the Ni amount is 50 mol% ≤ Ni amount ≤ 70 mol%.

[0028] Lithium transition metal composite oxides further contain Mn. The amount of Mn contained in lithium transition metal composite oxides satisfies 20 mol% ≤ Mn amount ≤ 60 mol% relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide. Lithium nickelate (LiNiO) ２ By replacing Ni in the battery with Mn instead of Co, it is possible to reduce costs while suppressing the decrease in battery capacity.

[0029] The lithium transition metal composite oxide may further contain Me (Me is at least one element selected from elements other than Li, Ni, Mn, and O). The amount of Me contained in the lithium transition metal composite oxide satisfies 0 mol% ≤ Me amount ≤ 10 mol% with respect to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide. Preferably, Me is at least one element selected from the group consisting of Co, Al, B, Mg, Si, Ca, Sr, Ti, Fe, Zr, Nb, Mo, Sn, W, and Bi. From the viewpoint of improving initial charge-discharge efficiency, it is preferable that Me contains Co.

[0030] The composition of lithium transition metal composite oxides is given by the general formula Li １＋ａ Ni ｂ Mn ｃ Me ｄ O ｅ(In the formula, 0 ≤ a ≤ 0.15, 0.4 ≤ b ≤ 0.8, 0.2 ≤ c ≤ 0.6, 0 ≤ d ≤ 0.1, b + c + d = 1, Me is at least one element selected from elements other than Li, Ni, Mn, and O, and e is a value that satisfies electrical neutrality). The molar ratio of O (e) is a value that satisfies electrical neutrality. In other words, it is a value that satisfies the valence of O in the positive electrode active material. The molar ratio of O (e) is, for example, 2.00 or more and 2.15 or less (2.00 ≤ e ≤ 2.15). The proportion of metal elements contained in lithium transition metal composite oxides can be measured, for example, by inductively coupled plasma atomic emission spectrometer (ICP-AES).

[0031] Lithium transition metal composite oxides have a layered structure. Examples of layered structures of lithium transition metal composite oxides include layered structures belonging to space group R-3m and layered structures belonging to space group C2 / m. From the viewpoint of increasing capacity and ensuring stability of the crystal structure, it is preferable for lithium transition metal composite oxides to have a layered structure belonging to space group R-3m. The layered structure of lithium transition metal composite oxides includes, for example, a transition metal layer and a Li layer. The charge and discharge reactions of the battery proceed as Li ions present in the Li layer reversibly move in and out.

[0032] In the layered structure of lithium transition metal composite oxides, the proportion of metal elements other than Li present in the Li layer is between 3 mol% and 10 mol% of the total number of moles of metal elements other than Li in the lithium transition metal composite oxide. If the proportion of metal elements other than Li in the Li layer is less than 3 mol%, the stability of the layered structure when Li ions are extracted from the Li layer decreases, which may reduce battery capacity. Furthermore, if the proportion of metal elements other than Li in the Li layer exceeds 10 mol%, the diffusivity of Li ions in the Li layer decreases, which may reduce battery capacity. The metal elements other than Li present in the Li layer are mainly Ni, but other metal elements may also be included.

[0033] 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 powder X-ray diffraction measurement of lithium transition metal composite oxides. For Rietveld analysis of X-ray diffraction patterns, for example, Rietveld analysis software such as SmartLab Studio II (Rigaku Corporation) can be used. The X-ray diffraction pattern is obtained, for example, 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 pseudo-Voigt function Constraints: Li(3a) + Ni(3a) = 1 Ni(3a) + Ni(3b) = y (y is the respective Ni content) ICSD No.: 98-009-4814

[0034] Lithium transition metal composite oxides contain secondary particles formed by the aggregation of primary particles. The particle size of the primary particles 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 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 secondary particles can be measured, for example, using a laser diffraction particle size distribution analyzer (e.g., Microtrac-Bell MT3000II) with water as the dispersion medium, but it can also be measured by image analysis using an SEM.

