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

Abstract

The present invention addresses the problem of providing a lithium-ion secondary battery that has excellent cycle characteristics and corrosion resistance, 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 a lithium-ion secondary battery.　This electrolytic solution for a lithium ion secondary battery in which an electrolyte is dissolved in a non-aqueous solvent is characterized by including cyanomethyl formate and / or 2-cyanoethyl formate. This lithium ion secondary battery comprises a positive electrode, a negative electrode, a separator, and the electrolytic solution for a lithium ion secondary battery.

Description

[Technical Field]

[0001] The present invention relates to an electrolyte for lithium-ion secondary batteries that is excellent in battery characteristics such as battery cycle life and also possesses safety features such as corrosion resistance, 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, to improve cycle life and high-temperature storage characteristics, lithium composite oxide cathodes containing Ni, graphite materials, and Li4Ti5O2 are being developed. 12 Lithium-ion secondary batteries, which use titanium oxide anodes such as LTO (hereinafter also referred to as LTO) and non-aqueous electrolytes containing lithium bis(fluorosulfonyl)imide (hereinafter also referred to as LiFSI) as the electrolyte, have come to be used as secondary batteries for automobiles.

[0003] However, Patent Document 1 points out that when a non-aqueous electrolyte using an imide-based lithium salt such as LiFSI is operated at a high voltage exceeding 4.2V, it can corrode the aluminum used as the positive electrode current collector in lithium-ion secondary batteries.

[0004] Patent Document 2 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.).

[0005] However, the formic acid 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, which have a -C≡N group, are not shown anywhere. Furthermore, Patent Document 3 does not mention cyanomethyl formate at all, and although an organic electrolyte using 2-cyanoethyl formate as a solvent is proposed (Example 1), it is stated that cyanoalkyl formate can be used in applications such as electric double-layer capacitors and electrolytic capacitors, but is unsuitable for lithium batteries and lithium-ion secondary batteries because it is easily decomposed by reaction with lithium salts or during charging and discharging (paragraph

[0012] ).

[0006] Generally, ester compounds containing formic acid, sulfuric acid, or halogen elements, which are strong acids, can cause metal corrosion. Therefore, caution must be exercised regarding corrosion when lithium-ion secondary batteries using these compounds in a non-aqueous electrolyte are exposed to high voltages exceeding 4.2V or temperatures above room temperature. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2014-203748 [Patent Document 2] Patent No. 4899862 [Patent Document 3] Patent No. 4221088 [Overview of the project] [Problems that the invention aims to solve]

[0008] The present invention aims to solve the aforementioned problems and provide a lithium-ion secondary battery that is excellent in cycle characteristics, which are important for secondary batteries used in vehicles such as electric vehicles, as well as in corrosion resistance. It also aims to provide an electrolyte that can be used to manufacture such a lithium-ion secondary battery. [Means for solving the problem]

[0009] As a result of intensive studies by the present inventors, in a lithium-ion secondary battery including 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, it has been found that the cycle characteristics of the battery can be improved and a lithium-ion secondary battery excellent in corrosion resistance can be obtained. Conventionally, it has not been known at all that adding cyanomethyl formate and / or 2-cyanoethyl formate to a non-aqueous solvent can improve the cycle characteristics of the battery as an electrolyte for a lithium-ion secondary battery and can produce a battery excellent in corrosion resistance. The electrolyte found by the present inventors is an electrolyte for a lithium-ion secondary battery for use in the application of an electrolyte of a lithium-ion secondary battery, and the lithium-ion secondary battery obtained by using this electrolyte for a lithium-ion secondary battery is excellent in corrosion resistance even when used at a high voltage exceeding 4.2V and is excellent in corrosion resistance even when used at a temperature of room temperature or higher.

[0010] Furthermore, the present inventors have found that by combining one or more selected from the group consisting of the phosphonate ester compound (I), carbonate ester compound (II), oxalate ester compound (III), and methanesulfonate ester compound (IV) described in the present invention with a non-aqueous electrolyte containing cyanomethyl formate and / or 2-cyanoethyl formate, the corrosion resistance in a lithium-ion secondary battery can be further improved.

