Electrolytic solution for lithium ion secondary batteries, and lithium ion secondary battery

Abstract

The present disclosure addresses the problem of providing lithium ion secondary batteries that have excellent cycle characteristics which are important for secondary batteries for vehicles such as electric vehicles, and providing an electrolytic solution that can be used for production of such lithium ion secondary batteries. This electrolytic solution is for lithium ion secondary batteries, has an electrolyte dissolved in a non-aqueous solvent, and contains cyanomethyl formate and vinylene carbonate. This lithium ion secondary battery comprises a positive electrode, a negative electrode, a separator, and the electrolytic solution for lithium ion secondary batteries.

Description

Electrolyte for lithium ion secondary battery and lithium ion secondary battery

[0001] The present disclosure relates to an electrolyte for a lithium ion secondary battery that is excellent in battery characteristics such as the cycle life of the battery, and a lithium ion secondary battery that includes the electrolyte.

[0002] In recent years, lithium-ion secondary batteries (hereinafter referred to as LIBs) have been developed to have higher energy densities and voltages. In particular, developments are being carried out to improve their cycle life and high-temperature storage characteristics.

[0003] In Patent Document 1, an electrolyte LiPF is dissolved in a non-aqueous solvent (e.g., EC, PC, MEC, etc.). ６ and LiBF ４ A lithium ion secondary battery has been proposed that exhibits excellent cycle characteristics by using a non-aqueous electrolyte containing a formic acid ester instead of a non-aqueous electrolyte containing a formic acid ester.

[0004] However, the formic acid esters described in Patent Document 1 are compounds having a hydrocarbon group such as octyl formate, allyl formate, and 2-propynyl formate, and cyanomethyl formate having a —C≡N group is not disclosed anywhere in Patent Document 1.

[0005] Japanese Patent No. 4899862

[0006] An electrolyte solution for a lithium ion secondary battery according to one embodiment of the present disclosure is an electrolyte solution for a lithium ion secondary battery in which an electrolyte salt is dissolved in a non-aqueous solvent, and contains cyanomethyl formate and vinylene carbonate.

[0007] A lithium ion secondary battery according to one embodiment of the present disclosure includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte solution in which an electrolyte salt is dissolved in a non-aqueous solvent, and the lithium ion secondary battery electrolyte solution in which the electrolyte salt is dissolved in the non-aqueous solvent contains cyanomethyl formate and vinylene carbonate.

[0008] FIG. 1 is a cross-sectional view showing the structure of a lithium ion secondary battery.

[0009] The following provides examples of embodiments and configurations of the present disclosure, but the present disclosure is not limited to these, and all embodiments that are in line with the intent of the claims, problem-solving means, effects of the invention, etc. are included in the present disclosure.

[0010] The present disclosure solves the above-mentioned problems and provides a lithium-ion secondary battery with excellent cycle characteristics, which are important for secondary batteries for in-vehicle use such as electric vehicles, residential storage batteries, or large-scale power storage systems, and also provides an electrolyte solution capable of producing such a lithium-ion secondary battery.

[0011] The electrolyte solution for a lithium ion secondary battery is a non-aqueous electrolyte solution in which an electrolyte salt is dissolved in a non-aqueous solvent. The electrolyte solution for a lithium ion secondary battery of the present disclosure contains cyanomethyl formate and vinylene carbonate (VC). By using the electrolyte solution for a lithium ion secondary battery of the present disclosure, a lithium ion secondary battery with excellent cycle characteristics can be produced.

[0012] The electrolyte salt is not particularly limited, but for example, SO ２ LiN(SO ２ F) ２ (hereinafter also referred to as LiFSI), etc. ３ LiOSO having a group ２ F et al., SO ４ LiOSO having a group ３ CH ３ , LiOSO ３ C ２ H ５ LiPF with phosphorus (P) ６ , LiPO ２ F ２ , lithium difluorobis(oxalato)phosphate (LiDFOP), etc., and boron (B)-containing LiBF ４ , lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), and arsenic (As)-containing LiAsF ６ In the present disclosure, one type of electrolyte salt may be used, or two or more types may be used in combination.