[0035] Inorganic sulfur compounds exist on the surface of primary particles of lithium transition metal composite oxides, or at the grain boundaries between primary particles of lithium transition metal composite oxides. The inorganic sulfur compounds may be uniformly dispersed or present only in a portion of the material. Here, the surface of primary particles of lithium transition metal composite oxides includes the surface of secondary particles. In other words, the surface of primary particles of lithium transition metal composite oxides includes the surface of primary particles located within the lithium transition metal composite oxide and the surface of primary particles exposed to the surface of the lithium transition metal composite oxide. Furthermore, the surface of primary particles of lithium transition metal composite oxides refers to the particle surface and its vicinity, for example, the region within 150 nm of the particle surface. The presence of inorganic sulfur compounds on the surface of primary particles of lithium transition metal composite oxides, or at the grain boundaries between primary particles of lithium transition metal composite oxides, can be confirmed by energy-dispersive X-ray spectroscopy (TEM-EDX).

[0036] The amount of sulfur (S) contained in the inorganic sulfur compound is between 0.01 mol% and 1 mol% relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide. By keeping the amount of S in the inorganic sulfur compound within the range of 0.01 mol% to 1 mol%, the initial charge-discharge efficiency of the positive electrode active material containing a lithium transition metal composite oxide with a Ni content of 40% to 80% can be improved. An inorganic sulfur compound is a sulfur compound that does not contain carbon (C). Sulfur compounds are, for example, sulfates, which are compounds formed by the bonding of metal elements eluted from Li or lithium transition metal composite oxides with sulfate ions. The amount of S contained in the inorganic sulfur compound can be measured, for example, by an inductively coupled plasma atomic emission spectrometer (ICP-AES). More specifically, by ICP, the amount of S S1 contained in the positive electrode active material containing the lithium transition metal composite oxide and the inorganic sulfur compound, and the amount of S S2 contained in the lithium transition metal composite oxide are measured, and the amount of S contained in the inorganic sulfur compound is calculated by subtracting S2 from S1. Furthermore, the amount of sulfur contained in the inorganic sulfur compound can be adjusted by the concentration of sulfuric acid used during washing.

[0037] Next, an example of a method for producing a positive electrode active material according to this embodiment will be described. The method for producing a positive electrode active material includes, for example, a mixing step of mixing a metal compound containing at least Ni with a Li compound to obtain a mixture; a calcination step of calcining the mixture to obtain a lithium transition metal composite oxide; a washing step of washing the lithium transition metal composite oxide with sulfuric acid to obtain a cake-like composition; and a heat treatment step of heat treating the cake-like composition.

[0038] Examples of Ni-containing metal compounds include Ni-containing metal hydroxides, Ni-containing metal oxides, and Ni-containing metal carbonate compounds. While not particularly limited, for example, a solution of a metal salt containing Ni, Co, Mn, etc., and an alkaline solution such as sodium hydroxide can be separately added dropwise to a reaction vessel stirring a pH-adjusted solution, thereby adjusting the pH to the alkaline side (e.g., 8.5 to 12.5) to precipitate (coprecipitation) a composite hydroxide and obtain a Ni-containing metal hydroxide. Alternatively, a Ni-containing metal oxide can be produced by calcining the Ni-containing metal hydroxide. The calcination temperature is not particularly limited, but is, for example, in the range of 250°C to 600°C.

[0039] Next, a mixture is obtained by mixing a metal compound containing at least Ni with a Li compound. Examples of Li 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, LiF, etc. Additionally, Me compounds may be mixed during the mixing process. Examples of Me compounds include SiO, SiO ２ Ca(OH) ２ , CaO, Sr(OH) ２ , SrO, TiO ２ , Ti(OH) ４ Fe(OH) ２ Fe ２ O ３ , ZrO ２ , Nb２ O ５ , Nb ２ O ５ nH ２ O, Li ２ MoO ４ MoO ３ , H ２ MoO ４ , SnO ２ WO ３ Li ２ WO ４ , Bi(OH) ３ , Bi ２ O ３ These are some examples.

[0040] A lithium transition metal composite oxide is obtained by calcining the mixture. Calcination is carried out, for example, in air or under an oxygen stream. Calcination under an oxygen stream is carried out, for example, 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 heating rate is in the range of 0.1 L / min to 4 L / min per unit area, or 1 L / min or more per 1 kg of mixture. In the firing conditions, the first set temperature is set to 450°C or lower, the holding time at the first set temperature is in the range of 0 hours to 8 hours, and the heating rate below 450°C is in the range of more than 1.5°C / min and 6.0°C / min or lower. The second set temperature is set to 450°C to 680°C, the holding time at the second set temperature is in the range of 0 hours to 8 hours, and the heating rate above 450°C to 680°C is in the range of more than 1.0°C / min and 4.5°C / min or lower. The maximum temperature reached is in the range of 690°C to 1000°C. The heating rate from above 680°C to the maximum temperature reached may be, for example, 0.1°C / min to 3.5°C / min. The holding time at the maximum temperature reached may be 1 hour to 10 hours or lower. Furthermore, this firing step may be a multi-stage firing process, and multiple settings may be set for each temperature range, as long as they are within the range specified above.