[0011] That is, the present invention is specified by the following matters. (1) An electrolyte for a lithium-ion secondary battery in which an electrolyte is dissolved in a non-aqueous solvent, characterized by containing cyanomethyl formate and / or 2-cyanoethyl formate. (2) A phosphonate ester compound represented by the following formula (I):

Chemical formula

[0012] The electrolyte for lithium-ion secondary batteries of the present invention, comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte in which the electrolyte is dissolved in a non-aqueous solvent, can improve the cycle characteristics of the lithium-ion secondary battery by containing cyanomethyl formate and / or 2-cyanoethyl formate in the non-aqueous electrolyte. This makes it possible to manufacture lithium-ion secondary batteries with excellent cycle characteristics. Furthermore, the corrosion resistance of the lithium-ion secondary battery can be improved by containing cyanomethyl formate and / or 2-cyanoethyl formate in the non-aqueous electrolyte. This makes it possible to manufacture lithium-ion secondary batteries with excellent corrosion resistance. Moreover, by combining cyanomethyl formate and / or 2-cyanoethyl formate with one or more compounds selected from the group consisting of phosphonic acid ester compounds (I), carbonate ester compounds (II), oxalic acid ester compounds (III), and methanesulfonic acid ester compounds (IV) in the present invention, it is possible to manufacture lithium-ion secondary batteries with even better corrosion resistance. [Modes for carrying out the invention]

[0013] The embodiments and configurations of the present invention are illustrated below, but the present invention is not limited to these, and is included in the present invention as long as it is in line with the intent of the claims, means of solving the problem, effects of the invention, etc.

[0014] A non-aqueous electrolyte consists of an electrolyte and a non-aqueous solvent. The electrolyte in this invention is not particularly limited, but examples of electrolyte salts include lithium salts such as LiN(SO2F)2 (hereinafter also referred to as LiFSI) having an SO2 group, LiOSO2F having an SO3 group, LiOSO3CH3 and LiOSO3C2H5 having an SO4 group, LiPF6, LiPO2F2, and lithium difluorobis(oxalato)phosphate (LiDFOP) having phosphorus (P), LiBF4, lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB) having boron (B), and LiAsF6 having arsenic (As). In this invention, one electrolyte may be used, or two or more may be used in mixture form.

[0015] In this invention, by adding cyanomethyl formate and / or 2-cyanoethyl formate to the non-aqueous electrolyte, corrosion resistance to metals such as aluminum is improved, allowing for the use of LiFSI, which has high chemical thermal stability and can improve battery performance at high temperatures. The lithium salt may be used alone or in combination of two or more types. It is preferable to add a certain amount of Li salts other than LiFSI (hereinafter also referred to as other Li salts) because they have the effect of supplementally improving battery performance at low temperatures. Preferred combinations of these lithium salts include two types of combinations: a lithium salt having an SO2 group and a lithium salt having phosphorus (P), or three types of combinations: a lithium salt having an SO2 group, a lithium salt having an SO4 group, and a lithium salt having phosphorus (P). Specifically, three types of combinations of LiFSI, LiPF6, and LiPO2F2, or four types of combinations of LiFSI, LiiOSO3CH3, LiPF6, and LiPO2F2 are more preferred. 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. Furthermore, the total electrolyte concentration is preferably 0.5 to 3 mol per 1 L of the total volume of the electrolyte solution for the lithium-ion secondary battery of the present invention, and more preferably 1 to 2 mol.

[0016] The non-aqueous solvent in the present invention is not particularly limited, but examples include cyclic carbonates and linear carbonates. Suitable examples of cyclic carbonates include ethylene carbonate (EC), fluoroethylene carbonate (FEC), vinylene carbonate (VC), and prolene carbonate (PC). Suitable examples of linear carbonates include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). In the present invention, these solvents may be used as the main solvent, and other solvents may be used in a mixture. Suitable secondary solvents to be used in combination with the main solvent include cyclic compounds such as γ-butyrolactone and 1,3-propanesultone (PS), which have an effect of improving ionic conductivity, and chain compounds with lower viscosity than DMC, such as ethyl formate, propyl formate, isopropyl formate, and propargyl formate (2-propynyl formate).

[0017] These solvents may be used individually or in combination of two or more. Suitable combinations of these cyclic carbonates include, for example, combinations of two types such as EC and VC, EC and FEC, PC and FEC, EC and PC, and PC and DMC, and other solvents may be added to these two-type combinations. Examples of combinations of three types include combinations of EC, PC and VC, EC, PC and FEC, EC, PC and DMC, and EC, EMC and DMC, and other solvents may be added to these three-type combinations. Examples of combinations of four types include combinations of EC, PC, FEC and VC, and EC, PC, DMC and EMC, and other solvents may be added to these four-type combinations.

