Non-aqueous electrolyte secondary battery

Abstract

In this non-aqueous electrolyte secondary battery, a positive electrode comprises, as a positive electrode active material, a lithium-transition metal compound oxide containing Ni, Nb, and an optional component, Co. The content of Ni in the lithium-transition metal compound oxide is 80 mol% or more relative to the total molar number of metallic elements excluding Li, the content of Nb is 0.35 mol% or less relative to the total molar number of metallic elements excluding Li, and the content of Co is 5 mol% or less relative to the total molar number of metallic elements excluding Li. A negative electrode has a negative electrode mixture layer containing a negative electrode active material, and a film containing Nb formed on a surface of the negative electrode mixture layer. The content of Nb in the film relative to the total mass of the negative electrode mixture layer and the film is 10 to 3000 ppm.

Description

[Technical field]

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery, and in particular to a non-aqueous electrolyte secondary battery that includes a lithium transition metal composite oxide containing Ni as a positive electrode active material. [Background technology]

[0002] In recent years, lithium transition metal composite oxides with a high Ni content have been attracting attention as a positive electrode active material with high energy density. For example, Patent Document 1 describes a compound represented by the general formula Li x Ni 1-y-z-v-w Co y Al z M 1 v M 2 w The lithium transition metal composite oxide is represented by the formula: 1 is at least one selected from Mn, Ti, Y, Nb, Mo, and W, and element M 2 is at least Mg and Ca. Patent Document 2 discloses a lithium transition metal composite oxide containing Ni, Mn, and Co, the composite oxide containing at least one selected from Mo, W, Nb, Ta, and Re. Since Co is expensive, it is desired to reduce the amount of Co used. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2006-310181 A [Patent Document 2] JP 2009-289726 A Summary of the Invention

[0004] However, in a lithium transition metal composite oxide with a high Ni content, when the Co content is reduced, the structure of the composite oxide becomes unstable, and side reactions with the electrolyte tend to occur on the particle surface of the composite oxide. As a result, a large amount of electrolyte decomposition products are generated, and a coating of the decomposition products is formed on the negative electrode surface, which is thought to deteriorate the charge / discharge cycle characteristics of the battery. Note that the technologies disclosed in Patent Documents 1 and 2 still have room for improvement in terms of cycle characteristics.

[0005] An object of the present disclosure is to suppress a capacity decrease accompanying charge and discharge in a nonaqueous electrolyte secondary battery that uses a lithium transition metal composite oxide having a high Ni content and a low Co content as a positive electrode active material.

[0006] A nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure is a nonaqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a nonaqueous electrolyte, the positive electrode including a lithium transition metal composite oxide including Ni, Nb, and an optional component Co. The lithium transition metal composite oxide includes a Ni content of 80 mol % or more relative to the total number of moles of metal elements excluding Li, a Nb content of 0.35 mol % or less relative to the total number of moles of metal elements excluding Li, and a Co content of 5 mol % or less relative to the total number of moles of metal elements excluding Li. The negative electrode includes a negative electrode mixture layer including a negative electrode active material, and a coating including Nb formed on a surface of the negative electrode mixture layer, and the Nb content in the coating is 10 ppm to 3000 ppm relative to the total mass of the negative electrode mixture layer and the coating.

[0007] According to one aspect of the present disclosure, in a non-aqueous electrolyte secondary battery using a lithium transition metal composite oxide having a high Ni content and a low Co content as a positive electrode active material, it is possible to suppress a capacity decrease accompanying charge and discharge. The non-aqueous electrolyte secondary battery according to the present disclosure has excellent charge and discharge cycle characteristics. [Brief description of the drawings]

[0008] [Figure 1] 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] As described above, lithium transition metal composite oxides with a high Ni content and a low Co content have an unstable structure, and therefore side reactions with the electrolyte tend to occur on the particle surface, which is believed to cause a decrease in the charge-discharge cycle characteristics of the battery. The present inventors have found that by using a lithium transition metal composite oxide containing a specific amount of Nb, a good quality coating containing Nb derived from the positive electrode is formed on the negative electrode surface, thereby improving the charge-discharge cycle characteristics. The improvement effect of the cycle characteristics is particularly noticeable under high voltage.

