Electrolyte solution for lithium ion secondary battery and lithium ion secondary battery

Abstract

The present invention addresses the problem of providing a lithium ion secondary battery having excellent cycle characteristics and corrosion resistance, which are important in an in-vehicle secondary battery such as an electric vehicle, and also addresses the problem of providing an electrolyte solution with which it is possible to produce such a lithium ion secondary battery. An electrolyte solution for a lithium ion secondary battery, which is obtained by dissolving an electrolyte in a non-aqueous solvent, and which is characterized in that the electrolyte solution for a lithium ion secondary battery contains cyanomethyl formate and / or 2-cyanoethyl formate. A lithium ion secondary battery provided with a positive electrode, a negative electrode, a separator, and the electrolyte solution for lithium ion secondary batteries.

Description

Technical Field

[0001] The present invention relates to an electrolyte for a lithium ion secondary battery which is excellent in battery characteristics such as battery cycle life and also has safety such as battery corrosion resistance, and a lithium ion secondary battery including the electrolyte. Background Art

[0002] In recent years, lithium-ion secondary batteries (hereinafter referred to as LIBs) have been advancing towards higher energy density and higher voltage. Among them, positive electrodes containing lithium composite oxides containing Ni, graphite materials or Li4Ti5O 12 Lithium ion secondary batteries having a negative electrode of titanium oxide such as tantalum (hereinafter also referred to as LTO) and a non-aqueous electrolyte solution containing lithium bis(fluorosulfonyl)imide (hereinafter also referred to as LiFSI) as an electrolyte are increasingly being used as secondary batteries for vehicles.

[0003] However, Patent Document 1 points out the problem that aluminum used as a positive electrode current collector of a lithium ion secondary battery corrodes when operating at a high voltage of more than 4.2 V in a non-aqueous electrolyte solution containing an imide-based lithium salt such as LiFSI as an electrolyte.

[0004] Patent document 2 proposes a lithium ion secondary battery that exhibits excellent cycle characteristics by using a non-aqueous electrolyte containing formate compared to a non-aqueous electrolyte in which electrolytes LiPF6 and LiBF4 are dissolved in a non-aqueous solvent (such as EC, PC, MEC, etc.).

[0005] However, the formate esters described in Patent Document 2 are compounds having hydrocarbon groups such as octyl formate, allyl formate, and 2-propynyl formate, and cyanomethyl formate and 2-cyanoethyl formate having a -C≡N group are not shown at all. In addition, Patent Document 3 also does not record cyanomethyl formate at all, but proposes an organic electrolyte solution using 2-cyanoethyl formate as a solvent (Example 1), and records that cyanoalkyl formate can be used in double-layer capacitors and electrolytic capacitors, but is easily decomposed in the reaction with lithium salts or in charging and discharging, and is not suitable for lithium batteries and lithium ion secondary batteries (Paragraph

[0012] ).

[0006] Generally, ester compounds containing any of the strong acids formic acid, sulfuric acid, and halogen elements may cause metal corrosion. When lithium-ion secondary batteries using these compounds as non-aqueous electrolytes are exposed to high voltages greater than 4.2V or temperatures above room temperature, care must be taken to prevent corrosion.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent document 1: Japanese Patent Application Laid-Open No. 2014-203748.

[0010] Patent document 2: Japanese Patent No. 4899862.

[0011] Patent document 3: Japanese Patent No. 4221088. Summary of the invention

[0012] Problems to be solved by the invention

[0013] The object of the present invention is to solve the above-mentioned problems and provide a lithium ion secondary battery having excellent cycle characteristics and corrosion resistance, which are important in secondary batteries for vehicles such as electric vehicles. In addition, the object of the present invention is to provide an electrolyte solution that can be used to make such a lithium ion secondary battery.

[0014] Means for solving problems

[0015] The inventors have conducted intensive research and found that in a lithium ion secondary battery having a positive electrode, a negative electrode, a separator and a non-aqueous electrolyte in which an electrolyte is dissolved in a non-aqueous solvent, by adding cyanomethyl formate (hereinafter, also referred to as CMF) and / or 2-cyanoethyl formate (hereinafter, also referred to as CEF) to the non-aqueous electrolyte, the cycle characteristics of the battery can be improved and a lithium ion secondary battery with excellent corrosion resistance can be obtained. In the past, it was completely unknown that when cyanomethyl formate and / or 2-cyanoethyl formate were added to a non-aqueous solvent, as an electrolyte for lithium ion secondary batteries, the cycle characteristics of the battery could be improved and a battery with excellent corrosion resistance could be made. The electrolyte discovered by the inventors is an electrolyte for lithium ion secondary batteries for the purpose of an electrolyte for lithium ion secondary batteries. The lithium ion secondary battery obtained by using the electrolyte for lithium ion secondary batteries also has excellent corrosion resistance when used under high voltage conditions greater than 4.2V, and also has excellent corrosion resistance when used above room temperature.

[0016] Furthermore, the inventors have discovered that the corrosion resistance of lithium-ion secondary batteries can be further improved by combining one or more selected from the group consisting of the phosphonate compound (I), carbonate compound (II), oxalate compound (III) and methanesulfonate compound (IV) described in the present invention with a non-aqueous electrolyte containing cyanomethyl formate and / or 2-cyanoethyl formate.

[0017] That is, the present invention is defined by the following aspects.

[0018] (1) An electrolyte for a lithium ion secondary battery, wherein the electrolyte for a lithium ion secondary battery is a lithium ion secondary battery electrolyte in which an electrolyte is dissolved in a non-aqueous solvent, characterized in that the electrolyte for a lithium ion secondary battery contains cyanomethyl formate and / or 2-cyanoethyl formate.

