Lithium ion battery electrolyte and lithium ion battery

Abstract

The application belongs to the technical field of lithium ion batteries, and particularly relates to a lithium ion battery electrolyte, which comprises an organic solvent, a lithium salt, a first additive and a second additive; the first additive is tris(2-cyanoethyl)borate; and the second additive is a silane additive. The electrolyte is compounded by the first additive and the second additive, and can effectively improve the high-temperature storage and cycle performance, low-temperature discharge and cycle performance of the lithium ion battery under high voltage (4.5 V). Meanwhile, the application further provides a lithium ion battery electrolyte.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and particularly relates to a lithium ion battery electrolyte and a lithium ion battery. BACKGROUND

[0002] Tris(2-cyanoethyl)borate has been used in the field of lithium ion batteries since 2020. The use of tris(2-cyanoethyl)borate in electrolyte can be traced back to the following comparison documents in China:

[0003] D1: CN202011121295.X discloses a kind of electrolyte for lithium ion battery and lithium ion battery including the electrolyte, the electrolyte includes non-aqueous organic solvent, additive and lithium salt, the additive includes 3-methyl-2-thio imidazoline-1-methyl formate, tris(2-cyanoethyl)borate and tris(trimethylsilyl) phosphate;The lithium ion battery has good high and low temperature charge-discharge performance while having high safety.

[0004] D2: CN202110634242.6 discloses a kind of lithium ion battery, the lithium ion battery has excellent cycle life while having high energy density, the electrolyte functional additive in the non-aqueous electrolyte of the lithium ion battery includes tris(2-cyanoethyl)borate, which can be firmly adsorbed on the positive electrode surface (especially the positive electrode with high effective compaction density), stabilize the electrode / electrolyte two-phase interface, inhibit the dissolution of transition metal ions and the oxidation decomposition of electrolyte components, realize the stable battery system, so that the lithium ion battery realizes excellent cycle life while having high energy density.

[0005] The embodiments of the application respectively verify that the use of tris(2-cyanoethyl)borate alone in the dosage of 0.5-5% effectively improves the high-temperature cycle performance.

[0006] D3: CN202110632599.0 discloses a kind of lithium ion battery, the lithium ion battery prepared by the synergistic effect of the separator and the electrolyte under the combination of positive and negative electrode materials can effectively improve the safety performance of the battery while taking into account the low temperature performance of the battery. The lithium ion battery includes non-aqueous electrolyte;The non-aqueous electrolyte includes non-aqueous organic solvent, additive and lithium salt, the synergistic effect of the additive and the solvent in the electrolyte formula makes the battery take into account the high and low temperature performance, wherein tris(2-cyanoethyl)borate and trimethylsilyl-substituted methylsulfonyl amide compounds can jointly form a relatively thick and stable CEI protective film on the positive electrode surface, improve the stability of the positive electrode material at high temperature and high voltage, prevent the electrolyte from being oxidized on the positive electrode surface, and reduce the heat release of side reactions.

[0007] D4: CN202111479167.7 discloses an electrolyte for lithium ion battery using silicon-carbon material as negative electrode and lithium ion battery comprising the electrolyte. The electrolyte is composed of lithium salt, non-aqueous organic solvent and additive. The additive is one or both of tris (hexafluoroisopropyl) borate and tris (2-cyanoethyl) borate. The lithium salt is at least one of LiPF6, LiBF4, LiClO4, LiTFSI and LiFSI, preferably LiPF6; the concentration is 0.8-1.5 mol / L, preferably 1.0-1.2 mol / L. The non-aqueous organic solvent is at least two of ethylene carbonate, dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate, preferably a combination of ethylene carbonate and dimethyl carbonate.

[0008] The advantage of D1 is that high and low temperature performance and safety performance are considered.

[0009] The advantage of D2 is that the high temperature cycle performance is effectively improved.

[0010] The advantage of D3 is that the positive electrode film formation is promoted, and the stability of the positive electrode material at high temperature and high voltage is improved.

[0011] The advantage of D4 is that the cycle life of the battery at room temperature and high temperature is prolonged.

[0012] It can be seen that the use of single or combination with other additives has potential in improving high and low temperature performance.

[0013] The main purpose of the case is to find other additives that can be compounded with tris (2-cyanoethyl) borate to improve the high temperature storage, cycle performance at high voltage (4.5V) and low temperature discharge and cycle performance of lithium ion battery. SUMMARY

[0014] The purpose of the present application is to provide a lithium ion battery electrolyte, which uses a combination of first and second additives, which can effectively improve the low temperature discharge, high temperature storage and room temperature cycle performance of LiCoO2 battery at high voltage (4.5V).

