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

Abstract

This invention provides a positive electrode active material for non-aqueous electrolyte secondary batteries containing a lithium transition metal composite oxide with a high Ni content, which contributes to improving the charge-discharge cycle characteristics of the battery. [Solution] The positive electrode active material for a non-aqueous electrolyte secondary battery comprises a lithium transition metal composite oxide containing at least 80 mol% or more of Ni and Al relative to the total number of moles of metal elements excluding Li, and a surface modification layer formed on the surface of the primary particles of the lithium transition metal composite oxide and containing at least Ca.

Description

[Technical Field]

[0001] This disclosure relates to a positive electrode active material for a non-aqueous electrolyte secondary battery, and to a non-aqueous electrolyte secondary battery. [Background technology]

[0002] In recent years, lithium transition metal composite oxides with a high Ni content have attracted attention as high-energy-density cathode active materials. For example, Patent Document 1 describes a material with the general formula Li x Ni y Co z M m It consists of a lithium transition metal composite oxide represented by O2 (wherein M is an element selected from Ba, Sr, and B, with 0.9≦x≦1.1, 0.5≦y≦0.95, 0.05≦z≦0.5, 0.0005≦m≦0.02), and has a BET specific surface area of ​​0.8 m². 2 A positive electrode active material for non-aqueous electrolyte secondary batteries with a content of less than / g is disclosed.

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

[0004] [Patent Document 1] Japanese Patent Publication No. 2003-100295 [Patent Document 2] Japanese Patent Publication No. 2018-129221 [Overview of the project] [Problems that the invention aims to solve]

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

[0006] A method for manufacturing a positive electrode active material for a non-aqueous electrolyte secondary battery, which is one aspect of the present disclosure, includes a first step of obtaining a lithium transition metal composite oxide containing at least 80 mol% of Ni and Al with respect to the total number of moles of metal elements excluding Li, a second step of mixing the composite oxide obtained in the first step, a lithium compound, and a calcium compound to obtain a mixture, and a third step of firing the mixture. The third step includes a first firing step of firing at a first heating rate to a first set temperature of 450 °C or higher and 680 °C or lower under an oxygen stream, and a second firing step of firing at a second heating rate slower than the first heating rate to a second set temperature of higher than 680 °C and 800 °C or lower under an oxygen stream. Thereby, a surface modification layer containing at least Ca is formed on the surface of the primary particles of the lithium transition metal composite oxide.

[0007] A non-aqueous electrolyte secondary battery, which is 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.

[0008] A positive electrode active material for a non-aqueous electrolyte secondary battery, which is one aspect of the present disclosure, contains a lithium transition metal composite oxide with a high Ni content and can contribute to improving the charge-discharge cycle characteristics of the battery. According to the positive electrode active material for a non-aqueous electrolyte secondary battery, which is one aspect of the present disclosure, a high-capacity non-aqueous electrolyte secondary battery that suppresses a decrease in battery capacity accompanying charge and discharge can be provided.

Brief Description of the Drawings

[0009] [Figure 1] It is a cross-sectional view of a non-aqueous electrolyte secondary battery which is an example of an embodiment.

Mode for Carrying Out the Invention

[0010] The layered structure of lithium transition metal composite oxides contained in the positive electrode active material consists of a transition metal layer such as Ni, a Li layer, and an oxygen layer. The reversible movement of Li ions in the Li layer drives the charge and discharge reactions of the battery. When lithium transition metal composite oxides with a high Ni content are used, many Li ions are extracted from the Li layer during battery charging, which can destabilize the layered structure. On the surface of the lithium transition metal composite oxide with an unstable layered structure, a modified layer is formed by a reaction with the electrolyte. Further structural changes in the lithium transition metal composite oxide proceed starting from the modified layer, causing the battery capacity to gradually decrease with charging and discharging.

[0011] Therefore, the present inventors conducted intensive studies to solve the above problems and found that a positive electrode active material comprising a lithium transition metal composite oxide containing a predetermined amount of Al and a surface modification layer containing Ca on its surface can suppress the decrease in battery capacity associated with charging and discharging because the reaction with the electrolyte on the surface is suppressed and the layered structure is stabilized due to the synergistic effect of Al and Ca. Since Al does not undergo an oxidation state change during charging and discharging, it is presumed that its inclusion in the transition metal layer stabilizes the structure of the transition metal layer. Furthermore, it is presumed that Ca suppresses the erosion of the surface modification layer by the electrolyte through electronic interactions.

