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

Abstract

To provide a positive electrode active material for a non-aqueous electrolyte secondary battery, including a lithium transition metal composite oxide having a high Ni content, and which contributes to improvement in charge-discharge cycle characteristics of a battery.SOLUTION: A positive electrode active material for a non-aqueous electrolyte secondary battery, includes: a lithium transition metal composite oxide having secondary particles formed of aggregating primary particles; and a surface modification compound that is present on at least a surface of the secondary particles, and contains at least one of Ca and Sr. The lithium transition metal composite oxide contains 70 mol % or more of Ni with respect to a total number of moles of metal elements excluding Li. A total amount of Ca and Sr in the surface modifying compound is 0.5 mol % or less with respect to a total number of moles of the metal elements excluding Li in the lithium transition metal composite oxide.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] In recent years, lithium transition metal composite oxides with a high Ni content have been attracting attention as a positive electrode active material with high energy density. For example, Patent Document 1 discloses a compound represented by the general formula Li x Ni y Co z M m The present invention discloses a positive electrode active material for a non-aqueous electrolyte secondary battery, which comprises a lithium transition metal composite oxide represented by the formula: O2 (wherein M is an element selected from Ba, Sr, and B, and 0.9≦x≦1.1, 0.5≦y≦0.95, 0.05≦z≦0.5, and 0.0005≦m≦0.02), and has a BET specific surface area of ​​0.8 m2 / g or less.

[0003] Furthermore, Patent Document 2 discloses a positive electrode active material for a non-aqueous electrolyte secondary battery, which has an α-NaFeO structure, contains one or more transition metal elements selected from the group consisting of Mn, Ni, and Co, and has an alkaline earth metal and W present on the particle surfaces of a lithium transition metal composite oxide. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-100295 [Patent Document 2] Japanese Patent Application Publication No. 2018-129221 Summary of the Invention

[0005] When a lithium transition metal composite oxide with a high Ni content is used as the positive electrode active material for a nonaqueous electrolyte secondary battery, the amount of Li extracted during charging is large, which causes the layered crystal structure to break down and the capacity to decrease with repeated charge and discharge. Note that the technologies disclosed in Patent Documents 1 and 2 still have room for improvement in terms of charge and discharge cycle characteristics.

[0006] A positive electrode active material for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a lithium transition metal composite oxide having secondary particles formed by aggregation of primary particles, and a surface modifying compound containing at least one of Ca and Sr, present at least on the surfaces of the secondary particles. The lithium transition metal composite oxide is characterized in that it contains 70 mol % or more of Ni relative to the total number of moles of metal elements excluding Li, and the total amount of Ca and Sr in the surface modifying compound is 0.5 mol % or less relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

[0007] A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a lithium transition metal composite oxide synthesis step of mixing and firing a transition metal oxide containing 70 mol % or more of Ni with a Li compound to obtain a lithium transition metal composite oxide; a washing step of washing the lithium transition metal composite oxide with water and dehydrating it to obtain a cake-like composition; and a heat treatment step of heat-treating the cake-like composition to obtain a positive electrode active material for a non-aqueous electrolyte secondary battery, wherein at least one of a Ca compound and a Sr compound is added to the cake-like composition during the washing step or after the washing step and before the heat treatment step.

[0008] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is characterized by including a positive electrode containing the above-described positive electrode active material for a non-aqueous electrolyte secondary battery, a negative electrode, and a non-aqueous electrolyte.

[0009] According to the positive electrode active material for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure, it is possible to provide a high-capacity non-aqueous electrolyte secondary battery in which the decrease in battery capacity due to charging and discharging is suppressed. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment. [Figure 2] FIG. 2 is a schematic diagram showing a cross section of a positive electrode active material according to an embodiment. [Figure 3] FIG. 3 shows the X-ray diffraction patterns of Example 18, SrO, and CaO. DETAILED DESCRIPTION OF THE INVENTION

[0011] The layered structure of the lithium transition metal composite oxide contained in the positive electrode active material contains a transition metal layer containing Ni and other metals, a Li layer, and an oxygen layer. The reversible movement of Li ions in the Li layer allows the battery's charge / discharge reactions to proceed. When using a lithium transition metal composite oxide with a high Ni content, many Li ions are extracted from the Li layer during battery charging, causing the layered structure to collapse and leading to a decrease in battery capacity. Furthermore, lithium transition metal composite oxides with a high Ni content have high activity near the particle surface, making the structure prone to instability. This makes them susceptible to the formation of a surface degradation layer or erosion due to reactions with the electrolyte, leading to a decrease in battery capacity.