[0018] When the cyclic carbonate in the non-aqueous electrolyte according to the present invention contains a linear carbonate, the ratio of cyclic carbonate to linear carbonate (by volume) is preferably 10:90 to 50:50, and more preferably 20:80 to 40:60, from the viewpoint of improving electrochemical properties over a wide range of temperatures from high to low.

[0019] The cyanomethyl formate used in this invention has two distinctive features. First, its molecular weight is smaller than that of DMC, which is used as one of the main solvents (90). Furthermore, its molecular weight is 85, which is smaller than that of VC, an additive currently used worldwide (86). Since the molar amount of an additive acts electrochemically, 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. Similarly, 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 and / or 2-cyanoethyl formate in the electrolyte for lithium-ion secondary batteries of the present invention is not particularly limited, but if it is too low, the formation of a protective film on the negative electrode will be insufficient, which will affect the cycle characteristics and corrosion resistance. Therefore, the appropriate content of cyanomethyl formate and / or 2-cyanoethyl formate is preferably 0.01% by mass or more, more preferably 0.1% by mass or more, and may also be 0.5% by mass or more, based on the total mass of the electrolyte for lithium-ion secondary batteries of the present invention. Furthermore, the upper limit is preferably 80% by mass or less, preferably 60% by mass or less, more preferably 30% by mass or less, more preferably 10% by mass or less, more preferably 5% by mass or less, and may also be 3% by mass or less, based on the total mass of the electrolyte for lithium-ion secondary batteries. Preferred ranges for 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 include 0.01 to 80% by mass, 0.01 to 60% by mass, 0.01 to 30% by mass, 0.01 to 10% by mass, 0.01 to 5% by mass, 0.01 to 3% by mass, 0.1 to 80% by mass, 0.1 to 60% by mass, 0.1 to 30% by mass, 0.1 to 10% by mass, 0.1 to 5% by mass, 0.1 to 3% by mass, 0.5 to 10% by mass, 0.5 to 5% by mass, and 0.5 to 3% by mass. By containing cyanomethyl formate and / or 2-cyanoethyl formate in the electrolyte for lithium-ion secondary batteries of the present invention, when used in lithium-ion secondary batteries, the cycle characteristics of the lithium-ion secondary batteries can be improved. Furthermore, the corrosion resistance of the lithium-ion secondary batteries can be improved. The above numerical ranges for the content of cyanomethyl formate and / or 2-cyanoethyl formate indicate the numerical range for each when cyanomethyl formate or 2-cyanoethyl formate is used individually, and the numerical range for the total of both when both cyanomethyl formate and 2-cyanoethyl formate are used.

[0021] The electrolyte for lithium-ion secondary batteries 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 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).

[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.)

[0023] The phosphonic acid ester compounds represented by formula (I) are the 24 types shown in Table 1.

[0024] [Table 1]

[0025] [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).)

[0026] The carbonate ester compounds represented by formula (II) are the eight types shown in Table 2.

[0027] [Table 2]

[0028] [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).)

[0029] The oxalate ester compounds represented by formula (III) are the four types shown in Table 3.

[0030] [Table 3]

[0031] [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).)

[0032] The methanesulfonic acid ester compounds represented by formula (IV) are the four types shown in Table 4.

[0033] [Table 4]

[0034] 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 mass or more, more preferably 0.1% by mass or more, and most preferably 0.5% by mass or more, relative to the total mass of the electrolyte for the lithium-ion secondary battery of the present invention. Furthermore, the upper limit is preferably 10% by mass or less, more preferably 8% by mass or less, and most preferably 5% by mass or less, relative to the total mass of the electrolyte for the lithium-ion secondary battery of the present invention. Preferred ranges for the total content of these compounds include 0.01 to 10% by mass, 0.1 to 8% by mass, and 0.5 to 5% by mass, relative to the total mass of the electrolyte for the lithium-ion secondary battery of the present invention. The electrolyte for lithium-ion secondary batteries of the present invention contains at least one compound selected from the phosphonic acid ester compound, carbonate ester compound, oxalic acid ester compound, and methanesulfonic acid ester compound of the present invention. When used in a lithium-ion secondary battery, it can further improve the corrosion resistance of the lithium-ion secondary battery, 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 / or 2-cyanoethyl formate, and the compounds represented by formulas (I), (II), (III), and (IV), the electrolyte for lithium-ion secondary batteries of the present invention may contain other components within the range that can be used as an electrolyte.