[0012] Hereinafter, an example of an embodiment of a non-aqueous electrolyte secondary battery according to this disclosure will be described in detail. In the following, a cylindrical battery in which a wound electrode body is housed in a cylindrical battery case will be given as an example, but the electrode body is not limited to the wound type and may be a laminated type in which multiple positive electrodes and multiple negative electrodes are stacked alternately one by one with a separator in between. Furthermore, the battery case is not limited to a cylindrical shape and may be, for example, rectangular, coin-shaped, etc., or may be a battery case made of a laminate sheet including a metal layer and a resin layer.

[0013] Figure 1 is a cross-sectional view of a non-aqueous electrolyte secondary battery 10, which is an example of an embodiment. As illustrated in Figure 1, the non-aqueous electrolyte secondary battery 10 comprises 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 around a separator 13. The battery case 15 consists of a bottomed cylindrical outer casing 16 and a sealing body 17 that closes the opening of the outer casing 16.

[0014] The electrode body 14 consists 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 both the longitudinal and width (short-side) directions. The two separators 13 are formed to be at least slightly larger than the positive electrode 11 and are arranged, for example, to sandwich the positive electrode 11.

[0015] The non-aqueous electrolyte secondary battery 10 includes insulating plates 18 and 19 positioned above and below the electrode body 14, respectively. In the example shown in Figure 1, a positive electrode tab 20 attached to the positive electrode 11 extends towards the sealing body 17 through a through-hole in the insulating plate 18, and a negative electrode tab 21 attached to the negative electrode 12 extends towards the bottom of the outer casing 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, which is electrically connected to the bottom plate 23, becomes the positive electrode terminal. The negative electrode tab 21 is connected to the inner surface of the bottom of the outer casing 16 by welding or the like, and the outer casing 16 becomes the negative electrode terminal.

[0016] The outer container 16 is, for example, a metal container with a bottomed cylindrical shape. A gasket 28 is provided between the outer container 16 and the sealing body 17, sealing the internal space of the battery case 15. The outer container 16 has a grooved portion 22 that supports the sealing body 17, which is formed, for example, by pressing the side portion from the outside. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer container 16, and its upper surface supports the sealing body 17.

[0017] 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 stacked in order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective centers, with the insulating member 25 interposed between their respective peripheries. When the internal pressure of the battery rises due to abnormal heat generation, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 towards the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0018] The following describes in detail the positive electrode 11, negative electrode 12, separator 13, and non-aqueous electrolyte that constitute the non-aqueous electrolyte secondary battery 10, with particular attention paid to the positive electrode active material contained in the positive electrode composite layer 31 that constitutes the positive electrode 11.

[0019] [Positive electrode] The positive electrode 11 comprises 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 made of a metal foil that is stable in the potential range of the positive electrode 11, such as aluminum or an aluminum alloy, or a film with the metal arranged on its surface. The 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 manufactured by applying a positive electrode slurry containing a positive electrode active material, a conductive material, and a binder 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.

[0020] Examples of the conductive material contained in the positive electrode composite layer 31 include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of the binder contained in the positive electrode composite 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), or the like.

[0021] The positive electrode active material includes a lithium transition metal composite oxide and a surface modification layer formed on the surface of the primary particles of the lithium transition metal composite oxide and containing at least Ca. The lithium transition metal composite oxide contains at least 80 mol% of Ni and Al with respect to the total molar number of metal elements excluding Li. By setting the content of Ni with respect to the total molar number of metal elements excluding Li in the lithium transition metal composite oxide to 80 mol% or more, a high-capacity battery can be obtained.

[0022] The content of Ni with respect to the total molar number of metal elements excluding Li in the lithium transition metal composite oxide is preferably 90 mol% or more. Thereby, a higher-capacity battery can be obtained. On the other hand, when the content of Ni exceeds 96 mol% with respect to the total molar number of metal elements excluding Li, the contents of Al and Ca become too small to ensure the stability of the layered structure and surface structure of the lithium transition metal composite oxide.

