Non-aqueous electrolyte secondary battery

Abstract

Provided is a non-aqueous electrolyte secondary battery having improved charge / discharge cycle characteristics and safety. A positive electrode active material for the non-aqueous electrolyte secondary battery that is one aspect of the present disclosure is provided with a positive electrode, a negative electrode, and a non-aqueous electrolyte, the positive electrode having a positive electrode current collector and a positive electrode mixture layer that contains a positive electrode active material and that is formed on the surface of the positive electrode current collector, the positive electrode active material including a lithium-containing composite oxide represented by a prescribed general formula, the lithium-containing composite oxide having secondary particles formed by aggregation of primary particles, Ca being present on the surfaces and in the interiors of the secondary particles, and the proportion of Ca present on the surfaces of the secondary particles being 12-58% relative to the total amount of Ca present on the surfaces and in the interiors of the secondary particles.

Description

[Technical Field]

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery. [Background technology]

[0002] In recent years, lithium-containing composite oxides such as lithium nickel cobalt oxide have been used as positive electrode active materials in order to increase capacity. However, if the Ni content in the lithium-containing composite oxide is increased in order to further increase capacity, the reactivity of the surface of the lithium-containing composite oxide increases, which in turn increases the reactivity with the electrolyte, and this can lead to a greater rate of decrease in battery capacity due to repeated charge and discharge. Patent Document 1 discloses a technology for suppressing the decrease in charge and discharge cycle characteristics by attaching a compound such as CaO to the surface of the lithium-containing composite oxide. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 08-236114 Summary of the Invention [Problem to be solved by the invention]

[0004] As a result of intensive studies by the present inventors, it has been found that even if Ca is present only on the surface of the lithium-containing composite oxide, the reaction between the lithium-containing composite oxide and the electrolyte cannot be sufficiently reduced, and the safety of the battery may be reduced. Patent Document 1 does not consider the safety of the battery, and there is still room for improvement.

[0005] Therefore, an object of the present disclosure is to provide a nonaqueous electrolyte secondary battery that achieves both good charge / discharge cycle characteristics and safety. [Means for solving the problem]

[0006] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode has a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector and containing a positive electrode active material. The positive electrode active material contains a lithium-containing composite oxide represented by the general formula Li a Ni b Co c Al d M e Ca f O g (where 0.9 ≦ a ≦ 1.2, 0.8 ≦ b ≦ 0.95, 0 < c ≦ 0.1, 0 < d ≦ 0.1, 0 ≦ e ≦ 0.1, b + c + d + e = 1, 0.0005 ≦ f / (b + c + d + e + f) ≦ 0.01, 1.9 ≦ g ≦ 2.1, and M is at least one element selected from Mn, Fe, Ti, Si, Nb, Zr, Mo, and Zn). The lithium-containing composite oxide has secondary particles formed by aggregation of primary particles, Ca is present on the surface and inside of the secondary particles, and the ratio of Ca present on the surface of the secondary particles is 12% to 58% with respect to the total amount of Ca present on the surface and inside of the secondary particles).

Advantages of the Invention

[0007] According to one aspect of the present disclosure, in a non-aqueous electrolyte secondary battery, it is possible to achieve both charge-discharge cycle characteristics and safety.

Brief Description of the Drawings

[0008] [Figure 1] It is an axial cross-sectional view of a cylindrical secondary battery which is an example of an embodiment.

Modes for Carrying Out the Invention

[0009] An example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail below. Hereinafter, a cylindrical battery in which a wound electrode assembly is housed in a cylindrical battery case will be exemplified. However, the electrode assembly is not limited to the wound type, and may be a laminated type in which multiple positive electrodes and multiple negative electrodes are alternately stacked one by one with separators interposed therebetween. Furthermore, the battery case is not limited to a cylindrical shape, and may be, for example, a prismatic or coin-shaped case, or a pouch-shaped case made of a laminate sheet including a metal layer and a resin layer.