[0019] (2) The electrolyte for lithium ion secondary batteries as described in (1) above, characterized in that the electrolyte for lithium ion secondary batteries contains at least one selected from the group consisting of a phosphonate compound represented by the following formula (I), a carbonate compound represented by the following formula (II), an oxalate compound represented by the following formula (III), and a methanesulfonate compound represented by the following formula (IV),

[0020]

[0021] (wherein, A and B are independently, A represents methyl, ethyl, 2-cyanoethyl, 1-cyanoethyl, 2-cyano-2-propyl or 2-propynyl, and B represents methyl, ethyl, vinyl or cyanomethyl),

[0022]

[0023] (wherein, C and D are independently, C represents methyl or ethyl, and D represents 2-cyanoethyl, 1-cyanoethyl, 2-cyano-2-propyl or 2-propynyl),

[0024]

[0025] (wherein E represents 2-cyanoethyl, 1-cyanoethyl, 2-cyano-2-propyl or 2-propynyl),

[0026]

[0027] (wherein F represents 2-cyanoethyl, 1-cyanoethyl, 2-cyano-2-propyl or 2-propynyl).

[0028] (3) An electrolyte for a lithium ion secondary battery as described in (2) above, characterized in that the total content of the phosphonate compound represented by formula (I), the carbonate compound represented by formula (II), the oxalate compound represented by formula (III), and the methanesulfonate compound represented by formula (IV) is 0.01 to 10% by mass.

[0029] (4) A lithium ion secondary battery comprising a positive electrode, a negative electrode, a separator, and the electrolyte for lithium ion secondary battery according to any one of (1) to (3) above.

[0030] Effects of the Invention

[0031] For the electrolyte for lithium ion secondary batteries of the present invention, in a lithium ion secondary battery having a positive electrode, a negative electrode, a separator and a non-aqueous electrolyte in which an electrolyte is dissolved in a non-aqueous solvent, by containing cyanomethyl formate and / or 2-cyanoethyl formate in the non-aqueous electrolyte, the cycle characteristics of the lithium ion secondary battery can be improved. Thus, a lithium ion secondary battery with excellent cycle characteristics can be made. In addition, by containing cyanomethyl formate and / or 2-cyanoethyl formate in the non-aqueous electrolyte, the corrosion resistance of the lithium ion secondary battery can be improved. Thus, a lithium ion secondary battery with excellent corrosion resistance can be made. Further, for the electrolyte for lithium ion secondary batteries of the present invention, by combining one or more selected from the group consisting of the phosphonate compound (I), the carbonate compound (II), the oxalate compound (III) and the methanesulfonate compound (IV) in the present invention with cyanomethyl formate and / or 2-cyanoethyl formate, a lithium ion secondary battery with better corrosion resistance can be made. DETAILED DESCRIPTION

[0032] The following examples illustrate the embodiments and configurations of the present invention, but the present invention is not limited thereto, and all are included in the present invention as long as they meet the intent described in the claims, technical means for solving the problems, effects of the invention, etc.

[0033] The non-aqueous electrolyte solution includes an electrolyte and a non-aqueous solvent. As the electrolyte in the present invention, there is no particular limitation, for example, LiN (SO2F) 2 (hereinafter, also referred to as LiFSI) with a SO2 group, LiOSO2F with a SO3 group, LiOSO3CH3, LiOSO3C2H5 with a SO4 group, LiPF6, LiPO2F2, lithium difluorobis (oxalate) phosphate (LiDFOP) with phosphorus (P), LiBF4 with boron (B), lithium bis (oxalate) borate (LiBOB), lithium difluoro (oxalate) borate (LiDFOB), LiAsF6 with arsenic (As) and other lithium salts and other electrolyte salts can be cited. In the present invention, one electrolyte can be used, or two or more electrolytes can be used in combination.

[0034] In the present invention, by adding cyanomethyl formate and / or 2-cyanoethyl formate to the non-aqueous electrolyte, the corrosion resistance to metals such as aluminum is improved, and therefore, LiFSI with high chemical thermal stability and capable of improving battery performance under high temperature conditions can be mostly used. The lithium salt can be used alone or in combination with two or more. In Li salts other than LiFSI (hereinafter also referred to as other Li salts), it is preferred to add a prescribed amount to have the effect of auxiliaryly improving battery performance under low temperature conditions. As a preferred combination of these lithium salts, a combination of two types of lithium salts with SO2 groups and lithium salts with phosphorus (P) or a combination of three types of lithium salts with SO2 groups, lithium salts with SO4 groups, and lithium salts with phosphorus (P). Specifically, a combination of three types of LiFSI, LiPF6, and LiPO2F2, and a combination of four types of LiFSI, LiiOSO3CH3, LiPF6, and LiPO2F2 are more preferred. When LiFSI and other Li salts are used, the mass ratio of LiFSI to other Li salts is preferably 100:0 to 1:99, more preferably 100:0 to 50:50, and most preferably 100:0 to 70:30. In addition, the total concentration of the electrolyte is preferably dissolved in the range of 0.5 to 3 mol, more preferably 1 to 2 mol, relative to the total volume 1L of the electrolyte for the lithium ion secondary battery of the present invention.

[0035] As the non-aqueous solvent in the present invention, it is not particularly limited, for example, cyclic carbonate, chain carbonate etc. can be cited. As cyclic carbonate, for example, preferably ethylene carbonate (EC), fluoroethylene carbonate (FEC), vinylene carbonate (VC), propylene carbonate (PC) etc. are cited. As chain carbonate, for example, preferably dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC) etc. are cited. As the non-aqueous solvent in the present invention, these solvents can also be used as the main solvent and other solvents are used as secondary solvents. As other secondary solvents used in combination with the main solvent, preferably γ-butyrolactone, 1,3-propane sultone (PS) etc. of the cyclic compound with the effect of improving ion conductivity, ethyl formate, propyl formate, isopropyl formate, propargyl formate (2-propynyl formate) etc. of the chain compound with viscosity lower than DMC are cited.