[0013] As the electrolyte salt, the lithium salt may be used alone or in combination of two or more kinds. Examples of the lithium salt include LiFSI and LiPF ６ , LiN(SO ２ F) ２ , LiPO ２ F ２ , LiOSO ３ CH ３ If the concentration of the electrolyte salt is too low, the lithium ions moving through the electrolyte solution are few, and the battery performance is reduced. On the other hand, if the concentration of the electrolyte salt is too high, the viscosity of the electrolyte solution increases, making it difficult for the lithium ions to move, and the battery performance is reduced. The electrolyte salt is LiPF ６ and LiN(SO ２ F) ２ The total concentration of the electrolyte salt may be in the range of 0.5 to 3 mol, or in the range of 0.8 to 2 mol, per 1 L of volume of the non-aqueous solvent.

[0014] In the present disclosure, LiFSI can be used in large amounts because it has high chemical thermal stability and can improve battery performance at high temperatures due to improved corrosion resistance against metals such as aluminum. It is preferable to add a certain amount of Li salts other than LiFSI because they have the effect of supplementarily improving battery performance at low temperatures. A suitable combination of these lithium salts is SO ２ a combination of two types of lithium salts having a group and a lithium salt having phosphorus (P), or SO ２ Lithium salts with a group, SO ４ A combination of three types of lithium salts, lithium salts having a group, and lithium salts having phosphorus (P) is preferred.

[0015] 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.

[0016] The nonaqueous solvent in the present disclosure is not particularly limited as long as it is a solvent generally used as a nonaqueous solvent for lithium-ion battery electrolytes, but examples thereof include cyclic carbonates, chain carbonates, and the like. Suitable examples of cyclic carbonates include ethylene carbonate (EC), fluoroethylene carbonate (FEC), and propylene carbonate (PC). Suitable examples of chain carbonates include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). The nonaqueous solvent may also contain the cyclic compound gamma-butyrolactone (GBL). The nonaqueous solvent may also contain at least two selected from ethylene carbonate (EC), propylene carbonate (PC), and gamma-butyrolactone (GBL). EC has a flash point of 143°C, and PC has a flash point of 133°C, making it a cyclic carbonate with a flash point of 80°C or higher. Furthermore, GBL has a flash point of 100°C, and is a cyclic compound with a flash point of 80°C or higher. The non-aqueous solvent contains at least two selected from EC, PC, and GBL in a total amount of 80 to 100% by volume. The above volume % is a ratio to the total volume of the non-aqueous solvent. In the present disclosure, the non-aqueous solvent preferably contains at least two selected from EC, PC, and GBL in a total amount of 90 to 100% by volume.

[0017] If the volume ratio of EC in the nonaqueous electrolyte is too high, the electrolyte will become solid at room temperature. Therefore, the volume ratio of EC to PC and GBL in the nonaqueous solvent may be EC / (PC+GBL) = 65 / 35 to 10 / 90, or 40 / 60 to 20 / 80.

[0018] The nonaqueous electrolyte solution of the present disclosure may contain additives in addition to the nonaqueous solvent. Examples of additives that can be mixed with the main solvent include fluoroethylene carbonate, succinic anhydride, maleic anhydride, biphenyl, and LiBF. ２ (C ２ O ４ ), LiB(C ２ O ４ ) ２(LiBOB), 1,3-propane sultone, and ethylene sulfate. The nonaqueous solvent may contain 10% by weight or less of the additive, 5% by weight or less, or 3% by weight or less, based on 100% by weight of the entire nonaqueous solvent.

[0019] The cyanomethyl formate used in this disclosure has two characteristics. First, its molecular weight is smaller than that of DMC, which is used as one of the main solvents (90). Furthermore, its molecular weight (85) is smaller than that of VC, an additive currently used worldwide (86). Because additives act electrochemically in molar amounts, achieving high additive effects with small amounts is important from both performance and cost perspectives. Second, its oxidative decomposition potential is 4.85 V for VC, while that of cyanomethyl formate is 5.4 V. Its reductive decomposition potential is 0.8 V for VC, while that of cyanomethyl formate is 1.1 V. This means that cyanomethyl formate is a compound that is more resistant to oxidative decomposition and more easily reduced than VC. With the current trend toward higher voltages, it is important that cyanomethyl formate is resistant to oxidative decomposition on a Ni positive electrode. Furthermore, its rapid reductive decomposition and formation of a protective coating on a highly active graphite negative electrode are advantageous from a performance perspective.

