Nonaqueous electrolyte for secondary battery and nonaqueous electrolyte secondary battery

Abstract

This nonaqueous electrolyte secondary battery is provided with a positive electrode, a negative electrode, and a nonaqueous electrolyte. The nonaqueous electrolyte contains a carboxylic acid ester and lithium bisoxalato borate. The concentration of the carboxylic acid ester is not less than 0.01% by volume but less than 10% by volume relative to the volume of the nonaqueous solvent. In addition, the concentration of the lithium bisoxalato borate is not less than 0.01 M but less than 0.2 M.

Description

[Technical Field]

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

[0002] In non-aqueous electrolyte secondary batteries such as lithium ion batteries, it is known that the non-aqueous electrolyte has a significant effect on battery performance such as input / output characteristics, capacity, and cycle characteristics. For example, Patent Document 1 discloses a non-aqueous electrolyte secondary battery equipped with a non-aqueous electrolyte containing a chain carboxylic acid ester in an amount of 3 to 30% by volume relative to the volume of the non-aqueous solvent. Patent Document 1 describes the effect of obtaining excellent low-temperature output characteristics. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. WO2016 / 084357 Summary of the Invention

[0004] In non-aqueous electrolyte secondary batteries, gas may be generated due to decomposition of the non-aqueous solvent during charging and discharging, and degassing may be necessary. Furthermore, if the amount of gas generated becomes large, problems such as battery swelling due to the gas or a decrease in capacity due to gas being trapped between the electrodes may occur. The carboxylic acid ester used in the non-aqueous electrolyte secondary battery of Patent Document 1 has a high dielectric constant and low viscosity, which contributes to improving input / output characteristics, but is prone to reductive decomposition during initial charging. Therefore, non-aqueous electrolyte secondary batteries using carboxylic acid esters have the problem of generating a large amount of gas.

[0005] The non-aqueous electrolyte for a secondary battery according to the present disclosure is a non-aqueous electrolyte containing a non-aqueous solvent, and includes a carboxylic acid ester and lithium bis(oxalato)borate, wherein the concentration of the carboxylic acid ester is 0.01% by volume or more and less than 10% by volume relative to the volume of the non-aqueous solvent, and the concentration of the lithium bis(oxalato)borate is 0.01 M or more and less than 0.2 M. Note that the volume ratios here are values ​​at 25° C. and 1 atmosphere.

[0006] The non-aqueous electrolyte secondary battery according to the present disclosure includes the non-aqueous electrolyte, a positive electrode, and a negative electrode.

[0007] The nonaqueous electrolyte for secondary batteries according to the present disclosure can suppress the amount of gas generation in nonaqueous electrolyte secondary batteries using carboxylic acid esters. Nonaqueous electrolyte secondary batteries including the nonaqueous electrolyte according to the present disclosure have excellent input / output characteristics and generate little gas during initial charging. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a perspective view showing the appearance of a nonaqueous electrolyte secondary battery as an example of the embodiment. [Figure 2] FIG. 2 is a perspective view of an electrode assembly according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] The present inventors conducted extensive research to develop a nonaqueous electrolyte secondary battery that exhibits excellent input / output characteristics and generates little gas during initial charging, and as a result, discovered that the desired battery performance can be achieved by adding specific amounts of a carboxylic acid ester and lithium bis(oxalato)borate to a nonaqueous electrolyte. As described above, while carboxylic acid esters contribute to improving input / output characteristics, they are prone to reductive decomposition during initial charging. However, the nonaqueous electrolyte according to the present disclosure specifically suppresses gas generation.

[0010] Hereinafter, an example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail with reference to the drawings. It is anticipated from the beginning that the following embodiments and modifications may be selectively combined.

[0011] Fig. 1 is a perspective view showing the appearance of a nonaqueous electrolyte secondary battery 10 according to an embodiment, and Fig. 2 is a perspective view of an electrode assembly 11 constituting the nonaqueous electrolyte secondary battery 10. The nonaqueous electrolyte secondary battery 10 shown in Fig. 1 includes a bottomed, rectangular cylindrical outer can 14 as an outer casing, but the outer casing is not limited to this. The nonaqueous electrolyte secondary battery according to the present disclosure may be, for example, a cylindrical battery including a bottomed, cylindrical outer can, a coin-shaped battery including a coin-shaped outer can, or a laminate battery including an outer casing made of a laminate sheet including a metal layer and a resin layer.