[0036] These solvents can be used alone or in combination of two or more. As preferred combinations of these cyclic carbonates, examples of two combinations include a combination of EC and VC, a combination of EC and FEC, a combination of PC and FEC, a combination of EC and PC, a combination of PC and DMC, etc., and other solvents can also be added to these two combinations. As examples of three combinations, a combination of EC, PC and VC; a combination of EC, PC and FEC; a combination of EC, PC and DMC; a combination of EC, EMC and DMC, etc., and other solvents can also be added to these three combinations. As examples of four combinations, a combination of EC, PC, FEC and VC; a combination of EC, PC, DMC and EMC, etc. can be mentioned, and other solvents can also be added to these four combinations.

[0037] When the cyclic carbonate of the nonaqueous electrolyte of the present invention includes linear carbonate, for the ratio of cyclic carbonate and linear carbonate, from the viewpoint of improving the electrochemical characteristics under the temperature conditions of a wide range from high temperature to low temperature, cyclic carbonate: linear carbonate (volume ratio) is preferably 10:90 to 50:50, more preferably 20:80 to 40:60.

[0038] The cyanomethyl formate used in the present invention has two characteristics. The first is that it is lower than the molecular weight of DMC used as one of the main solvents, which is 90. In addition, it is 85, which is smaller than the molecular weight of VC, an additive currently used worldwide, which is 86. The molar amount of the additive plays a role in electrochemistry, so it is important to obtain a high additive effect with a small amount of addition, both from the performance and cost aspects. The second is that the oxidation decomposition potential of VC is 4.85V, compared with the oxidation decomposition potential of cyanomethyl formate is 5.4V, and the reduction decomposition potential of VC is 0.8V, compared with the reduction decomposition potential of cyanomethyl formate is 1.1V, so cyanomethyl formate is difficult to oxidatively decompose compared with VC and is an easily reduced compound. At present, in the process of high voltage development, it is important to be difficult to oxidatively decompose on the Ni positive electrode, and it is advantageous from the performance aspect to reduce and decompose as early as possible on the highly active graphite negative electrode to form a protective film.

[0039] The content of cyanomethyl formate and / or 2-cyanoethyl formate in the electrolyte for lithium ion secondary batteries of the present invention is not particularly limited. When it is too little, the formation of the protective coating of the negative electrode is insufficient, so the decline of the cycle characteristics and corrosion resistance will be affected. Therefore, relative to the gross mass of the electrolyte for lithium ion secondary batteries of the present invention, the appropriate content of cyanomethyl formate and / or 2-cyanoethyl formate is preferably 0.01 mass % or more, more preferably 0.1 mass % or more, and can also be 0.5 mass % or more. In addition, relative to the gross mass of the electrolyte for lithium ion secondary batteries, the upper limit is preferably 80 mass % or less, preferably 60 mass % or less, more preferably 30 mass % or less, more preferably 10 mass % or less, more preferably 5 mass % or less, and can also be 3 mass % or less. As the preferred range of the content of cyanomethyl formate and / or 2-cyanoethyl formate relative to the total mass of the electrolyte for lithium ion secondary batteries of the present invention, 0.01-80 mass%, 0.01-60 mass%, 0.01-30 mass%, 0.01-10 mass%, 0.01-5 mass%, 0.01-3 mass%, 0.1-80 mass%, 0.1-60 mass%, 0.1-30 mass%, 0.1-10 mass%, 0.1-5 mass%, 0.1-3 mass%, 0.5-10 mass%, 0.5-5 mass%, 0.5-3 mass%, etc. can be cited. The electrolyte for lithium ion secondary batteries of the present invention can improve the cycle characteristics of lithium ion secondary batteries when used for lithium ion secondary batteries by containing cyanomethyl formate and / or 2-cyanoethyl formate in the electrolyte. In addition, the corrosion resistance of lithium ion secondary batteries can be improved. When cyanomethyl formate or 2-cyanoethyl formate is used alone, the above numerical ranges related to the content of cyanomethyl formate and / or 2-cyanoethyl formate represent numerical ranges relative to them, and when both cyanomethyl formate and 2-cyanoethyl formate are used, it represents the numerical range of the total of the two.

[0040] In order to further improve the corrosion resistance of the lithium ion secondary battery, the electrolyte for the lithium ion secondary battery of the present invention preferably contains, in addition to cyanomethyl formate and / or 2-cyanoethyl formate, at least one compound selected from the group consisting of a phosphonate compound represented by the following formula (I), a carbonate compound represented by the following formula (II), an oxalate compound represented by the following formula (III), and a methanesulfonate compound represented by the following formula (IV).

[0041]

[0042] (In the formula, A and B are independently, A represents methyl, ethyl, 2-cyanoethyl (propionitrile), 1-cyanoethyl, 2-cyano-2-propyl or 2-propynyl (propargyl), and B represents methyl, ethyl, vinyl or cyanomethyl.)

[0043] The phosphonate compounds represented by the formula (I) are 24 compounds as shown in Table 1.

[0044] Table 1

[0045]

[0046]

[0047] (wherein, C and D are independently, C represents methyl or ethyl, and D represents 2-cyanoethyl (propionitrile), 1-cyanoethyl, 2-cyano-2-propyl or 2-propynyl (propargyl))

[0048] The carbonate compounds represented by the formula (II) are 8 kinds as shown in Table 2.

[0049] Table 2

[0050]

[0051]

[0052] (wherein E represents 2-cyanoethyl (propionitrile), 1-cyanoethyl, 2-cyano-2-propyl or 2-propynyl (propargyl))

[0053] The oxalate compounds represented by the formula (III) are four types as shown in Table 3.

