Manufacturing method of 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 manufacturing a positive electrode active material for a non-aqueous electrolyte secondary battery.

Background Art

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

[0003] Further, in Patent Document 2, a positive electrode active material for a non-aqueous electrolyte secondary battery having an α-NaFeO2 structure, containing one or more selected from the group consisting of Mn, Ni, and Co as transition metal elements, and having an alkaline earth metal and W present on the particle surface of the lithium transition metal composite oxide is disclosed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

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 of a non-aqueous electrolyte secondary battery, since a large amount of Li is extracted during charging, the layered crystal structure is broken and the capacity decreases when charge and discharge are repeated. In addition, the technologies disclosed in Patent Documents 1 and 2 still have room for improvement in terms of charge-discharge cycle characteristics.

[0006] The positive electrode active material for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes a lithium transition metal composite oxide having secondary particles formed by aggregation of primary particles, and a surface modification compound that is present at least on the 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 the total molar number of metal elements excluding Li, and the total amount of Ca and Sr in the surface modification compound is 0.5 mol% or less with respect to the total molar number of metal elements excluding Li in the lithium transition metal composite oxide.

[0007] The manufacturing method of the positive electrode active material for a non-aqueous electrolyte secondary battery according to one aspect 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 and a Li compound to obtain a lithium transition metal composite oxide, a washing step of washing the lithium transition metal composite oxide 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, and is characterized in that at least one of a Ca compound and an Sr compound is added to the cake-like composition during the washing step or before the heat treatment step after the washing step.

[0008] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes a positive electrode containing the 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 that suppresses a decrease in battery capacity associated with charge and discharge.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Mode for Carrying Out the Invention

[0011] In the layered structure of the lithium transition metal composite oxide contained in the positive electrode active material, there are a transition metal layer containing Ni or the like, a Li layer, and an oxygen layer. By the reversible entry and exit of Li ions existing in the Li layer, the charge and discharge reaction of the battery proceeds. When a lithium transition metal composite oxide having a large Ni content is used, many Li ions are extracted from the Li layer during charging of the battery, so that the layered structure collapses and the battery capacity decreases. In addition, a lithium transition metal composite oxide having a large Ni content has high activity in the vicinity of the particle surface and is likely to have an unstable structure. Therefore, generation and erosion of a surface deterioration layer are likely to occur due to a reaction with an electrolytic solution or the like, leading to a decrease in battery capacity.

[0012] Therefore, as a result of intensive studies to solve the above problems, the present inventors have found that erosion of the structural deterioration layer can be suppressed by protecting the surface of the lithium transition metal composite oxide with a compound containing at least one of Ca and Sr. As a result, the inventors have arrived at a positive electrode active material for a non-aqueous electrolyte secondary battery in the following embodiments, which suppresses a decrease in battery capacity accompanying charge and discharge while maintaining a high capacity.

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

[0014] Hereinafter, an example of an embodiment of a non-aqueous electrolyte secondary battery according to the present disclosure will be described in detail. Hereinafter, a cylindrical battery in which a wound electrode body is housed in a cylindrical battery case will be exemplified, but the electrode body is not limited to a wound type, and may be a laminated type in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated one by one with a separator interposed therebetween. Further, the battery case is not limited to a cylindrical shape, and may be, for example, a rectangular shape, a coin shape, etc., or 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 non-aqueous electrolyte secondary battery 10 which is an example of an embodiment. As illustrated in FIG. 1, the non-aqueous electrolyte secondary battery 10 includes an electrode body 14, a non-aqueous electrolyte (not shown), and a battery case 15 that houses the electrode body 14 and the non-aqueous electrolyte. The electrode body 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 is composed of a bottomed cylindrical outer can 16 and a sealing body 17 that closes the opening of the outer can 16.

[0016] The electrode body 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 in size than the positive electrode 11 in order to prevent precipitation of lithium. That is, the negative electrode 12 is formed longer than the positive electrode 11 in the longitudinal direction and the width direction (short side direction). The two separators 13 are formed to be at least slightly larger in size than the positive electrode 11 and are arranged, for example, so as to sandwich the positive electrode 11.

