Electrolytes for lithium-ion secondary batteries and lithium-ion secondary batteries

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

[Technical Field]

[0001] This disclosure relates to an electrolyte for lithium-ion secondary batteries that has excellent battery characteristics such as battery cycle life, and to a lithium-ion secondary battery equipped with the electrolyte. [Background technology]

[0002] In recent years, lithium-ion secondary batteries (hereinafter also referred to as LIBs) have seen advancements in energy density and voltage. In particular, development is underway to improve cycle life and high-temperature storage characteristics.

[0003] Patent Document 1 proposes a lithium-ion secondary battery that exhibits excellent cycle characteristics by using a non-aqueous electrolyte containing formic acid ester, instead of a non-aqueous electrolyte prepared by dissolving the electrolyte LiPF6 or LiBF4 in a non-aqueous solvent (e.g., EC, PC, MEC, etc.).

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

[0005] [Patent Document 1] Japanese Patent No. 4899862 [Overview of the project]

[0006] An electrolyte for a lithium-ion secondary battery according to one aspect of the present disclosure is an electrolyte 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 aspect of the present disclosure comprises a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte in which an electrolyte salt is dissolved in a non-aqueous solvent, wherein the lithium-ion secondary battery electrolyte in which the electrolyte salt is dissolved in a non-aqueous solvent contains cyanomethyl formate and vinylene carbonate. [Brief explanation of the drawing]

[0008] [Figure 1] This is a cross-sectional view showing the structure of a lithium-ion secondary battery. [Modes for carrying out the invention]

[0009] Examples of embodiments and configurations of this disclosure are given below, but this disclosure is not limited to these examples. Anything that is in line with the intent of the claims, means of solving the problem, effects of the invention, etc., is included in this disclosure.

[0010] This disclosure solves the aforementioned problems and provides a lithium-ion secondary battery with excellent cycle characteristics, which are important for secondary batteries used in electric vehicles and other automotive applications, residential battery storage systems, or large-scale energy storage systems. It also provides an electrolyte that can be used to manufacture such a lithium-ion secondary battery.

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

[0012] The electrolyte salt is not particularly limited. For example, lithium salts such as LiN(SO2F)2 having a SO2 group (hereinafter also referred to as LiFSI), LiOSO2F having a SO3 group, LiOSO3CH3, LiOSO3C2H5 having a SO4 group, LiPF6, LiPO2F2 having phosphorus (P), lithium difluorobis(oxalato)phosphate (LiDFOP), etc., LiBF4 having boron (B), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), LiAsF6 having arsenic (As), etc. can be mentioned. In the present disclosure, one kind of electrolyte salt may be used, or two or more kinds may be mixed and used.

[0013] As the electrolyte salt, the lithium salt may be used alone or in two or more kinds. As an example, the lithium salt is LiFSI, LiPF6, LiN(SO2F)2, LiPO2F2, LiOSO3CH3, etc. If the concentration of the electrolyte salt is too small, the battery performance will decrease because there are few lithium ions moving the electrolyte solution. Also, if the concentration of the electrolyte salt is too large, the viscosity of the electrolyte solution will increase and it will be difficult for lithium ions to move, resulting in a decrease in battery performance. The electrolyte salt may be at least one of LiPF6 and LiN(SO2F)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 the volume of the non-aqueous solvent.

[0014] In the present disclosure, since the corrosion resistance to metals such as aluminum is improved, LiFSI having high chemical thermal stability and capable of improving battery performance at high temperatures can be used in large amounts. For Li salts other than LiFSI, it is preferable to add a certain amount because it has an effect of assisting in improving battery performance at low temperatures. As a suitable combination of these lithium salts, a combination of two kinds of lithium salts having a SO2 group and a lithium salt having phosphorus (P), or a combination of three kinds of a lithium salt having a SO2 group, a lithium salt having a SO4 group, and a lithium salt having phosphorus (P) is preferable.