[0054] Table 3

[0055] E <![CDATA[-CH2CH2C≡N]]> <![CDATA[-C(CH3)2C≡N]]> <![CDATA[-CH(CH3)C≡N]]> <![CDATA[-CH2C≡CH]]>

[0056]

[0057] (wherein F represents 2-cyanoethyl (propionitrile), 1-cyanoethyl, 2-cyano-2-propyl or 2-propynyl (propargyl))

[0058] Table 4 shows the four types of methanesulfonate compounds represented by formula (IV).

[0059] Table 4

[0060] F <![CDATA[-CH2CH2C≡N]]> <![CDATA[-C(CH3)2C≡N]]> <![CDATA[-CH(CH3)C≡N]]> <![CDATA[-CH2C≡CH]]>

[0061] The reason why the above-mentioned phosphonate compound (I), carbonate compound (II), oxalate compound (III), and methanesulfonate compound (IV) are preferably used in combination is not beyond the scope of speculation. It is believed that a strong adsorption layer is formed on the surface of metals such as aluminum to prevent contact with corrosive compounds. Relative to the total mass of the electrolyte for lithium ion secondary batteries of the present invention, the total content of at least one selected from these 40 species, that is, a combination of one or more of two or more, is preferably 0.01 mass% or more, more preferably 0.1 mass% or more, and most preferably 0.5 mass% or more. In addition, relative to the total mass of the electrolyte for lithium ion secondary batteries, the upper limit is preferably 10 mass% or less, more preferably 8 mass% or less, and most preferably 5 mass% or less. As the preferred range of the total content of these compounds, 0.01-10 mass%, 0.1-8 mass%, 0.5-5 mass%, etc. can be cited relative to the total mass of the electrolyte for lithium ion secondary batteries of the present invention. The electrolyte for lithium ion secondary battery of the present invention can further improve the corrosion resistance of lithium ion secondary battery when used in lithium ion secondary battery by containing at least one compound selected from phosphonate compound, carbonate compound, oxalate compound and methanesulfonate compound in the present invention, and can further improve the corrosion resistance when used under high voltage conditions greater than 4.2V and the corrosion resistance when used under conditions above room temperature. In addition to containing electrolyte, non-aqueous solvent, cyanomethyl formate and / or 2-cyanoethyl formate and compounds represented by formula (I), (II), (III) and (IV), the electrolyte for lithium ion secondary battery of the present invention can also contain other components within the range that can be used as electrolyte.

[0062] According to the present invention, even a non-aqueous electrolyte containing 1,3-propane sultone, which is generally highly corrosive and has concerns about its use, can be used without impairing the corrosion resistance of LIB. When a graphite negative electrode is used in a battery, the 1,3-propane sultone has the effect of inhibiting the reductive decomposition of EC and PC on the graphite negative electrode, so it is preferably added in a range of 0.1 to 5% by mass relative to the total non-aqueous electrolyte.

[0063] In addition, according to the present invention, in the past, although there was an effect of inhibiting corrosion, as the amount of addition increased, the cycle characteristics decreased, so even if it contained dinitriles with a carbon chain length of 2 to 5 such as succinonitrile, glutaronitrile, adiponitrile, pimelonitrile, etc., hexamethylene diisocyanate (HMDI), 1,3-bis (isocyanate methyl) cyclohexane (a mixture of cis and trans), N, N'-diisopropylcarbodiimide (DIC), N, N'-dicyclohexylcarbodiimide (DCC) and other carbodiimides, it can be used without damaging the cycle characteristics of LIB. Preferably, these compounds are added in the range of 0.1 to 5 mass % relative to the total non-aqueous electrolyte.

[0064] The lithium ion secondary battery of the present invention comprises a positive electrode, a negative electrode, a separator and an electrolyte for the lithium ion secondary battery of the present invention. The positive electrode, the negative electrode and the separator in the present invention are not particularly limited as long as they can be used in lithium ion secondary batteries. As the separator in the present invention, it is most preferred to use a separator composed of a microporous membrane formed by a polyolefin material such as polypropylene and polyethylene, and a non-woven fabric separator can also be used. The porous sheet and the non-woven fabric can be a single layer or a multi-layer structure, and an oxide such as aluminum oxide can also be coated on the surface of the separator. The thickness of the separator needs to be as thin as possible to increase the volume energy density of the battery. Therefore, it is preferably less than 20 μm, and particularly preferably less than 10 μm.

[0065] As the negative electrode active material used in the negative electrode of the present invention, in order to increase the volume energy density, graphite materials such as natural graphite and artificial graphite, and carbon materials such as hard carbon and soft carbon are preferably used. In addition, in order to make the rapid charge and discharge good, it is preferred to use Li4Ti5O which does not expand and contract during charge and discharge. 12 Titanium oxides with spinel structures such as (LTO), TiNb2O7, Ti2Nb 10 O 29 of titanium oxide.

[0066] As the binder used in the negative electrode composite material, for example, ethylene-propylene-diene terpolymer (EPDM), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), copolymer of styrene and butadiene (SBR), copolymer of acrylonitrile and butadiene (NBR), carboxymethyl cellulose (CMC), etc. can be cited. For example, after these binders are kneaded with the negative electrode active material to form a slurry-like negative electrode composite material, the negative electrode composite material is applied to the copper foil or aluminum foil of the collector, dried, press-formed, and then, for example, heated at 80° C. under vacuum to make the negative electrode.