[0010] In addition, when a conventional lithium transition metal composite oxide is used, a coating containing a large amount of Li is likely to be formed on the negative electrode surface due to the decomposition products of the electrolyte, and it is assumed that the coating is one of the factors that deteriorate the charge-discharge cycle characteristics. In the nonaqueous electrolyte secondary battery according to the present disclosure, the formation of the coating is suppressed, and instead, a high-quality coating containing Nb is formed on the negative electrode surface, which is believed to have greatly improved the charge-discharge cycle characteristics.

[0011] In this specification, the expression "numeric value (A) to numerical value (B)" means numerical value (A) or more and numerical value (B) or less.

[0012] Hereinafter, an example of an embodiment of the positive electrode active material for nonaqueous electrolyte secondary batteries according to the present disclosure and a nonaqueous electrolyte secondary battery using the positive electrode active material will be described in detail. Hereinafter, a cylindrical battery in which a wound electrode body 14 is housed in a cylindrical outer can 16 with a bottom will be exemplified, but the outer can is not limited to a cylindrical outer can, and may be, for example, a rectangular outer can, or may be an outer can made of a laminate sheet including a metal layer and a resin layer. In addition, the electrode body may be a laminated type electrode body in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0013] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As illustrated in FIG. 1, the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an exterior can 16 that accommodates the electrode assembly 14 and the electrolyte. The electrode assembly 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape with the separator 13 interposed therebetween. The exterior can 16 is a cylindrical metal container with a bottom that is open on one axial side, and the opening of the exterior can 16 is closed by a sealing body 17. In the following description, for convenience of explanation, the sealing body 17 side of the battery is referred to as the top, and the bottom side of the exterior can 16 is referred to as the bottom.

[0014] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. For example, esters, ethers, nitriles, amides, and mixed solvents of two or more of these are used as the non-aqueous solvent. The non-aqueous solvent may contain a halogen-substituted product in which at least a part of the hydrogen of these solvents is replaced with a halogen atom such as fluorine. For the electrolyte salt, for example, a lithium salt such as LiPF6 is used. The electrolyte is not limited to a liquid electrolyte, and may be a solid electrolyte using a gel-like polymer or the like.

[0015] The positive electrode 11, the negative electrode 12, and the separator 13 constituting the electrode body 14 are all long strip-shaped bodies, and are alternately stacked in the radial direction of the electrode body 14 by being wound in a spiral shape. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the longitudinal direction and the width direction (short direction). The two separators 13 are formed to be at least slightly larger than the positive electrode 11, and are arranged to sandwich the positive electrode 11, for example. The electrode body 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0016] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1, the positive electrode lead 20 passes through a through hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer casing 16. The positive electrode lead 20 is connected to the lower surface of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is a top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as a positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom inner surface of the outer casing 16 by welding or the like, and the outer casing 16 serves as a negative electrode terminal.

[0017] A gasket 28 is provided between the exterior can 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior can 16 has a grooved portion 22 formed therein, with part of the side surface protruding inward, to support the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the exterior can 16, and supports the sealing body 17 on its upper surface. The sealing body 17 is fixed to the top of the exterior can 16 by the grooved portion 22 and the open end of the exterior can 16 which is crimped against the sealing body 17.

[0018] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are laminated in this order from the electrode body 14 side. Each member constituting the sealing body 17 has, for example, a disk 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 at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks so as to push the upper valve body 26 toward the cap 27, thereby cutting off the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0019] The positive electrode 11, the negative electrode 12, and the separator 13 that constitute the electrode assembly 14, and in particular the positive electrode active material that constitutes the positive electrode 11, will be described in detail below.