[0015] Meanwhile, the present application also provides a lithium ion battery electrolyte.

[0016] The technical solution of the present application is:

[0017] A lithium ion battery electrolyte, the electrolyte is composed of organic solvent, lithium salt and first additive, second additive, the first additive is tris (2-cyanoethyl) borate, the second additive is silane additive, the weight ratio of the first additive and the second additive is 0.1-10:1;

[0018] The silane additive has the following structural formula:

[0019]

[0020] At least one of R1, R2, R3, R4 is vinyl and the rest are each independently selected from H, alkyl, alkoxy or phenyl.

[0021] The alkyl group is preferably an alkyl group having a carbon number of 1-6; such as methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, etc.

[0022] The alkoxy group is generally methoxy or ethoxy; but does not exclude butoxy, etc.

[0023] Preferably, the weight ratio of the first additive and the second additive is 0.5-5:1.

[0024] More preferably, the weight ratio of the first additive and the second additive is 0.5-3:1.

[0025] In the above lithium ion battery electrolyte, the weight of the first additive is 0.1%-10% of the total weight of the electrolyte; preferably, the weight of the first additive is 0.5%-5% of the total weight of the electrolyte; more preferably, the weight of the first additive is 0.5%-3% of the total weight of the electrolyte.

[0026] The weight of the second additive is 0.1%-10% of the total weight of the electrolyte. Preferably, the weight of the second additive is 0.5%-5% of the total weight of the electrolyte; more preferably, the weight of the second additive is 0.5%-3% of the total weight of the electrolyte.

[0027] In the above lithium ion battery electrolyte, the weight of the first additive is 0.5%-2% of the total weight of the electrolyte;

[0028] The weight of the second additive is 0.5%-2% of the total weight of the electrolyte.

[0029] In some embodiments, the weight of the first additive is 0.1%, 0.3%, 0.5%, 1%, 2%, 3%, 4%, 5%, 7%, 8%, 10% of the total weight of the electrolyte;

[0030] The weight of the second additive is 0.1%, 0.3%, 0.5%, 1%, 2%, 3%, 4%, 5%, 7%, 8%, 10% of the total weight of the electrolyte;

[0031] In some embodiments, the first and second additives are present in amounts of 0.5% and 0.5%, 0.5% and 1%, 0.5% and 2%, 0.5% and 3%, 0.5% and 4%, 0.5% and 5%, 1% and 0.5%, 2% and 0.5%, 3% and 0.5%, 4% and 0.5%, 5% and 0.5%, respectively.

[0032] In the above lithium ion battery electrolyte, the second additive is tetraethenesilane, methyltrivinylsilane, dimethyldivinylsilane, methoxytrivinylsilane, dimethoxydivinylsilane, vinyltrimethoxysilane, vinylphenylmethylsilane.

[0033] In the above lithium ion battery electrolyte, a third additive is also included, the third additive is tris(trimethylsilyl)borate, the amount of the third additive is 0.1% to 1% of the total weight of the electrolyte.

[0034] As preferred in the present application, an appropriate amount of a fourth additive can also be added, the fourth additive is preferably controlled to be below 1% of the total weight of the electrolyte; one or more of the following: nitrile additive, aromatic additive, isocyanate additive, other triple bond-containing additive, S=O group-containing additive, cyclic acetal additive, other P-containing additive, cyclic anhydride additive, cyclic phosphazene additive, fluorine-containing additive;

[0035] More specifically, as follows:

[0036] one or two or more of acetonitrile, propionitrile, butanedinitrile, pentanedinitrile, hexanedinitrile, heptanedinitrile, octanedinitrile, and decanedinitrile; aromatic compounds having branched alkyl groups such as cyclohexylbenzene, fluorocyclohexylbenzene compounds (1-fluoro-2-cyclohexylbenzene, 1-fluoro-3-cyclohexylbenzene, 1-fluoro-4-cyclohexylbenzene), t-butylbenzene, t-amylbenzene, 1-fluoro-4-t-butylbenzene, and the like, biphenyl, terphenyl (ortho, meta, para), diphenyl ether, fluorobenzene, difluorobenzene (ortho, meta, para), anisole, 2,4-difluoroanisole, partially hydrogenated terphenyls (1,2-dicyclohexylbenzene, 2-phenylbicyclohexyl, 1,2-diphenylcyclohexane, o-cyclohexylbiphenyl), and the like; isocyanate compounds selected from one or two or more of methyl isocyanate, ethyl isocyanate, butyl isocyanate, phenyl isocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, octamethylene diisocyanate, 1,4-phenylene diisocyanate, 2-isocyanatoethyl acrylate, and 2-isocyanatoethyl methacrylate; compounds having a triple bond selected from one or two or more of 2-propynyl methyl carbonate, 2-propynyl acetate, 2-propynyl formate, 2-propynyl methacrylate, 2-propynyl methanesulfonate, 2-propynyl vinylsulfonate, 2-propynyl 2-(methanesulfonyloxy)propionate, bis(2-propynyl)oxalate, 2-propynyl methyl oxalate, 2-propynyl ethyl oxalate, glutaric acid bis(2-propynyl ester), 2-butyn-l,4-diyl dimethanesulfonate, 2-butyn-l,4-diyl diformate, and 2,4-hexadiyn-l,6-diyl dimethanesulfonate; sulfine compounds selected from one or two or more of 1,3-propane sulfine, 1,3-butane sulfine, 2,4-butane sulfine, 1,4-butane sulfine, 1,3-propene sulfine, 2,2-dioxo-l,2-oxathiolane-4-yl acetate, 5,5-dimethyl-l,2-oxathiolane-4-ketone 2,2-dioxide, and the like, ethylene sulfite, hexahydrobenzo[l,3,2]dioxathiolane-2-oxide (also called 1,2-cyclohexanediol cyclic sulfite), 5-vinyl-hexahydro-l,3,2-benzodioxathiolane-2-oxide, and the like, butane-2,3-diyl dimethanesulfonate, butane-l,4-diyl dimethanesulfonate, methylene methane disulfonate, and the like; sulfonic acid esters; divinyl sulfone, 1,2-bis(vinylsulfonyl)ethane, bis(2-vinylsulfonyl ethyl)ether, and the like; compounds having S=0 groups; cyclic acetal compounds selected from 1,3-dioxolane, 1,3-dioxane, 1,3,5-trioxane, and the like;one or two or more kinds of phosphorus-containing compounds selected from the group consisting of trimethyl phosphate, tributyl phosphate, trioctyl phosphate, tris (2, 2, 2-trifluoroethyl) phosphate, bis (2, 2, 2-trifluoroethyl) methyl phosphate, bis (2, 2, 2-trifluoroethyl) ethyl phosphate, bis (2, 2, 2-trifluoroethyl) 2, 2-difluoroethyl phosphate, bis (2, 2, 2-trifluoroethyl) 2, 2, 3, 3-tetrafluoropropyl phosphate, bis (2, 2-difluoroethyl) 2, 2, 2-trifluoroethyl phosphate, bis (2, 2, 3, 3-tetrafluoropropyl) 2, 2, 2-trifluoroethyl phosphate, (2, 2, 2-trifluoroethyl) (2, 2, 3, 3-tetrafluoropropyl) methyl phosphate, tris (1, 1, 1, 3, 3, 3-hexafluoropropane-2-yl) phosphate, methylene bis (methyl phosphonate), methylene bis (ethyl phosphonate), ethylene bis (methyl phosphonate), ethylene bis (ethyl phosphonate), butylene bis (methyl phosphonate), butylene bis (ethyl phosphonate), methyl 2- (dimethylphosphoryl) acetate, ethyl 2- (dimethylphosphoryl) acetate, methyl 2- (diethylphosphoryl) acetate, ethyl 2- (diethylphosphoryl) acetate, 2-propynyl 2- (dimethylphosphoryl) acetate, 2-propynyl 2- (diethylphosphoryl) acetate, methyl 2- (dimethoxyphosphoryl) acetate, ethyl 2- (dimethoxyphosphoryl) acetate, methyl 2- (diethoxyphosphoryl) acetate, ethyl 2- (diethoxyphosphoryl) acetate, 2-propynyl 2- (dimethoxyphosphoryl) acetate, 2-propynyl 2- (diethoxyphosphoryl) acetate, methyl pyrophosphate, and ethyl pyrophosphate; chain carboxylic acid anhydrides such as acetic anhydride, propionic anhydride, or cyclic acid anhydrides such as succinic anhydride, maleic anhydride, 2-allylsuccinic anhydride, glutaric anhydride, itaconic anhydride, 3-sulfo-propionic anhydride; cyclic phosphazene compounds such as methoxy pentafluorocyclotriphosphazene, ethoxy pentafluorocyclotriphosphazene, phenoxy pentafluorocyclotriphosphazene, or ethoxy heptafluorocyclotetraphosphazene; fluorinated compounds such as methyl ethyl fluorocarbonate, dimethyl fluorocarbonate, diethyl fluorocarbonate, ethyl propyl fluorocarbonate, propyl propyl fluorocarbonate, methyl propyl fluorocarbonate, ethyl methyl fluorocarbonate, or propyl methyl fluorocarbonate;