[0035] This invention makes it possible to use non-aqueous electrolytes containing 1,3-propanesultone, which are normally 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 mass of the total non-aqueous electrolyte.

[0036] Furthermore, the present invention 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, which have corrosion-inhibiting effects but whose use has been hesitated due to the deterioration of cycle characteristics as the amount added increases; 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), without impairing the cycle characteristics of the LIB. These compounds are preferably added in an amount of 0.1 to 5% by mass relative to the total non-aqueous electrolyte.

[0037] The lithium-ion secondary battery of the present invention comprises a positive electrode, a negative electrode, a separator, and the electrolyte for the lithium-ion secondary battery of the present invention. The positive electrode, negative electrode, and separator in the present invention are not particularly limited as long as they can be used in a lithium-ion secondary battery. As the separator in the present invention, 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 single-layer or multi-layer, 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.

[0038] In the present invention, suitable negative electrode active materials for increasing volumetric energy density include graphite materials such as natural graphite and artificial graphite, and carbon materials such as hard carbon and soft carbon. In addition, to improve rapid charging and discharging, Li4Ti5O2, which does not expand or contract during charging and discharging, is suitable. 12 Titanium oxides with spinel-type structures such as (LTO), TiNb2O7, and Ti2Nb 10 O 29The titanium oxide is preferably used.

[0039] Examples of the binder used for the negative electrode composite material include 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. The negative electrode is produced, for example, by kneading these binders with the negative electrode active material to form a slurry-like negative electrode composite material, then applying this negative electrode composite material to a copper foil or aluminum foil of a current collector, drying, and subjecting it to pressure molding, and then heat-treating it at 80 °C under vacuum, for example.

[0040] Examples of the positive electrode active material used for the positive electrode in the present invention include, for example, LiCoO2 (LCO), LiCo in which part of Co is replaced with Ni or 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 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 0.02Examples include O2(HE-LNMO). To increase the volumetric energy density, cathode active materials containing lithium composite oxides with an atomic ratio of 50% or more Ni, such as NCM523, NCM622, NCM811, NCA, and HE-LNMO, are preferably used. Furthermore, to improve rapid charging and discharging, LiMn2O4(LMO) with a spinel-type structure and LiFePO4(LFP) with an olivine-type structure are preferably used.

[0041] 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.

[0042] In this invention, suitable combinations of positive electrode active material and negative electrode active material include LCO and graphite, NCM523 and graphite, NCM622 and graphite, NCM811 and graphite, NCA and graphite, and HE-LNMO and graphite, in order to increase the volumetric energy density. Furthermore, suitable combinations include NCM811 and LTO, HE-LNMO and LTO, and LFP and LTO, in order to improve rapid charging and discharging.

[0043] 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, but aluminum foil or copper foil is preferred, and a porous current collector may be used to further improve the permeability of the electrolyte.

[0044] In the present invention, there are no particular restrictions on the solvent used as 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.

[0045] The structure of the lithium secondary battery of the present invention 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, etc. Furthermore, it can also be applied to clay-like (clay-like) pouch-type lithium-ion secondary batteries in which two electrode layers, a clay-like positive electrode and a negative electrode, are separated by a separator, rather than using sheet-like positive and negative electrodes. [Examples]