[0020] [Positive electrode] The positive electrode 11 has a positive electrode core and a positive electrode composite layer provided on the surface of the positive electrode core. For the positive electrode core, a foil of a metal such as aluminum that is stable in the potential range of the positive electrode 11, a film with the metal disposed on the surface layer, or the like can be used. The positive electrode composite layer contains a positive electrode active material, a binder, and a conductive material, and is preferably provided on both sides of the positive electrode core except for the part to which the positive electrode lead 20 is connected. The positive electrode 11 can be produced, for example, by applying a positive electrode composite slurry containing a positive electrode active material, a binder, a conductive material, and the like to the surface of the positive electrode core, drying the coating, and then compressing it to form a positive electrode composite layer on both sides of the positive electrode core.

[0021] Examples of the conductive material contained in the positive electrode mixture layer include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of the binder contained in the positive electrode mixture layer 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 cellulose derivatives such as carboxymethylcellulose (CMC) or its salt, and polyethylene oxide (PEO).

[0022] The positive electrode 11 contains a lithium transition metal composite oxide containing Ni, Nb, and an optional component Co. Hereinafter, for convenience of explanation, the lithium transition metal composite oxide is referred to as a "composite oxide (Z)". The composite oxide (Z) functions as a positive electrode active material. The composite oxide (Z) has a layered structure, for example, a layered structure belonging to the space group R-3m, or a layered structure belonging to the space group C2 / m. The positive electrode active material may be mainly composed of the composite oxide (Z) and substantially composed of only the composite oxide (Z). Note that the positive electrode active material may contain a composite oxide other than the composite oxide (Z) or other compounds within a range that does not impair the object of the present disclosure.

[0023] The composite oxide (Z) contains 80 mol% or more of Ni relative to the total number of moles of metal elements excluding Li. By making the Ni content 80 mol% or more, a battery with high energy density can be obtained. In the composite oxide (Z), the Ni content is 80 mol% or more relative to the total number of moles of metal elements excluding Li, and the Nb content is 0.35 mol% or less relative to the total number of moles of metal elements excluding Li. The Ni content may be 85 mol% or more, or may be 90 mol% or more, relative to the total number of moles of metal elements excluding Li.

[0024] When the composite oxide (Z) contains Co, the content of Co is 5 mol% or less based on the total number of moles of metal elements excluding Li. Since Co is expensive, it is preferable to use a small amount of Co. It is preferable that the composite oxide (Z) contains 2 mol% or less of Co based on the total number of moles of metal elements excluding Li, or contains substantially no Co. The term "substantially no Co" means that no Co is contained at all, or that Co is mixed in as an impurity (that is, that Co is mixed in to an extent that cannot be accurately quantified).

[0025] As described above, the content of Nb in the composite oxide (Z) is 0.35 mol% or less, preferably 0.30 mol% or less, based on the total number of moles of metal elements excluding Li. If the composite oxide (Z) contains Nb, the effect of improving the charge-discharge cycle characteristics can be obtained, but the content of Nb is preferably 0.05 mol% or more. In this case, the effect of improving the charge-discharge cycle characteristics is more remarkable. If the content of Nb exceeds 0.35 mol%, the resistance increases and the charge capacity decreases.

[0026] In the composite oxide (Z), Nb preferably forms a solid solution together with other metal elements such as Ni. It is preferable that 20% or more of the Nb contained in the composite oxide (Z) is solid-dissolved in the composite oxide, more preferably 80% or more of the Nb is solid-dissolved in the composite oxide, and particularly preferably that substantially all of the Nb is solid-dissolved. The amount of Nb in solid solution can be confirmed by energy dispersive X-ray spectroscopy (EDS). The Nb contained in the composite oxide (Z) is the Nb source for the coating formed on the negative electrode surface, and a part of it is dissolved and deposited on the negative electrode surface by charge and discharge, and is contained in the coating of the negative electrode.