[0012] Therefore, the present inventors conducted extensive research to solve the above problems and found that the corrosion of the structurally deteriorated layer can be suppressed by protecting the surface of a lithium transition metal composite oxide with a compound containing at least one of Ca and Sr. As a result, they came up with the following positive electrode active material for a non-aqueous electrolyte secondary battery, which maintains a high capacity while suppressing a decrease in battery capacity due to charge and discharge.

[0013] A positive electrode active material for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a lithium transition metal composite oxide having secondary particles formed by aggregation of primary particles, and a surface modifying compound containing at least one of Ca and Sr, present at least on the surfaces of the secondary particles. The lithium transition metal composite oxide is characterized in that it contains 70 mol % or more of Ni relative to the total number of moles of metal elements excluding Li, and the total amount of Ca and Sr in the surface modifying compound is 0.5 mol % or less relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

[0014] An example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail below. Hereinafter, a cylindrical battery in which a wound electrode assembly is housed in a cylindrical battery case will be exemplified. However, the electrode assembly is not limited to the wound type and may be a laminated type in which multiple positive electrodes and multiple negative electrodes are alternately stacked one by one with separators interposed therebetween. Furthermore, the battery case is not limited to a cylindrical shape and may be, for example, a prismatic or coin-shaped battery case, or may be a battery case made of a laminate sheet including a metal layer and a resin layer.

[0015] Fig. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As illustrated in Fig. 1, the nonaqueous electrolyte secondary battery 10 includes an electrode assembly 14, a nonaqueous electrolyte (not shown), and a battery case 15 that accommodates the electrode assembly 14 and the nonaqueous electrolyte. The electrode assembly 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween. The battery case 15 includes a cylindrical outer can 16 with a bottom, and a sealing member 17 that closes the opening of the outer can 16.

[0016] The electrode assembly 14 is composed of a long positive electrode 11, a long negative electrode 12, two long separators 13, a positive electrode tab 20 joined to the positive electrode 11, and a negative electrode tab 21 joined to the negative electrode 12. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the longitudinal direction and width direction (short direction). The two separators 13 are formed to be at least slightly larger than the positive electrode 11 and are arranged to sandwich the positive electrode 11, for example.

[0017] The nonaqueous electrolyte secondary battery 10 includes insulating plates 18 and 19 disposed above and below the electrode assembly 14. In the example shown in Fig. 1 , a positive electrode tab 20 attached to the positive electrode 11 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and a negative electrode tab 21 attached to the negative electrode 12 passes outside the insulating plate 19 and extends toward the bottom of the outer can 16. The positive electrode tab 20 is connected to the underside of a bottom plate 23 of the sealing body 17 by welding or the like, and a cap 27 of the sealing body 17 electrically connected to the bottom plate 23 serves as the positive electrode terminal. The negative electrode tab 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0018] The outer can 16 is, for example, a cylindrical metal container with a bottom. A gasket 28 is provided between the outer can 16 and the sealing body 17, sealing the internal space of the battery case 15. The outer can 16 has a grooved portion 22 that supports the sealing body 17, formed, for example, by pressing the side surface from the outside. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its upper surface.

[0019] The sealing body 17 has a structure in which, in order from the electrode body 14 side, a bottom plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0020] The positive electrode 11, negative electrode 12, separator 13, and nonaqueous electrolyte that constitute the nonaqueous electrolyte secondary battery 10 will be described in detail below, particularly the positive electrode active material contained in the positive electrode mixture layer 31 that constitutes the positive electrode 11.

[0021] [Positive electrode] The positive electrode 11 has a positive electrode current collector 30 and a positive electrode composite layer 31 formed on both sides of the positive electrode current collector 30. The positive electrode current collector 30 can be a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode 11, or a film with such a metal disposed on its surface. The positive electrode composite layer 31 contains a positive electrode active material, a conductive material, and a binder. The thickness of the positive electrode composite layer 31 is, for example, 10 μm to 150 μm on one side of the positive electrode current collector 30. The positive electrode 11 can be produced by applying a positive electrode slurry containing the positive electrode active material, the conductive material, the binder, etc. to the surface of the positive electrode current collector 30, drying the coating, and then compressing it to form the positive electrode composite layer 31 on both sides of the positive electrode current collector 30.

[0022] Examples of the conductive material contained in the positive electrode mixture layer 31 include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of the binder contained in the positive electrode mixture layer 31 include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. These resins may be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like.

[0023] 2 is a diagram schematically illustrating an example of a cross section of a positive electrode active material contained in positive electrode composite layer 31. The positive electrode active material includes a lithium transition metal composite oxide having secondary particles 50 formed by aggregation of primary particles 52, and a surface modifying compound 54 that is present at least on the surface of secondary particles 50 and contains at least one of Ca and Sr. This can suppress the generation and erosion of a structurally deteriorated layer on the surface of the lithium transition metal composite oxide due to reaction with the electrolyte, etc.