[0023] The lithium transition metal composite oxide has a layered structure. Examples of the layered structure of the lithium transition metal composite oxide include a layered structure belonging to the space group R-3m, a layered structure belonging to the space group C2 / m, and the like. Among these, a layered structure belonging to the space group R-3m is preferable in terms of high capacity, stability of the crystal structure, and the like.

[0024] The lithium transition metal composite oxide has the general formula Li a Ni x Al y Co z M w O2-b (In the formula, 0.95 < a < 1.05, 0.8 ≤ x ≤ 0.96, 0 < y ≤ 0.10, 0 ≤ z ≤ 0.15, 0 ≤ w ≤ 0.1, 0 ≤ b < 0.05, x + y + z + w = 1, and M is at least one element selected from Mn, Fe, Ti, Si, Nb, Zr, Mo, and Zn). The composite oxide can be represented as such. Note that 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 entire particles of the lithium transition metal composite oxide can be measured by an inductively coupled plasma optical emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), an energy dispersive X-ray analyzer (EDX), or the like.

[0025] a, which indicates the ratio of Li in the lithium transition metal composite oxide, preferably satisfies 0.95 ≤ a < 1.05, and more preferably satisfies 0.97 ≤ a ≤ 1.03. When a is less than 0.95, the battery capacity may decrease compared to the case where a satisfies the above range. When a is 1.05 or more, more Li compounds need to be added compared to the case where a satisfies the above range, which may not be economical from the perspective of production cost.

[0026] y, which indicates the content of Al with respect to the total molar number of the metal elements excluding Li in the lithium transition metal composite oxide, preferably satisfies 0 < y ≤ 0.10, and more preferably satisfies 0.03 ≤ y ≤ 0.07. Since the oxidation number of Al does not change during charge and discharge, it is considered that the structure of the transition metal layer is stabilized by being contained in the transition metal layer. On the other hand, when y > 0.10, Al impurities are generated and the battery capacity decreases. Al may be uniformly dispersed in the layered structure of the lithium transition metal composite oxide, or may be present in a part of the layered structure.

[0027] Co and M (where M is at least one element selected from Mn, Fe, Ti, Si, Nb, Zr, Mo, and Zn) are optional components. The z and w values, which represent the content 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 0 ≤ z ≤ 0.15 and 0 ≤ w ≤ 0.1, respectively. Since Co is expensive, it is preferable to reduce the Co content from the viewpoint of manufacturing costs.

[0028] Lithium transition metal composite oxides are, for example, secondary particles formed by the aggregation of multiple primary particles. The particle size of the primary particles constituting the secondary particles is, for example, 0.05 μm to 1 μm. The particle size of the primary particles is measured as the diameter of the circumscribed circle in the particle image observed by a scanning electron microscope (SEM). The surface modification layer exists on the surface of the primary particles. In other words, the surface modification layer exists on the surface of the secondary particles of the lithium transition metal composite oxide, or at the interface where the primary particles come into contact with each other.

[0029] Lithium transition metal composite oxides are particles with a volume-based median diameter (D50) of, for example, 3 μm to 30 μm, 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 the smallest particle size accounts for 50% in the volume-based particle size distribution, and is also called the median diameter. The particle size distribution of lithium transition metal composite oxides can be measured using a laser diffraction particle size distribution analyzer (e.g., Microtrac-Bell MT3000II) with water as the dispersion medium.

[0030] Lithium transition metal composite oxides have a surface layer located from the surface inward and a main body located inside the surface layer. The thickness of the surface layer is, for example, 1 nm to 5 nm.

[0031] The thickness of the surface modification layer is, for example, 0.1 nm to 5 nm. Within this range, the reaction with the electrolyte on the surface of the lithium transition metal composite oxide is suppressed, thereby suppressing the decrease in battery capacity associated with charging and discharging.

[0032] The surface modification layer contains at least Ca. The surface modification layer may contain, for example, Ca or a compound containing Ca. Examples of compounds containing Ca include CaO, Ca(OH)2, and CaCO3.