[0046] Next, the present invention will be specifically described with reference to examples and comparative examples, but these are not intended to limit the present invention in any way. In the examples, comparative examples, and Tables 5-8, 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 1-18] [Preparation of electrolyte solution] LiPF6 and LiFSI were dissolved in a non-aqueous solvent prepared by mixing PC and DMC in a 1:2 (volume ratio). To the resulting solution, cyanomethyl formate (CMF) (manufactured by Tokyo Chemical Industry Co., Ltd.) was added to prepare the electrolyte of Example 1-1. The prepared electrolyte contained 0.1% by mass of CMF, 0.5 mol / L of LiPF6, and 0.5 mol / L of LiFSI. Furthermore, the electrolytes of Examples 1-2 to 1-18 were prepared so that the CMF content of the prepared electrolyte was as shown in Table 5. In Table 5, the mass % of CMF and the mass % of compounds other than CMF represent their respective proportions to the total mass of the prepared electrolyte, and the M (mol / L) of the electrolyte in Table 5 represents their respective proportions to the total volume of the prepared electrolyte. In Examples 1-2 to 1-3, the electrolytes were prepared in the same manner as in Example 1-1, except for the CMF content. In Examples 1-4, PC and CMF were mixed in a 1:2 (volume ratio), and then LiPF6 and LiFSI were dissolved to obtain the amounts of CMF, LiPF6, and LiFSI listed 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 of EC and DMC mixed in a 1:2 (volume ratio) was used and the amount of CMF was listed in Table 5. In Examples 1-6 to 1-18, LiPF6 and LiFSI were dissolved in a non-aqueous solvent of PC and DMC mixed in a 1:2 (volume ratio), and then CMF and a compound in which A, B, C, D, E, and F in the compound represented by formula (I), (II), (III), or (IV) of the present invention are A, B, C, D, E, and F listed in the column for types of compounds other than CMF in Table 5 were added to prepare the electrolyte. The content of CMF and its compounds, as well as LiPF6 and LiFSI, in the prepared electrolyte is as shown in Table 5. Examples 1-6 to 1-18 are examples in which phosphonic acid esters, carbonate esters, oxalic acid esters, or methanesulfonic acid esters according to the present invention are added in addition to CMF.

[0048] [Comparative Examples 1-1 to 1-9] The electrolytes for Comparative Examples 1-1 to 1-9 were prepared without adding CMF. In Comparative Example 1-1, the electrolyte was prepared in the same manner as in Example 1-1, except that CMF was not added. In Comparative Example 1-2, the electrolyte 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 electrolytes 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 was prepared in the same manner as in Comparative Example 1-1, except that a non-aqueous solvent of EC and DMC mixed in a 1:2 (volume ratio) was used.

[0049] [Fabrication of lithium-ion secondary batteries (LIBs) and measurement of battery characteristics] A positive electrode composite was prepared by mixing 93% by mass of NCM523 (positive electrode active material), 3% by mass of acetylene black (conductive additive), and 4% by mass of polyvinylidene fluoride (binding agent) in a ratio of 1-methyl-2-pyrrolidone to form a slurry, which was then applied to aluminum foil. The positive electrode was then prepared by drying and pressure molding. Similarly, a negative electrode composite was prepared by adding 98% by mass of artificial graphite (negative electrode active material), 1% by mass of a styrene-butadiene copolymer (binding agent), and 1% by mass of carboxymethylcellulose to water and mixing to form a slurry, which was then applied to copper foil. The negative electrode sheet was then prepared by drying, pressure molding, and heat treatment. Then, a 20-micron microporous film consisting of three layers of polyethylene sandwiched between polypropylene was used as the separator, and coin batteries (coin-type LIBs: 20 mm in diameter, 3.2 mm thick) were fabricated by injecting the electrolytes of Examples 1-1 to 1-14 and Comparative Examples 1-1 to 1-9, respectively.

[0050] This coin cell was charged to an upper voltage limit of 4.3V at 25°C using a charge / discharge device ACD-MO1A (manufactured by Asuka Electronics) at a constant current and constant voltage 1C rate, and then discharged to a lower voltage limit of 3.0V at a 1C rate, repeating the charge / discharge cycle. The discharge capacity at the 5th cycle was calculated as a relative ratio compared to the discharge capacity at the 5th cycle of the case where CMF and other compounds were not added using a non-aqueous solvent mixed with EC and DMC in a 1:2 (volume ratio) (Comparative Examples 1-3). The cycle characteristic (%) was calculated by multiplying the obtained capacity (mAh / g) at the 50th cycle by the 5th cycle × 100. The results are shown in Table 5.