[0010] Fig. 1 is an axial cross-sectional view of a cylindrical secondary battery 10 that is one example of an embodiment. In the secondary battery 10 shown in Fig. 1, an electrode assembly 14 and a non-aqueous electrolyte (not shown) are housed in an exterior body 15. The electrode assembly 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween. For ease of explanation, the following description will be given with the sealing body 16 side referred to as "top" and the bottom side of the exterior body 15 referred to as "bottom."

[0011] The open end of the exterior body 15 is sealed with the sealing body 16, thereby sealing the interior of the secondary battery 10. Insulating plates 17 and 18 are provided above and below the electrode body 14. The positive electrode lead 19 extends upward through the through hole of the insulating plate 17 and is welded to the underside of the filter 22, which is the bottom plate of the sealing body 16. In the secondary battery 10, the cap 26, which is the top plate of the sealing body 16 and is electrically connected to the filter 22, serves as the positive electrode terminal. On the other hand, the negative electrode lead 20 extends through the through hole of the insulating plate 18 toward the bottom of the exterior body 15 and is welded to the inner bottom surface of the exterior body 15. In the secondary battery 10, the exterior body 15 serves as the negative electrode terminal. Note that if the negative electrode lead 20 is installed at the terminal end, the negative electrode lead 20 extends through the through hole of the insulating plate 18 toward the bottom of the exterior body 15 and is welded to the inner bottom surface of the exterior body 15.

[0012] Exterior body 15 is, for example, a cylindrical metal exterior can with a bottom. A gasket 27 is provided between exterior body 15 and sealing body 16, ensuring the airtightness of the interior of secondary battery 10. Exterior body 15 has a grooved portion 21 that supports sealing body 16, formed, for example, by pressing the side surface from the outside. Grooved portion 21 is preferably formed in an annular shape along the circumferential direction of exterior body 15, and supports sealing body 16 on its upper surface via gasket 27.

[0013] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26, which are stacked in this order from the electrode body 14 side. Each component of the sealing body 16 has, for example, a disk or ring shape, and all components except for the insulating member 24 are electrically connected to each other. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective centers, with the insulating member 24 interposed between their respective peripheral edges. If the internal pressure of the battery increases due to abnormal heat generation, for example, the lower valve body 23 may break, causing the upper valve body 25 to bulge toward the cap 26 and separate from the lower valve body 25, thereby cutting off the electrical connection between them. If the internal pressure continues to increase, the upper valve body 25 may break, and gas may be released from the opening 26a of the cap 26.

[0014] The positive electrode 11, negative electrode 12, separator 13, and non-aqueous electrolyte that constitute the secondary battery 10 will be described in detail below, particularly the positive electrode active material contained in the positive electrode mixture layer that constitutes the positive electrode 11.

[0015] [Positive electrode] The positive electrode has a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. The positive electrode mixture layer is preferably formed on both sides of the positive electrode current collector. The positive electrode current collector can be a foil of a metal such as aluminum that is stable within the potential range of the positive electrode, or a film with such a metal disposed on the surface layer. The positive electrode mixture layer contains, for example, a positive electrode active material, a binder, a conductive agent, etc. The positive electrode can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a binder, a conductive agent, etc. to the positive electrode current collector, drying it to form a positive electrode mixture layer, and then rolling this positive electrode mixture layer.

[0016] Examples of the conductive agent contained in the positive electrode active material layer include carbon-based particles such as carbon black (CB), acetylene black (AB), ketjen black, and graphite. These may be used alone or in combination of two or more kinds.

[0017] Examples of the binder contained in the positive electrode active material layer include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide-based resins, acrylic resins, and polyolefin-based resins. These may be used alone or in combination of two or more kinds.