[0012] As shown in FIGS. 1 and 2 , a nonaqueous electrolyte secondary battery 10 includes an electrode assembly 11, a nonaqueous electrolyte, a bottomed rectangular cylindrical outer can 14 that houses the electrode assembly 11 and the nonaqueous electrolyte, and a sealing plate 15 that closes the opening of the outer can 14. The nonaqueous electrolyte secondary battery 10 is a so-called prismatic battery. The electrode assembly 11 has a wound structure in which a positive electrode 20 and a negative electrode 30 are wound with a separator 40 interposed therebetween. The positive electrode 20, the negative electrode 30, and the separator 40 are all strip-shaped, long bodies, and the positive electrode 20 and the negative electrode 30 are wound with the separator 40 interposed therebetween. The electrode assembly may also be a stacked type in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked one by one with separators interposed therebetween.

[0013] The nonaqueous electrolyte secondary battery 10 includes a positive electrode terminal 12 electrically connected to the positive electrode 20 via a positive electrode current collector 25, and a negative electrode terminal 13 electrically connected to the negative electrode 30 via a negative electrode current collector 35. In this embodiment, the sealing plate 15 has an elongated rectangular shape, with the positive electrode terminal 12 disposed at one longitudinal end of the sealing plate 15 and the negative electrode terminal 13 disposed at the other longitudinal end of the sealing plate 15. The positive electrode terminal 12 and the negative electrode terminal 13 are external connection terminals electrically connected to other nonaqueous electrolyte secondary batteries 10, various electronic devices, etc., and are attached to the sealing plate 15 via insulating members.

[0014] For ease of explanation, the height direction of the outer can 14 will be referred to as the "vertical direction" of the nonaqueous electrolyte secondary battery 10, the sealing plate 15 side will be referred to as the "top," and the bottom side of the outer can 14 will be referred to as the "bottom." Additionally, the direction along the longitudinal direction of the sealing plate 15 will be referred to as the "lateral direction" of the nonaqueous electrolyte secondary battery 10.

[0015] The outer can 14 is a metal container in the shape of a rectangular cylinder with a bottom. An opening formed at the top end of the outer can 14 is closed, for example, by welding a sealing plate 15 to the edge of the opening. The sealing plate 15 is generally provided with a liquid injection portion 16 for injecting a non-aqueous electrolyte, a gas exhaust valve 17 that opens to exhaust gas in the event of a battery abnormality, and a current interruption mechanism. The outer can 14 and the sealing plate 15 are made of a metal material containing, for example, aluminum as a main component.

[0016] The electrode body 11 is a flat, wound electrode body including a flat portion and a pair of curved portions. The electrode body 11 is housed in the outer can 14 with the winding axis direction aligned with the lateral direction of the outer can 14 and the width direction of the electrode body 11, along which the pair of curved portions are aligned, aligned with the height direction of the battery. In this embodiment, a positive electrode side current collector formed by laminating the substrate exposed portion 23 of the positive electrode 20 at one axial end of the electrode body 11 and a negative electrode side current collector formed by laminating the substrate exposed portion 33 of the negative electrode 30 at the other axial end are formed, and each current collector is electrically connected to a terminal via a current collector. Note that an insulating electrode body holder (insulating sheet) may be disposed between the electrode body 11 and the inner surface of the outer can 14.

[0017] [Positive electrode] The positive electrode 20 includes a positive electrode core 21 and a positive electrode composite layer provided on the surface of the positive electrode core 21. The positive electrode core 21 can be a foil of a metal that is stable within the potential range of the positive electrode 20, such as aluminum or an aluminum alloy, or a film with such a metal disposed on the surface layer. The positive electrode composite layer includes a positive electrode active material, a conductive material, and a binder, and is preferably provided on both sides of the positive electrode core 21. The positive electrode 20 can be produced, for example, by applying a positive electrode composite slurry containing a positive electrode active material, a conductive material, a binder, etc., onto the positive electrode core 21, drying the coating, and then compressing it to form positive electrode composite layers on both sides of the positive electrode core 21.

[0018] A lithium transition metal composite oxide is used as the positive electrode active material. Metal elements contained in the lithium transition metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, and W. Among these, it is preferable to contain at least one of Ni, Co, and Mn. Examples of suitable composite oxides include lithium transition metal composite oxides containing Ni, Co, and Mn, and lithium transition metal composite oxides containing Ni, Co, and Al.