[0051] [Table 5]

[0052] The results in Table 5 show that in batteries using a graphite anode, even when using a PC-based electrolyte, excellent cycle characteristics were 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 example without CMF, destruction of the graphite anode occurred, and the battery could not be satisfactorily charged and discharged as a secondary battery (Comparative Example 1-1). Furthermore, even when n-octyl formic acid, which has the same formic acid group as CMF, was added, partial destruction of the graphite anode occurred, and sufficient cycle characteristics as a secondary battery could not be obtained (Comparative Example 1-2). Even when a compound selected from phosphonic acid esters, carbonate esters, oxalic acid esters, and methanesulfonic acid esters in the present invention was added, destruction of the graphite anode occurred when CMF was not added, and the battery could not be satisfactorily charged and discharged as a secondary battery (Comparative Examples 1-4 to 1-9). On the other hand, excellent cycle characteristics were obtained when these compounds were used in combination with CMF (Examples 1-6 to 1-18). Furthermore, the results of Comparative Examples 1-3, in which EC was used as a non-aqueous solvent, were used as a basis for determining the discharge capacity ratio at the 5th cycle. However, since EC is a solid at room temperature while PC is a liquid at low temperatures, improving the cycle characteristics of PC-based electrolytes can broaden the temperature range in which the electrolyte can be applied. In addition, although the cycle characteristics of Comparative Examples 1-3 were 98%, the capacity decreased after the 50th cycle. In contrast, in Examples 1-5, in which EC was used as a non-aqueous solvent, the decrease in capacity was minimal even after the 50th cycle.

[0053] [Examples 2-1 to 2-16] [Preparation of electrolyte solution] LiFSI was dissolved in a non-aqueous solvent prepared by mixing EC, EMC, and DMC in a 3:3:4 (volume ratio). CMF was added to the resulting solution to prepare the electrolyte of Example 2-1. The CMF content in the prepared electrolyte was 1.3% by mass, and the LiFSI content was 1.2 mol / L. In Example 2-2, the electrolyte was prepared in the same manner as in Example 2-1, except that LiPF6 was added instead of LiFSI as the electrolyte. In Example 2-3, the electrolyte was prepared in the same manner as in Example 2-1, except that a non-aqueous solvent prepared by mixing PC, EMC, and DMC in a 3:3:4 (volume ratio) was used. In Examples 2-4 to 2-16, LiFSI was dissolved in a non-aqueous solvent prepared by mixing EC, EMC, and DMC in a 3:3:4 (volume ratio). Then, CMF and compounds in which A, B, C, D, E, and F are represented by formulas (I), (II), (III), or (IV) in the present invention are A, B, C, D, E, and F as listed in the column for "Types of Compounds Other Than CMF" in Table 6 were added to prepare the electrolyte. The content of CMF, these compounds, and LiFSI in the prepared electrolyte is as shown in Table 6. Examples 2-4 to 2-16 are examples in which phosphonic acid esters, carbonate esters, oxalic acid esters, or methanesulfonic acid esters in the present invention were added in addition to CMF. The mass % of CMF and the mass % of compounds other than CMF in Table 6 represent the respective proportions to the total mass of the prepared electrolyte, and the M (mol / L) of the electrolyte in Table 6 represents the respective proportions to the total volume of the prepared electrolyte.

[0054] [Comparative Examples 2-1 to 2-4] The electrolytes for 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 Comparative Example 2-2, the electrolyte was prepared in the same manner as in Example 2-1, except that octyl formate was added in the amount shown in Table 6 instead of CMF. In Comparative Example 2-3, the electrolyte was prepared in the same manner as in Example 2-1, except that FEC was added in the amount shown in Table 6 instead of CMF. In Comparative Example 2-4, the electrolyte was prepared in the same manner as in Example 2-1, except that a compound in which A and B in the compound represented by formula (I) of this specification were as shown in Table 6 was added in the amount shown in Table 6 instead of CMF.

[0055] [Measurement of corrosion resistance] The measurements were performed using a triode beaker cell with Al, Li, and Li as the working electrode, counter electrode, and reference electrode, respectively. The electrolytes used were those from Examples 2-1 to 2-12 and Comparative Examples 2-1 to 2-4. The measurement conditions were 25°C, 5mV / sec, 4.5 to 3.0V, and the current value was determined for up to 10 cycles using cyclic voltammetry (CV). The obtained current value was quantified by multiplying the 10th cycle value by the 2nd cycle value by 100%. The results are shown in Table 6.