[0033] When compositional analysis is performed using energy-dispersive X-ray spectroscopy (TEM-EDX), the Ca content relative to the total number of moles of metal elements excluding Li in the surface modification layer can be between 1.5 mol% and 20 mol%. Within this range, the synergistic effect with Al can further improve the charge-discharge cycle characteristics of the battery. Here, the composition of the surface modification layer in the positive electrode active material, as well as the composition of the main body and surface layer of the lithium transition metal composite oxide, can be determined by analyzing each location in the cross-section of the primary particles of the positive electrode active material using TEM-EDX, thereby measuring the content of Ni, Co, Al, M, and Ca. Note that since the surface modification layer is thinner than the spot diameter of the irradiated electron beam, the composition of the surface layer is influenced by the composition of adjacent surface modification layers, and even if a trace amount of Ca is detected in the surface layer measurement results, it is considered that Ca is not actually present in the surface layer.

[0034] Furthermore, it is preferable that the X-ray diffraction pattern obtained by X-ray diffraction measurement of the lithium transition metal composite oxide does not contain any peaks originating from CaO. If CaO is present to an extent that can be detected by X-ray diffraction measurement, a decrease in charge / discharge capacity may occur. Here, the X-ray diffraction pattern is obtained, for example, by powder X-ray diffraction using a powder X-ray diffractometer (manufactured by Rigaku Corporation, product name "RINT-TTR", radiation source Cu-Kα) under the following conditions.

[0035] Measurement range: 15-120° Scan speed: 4° / min Analysis range: 30-120° Background: B-spline Profile function: Split type pseudo-Voigt function Constraint condition: Li(3a)+Ni(3a)=1 Ni(3a) + Ni(3b) = y (where y is the respective Ni content) ICSD No.: 98-009-4814 The surface modification layer may further contain Al. In other words, the surface modification layer may further contain at least one selected from Al or Al-containing compounds, and Ca and Al-containing compounds. An example of an Al-containing compound is Al2O3. Examples of Ca and Al-containing compounds are CaAl2O4 and Ca3Al2O6. The surface modification layer may further contain Li.

[0036] The molar ratio of Al to Ni in the surface modification layer, as determined by energy-dispersive X-ray spectroscopy (TEM-EDX), may be greater than the molar ratio of Al to Ni in the main body. This, combined with the synergistic effect with Ca, can further improve the battery's charge-discharge cycle characteristics.

[0037] Furthermore, when compositional analysis is performed by energy-dispersive X-ray spectroscopy (TEM-EDX), the molar ratio of Al to Ni in the surface modification layer is preferably at least twice the molar ratio of Al to Ni in the main body. Within this range, the charge-discharge cycle characteristics of the battery can be improved more significantly.

[0038] Next, an example of a method for producing a positive electrode active material including a lithium transition metal composite oxide and a surface modification layer will be described.

[0039] A method for producing a positive electrode active material includes, for example, a first step of obtaining a composite oxide containing Ni, Al, and an arbitrary metal element; a second step of mixing the composite oxide obtained in the first step with a lithium compound to obtain a mixture; and a third step of calcining the mixture. The composition and thickness parameters of the surface layer and surface modification layer in the finally obtained positive electrode active material are adjusted by controlling, for example, the mixing ratio of raw materials in the second step, the calcination temperature and time in the third step, etc.

[0040] In the first step, for example, an alkaline solution such as sodium hydroxide is added dropwise while stirring a solution of a metal salt containing Ni, Al, and an arbitrary metal element (Co, Mn, Fe, etc.) to adjust the pH to the alkaline side (e.g., 8.5 to 12.5), thereby precipitating (coprecipitation) a composite hydroxide containing Ni, Al, and an arbitrary metal element. The composite hydroxide is then calcined to obtain a composite oxide containing Ni, Al, and an arbitrary metal element. The calcination temperature is not particularly limited, but for example, it is in the range of 300°C to 600°C.

[0041] In the second step, the composite oxide obtained in the first step is mixed with a lithium compound and a calcium compound to obtain a mixture. Examples of lithium compounds include Li2CO3, LiOH, Li2O2, Li2O, LiNO3, LiNO2, Li2SO4, LiOH·H2O, LiH, and LiF. Examples of calcium compounds include Ca(OH)2, CaO, CaCO3, CaSO4, and Ca(NO3)2. The mixing ratio of the composite oxide obtained in the first step and the lithium compound is preferably such that, for example, the molar ratio of metal elements excluding Li to Li is in the range of 1:0.98 to 1:1.1, in order to easily adjust each of the above parameters to the specified range. Furthermore, the mixing ratio of the composite oxide obtained in the first step and the calcium compound is preferably such that, for example, the molar ratio of metal elements excluding Li to Ca is in the range of 1:0.0005 to 1:0.02, in order to easily adjust each of the above parameters to the specified range. In the second step, when mixing the composite oxide obtained in the first step with the lithium compound and the calcium compound, other metal raw materials may be added as needed. These other metal raw materials are oxides containing metal elements other than those that constitute the composite oxide obtained in the first step.