[0027] The composite oxide (Z) may contain a metal element other than Li, Ni, Nb, and Co. Examples of the metal element include Mn, Al, Zr, B, Mg, Fe, Cu, Zn, Sn, Na, K, Ba, Sr, Ca, W, Mo, and Si. Among them, the composite oxide (Z) preferably contains at least one of Mn and Al. When the composite oxide (Z) contains Mn, the content of Mn is preferably 1 to 10 mol% based on the total number of moles of the metal elements excluding Li. When the composite oxide (Z) contains Al, the content of Al is preferably 1 to 10 mol% based on the total number of moles of the metal elements excluding Li.

[0028] An example of a suitable composite oxide (Z) is represented by the general formula Li a Ni b Co c Al d Mn e Nb f O g (In the formula, 0.8≦a≦1.2, 0.80≦b<1, 0≦c≦0.05, 0≦d≦0.10, 0≦e≦0.10, 0< f ≦0.0035, 1≦ g ≦2). Preferably, b is a composite oxide represented by the formula: 0.85≦b<1, 0≦c≦0.02, 0< f ≦b<0.0030, more preferably 0.85≦b<0.95, 0≦c≦0.01, 0.0005≦ f ≦0.0030.

[0029] The content of the elements constituting the composite oxide (Z) 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.

[0030] The complex oxide (Z) is, for example, a secondary particle formed by agglomeration of a plurality of primary particles. The particle size of the primary particles is generally 0.05 μm to 1 μm. The volume-based median diameter (D50) of the complex oxide (Z) is, for example, 3 μm to 30 μm, preferably 5 μm to 25 μm. D50 means the particle size at which the cumulative frequency is 50% from the smallest particle size in the volume-based particle size distribution, and is also called the median diameter. The particle size distribution of the complex oxide (Z) can be measured using a laser diffraction type particle size distribution measuring device (for example, MT3000II manufactured by Microtrack Bell Co., Ltd.) using water as a dispersion medium.

[0031] The composite oxide (Z) can be synthesized by mixing and firing a compound containing Ni, Al, Mn, etc., a compound containing Nb, and a Li source such as lithium hydroxide (LiOH). A compound containing Ni, Al, Mn, etc. and a compound containing Nb may be mixed and fired to synthesize a composite oxide containing Ni and Nb, and then a Li source may be added and fired again. Examples of Nb-containing compounds include niobium hydroxide, niobium oxide, lithium niobate, and niobium chloride. The firing is performed, for example, in an oxygen atmosphere at a temperature of 600°C to 800°C.

[0032] [Negative electrode] The negative electrode 12 has a negative electrode core and a negative electrode composite layer provided on the surface of the negative electrode core. For the negative electrode core, a foil of a metal such as copper that is stable in the potential range of the negative electrode 12, a film with the metal disposed on the surface layer, or the like can be used. The negative electrode composite layer contains a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode core except for the part to which the negative electrode lead 21 is connected. The negative electrode 12 can be produced, for example, by applying a negative electrode composite slurry containing a negative electrode active material and a binder to the surface of the negative electrode core, drying the coating, and then compressing it to form a negative electrode composite layer on both sides of the negative electrode core.

[0033] The negative electrode mixture layer contains, as the negative electrode active material, for example, a carbon-based active material that reversibly absorbs and releases lithium ions. Suitable carbon-based active materials are graphites such as natural graphite, such as flake graphite, lump graphite, and earthy graphite, and artificial graphite, such as lump artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). In addition, the negative electrode active material may be a Si-based active material composed of at least one of Si and a Si-containing compound, or a carbon-based active material and a Si-based active material may be used in combination.

[0034] The binder contained in the negative electrode mixture layer may be, as in the case of the positive electrode 11, a fluororesin, PAN, polyimide, acrylic resin, polyolefin, or the like, but it is preferable to use styrene-butadiene rubber (SBR). The negative electrode mixture layer preferably further contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. Among them, it is preferable to use SBR in combination with CMC or a salt thereof, or PAA or a salt thereof.