[0020] The content of cyanomethyl formate in the electrolyte for lithium ion secondary batteries of the present disclosure is not particularly limited, but if it is too low, the formation of a protective film on the negative electrode will be insufficient, resulting in reduced cycle characteristics. If the content of cyanomethyl formate is too high, the protective film on the negative electrode will become thick, increasing the resistance of the negative electrode and resulting in reduced cycle characteristics. The content of cyanomethyl formate relative to the total weight of the non-aqueous solvent is not particularly limited, but may be 0.01 to 10 wt %, 0.01 to 7 wt %, or 0.1 to 5 wt %. By including cyanomethyl formate in the electrolyte, the electrolyte for lithium ion secondary batteries of the present disclosure can improve the corrosion resistance of lithium ion secondary batteries.

[0021] In order to further improve corrosion resistance in lithium ion secondary batteries, the electrolyte solution for lithium ion secondary batteries of the present disclosure may contain, in addition to cyanomethyl formate, at least one compound selected from the group consisting of a phosphonate ester compound represented by formula (I) below, a carbonate ester compound represented by formula (II) below, an oxalate ester compound represented by formula (III) below, and a methanesulfonate ester compound represented by formula (IV) below.

[0022] (In the formula, A and B each independently represent a methyl group, an ethyl group, a 2-cyanoethyl group (propionitrile group), a 1-cyanoethyl group, a 2-cyano-2-propyl group, or a 2-propynyl group (propargyl group), and B represents a methyl group, an ethyl group, a vinyl group, or a cyanomethyl group.) There are 24 types of phosphonate ester compounds represented by formula (I) as shown in Table 1.

[0023] (In the formula, C and D each independently represent a methyl group or an ethyl group, and D represents a 2-cyanoethyl group (propionitrile group), a 1-cyanoethyl group, a 2-cyano-2-propyl group, or a 2-propynyl group (propargyl group).) The carbonate ester compounds represented by formula (II) are eight types shown in Table 2.

[0024] (In the formula, E represents a 2-cyanoethyl group (propionitrile group), a 1-cyanoethyl group, a 2-cyano-2-propyl group, or a 2-propynyl group (propargyl group).) The four oxalic acid ester compounds represented by formula (III) are shown in Table 3.

[0025] (In the formula, F represents a 2-cyanoethyl group (propionitrile group), a 1-cyanoethyl group, a 2-cyano-2-propyl group, or a 2-propynyl group (propargyl group).) The four methanesulfonate compounds represented by formula (IV) are shown in Table 4.

[0026] The reason why the combination of the phosphonate ester compound (I), carbonate ester compound (II), oxalate ester compound (III), and methanesulfonate ester compound (IV) is preferred is merely speculative, but it is thought that they form a strong adsorption layer on metal surfaces such as aluminum, preventing contact with corrosive compounds. The total content of at least one compound selected from these 40 compounds, i.e., one or a combination of two or more compounds, is preferably 0.01 wt % or more, more preferably 0.1 wt % or more, and most preferably 0.5 wt % or more, based on the total mass of the lithium-ion secondary battery electrolyte solution of the present disclosure. The upper limit is preferably 10 wt % or less, more preferably 7 wt % or less, and most preferably 5 wt % or less, based on the non-aqueous solvent. Preferred ranges for the total content of these compounds include 0.01 to 10 wt %, 0.01 to 7 wt %, and 0.1 to 5 wt %, based on the total mass of the lithium-ion secondary battery electrolyte solution of the present disclosure. The electrolyte solution for lithium ion secondary batteries of the present disclosure contains at least one compound selected from the phosphonate ester compounds, carbonate ester compounds, oxalate ester compounds, and methanesulfonate ester compounds of the present disclosure, and when used in a lithium ion secondary battery, it can further improve the corrosion resistance of the lithium ion secondary battery, particularly the corrosion resistance when used at a high voltage of more than 4.2 V and the corrosion resistance when used at room temperature or higher. The electrolyte solution for lithium ion secondary batteries of the present disclosure may contain other components in addition to the electrolyte, non-aqueous solvent, cyanomethyl formate, and compounds represented by formulas (I), (II), (III), and (IV), to the extent that they can be used as an electrolyte.