[0042] In the third step, the mixture obtained in the second step is calcined at a predetermined temperature and time to obtain the positive electrode active material according to this embodiment. The calcination of the mixture in the third step comprises a multi-stage calcination process, which includes, for example, a first calcination step in which the mixture is calcined in a calcination furnace under an oxygen flow to a first set temperature of 450°C to 680°C at a first heating rate, 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 flow to a second set temperature of over 680°C and up to 800°C at a second heating rate. Here, the first heating rate is in the range of 1.5°C / min to 5.5°C / min, and the second heating rate is slower than the first heating rate, in the range of 0.1°C / min to 3.5°C / min. Through such multi-stage calcination, the composition and thickness parameters of the surface layer and surface modification layer in the positive electrode active material of this embodiment obtained can be adjusted to the ranges specified above. The first and second heating rates may be set multiple times for each temperature range, as long as they are within the ranges specified above. The holding time of the first set temperature in the first firing process is preferably 5 hours or less, and more preferably 3 hours or less, in order to adjust the parameters of the lithium transition metal composite oxide to the ranges specified above. The holding time of the first set temperature is the time after the first set temperature is reached and the first set temperature is maintained. The holding time of the second set temperature in the second firing process is preferably 1 to 10 hours, and more preferably 1 to 5 hours, in order to adjust the parameters of the lithium transition metal composite oxide to the ranges specified above. The holding time of the second set temperature is the time after the second set temperature is reached and the second set temperature is maintained. When firing the mixture, in order to adjust the parameters to the ranges specified above, for example, it may be carried out in an oxygen stream with an oxygen concentration of 60% or more, and the flow rate of the oxygen stream should be set to 10 cm in the firing furnace. 3 The flow rate can be in the range of 0.2 mL / min to 4 mL / min per unit area, and 0.3 L / min or more per 1 kg of mixture.

[0043] The mole fraction of the metal elements contained in the positive electrode active material obtained above was measured by inductively coupled plasma (ICP) emission spectroscopy, and the general formula is Li a Ni x Al y Co z M w Caα O 2-b (where 0.95 < a < 1.05, 0.8 ≤ x ≤ 0.96, 0 < y ≤ 0.10, 0 ≤ z ≤ 0.15, 0 ≤ w ≤ 0.1, 0.0005 ≤ α ≤ 0.02, 0 ≤ b < 0.05, x + y + z + w = 1, and M is at least one element selected from Mn, Fe, Ti, Si, Nb, Zr, Mo, and Zn). Note that Ca is not dissolved in the lithium transition metal composite oxide but is contained in the surface modification layer present on the surface of the lithium transition metal composite oxide. Also, a part of Al may be contained in the surface modification layer.

[0044] [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, 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 produced 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.

[0045] 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, or artificial graphite such as graphitized mesophase carbon microbeads. Also, as the negative electrode active material, metals that alloy with Li such as Si and Sn, metal compounds containing Si, Sn, etc., lithium titanium composite oxides, etc. may be used. Further, those provided with a carbon coating may be used. For example, SiO x (0.5 ≤ x ≤ 1.6), or Li 2y SiO (2+y) (0 < y < 2), a Si-containing compound in which fine particles of Si are dispersed in a lithium silicate phase, etc. may be used in combination with graphite.

[0046] The binder included in the negative electrode composite layer 41 may be a fluororesin such as PTFE or PVdF, PAN, polyimide, acrylic resin, or polyolefin, similar to the case of the positive electrode 11, but styrene-butadiene rubber (SBR) is preferably used. The negative electrode composite layer 41 may also contain CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, or polyvinyl alcohol (PVA).