[0015] When using LiFSI and other Li salts, 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 non-aqueous solvent in this disclosure is not particularly limited as long as it is commonly used as a non-aqueous solvent for lithium-ion battery electrolytes, but examples include cyclic carbonates and linear carbonates. Suitable examples of cyclic carbonates include ethylene carbonate (EC), fluoroethylene carbonate (FEC), and prolene carbonate (PC). Suitable examples of linear carbonates include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). It may also contain the cyclic compound γ-butyrolactone (GBL). It may also contain at least two selected from ethylene carbonate (EC), propylene carbonate (PC), and γ-butyrolactone (GBL). The flash point of EC is 143°C, the flash point of PC is 133°C, and it is 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 80-100% by volume of at least two selected from EC, PC, and GBL. The above volume percentage is a ratio to the total volume of the non-aqueous solvent. In this disclosure, it is preferable that the non-aqueous solvent contains 90-100% by volume of at least two selected from EC, PC, and GBL.

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

[0018] The non-aqueous electrolyte in this disclosure may be used by mixing additives with the non-aqueous solvent. Other additives used in combination with the main solvent include at least one selected from fluoroethylene carbonate, succinic anhydride, maleic anhydride, biphenyl, LiBF2(C2O4), LiB(C2O4)2(LiBOB), 1,3-propanesultone, and ethylene sulfate. The additive may be present in an amount of 10% by weight or less, 5% by weight or less, or 3% by weight or less, relative to 100% by weight of the total non-aqueous solvent.

[0019] The cyanomethyl formate used in this disclosure has two key characteristics. First, its molecular weight is smaller than that of DMC, which is used as one of the main solvents (90). It is also smaller than that of VC, an additive currently used worldwide (86), at 85. Since additives act electrochemically in molar quantities, obtaining a high additive effect with a small amount is important from both a performance and cost perspective. Second, while the oxidative decomposition potential of VC is 4.85V, that of cyanomethyl formate is 5.4V. Furthermore, while the reductive decomposition potential of VC is 0.8V, that of cyanomethyl formate is 1.1V. Therefore, cyanomethyl formate is a compound that is less susceptible to oxidative decomposition and more susceptible to reduction than VC. Currently, with the increasing use of high voltages, resistance to oxidative decomposition on Ni cathodes is important, and early reductive decomposition on highly active graphite anodes to form a protective film is advantageous from a performance perspective.

[0020] The content of cyanomethyl formate in the electrolyte for lithium-ion secondary batteries of this 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 a decrease in cycle characteristics. If the content of cyanomethyl formate is too high, the protective film on the negative electrode will become thicker, increasing the resistance of the negative electrode and leading to a decrease in 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% by weight, 0.01 to 7% by weight, or 0.1 to 5% by weight. The electrolyte for lithium-ion secondary batteries of this disclosure can improve the corrosion resistance of the lithium-ion secondary battery by containing cyanomethyl formate in the electrolyte.

[0021] The electrolyte for lithium-ion secondary batteries of this disclosure may contain, in addition to cyanomethyl formate, at least one compound selected from the group consisting of a phosphonic acid ester compound represented by the following formula (I), a carbonate ester compound represented by the following formula (II), an oxalic acid ester compound represented by the following formula (III), and a methanesulfonic acid ester compound represented by the following formula (IV), in order to further improve the corrosion resistance in lithium-ion secondary batteries.

[0022] [ka] (In the formula, A and B 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.) The phosphonic acid ester compounds represented by formula (I) are the 24 types shown in Table 1. [Table 1]

[0023] [ka] (In the formula, C and D independently represent either a methyl group or an ethyl group, and D represents either 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 the eight types shown in Table 2. [Table 2]

[0024] [ka] (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 oxalate ester compounds represented by formula (III) are the four types shown in Table 3. [Table 3]

[0025] [ka] (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 methanesulfonic acid ester compounds represented by formula (IV) are the four types shown in Table 4.