[0027] The present disclosure makes it possible to use non-aqueous electrolytes containing 1,3-propane sultone, which is usually highly corrosive and has been avoided for use, without impairing the corrosion resistance of LIB. When a graphite negative electrode is used in a battery, 1,3-propane sultone has the effect of suppressing the reductive decomposition of EC and PC on the graphite negative electrode, and therefore it is preferable to add 1,3-propane sultone in an amount ranging from 0.1 to 5 wt % relative to the total non-aqueous electrolyte.

[0028] Furthermore, according to the present disclosure, even non-aqueous electrolytes containing dinitriles with a carbon chain length of 2 to 5, such as succinonitrile, glutaronitrile, adiponitrile, and pimelonitrile, isocyanates such as hexamethylene diisocyanate (HMDI) and 1,3-bis(isocyanatomethyl)cyclohexane (a mixture of cis- and trans-), and carbodiimides such as N,N'-diisopropylcarbodiimide (DIC) and N,N'-dicyclohexylcarbodiimide (DCC), which have traditionally been used reluctantly due to their corrosion-inhibiting effects but the resulting degradation of cycle characteristics as the amount added increases, can now be used without impairing the cycle characteristics of LIBs. These compounds are preferably added in an amount of 0.1 to 5 wt % relative to the total amount of the non-aqueous electrolyte.

[0029] In the electrolyte solution for lithium ion secondary batteries of the present disclosure, the content of vinylene carbonate relative to the total weight of the non-aqueous solvent may be 0.01 to 10 wt %, 0.01 to 7 wt %, or 0.1 to 5 wt %. If the content of vinylene carbonate is too low, the formation of a protective coating on the negative electrode will be insufficient, resulting in reduced cycle characteristics. If the content of vinylene carbonate is too high, the protective coating on the negative electrode will become thick, increasing the resistance of the negative electrode and resulting in reduced cycle characteristics. By including cyanomethyl formate and vinylene carbonate in the electrolyte solution, the electrolyte solution for lithium ion secondary batteries of the present disclosure can improve the cycle characteristics of the lithium ion secondary battery when used in the battery.

[0030] The electrolyte solution for a lithium ion secondary battery according to the present disclosure may further contain at least one of trioctyl phosphate, trifluoroacetic acid ester, and pivalic acid ester having an alcohol group carbon chain length of 6 to 8. Trioctyl phosphate, trifluoroacetic acid ester, and pivalic acid ester having an alcohol group carbon chain length of 6 to 8 are preferably added in a certain amount because they have the effect of increasing the permeability of the electrolyte solution into the separator and reducing the interfacial resistance between the separator and the electrode. The content of each of the trioctyl phosphate, trifluoroacetic acid ester, and pivalic acid ester having an alcohol group carbon chain length of 6 to 8 may be 0.1 to 3 wt %, 0.5 to 2 wt %, or 1 to 1.5 wt %, based on 100 wt % of the nonaqueous solvent.

[0031] The trifluoroacetic acid ester is not particularly limited, and may be at least one selected from n-hexyl trifluoroacetate, 2-ethylhexyl trifluoroacetate, n-heptyl trifluoroacetate, n-octyl trifluoroacetate, 2-octyl trifluoroacetate, 3-octyl trifluoroacetate, and 4-octyl trifluoroacetate.

[0032] The pivalic acid ester is not particularly limited, and may be at least one selected from n-hexyl pivalate, 2-ethylhexyl pivalate, n-heptyl pivalate, n-octyl pivalate, 2-octyl pivalate, 3-octyl pivalate, and 4-octyl pivalate.

[0033] The lithium ion secondary battery of the present disclosure includes a positive electrode, a negative electrode, a separator, and the lithium ion secondary battery electrolyte of the present disclosure. The positive electrode, negative electrode, and separator of the present disclosure are not particularly limited as long as they are usable in lithium ion secondary batteries. The separator of the present disclosure is most preferably a separator made of a microporous membrane formed from a polyolefin material such as polypropylene or polyethylene, but a nonwoven fabric separator can also be used. The porous sheet or nonwoven fabric may have a single-layer or multilayer structure, and the separator surface may be coated with an oxide such as alumina. The thickness of the separator must be as thin as possible to increase the volumetric energy density of the battery. Therefore, a thickness of 20 μm or less is preferred, and a thickness of 10 μm or less is particularly preferred.