[0035] The negative electrode 12 has a coating containing Nb (hereinafter, sometimes referred to as "negative electrode coating") formed on the surface of the negative electrode mixture layer. It is considered that the negative electrode coating is formed by depositing Nb in the complex oxide (Z) dissolved by charging and discharging on the surface of the negative electrode mixture layer. That is, the negative electrode coating contains Nb derived from the complex oxide (Z). The negative electrode coating is formed, for example, by 10 or fewer cycles of charging and discharging. By using the complex oxide (Z) containing a specific amount of Nb and forming a good quality coating containing Nb derived from the positive electrode on the surface of the negative electrode, the capacity decrease due to charging and discharging is suppressed, and good cycle characteristics are obtained. The presence of the negative electrode coating can be confirmed, for example, by X-ray photoelectron spectroscopy (XPS).

[0036] The content of Nb in the negative electrode coating is 10 ppm to 3000 ppm with respect to the total mass of the negative electrode composite layer and the coating. If the content of Nb is less than 10 ppm or exceeds 3000 ppm, the effect of improving the charge-discharge cycle characteristics cannot be obtained. The content of Nb in the negative electrode coating can be controlled by the composition of the composite oxide (Z), particularly the content of Nb, and the charge-discharge conditions. For example, the content of Nb tends to increase when the end-of-charge voltage is increased and the depth of discharge is deepened.

[0037] The negative electrode coating may further contain Ni. It is considered that Ni in the complex oxide (Z) dissolved by charging and discharging is deposited on the surface of the negative electrode mixture layer together with Nb to form the negative electrode coating. That is, the negative electrode coating contains Ni derived from the complex oxide (Z). The mass ratio (Nb / Ni) of Nb to Ni in the coating is preferably 0.3 to 2. If the Nb / Ni ratio is within this range, the effect of improving the cycle characteristics can be enhanced. The Nb / Ni ratio can be controlled by the composition of the complex oxide (Z), particularly the ratio of the contents of Nb and Ni, as well as the charge / discharge conditions, etc.

[0038] The negative electrode coating may contain metal elements other than Nb and Ni. The negative electrode coating contains, for example, metal elements such as Nb and Ni, and an organic substance that is a decomposition product of the electrolyte. The contents of Nb and Ni in the negative electrode coating and the mass ratio of Nb / Ni can be determined by removing the negative electrode from the battery after charging and discharging, dissolving the negative electrode mixture layer, and analyzing the solution by ICP-AES.

[0039] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. The material of the separator 13 is preferably a polyolefin such as polyethylene or polypropylene, or cellulose. The separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer or the like may be formed on the surface of the separator. EXAMPLES

[0040] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0041] <Example 1> [Synthesis of lithium transition metal composite oxide (positive electrode active material)] General formula Ni 0.84 Co 0.01 Al 0.052 Mn 0.098 The composite oxide was mixed with niobium hydroxide (Nb2O5·nH2O) so that the Nb content was 0.05 mol% relative to the total amount of Ni, Co, Al, and Mn in the composite oxide represented by O2, and lithium hydroxide (LiOH) was further mixed so that the molar ratio of Li to the total amount of Ni, Co, Al, Mn, and Nb was 1:1.03. The mixture was placed in a sintering furnace and sintered under an oxygen flow with an oxygen concentration of 95% (10 cm 3 The mixture was calcined from room temperature to 650°C at a temperature rise rate of 2.0°C / min (flow rate of 2 mL / min per kg and 5 L / min per kg of mixture). The mixture was then calcined from 650°C to 715°C at a temperature rise rate of 0.5°C / min, and the calcined product was washed with water to obtain a lithium transition metal composite oxide. The composition of the lithium transition metal composite oxide was analyzed by ICP-AES, and the results were as follows: Li 0.973 Ni 0.8396 Co 0.0100 Al 0.0520 Mn 0.0980 Nb 0.0005 It was O2.

[0042] [Preparation of positive electrode] The above-mentioned lithium transition metal composite oxide was used as the positive electrode active material. The positive electrode active material, acetylene black, and polyvinylidene fluoride (PVdF) were mixed in a solid content mass ratio of 95:3:2, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added, and then the mixture was kneaded to prepare a positive electrode composite slurry. The positive electrode composite slurry was applied to both sides of a positive electrode core made of aluminum foil, and the coating film was dried. The coating film was then rolled using a roller 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 in which the surface of the positive electrode core was exposed was provided in a part of the positive electrode.