[0019] Examples of conductive materials contained in the positive electrode mixture layer include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of binders contained in the positive electrode mixture layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, and polyolefin resins. These resins may be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, and polyethylene oxide (PEO).

[0020] [Negative electrode] The negative electrode 30 has a negative electrode core 31 and a negative electrode composite layer provided on the surface of the negative electrode core 31. For the negative electrode core 31, a foil of a metal such as copper that is stable within the potential range of the negative electrode 30, or a film with such a metal disposed on the surface layer, 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 31. The negative electrode 30 can be produced, for example, by applying a negative electrode composite slurry containing a negative electrode active material, a conductive material, a binder, etc. to the surface of the negative electrode core 31, drying the coating, and then compressing it to form a negative electrode composite layer on both sides of the negative electrode core 31.

[0021] 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 include natural graphite such as flake graphite, lump graphite, and amorphous graphite, and artificial graphite such as lump graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). The negative electrode active material may also be a Si-based active material composed of at least one of Si and a Si-containing compound, or a combination of a carbon-based active material and a Si-based active material.

[0022] As in the case of the positive electrode 20, the conductive material contained in the negative electrode mixture layer can be a carbon material such as carbon black, acetylene black, ketjen black, or graphite. As in the case of the positive electrode 20, the binder contained in the negative electrode mixture layer can be a fluororesin, PAN, polyimide, acrylic resin, polyolefin, or the like, but styrene-butadiene rubber (SBR) is preferred. Furthermore, 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 these, it is preferable to use SBR in combination with CMC or a salt thereof, or PAA or a salt thereof.

[0023] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 40. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 40 include polyethylene, polypropylene, polyolefins such as copolymers of ethylene and α-olefins, and cellulose. The separator 40 may have either a single-layer structure or a laminated structure. A heat-resistant layer containing inorganic particles, or a heat-resistant layer made of a highly heat-resistant resin such as aramid resin, polyimide, or polyamideimide, may be formed on the surface of the separator 40.

[0024] [Non-aqueous electrolyte] The non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt. Examples of the non-aqueous solvent that can be used include ethers, esters, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. Examples of the halogen-substituted compound include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP).

[0025] 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, methylphenyl ether, and the like. Examples of suitable ethers include chain ethers such as ethyl 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.

[0026] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate, and chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate. Among these, it is preferable to use at least one selected from EC, EMC, and DMC, and it is particularly preferable to use a mixed solvent of EC, EMC, and DMC. For example, the EC content is 20% by volume or more and 30% by volume or less relative to the volume of the non-aqueous medium. For example, the EMC and DMC content is 30% by volume or more and 40% by volume or less relative to the volume of the non-aqueous medium.

[0027] The non-aqueous solvent also contains a carboxylic acid ester as an essential component. The carboxylic acid ester may be a cyclic carboxylic acid ester such as γ-butyrolactone (GBL) or γ-valerolactone (GVL), but is preferably a chain carboxylic acid ester. The carboxylic acid ester is contained in an amount of 0.01% by volume or more and less than 10% by volume relative to the volume of the non-aqueous solvent. Adding 0.01% by volume or more of a carboxylic acid ester, particularly a chain carboxylic acid ester, to the non-aqueous electrolyte improves the input / output characteristics of the battery.

[0028] The content of the carboxylic acid ester is preferably 0.1% by volume or more, more preferably 0.5% by volume or more, and particularly preferably 1% by volume or more, relative to the volume of the non-aqueous solvent. The upper limit of the content of the carboxylic acid ester is preferably 8% by volume, more preferably 6% by volume, and particularly preferably 5% by volume, relative to the volume of the non-aqueous solvent. Note that if the amount of the carboxylic acid ester added is 10% by volume or more, it becomes difficult to suppress the amount of gas generated.

[0029] The chain carboxylic acid ester is preferably a compound having 3 to 10 carbon atoms. Specific examples include methyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, n-propyl propionate, isopropyl propionate, n-butyl propionate, isobutyl propionate, t-butyl propionate, methyl butyrate, ethyl butyrate, n-propyl butyrate, isopropyl butyrate, methyl isobutyrate, ethyl isobutyrate, n-propyl isobutyrate, and isopropyl isobutyrate. Among these, at least one selected from methyl acetate, ethyl acetate, methyl propionate, and ethyl propionate is preferred, with methyl acetate and methyl propionate being particularly preferred.