[0024] The secondary particles 50 are particles having a volume-based median diameter (D50) of preferably 3 μm to 30 μm, more preferably 5 μm to 25 μm, and particularly preferably 7 μm to 15 μm. D50 refers to the particle size at which the cumulative frequency of particles in the volume-based particle size distribution is 50% from the smallest particle size, and is also called the median diameter. The particle size distribution of the secondary particles 50 of the lithium transition metal composite oxide can be measured using a laser diffraction particle size distribution analyzer (for example, the MT3000II manufactured by Microtrack Bell Corporation) using water as a dispersion medium.

[0025] The particle size of the primary particles 52 constituting the secondary particles 50 is, for example, 0.05 μm to 1 μm. The particle size of the primary particles 52 is measured as the diameter of a circumscribed circle in a particle image observed with a scanning electron microscope (SEM).

[0026] The lithium transition metal composite oxide may have, for example, a layered structure belonging to the space group R-3m or a layered structure belonging to the space group C2 / m. Among these, a layered structure belonging to the space group R-3m is preferred in terms of high capacity, stability of the crystal structure, etc. The layered structure of the lithium transition metal composite oxide includes a transition metal layer, a Li layer, and an oxygen layer.

[0027] The lithium transition metal composite oxide preferably contains 70 mol% or more of Ni relative to the total number of moles of metal elements excluding Li, and 80 mol% or more of Ni relative to the total number of moles of metal elements excluding Li. By making the Ni content 70 mol% or more, a high-capacity battery can be obtained. Furthermore, by making the Ni content 80 mol% or more, the effect of improving cycle characteristics by stabilizing the structure of the lithium transition metal composite oxide can be easily obtained. The Ni content is preferably 95 mol% or less. If the Ni content exceeds 95 mol%, the layered structure of the lithium transition metal composite oxide becomes unstable.

[0028] Lithium transition metal composite oxides have the general formula Li a Ni 1-x-y Co x M yO2 (wherein 0.97≦a≦1.20, 0≦x≦0.2, 0≦y≦0.1, and M is at least one element selected from Mn, W, Mg, Mo, Nb, Ti, Si, and Al). The positive electrode active material may contain a lithium transition metal composite oxide other than that represented by the above general formula or other compounds, as long as the object of the present disclosure is not impaired. The molar fraction of the metal elements contained in the lithium transition metal composite oxide can be measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), an energy dispersive X-ray analyzer (EDX), or the like.

[0029] The ratio of Li in the lithium transition metal composite oxide, a, satisfies 0.97≦a<1.20, and preferably 0.97≦a≦1.05. If a is less than 0.97, the battery capacity may be reduced compared to when a satisfies the above range. If a is more than 1.20, the charge-discharge cycle characteristics may be reduced compared to when a satisfies the above range.

[0030] Co and M (M is at least one element selected from Mn, W, Mg, Mo, Nb, Ti, Si, and Al) are optional components. x and y, which represent the contents of Co and M relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide, preferably satisfy the relationships 0≦x≦0.2 and 0≦y≦0.1, respectively. Co is expensive, so from the viewpoint of production costs, it is preferable to keep the Co content low.

[0031] The total amount of Ca and Sr in the surface modification compound is 0.5 mol % or less relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide. This can improve charge-discharge cycle characteristics. Furthermore, the total amount of Ca and Sr in the surface modification compound is preferably 0.03 mol % or more relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide. The surface modification compound may contain Ca in the form of a compound such as CaO, Ca(OH)2, or CaCO3, or may contain Sr in the form of a compound such as SrO, Sr(OH)2, or SrCO3.

[0032] At least 95 mol % of the surface-modifying compound may be present in a range extending from the surface of the secondary particles to the inside by the average particle diameter of the primary particles. By having the majority, i.e., at least 95 mol %, of the surface-modifying compound present in the vicinity of the surface of the secondary particles of the lithium transition metal composite oxide, it is possible to efficiently suppress the formation and erosion of a structurally deteriorated layer on the surface of the lithium transition metal composite oxide due to reaction with the electrolyte, etc.

[0033] Alternatively, 95 mol % or more of the surface modifying compound may be present on the surfaces of the primary particles that form the surfaces of the secondary particles. By having the majority, i.e., 95 mol % or more, of the surface modifying compound present in the vicinity of the surfaces of the secondary particles of the lithium transition metal composite oxide, it is possible to efficiently suppress the formation and erosion of a structurally deteriorated layer on the surface of the lithium transition metal composite oxide due to reaction with the electrolyte, etc.