[0041] A cake-like composition is obtained by washing a lithium transition metal composite oxide with sulfuric acid at a concentration of 0.0001 mol / L to 1 mol / L and then dehydrating it. The washing step is carried out, for example, using a 3 L reaction vessel, under conditions such as a solid-liquid ratio of 300 g / L to 2000 g / L, a washing time of 1 minute to 1 hour, and a stirring speed of 100 rpm or more. If the size of the reaction vessel is changed, the washing time and stirring speed may be changed. The water content of the cake-like composition obtained by dehydration is, for example, 10% or less, but may also be 8% or less.

[0042] A positive electrode active material is obtained by heat-treating a cake-like composition. The heat treatment of the cake-like composition is carried out at a temperature of, for example, 150°C to 500°C. The atmosphere during the heat treatment may be a vacuum, an oxygen stream, or air. Also, during the heat treatment, for example, tungsten oxide (WO) ３ ), lithium tungstate (Li ２ WO ４ Li ４ WO ５ Li ６ W ２ O ９ ), boric acid (H ３ BO ３ ), lithium borate (Li ２ B ４ O ７ Li ３ BO ３ LiB ３ O ５ LiBO ２ ), lithium phosphate (Li ３－ｘ H ｘ PO ４ Additions such as (0 ≤ x ≤ 3) may also be added.

[0043] [Negative Electrode] The negative electrode 12 may, for example, have a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector, or a metallic Li foil may be used as the negative electrode 12. Alternatively, the negative electrode 12 may have a negative electrode current collector, and lithium metal may be deposited on the surface of the negative electrode current collector by charging. When the negative electrode 12 has a negative electrode mixture layer, it is preferable that the negative electrode mixture layer is formed on both sides of the negative electrode current collector. For the negative electrode current collector, a foil of a metal that is stable in the potential range of the negative electrode 12, such as copper or a copper alloy, or a film with the metal arranged on the surface layer, can be used. The thickness of the negative electrode current collector 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 150 μm or less on one side of the negative electrode current collector. The negative electrode 12 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 a negative electrode current collector, drying the coating, and then rolling it to form a negative electrode mixture layer on both sides of the negative electrode current collector.

[0044] 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.

[0045] 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.

[0046] [Separator] The separator 13 is made of a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyethylene, polyolefins such as polypropylene, and cellulose. The separator 13 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 13.

[0047] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. 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 11, the negative electrode 12, or the separator 13 with a slurry containing the filler.

[0048] [Non-aqueous electrolytes] Non-aqueous electrolytes, for example, have lithium ion conductivity. Non-aqueous electrolytes may be liquid electrolytes (electrolytes) or solid electrolytes.

[0049] 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).

[0050] 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).

[0051] Examples of the above ethers include cyclic ethers such as 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, crown ether, etc., and chain ethers such as 1,2 - dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o - dimethoxybenzene, 1,2 - diethoxyethane, 1,2 - dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1 - dimethoxymethane, 1,1 - diethoxyethane, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, etc.

[0052] The electrolyte salt is preferably a lithium salt. Examples of the lithium salt include LiClO ４ , LiBF [[ID=(6]] ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０ , lithium lower aliphatic carboxylate, LiCl, LiBr, LiI, phosphate, borate, imide salt. Examples of the phosphate include lithium difluorophosphate (LiPO ２ F ２), lithium difluorobis(oxalato)phosphate (LiDFBOP), 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 ), lithium bistrifluoromethanesulfonylimide (LiN(CF ３ SO ２ ), ２ ), lithium trifluoromethanesulfonato nonafluorobutanesulfonylimide (LiN(CF ３ SO ２ )(C ４ F ９ SO ２ )), lithium bispentafluoroethanesulfonylimide (LiN(C ２ F ５ SO ２ ), ２ ), etc. Among these, from the viewpoints of ionic conductivity, electrochemical stability, etc., it is preferable to use LiPF ６ . The concentration of the lithium salt may be, for example, 4 mol or less per 1 L of the non-aqueous solvent, may be 3 mol or less, preferably 1.8 mol or less, and more preferably 0.8 mol or more and 1.8 mol or less.