[0067] Examples of the positive electrode active material used in the positive electrode of the present invention include LiCoO2 (LCO), LiCoO2 in which a portion of Co is replaced with Ni, and the like. 1 / 3 Ni 1 / 3 Mn 1 / 3 O2、LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523), LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), LiNi 0.8 Co0.15 Al 0.05 O2(NCA), LiNi without Co 0.5 Mn 1.5 O4、LiNi 0.8 Mn 0.13 Ti 0.02 Mg 0.02 Nb 0.01 Mo 0.02 O2 (HE-LNMO), etc. In order to improve the volume energy density, as the positive electrode active material of the lithium composite oxide containing Ni with an atomic ratio of more than 50%, NCM523, NCM622, NCM811, NCA, HE-LNMO, etc. are preferably used. In addition, in order to make fast charging and discharging good, LiMn2O4 (LMO) with a spinel structure and LiFePO4 (LFP) with an olivine structure are preferably used.

[0068] As the conductive aid used in the positive electrode composite material, for example, carbon black such as acetylene black and Ketjen black, carbon nanotubes, carbon fibers, activated carbon, graphite and other known or commercially available conductive aids can be cited. As a binder, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVFF), copolymers of styrene and butadiene (SBR), copolymers of acrylonitrile and butadiene (NBR), carboxymethyl cellulose (CMC) and the like can be cited. For example, after these conductive aids and binders are kneaded with the positive electrode active material to form a slurry-like positive electrode composite material, the positive electrode composite material is applied to an aluminum foil as a collector, dried, press-formed, and then, for example, heated at 80°C under vacuum to make a positive electrode. As long as the battery can be assembled without using a binder, a binder may not be used.

[0069] As the combination of the positive electrode active material and the negative electrode active material in the present invention, in order to improve the volume energy density, preferably there is a combination of LCO and graphite, NCM523 and graphite, NCM622 and graphite, NCM811 and graphite, NCA and graphite, HE-LNMO and graphite, etc. In addition, in order to make fast charge and discharge good, preferably there is a combination of NCM811 and LTO, HE-LNMO and LTO, LFP and LTO, etc.

[0070] The other materials in the lithium ion secondary battery of the present invention are not particularly limited as long as they can be used in lithium ion secondary batteries. The current collector used in the present invention is not particularly limited, preferably aluminum foil, copper foil, or a porous current collector can be used to improve the permeability of the electrolyte.

[0071] In the present invention, the solvent used for the binder is not particularly limited, and various solvents can be selected according to the active material or binder used. Specifically, when PVDF is used as the binder, N-methyl-2-pyrrolidone is preferably used as the solvent. On the other hand, when a rubber-based binder such as styrene-butadiene rubber (SBR), polytetrafluoroethylene, polyvinyl alcohol, carboxymethyl cellulose (CMC) is used, water is preferably cited as the solvent.

[0072] The structure of the lithium secondary battery of the present invention is not particularly limited, and the shape of the secondary battery having a positive electrode, a negative electrode and a separator can include a coin-shaped battery, a cylindrical battery, a square battery, a pouch-shaped battery, etc. In addition, it is also applicable to a clay-shaped (clay-shaped) pouch-shaped lithium ion secondary battery in which two electrode layers of a clay-shaped positive electrode and a negative electrode are separated by a separator instead of a sheet-shaped positive electrode and a negative electrode.

[0073] Example

[0074] Then, examples and comparative examples are listed to specifically illustrate the present invention, but these do not limit the present invention in any way. In addition, in the examples, comparative examples, and Tables 5 to 8, PC represents propylene carbonate, DMC represents dimethyl carbonate, EC represents ethylene carbonate, EMC represents ethyl methyl carbonate, FEC represents fluoroethylene carbonate, PS represents 1,3-propane sultone, HMDI represents hexamethylene diisocyanate, DCC represents N,N'-dicyclohexylcarbodiimide, LiFSI represents LiN(SO2F)2, and NCM523 represents LiNi 0.5 Co 0.2 Mn 0.3 O2, NCM811 represents LiNi 0.8 Co 0.1 Mn 0.1 O2, LCO means LiCoO2.

[0075] [Examples 1-1 to 1-18]

[0076] 〔Preparation of electrolyte〕

[0077] LiPF6 and LiFSI are dissolved in a non-aqueous solvent in which PC and DMC are mixed in a ratio of 1:2 (volume ratio). To the solution thus obtained, cyanomethyl formate (CMF) (manufactured by Tokyo Chemical Industry Co., Ltd.) is added to prepare the electrolyte of Example 1-1. The content of CMF in the prepared electrolyte is 0.1 mass %, the content of LiPF6 is 0.5 mol / L, and the content of LiFSI is 0.5 mol / L. Furthermore, electrolytes of Examples 1-2 to 1-18 are prepared so that the content of CMF in the prepared electrolyte is as described in Table 5. The mass % of CMF and the mass % of compounds other than CMF in Table 5 represent the respective proportions relative to the total mass of the prepared electrolyte, and the M (mol / L) of the electrolyte in Table 5 represents the respective proportions relative to the total volume of the prepared electrolyte. In Examples 1-2 to 1-3, except for the content of CMF, the electrolyte is prepared in the same manner as in Example 1-1. In Example 1-4, PC and CMF were mixed at a volume ratio of 1:2, and then LiPF6 and LiFSI were dissolved, and the contents of CMF, LiPF6 and LiFSI were set to the amounts described in Table 5. In Example 1-5, an electrolyte was prepared in the same manner as in Example 1-1, except that a non-aqueous solvent in which EC and DMC were mixed at a volume ratio of 1:2 was used, and the content of CMF was set to the amount described in Table 5. In Examples 1-6 to 1-18, LiPF6 and LiFSI were dissolved in a non-aqueous solvent in which PC and DMC were mixed at a volume ratio of 1:2, and then CMF and the compounds represented by formula (I), (II), (III) or (IV) in the present invention were added, wherein A, B, C, D, E and F were the compounds described in the column of the type of compound other than CMF in Table 5 to prepare an electrolyte. The contents of CMF and these compounds as well as LiPF6 and LiFSI in the prepared electrolyte are shown in Table 5. Examples 1-6 to 1-18 are examples in which the phosphonate, carbonate, oxalate or methanesulfonate of the present invention is added in addition to CMF.