[0034] The state of the surface modifying compound in the positive electrode active material can be measured by the following procedure. (1) Secondary particles of the positive electrode active material are processed, for example, by an ion milling device (for example, IM4000PLUS manufactured by Hitachi High-Technologies Corporation) to expose a cross section of the positive electrode active material. (2) A backscattered electron image of the cross section of the exposed positive electrode active material is taken using a scanning electron microscope at a magnification of 500 to 2000 times. (3) The cross-sectional image obtained above is imported into a computer, and the distribution state of the surface-modifying compound is measured in terms of area using image analysis software (e.g., ImageJ, manufactured by the National Institutes of Health, USA). The state of the surface-modifying compound is determined from the average value of the values ​​measured for 10 positive electrode active materials.

[0035] It is preferable that peaks derived from CaO and SrO are not present in the X-ray diffraction pattern obtained by X-ray diffraction measurement of the positive electrode active material. If peaks derived from CaO or SrO are present, a decrease in battery capacity may occur. Here, the X-ray diffraction pattern is obtained by powder X-ray diffraction under the following conditions using, for example, a powder X-ray diffractometer (manufactured by Rigaku Corporation, product name "RINT-TTR", radiation source Cu-Kα).

[0036] Measurement range: 15-120° Scan speed: 4° / min Analysis range: 30-120° Background: B-splines Profile function: Split pseudo-Voigt function Constraint condition: Li(3a)+Ni(3a)=1 Ni(3a) + Ni(3b) = y (y is the Ni content in each element) ICSD No.:98-009-4814

[0037] The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery includes a lithium transition metal composite oxide synthesis step of mixing a transition metal oxide containing 70 mol % or more of Ni with a Li compound and calcining the mixture to obtain a lithium transition metal composite oxide; a washing step of washing the lithium transition metal composite oxide with water and dehydrating it to obtain a cake-like composition; and a heat treatment step of heat-treating the cake-like composition to obtain a positive electrode active material for a non-aqueous electrolyte secondary battery, wherein at least one of a Ca compound and a Sr compound is added to the cake-like composition during the washing step or after the washing step and before the heat treatment step.

[0038] In the lithium transition metal composite oxide synthesis process, for example, an alkaline solution such as sodium hydroxide is added dropwise to a stirred solution of a metal salt containing Ni and an optional metal element (such as Co) to adjust the pH to the alkaline side (e.g., 8.5 to 12.5), thereby precipitating (co-precipitating) a transition metal hydroxide containing Ni and the optional metal element, and the transition metal hydroxide is then calcined to obtain a transition metal oxide containing Ni and the optional metal element. The calcination temperature is not particularly limited, but is, for example, in the range of 300°C to 600°C.

[0039] The above-mentioned transition metal oxide and a Li compound are dry-mixed to obtain a lithium transition metal composite oxide. Examples of Li compounds include Li2CO3, LiOH, Li2O2, Li2O, LiNO3, LiNO2, Li2SO4, LiOH·H2O, LiH, and LiF. The mixing ratio of the transition metal oxide and the Li compound is preferably such that the molar ratio of metal elements other than Li to Li is in the range of 1:0.97 to 1:1.2, in order to easily adjust each of the above parameters to the specified ranges. Other metal materials may be added as needed. The other metal materials are oxides containing metal elements other than the metal elements constituting the transition metal oxide. The calcination of the mixture may also include a multi-stage calcination process, including a first calcination step in which the mixture is calcined in a calcination furnace under an oxygen stream at a first temperature increase rate to a first set temperature of 450°C to 680°C, and a second calcination step in which the calcined product obtained in the first calcination step is calcined in a calcination furnace under an oxygen stream at a second temperature increase rate to a second set temperature of more than 680°C to 800°C. Here, the first temperature increase rate is in the range of 1.5°C / min to 5.5°C / min, and the second temperature increase rate is slower than the first temperature increase rate and is in the range of 0.1°C / min to 3.5°C / min. By performing such multi-stage calcination, the state of the surface modifier compound in the finally obtained cathode active material of this embodiment can be adjusted to the specified range. Note that the first temperature increase rate and the second temperature increase rate may be set multiple times for each temperature range, as long as they are within the specified range. The holding time of the first set temperature in the first firing step is preferably 0 to 5 hours, more preferably 0 to 3 hours, in order to adjust each of the above parameters of the lithium transition metal composite oxide to the specified ranges. The holding time of the first set temperature is the time for which the first set temperature is maintained after the first set temperature is reached. The holding time of the second set temperature in the second firing step is preferably 1 to 10 hours, more preferably 1 to 5 hours, in order to adjust each of the above parameters of the lithium transition metal composite oxide to the specified ranges. The holding time of the second set temperature is the time for which the second set temperature is maintained after the second set temperature is reached.When firing the mixture, the above parameters are adjusted to fall within the ranges specified above, and the firing can be carried out, for example, in an oxygen stream with an oxygen concentration of 60% or more, with the flow rate of the oxygen stream being in the range of 0.2 mL / min to 4 mL / min per 10 cm3 of the firing furnace and 0.3 L / min or more per 1 kg of the mixture.