[0053] The non-aqueous electrolyte may contain an additive. Examples of the additive include unsaturated carbonic esters, acid anhydrides, phenolic compounds, benzene compounds, nitrile compounds, isocyanate compounds, sultone compounds, sulfuric acid compounds, borate ester compounds, phosphate ester compounds, phosphite ester compounds, etc.

[0054] 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.

[0055] Examples of phenolic compounds include phenol and hydroxytoluene. Examples of benzene compounds include fluorobenzene, hexafluorobenzene, and cyclohexylbenzene (CHB).

[0056] 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.

[0057] 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.

[0058] 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.

[0059] <Example 1-1> [Preparation of positive electrode active material] [Ni obtained by coprecipitation method ０．７ Mn ０．３ ] (OH) ２The composite hydroxide represented by was calcined at 400°C for 8 hours to obtain a metal compound containing Ni and Mn. Next, lithium hydroxide monohydrate (LiOH·H) was prepared so that the molar ratio of Li to the total number of moles of Ni and Mn was 107 mol%. ２ O) was mixed to obtain a mixture. This mixture was heated under an oxygen stream with an oxygen concentration of 95% (flow rate of 3 L / min per 1 kg of mixture) at a heating rate of 5°C / min from room temperature to 300°C, and then heated from 300°C to 670°C at a heating rate of 2°C / min. After that, the temperature was raised from 670°C to 890°C at a heating rate of 1°C / min and held for 10 hours to obtain a lithium transition metal composite oxide. Using a 3 L reaction vessel, the lithium transition metal composite oxide was added to sulfuric acid with a concentration of 0.38 mol / L so that the solid-liquid ratio was 1250 g / L, and after washing at a stirring speed of 400 rpm for 5 minutes, it was dehydrated to obtain a cake-like composition. Furthermore, this cake-like composition was heat-treated for 2 hours in a vacuum atmosphere at a temperature of 180°C and a pressure of 10 Pa to obtain the positive electrode active material of Example 1-1.

[0060] Measurement of the obtained lithium transition metal composite oxide using inductively coupled plasma atomic emission spectroscopy (ICP-AES) confirmed the presence of the elements shown in Table 1 below. Furthermore, TEM-EDX measurements confirmed the presence of inorganic sulfates on the primary particle surface or at the grain boundaries between primary particles of the lithium transition metal composite oxide. X-ray diffraction measurements revealed that the proportion of non-Li metal elements in the Li layer relative to the total molar amount of non-Li metal elements in the lithium transition metal composite oxide was 6.0 mol%. Additionally, the amount of S contained in the inorganic sulfur compound measured by ICP-AES was 0.2 mol% relative to the total molar amount of non-Li metal elements in the lithium transition metal composite oxide. Note that the amount of S in the inorganic sulfur compound can be adjusted by the concentration of sulfuric acid used during washing.

[0061] [Preparation of the positive electrode] 95 parts by mass of the above positive electrode active material, 3 parts by mass of acetylene black (AB), and 2 parts by mass of polyvinylidene fluoride (PVDF) were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to both sides of a positive electrode current collector made of aluminum foil, and after the coating film was dried, the coating film was rolled out with a rolling mill and cut to a predetermined electrode size to produce the positive electrode. An exposed portion was provided on a part of the positive electrode in which the surface of the positive electrode current collector was exposed.

[0062] [Fabrication of the negative electrode] Natural graphite was used as the negative electrode active material. The negative electrode active material, sodium carboxymethylcellulose (CMC-Na), and styrene-butadiene rubber (SBR) were mixed in an aqueous solution in a solid content mass ratio of 100:1:1 to prepare a negative electrode mixture slurry. Next, the negative electrode mixture slurry was applied to both sides of a negative electrode current collector made of copper foil, and after the coating film was dried, the coating film was rolled using a rolling mill and cut to a predetermined electrode size to produce the negative electrode. An exposed portion was provided on a part of the negative electrode in which the surface of the negative electrode current collector was exposed.

[0063] [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.

[0064] [Preparation of Test Cell] A positive electrode lead was attached to the exposed portion of the positive electrode, and a negative electrode lead was attached to the exposed portion of the negative electrode. The positive and negative electrodes were wound in a spiral shape via a polyolefin separator, and then press-molded radially to produce a flat, wound electrode body. This electrode body was housed in an outer casing made of aluminum laminate sheet, the non-aqueous electrolyte was injected, and the opening of the outer casing was sealed to obtain a test cell.