[0034] The negative electrode active material used in the negative electrode of the present disclosure may contain at least one material selected from graphite, non-graphitizable carbon, silicon, silicon oxide, lithium titanate, and tin. To increase the volumetric energy density, the negative electrode active material may be a graphite material such as natural graphite or artificial graphite, or a carbon material such as hard carbon or soft carbon. Furthermore, to improve rapid charge / discharge, a Li-based material that does not expand or contract during charge / discharge may be used. ４ Ti ５ O １２ Titanium oxides with spinel structure such as (LTO) and TiNb ２ O ７ , Ti ２ Nb １０ O ２９ Titanium oxide may also be used.

[0035] Examples of binders used in the negative electrode composite include ethylene propylene diene terpolymer (EPDM), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene and butadiene copolymer (SBR), acrylonitrile and butadiene copolymer (NBR), carboxymethyl cellulose (CMC), etc. The negative electrode is produced, for example, by kneading the negative electrode active material with these binders to form a slurry negative electrode composite, applying this negative electrode composite to a copper foil or aluminum foil current collector, drying, pressure molding, and then heat treating, for example, under vacuum at 80°C.

[0036] The positive electrode active material used in the positive electrode of the present disclosure may include a lithium metal compound containing at least one metal element selected from the group consisting of cobalt, nickel, manganese, and iron. The positive electrode active material may be, for example, LiCoO ２ (LCO), LiCo in which part of the Co is replaced with Ni １／３ Ni １／３ Mn １／３ O ２ , LiNi ０．５ Co ０．２ Mn ０．３ O ２ (NCM523), LiNi ０．６ Co ０．２ Mn ０．２ O ２ (NCM622), LiNi ０．８ Co ０．１ Mn ０．１ O ２ (NCM811), LiNi ０．８ Co ０．１５ Al ０．０５ O ２ (NCA), Co-free LiNi ０．５ Mn １．５ O ４ , LiNi ０．８ Mn ０．１３ Ti ０．０２ Mg ０．０２ Nb ０．０１ Mo ０．０２ O ２In order to increase the volumetric energy density, NCM523, NCM622, NCM811, NCA, HE-LNMO, etc. may be used as a positive electrode active material containing a lithium composite oxide with an atomic ratio of Ni of 50% or more. In addition, in order to improve rapid charge and discharge, LiMn ２ O ４ (LMO), LiFePO with olivine structure ４ (LFP) may also be used.

[0037] Examples of conductive additives used in the positive electrode composite include known or commercially available conductive additives such as carbon black (e.g., acetylene black, Ketjen black), carbon nanotubes, carbon fiber, activated carbon, and graphite. Examples of binders include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVFF), styrene-butadiene copolymer (SBR), acrylonitrile-butadiene copolymer (NBR), and carboxymethyl cellulose (CMC). The positive electrode is prepared, for example, by kneading the conductive additive and binder with the positive electrode active material to form a slurry-like positive electrode composite, applying the positive electrode composite to an aluminum foil current collector, drying, pressure molding, and then heat-treating the composite at, for example, 80°C under vacuum. If the battery can be assembled without using a binder, the binder need not be used.

[0038] In the present disclosure, the combination of the positive electrode active material and the negative electrode active material may be, for example, LCO and graphite, NCM523 and graphite, NCM622 and graphite, NCM811 and graphite, NCA and graphite, or HE-LNMO and graphite, in order to increase the volumetric energy density. Also, in order to improve rapid charge / discharge, the combination may be, for example, NCM811 and LTO, HE-LNMO and LTO, or LFP and LTO.

[0039] Other materials in the lithium ion secondary battery of the present disclosure are not particularly limited as long as they can be used in lithium ion secondary batteries. The current collector used in the present disclosure is not particularly limited, but aluminum foil or copper foil is suitable, and a porous current collector may also be used to further improve the permeability of the electrolyte.

[0040] In the present disclosure, the solvent used for the binder is not particularly limited, and various solvents can be selected depending on the active material or binder used. Specifically, when PVDF is used as the binder, it is preferable to use N-methyl-2-pyrrolidone as the solvent, while when a rubber-based binder such as styrene butadiene rubber (SBR), polytetrafluoroethylene, polyvinyl alcohol, or carboxymethyl cellulose (CMC) is used, water is a suitable solvent.

[0041] The structure of the lithium ion secondary battery of the present disclosure is not particularly limited, but examples of the shape of the secondary battery having a positive electrode, a negative electrode, and a separator include a coin-type battery, a cylindrical battery, a prismatic battery, and a pouch-type battery.