[0040] In the washing step, the lithium transition metal composite oxide is washed with water and dehydrated to obtain a cake-like composition. By washing with water, unreacted Li compounds added in the lithium transition metal composite oxide synthesis step and impurities other than the lithium compounds can be removed. For example, 300 g to 5000 g of lithium transition metal composite oxide may be added to 1 L of water for washing. Washing with water can be repeated multiple times. Dehydration after washing with water can be performed using, for example, a filter press. By dehydrating, the moisture content of the cake-like composition after the washing step can be reduced to 10 wt% or less. The moisture content of the cake-like composition is calculated by drying 10 g of the cake-like composition by leaving it to stand in a vacuum at 120°C for 2 hours, and dividing the change in weight of the cake-like composition before and after drying by the weight of the cake-like composition before drying.

[0041] At least one of a Ca compound and a Sr compound is added to the cake-like composition during the washing process or after the washing process and before the heat treatment process. Examples of Ca compounds include CaCl2, Ca(OH)2, CaO, CaCO3, CaSO4, and Ca(NO3)2. Examples of Sr compounds include SrCl2, Sr(OH)2, Sr(OH)2·8H2O, SrO, SrCO3, SrSO4, and Sr(NO3)2. From the viewpoint of dispersibility of Ca or Sr on the surface of the lithium transition metal composite oxide, a method of adding an aqueous solution in which a Ca compound or Sr compound is dissolved is preferred. When adding an aqueous solution, a Ca compound or Sr compound with high solubility in water is preferred from the viewpoint of easy preparation of the aqueous solution. The water content of the cake-like composition when adding the Ca compound or Sr compound is preferably 2 wt% or more, more preferably 4 wt% or more, from the viewpoint of dispersibility of Ca or Sr on the surface of the lithium transition metal composite oxide.

[0042] A W compound or W-containing solution may be added to the cake-like composition after the washing step and before the heat treatment step. This further suppresses the formation and erosion of a structurally deteriorated layer on the surface of the lithium-transition metal composite oxide due to reaction with the electrolyte, etc., and improves charge-discharge cycle characteristics. Li compounds remain in the cake-like composition after the washing step and before the heat treatment step, and the remaining Li compounds dissolve in the water contained in the cake-like composition to form an alkaline aqueous solution. When a W compound is added to the cake-like composition, the W compound dissolves in the alkaline aqueous solution and spreads over the entire surface of the lithium-containing transition metal oxide. Examples of W compounds include tungsten oxide (WO), lithium tungstate (LiWO, LiWO, LiWO). The amount of W added may be 0.5 mol% or less, preferably 0.3 mol% or less, based on the total number of moles of metal elements excluding Li in the lithium-transition metal composite oxide. Furthermore, when a W-containing solution is added to the cake-like composition, the W concentration in the W-containing solution is, for example, 0.05 mol / L or more, and preferably 0.1 mol / L to 1 mol / L. The W-containing solution is not particularly limited as long as it contains W, but is preferably one in which a W compound that is easily soluble in alkaline solutions, such as tungsten oxide, lithium tungstate, or ammonium tungstate, is dissolved in an aqueous solution of lithium hydroxide.

[0043] The heat treatment step is a step in which the cake-like composition is heat-treated at a temperature of 600°C or less, particularly preferably 250°C or less. There are no particular limitations on the temperature, as long as it is possible to evaporate the moisture in the cake-like composition at 600°C or less, but from the viewpoint of efficiency, a temperature of 100°C or more is preferred, and 150°C or more is more preferred. The atmosphere in the heat treatment step can be, for example, a vacuum. The heat treatment time in the heat treatment step is not particularly limited, but is preferably 0.5 to 10 hours in order to sufficiently evaporate the moisture in the cake-like composition.

[0044] The molar fraction of the metal element contained in the positive electrode active material obtained above was measured by inductively coupled plasma (ICP) emission spectroscopy, and the molar fraction of the metal element contained in the positive electrode active material was determined by the general formula Li a Ni 1-x-y Co x My Ca α Sr β O2 (where 0.97 ≤ a ≤ 1.20, 0 ≤ x ≤ 0.2, 0 ≤ y ≤ 0.1, 0 < α + β ≤ 0.005, and M is at least one element selected from Mn, W, Mg, Mo, Nb, Ti, Si, and Al). Note that Ca and Sr are contained in the surface modification compound present on the surface of the lithium transition metal composite oxide.