[0018] The positive electrode active material contained in the positive electrode active material layer is represented by the general formula Li a Ni b Co c Al d M e Ca f O g (where 0.9 ≦ a ≦ 1.2, 0.8 ≦ b ≦ 0.95, 0 < c ≦ 0.1, 0 < d ≦ 0.1, 0 ≦ e ≦ 0.1, b + c + d + e = 1, 0.0005 ≦ f / (b + c + d + e + f) ≦ 0.01, 1.9 ≦ g ≦ 2.1, and M is at least one element selected from Mn, Fe, Ti, Si, Nb, Zr, Mo, and Zn) and contains a lithium-containing composite oxide. By using a Ni-Co-Al-based lithium-containing composite oxide, it is possible to suppress cation mixing in which Ni enters the Li site while increasing the capacity of the battery. In addition, the positive electrode active material may contain a lithium-containing composite oxide other than that represented by the above general formula or other compounds as long as the object of the present disclosure is not impaired. The molar fraction of the metal elements contained in the lithium-containing composite oxide is measured by inductively coupled high-frequency plasma emission spectrometry (ICP-AES).

[0019] The ratio a of Li in the lithium-containing composite oxide preferably satisfies 0.9 ≦ a ≦ 1.2, and more preferably satisfies 0.95 ≦ a ≦ 1.05. When a is less than 0.9, the battery capacity may decrease compared to the case where a satisfies the above range. When a exceeds 1.2, it may lead to a decrease in charge-discharge cycle characteristics compared to the case where a satisfies the above range.

[0020] The ratio b of Ni to the total molar number of metal elements excluding Li and Ca in the lithium-containing composite oxide preferably satisfies 0.8 ≦ b ≦ 0.96, and more preferably satisfies 0.88 ≦ b ≦ 0.92. By setting b to 0.8 or more, a high-capacity battery can be obtained. Also, by setting b to 0.96 or less, other elements such as Co and Al can be included, so cation mixing can be suppressed.

[0021] The ratio c of Co to the total molar number of metal elements excluding Li and Ca in the lithium-containing composite oxide preferably satisfies 0 < c ≦ 0.10, and more preferably satisfies 0.04 ≦ c ≦ 0.06.

[0022] The ratio d of Al to the total molar number of metal elements excluding Li and Ca in the lithium-containing composite oxide preferably satisfies 0 < d ≦ 0.10, and more preferably satisfies 0.04 ≦ d ≦ 0.06. Since Al does not cause a change in oxidation number even 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. When d exceeds 0.10, Al impurities may be generated and the battery capacity may decrease.

[0023] M (M is at least one element selected from Mn, Fe, Ti, Si, Nb, Zr, Mo, and Zn) is an optional component. The ratio e of M to the total molar number of metal elements excluding Li and Ca in the lithium-containing composite oxide preferably satisfies 0 ≦ e ≦ 0.1.

[0024] f / (b+c+d+e+f), which indicates the ratio of Ca to the total number of moles of metal elements excluding Li in the lithium-containing composite oxide, preferably satisfies 0.0005≦f / (b+c+d+e+f)≦0.01, more preferably satisfies 0.001≦f / (b+c+d+e+f)≦0.005, and particularly preferably satisfies 0.0015≦f / (b+c+d+e+f)≦0.0045.

[0025] The lithium-containing composite oxide has secondary particles formed by aggregation of primary particles, and Ca is present on the surface and inside of the secondary particles. Here, the presence of Ca inside the secondary particles means that Ca is present between the primary particles that make up the secondary particles.

[0026] The secondary particles of the lithium-containing composite oxide are particles having a volume-based median diameter (D50) of preferably 3 μm to 30 μm, more preferably 5 μm to 25 μm, and particularly preferably 7 μm to 15 μm. D50 refers to the particle size at which the cumulative frequency of particles in the volume-based particle size distribution is 50% from the smallest particle size, and is also called the median diameter. The particle size distribution of the secondary particles of the lithium-containing composite oxide can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II manufactured by Microtrack Bell Corporation) using water as a dispersion medium.

[0027] The particle size of the primary particles that make up 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 a particle image observed with a scanning electron microscope (SEM).