[0026] [Table 4] The reason why the combined use of the above-mentioned phosphonic acid ester compound (I), carbonate ester compound (II), oxalic acid ester compound (III), and methanesulfonic acid ester compound (IV) is preferable is 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 of these 40 compounds, i.e., one or more combinations of one or more compounds, is preferably 0.01% by weight or more, more preferably 0.1% by weight or more, and most preferably 0.5% by weight or more, relative to the total mass of the electrolyte for lithium-ion secondary batteries of this disclosure. Furthermore, the upper limit is preferably 10% by weight or less, more preferably 7% by weight or less, and most preferably 5% by weight or less, relative to the non-aqueous solvent. Preferred ranges for the total content of these compounds include 0.01 to 10% by weight, 0.01 to 7% by weight, and 0.1 to 5% by weight, relative to the total mass of the electrolyte for lithium-ion secondary batteries of this disclosure. The electrolyte for lithium-ion secondary batteries of this disclosure contains at least one compound selected from the phosphonic acid ester compounds, carbonate ester compounds, oxalic acid ester compounds, and methanesulfonic acid ester compounds of this disclosure. When used in lithium-ion secondary batteries, it can further improve the corrosion resistance of the lithium-ion secondary batteries, particularly in use at high voltages exceeding 4.2V and at temperatures above room temperature. In addition to the electrolyte, non-aqueous solvent, cyanomethyl formate, and the compounds represented by formulas (I), (II), (III), and (IV), the electrolyte for lithium-ion secondary batteries of this disclosure may contain other components to the extent that they can be used as an electrolyte.

[0027] This disclosure makes it possible to use non-aqueous electrolytes containing 1,3-propanesultone, which are usually highly corrosive and have been avoided in use, without impairing the corrosion resistance of LIBs. When a graphite anode is used in the battery, 1,3-propanesultone has the effect of suppressing the reductive decomposition of EC and PC on the graphite anode, so it is preferable to add it in the range of 0.1 to 5% by weight of the total non-aqueous electrolyte.

[0028] Furthermore, this disclosure makes it possible to use non-aqueous electrolytes containing dinitriles with carbon chain lengths of 2 to 5, such as succinonitrile, glutaronitrile, adiponitrile, and pimeronitrile; isocyanates such as hexamethylene diisocyanate (HMDI) and 1,3-bis(isocyanate-methyl)cyclohexane (a mixture of cis- and trans-); and carbodiimides such as N,N'-diisopropylcarbodiimide (DIC) and N,N'-dicyclohexylcarbodiimide (DCC), which have conventionally had corrosion-inhibiting effects but whose use has been hesitated due to the deterioration of cycle characteristics as the amount added increases, without impairing the cycle characteristics of the LIB. It is preferable to add these compounds in an amount of 0.1 to 5% by weight relative to the total non-aqueous electrolyte.

[0029] In the electrolyte for lithium-ion secondary batteries of this disclosure, the content of vinylene carbonate relative to the total weight of the non-aqueous solvent may be 0.01 to 10% by weight, 0.01 to 7% by weight, or 0.1 to 5% by weight. If the vinylene carbonate content is too low, the formation of a protective film on the negative electrode will be insufficient, resulting in a decrease in cycle characteristics. If the vinylene carbonate content is too high, the protective film on the negative electrode will become thicker, increasing the resistance of the negative electrode and leading to a decrease in cycle characteristics. By containing cyanomethyl formate and vinylene carbonate in the electrolyte for lithium-ion secondary batteries of this disclosure, when used in lithium-ion secondary batteries, the cycle characteristics of the lithium-ion secondary batteries can be improved.

[0030] The electrolyte for lithium-ion secondary batteries of this disclosure may further contain at least one of trioctyl phosphate, trifluoroacetic acid, and pivalophosphate with an alcohol group carbon chain length of 6 to 8. It is preferable to add a certain amount of trioctyl phosphate, trifluoroacetic acid, and pivalophosphate with an alcohol group carbon chain length of 6 to 8 because they have the effect of increasing the permeability of the electrolyte to the separator and lowering the interfacial resistance between the separator and the electrode. The respective contents of trioctyl phosphate, trifluoroacetic acid, and pivalophosphate with an alcohol group carbon chain length of 6 to 8 may be 0.1 to 3% by weight, 0.5 to 2% by weight, or 1 to 1.5% by weight, with the non-aqueous solvent being 100% by weight.