[0042] An example of the structure of a pouch-type lithium ion secondary battery will be described with reference to Fig. 1 . Fig. 1 is a cross-sectional view showing the structure of a lithium ion secondary battery 14. As shown in Fig. 1 , in the lithium ion secondary battery 14, the negative electrode active material layer 11b may be a layer of a negative electrode material that is a mixture of a negative electrode active material 11c, a conductive additive 11d, and an electrolyte solution 11f. In addition, in the lithium ion secondary battery 14, the positive electrode active material layer 12b may be a layer of a positive electrode material that is a mixture of a positive electrode active material 12c, a conductive additive 12d, and an electrolyte solution 12f.

[0043] The negative electrode material may be composed of a negative electrode active material 11c, a conductive additive 11d, and an electrolyte solution 11f. The negative electrode 11 may be an electrode in which the negative electrode material is coated on a current collector 11a. The positive electrode material may be composed of a positive electrode active material 12c, a conductive additive 12d, and an electrolyte solution 12f. The positive electrode 12 may be an electrode in which the positive electrode material is coated on a current collector 12a.

[0044] The lithium ion secondary battery 14 may further include a separator 13. The negative electrode 11, the positive electrode 12, and the separator 13 may be positioned such that the negative electrode active material layer 11b and the positive electrode active material layer 12b are in contact with the separator 13. That is, the lithium ion secondary battery 14 may have a structure in which the negative electrode 11 and the positive electrode 12 are stacked with the separator 13 interposed therebetween. The separator 13 may function as an insulating member that insulates the negative electrode 11 and the positive electrode 12. The separator 13 may be, for example, a sheet-like nonwoven fabric or a porous material. Therefore, the first electrolytic solution and the second electrolytic solution can permeate the separator 13.

[0045] The present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present disclosure.

[0046] Next, the present disclosure will be specifically described with reference to examples and comparative examples, but these do not limit the present disclosure in any way. In the examples and comparative examples, 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, and LiFSI represents LiN(SO ２ F) ２ , NCM523 is LiNi ０．５ Co ０．２ Mn ０．３ O ２ , NCM811 is LiNi ０．８ Co ０．１ Mn ０．１ O ２ , LCO is LiCoO ２ Represents.

[0047] Examples 1-1 and 1-2 Electrolyte solutions were prepared according to the compounds and contents shown in Table 5. In Table 5, EC represents ethylene carbonate, PC represents propylene carbonate, and GBL represents γ-butyrolactone.

[0048] LiPF was added to a non-aqueous solvent containing EC / (PC+GBL) mixed at a volume ratio of 1:4. ６ Cyanomethyl formate (CMF) (manufactured by Tokyo Chemical Industry Co., Ltd.) and vinylene carbonate (VC) were added to the solution obtained in this way to prepare the electrolyte solutions of Examples 1-1 and 1-2. The content of CMF in the electrolyte solution of Example 1-1 was 0.5 wt %, the content of VC was 1.5 wt %, and LiPF ６ The content of CMF in the electrolyte solution of Example 1-2 was 0.25% by weight, the content of VC was 0.75% by weight, and the content of LiPF ６ The content was 1 mol / L.

[0049] [Comparative Example 1-1] An electrolytic solution of Comparative Example 1-1 was prepared without adding CMF. In Comparative Example 1-1, an electrolytic solution was prepared in the same manner as in Examples 1-1 and 1-2, except that CMF was not added.

[0050] The positive electrode used NCM523 as the active material, acetylene black as the conductive material, and PVDF as the binder. The mass ratio of the positive electrode active material, conductive material, and binder was 92:5:3. The positive electrode was coated on aluminum foil, dried, and pressure-molded. The coating weight of the positive electrode was 0.010 g / cm. ２ The negative electrode used natural graphite as the active material and SBR and CMC as the binder, with a mass ratio of the negative electrode active material to the binder of 98:1:1. The negative electrode was coated on copper foil, dried, and pressure-molded. The coating weight of the negative electrode was 0.007 g / cm. ２ Using the positive electrode, the negative electrode, and a separator, and using the electrolyte solutions of Examples 1-1 to 1-2 and Comparative Example 1-1, coin batteries were fabricated so that the design capacity of the battery was 2 mAh.