[0028] The proportion of Ca present on the surface of the secondary particles of the lithium-containing composite oxide (hereinafter referred to as the Ca surface abundance ratio) is preferably 12% to 58%, more preferably 12% to 31%, and even more preferably 12% to 20%, of the total amount of Ca present on the surface and inside the secondary particles. By setting the Ca surface abundance ratio within this range, both charge / discharge cycle characteristics and safety can be achieved. It is presumed that the Ca present on the surface of the secondary particles suppresses reaction with the electrolyte and thereby suppresses an increase in the battery's internal resistance due to repeated charge / discharge, and that the Ca present inside the secondary particles suppresses thermal decomposition of the positive electrode active material.

[0029] The surface abundance of Ca is measured as follows. (1) Measurement of the total amount of Ca present on the surface and inside of secondary particles 10 mL of aqua regia was added dropwise to 0.2 g of the positive electrode active material powder, followed by 2.5 mL of hydrofluoric acid, which was then heated to completely dissolve the powder, to prepare an aqueous solution. The aqueous solution was then adjusted to a constant volume of 100 mL with ion-exchanged water, and the Ca concentration was measured by ICP-AES, and the result was taken as the total amount of Ca present on the surface and inside of the secondary particles. (2) Measurement of the amount of Ca present on the surface of secondary particles 4 g of the positive electrode active material powder was stirred in 400 mL of 0.01 mol / L sodium hydroxide solution at 40°C for 5 minutes, and then filtered through a syringe filter with a pore size of 0.45 μm to obtain a filtrate. 2.5 mL of aqua regia was added dropwise to 4 mL of the filtrate, followed by 0.6 mL of hydrofluoric acid, and the filtrate was heated to completely dissolve the powder remaining in the filtrate, producing an aqueous solution. The aqueous solution was then adjusted to a constant volume of 100 mL with ion-exchange water, and the Ca concentration was measured using ICP-AES. The result was used as the total amount of Ca present on the surface of the secondary particles. (3) Calculation of the surface abundance of Ca Using the above measurement results, the surface abundance ratio of Ca is calculated according to the following formula. Ca surface abundance ratio = (amount of Ca present on the surface of secondary particles) / (total amount of Ca present on the surface and inside of secondary particles)

[0030] On the surface of the secondary particles and inside the secondary particles, Ca may be present in the form of a Ca compound containing Ca. Examples of Ca compounds include CaO, Ca(OH)2, and CaCO3.

[0031] Next, an example of a method for producing a lithium-containing composite oxide will be described.

[0032] The method for producing a lithium-containing composite oxide includes the steps of: mixing a composite oxide containing at least Ni, Co, and Al with a Li raw material such as LiOH, Li2O, or Li2CO3, and a Ca raw material such as Ca(OH)2, CaO, or CaCO3; calcining the mixture to obtain a calcined product; washing the calcined product with water and dehydrating it to obtain a cake-like composition having a predetermined moisture content; and heat-treating the cake-like composition to obtain the lithium-containing composite oxide.

[0033] <Baked product synthesis process> First, a composite oxide containing Ni, Co, and Al, a Li raw material such as lithium hydroxide (LiOH) or lithium carbonate, and a Ca raw material such as CaO, Ca(OH)2, or CaCO3 are prepared. The composite oxide can be obtained, for example, by heat-treating a composite hydroxide such as a nickel-cobalt-aluminum composite hydroxide obtained by coprecipitation. Next, the composite oxide, the Li raw material, and the Ca raw material are mixed, and the mixture is calcined and then pulverized to obtain calcined particles. The inventors' studies have revealed that the surface abundance of Ca can be adjusted by adjusting the calcination conditions. For example, the surface abundance of Ca can be reduced by increasing the calcination temperature. It is believed that increasing the calcination temperature promotes the reaction between Ca and Li, etc., inside the secondary particles of the lithium-containing composite oxide.