[0047] [Separator] For example, a porous sheet having ion permeability and insulating properties can be used for the separator 13. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyethylene, polyolefins such as polypropylene, and cellulose. The separator 13 may have a single-layer structure or a laminated structure. Furthermore, the surface of the separator 13 may be provided with a highly heat-resistant resin layer such as aramid resin, and a filler layer containing an inorganic compound filler.

[0048] [Non-aqueous electrolytes] Non-aqueous electrolytes include, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixtures of two or more of these. The non-aqueous solvent may contain halogen-substituted solvents in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of halogen-substituted solvents include fluorinated cyclic carbonate esters such as fluoroethylene carbonate (FEC), fluorinated linear carbonate esters, and fluorinated linear carboxylic acid esters such as methyl fluoropropionate (FMP).

[0049] Examples of the above esters include cyclic carbonate esters such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; linear carbonate esters such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and linear carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0050] Examples of the above ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methylphenyl ether. Examples include chain ethers such as ethylphenyl ether, butylphenyl ether, pentylphenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0051] The electrolyte salt is preferably a lithium salt. Examples of lithium salts include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), and 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, borates such as Li2B4O7, Li(B(C2O4)F2), imide salts such as LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 SO2){l, m are integers of 0 or more}, etc. Examples of the lithium salt include these. 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.

Examples

[0052] Hereinafter, the present disclosure will be further described by examples and comparative examples, but the present disclosure is not limited to the following examples.

[0053] [Preparation of positive electrode active material] <Example 1> General formula Ni 0.90 Co 0.05 Al 0.05 O2, the metal composite oxide and calcium hydroxide (Ca(OH)2) were mixed so that the content of Ca was 0.1 mol% with respect to the total amount of Ni, Co, and Al, and further, lithium hydroxide monohydrate (LiOH·H2O) was mixed so that the molar ratio of the total amount of Ni, Co, Al, and Ca to Li was 1:1.02. The mixture was fired from room temperature to 650 °C at a heating rate of 2 °C / min under an oxygen stream with an oxygen concentration of 95% (flow rate of 10 L / min per 1 kg of the mixture), and then fired from 650 °C to 720 °C at a heating rate of 1 °C / min. The fired product was washed with water to remove impurities, and the positive electrode active material of Example 1 was obtained. As a result of analyzing the composition of the positive electrode active material of Example 1 by ICP-AES, Li 0.99Ni 0.899 Co 0.05 Al 0.05 Ca 0.001 It was O2.

[0054] <Example 2> General formula Ni 0.90 Co 0.05 Al 0.05 The positive electrode active material of Example 2 was obtained in the same manner as in Example 1, except that the metal composite oxide and calcium hydroxide (Ca(OH)2) were mixed so that the Ca content was 0.15 mol% relative to the total amount of Ni, Co, and Al in the metal composite oxide represented by O2. The composition of the obtained positive electrode active material of Example 2 was Li 0.99 Ni 0.899 Co 0.05 Al 0.05 Ca 0.0015 It was O2.

[0055] <Comparative Example> A positive electrode active material was obtained in the same manner as in Example 1, except that calcium hydroxide (Ca(OH)2) was not mixed, and the material was calcined from room temperature to 650°C at a heating rate of 3.0°C / min, followed by calcination from 650°C to 720°C at a heating rate of 1°C / min. The composition of the obtained positive electrode active material was Li 0.99 Ni 0.90 Co 0.05 Al 0.05 The substance was O2. This was used as the positive electrode active material for the comparative example. Furthermore, 0.01 mol% Ca was detected in the comparative example. This amount of Ca is significantly less than the amount contained in Examples 1 and 2, and therefore is not expected to affect the experimental results.

[0056] TEM-EDX measurements were performed on the positive electrode active materials of Examples 1 and 2 and the Comparative Example, and compositional analysis was conducted on the main body, surface layer, and surface modification layer of the lithium transition metal composite oxide. For the main body, measurements were taken at an internal position at a distance of 15 nm or more from the surface of the lithium transition metal composite oxide. For Examples 1 and 2, measurements were performed at two different observation points, the first and second observation points, respectively. The results are shown in Table 1. In Table 1, the molar percentages of Ni, Co, and Al are listed with the sum of Ni, Co, and Al being 100. X-ray diffraction measurements were also performed on Examples 1 and 2 and the Comparative Example, but no peaks originating from CaO were found in the X-ray diffraction patterns of any of them.