[0056] [Table 6]

[0057] The results in Table 6 show that in the examples where CMF was added to the electrolyte (Examples 2-1, 2-2, and 2-3), the numerical value representing corrosion resistance decreased compared to the example where CMF was not added (Comparative Example 2-1), resulting in improved corrosion resistance. Furthermore, in the examples where a compound selected from the phosphonic acid esters, carbonate esters, oxalic acid esters, and methanesulfonic acid esters of the present invention was added in addition to CMF (Examples 2-4 to 2-16), the numerical value representing corrosion resistance decreased further, resulting in even greater corrosion resistance. Thus, the results in Table 6 show that by using CMF, the numerical value representing corrosion resistance decreases, improving corrosion resistance in LIBs. In addition, it was found that by combining CMF with a compound selected from the phosphonic acid esters, carbonate esters, oxalic acid esters, and methanesulfonic acid esters of the present invention as an auxiliary, the numerical value representing corrosion resistance decreases further, further improving corrosion resistance in LIBs.

[0058] On the other hand, in Comparative Example 2-2, which added n-octyl formic acid described in Patent Document 2, which has the same formic acid group as CMF; Comparative Example 2-3, which added FEC, which has a fluorine group; and Comparative Example 2-4, which has the same phosphonic acid skeleton but B is -CH2COOC2H5, which is outside the scope of the present invention, it was found that the corrosion resistance was significantly worse in all cases. From this, it can be said that not just any formic acid group or phosphonic acid skeleton will do, but that the combination with a specific skeleton is important.

[0059] [Example 3-1] LiFSI, LiPF6, LiPO2F2, and LiiOSO3CH3 were dissolved in a non-aqueous solvent prepared by mixing EC, PC, PS, EMC, and DMC in a volume ratio of 2:1:0.5:3:3.5. To this solution, CMF and a compound whose C and D components are those listed in the "Types of Compounds Other Than CMF" column of Table 7 were added to prepare the electrolyte. The content of CMF, compounds other than CMF, and LiFSI, LiPF6, LiPO2F2, and LiiOSO3CH3 in the prepared electrolyte was as shown in Table 7. The mass percentages of CMF and compounds other than CMF in Table 7 represent their respective proportions to the total mass of the prepared electrolyte, and the M (mol / L) of the electrolyte in Table 7 represents their respective proportions to the total volume of the prepared electrolyte. A coin cell similar to that in Example 1-1 was constructed using the prepared electrolyte, the positive electrode active material, and the negative electrode active material listed in Table 7, and the battery characteristics were measured. Furthermore, a triode beaker cell similar to that in Example 2-1 was prepared, and its corrosion resistance was measured. The results are shown in Table 7.

[0060] [Example 3-2] LiFSI was dissolved in a non-aqueous solvent prepared by mixing PC, FEC, and CMF in a 1:1:8 (volume ratio). To the resulting solution, compounds in which A and B in the compound represented by formula (I) in the present invention are A and B listed in the column for types of compounds other than CMF in Table 7 were added to prepare an electrolyte. The contents of CMF, compounds other than CMF, and LiFSI in the prepared electrolyte were as shown in Table 7. A coin cell was fabricated using the prepared electrolyte, the positive electrode active material and the negative electrode active material listed in Table 7. Acetylene black was used as the conductive material and PVDF as the binder for the negative electrode active material, with a mass ratio of negative electrode active material, conductive material and binder of 85:10:5, and N-methylpyrrolidone was used as the solvent to prepare a slurry. After coating the prepared slurry onto aluminum foil, it was dried, pressure molded, and heat treated to obtain a negative electrode sheet, and a coin cell similar to that in Example 3-1 was fabricated. The charge and discharge conditions were as follows: at 45°C, the battery was charged at a constant current and constant voltage 1C rate up to an upper voltage limit of 2.8V, and then discharged at a 1C rate down to a lower voltage limit of 1.4V to measure its characteristics. Corrosion resistance was measured using a three-electrode beaker cell similar to that used in Example 2-1. The results are shown in Table 7.

[0061] [Table 7]

[0062] Table 7 shows that, in addition to highly corrosive LiFSI, non-aqueous electrolytes containing fluorine-containing compounds FEC and SO3-containing compounds 1,3-propanesultone (PS) also exhibit improved cycle characteristics and corrosion resistance.