[0043] [Preparation of 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 at a solid content mass ratio of 100:1:1 to prepare a negative electrode composite slurry. The negative electrode composite slurry was applied to both sides of a negative electrode core made of copper foil, and the coating film was dried. The coating film was then rolled using a roller and cut to a predetermined electrode size to obtain a negative electrode in which a negative electrode composite layer was formed on both sides of the negative electrode core. An exposed portion was provided in part of the negative electrode, where the surface of the negative electrode core was exposed.

[0044] [Preparation of non-aqueous electrolyte] A non-aqueous electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF6) at a concentration of 1.2 mol / L in a mixed solvent of ethylene carbonate (EC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC) in a volume ratio of 3:3:4.

[0045] [Preparation of test cell (non-aqueous electrolyte secondary battery)] 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 polyolefin separator interposed therebetween to prepare a wound electrode body. This electrode body was housed in an exterior body made of an aluminum laminate sheet, and the nonaqueous electrolyte was poured into it, and the opening of the exterior body was sealed to obtain a test cell.

[0046] For the above test cells, the Nb content in the coating formed on the surface of the negative electrode, the Nb / Ni ratio, and the capacity retention rate after the cycle test were evaluated by the following methods, and the evaluation results are shown in Table 1 (the same applies to the test cells of the Examples, Comparative Examples, and Reference Examples described later).

[0047] [Evaluation of negative electrode coating] The test cell after the cycle test described later was disassembled, the negative electrode was taken out, and the negative electrode composite layer was dissolved, and the Nb content and Nb / Ni ratio in the coating formed on the surface of the negative electrode composite layer were determined by ICP-AES. The Nb content in the coating was 98 ppm relative to the total mass of the negative electrode composite layer and the coating. The Nb / Ni ratio in the coating was 0.30.

[0048] [Evaluation of capacity retention rate after cycle testing] The above test cell was charged at a constant current of 0.5 It in a temperature environment of 25°C until the battery voltage reached 4.1 V, and then charged at a constant voltage until the current value at 4.1 V reached 1 / 50 It. Thereafter, the cell was discharged at a constant current of 0.5 It until the battery voltage reached 2.85 V. This charge / discharge cycle was repeated 100 times. The discharge capacity at the first cycle and the discharge capacity at the 100th cycle of the cycle test were determined, and the capacity retention rate was calculated by the following formula.

[0049] Capacity retention rate (%) = (100th cycle discharge capacity ÷ 1st cycle discharge capacity) × 100 <Example 2> In the synthesis of the positive electrode active material, the general formula Ni 0.84 Co 0.008 Al 0.052 Mn 0.1 A test cell was fabricated in the same manner as in Example 1, except that a composite oxide represented by NbO2 was used and mixed with Nb2O5·nH2O so that the Nb content was 0.15 mol %. Then, the performance of the test cell was evaluated.

[0050] <Example 3> In the synthesis of the positive electrode active material, the general formula Ni 0.88 Co 0.01 Al 0.052 Mn 0.058 A test cell was fabricated in the same manner as in Example 1, except that a composite oxide represented by Nb2O5·NH2O was used and mixed with Nb2O5·NH2O so that the Nb content was 0.12 mol %. A performance evaluation was then carried out on the test cell.

[0051] <Example 4> A test cell was prepared in the same manner as in Example 3, except that in the synthesis of the positive electrode active material, the composite oxide and Nb2O5·nH2O were mixed so that the Nb content was 0.20 mol %. Then, the performance was evaluated.

[0052] <Example 5> In the synthesis of the positive electrode active material, the general formula Ni 0.9 Co 0.01 Al 0.052 Mn 0.038 A test cell was fabricated in the same manner as in Example 1, except that a composite oxide represented by Nb2O5·NH2O was used and mixed with Nb2O5·NH2O so that the Nb content was 0.22 mol %. A performance evaluation was then carried out on the test cell.