[0037] In the above-described lithium ion battery electrolyte, the organic solvent can be a chain and / or cyclic carbonate. Chain carbonates can include dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, diisopropyl carbonate, n-propyl isopropyl carbonate, methyl ethyl carbonate, methyl n-propyl carbonate, methyl n-butyl carbonate, methyl isobutyl carbonate, methyl t-butyl carbonate, ethyl n-propyl carbonate, ethyl n-butyl carbonate, ethyl isobutyl carbonate, ethyl t-butyl carbonate, and the like. Cyclic carbonates can include ethylene carbonate, propylene carbonate, and butylene carbonate.

[0038] In some embodiments of the present application, the solvent is selected from two or more of ethylene carbonate, propylene carbonate, butyric carbonate, fluoroethylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, and methyl propyl carbonate.

[0039] The above description of the organic solvent does not mean that the above solvent system cannot contain other types of solvents, which are common solvents that can be selected for the lithium salt, such as cyclic carboxylic acid esters, chain carboxylic acid esters, ether compounds, and sulfone compounds, which can be added. In the present embodiment, the addition amount of the above cyclic carboxylic acid esters, chain carboxylic acid esters, ether compounds, and sulfone compounds should not exceed 30% by weight of the solvent.

[0040] Specific examples of the cyclic carboxylic acid ester can be selected from γ-butyrolactone, γ-valerolactone, γ-hexalactone, ε-hexalactone, and the like. This can avoid a decrease in conductivity, inhibit an increase in negative electrode resistance, and easily achieve a good range of large-current discharge characteristics of the nonaqueous electrolyte secondary battery.

[0041] The chain carboxylic acid ester is preferably a chain carboxylic acid ester having 3 to 7 carbon atoms. Specific examples can include methyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, n-propyl propionate, isopropyl propionate, n-butyl propionate, isobutyl propionate, t-butyl propionate, methyl butyrate, ethyl butyrate, n-propyl butyrate, isopropyl butyrate, methyl isobutyrate, ethyl isobutyrate, n-propyl isobutyrate, isopropyl isobutyrate, and the like. The chain carboxylic acid ester can inhibit an increase in negative electrode resistance, and achieve a good range of large-current discharge characteristics and cycle characteristics of the nonaqueous electrolyte battery.