[0017] The non-aqueous electrolyte secondary battery 10 includes insulating plates 18 and 19 disposed above and below the electrode body 14, respectively. In the example shown in FIG. 1, the positive electrode tab 20 attached to the positive electrode 11 extends toward the sealing body 17 through the through hole of the insulating plate 18, and the negative electrode tab 21 attached to the negative electrode 12 extends toward the bottom side of the outer can 16 through the outside of the insulating plate 19. The positive electrode tab 20 is connected to the lower surface of the bottom plate 23 of the sealing body 17 by welding or the like, and the 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 surface of the bottom 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 metal container having a bottomed cylindrical shape. A gasket 28 is provided between the outer can 16 and the sealing body 17, and the internal space of the battery case 15 is sealed. The outer can 16 has, for example, a groove portion 22 formed by pressing the side surface from the outside to support the sealing body 17. The groove 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 a bottom plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are laminated in this order from the electrode body 14 side. Each member constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each member except 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 the central portions thereof, and the insulating member 25 is interposed between the peripheral edges thereof. When the internal pressure of the battery rises due to abnormal heat generation, the lower valve body 24 is deformed and broken so as to push up the upper valve body 26 toward the cap 27 side, and the current path between the lower valve body 24 and the upper valve body 26 is interrupted. When the internal pressure further rises, the upper valve body 26 is broken, and gas is discharged from the opening of the cap 27.

[0020] Hereinafter, the positive electrode 11, the negative electrode 12, the separator 13, and the non-aqueous electrolyte constituting the non-aqueous electrolyte secondary battery 10 will be described in detail, particularly the positive electrode active material contained in the positive electrode composite material layer 31 constituting the positive electrode 11.

[0021] [Positive Electrode] The positive electrode 11 includes a positive electrode current collector 30 and a positive electrode mixture layer 31 formed on both surfaces of the positive electrode current collector 30. As the positive electrode current collector 30, a foil of a metal stable within the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a film having such a metal disposed on its surface layer can be used. The positive electrode mixture layer 31 contains a positive electrode active material, a conductive material, and a binder. The thickness of the positive electrode mixture 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 manufactured by applying a positive electrode slurry containing a positive electrode active material, a conductive material, a binder, etc. to the surface of the positive electrode current collector 30, drying the coating film, and then compressing it to form the positive electrode mixture layer 31 on both surfaces 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 its salt, polyethylene oxide (PEO), etc.

[0023] FIG. 2 is a diagram schematically showing an example of a cross-section of the positive electrode active material contained in the positive electrode mixture 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 modification compound 54 present at least on the surface of the secondary particles 50 and containing at least one of Ca and Sr. Thereby, the generation and erosion of the structural deterioration layer on the surface of the lithium transition metal composite oxide due to reaction with the electrolytic solution or the like can be suppressed.

[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 means the particle diameter at which the cumulative frequency in the volume-based particle size distribution reaches 50% from the smaller particle size side, 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 type particle size distribution measuring device (for example, MT3000II manufactured by Microtrac Bell Corporation) with water as the dispersion medium.

[0025] The particle diameter of the primary particles 52 constituting the secondary particles 50 is, for example, 0.05 μm to 1 μm. The particle diameter of the primary particles 52 is measured as the diameter of the circumscribed circle in the particle image observed by 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, a layered structure belonging to the space group C2 / m, etc. Among these, in terms of high capacity, stability of the crystal structure, etc., it is preferably a layered structure belonging to the space group R-3m. The layered structure of the lithium transition metal composite oxide includes a transition metal layer, a Li layer, and an oxygen layer.