[0053] <Example 6> In the synthesis of the positive electrode active material, the general formula Ni 0.9 Al 0.05 Mn 0.05 A test cell was fabricated in the same manner as in Example 1, except that a composite oxide represented by NbO2 was used and mixed with Nb2O5·nH2O so that the Nb content was 0.25 mol %. Then, the performance was evaluated.

[0054] <Example 7> In the synthesis of the positive electrode active material, the general formula Ni 0.912 Al 0.05 Mn 0.038 A test cell was produced in the same manner as in Example 6, except that the composite oxide represented by O2 was used, and the performance was evaluated.

[0055] <Example 8> In the synthesis of the positive electrode active material, the general formula Ni 0.915 Al 0.055 Mn 0.03 A test cell was produced in the same manner as in Example 6, except that the composite oxide represented by O2 was used, and the performance was evaluated.

[0056] <Example 9> In the synthesis of the positive electrode active material, the general formula Ni 0.92 Al 0.057 Mn 0.023A test cell was fabricated in the same manner as in Example 1, except that a composite oxide represented by Nb2O5·O2 was used and mixed with Nb2O5·nH2O so that the Nb content was 0.20 mol %. Then, the performance of the test cell was evaluated.

[0057] <Example 10> In the synthesis of the positive electrode active material, the general formula Ni 0.92 Al 0.055 Mn 0.025 A test cell was prepared in the same manner as in Example 1, except that a composite oxide represented by the formula (I) was used and mixed with Nb2O5·nH2O so that the Nb content was 0.31 mol %. In addition, the charge / discharge voltage range in the cycle test was changed to 4.2V to 2.5V.

[0058] <Example 11> In the synthesis of the positive electrode active material, the general formula Ni 0.93 Al 0.057 Mn 0.013 A test cell was prepared in the same manner as in Example 1, except that a composite oxide represented by the formula (I) was used and mixed with Nb2O5·nH2O so that the Nb content was 0.34 mol %. In addition, the charge / discharge voltage range in the cycle test was changed to 4.2V to 2.5V.

[0059] <Comparative Example 1> A test cell was produced in the same manner as in Example 1, except that Nb2O5 was not added in the synthesis of the positive electrode active material, and the performance was evaluated.

[0060] <Comparative Example 2> A test cell was produced in the same manner as in Example 3, except that Nb2O5 was not added in the synthesis of the positive electrode active material, and the performance was evaluated.

[0061] <Comparative Example 3> A test cell was produced in the same manner as in Example 6, except that Nb2O5 was not added in the synthesis of the positive electrode active material, and the performance was evaluated.

[0062] <Comparative Example 4> A test cell was produced in the same manner as in Example 7, except that Nb2O5 was not added in the synthesis of the positive electrode active material, and the performance was evaluated.

[0063] <Comparative Example 5> A test cell was produced in the same manner as in Example 8, except that Nb2O5 was not added in the synthesis of the positive electrode active material, and the performance was evaluated.

[0064] <Comparative Example 6> A test cell was produced in the same manner as in Example 9, except that Nb2O5 was not added in the synthesis of the positive electrode active material, and the performance was evaluated.

[0065] <Comparative Example 7> Except for not adding Nb2O5 in the synthesis of the positive electrode active material, a test cell was prepared in the same manner as in Example 9. In addition, the charge / discharge voltage range in the cycle test was changed to 4.2V to 2.5V.

[0066] <Comparative Example 8> A test cell was produced in the same manner as in Example 11, except that Nb2O5 was not added in the synthesis of the positive electrode active material, and the performance was evaluated.

[0067] <Reference example 1> In the synthesis of the positive electrode active material, the general formula Ni 0.6 Co 0.2 Mn 0.2 A test cell was fabricated in the same manner as in Example 1, except that a composite oxide represented by O2 was used and Nb2O5 was not added, and performance evaluation was carried out.

[0068] <Reference example 2> A test cell was prepared in the same manner as in Reference Example 1, except that in the synthesis of the positive electrode active material, Nb2O5·nH2O was added so that the Nb content was 0.25 mol %, and the performance was evaluated.