[0034] <Cake-like composition preparation step> Next, the fired product is washed with water and dehydrated to obtain a cake-like composition. The fired product may be particulate material obtained in the synthesis step described above. By washing with water, unreacted Li raw material added in the synthesis step of the fired product and impurities other than the Li raw material can be removed. For example, 300 g to 5000 g of fired product can be added to 1 L of water for washing. Washing with water can be repeated multiple times. Dehydration after washing can be performed, for example, using a filter press. The moisture content of the cake-like composition after washing (hereinafter referred to as cake moisture content) can be adjusted by adjusting the dehydration conditions. The inventors' studies have found that increasing the cake moisture content can increase the surface Ca abundance ratio of the lithium-containing composite oxide. By adjusting the cake moisture content within a predetermined range, the surface Ca abundance ratio can be adjusted to 12% to 58%. The cake moisture content can be calculated by drying 10 g of the cake-like composition by leaving it to stand in a vacuum at 120°C for 2 hours, and dividing the change in mass of the cake-like composition before and after drying by the mass of the cake-like composition before drying.

[0035] <Lithium-containing composite oxide synthesis process> The cake-like composition can be heat-treated to obtain a lithium-containing composite oxide. The heat-treatment conditions are not particularly limited, but for example, the heat-treatment temperature can be 150°C to 400°C, and the heat-treatment time can be 0.5 hours to 15 hours.

[0036] [Negative electrode] The negative electrode has a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. The negative electrode mixture layer is preferably formed on both sides of the negative electrode current collector. The negative electrode current collector can be a foil of a metal such as copper that is stable within the potential range of the negative electrode, or a film with such a metal disposed on the surface layer. The negative electrode mixture layer contains a negative electrode active material and preferably also contains a thickener, a binder, etc. The negative electrode can be produced, for example, by applying a negative electrode mixture slurry, in which the negative electrode active material, the thickener, and the binder are dispersed in water at a predetermined mass ratio, onto the negative electrode current collector, drying the coating, and then rolling it to form a negative electrode mixture layer on both sides of the negative electrode current collector.

[0037] As the negative electrode active material, a carbon material capable of absorbing and releasing lithium ions can be used, and in addition to graphite, non-graphitizable carbon, graphitizable carbon, fibrous carbon, coke, carbon black, etc. Furthermore, as a non-carbon-based material, silicon, tin, and alloys and oxides mainly containing these can be used.

[0038] As in the case of the positive electrode, a fluorine-based resin or the like can be used as the binder, but styrene-butadiene copolymer (SBR) or a modified product thereof may also be used. As the thickener, carboxymethyl cellulose (CMC) or the like can be used.

[0039] [Separator] The separator 13 may be, for example, a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include microporous membranes, woven fabrics, and nonwoven fabrics. Suitable materials for the separator include olefin-based resins such as polyethylene and polypropylene, and cellulose. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin-based resin. Alternatively, the separator 13 may be a multilayer separator including a polyethylene layer and a polypropylene layer, and the surface of the separator 13 may be coated with a material such as an aramid-based resin or ceramic.

[0040] [Non-aqueous electrolyte] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The non-aqueous electrolyte is not limited to a liquid electrolyte (electrolytic solution) and may be a solid electrolyte using a gel polymer or the like. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted product in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine.

[0041] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone and γ-valerolactone; and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate.

[0042] Examples of the ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, and methyl phenyl ether. and chain ethers such as ethyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0043] As the halogen-substituted compound, it is preferable to use fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, fluorinated chain carboxylates such as methyl fluoropropionate (FMP), and the like.

[0044] 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), LiPF 6-x (C n F 2n+1 ) x (1 < x < 6, n is 1 or 2), LiB 10 Cl 10 、LiCl、LiBr、LiI、lithium chloroborane, lithium lower aliphatic carboxylic acid, borate salts 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 1 or more}, etc. 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 preferably 0.8 to 1.8 mol per 1 L of the solvent.

Examples

[0045] Hereinafter, the present disclosure will be further described by examples, but the present disclosure is not limited to these examples.