[0065] [Evaluation of Initial Charge / Discharge Efficiency] The test cell was charged at a constant current of 0.2 It to a cell voltage of 4.5 V (vsLi) under a temperature environment of 25°C. Then, it was charged at a constant voltage of 4.5 V (vsLi) until the current value was 0.02 It. After 1 hour, it was discharged at a constant current of 0.2 It to a cell voltage of 2.5 V (vsLi). The charging capacity and discharging capacity at this time were measured, and the initial charge / discharge efficiency of the test cell was calculated using the following formula: Initial charge / discharge efficiency = Discharge capacity / Charging capacity

[0066] <Comparative Example 1-1> A test cell was prepared and evaluated in the same manner as in Example 1-1, except that the positive electrode active material was washed with water instead of sulfuric acid.

[0067] <Comparative Example 1-2> A test cell was prepared and evaluated in the same manner as in Example 1-1, except that the positive electrode active material was washed with boric acid at a concentration of 0.064 mol / L instead of sulfuric acid.

[0068] <Comparative Example 1-3> A test cell was prepared and evaluated in the same manner as in Example 1-1, except that the positive electrode active material was washed with phosphoric acid at a concentration of 0.38 mol / L instead of sulfuric acid.

[0069] <Comparative Example 1-4> A test cell was prepared and evaluated in the same manner as in Example 1-1, except that the positive electrode active material was washed with nitric acid at a concentration of 0.38 mol / L instead of sulfuric acid.

[0070] Table 1 shows the evaluation results of the test cells for Example 1-1 and Comparative Examples 1-1 to 1-4. As shown in Table 1, the initial charge-discharge efficiency of the test cell for Example 1-1 was higher than that of the test cells for Comparative Examples 1-1 to 1-4. Furthermore, measurements by TEM-EDX confirmed the presence of inorganic sulfates on the primary particle surface or at the grain boundaries between primary particles in Example 1-1, while confirming the absence of inorganic sulfates on the primary particle surface or at the grain boundaries between primary particles in Comparative Examples 1-1 to 1-4. This indicates that inorganic sulfates are formed by the sulfuric acid used as the cleaning solution.

[0071]

[0072] <Example 2-1> A test cell was prepared and evaluated in the same manner as in Example 1-1, except that the positive electrode active material was prepared as follows: (1) In the preparation of the positive electrode active material, [Ni ０．６ Mn ０．４ ] (OH) ２ A metal compound containing Ni and Mn was obtained using a composite hydroxide represented by (2) Lithium hydroxide monohydrate (LiOH·H) such that the molar ratio of Li to the total number of moles of Ni and Mn is 111 mol%. ２ (O) was mixed to obtain a mixture. (3) The firing atmosphere was changed to air and the maximum temperature was changed to 900°C.

[0073] <Comparative Example 2-1> A test cell was prepared and evaluated in the same manner as in Example 2-1, except that the positive electrode active material was washed with water instead of sulfuric acid.

[0074] Table 2 shows the evaluation results of the test cells for Example 2-1 and Comparative Example 2-1. In addition to the evaluation results, Table 2 also shows the following information. Similarly, the following information is also shown in Tables 3 to 6 described later. (1) Composition of lithium transition metal composite oxide calculated from the ICP-AES measurement results (2) Amount of S contained in inorganic sulfate present on the primary particle surface or grain boundaries between primary particles of lithium transition metal composite oxide (3) Percentage of metal elements other than Li present in the Li layer of the layered structure (4) Cleaning solution used in the cleaning step

[0075] As shown in Table 2, the initial charge-discharge efficiency of the test cell in Example 2-1 was higher than that of the test cell in Comparative Example 2-1. Furthermore, TEM-EDX measurements confirmed the presence of inorganic sulfate on the primary particle surface or at the grain boundaries between primary particles in Example 2-1. Since the concentration of sulfuric acid used in Example 2-1 was the same as that used in Example 1-1, the amount of S contained in the inorganic sulfur compound is estimated to be 0.2 mol% of the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide, the same as in Example 1-1. In Comparative Example 2-1, it was confirmed that inorganic sulfate was not present on the primary particle surface or at the grain boundaries between primary particles in the lithium transition metal composite oxide.