[0051] This coin battery was charged at 25°C at a constant current and voltage (CC / CV mode) at a 1C rate up to an upper voltage of 4.2 V, and the voltage was maintained at 4.2 V until the final current reached 0.03 mA or less. The battery was then discharged at a 1C rate down to a lower voltage of 3.0 V, and the cycle characteristics were measured. The results are shown in Table 5. The discharge capacity after 50 charge / discharge cycles is shown as the 50th cycle discharge capacity.

[0052] The results in Table 5 show that adding both CMF and VC to the electrolyte solution resulted in better cycle characteristics than when CMF or VC was added alone (Examples 1-1 to 1-2). [Examples 2-1 to 2-7] Electrolytes were prepared according to the compounds and contents listed in Tables 6 and 7. In Tables 6 and 7, EC stands for ethylene carbonate, PC stands for propylene carbonate, GBL stands for γ-butyrolactone, and LiFSI stands for LiN(SO ２ F) ２ TOP represents trioctyl phosphate, CF-2EH represents 2-ethylhexyl trifluoroacetate, PV-2EH represents 2-ethylhexyl pivalate, MA represents maleic anhydride, BP represents biphenyl, and LiBOB represents LiB(C ２ O ４ ) ２ Represents.

[0053] LiPF was added to a non-aqueous solvent containing EC / (PC+GBL) mixed at a volume ratio of 1:4. ６ Or LiFSI was dissolved in the solution. Cyanomethyl formate (CMF) (manufactured by Tokyo Chemical Industry Co., Ltd.) and vinylene carbonate (VC) were added to the solution obtained in this way to prepare the electrolyte solutions of Examples 2-1 to 2-7. The CMF content in the electrolyte solutions of Examples 2-1 to 2-3 was 0.75 wt %, the VC content was 2.25 wt %, and LiPF ６ The content of CMF was 1 mol / L. The electrolytic solutions of Examples 2-4, 2-6, and 2-7 had a CMF content of 1 wt %, a VC content of 1 wt %, and a LiFSI content of 1 mol / L. The electrolytic solution of Example 2-5 had a CMF content of 7 wt %, a VC content of 7 wt %, and a LiFSI content of 1 mol / L. Furthermore, the compounds other than CMF and VC listed in Tables 6 and 7 were added in the amounts shown in the tables to prepare the electrolytic solutions of Examples 2-1 to 2-7.

[0054] [Comparative Examples 2-1 to 2-4] The electrolytic solutions of Comparative Examples 2-1 to 2-3 were prepared without adding CMF. The electrolytic solution of Comparative Example 2-4 was prepared without adding VC. In Comparative Examples 2-1, 2-2, and 2-3, the electrolytic solutions were prepared in the same manner as in Examples 2-1, 2-2, and 2-3, respectively, except that CMF was not added. In Comparative Example 2-4, the electrolytic solution was prepared in the same manner as in Example 2-4, except that VC was not added.

[0055] [Fabrication of Lithium Ion Secondary Battery (LIB) and Measurement of Battery Characteristics] The positive electrode contains LiFePO as the active material. ４ The positive electrode active material (LFP), carbon black as a conductive material, and the above-mentioned electrolyte were used, and the mass ratio of the positive electrode active material, conductive material, and electrolyte was 72:1:27. The positive electrode coating weight was 0.075 g / cm. ２ The negative electrode used artificial graphite as the active material, carbon black as the conductive material, and the above-mentioned electrolyte solution, and the mass ratio of the negative electrode active material, conductive material, and electrolyte solution was 62:3:35. The negative electrode was coated on copper foil. The coating weight of the negative electrode was 0.051 g / cm. ２ Using the positive electrode, the negative electrode, and a separator, and using the electrolyte solutions of Examples 2-1 to 2-7 and Comparative Examples 2-1 to 2-4, pouch-type batteries were fabricated so that the design capacity of the battery was 111 mAh.

[0056] This pouch-type battery was charged at 45°C with a constant current and voltage (CC / CV mode) at a 0.5C rate to an upper voltage limit of 3.6 V, and the voltage was maintained at 3.6 V for 30 minutes. The battery was then discharged at a 0.5C rate to a lower voltage limit of 2.0 V, and the cycle characteristics were measured by repeating charge and discharge. The cycle characteristics (%) were calculated by multiplying the obtained capacity (mAh / g) by the 50th cycle / 1st cycle times 100. The results are shown in Tables 6 and 7.