[0069] <Reference example 3> In the synthesis of the positive electrode active material, the general formula Ni 0.7 Co 0.2 Al 0.01Mn 0.09 Except for using a composite oxide represented by O2 and not adding Nb2O5, a test cell was fabricated in the same manner as in Example 1. In addition, the charge / discharge voltage range in the cycle test was changed to 4.2V to 2.5V.

[0070] <Reference example 4> A test cell was prepared in the same manner as in Reference Example 3, except that in the synthesis of the positive electrode active material, Nb2O5·nH2O was added so that the Nb content was 0.25 mol %, and the performance was evaluated.

[0071] [Table 1]

[0072] As shown in Table 1, all of the test cells of the Examples have higher capacity retention rates after cycle testing and excellent charge-discharge cycle characteristics than the corresponding test cells of the Comparative Examples (Example 1 vs. Comparative Example 1, Example 3 vs. Comparative Example 2, Example 6 vs. Comparative Example 3, Example 7 vs. Comparative Example 4, Example 8 vs. Comparative Example 5, Example 9 vs. Comparative Examples 6 and 7, and Example 11 vs. Comparative Example 8). The test cells of the Examples use a positive electrode active material containing 0.05 to 0.34 mol % (externally added) of Nb, and a coating containing Nb derived from the positive electrode active material is formed on the negative electrode surface. On the other hand, in the test cells of the Comparative Examples, Nb is not contained in the positive electrode active material, and no coating containing Nb is present on the negative electrode surface. In other words, the charge-discharge cycle characteristics of the battery are greatly improved by the positive electrode active material containing a specific amount of Nb and the negative electrode coating containing a specific amount of Nb derived from the positive electrode active material.

[0073] As shown in Reference Examples 1 to 4, when a positive electrode active material having a Ni content of less than 80 mol % and a Co content of 5 mol % or more is used, even if Nb is added to the positive electrode active material and a coating containing Nb is formed on the negative electrode surface, the capacity retention rate after the cycle test does not change and no improvement effect on the charge-discharge cycle characteristics is obtained. [Explanation of symbols]

[0074] 10 Secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode body 16 Outer can 17 Sealing body 18,19 Insulating plate 20 Positive lead 21 Negative lead 22 Grooved part 23 Internal terminal board 24 Lower valve body 25 Insulating materials 26 Upper valve 27 Cap 28 Gasket

Claims

1. A non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, The positive electrode comprises a lithium transition metal composite oxide containing Ni, Nb, and optionally Co; In the lithium transition metal composite oxide, The content of Ni is 80 mol % or more based on the total number of moles of metal elements excluding Li, The content of Nb is 0.35 mol% or less based on the total number of moles of metal elements excluding Li, The lithium transition metal composite oxide contains 2 mol % or less of Co based on the total number of moles of metal elements excluding Li, or contains substantially no Co; The negative electrode has a negative electrode mixture layer including a negative electrode active material, and a coating containing Nb formed on a surface of the negative electrode mixture layer, the Nb content in the coating relative to the total mass of the negative electrode mixture layer and the coating is 10 ppm to 3000 ppm.

2. The non-aqueous electrolyte secondary battery as described in claim 1, wherein the lithium transition metal composite oxide contains substantially no Co.

3. The non-aqueous electrolyte secondary battery as described in claim 1 or 2, wherein the negative electrode active material includes a carbon-based active material.

4. 4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the lithium transition metal composite oxide has a Ni content of 85 mol % or more.

5. 5. The nonaqueous electrolyte secondary battery according to claim 1, wherein the coating further contains Ni.

6. 6. The nonaqueous electrolyte secondary battery according to claim 5, wherein a mass ratio of Nb to Ni (Nb / Ni) in the coating is 0.3 to 2.

7. 7. The nonaqueous electrolyte secondary battery in accordance with claim 1, wherein the lithium transition metal composite oxide further contains at least one of Mn and Al.

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