[0063] [Example 4-1] LiFSI, LiPF6, and LiiOSO3CH3 were dissolved in a non-aqueous solvent prepared by mixing EC, EMC, and DMC in a 3:3:3 (volume ratio). To the resulting solution, CMF and a compound whose C and D components in formula (II) of the present invention are those listed in the "Types of Compounds Other Than CMF" column of Table 8 were added to prepare an electrolyte. The content of CMF, compounds other than CMF, LiFSI, and LiPF6 in the prepared electrolyte is as shown in Table 8. The mass % of CMF and the mass % of compounds other than CMF in Table 8 represent their respective proportions to the total mass of the prepared electrolyte, and the M (mol / L) of the electrolyte in Table 8 represents their respective proportions to the total volume of the prepared electrolyte. A coin cell similar to that in Example 1-1 was prepared using the prepared electrolyte and the positive and negative electrode active materials listed in Table 8. The charge and discharge conditions were 25°C, constant current and constant voltage at a 1C rate. The battery was charged up to an upper voltage limit of 4.45V, and then discharged at a 1C rate down to a lower voltage limit of 3.0V to measure its characteristics. In addition, a three-electrode beaker cell similar to that in Example 2-1 was prepared, and its corrosion resistance was measured. The results are shown in Table 8.

[0064] [Example 4-2] The electrolyte was prepared in the same manner as in Example 4-1, except that a portion of the EMC was replaced with ethyl formate, and LiFSI, LiPF6, and LiiOSO3CH3 were dissolved in a non-aqueous solvent prepared by mixing EC, EMC, DMC, and ethyl formate in a volume ratio of 3:2:4:1. The battery characteristics and corrosion resistance were then measured. The content of each component in the electrolyte and the measurement results are shown in Table 8. In Example 4-2, a compound in which E in the compound represented by formula (III) is the E listed in the column for types of compounds other than CMF in Table 8, and a compound in which F in the compound represented by formula (IV) is the F listed in the column for types of compounds other than CMF in Table 8 were added.

[0065] [Table 8]

[0066] [Example 4-3] The electrolyte was prepared in the same manner as in Example 4-2, except that CMF was replaced with 2-cyanoethyl formate (CEF), and the battery characteristics and corrosion resistance were measured. The content of each component in the electrolyte and the measurement results are shown in Table 9.

[0067] [Table 9]

[0068] Tables 8 and 9 show that even batteries with a high voltage limit of 4.45V exhibit improved cycle characteristics and corrosion resistance. Based on the results in Tables 7 to 9, it is expected that even when using a large number of LIBs, such as in automotive batteries, they will maintain excellent cycle characteristics and corrosion resistance over long periods. [Industrial applicability]

[0069] By using the non-aqueous electrolyte of the present invention, it is possible to achieve excellent battery characteristics such as battery cycle performance, as well as safety features such as corrosion resistance. The contribution of this invention to the long-term use of numerous lithium-ion secondary batteries, such as those used in automobiles, is immeasurable.

Claims

1. An electrolyte for lithium-ion secondary batteries, wherein the electrolyte is dissolved in a non-aqueous solvent, and the electrolyte contains cyanomethyl formate and / or 2-cyanoethyl formate. Phosphonic acid ester compounds represented by the following formula (I): 【Chemistry 1】 (In the formula, A and B independently represent a methyl group, an ethyl group, a 2-cyanoethyl group, a 1-cyanoethyl group, a 2-cyano-2-propyl group, or a 2-propynyl group, and B represents a methyl group, an ethyl group, a vinyl group, or a cyanomethyl group.) Carbonate ester compounds represented by the following formula (II): 【Chemistry 2】 (In the formula, C and D independently represent a methyl group or an ethyl group, and D represents a 2-cyanoethyl group, a 1-cyanoethyl group, a 2-cyano-2-propyl group, or a 2-propynyl group.) Oxalate ester compounds represented by the following formula (III): 【Transformation 3】 (In the formula, E represents a 2-cyanoethyl group, a 1-cyanoethyl group, a 2-cyano-2-propyl group, or a 2-propynyl group.) and Methanesulfonic acid ester compounds represented by the following formula (IV): 【Chemistry 4】 It contains at least one selected from the group consisting of (wherein F represents a 2-cyanoethyl group, a 1-cyanoethyl group, a 2-cyano-2-propyl group, or a 2-propynyl group), The content of cyanomethyl formate and / or 2-cyanoethyl formate is 0.1 to 80% by mass. An electrolyte for lithium-ion secondary batteries, characterized in that the total content of the phosphonic acid ester compound represented by formula (I), the carbonate ester compound represented by formula (II), the oxalic acid ester compound represented by formula (III), and the methanesulfonic acid ester compound represented by formula (IV) is 0.1 to 10% by mass.

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

Images