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

[0032] The pivalate ester is not particularly limited, but 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 this disclosure comprises a positive electrode, a negative electrode, a separator, and an electrolyte for a lithium-ion secondary battery of this disclosure. The positive electrode, negative electrode, and separator in this disclosure are not particularly limited as long as they can be used in a lithium-ion secondary battery. As the separator in this disclosure, it is most preferable to use 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 be a single layer or a multilayer structure, and the separator surface may be coated with an oxide such as alumina. The thickness of the separator needs to be as thin as possible in order to increase the volumetric energy density of the battery. For this reason, it is preferably 20 μm or less, and particularly preferably 10 μm or less.

[0034] The negative electrode active material used in the negative electrode in this disclosure may include at least one 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 include graphite materials such as natural graphite or artificial graphite, or carbon materials such as hard carbon or soft carbon. Furthermore, to improve rapid charging and discharging, Li4Ti5O, which does not expand or contract during charging and discharging, may be used. 12 Titanium oxides with spinel-type structures such as (LTO), TiNb2O7, and Ti2Nb 10 O 29 Titanium oxide may also be used.

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

[0036] As the positive electrode active material used in the positive electrode in the present disclosure, a lithium metal compound containing at least one metal element selected from the group consisting of cobalt, nickel, manganese, and iron may be included. The positive electrode active material is, for example, LiCoO2 (LCO), LiCo in which a part of Co is replaced by Ni 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 Co 0.15 Al 0.05 O2 (NCA), LiNi without using 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. may be used. In order to increase the volumetric energy density, NCM523, NCM622, NCM811, NCA, HE-LNMO, etc. may be used as the positive electrode active material containing a lithium composite oxide in which Ni is 50% or more as an atomic ratio. Further, in order to improve rapid charge and discharge, LiMn2O4 (LMO) having a spinel structure and LiFePO4 (LFP) having an olivine structure may be used.

[0037] Examples of conductive additives used in positive electrode composites include known or commercially available conductive additives such as acetylene black, Ketjenblack, carbon black, carbon nanotubes, carbon fibers, activated carbon, and graphite. Examples of binders include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVFF), styrene-butadiene copolymer (SBR), acrylonitrile-butadiene copolymer (NBR), and carboxymethylcellulose (CMC). The positive electrode is manufactured, for example, by kneading these conductive additives and binders into a slurry-like positive electrode composite material with the positive electrode active material, then coating this positive electrode composite material onto aluminum foil as a current collector, drying, pressure molding, and then heat-treating it, for example, under vacuum at 80°C. If the battery can be assembled without using a binder, then a binder may be omitted.

[0038] In this disclosure, the combinations of positive and negative electrode active materials may include LCO and graphite, NCM523 and graphite, NCM622 and graphite, NCM811 and graphite, NCA and graphite, HE-LNMO and graphite, etc., in order to increase the volumetric energy density. In addition, combinations such as NCM811 and LTO, HE-LNMO and LTO, LFP and LTO, etc., may be used to improve rapid charging and discharging.

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

[0040] In this disclosure, there are no particular restrictions on the solvent used for the binder, and various solvents can be selected depending on the active material or binder used. Specifically, when PVDF is used as the binder, N-methyl-2-pyrrolidone is preferably used as the solvent, while when rubber-based binders such as styrene-butadiene rubber (SBR), polytetrafluoroethylene, polyvinyl alcohol, or carboxymethylcellulose (CMC) are used, water is preferably used as the solvent.

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

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

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

[0044] The lithium-ion secondary battery 14 may further include a separator 13. The negative electrode 11, positive electrode 12, and 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 in between. The separator 13 may function as an insulating member that insulates the negative electrode 11 and the positive electrode 12. For example, a sheet-like nonwoven fabric or a porous material may be used for the separator 13. Therefore, the first electrolyte and the second electrolyte can penetrate the separator 13.

[0045] This disclosure is not limited to the embodiments described above, 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 this disclosure. [Examples]

[0046] Next, the present disclosure will be specifically explained with reference to examples and comparative examples, but these are not intended to limit the present disclosure in any way. In the examples and comparative examples, PC represents prolene carbonate, DMC represents dimethyl carbonate, EC represents ethylene carbonate, EMC represents ethyl methyl carbonate, FEC represents fluoroethylene carbonate, PS represents 1,3-propanesultone, 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 is LiNi 0.8 Co 0.1 Mn 0.1 O2 and LCO represent LiCoO2.