[0078] [Comparative Examples 1-1 to 1-9]

[0079] The electrolyte solutions of Comparative Examples 1-1 to 1-9 were prepared without adding CMF. In Comparative Example 1-1, the electrolyte solution was prepared in the same manner as in Example 1-1, except that CMF was not added. In Comparative Example 1-2, the electrolyte solution was prepared in the same manner as in Example 1-1, except that octyl formate was added in the amount shown in Table 5 instead of CMF. In Comparative Examples 1-4, 1-5, 1-6, 1-7, 1-8 and 1-9, the electrolyte solutions were prepared in the same manner as in Examples 1-6, 1-7, 1-8, 1-9, 1-11 and 1-12, respectively, except that CMF was not added. In Comparative Example 1-3, the electrolyte solution was prepared in the same manner as in Comparative Example 1-1, except that a non-aqueous solvent in which EC and DMC were mixed in a ratio of 1:2 (volume ratio) was used.

[0080] 〔Manufacturing of lithium-ion secondary batteries (LIB) and measurement of battery characteristics〕

[0081] According to the ratio of 93% by mass of NCM523 (positive electrode active material), 3% by mass of acetylene black (conductive aid), and 4% by mass of polyvinylidene fluoride (binder), 1-methyl-2-pyrrolidone is added thereto to make a slurry, and a positive electrode composite material is prepared and coated on an aluminum foil. Then, it is dried and press-formed to prepare a positive electrode. Similarly, 98% by mass of artificial graphite (negative electrode active material), 1% by mass of a copolymer of styrene and butadiene as a binder, and 1% by mass of carboxymethyl cellulose are added to water and mixed to form a slurry, and a negative electrode composite material is prepared and coated on a copper foil. Then, it is dried, press-formed, and heat-treated to prepare a negative electrode sheet. Then, a 3-layer microporous film with a thickness of 20 microns and a polyethylene sandwiched between polypropylene is used as the separator, and the electrolytes of Examples 1-1 to 1-14 and Comparative Examples 1-1 to 1-9 are injected respectively to make coin-type batteries (coin-type LIB: diameter 20 mm, thickness 3.2 mm).

[0082] Using a charge and discharge device ACD-MO1A (manufactured by ASKA Electronic Co., Ltd.), the coin-type battery was charged at a constant current and constant voltage of 1C rate to an upper limit voltage of 4.3V at 25°C, and then discharged at a 1C rate to a lower limit voltage of 3.0V, and the charge and discharge were repeated. The discharge capacity of the 5th cycle was calculated as a relative ratio compared with the discharge capacity of the 5th cycle when a non-aqueous solvent in which EC and DMC were mixed at a volume ratio of 1:2 (volume ratio) and CMF and compounds other than CMF were not added (Comparative Examples 1-3). For the cycle characteristics (%), the obtained capacity (mAh / g) was digitized as the 50th cycle / 5th cycle × 100. The results are shown in Table 5.

[0083] Table 5

[0084]

[0085] According to the results of Table 5, in batteries using graphite negative electrodes, when using PC-based electrolytes, excellent cycle characteristics are obtained by adding CMF to the electrolyte (Examples 1-1 to 1-4 and 1-6 to 1-18). On the other hand, in the examples where CMF is not added, the graphite negative electrode is damaged, and the secondary battery cannot be charged and discharged satisfactorily (Comparative Example 1-1). In addition, even if n-octyl formate having the same formic acid group as CMF is added, the graphite negative electrode is partially damaged, and sufficient cycle characteristics cannot be obtained as a secondary battery (Comparative Example 1-2). Even when a compound selected from the phosphonate, carbonate, oxalate and methanesulfonate of the present invention is added, the graphite negative electrode is damaged when CMF is not added, and the secondary battery cannot be charged and discharged satisfactorily (Comparative Examples 1-4 to 1-9). On the other hand, excellent cycle characteristics are obtained by also using these compounds in combination with CMF (Examples 1-6 to 1-18). It should be noted that the results of Comparative Examples 1-3 in which EC was used as a non-aqueous solvent were used as a reference for determining the discharge capacity ratio of the 5th cycle. EC is solid at room temperature, while PC is liquid at low temperature. Therefore, the cycle characteristics of PC-based electrolytes can be improved, which can expand the temperature range of application of electrolytes. In addition, the cycle characteristics of Comparative Examples 1-3 were 98%, and the capacity decreased after the 50th cycle. In contrast, the capacity decrease of Example 1-5 in which EC was used as a non-aqueous solvent was also small after the 50th cycle.

[0086] [Examples 2-1 to 2-16]

[0087] 〔Preparation of electrolyte〕

[0088] LiFSI is dissolved in a non-aqueous solvent in which EC, EMC and DMC are mixed in a ratio of 3:3:4 (volume ratio). CMF is added to the solution thus obtained to prepare the electrolyte of Example 2-1. The content of CMF in the prepared electrolyte is 1.3% by mass, and the content of LiFSI is 1.2 mol / L. In Example 2-2, LiPF6 is added as an electrolyte instead of LiFSI, and the electrolyte is prepared in the same manner as in Example 2-1. In Example 2-3, an electrolyte is prepared in the same manner as in Example 2-1, except that a non-aqueous solvent in which PC, EMC and DMC are mixed in a ratio of 3:3:4 (volume ratio) is used. In Examples 2-4 to 2-16, LiFSI is dissolved in a non-aqueous solvent in which EC, EMC and DMC are mixed in a ratio of 3:3:4 (volume ratio), and then CMF and the compounds represented by formula (I), (II), (III) or (IV) of the present invention are added, wherein A, B, C, D, E and F are the compounds A, B, C, D, E and F recorded in the column of the type of compounds other than CMF in Table 6 to prepare an electrolyte. The contents of CMF, these compounds and LiFSI in the prepared electrolyte are as described in Table 6, and Examples 2-4 to 2-16 are examples of adding phosphonates, carbonates, oxalates or methanesulfonates in the present invention in addition to CMF. The mass % of CMF and the mass % of compounds other than CMF in Table 6 represent the respective proportions relative to the total mass of the prepared electrolyte, and the M (mol / L) of the electrolyte in Table 6 represents the respective proportions relative to the total volume of the prepared electrolyte.