[0057] The results in Tables 6 and 7 show that adding both CMF and VC to the electrolyte solution resulted in better cycle characteristics than adding only CMF or only VC (Examples 2-1 to 2-7).

[0058] Use of the nonaqueous electrolyte solution of the present disclosure makes it possible to improve battery characteristics such as the cycle characteristics of the battery. The contribution of the present disclosure to the long-term use of a large number of lithium-ion secondary batteries, such as automotive batteries, is immeasurable.

[0059] REFERENCE SIGNS LIST 11 negative electrode 11a current collector 11b negative electrode active material layer 11c negative electrode active material 11d conductive additive 11f electrolyte 12 positive electrode 12a current collector 12b positive electrode active material layer 12c positive electrode active material 12d conductive additive 12f electrolyte 13 separator 14 lithium ion secondary battery

Claims

1. An electrolyte solution for a lithium-ion secondary battery in which an electrolyte salt is dissolved in a non-aqueous solvent, characterized in that it contains cyano methyl formate and vinylene carbonate.

2. The electrolyte solution for a lithium-ion secondary battery according to Claim 1, characterized in that, based on 100% by weight of the non-aqueous solvent, it contains 0.01 to 7% by weight of the cyano methyl formate and 0.01 to 7% by weight of the vinylene carbonate.

3. The electrolyte solution for a lithium-ion secondary battery according to Claim 1 or 2, characterized in that the non-aqueous solvent contains at least two selected from ethylene carbonate, propylene carbonate, and γ-butyrolactone.

4. The electrolyte solution for a lithium-ion secondary battery according to Claim 3, characterized in that the non-aqueous solvent contains the ethylene carbonate, the propylene carbonate, and the γ-butyrolactone in a volume ratio of ethylene carbonate / (propylene carbonate + γ-butyrolactone) = 65 / 35 to 10 / 90.

5. As the electrolyte salt, LiPF ６ and LiN(SO ２ F) ２ The electrolyte for a lithium-ion secondary battery according to claim 1 or 2, characterized by containing at least one of them.

6. The electrolyte solution for a lithium-ion secondary battery according to Claim 1 or 2, characterized in that the electrolyte salt is contained at 0.5 to 3 mol / L with respect to the non-aqueous solvent.

7. The electrolyte solution for a lithium-ion secondary battery according to Claim 1 or 2, characterized in that, based on 100% by weight of the non-aqueous solvent, it further contains at least one of 0.1 to 3% by weight of trioctyl phosphate, 0.1 to 3% by weight of trifluoroacetate ester, and 0.1 to 3% by weight of pivalate ester having an alcohol group carbon chain length of 6 to 8.

8. The electrolyte solution for a lithium-ion secondary battery according to Claim 7, characterized in that the trifluoroacetate ester is at least one selected from n-hexyl trifluoroacetate, 2-ethylhexyl trifluoroacetate, n-heptyl trifluoroacetate, n-octyl trifluoroacetate, 2-octyl trifluoroacetate, 3-octyl trifluoroacetate, and 4-octyl trifluoroacetate.

9. The electrolyte solution for a lithium-ion secondary battery according to Claim 7, characterized in that the pivalate ester is at least one selected from n-hexyl pivalate, 2-ethylhexyl pivalate, n-heptyl pivalate, n-octyl pivalate, 2-octyl pivalate, 3-octyl pivalate, and 4-octyl pivalate.

10. The non-aqueous solvent further contains at least one selected from fluoroethylene carbonate, succinic anhydride, maleic anhydride, biphenyl, LiBF ２ (C ２ O ４ )、LiB(C ２ O ４ ) ２ (LiBOB), 1,3 - propane sultone, and ethylene sulfate, in a combined amount of 1% by weight or more and 10% by weight or less. The electrolyte for a lithium-ion secondary battery according to claim 1 or 2, characterized in that it further contains the above components. **Claim 11** A lithium-ion secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte for a lithium-ion secondary battery according to claim 1 or 2. **Claim 12** The positive electrode includes, as a positive electrode active material, a lithium metal compound containing at least one metal element selected from the group consisting of cobalt, nickel, manganese, and iron. The lithium-ion secondary battery according to claim 11, wherein the negative electrode includes at least one selected from the group consisting of graphite, graphitizable carbon, silicon, silicon oxide, lithium titanate, and tin as a negative electrode active material.