[0042] The ether compound is preferably a linear ether having 3 to 10 carbon atoms, in which part of the hydrogen is optionally replaced with fluorine, and a cyclic ether having 3 to 6 carbon atoms. As the linear ether having 3 to 10 carbon atoms, there are mentioned diethyl ether, di(2-fluoroethyl) ether, di(2,2-difluoroethyl) ether, di(2,2,2-trifluoroethyl) ether, ethyl(2-fluoroethyl) ether, ethyl(2,2,2-trifluoroethyl) ether, ethyl(l,l,2,2-tetrafluoroethyl) ether, (2-fluoroethyl)(2,2,2-trifluoroethyl) ether, (2-fluoroethyl)(l,l,2,2-tetrafluoroethyl) ether, (2,2,2-trifluoroethyl)(l,l,2,2-tetrafluoroethyl) ether, ethyl-n-propyl ether, ethyl(3-fluoro-n-propyl) ether, ethyl(3,3,3-trifluoro-n-propyl) ether, ethyl(2,2,3,3-tetrafluoro-n-propyl) ether, ethyl(2,2,3,3,3-pentafluoro-n-propyl) ether, 2-fluoroethyl-n-propyl ether, (2-fluoroethyl)(3-fluoro-n-propyl) ether, (2-fluoroethyl)(3,3,3-trifluoro-n-propyl) ether, (2-fluoroethyl)(2,2,3,3-tetrafluoro-n-propyl) ether, (2-fluoroethyl)(2,2,3,3,3-pentafluoro-n-propyl) ether, 2,2,2-trifluoroethyl-n-propyl ether, (2,2,2-trifluoroethyl)(3-fluoro-n-propyl) ether, (2,2,2-trifluoroethyl)(3,3,3-trifluoro-n-propyl) ether, (2,2,2-trifluoroethyl)(2,2,3,3-tetrafluoro-n-propyl) ether, (2,2,2-trifluoroethyl)(2,2,3,3,3-pentafluoro-n-propyl) ether, l,l,2,2-tetrafluoroethyl-n-propyl ether, (l,l,2,2-tetrafluoroethyl)(3-fluoro-n-propyl) ether, (l,l,2,2-tetrafluoroethyl)(3,3,3-trifluoro-n-propyl) ether, (l,l,2,2-tetrafluoroethyl)(2,2,3,3-tetrafluoro-n-propyl) ether, (l,l,2,2-tetrafluoroethyl)(2,2,3,3,3-pentafluoro-n-propyl) ether, di-n-propyl ether, (n-propyl)(3-fluoro-n-propyl) ether, (n-propyl)(3,3,3-trifluoro-n-propyl) ether, (n-propyl)(2,2,3,3-tetrafluoro-n-propyl) ether, (n-propyl)(2,2,3,3,3-pentafluoro-n-propyl) ether, di(3-fluoro-n-propyl) ether, (3-fluoro-n-propyl)(3,3,3-trifluoro-n-propyl) ether, (3-fluoro-n-propyl)(2,2,3,3-tetrafluoro-n-propyl) ether, (3-fluoro-n-propyl)(2,2,3,3,3-pentafluoro-n-propyl) ether, di(3,3,3-trifluoro-n-propyl) ether, (3,3,3-trifluoro-n-propyl)(2,2,3,3-tetrafluoro-n-propyl) ether, (3,3,3-trifluoro-n-propyl)(2,2,3,3,3-pentafluoro-n-propyl) ether, di(2,2,3,3-tetrafluoro-n-propyl) ether, (2,2,3,3-tetrafluoro-n-propyl)(2,2,3,3,3-pentafluoro-n-propyl) ether,3-pentafluoro-n-propyl) ether, di-n-butyl ether, dimethoxymethane, methoxyethoxymethane, methoxy(2-fluoroethoxy)methane, methoxy(2,2,2-trifluoroethoxy)methane, methoxy(l, 1,2,2-tetrafluoroethoxy)methane, diethoxymethane, ethoxy(2-fluoroethoxy)methane, ethoxy(2,2,2-trifluoroethoxy)methane, ethoxy(l, 1,2,2-tetrafluoroethoxy)methane, di(2-fluoroethoxy)methane, (2-fluoroethoxy)(2,2,2-trifluoroethoxy)methane, (2-fluoroethoxy)(l, 1,2,2-tetrafluoroethoxy)methane, di(2,2,2-trifluoroethoxy)methane, (2,2,2-trifluoroethoxy)(l, 1,2,2-tetrafluoroethoxy)methane, di(l, 1,2,2-tetrafluoroethoxy)methane, dimethoxyethane, methoxyethoxyethane, methoxy(2-fluoroethoxy)ethane, methoxy(2,2,2-trifluoroethoxy)ethane, methoxy(l, 1,2,2-tetrafluoroethoxy)ethane, diethoxyethane, ethoxy(2-fluoroethoxy)ethane, ethoxy(2,2,2-trifluoroethoxy)ethane, ethoxy(l, 1,2,2-tetrafluoroethoxy)ethane, di(2-fluoroethoxy)ethane, (2-fluoroethoxy)(2,2,2-trifluoroethoxy)ethane, (2-fluoroethoxy)(l, 1,2,2-tetrafluoroethoxy)ethane, di(2,2,2-trifluoroethoxy)ethane, (2,2,2-trifluoroethoxy)(l, 1,2,2-tetrafluoroethoxy)ethane, di(l, 1,2,2-tetrafluoroethoxy)ethane, ethyleneglycol di-n-propyl ether, ethyleneglycol di-n-butyl ether, diethyleneglycol dimethyl ether, and the like; as the cyclic ether having 3 to 6 carbon atoms, there can be mentioned tetrahydrofuran, 2-methyltetrahydrofuran, 3-methyltetrahydrofuran, 1,3-dioxolane, 2-methyl-l,3-dioxolane, 4-methyl-l,3-dioxolane, 1,4-dioxolane, and the like, and fluorinated compounds thereof;