[0089] [Comparative Examples 2-1 to 2-4]

[0090] The electrolytes of Comparative Examples 2-1 to 2-4 were prepared without adding CMF. In Comparative Example 2-1, the electrolyte was prepared in the same manner as in Example 2-1 except that CMF was not added. In addition, in Comparative Example 2-2, the electrolyte was prepared in the same manner as in Example 2-1 except that octyl formate was added in a manner to replace CMF in a content shown in Table 6. In Comparative Example 2-3, the electrolyte was prepared in the same manner as in Example 2-1 except that FEC was added in a manner to replace CMF in a content shown in Table 6. In Comparative Example 2-4, the electrolyte was prepared in the same manner as in Example 2-1 except that A and B in the compound represented by formula (I) of the present specification were set to the compounds described in Table 6 instead of CMF in a manner to replace CMF in a content shown in Table 6.

[0091] [Determination of Corrosion Resistance]

[0092] The measurement was performed using a three-electrode beaker cell in which the working electrode, counter electrode, and reference electrode were Al, Li, and Li, respectively. As the electrolyte, the electrolytes of Examples 2-1 to 2-12 and Comparative Examples 2-1 to 2-4 were used. The measurement conditions were 25°C, 5mV / sec, 4.5 to 3.0V, and the current value until 10 cycles was obtained by cyclic voltammetry (CV). The obtained current value was digitized as the 10th cycle / 2nd cycle × 100%. The results are shown in Table 6.

[0093] Table 6

[0094]

[0095] According to the results of Table 6, in the examples in which CMF was added to the electrolyte (Examples 2-1, 2-2 and 2-3), the numerical value indicating corrosion resistance was reduced and the corrosion resistance was improved compared to the example in which CMF was not added (Comparative Example 2-1). Furthermore, in the examples in which compounds selected from the phosphonates, carbonates, oxalates and methanesulfonates of the present invention were added in addition to CMF (Examples 2-4 to 2-16), the numerical value indicating corrosion resistance was reduced and the corrosion resistance was further improved. As described above, according to the results of Table 6, it was found that by using CMF, the numerical value indicating corrosion resistance was reduced, and the corrosion resistance of LIB could be improved. In addition, it was found that by auxiliaryly combining compounds selected from the phosphonates, carbonates, oxalates and methanesulfonates described in the present invention with CMF, the numerical value indicating corrosion resistance was further reduced, and the corrosion resistance of LIB could be further improved.

[0096] On the other hand, it is known that the corrosion resistance is significantly deteriorated in Comparative Example 2-2 in which n-octyl formate described in Patent Document 2 having the same formic acid group as CMF is added, Comparative Example 2-3 in which FEC having a fluorine group is added, and Comparative Example 2-4 in which B is -CH2COOC2H5, which is outside the scope of the present invention, although having the same phosphonic acid skeleton. Therefore, it is not enough to have a formic acid group and a phosphonic acid skeleton, but what is important is the combination with a specific skeleton.

[0097] [Example 3-1]

[0098] LiFSI, LiPF6, LiPO2F2 and LiOSO3CH3 are dissolved in a non-aqueous solvent prepared by mixing EC, PC, PS, EMC and DMC in a ratio of 2:1:0.5:3:3.5 (volume ratio). To the solution thus obtained, CMF and the compound represented by formula (II) in the present invention, wherein C and D are the compounds C and D recorded in the column of the type of compound other than CMF in Table 7, are added to prepare an electrolyte. The contents of CMF and compounds other than CMF and LiFSI, LiPF6, LiPO2F2 and LiOSO3CH3 in the prepared electrolyte are as shown in Table 7. The mass % of CMF and the mass % of compounds other than CMF in Table 7 represent the respective proportions relative to the total mass of the prepared electrolyte, and the M (mol / L) of the electrolyte in Table 7 represents the respective proportions relative to the total volume of the prepared electrolyte. The prepared electrolyte, the positive electrode active material and the negative electrode active material recorded in Table 7 are used to make a coin-type battery the same as in Example 1-1, and the battery characteristics are measured. In addition, a three-electrode beaker cell similar to that of Example 2-1 was prepared and the corrosion resistance was measured. The results are shown in Table 7.

[0099] [Example 3-2]

[0100] LiFSI is dissolved in a non-aqueous solvent in which PC, FEC and CMF are mixed in a ratio of 1:1:8 (volume ratio). To the solution thus obtained, A and B in the compound represented by formula (I) in the present invention are compounds A and B recorded in the type column of compounds other than CMF in Table 7 to prepare an electrolyte. The contents of CMF, compounds other than CMF and LiFSI in the prepared electrolyte are as shown in Table 7. A coin-type battery is made using the prepared electrolyte, the positive electrode active material and the negative electrode active material recorded in Table 7. Acetylene black is used as the conductive material of the negative electrode active material, PVDF is used as the binder, the mass ratio of the negative electrode active material, the conductive material and the binder is 85:10:5, and N-methylpyrrolidone is used as the solvent to make a slurry. After applying the prepared slurry on aluminum foil, it is dried, press-formed and heat-treated to obtain a negative electrode sheet, and a coin-type battery the same as Example 3-1 is made. The charge and discharge conditions were: charging at a constant current and constant voltage of 1C rate at 45°C to an upper limit voltage of 2.8V, and then changing to discharging at a 1C rate to a lower limit voltage of 1.4V, and measuring the battery characteristics. In addition, a three-electrode cup-shaped battery similar to Example 2-1 was prepared and the corrosion resistance was measured. The results are shown in Table 7.