[0027] The lithium transition metal composite oxide preferably contains 70 mol% or more of Ni based on the total number of moles of metal elements excluding Li, and preferably contains 80 mol% or more of Ni based on the total number of moles of metal elements excluding Li. By setting the Ni content to 70 mol% or more, a high-capacity battery can be obtained. Also, by setting the Ni content to 80 mol% or more, the effect of improving the cycle characteristics due to the stabilization of the structure of the lithium transition metal composite oxide is 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] The lithium transition metal composite oxide has the general formula Li a Ni 1-x-y Co x M yO2 (where 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). Note that the positive electrode active material may contain a lithium transition metal composite oxide other than those 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 by 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, represented by a, satisfies 0.97 ≤ a < 1.20, and preferably satisfies 0.97 ≤ a ≤ 1.05. When a is less than 0.97, the battery capacity may decrease compared to the case where a satisfies the above range. When a exceeds 1.20, it may lead to a decrease in charge-discharge cycle characteristics compared to the case where a satisfies the above range.

[0030] Co and M (where M is at least one element selected from Mn, W, Mg, Mo, Nb, Ti, Si, and Al) are optional components. The contents of Co and M, represented by x and y, respectively, with respect to the total molar number of the metal elements excluding Li in the lithium transition metal composite oxide preferably satisfy 0 ≤ x ≤ 0.2 and 0 ≤ y ≤ 0.1. Since Co is expensive, it is preferable to suppress the Co content from the perspective of manufacturing cost.

[0031] The total amount of Ca and Sr in the surface modification compound is 0.5 mol% or less with respect to the total molar number of the metal elements excluding Li in the lithium transition metal composite oxide. Thereby, the charge-discharge cycle characteristics can be improved. Also, the total amount of Ca and Sr in the surface modification compound is preferably 0.03 mol% or more with respect to the total molar number of the metal elements excluding Li in the lithium transition metal composite oxide. The surface modification compound may contain Ca in the state of a compound such as CaO, Ca(OH)2, CaCO3, etc., and may contain Sr in the state of a compound such as SrO, Sr(OH)2, SrCO3, etc.

[0032] It is possible that 95 mol% or more of the surface-modifying compound is present in the range from the surface of the secondary particles to the inside by the average particle diameter of the primary particles. Since most of 95 mol% or more of the surface-modifying compound is present in the vicinity of the surface of the secondary particles of the lithium transition metal composite oxide, generation and erosion of a structural deterioration layer on the surface of the lithium transition metal composite oxide due to reaction with an electrolytic solution or the like can be efficiently suppressed.

[0033] Alternatively, 95 mol% or more of the surface-modifying compound may be present on the surface of the primary particles that form the surface of the secondary particles. Since most of 95 mol% or more of the surface-modifying compound is present in the vicinity of the surface of the secondary particles of the lithium transition metal composite oxide, generation and erosion of a structural deterioration layer on the surface of the lithium transition metal composite oxide due to reaction with an electrolytic solution or the like can be efficiently suppressed.

[0034] The state of existence 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 with, for example, an ion milling device (for example, IM4000PLUS manufactured by Hitachi High-Tech Corporation) to expose a cross section of the positive electrode active material. (2) Using a scanning electron microscope, a backscattered electron image of the exposed cross section of the positive electrode active material is taken. The magnification when taking the backscattered electron image is 500 to 2000 times. (3) The cross-sectional image obtained as described above is taken into a computer, and the distribution state of the surface-modifying compound is measured by area using image analysis software (for example, ImageJ manufactured by the National Institutes of Health, USA). The state of existence of the surface-modifying compound is measured from the average value of the values measured for 10 positive electrode active materials.

[0035] It is preferable that there are no peaks derived from CaO and SrO in the X-ray diffraction pattern obtained by X-ray diffraction measurement of the positive electrode active material. When there are peaks derived from CaO or SrO, a decrease in battery capacity or the like may occur. Here, the X-ray diffraction pattern is obtained by the powder X-ray diffraction method under the following conditions using, for example, a powder X-ray diffractometer (manufactured by Rigaku Corporation, trade name "RINT-TTR", radiation source Cu-Kα).

[0036] Measurement range: 15 - 120° Scan speed: 4° / min Analysis range: 30 - 120° Background: B-spline Profile function: Split pseudo-Voigt function Constraint condition: Li(3a) + Ni(3a) = 1 Ni(3a) + Ni(3b) = y (y is the Ni content ratio of each) 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 and a Li compound and firing them 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, and adding at least one of a Ca compound and an Sr compound to the cake-like composition during the washing step or before the heat treatment step after the washing step.