[0046] [Preparation of positive electrode active material] [Example 1]< General formula Ni 0.91 Co 0.04 Al 0.05A composite oxide represented by O2, Ca(OH)2, and LiOH were mixed so that the molar ratio of the total amount of Ni, Co, and Al to Ca and Li was 1:0.0028:1.02, and the mixture was fired to obtain a fired product. The fired product was then washed with water and dehydrated using a filter press to obtain a cake-like composition with a predetermined moisture content. The cake-like composition was then heat-treated under an oxygen stream with an oxygen concentration of 95% (flow rate of 5 L / min per kg of mixture) at a heating rate of 2°C / min from room temperature to 650°C, and then at a heating rate of 1°C / min from 650°C to 800°C to obtain the cathode active material of Example 1. Analysis of the cathode active material of Example 1 by ICP-AES revealed that its composition was LiNi 0.91 Co 0.04 Al 0.05 Ca 0.0028 It was O2.

[0047] [Preparation of positive electrode] 100 parts by mass of the above positive electrode active material, 1 part by mass of acetylene black (AB) as a conductive agent, and 0.9 parts by mass of polyvinylidene fluoride (PVdF) as a binder were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to both sides of a positive electrode current collector made of aluminum foil, and after drying the coating, the coating was rolled using a rolling roller and cut to a predetermined electrode size to obtain a positive electrode with a positive electrode mixture layer formed on both sides of the positive electrode current collector. In addition, an exposed portion was provided in part of the positive electrode where the surface of the positive electrode current collector was exposed.

[0048] [Preparation of negative electrode] Graphite and SiO were mixed to a concentration of 94 parts by weight and 6 parts by weight, respectively, to form the negative electrode active material. 95 parts by weight of the negative electrode active material was mixed with 3 parts by weight of carboxymethyl cellulose (CMC) as a thickener and 2 parts by weight of styrene-butadiene rubber (SBR) as a binder, and an appropriate amount of water was added to prepare a negative electrode mixture slurry. The negative electrode mixture slurry was applied to both sides of a negative electrode current collector made of copper foil, and the coating was dried. The coating was then rolled using a rolling roller and cut to the desired electrode size to obtain a negative electrode with a negative electrode mixture layer formed on both sides of the negative electrode current collector. An exposed portion was provided on a portion of the negative electrode, exposing the surface of the negative electrode current collector.

[0049] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30:70. Lithium hexafluorophosphate (LiPF6) was added to the mixed solvent to a concentration of 1 mol / L to prepare a non-aqueous electrolyte.

[0050] [Test cell construction] An aluminum lead was attached to the exposed portion of the positive electrode, and a nickel lead was attached to the exposed portion of the negative electrode, and the positive and negative electrodes were spirally wound with a polyethylene microporous membrane separator interposed therebetween, and then pressed radially to produce a flat wound electrode assembly. This electrode assembly was housed in an outer casing, and the nonaqueous electrolyte was poured into it. The opening of the outer casing was then sealed to obtain a test cell.

[0051] [ARC Exam] The test cell was charged at a constant current of 0.3 It at 25°C until the battery voltage reached 4.2 V, and then at a constant voltage of 4.2 V until the current reached 0.05 It. The test cell was then placed in an adiabatic runaway calorimeter (ARC) and the self-heating rate (°C / min) of the test cell was measured by monitoring the cell temperature with a thermocouple attached to the test cell. Specifically, the test cell temperature was repeatedly measured while increasing the temperature at a rate of 5°C / min. When the self-heating rate reached 1°C / min based on the Arrhenius plot, the control was switched to adiabatic control and continued until heat generation occurred. The battery temperature (°C) at which the self-heating rate of the test cell reached 2°C / min was defined as the thermal runaway temperature.

[0052] [Capacity retention rate evaluation] The test cell was charged at a constant current of 0.5 It in a 25°C environment until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current value reached 1 / 50 It. It was then discharged at a constant current of 0.5 It until the battery voltage reached 2.5 V. This charge / discharge cycle was counted as one cycle, and 400 cycles were performed. The capacity retention rate of the test cell during the charge / discharge cycles was calculated using the following formula. Capacity retention rate = (discharge capacity at 400th cycle / discharge capacity at 1st cycle) x 100

[0053] <Example 2> A test cell was produced and evaluated in the same manner as in Example 1, except that the moisture content of the cake was increased in the production of the positive electrode active material.