[0076]

[0077] <Example 3-1> A test cell was prepared and evaluated in the same manner as in Example 1-1, except that the positive electrode active material was prepared as follows: (1) In the preparation of the positive electrode active material, [Ni ０．５ Mn ０．５ ] (OH) ２ A metal compound containing Ni and Mn was obtained using a composite hydroxide represented by (2) Lithium hydroxide monohydrate (LiOH·H) such that the molar ratio of Li to the total number of moles of Ni and Mn is 111 mol%. ２ (O) was mixed to obtain a mixture. (3) The firing atmosphere was changed to air and the maximum temperature was changed to 1000°C.

[0078] <Comparative Example 3-1> A test cell was prepared and evaluated in the same manner as in Example 3-1, except that the positive electrode active material was washed with water instead of sulfuric acid. The obtained lithium transition metal composite oxide was measured by ICP-AES, and the elements shown in Table 1 below were confirmed. Furthermore, TEM-EDX measurement confirmed that inorganic sulfates were not present on the primary particle surface or at the grain boundaries between primary particles of the lithium transition metal composite oxide. In addition, X-ray diffraction measurement showed that the proportion of metal elements other than Li in the Li layer relative to the total molar amount of metal elements other than Li in the lithium transition metal composite oxide was 7.1 mol%.

[0079] Table 3 shows the evaluation results of the test cells for Example 3-1 and Comparative Example 3-1. As shown in Table 3, the initial charge-discharge efficiency of the test cell in Example 3-1 was higher than that of the test cell in Comparative Example 3-1. Furthermore, TEM-EDX measurements confirmed the presence of inorganic sulfate on the primary particle surface or at the grain boundaries between primary particles in Example 3-1. Since the concentration of sulfuric acid used in Example 3-1 was the same as that used in Example 1-1, the amount of S contained in the inorganic sulfur compound is estimated to be 0.2 mol% of the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide, the same as in Example 1-1. In Comparative Example 3-1, it was confirmed that inorganic sulfate was not present on the primary particle surface or at the grain boundaries between primary particles in the lithium transition metal composite oxide.

[0080]

[0081] <Example 4-1> A test cell was prepared and evaluated in the same manner as in Example 1-1, except that the positive electrode active material was prepared as follows: (1) In the preparation of the positive electrode active material, [Ni ０．６ Mn ０．３５ Co ０．０５ ] (OH) ２ A composite hydroxide represented by was used to obtain a metal compound containing Ni, Mn, and Co. (2) The calcination atmosphere was changed to air, and the maximum temperature was changed to 900°C.

[0082] <Comparative Example 4-1> A test cell was prepared and evaluated in the same manner as in Example 4-1, except that the positive electrode active material was washed with water instead of sulfuric acid.

[0083] As shown in Table 4, the initial charge-discharge efficiency of the test cell in Example 4-1 was higher than that of the test cell in Comparative Example 4-1. Furthermore, TEM-EDX measurements confirmed the presence of inorganic sulfate on the primary particle surface or at the grain boundaries between primary particles in Example 4-1. Since the concentration of sulfuric acid used in Example 4-1 was the same as that used in Example 1-1, the amount of S contained in the inorganic sulfur compound is estimated to be 0.2 mol% of the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide, the same as in Example 1-1. In Comparative Example 4-1, it was confirmed that inorganic sulfate was not present on the primary particle surface or at the grain boundaries between primary particles in the lithium transition metal composite oxide.

[0084]

[0085] <Example 5-1> A test cell was prepared and evaluated in the same manner as in Example 1-1, except that the positive electrode active material was prepared as follows: (1) In the preparation of the positive electrode active material, [Ni ０．７ Mn ０．２５ Co ０．０５ ] (OH) ２ A metal compound containing Ni and Mn was obtained using the composite hydroxide represented by [formula]. (2) The maximum temperature was changed to 850°C.

[0086] <Comparative Example 5-1> A test cell was prepared and evaluated in the same manner as in Example 5-1, except that the positive electrode active material was washed with water instead of sulfuric acid.