[0047] [Examples 1-1 to 1-2] The electrolyte was prepared according to the compounds and their contents listed in Table 5. In Table 5, EC represents ethylene carbonate, PC represents propylene carbonate, and GBL represents γ-butyrolactone.

[0048] LiPF6 was dissolved in a non-aqueous solvent prepared by mixing EC / (PC+GBL) in a 1:4 (volume ratio). To the resulting solution, cyanomethyl formate (CMF) (manufactured by Tokyo Chemical Industry Co., Ltd.) and vinylene carbonate (VC) were added to prepare the electrolytes of Examples 1-1 and 1-2. The electrolyte of Example 1-1 contained 0.5% by weight of CMF, 1.5% by weight of VC, and 1 mol / L of LiPF6. The electrolyte of Example 1-2 contained 0.25% by weight of CMF, 0.75% by weight of VC, and 1 mol / L of LiPF6.

[0049] [Comparative Example 1-1] The electrolyte for Comparative Example 1-1 was prepared without adding CMF. The electrolyte for Comparative Example 1-1 was prepared in the same manner as in Examples 1-1 to 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. This was applied to aluminum foil, dried, and pressure-molded. The coating weight of the positive electrode was 0.010 g / cm³. 2 The negative electrode was constructed using natural graphite as the active material and SBR and CMC as the binder, with a mass ratio of 98:1:1 for the negative electrode active material to the binder. This was applied to copper foil, dried, and then pressure-molded. The coating weight of the negative electrode was 0.007 g / cm³. 2 Using the positive electrode, the negative electrode, and the separator, coin-type batteries were fabricated using the electrolytes of Examples 1-1 to 1-2 and Comparative Example 1-1, respectively, so that the design capacity of the battery was 2 mAh.

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

[0052] [Table 5] The results in Table 5 show that adding both CMF and VC to the electrolyte resulted in superior cycle characteristics compared to adding CMF alone or VC alone (Examples 1-1 to 1-2). [Examples 2-1 to 2-7] The electrolyte was prepared according to the compounds and their contents listed in Tables 6 and 7. In Tables 6 and 7, EC represents ethylene carbonate, PC represents propylene carbonate, GBL represents γ-butyrolactone, LiFSI represents LiN(SO2F)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(C2O4)2.

[0053] LiPF6 or LiFSI was dissolved in a non-aqueous solvent prepared by mixing EC / (PC+GBL) in a 1:4 (volume ratio). To the resulting solution, cyanomethyl formate (CMF) (manufactured by Tokyo Chemical Industry Co., Ltd.) and vinylene carbonate (VC) were added to prepare the electrolytes of Examples 2-1 to 2-7. In the electrolytes of Examples 2-1 to 2-3, the CMF content was 0.75% by weight, the VC content was 2.25% by weight, and the LiPF6 content was 1 mol / L. In the electrolytes of Examples 2-4, 2-6, and 2-7, the CMF content was 1% by weight, the VC content was 1% by weight, and the LiFSI content was 1 mol / L. In the electrolyte of Example 2-5, the CMF content was 7% by weight, the VC content was 7% by weight, and the LiFSI content was 1 mol / L. Furthermore, compounds other than CMF and VC listed in Tables 6 and 7 were added in the amounts shown in the tables to prepare the electrolytes for Examples 2-1 to 2-7.

[0054] [Comparative Examples 2-1 to 2-4] The electrolytes for Comparative Examples 2-1 to 2-3 were prepared without adding CMF. The electrolyte for Comparative Example 2-4 was prepared without adding VC. Comparative Examples 2-1, 2-2, and 2-3 were prepared in the same manner as Examples 2-1, 2-2, and 2-3, except that CMF was not added. Comparative Example 2-4 was prepared in the same manner as Example 2-4, except that VC was not added.