[0054] Example 3 In preparing the positive electrode active material, the amount of Ca(OH)2 added was changed so that the molar ratio of the total amount of Ni, Co, and Al to Ca was 1:0.0017. A test cell was prepared and evaluated in the same manner as in Example 1.

[0055] Example 4 In preparing the positive electrode active material, the amount of Ca(OH)2 added was changed so that the molar ratio of the total amount of Ni, Co, and Al to Ca was 1:0.0041. A test cell was prepared and evaluated in the same manner as in Example 1.

[0056] <Comparative Example 1> In preparing the positive electrode active material, the amount of Ca(OH)2 added was changed so that the molar ratio of the total amount of Ni, Co, and Al to Ca was 1:0.0025, and the moisture content of the cake was made higher than in Example 2. Except for this, a test cell was prepared and evaluated in the same manner as in Example 1.

[0057] <Comparative Example 2> In preparing the positive electrode active material, the amount of Ca(OH)2 added was changed so that the molar ratio of the total amount of Ni, Co, and Al to Ca was 1:0.0024, and the firing temperature was increased. Except for this, a test cell was prepared and evaluated in the same manner as in Example 1.

[0058] The evaluation results of each test cell of the Examples and Comparative Examples are shown in Table 1. In Table 1, the results of the Examples and Comparative Examples are shown as relative values ​​when the capacity retention rate (%) and thermal runaway temperature (°C) of the test cell of Comparative Example 1 are set to 100. Table 1 also shows the Ca element content (mol%) relative to the total number of moles of metal elements excluding Li in the cake-like composition, the cake moisture content, the firing temperature, and the surface abundance rate of Ca. The cake moisture content is expressed as a relative evaluation of "+2," "+1," and "0 (reference)" in descending order of moisture content, with the cake moisture content of Example 1 set to "0 (reference)." The firing temperature is expressed as a relative evaluation of "+1," with the conditions of Example 1 set to "0 (reference)."

[0059] [Table 1]

[0060] In Examples 1 to 4, which contain a predetermined proportion of Ca and have a surface abundance ratio of Ca within a predetermined range, the thermal runaway temperature is higher than that of Comparative Example 1 while maintaining a capacity retention rate equivalent to that of Comparative Example 1, thereby achieving both good charge-discharge cycle characteristics and safety. On the other hand, the thermal runaway temperature of Comparative Example 2, in which the surface abundance ratio of Ca is less than 12%, is lower than that of Comparative Example 1. [Explanation of symbols]

[0061] 10 secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 15 exterior body, 16 sealing body, 17, 18 insulating plate, 19 positive electrode lead, 20 negative electrode lead, 21 grooved portion, 22 filter, 23 lower valve body, 24 insulating member, 25 upper valve body, 26 cap, 26a opening, 27 gasket

Claims

1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, the positive electrode has a positive electrode current collector and a positive electrode mixture layer formed on a surface of the positive electrode current collector and containing a positive electrode active material, The positive electrode active material has the general formula Li ａ Ni ｂ Co ｃ Al ｄ M ｅ Ca ｆ O ｇ (wherein 0.9≦a≦1.2, 0.8≦b≦0.95, 0<c≦0.1, 0<d≦0.1, 0≦e≦0.1, b+c+d+e=1, 0.0005≦f / (b+c+d+e+f)≦0.01, 1.9≦g≦2.1, and M is at least one element selected from Mn, Fe, Ti, Si, Nb, Zr, Mo, and Zn), the lithium-containing composite oxide has secondary particles formed by aggregation of primary particles, Ca is present on the surfaces and inside the secondary particles, and the ratio of Ca present on the surfaces of the secondary particles is 12% to 58% with respect to the total amount of Ca present on the surfaces and inside the secondary particles.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein a ratio of Ca present on the surfaces of the secondary particles to a total amount of Ca present on the surfaces and inside the secondary particles is 12% to 31%.

3. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein a ratio of Ca present on the surfaces of the secondary particles to a total amount of Ca present on the surfaces and inside the secondary particles is 12% to 20%.

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