[0043] In the case where the negative electrode active material is a carbonaceous material, the ether compound is less likely to be co-intercalated with lithium ions along with the lithium ions, and thus the capacity decrease problem can be avoided;

[0044] The sulfone compound can be selected from dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, n-propyl methyl sulfone, isopropyl methyl sulfone, n-butyl methyl sulfone, t-butyl methyl sulfone, monofluoromethyl methyl sulfone, difluoromethyl methyl sulfone, trifluoromethyl methyl sulfone, monofluoroethyl methyl sulfone, difluoroethyl methyl sulfone, trifluoroethyl methyl sulfone, pentafluoroethyl methyl sulfone, ethyl monofluoromethyl sulfone, ethyl difluoromethyl sulfone, ethyl trifluoromethyl sulfone, ethyl trifluoroethyl sulfone, ethyl pentafluoroethyl sulfone, trifluoromethyl n-propyl sulfone, trifluoromethyl isopropyl sulfone, trifluoroethyl n-butyl sulfone, trifluoroethyl t-butyl sulfone, trifluoromethyl n-butyl sulfone, trifluoromethyl t-butyl sulfone, and the like. In the presence of the sulfone compound as an auxiliary solvent, the cycle performance and cycle retention performance of the battery can be improved, the solution viscosity can be reduced, and the electrochemical performance can be improved.

[0045] In the above-described lithium ion battery electrolyte, as the electrolyte in the nonaqueous electrolyte of the present application, there is no particular limitation as long as it is a known lithium salt used in this use, and any of the following lithium salts can be used. Specifically, the following lithium salts can be mentioned.

[0046] For example, inorganic lithium salts such as LiPF6, LiBF4, LiClO4, LiAlF4, LiSbF6, LiTaF6, LiWF7, and the like; lithium tungstate such as LiWOF5;

[0047] For example, lithium carboxylate salts such as HCO2Li, CH3CO2Li, CH2FCO2Li, CHF2CO2Li, CF3CO2Li, CF3CH2CO2Li, CF3CF2CO2Li, CF3CF2CF2CO2Li, and the like;

[0048] For example, lithium sulfonate salts such as FSO3Li, CH3SO3Li, CH2FSO3Li, CHF2SO3Li, CF3SO3Li, CF3CF2SO3Li, CF3CF2CF2SO3Li, CF3CF2CF2CF2SO3Li, and the like;

[0049] For example, lithium imide salts such as LiN(FCO)2, LiN(FCO)(FSO2), LiN(FSO2)2, LiN(FSO2)(CF3SO2), LiN(CF3SO2)2, LiN(C2F5SO2)2, cyclic 1,2-perfluoroethane disulfonimide lithium, cyclic 1,3-perfluoropropane disulfonimide lithium, LiN(CF3SO2)(C4F9SO2), and the like;

[0050] For example, methylated lithium salts such as LiC(FSO2)3, LiC(CF3SO2)3, LiC(C2F5SO2)3, and the like;

[0051] lithium difluorooxalato borate, lithium bis(oxalato)borate, and the like;

[0052] lithium tetrafluorooxalato phosphate, lithium difluorobis(oxalato) phosphate, lithium tris(oxalato) phosphate, and the like;

[0053] LiPF4(C2F5)2, LiPF4(CF3SO2)2, LiPF4(C2F5SO2)2, LiBF3CF3, LiBF3C2F5, LiBF3C3F7, LiBF2(CF3)2, LiBF2(C2F5)2, LiBF2(CF3SO2)2, LiBF2(C2F5SO2)2, and the like.

[0054] These lithium salts can be used alone or in combination of two or more.

[0055] In a more preferred embodiment of the present application, the lithium salt is at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bisoxalate borate, lithium difluorooxalate borate, lithium bis(trifluoromethylsulfonyl)imide, lithium bisfluorosulfonylimide.

[0056] In the above-mentioned lithium ion battery electrolyte, the lithium salt is 10% to 20% of the total weight of the electrolyte.

[0057] Meanwhile, the present application also discloses a lithium ion battery, which is composed of a positive electrode, a negative electrode, a separator arranged between the positive electrode and the negative electrode, and the above-mentioned lithium ion battery electrolyte.