[0038] In the lithium transition metal composite oxide synthesis step, for example, while stirring a solution of a metal salt containing Ni and an arbitrary metal element (such as Co), an alkaline solution such as sodium hydroxide is dropped, and the pH is adjusted to the alkaline side (for example, 8.5 - 12.5) to precipitate (co-precipitate) a transition metal hydroxide containing Ni and an arbitrary metal element, and by firing the transition metal hydroxide, a transition metal oxide containing Ni and an arbitrary metal element can be obtained. The firing temperature is not particularly limited, but is, for example, in the range of 300°C to 600°C.

[0039] The above transition metal oxide and a Li compound are dry-mixed to obtain a lithium transition metal composite oxide. Examples of the Li compound include Li2CO3, LiOH, Li2O2, Li2O, LiNO3, LiNO2, Li2SO4, LiOH·H2O, LiH, LiF, etc. The mixing ratio of the transition metal oxide and the Li compound is preferably such that the molar ratio of the metal element excluding Li to Li is in the range of 1:0.97 to 1:1.2 in terms of facilitating the adjustment of the above parameters within the specified range. Other metal raw materials may be added as necessary. The other metal raw materials are oxides or the like containing metal elements other than the metal elements constituting the transition metal oxide. Further, the firing of the above mixture includes, for example, a first firing step of firing in a firing furnace under an oxygen stream to a first set temperature of 450°C or higher and 680°C or lower at a first heating rate, and a second firing step of firing the fired product obtained in the first firing step in the firing furnace under an oxygen stream to a second set temperature exceeding 680°C and 800°C or lower at a second heating rate, and may be provided with a multi-step firing process. Here, the first heating rate is in the range of 1.5°C / min or higher and 5.5°C / min or lower, and the second heating rate is slower than the first heating rate and is in the range of 0.1°C / min or higher and 3.5°C / min or lower. By such multi-step firing, in the positive electrode active material of the present embodiment finally obtained, the state of existence of the surface modification compound and the like can be adjusted within the specified range. Note that the first heating rate and the second heating rate may be set in plural for each temperature region 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 hours or more and 5 hours or less, more preferably 0 hours or more and 3 hours or less, in terms of adjusting the above parameters of the lithium transition metal composite oxide within the specified range. The holding time of the first set temperature is the time for maintaining the first set temperature after reaching the first set temperature. The holding time of the second set temperature in the second firing step is preferably 1 hour or more and 10 hours or less, more preferably 1 hour or more and 5 hours or less, in terms of adjusting the above parameters of the lithium transition metal composite oxide within the specified range. The holding time of the second set temperature is the time for maintaining the second set temperature after reaching the second set temperature.When firing the mixture, in terms of adjusting each of the above parameters within the specified ranges, for example, it can be carried out in an oxygen stream with an oxygen concentration of 60% or more, and the flow rate of the oxygen stream can be 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 above lithium transition metal composite oxide is washed with water and dehydrated to obtain a cake-like composition. By washing with water, unreacted portions of the Li compound added in the lithium transition metal composite oxide synthesis step and impurities other than the lithium compound can be removed. When washing with water, for example, 300 g to 5000 g of the lithium transition metal composite oxide may be added per 1 L of water. Washing with water can also be repeated a plurality of times. Dehydration after washing with water may be carried out, for example, using a filter press. By dehydration, the water content of the cake-like composition after the washing step can be made 10 wt% or less. The water content of the cake-like composition is calculated by allowing 10 g of the cake-like composition to stand in a vacuum at 120 °C for 2 hours for drying and dividing the weight change of the cake-like composition before and after drying by the weight of the cake-like composition before drying.