[0055] [Fabrication of lithium-ion secondary batteries (LIBs) and measurement of battery characteristics] The positive electrode used LiFePO4 (LFP) as the active material, carbon black as the conductive material, and the above-mentioned electrolyte. The mass ratio of the positive electrode active material, conductive material, and electrolyte was 72:1:27, and it was coated onto aluminum foil. The coating weight of the positive electrode was 0.075 g / cm³. 2 The negative electrode used artificial graphite as the active material, carbon black as the conductive material, and the above-mentioned electrolyte. The mass ratio of the negative electrode active material, conductive material, and electrolyte was 62:3:35, and it was coated onto copper foil. The coating weight of the negative electrode was 0.051 g / cm³. 2 Using the positive electrode, the negative electrode, and the separator, pouch-type batteries were fabricated using the electrolytes of Examples 2-1 to 2-7 and Comparative Examples 2-1 to 2-4, respectively, so that the design capacity of the battery was 111 mAh.

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

[0057] [Table 7] The results in Tables 6 and 7 show that adding both CMF and VC to the electrolyte resulted in superior cycle characteristics compared to adding CMF alone or VC alone (Examples 2-1 to 2-7). [Industrial applicability]

[0058] By using the non-aqueous electrolyte of this disclosure, it has become possible to improve battery characteristics such as battery cycle characteristics. The contribution of this disclosure to the long-term use of a large number of lithium-ion secondary batteries, such as those used in automobiles, is immeasurable. [Explanation of Symbols]

[0059] 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 rechargeable batteries

Claims

1. An electrolyte for lithium-ion secondary batteries, wherein an electrolyte salt is dissolved in a non-aqueous solvent, and which contains cyanomethyl formate and vinylene carbonate. An electrolyte for lithium-ion secondary batteries, characterized by containing 100% by weight of the non-aqueous solvent, 0.1 to 10% by weight of the cyanomethyl formate, and 0.1 to 10% by weight of the vinylene carbonate.

2. The electrolyte for lithium-ion secondary batteries according to claim 1, characterized in that the non-aqueous solvent comprises at least two selected from ethylene carbonate, propylene carbonate, and γ-butyrolactone.

3. The electrolyte for lithium-ion secondary batteries according to claim 2, 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.

4. As the electrolyte salt, LiPF ６ and LiN(SO ２ F) ２ The electrolyte for lithium-ion secondary batteries according to claim 1 or 2, characterized by comprising at least one of the following.

5. The electrolyte for lithium-ion secondary batteries according to claim 1 or 2, characterized in that the electrolyte salt is contained in an amount of 0.5 to 3 mol / L relative to the non-aqueous solvent.

6. The electrolyte for lithium-ion secondary batteries according to claim 1 or 2, further comprising 0.1 to 3% by weight of trioctyl phosphate, 0.1 to 3% by weight of trifluoroacetic acid ester, and 0.1 to 3% by weight of at least one pivalic acid ester having an alcohol group carbon chain length of 6 to 8, with the non-aqueous solvent being 100% by weight.

7. The electrolyte for lithium-ion secondary batteries according to claim 6, characterized in that the trifluoroacetic acid ester is at least one selected from n-hexyl trifluoroacetic acid, 2-ethylhexyl trifluoroacetic acid, n-heptyl trifluoroacetic acid, n-octyl trifluoroacetic acid, 2-octyl trifluoroacetic acid, 3-octyl trifluoroacetic acid, and 4-octyl trifluoroacetic acid.

8. The electrolyte for lithium-ion secondary batteries according to claim 6, characterized in that the pivalic acid ester is at least one selected from n-hexyl pivalic acid, 2-ethylhexyl pivalic acid, n-heptyl pivalic acid, n-octyl pivalic acid, 2-octyl pivalic acid, 3-octyl pivalic acid, and 4-octyl pivalic acid.

9. 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 by further containing the same.

10. A lithium-ion secondary battery comprising a positive electrode, a negative electrode, a separator, and the electrolyte for lithium-ion secondary batteries described in claim 1 or 2.

11. The positive electrode includes a lithium metal compound containing at least one metal element selected from the group consisting of cobalt, nickel, manganese, and iron as the positive electrode active material. The lithium-ion secondary battery according to claim 10, characterized in that the negative electrode includes at least one selected from graphite, non-graphitizable carbon, silicon, silicon oxide, lithium titanate, and tin as the negative electrode active material.

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