[0057] [Table 1]

[0058] Small amounts of Ca were detected in the surface layer at the second measurement point in Example 1, and at the first and second measurement points in Example 2. However, as mentioned above, it is considered that Ca is not actually present in the surface layer. Separately performed electron energy loss spectroscopy (TEM-EELS) also confirmed the presence of Ca only in the surface modification layer. In other words, in Examples 1 and 2, Ca was present only in the surface modification layer. On the other hand, in the comparative example where Ca was not added, Ca was not detected in any part. Furthermore, in Examples 1 and 2, Al was present in the order of surface modification layer, surface layer, and main body. On the other hand, in the comparative example, the Al content was approximately the same in all parts.

[0059] Next, test cells were prepared using the positive electrode active materials of Examples 1 and 2 and the Comparative Example, as follows.

[0060] [Fabrication of the positive electrode] 95 parts by mass of the positive electrode active material from Examples 1 and 2 and the Comparative Example, 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 mixture 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 15 μm thick aluminum foil, and after the coating film was dried, the coating film was rolled using a rolling mill and cut to a predetermined electrode size to obtain a positive electrode in which a positive electrode composite layer was formed on both sides of the positive electrode core. An exposed portion was provided on a part of the positive electrode in which the surface of the positive electrode core was exposed. Positive electrodes were prepared in the same manner for the other examples and comparative examples.

[0061] [Fabrication of the negative electrode] Natural graphite was used as the negative electrode active material. The negative electrode active material, sodium carboxymethylcellulose (CMC-Na), and styrene-butadiene rubber (SBR) were mixed in an aqueous solution in a solid content mass ratio of 100:1:1 to prepare a negative electrode mixture slurry. This negative electrode mixture slurry was applied to both sides of a negative electrode core made of copper foil, and after the coating film was dried, the coating film was rolled using a rolling mill and cut to a predetermined electrode size to obtain a negative electrode in which a negative electrode mixture layer was formed on both sides of the negative electrode core. An exposed portion was provided on a part of the negative electrode in which the surface of the negative electrode core was exposed.

[0062] [Preparation of non-aqueous electrolytes] Ethylene carbonate (EC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 3:3:4. Lithium hexafluoride phosphate (LiPF6) was dissolved in this mixed solvent to a concentration of 1.2 mol / liter to prepare a non-aqueous electrolyte.

[0063] [Preparation of test cells] Aluminum leads were attached to the exposed portion of the positive electrode containing the positive electrode active material of Examples 1 and 2 and the Comparative Example, and nickel leads were attached to the exposed portion of the negative electrode. The positive and negative electrodes were then wound in a spiral shape via a polyolefin separator, and then press-molded radially to produce a flattened wound electrode body. This electrode body was housed in an outer casing, the non-aqueous electrolyte was injected, and the opening of the outer casing was sealed to obtain a test cell.

[0064] [Evaluation of capacity retention rate] The following cycle tests were performed on batteries fabricated by incorporating the positive electrodes containing the positive electrode active materials of Examples 1 and 2 and the Comparative Example. The discharge capacity at the 1st cycle and the discharge capacity at the 30th cycle were determined, and the capacity retention rate was calculated using the following formula.

[0065] Capacity retention rate (%) = (Discharge capacity at 30th cycle ÷ Discharge capacity at 1st cycle) × 100 <Cycle Testing> The test cell was charged with a constant current of 0.2It at a temperature of 25°C until the battery voltage reached 4.2V, and then charged with a constant voltage until the current value was reduced to 1 / 100It at 4.2V. After that, it was discharged with a constant current of 0.2It until the battery voltage reached 2.5V. This charge-discharge cycle was repeated 30 times.

[0066] Table 2 shows the volume retention rates for Examples 1 and 2 and the Comparative Example. The volume retention rates of the test cells in Examples 1 and 2 shown in Table 2 are expressed relatively, with the volume retention rate of the test cell in Comparative Example 1 set to 100%.