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

[0042] A W compound or a W-containing solution may be added to the cake-like composition after the washing step and before the heat treatment step. By doing so, the formation and erosion of the structural deterioration layer on the surface of the lithium transition metal composite oxide due to the reaction with the electrolytic solution or the like can be further suppressed, and the charge-discharge cycle characteristics can be improved. A Li compound remains in the cake-like composition after the washing step and before the heat treatment step, and the remaining Li compound dissolves 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 the W compound include tungsten oxide (WO3), lithium tungstate (Li2WO4, Li4WO5, Li6W2O9), etc. 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. 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, preferably 0.1 mol / L to 1 mol / L. The W-containing solution is not particularly limited as long as it contains W, but a solution obtained by dissolving a W compound that is easily soluble in an alkaline solution, such as tungsten oxide, lithium tungstate, ammonium tungstate, etc., in an aqueous solution of lithium hydroxide is preferred.

[0043] The heat treatment step is a step of heat-treating the cake-like composition at a temperature of 600 °C or lower, particularly preferably 250 °C or lower. It is not particularly limited as long as it is a temperature at which the moisture of the cake-like composition can be evaporated at 600 °C or lower, but from the viewpoint of efficiency, 100 °C or higher is preferred, and 150 °C or higher 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 of the cake-like composition.

[0044] The molar fraction of the metal elements contained in the positive electrode active material obtained above is measured by inductively coupled plasma (ICP) emission spectroscopy, and 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. As the negative electrode current collector 40, a foil of a metal stable within the potential range of the negative electrode 12 such as copper or a copper alloy, or a film having such a metal disposed on the surface layer can be used. The negative electrode 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 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, carbon materials such as graphite are 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. Further, as the negative electrode active material, metals that alloy with Li such as Si and Sn, metal compounds containing Si, Sn, etc., and lithium titanium composite oxides 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) may be used in combination with graphite.

[0047] As for the binder contained in the negative electrode composite layer 41, similar to the case of the positive electrode 11, a fluorine-containing resin such as PTFE or PVdF, PAN, polyimide, acrylic resin, polyolefin, etc. may be used, but preferably styrene-butadiene rubber (SBR) is used. Further, the negative electrode composite layer 41 may contain CMC or its salt, polyacrylic acid (PAA) or its salt, polyvinyl alcohol (PVA), etc.

[0048] [Separator] For the separator 13, for example, a porous sheet having ion permeability and insulation is used. Specific examples of the porous sheet include microporous thin films, woven fabrics, non-woven fabrics, etc. As the material of the separator 13, polyolefins such as polyethylene and polypropylene, cellulose, etc. are suitable. The separator 13 may have a single-layer structure or a laminated structure. Further, a resin layer with high heat resistance such as an aramid resin or a filler layer containing an inorganic compound filler may be provided on the surface of the separator 13.

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

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

[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 LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiMnCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), LiPF6-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 can be mentioned. As the lithium salt, these 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 way of 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 was 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 was 0.06 mol% with respect to the total number of moles 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 number of moles of metal elements excluding Li.

[0055] [Fabrication of the positive electrode] 95 parts by mass of the above 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 were mixed, and this was mixed with N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode slurry. Next, the slurry was applied to a positive electrode current collector made of an aluminum foil with a thickness of 15 μm. After drying the coating film, the coating film was rolled by a rolling roller and cut into a predetermined electrode size to obtain a positive electrode in which positive electrode composite layers were formed on both sides of the positive electrode core. In addition, an exposed portion where the surface of the positive electrode core was exposed was provided in a part of the positive electrode.

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

[0057] [Fabrication of test cell] An aluminum lead was attached to the exposed portion of the above positive electrode, and a nickel lead was attached to a lithium metal foil as a negative electrode. After winding the positive electrode and the negative electrode in a spiral shape through a polyolefin separator, it was press-molded in the radial direction to fabricate a flat wound electrode body. This electrode body was housed in an exterior body composed of an aluminum laminate sheet, and after injecting the above non-aqueous electrolyte, the opening of the exterior body was sealed to obtain a test cell.

[0058] [Evaluation of capacity retention rate] The following cycle test was performed on the above test cell. The discharge capacity of the first cycle and the discharge capacity of the 30th cycle of the cycle test were obtained, and the capacity retention rate was calculated by the following formula.