[0101] Table 7

[0102]

[0103] According to Table 7, it was found that even a non-aqueous electrolyte solution containing a fluorine-containing compound FEC and 1,3-propane sultone (PS) as a SO 3 -containing compound in addition to the highly corrosive LiFSI can improve the cycle characteristics and corrosion resistance.

[0104] [Example 4-1]

[0105] LiFSI, LiPF6 and LiOSO3CH3 are dissolved in a non-aqueous solvent prepared by mixing EC, EMC and DMC in a ratio of 3:3:3 (volume ratio). To the solution thus obtained, CMF and the compound represented by formula (II) in the present invention, in which C and D are the compounds C and D recorded in the column of the types of compounds other than CMF in Table 8, are added to prepare an electrolyte. The contents of CMF and compounds other than CMF, as well as LiFSI and LiPF6 in the prepared electrolyte are as shown in Table 8. The mass % of CMF and the mass % of compounds other than CMF in Table 8 represent the respective proportions relative to the total mass of the prepared electrolyte, and the M (mol / L) of the electrolyte in Table 8 represents the respective proportions relative to the total volume of the prepared electrolyte. The prepared electrolyte, the positive electrode active material and the negative electrode active material described in Table 8 were used to prepare the same coin-shaped battery as in Example 1-1. The charging and discharging conditions were as follows: charging at a constant current and constant voltage of 1C rate to an upper limit voltage of 4.45V at 25°C, and then changing to discharging at a 1C rate to a lower limit voltage of 3.0V, and the battery characteristics were measured. In addition, a three-electrode cup-shaped battery was prepared in the same manner as in Example 2-1, and the corrosion resistance was measured. The results are shown in Table 8.

[0106] [Example 4-2]

[0107] A portion of EMC was replaced with ethyl formate, and LiFSI, LiPF6, and LiOSO3CH3 were dissolved in a non-aqueous solvent prepared by mixing EC, EMC, DMC, and ethyl formate at a volume ratio of 3:2:4:1. The electrolyte was prepared in the same manner as in Example 4-1, and the battery characteristics and corrosion resistance were measured. The content of each component in the electrolyte and the results of the measurement are shown in Table 8. In Example 4-2, a compound represented by formula (III) in which E is E listed in the "Type of Compounds Other Than CMF" column of Table 8, and a compound represented by formula (IV) in which F is F listed in the "Type of Compounds Other Than CMF" column of Table 8 were added.

[0108] Table 8

[0109]

[0110] [Example 4-3]

[0111] The electrolyte solution was prepared in the same manner as in Example 4-2 except that CMF was changed to 2-cyanoethyl formate (CEF), and the battery characteristics and corrosion resistance were measured. Table 9 shows the contents of the components in the electrolyte solution and the results of the measurements.

[0112] Table 9

[0113]

[0114] According to Tables 8 and 9, it was found that the cycle characteristics and corrosion resistance were improved even for a high voltage battery with an upper limit voltage of 4.45 V. According to the results of Tables 7 to 9, it is expected that even when a large number of LIBs such as automotive batteries are used, excellent cycle characteristics and corrosion resistance are maintained for a long time.

[0115] Industrial Applicability

[0116] By using the non-aqueous electrolyte of the present invention, the battery has excellent battery characteristics such as cycle characteristics, and can also have safety such as corrosion resistance. According to the present invention, in the case of long-term use of a large number of lithium-ion secondary batteries such as automotive batteries, the contribution of the present invention is immeasurable.

Claims

1. An electrolyte for a lithium ion secondary battery, wherein the electrolyte for a lithium ion secondary battery is an electrolyte for a lithium ion secondary battery dissolved in a non-aqueous solvent, characterized in that: The electrolyte for lithium ion secondary batteries contains cyanomethyl formate and / or 2-cyanoethyl formate.

2. The electrolyte for lithium ion secondary battery according to claim 1, characterized in that The electrolyte for lithium ion secondary batteries contains at least one selected from the group consisting of a phosphonate compound represented by the following formula (I), a carbonate compound represented by the following formula (II), an oxalate compound represented by the following formula (III), and a methanesulfonate compound represented by the following formula (IV), In formula (I), A and B are independently, A represents methyl, ethyl, 2-cyanoethyl, 1-cyanoethyl, 2-cyano-2-propyl or 2-propynyl, B represents methyl, ethyl, vinyl or cyanomethyl, In formula (II), C and D are independently methyl or ethyl, and D is 2-cyanoethyl, 1-cyanoethyl, 2-cyano-2-propyl or 2-propynyl. In formula (III), E represents 2-cyanoethyl, 1-cyanoethyl, 2-cyano-2-propyl or 2-propynyl, In the formula (IV), F represents a 2-cyanoethyl group, a 1-cyanoethyl group, a 2-cyano-2-propyl group or a 2-propynyl group.

3. The electrolyte for lithium ion secondary battery according to claim 2, characterized in that The total content of the phosphonate compound represented by formula (I), the carbonate compound represented by formula (II), the oxalate compound represented by formula (III), and the methanesulfonate compound represented by formula (IV) is 0.01 to 10% by mass.

4. A lithium ion secondary battery, wherein: The lithium ion secondary battery comprises a positive electrode, a negative electrode, a separator, and the electrolyte for a lithium ion secondary battery according to any one of claims 1 to 3.

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