[0087] As shown in Table 5, the initial charge-discharge efficiency of the test cell in Example 5-1 was higher than that of the test cell in Comparative Example 5-1. Furthermore, TEM-EDX measurements confirmed the presence of inorganic sulfate on the primary particle surface or at the grain boundaries between primary particles in Example 5-1. Since the concentration of sulfuric acid used in Example 5-1 was the same as that used in Example 1-1, the amount of S contained in the inorganic sulfur compound is estimated to be 0.2 mol% of the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide, the same as in Example 1-1. In Comparative Example 5-1, it was confirmed that inorganic sulfate was not present on the primary particle surface or at the grain boundaries between primary particles in the lithium transition metal composite oxide.

[0088]

[0089] <Reference Example 1-1> A test cell was prepared and evaluated in the same manner as in Example 1-1, except that the positive electrode active material was prepared as follows: (1) In the preparation of the positive electrode active material, [Ni ０．８２ Mn ０．１８ ] (OH) ２ A metal compound containing Ni and Mn was obtained using the composite hydroxide represented by [formula]. (2) The maximum temperature was changed to 800°C.

[0090] <Reference Example 1-2> A test cell was prepared and evaluated in the same manner as in Reference Example 1-1, except that the positive electrode active material was washed with water instead of sulfuric acid.

[0091] Measurements using TEM-EDX confirmed the presence of inorganic sulfates on the primary particle surface or at the grain boundaries between primary particles in Reference Example 1-1. Since the concentration of sulfuric acid used in Reference Example 1-1 was the same as that used in Example 1-1, the amount of S contained in the inorganic sulfur compound is estimated to be 0.2 mol% of the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide, the same as in Example 1-1. In Reference Example 1-2, it was confirmed that inorganic sulfates were not present on the primary particle surface or at the grain boundaries between primary particles in the lithium transition metal composite oxide. However, as shown in Table 6, the initial charge-discharge efficiency of the test cell in Reference Example 1-1 and the initial charge-discharge efficiency of the test cell in Reference Example 1-2 were approximately the same.

[0092]

[0093] 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer casing, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved section, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket

Claims

1. General formula Li １＋ａ Ni ｂ Mn ｃ Me ｄ O ｅ A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, comprising the step of washing a lithium transition metal composite oxide having a composition represented by the formula (wherein 0 ≤ a ≤ 0.15, 0.4 ≤ b ≤ 0.8, 0.2 ≤ c ≤ 0.6, 0 ≤ d ≤ 0.1, b + c + d = 1, Me is at least one element selected from elements other than Li, Ni, Mn, and O, and e is a value that satisfies electrical neutrality) with sulfuric acid at a concentration of 0.0001 mol / L or more and 1 mol / L or less.

2. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium transition metal composite oxide has a layered structure, and the proportion of metal elements other than Li present in the Li layer of the layered structure is 3 mol% or more and 10 mol% or less with respect to the total number of moles of metal elements other than Li in the lithium transition metal composite oxide.

3. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the Me is at least one element selected from the group consisting of Co, Al, B, Mg, Si, Ca, Sr, Ti, Fe, Zr, Nb, Mo, Sn, W, and Bi.

4. A positive electrode active material for a non-aqueous electrolyte secondary battery, comprising a lithium transition metal composite oxide containing secondary particles formed by aggregation of primary particles, and an inorganic sulfur compound present on the surface of the primary particles or at grain boundaries between the primary particles, wherein the composition of the lithium transition metal composite oxide is represented by the general formula Li １＋ａ Ni ｂ Mn ｃ Me ｄ O ｅ (where 0 ≦ a ≦ 0.15, 0.4 ≦ b ≦ 0.8, 0.2 ≦ c ≦ 0.6, 0 ≦ d ≦ 0.1, b + c + d = 1, Me is at least one element selected from elements other than Li, Ni, Mn, and O, and e is a value satisfying electrical neutrality). The lithium transition metal composite oxide has a layered structure, and the ratio of metal elements other than Li present in the Li layer of the layered structure is 3 mol% or more and 10 mol% or less with respect to the total molar number of metal elements excluding Li in the lithium transition metal composite oxide. The amount of S contained in the inorganic sulfur compound is 0.01 mol% or more and 1 mol% or less with respect to the total molar number of metal elements excluding Li in the lithium transition metal composite oxide. A positive electrode active material for a non-aqueous electrolyte secondary battery.

5. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 4, wherein Me is at least one element selected from the group consisting of Co, Al, B, Mg, Si, Ca, Sr, Ti, Fe, Zr, Nb, Mo, Sn, W, and Bi.

6. 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 claim 4 or 5, a negative electrode, and a non-aqueous electrolyte.

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