[0045] [Negative electrode] The negative electrode 12 has a negative electrode current collector 40 and a negative electrode composite layer 41 formed on both surfaces of the negative electrode current collector 40. For the negative electrode current collector 40, a foil of a metal stable within the potential range of the negative electrode 12 such as copper or a copper alloy, a film having such a metal disposed on the surface layer, etc. can be used. The negative electrode composite layer 41 contains a negative electrode active material and a binder. The thickness of the negative electrode composite layer 41 is, for example, 10 μm to 150 μm on one side of the negative electrode current collector 40. The negative electrode 12 can be manufactured by applying a negative electrode composite slurry containing a negative electrode active material, a binder, etc. to the surface of the negative electrode current collector 40, drying the coating film, and then rolling it to form the negative electrode composite layer 41 on both surfaces of the negative electrode current collector 40.

[0046] The negative electrode active material contained in the negative electrode composite layer 41 is not particularly limited as long as it can reversibly occlude and release lithium ions. Generally, a carbon material such as graphite is used. The graphite may be any of natural graphite such as flake graphite, massive graphite, and earthy graphite, artificial massive graphite, and artificial graphite such as graphitized mesophase carbon microbeads. Also, as the negative electrode active material, a metal that alloys with Li such as Si or Sn, a metal compound containing Si or Sn, a lithium titanium composite oxide, etc. may be used. Further, those provided with a carbon coating may be used. For example, a Si-containing compound represented by SiO x (0.5 ≤ x ≤ 1.6), or Li 2y SiO (2+y) A Si-containing compound in which fine particles of Si are dispersed in a lithium silicate phase represented by (0 < y < 2), etc. may be used in combination with graphite.

[0047] The binder contained in negative electrode mixture layer 41 may be a fluorine-containing resin such as PTFE or PVdF, PAN, polyimide, acrylic resin, or polyolefin, as in the case of positive electrode 11, but is preferably styrene-butadiene rubber (SBR). Negative electrode mixture layer 41 may also contain CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like.

[0048] [Separator] The separator 13 may be, for example, a porous sheet having ion permeability and insulating properties. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a laminated structure. Furthermore, a highly heat-resistant resin layer such as an aramid resin, or a filler layer containing an inorganic compound filler may be provided on the surface of the separator 13.

[0049] [Non-aqueous electrolyte] The non-aqueous electrolyte includes, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. Examples of the halogen-substituted compound include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP).

[0050] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0051] Examples of the ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methyl phenyl ether. and chain ethers such as ethyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0052] The electrolyte salt is preferably a lithium salt. Examples of lithium salts include LiBF, LiClO, LiPF, LiAsF, LiSbF, LiMnCl, LiSCN, LiCF, SO, LiCF, CO, Li(P(C), O)F, and LiPF.6-x (C n F 2n+1 ) x (1 < x < 6, n is 1 or 2), LiB 10 Cl 10 , LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylate, borate salts such as Li2B4O7, Li(B(C2O4)F2), LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 SO2){l, m are integers of 0 or more}, and imide salts such as these may be mentioned. The lithium salt may be used alone or in combination of multiple kinds. Among these, from the viewpoints of ionic conductivity, electrochemical stability, etc., it is preferable to use LiPF6. The concentration of the lithium salt is, for example, 0.8 mol to 1.8 mol per 1 L of the non-aqueous solvent. Further, vinylene carbonate or a propane sultone-based additive may be added.

Example

[0053] Hereinafter, the present disclosure will be further described by examples and comparative examples, but the present disclosure is not limited to the following examples.

[0054] [Production of Cathode Active Material] <Example 1> General formula Ni 0.91 Co 0.045 Al 0.045 O2, lithium hydroxide monohydrate (LiOH·H2O) was mixed so that the molar ratio of the total amount of Ni, Co, and Al of the transition metal oxide represented by and Li would be 1:1.03, and fired to obtain a lithium transition metal composite oxide. Next, in the washing step, the obtained lithium transition metal composite oxide was washed with water to obtain a cake-like composition. An Sr-containing aqueous solution was added to the cake-like composition so that Sr would be 0.06 mol% with respect to the total molar number of metal elements excluding Li in the lithium transition metal composite oxide. Further, the cake-like composition to which Sr was added was heat-treated in a vacuum at 200 °C for 3 hours to obtain the cathode active material of Example 1. The amount of Sr deposited measured by ICP-AES was 0.06 mol% with respect to the total molar number of metal elements excluding Li.