[0058] In the above-mentioned lithium ion battery electrolyte, the positive electrode material is selected from lithium transition metal oxides, wherein the lithium transition metal oxides are LiCoO2, LiMn2O4, LiMnO2, Li2MnO4, LiFePO4, Li 1+a Mn 1-x M x O2, LiCo 1- x M x O2, LiFe 1-x M x PO4, Li2Mn 1-x O4, wherein M is one or more selected from Ni, Co, Mn, Al, Cr, Mg, Zr, Mo, V, Ti, B, F, 0≤a<0.2, 0≤x<1; and the negative electrode material is at least one of graphite, soft carbon, hard carbon, silicon, silicon oxide compound, silicon-carbon composite.

[0059] The present application has the following advantages:

[0060] The silane represented by tris(2-cyanoethyl)borate and tetraethenylsilane can effectively improve the high-temperature storage, cycle performance, low-temperature discharge and cycle performance of the lithium ion battery.

[0061] The tris(2-cyanoethyl)borate can form a stable low-impedance interface film on the surface of the positive electrode sheet and inhibit the dissolution of transition metal ions, which not only enables lithium ions to be more easily reversible extracted / inserted but also protects the material, thereby improving the reversible capacity and cycle stability of the positive electrode; the silane represented by tetraethenylsilane can improve the cycle and storage performance of the battery at 45-60℃ high temperature in the electrolyte.

[0062] Experiments prove that the film-forming function of tris(2-cyanoethyl)borate and the high-temperature improvement function of the silane can synergistically improve the high-temperature and low-temperature performance, and the improvement is significant compared with the use of tris(2-cyanoethyl)borate alone, especially in the low-temperature performance. DETAILED DESCRIPTION

[0063] The technical solutions of the present application will be further described in detail below in combination with specific embodiments, but do not constitute any limitation on the present application.

[0064] Example 1

[0065] 1. Preparation of electrolyte: mix ethylene carbonate (EC), diethyl carbonate (DEC) and methyl ethyl carbonate (EMC) according to the mass ratio of EC:DEC:EMC = 1:1:1, then add 1 mol of lithium hexafluorophosphate (LiPF6) after mixing, and then add 1% of tris(2-cyanoethyl)borate and 1% of tetraethenylsilane after the lithium salt is completely dissolved.

[0066] 2. Preparation of positive electrode sheet: mix positive electrode material lithium cobaltate LiCoO2, conductive agent conductive carbon black (Super P), adhesive polyvinylidene fluoride (PVDF) and carbon nanotube (CNT) according to the mass ratio of 95:2.3:2:0.7 to uniformly prepare lithium ion battery positive electrode slurry with a certain viscosity, coat on both sides of the current collector aluminum foil, dry at 85℃, then cold press; then edge cutting, sheet cutting, striping, striping under vacuum condition at 95℃ for 12 hours, weld the tabs, and prepare the required lithium ion battery positive electrode sheet with a surface density of 33mg / cm 2 .

[0067] 3. Preparation of the negative electrode sheet: graphite, conductive agent Super P, thickening agent sodium carboxymethyl cellulose (CMC), and adhesive styrene-butadiene rubber emulsion (SBR) are mixed in a ratio of 95:1.5:1.0:2.5 by mass to form a slurry, which is uniformly mixed. The mixed slurry is coated on both sides of a copper foil, and then dried at 85°C and cold-pressed. Then, the edges are trimmed, the sheet is cut, and the sheet is divided into strips. The divided strips are dried at 85°C for 12 hours under vacuum, and the tabs are welded to produce a lithium ion battery negative electrode sheet that meets the requirements, with a surface density of 21.1 mg / cm 2 .

[0068] 4. Preparation of the lithium ion battery: the positive electrode sheet, the negative electrode sheet, and the separator prepared according to the above process are stacked to produce a lithium ion battery with a thickness of 4.7 mm, a width of 55 mm, and a length of 60 mm. The theoretical capacity is 1800 mAh. The battery is vacuum baked at 75°C for 10 hours, and the above electrolyte is injected. After standing for 24 hours, the battery is charged at a constant current of 0.1 C (180 mA) to 4.5 V, and then charged at a constant voltage of 4.5 V until the current drops to 0.05 C (90 mA). Then, the battery is discharged at 0.2 C (360 mA) to 3.0 V, and the charging and discharging process is repeated twice. Finally, the battery is charged at 0.2 C (360 mA) to 3.8 V to complete the battery production.