[0030] The non-aqueous electrolyte also contains lithium bis(oxalato)borate (LiBOB) as an essential component. The concentration of LiBOB in the non-aqueous electrolyte is 0.01M (mol / L) or more and less than 0.2M. Adding 0.01M or more of LiBOB to the non-aqueous electrolyte specifically suppresses the decomposition of the carboxylic acid ester, significantly reducing the amount of gas generated during initial charging. The combined use of the carboxylic acid ester and LiBOB enables both improved input / output characteristics of the battery and reduced gas generation.

[0031] The concentration of LiBOB in the nonaqueous electrolyte is preferably 0.015 M or more, more preferably 0.018 M or more, and particularly preferably 0.020 M or more. The upper limit of the LiBOB concentration is preferably 0.15 M, more preferably 0.10 M, and particularly preferably 0.08 M. Note that adding LiBOB at a concentration of 0.2 M or more only results in a small effect in reducing the amount of gas generation, and an excessive amount of LiBOB degrades input / output characteristics.

[0032] In addition to LiBOB, the nonaqueous electrolyte preferably contains other lithium salts as electrolyte salts, such as LiBF, LiClO, LiPF, LiAsF, LiSbF, LiAlCl, LiSCN, LiFSO, LiCFSO, LiCFCO, Li(P(C0)F), Li(P(C0)F), Li(P(C0)), and LiPF. 6-x (C n F 2n+1 ) x (1 <x<6,nは1または2)、LiB 10 Cl 10 Examples of suitable lithium borates include LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylates, and borates such as LiBO and Li(B(C0)F). Of these, LiPF is preferred. The concentration of LiPF is preferably higher than that of LiBOB.

[0033] An example of a suitable non-aqueous electrolyte contains the following components:

[0034] <Non-aqueous solvent> 1% to 5% by volume of at least one of methyl acetate and methyl propionate 20% to 30% by volume of EC EMC between 30% and 40% by volume 30% to 40% by volume of DMC <Lithium salt> LiBOB between 0.02M and 0.08M 0.5M to 1.5M LiPF6 <Example> The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0035] Example 1 [Preparation of positive electrode] The positive electrode active material is LiNi 1 / 3 Co 1 / 3 Mn 1 / 3A lithium transition metal composite oxide represented by O2 was used. The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a solids mass ratio of 90:7:3, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode composite slurry. Next, the positive electrode composite slurry was applied to both sides of a positive electrode core made of aluminum foil, the coating was dried and compressed, and then cut to the specified electrode size (50 x 234 mm). The coating was then peeled off from the area where the aluminum lead was attached, resulting in a positive electrode with a positive electrode composite layer formed on both sides of the positive electrode core.

[0036] [Preparation of negative electrode] Graphite was used as the negative electrode active material. The negative electrode active material, carboxymethyl cellulose (CMC), and styrene butadiene rubber (SBR) were mixed in a solids mass ratio of 98:1:1, and water was used as a dispersion medium to prepare a negative electrode composite slurry. Next, the negative electrode composite slurry was applied to both sides of a negative electrode core made of copper foil, the coating was dried, compressed with a predetermined force, and then cut to the specified electrode size (52 × 330 mm). Further, the coating at the portion where the nickel lead was attached was peeled off, resulting in a negative electrode with a negative electrode composite layer formed on both sides of the negative electrode core.

[0037] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and methyl propionate (MP) were mixed in a volume ratio of 25:37:35:3 (25°C, 1 atmosphere). LiPF6 and lithium bis(oxalato)borate (LiBOB) were added to the mixed solvent to a concentration of 1.15 M and 0.025 M, respectively, to obtain a nonaqueous electrolyte solution.

[0038] [Test cell construction] An aluminum lead was attached to the exposed core portion of the positive electrode, and a nickel lead was attached to the exposed core portion of the negative electrode, and the positive and negative electrodes were spirally wound with a separator interposed therebetween, and then pressed radially to produce a flat wound electrode assembly. This electrode assembly was housed in an exterior body made of an aluminum laminate sheet, and the nonaqueous electrolyte was poured into it. The opening of the exterior body was then sealed to obtain a test cell (nonaqueous electrolyte secondary battery).