[0067] [Table 2]

[0068] As shown in Table 2, Examples 1 and 2, which used a positive electrode active material containing Ca in the surface modification layer, showed a higher capacity retention rate than the comparative example, which used a positive electrode active material that did not contain Ca in the surface modification layer. [Explanation of symbols]

[0069] 10 Nonaqueous electrolyte secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode body 15 Battery Case 16 outer cans 17 Sealing body 18,19 Insulating board 20 Positive Tabs 21 Negative electrode tab 22 Grooved section 23 Bottom plate 24 Lower valve body 25 Insulating material 26 Upper valve body 27 caps 28 Gaskets 30 Positive electrode current collector 31. Positive electrode composite layer 40 Negative electrode current collector 41 Negative electrode composite layer

Claims

1. A first step to obtain a lithium transition metal composite oxide containing at least 80 mol% or more of Ni and Al relative to the total number of moles of metal elements other than Li, The second step involves mixing the composite oxide obtained in the first step with a lithium compound and a calcium compound to obtain a mixture. The process includes a third step of calcining the mixture, The third step is, A first firing process in which the product is fired under an oxygen stream to a first set temperature of 450°C to 680°C at a first heating rate, The process includes a second firing step in which the product is fired under an oxygen stream to a second set temperature of over 680°C but not exceeding 800°C at a second heating rate slower than the first heating rate. This provides a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, comprising forming a surface modification layer containing at least Ca on the surface of primary particles of the lithium transition metal composite oxide.

2. In the second step, the calcium compound is Ca(OH) ２ CaO, CaCO ３ CaSO ４ Ca(NO ３ ) ２ A 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 at least one of the following.

3. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein in the second step, the mixing ratio of the composite oxide obtained in the first step and the calcium compound is such that the molar ratio of metal elements excluding Li to Ca is in the range of 1:0.0005 to 1:0.

02.

4. A 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, in the third step, washing with water is performed after the firing.

5. A 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 in the third step, the first heating rate is in the range of 1.5°C / min or more and 5.5°C / min or less.

6. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, wherein in the third step, the second heating rate is in the range of 0.1°C / min or more and 3.5°C / min or less.

7. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 6, wherein in the third step, after reaching the first set temperature, the first set temperature is maintained for 5 hours or less.

8. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 7, wherein in the third step, after reaching the second set temperature, the second set temperature is maintained for 1 hour or more and 10 hours or less.

9. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 8, wherein the Ca is not solid-dissolved in the lithium transition metal composite oxide.

10. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 9, wherein the surface modification layer further contains Al.

11. The lithium transition metal composite oxide has a general formula Li ａ Ni ｘ Al ｙ Co ｚ M ｗ O ２－ｂ (where 0.95 < a < 1.05, 0.8 ≤ x ≤ 0.96, 0 < y ≤ 0.10, 0 ≤ z ≤ 0.15, 0 ≤ w ≤ 0.1, 0 ≤ b < 0.05, x + y + z + w = 1, and M is at least one element selected from Mn, Fe, Ti, Si, Nb, Zr, Mo, and Zn). A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 10.

12. The lithium transition metal composite oxide has a surface layer located on the interior side of the surface and a main body located on the interior side of the surface layer. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 11, wherein the molar ratio of Al to Ni in the surface modification layer, as determined by compositional analysis using energy-dispersive X-ray spectroscopy (TEM-EDX), is greater than the molar ratio of Al to Ni in the main body.

13. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 12, wherein the molar ratio of Al to Ni in the surface modification layer, as determined by compositional analysis using energy-dispersive X-ray spectroscopy (TEM-EDX), is at least twice the molar ratio of Al to Ni in the main body.

14. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 13, wherein the content of Ca relative to the total number of moles of metal elements excluding Li in the surface modification layer, as determined by compositional analysis by energy-dispersive X-ray spectroscopy (TEM-EDX), is 1.5 mol% to 20 mol%.

15. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 14, wherein the content of Ni relative to the total number of moles of metal elements other than Li in the lithium transition metal composite oxide is 90 mol% or more.

16. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 15, wherein the X-ray diffraction pattern obtained by X-ray diffraction measurement of the lithium transition metal composite oxide including the surface modification layer does not contain any peaks originating from CaO.

17. A method for manufacturing a non-aqueous electrolyte secondary battery, comprising manufacturing a battery using a positive electrode prepared by the method for manufacturing a positive electrode active material for a non-aqueous electrolyte secondary battery described in any one of claims 1 to 16, a negative electrode, and a non-aqueous electrolyte.

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