[0059] Capacity retention rate (%) = (Discharge capacity of the 30th cycle ÷ Discharge capacity of the first cycle) × 100 <Cycle test> The test cells were subjected to constant current charging at a constant current of 0.2It until the battery voltage reached 4.3V in a temperature environment of 25°C, and then constant voltage charging was performed until the current value reached 1 / 100It at 4.3V. Thereafter, constant current discharging was performed at a constant current of 0.2It until the battery voltage reached 2.5V. 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 in the washing step such that the molar percentage of Sr was 0.03% with respect to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

[0061] <Example 3> 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 in the washing step.

[0062] <Example 4> Test cells were prepared and evaluated in the same manner as in Example 1, except that a Ca-containing aqueous solution was added such that the molar percentage of Ca was 0.06% with respect 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 and evaluated in the same manner as in Example 4, except that a Ca-containing aqueous solution was added during the water washing in the washing step.

[0064] <Example 6> 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 in the washing step such that the molar percentage of Sr was 0.03% with respect to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide, and further, a Ca-containing aqueous solution was added such that the molar percentage of Ca was 0.03% with respect 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 and fired so that the molar ratio of the total amount of Ni, Co, and Al in the transition metal oxide represented by O2 to Li was 1:1.03 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 Sr was 0.05 mol% with respect to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide. After filtration, further, WO3 was added so that W was 0.1 mol% with respect to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide to obtain a cake-like composition. Furthermore, the cake-like composition added with Sr and W was heat-treated in a vacuum at 200 °C for 3 hours to obtain the positive electrode active material of Example 7. The adhesion amounts of Sr and W measured by ICP-AES were 0.05 mol% and 0.1 mol%, respectively, with respect to the total number of 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% with respect 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% with respect 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% with respect to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide.

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

[0070] <Example 12> Test cells were prepared and evaluated in the same manner as in Example 9, except that WO3 was added so that W was 0.15 mol% with respect 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 an aqueous solution containing Ca was added so that Ca was 0.05 mol% with respect 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 an aqueous solution containing Ca was added so that Ca was 0.1 mol% with respect 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 an aqueous solution containing Ca was added during the water washing in the washing step.

[0074] <Example 16> Test cells were prepared and evaluated in the same manner as in Example 15, except that an aqueous solution containing Ca was added so that Ca was 0.2 mol% with respect 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 an aqueous solution containing Ca was added so that Ca was 0.3 mol% with respect 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 an aqueous solution containing Ca was added so that Ca was 0.5 mol% with respect 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 aqueous solution containing Sr was added so that Sr was 0.05 mol% with respect to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide, and further an aqueous solution containing Ca was added so that Ca was 0.05 mol% with respect 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 an aqueous solution containing Sr was not added.

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

[0080] <Comparative Example 3> Test cells were prepared and evaluated in the same manner as in Example 15, except that an aqueous solution containing Ca was added so that Ca was 0.8 mol% with respect 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 to 2. The evaluation results of the capacity retention rates shown in Tables 1 to 2 are relatively represented with the capacity retention rates of the test cells of Comparative Examples 1 and 2 as 100%. Also shown in Tables 1 to 2 are the timing of addition of Ca or Sr, the added element and its amount, 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 a higher capacity retention rate than Comparative Example 1, and Examples 7 to 19 had a higher capacity retention rate than Comparative Example 2. In addition, for any of the examples, there were no peaks derived from SrO and CaO in the X-ray diffraction pattern. As an example, the X-ray diffraction patterns of Example 18, SrO, and CaO are shown in FIG. 3.

Description of Reference Numerals

[0085] 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 15 Battery case, 16 Exterior 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 material layer, 40 Negative electrode current collector, 41 Negative electrode composite material 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 and a Li compound and firing them 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 obtaining a positive electrode active material for a non-aqueous electrolyte secondary battery by heat-treating the cake-like composition, and To the cake-like composition during the washing step or before the heat treatment step after the washing step, 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 at least one of Sr(NO ３ ), ２ is added. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery.

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

3. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein no peaks derived from CaO and SrO are present in the X-ray diffraction pattern by X-ray diffraction measurement.

4. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the water content of the cake-like composition is 2 wt% or more.

5. To the cake-like composition in the washing step, 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 at least one of Sr(NO ３ ), ２ is added. 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.

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