[0055] [Preparation of positive electrode] A positive electrode slurry was prepared by mixing 95 parts by mass of the positive electrode active material, 3 parts by mass of acetylene black as a conductive material, and 2 parts by mass of polyvinylidene fluoride as a binder, and then mixing this with N-methyl-2-pyrrolidone (NMP). The slurry was then applied to a positive electrode current collector made of aluminum foil with a thickness of 15 μm. After drying the coating, the coating was rolled with a rolling roller and cut to a predetermined electrode size to obtain a positive electrode with a positive electrode composite layer formed on both sides of the positive electrode core. An exposed portion was provided in part of the positive electrode, exposing the surface of the positive electrode core.

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

[0057] [Test cell construction] An aluminum lead was attached to the exposed portion of the positive electrode, and a nickel lead was attached to the lithium metal foil as the negative electrode. The positive and negative electrodes were spirally wound with a polyolefin separator interposed therebetween, and then pressed radially to produce a flat wound electrode assembly. This electrode assembly was housed in an exterior body made of an aluminum laminate sheet, and the nonaqueous electrolyte was poured into it. The opening of the exterior body was then sealed to obtain a test cell.

[0058] [Capacity retention rate evaluation] The test cell was subjected to the following cycle test. The discharge capacity at the first cycle and the discharge capacity at the 30th cycle of the cycle test were determined, and the capacity retention rate was calculated using the following formula.

[0059] Capacity retention rate (%) = (30th cycle discharge capacity ÷ 1st cycle discharge capacity) × 100 <Cycle test> The test cell was charged at a constant current of 0.2 It at a temperature of 25°C until the battery voltage reached 4.3 V, and then charged at a constant voltage until the current value reached 1 / 100 It at 4.3 V. It was then discharged at a constant current of 0.2 It until the battery voltage reached 2.5 V. This charge / discharge cycle was repeated 30 times.

[0060] <Example 2> Test cells were prepared and evaluated in the same manner as in Example 1, except that an Sr-containing aqueous solution was added during the water washing step so that Sr was 0.03 mol % relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

[0061] Example 3 Test cells were fabricated and evaluated in the same manner as in Example 1, except that an Sr-containing aqueous solution was added during the water washing in the cleaning process.

[0062] Example 4 Test cells were prepared and evaluated in the same manner as in Example 1, except that the Ca-containing aqueous solution was added so that the Ca content was 0.06 mol % relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

[0063] <Example 5> Test cells were prepared in the same manner as in Example 4, except that a Ca-containing aqueous solution was added during the water washing in the cleaning process, and the test cells were evaluated.

[0064] Example 6 Test cells were fabricated and evaluated in the same manner as in Example 1, except that during the water washing in the cleaning step, an Sr-containing aqueous solution was added so that Sr was 0.03 mol % relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide, and further, an aqueous Ca-containing solution was added so that Ca was 0.03 mol % relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

[0065] Example 7 General formula Ni0.91 Co 0.045 Al 0.045 Lithium hydroxide monohydrate (LiOH·H2O) was mixed with the transition metal oxide represented by 02 in a molar ratio of 1:1.03 to the total amount of Ni, Co, and Al in the transition metal oxide represented by 02, and calcined to obtain a lithium transition metal composite oxide. Next, while washing the lithium transition metal composite oxide with water, an Sr-containing aqueous solution was added so that the Sr content was 0.05 mol% relative to the total moles of metal elements excluding Li in the lithium transition metal composite oxide. After filtration, WO3 was added so that the W content was 0.1 mol% relative to the total moles of metal elements excluding Li in the lithium transition metal composite oxide to obtain a cake-like composition. This cake-like composition with added Sr and W was then heat-treated in a vacuum at 200°C for 3 hours to obtain the cathode active material of Example 7. The deposition amounts of Sr and W measured by ICP-AES were 0.05 mol% and 0.1 mol%, respectively, relative to the total moles of metal elements excluding Li.

[0066] Example 8 Test cells were prepared and evaluated in the same manner as in Example 7, except that an Sr-containing aqueous solution was added so that Sr was 0.1 mol % relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

[0067] Example 9 Test cells were prepared and evaluated in the same manner as in Example 7, except that an Sr-containing aqueous solution was added so that Sr was 0.1 mol % relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

[0068] Example 10 Test cells were prepared and evaluated in the same manner as in Example 7, except that an Sr-containing aqueous solution was added so that Sr was 0.15 mol % relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

[0069] Example 11 Test cells were fabricated and evaluated in the same manner as in Example 9, except that WO3 was added so that W was 0.05 mol % relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

[0070] Example 12 Test cells were fabricated and evaluated in the same manner as in Example 9, except that WO3 was added so that W was 0.15 mol % relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

[0071] Example 13 Test cells were prepared and evaluated in the same manner as in Example 7, except that a Ca-containing aqueous solution was added so that Ca was 0.05 mol % relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

[0072] Example 14 Test cells were prepared and evaluated in the same manner as in Example 8, except that a Ca-containing aqueous solution was added so that Ca was 0.1 mol % relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

[0073] Example 15 Test cells were prepared and evaluated in the same manner as in Example 14, except that a Ca-containing aqueous solution was added during the water washing in the cleaning process.