[0039] [Evaluation of gas generation rate] The test cell, whose volume was measured by Archimedes' method, was initially charged (CCCV charging until the battery voltage reached 3.7 V) in a temperature environment of 25°C, and then subjected to an aging treatment in which the cell was left in this charged state for 11 hours in a temperature environment of 75°C. The volume of the test cell after the aging treatment was measured by Archimedes' method, and the amount of gas generated was calculated from the difference with the volume before the initial charge. The amount of gas generated is shown in Table 1 as a relative value, with the amount of gas generated in the test cell of Reference Example 2 described below being set to 100 (the same applies to the following Examples, etc.).

[0040] <Example 2> A test cell was produced in the same manner as in Example 1, except that the concentration of LiPF6 in the preparation of the non-aqueous electrolyte was changed to 0.9M, and the amount of gas generated was evaluated.

[0041] Example 3 A test cell was produced in the same manner as in Example 2, except that in preparing the non-aqueous electrolyte, vinylene carbonate (VC) was added to give a concentration of 0.3 mass % relative to the mass of the non-aqueous electrolyte, and the amount of gas generated was evaluated.

[0042] Example 4 A test cell was produced in the same manner as in Example 2, except that the concentration of LiBOB in the preparation of the non-aqueous electrolyte was changed to 0.04M, and the amount of gas generated was evaluated.

[0043] <Comparative Example 1> A test cell was produced in the same manner as in Example 2, except that LiBOB was not added in the preparation of the non-aqueous electrolyte, and the amount of gas generated was evaluated.

[0044] <Comparative Example 2> A test cell was produced in the same manner as in Comparative Example 1, except that in preparing the non-aqueous electrolyte, VC was added to a concentration of 0.3 mass % relative to the mass of the non-aqueous electrolyte, MP was not added, and the volume ratio of EC, EMC, and DMC was 26:38:36, and the amount of gas generated was evaluated.

[0045] <Comparative Example 3> A test cell was produced in the same manner as in Example 1, except that in preparing the non-aqueous electrolyte, LiBOB and MP were not added and the volume ratio of EC, EMC, and DMC was set to 30:30:40, and the amount of gas generated was evaluated.

[0046] <Reference example 1> A test cell was produced in the same manner as in Example 1, except that in the preparation of the non-aqueous electrolyte, the concentration of LiBOB was changed to 0.07M, MP was not added, and the volume ratio of EC, EMC, and DMC was set to 30:30:40, and the amount of gas generated was evaluated.

[0047] <Reference example 2> A test cell was produced in the same manner as in Reference Example 1, except that in the preparation of the non-aqueous electrolyte, the concentration of LiBOB was changed to 0.05M and VC was added so as to have a concentration of 0.3 mass% relative to the mass of the non-aqueous electrolyte, and the amount of gas generated was evaluated.

[0048] [Table 1]

[0049] As shown in Table 1, the amount of gas generated during initial charging was significantly reduced in all of the test cells of the examples compared to the test cells of the comparative examples. MP, a carboxylic acid ester, contributes to improving input / output characteristics but is prone to reductive decomposition during initial charging. However, in the test cells of the examples, gas generation was suppressed to a level equal to or greater than that of the test cells of the reference examples, which do not contain MP. Note that, because the test cells of the reference examples do not contain MP, their input / output characteristics were inferior to those of the test cells of the examples. [Explanation of symbols]

[0050] 10 Nonaqueous electrolyte secondary battery 11 Electrode body 12 Positive terminal 13 Negative terminal 14 Outer can 15 Sealing plate 16 Injection section 17 Gas exhaust valve 20 positive electrode 21 Positive electrode core 23,33 Exposed core part 25 Positive electrode current collector 30 negative electrode 31 Negative electrode core 35 Negative electrode current collector 40 Separator

Claims

1. A non-aqueous electrolyte containing a non-aqueous solvent, a carboxylic acid ester; lithium bis(oxalato)borate; Including, a concentration of the carboxylic acid ester is 1% by volume or more and less than 10% by volume relative to the volume of the non-aqueous solvent; The concentration of the lithium bis(oxalato)borate is 0.01 M or more and less than 0.2 M, The non-aqueous electrolyte for a secondary battery, wherein the carboxylic acid ester is at least one selected from the group consisting of methyl acetate, ethyl acetate, methyl propionate, and ethyl propionate.

2. 2. The non-aqueous electrolyte for a secondary battery according to claim 1, wherein the carboxylic acid ester is methyl propionate.

3. The nonaqueous electrolyte for a secondary battery according to claim 1 or 2; A positive electrode and a negative electrode; A non-aqueous electrolyte secondary battery comprising:

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