[0074] Example 16 Test cells were prepared and evaluated in the same manner as in Example 15, except that the Ca-containing aqueous solution was added so that the Ca content was 0.2 mol % relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

[0075] Example 17 Test cells were prepared and evaluated in the same manner as in Example 15, except that the Ca-containing aqueous solution was added so that the Ca content was 0.3 mol % relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

[0076] Example 18 Test cells were prepared and evaluated in the same manner as in Example 15, except that a Ca-containing aqueous solution was added so that Ca was 0.5 mol % relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

[0077] Example 19 Test cells were prepared and evaluated in the same manner as in Example 15, except that an Sr-containing aqueous solution was added so that Sr was 0.05 mol % relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide, and further, an Ca-containing aqueous solution was added so that Ca was 0.05 mol % relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

[0078] <Comparative Example 1> Test cells were prepared and evaluated in the same manner as in Example 1, except that the Sr-containing aqueous solution was not added.

[0079] <Comparative Example 2> Test cells were prepared and evaluated in the same manner as in Example 7, except that the Sr-containing aqueous solution was not added.

[0080] <Comparative Example 3> Test cells were prepared and evaluated in the same manner as in Example 15, except that the Ca-containing aqueous solution was added so that the Ca content was 0.8 mol % relative to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

[0081] The capacity retention rates of the examples and comparative examples are shown in Tables 1 and 2. The evaluation results of the capacity retention rates shown in Tables 1 and 2 are expressed relative to the capacity retention rates of the test cells of Comparative Examples 1 and 2, which are set to 100%. Tables 1 and 2 also show the timing of adding Ca or Sr, the added elements and their amounts, and the added amount of W.

[0082] [Table 1]

[0083] [Table 2]

[0084] As shown in Tables 1 and 2, Examples 1 to 6 had higher capacity retention rates than Comparative Example 1, and Examples 7 to 19 had higher capacity retention rates than Comparative Example 2. Note that, for none of the Examples, peaks derived from SrO and CaO were present in the X-ray diffraction patterns. As an example, the X-ray diffraction patterns of Example 18, SrO, and CaO are shown in Figure 3. [Explanation of symbols]

[0085] 10 non-aqueous electrolyte secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 15 battery case, 16 outer can, 17 sealing body, 18, 19 insulating plate, 20 positive electrode tab, 21 negative electrode tab, 22 grooved portion, 23 bottom plate, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cap, 28 gasket, 30 positive electrode current collector, 31 positive electrode composite layer, 40 negative electrode current collector, 41 negative electrode composite layer, 50 secondary particles, 51 primary particles, 54 surface modification compound

Claims

1. a lithium transition metal composite oxide synthesis step of mixing a transition metal oxide containing 70 mol % or more of Ni with a Li compound and calcining the mixture to obtain a lithium transition metal composite oxide; a washing step of washing the lithium transition metal composite oxide with water and dehydrating it to obtain a cake-like composition; a heat treatment step of heat-treating the cake-like composition to obtain a positive electrode active material for a non-aqueous electrolyte secondary battery, During the washing step or after the washing step and before the heat treatment step, the cake-like composition is added with CaCl ２ , Ca(OH) ２ , CaO, CaCO ３ , CaSO ４ , Ca(NO ３ ) ２ , SrCl ２ , Sr(OH) ２ , Sr(OH) ２ ・8H ２ O, SrO, SrCO ３ , SrSO ４ , and Sr(NO ３ ) ２ and a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, the method comprising adding at least one of the following.

2. The lithium transition metal composite oxide has the general formula Li ａ Ni １－ｘ－ｙ Co ｘ M ｙ O ２ 2. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the positive electrode active material is represented by the formula (wherein 0.97≦a≦1.20, 0≦x≦0.2, 0≦y≦0.1, and M is at least one element selected from Mn, W, Mg, Mo, Nb, Ti, Si, and Al).

3. 3. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein peaks derived from CaO and SrO are not present in an X-ray diffraction pattern obtained by X-ray diffraction measurement.

4. 4. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the cake-like composition has a water content of 2 wt % or more.

5. The cake-like composition during the washing step is added with CaCl ２ , Ca(OH) ２ , CaO, CaCO ３ , CaSO ４ , Ca(NO ３ ) ２ , SrCl ２ , Sr(OH) ２ , Sr(OH) ２ ・8H ２ O, SrO, SrCO ３ , SrSO ４ , and Sr(NO ３ ) ２ The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein at least one of the following is added.

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