[0069] Examples 2-17 and Comparative Examples 1-3 are substantially the same as Example 1, except for the electrolyte formulation and the positive electrode material. In Example 17, although the positive electrode material is replaced by NCM ternary LiNi 0.5 Mn 0.3 Co 0.2 O2, the battery capacity is still 1800 mAh.

[0070] The electrolyte formulation and the positive electrode material of Examples 2-17 and Comparative Examples 1-3 are shown in Table 1.

[0071] Table 1: Electrolyte formulation and positive electrode material of examples and comparative examples

[0072]

[0073]

[0074]

[0075] High temperature performance test of lithium ion battery

[0076] The lithium ion batteries in Examples 1-17 and Comparative Examples 1-3 are subjected to high temperature cycle performance and high temperature storage performance tests, and the test methods are as follows;

[0077] High temperature cycle performance: at 55℃, the lithium ion battery was first charged at 1C constant current to 4.5V, charged at 4.5V constant voltage to 0.05C, then discharged at 1C constant current to 3V, and 500 cycles of charge and discharge test were carried out, and the discharge capacity of the 500th cycle was recorded.

[0078] Capacity retention rate = (500th discharge capacity / 1st discharge capacity) x 100%

[0079] High temperature storage performance: at room temperature, the lithium ion battery was first charged at 1C constant current to 4.5V, charged at 4.5V constant voltage to 0.05C, and the thickness of the lithium ion battery was tested at this time and recorded as H0; then it was placed in a 60℃ oven for 30 days, and then the thickness was tested and recorded as H1; the lithium ion battery was taken out and cooled to room temperature, and then discharged at 1C to 3.0V, and the discharge capacity was recorded.

[0080] High temperature storage capacity retention rate = (discharge capacity after storage / discharge capacity before storage) x 100%;

[0081] Thickness expansion rate = (H1-H0) / H0 x 100%.

[0082] Lithium ion battery low temperature performance test

[0083] The lithium ion batteries in Examples 1-17 and Comparative Examples 1-3 were tested for low temperature cycle performance and low temperature storage performance, and the test methods were as follows:

[0084] Low temperature cycle performance test: at -20℃, the lithium ion battery was first charged at 0.2C constant current to 4.5V, charged at 4.5V constant voltage to 0.05C, and then discharged at 0.2C to 3.0V, and 100 cycles of charge and discharge test were carried out, and the discharge capacity of the 100th cycle was recorded.

[0085] -20℃ cycle capacity retention rate = (100th discharge capacity / 1st discharge capacity) x 100%

[0086] Low temperature discharge performance test: at room temperature, the lithium ion battery was first charged at 1C constant current to 4.5V, charged at 4.5V constant voltage to 0.05C, then the battery was placed in a -20℃ low temperature cabinet, and the standing time was >4h, and then discharged at 0.5C to 3.0V when the battery temperature dropped to -20℃.

[0087] -20℃ discharge capacity retention rate = (-20℃ 0.5C discharge capacity / room temperature discharge capacity) x 100%

[0088] The test results are shown in Table 1 below:

[0089] Table 1 Test results of high temperature performance and low temperature performance of lithium ion battery

[0090]

[0091]

[0092] Result analysis:

[0093] 1. From the comparative example 1 and comparative example 2, it can be seen that in the LiCoO2 / graphite system, the more the amount of tris(2-cyanoethyl)borate used, the higher the high-temperature and low-temperature performance. From the comparative example 2 and comparative example 3, it can be seen that the combination of tris(2-cyanoethyl)borate and tris(trimethylsilyl)borate is not as good as the use of tris(2-cyanoethyl)borate alone.

[0094] 2. From the comparison of example 1 and comparative examples 1-3, it can be found that the simple addition of tetraethylenylsilane can significantly improve the high-temperature cycle, storage, low-temperature discharge and cycle performance of example 1 compared with comparative example 1. Compared with comparative example 2, the use of tetraethylenylsilane instead of tris(2-cyanoethyl)borate has the same conclusion as comparative example 1. Compared with comparative example 3, the use of tetraethylenylsilane instead of tris(trimethylsilyl)borate can also exhibit similar effects as example 1, comparative example 1 and comparative example 2, indicating that tetraethylenylsilane and tris(2-cyanoethyl)borate have strong internal synergistic effect.

[0095] 3. From the comparison of examples 1-5, it can be found that the excessive use of tetraethylenylsilane, the excessive use of tris(2-cyanoethyl)borate, and the excessive use of both tetraethylenylsilane and tris(2-cyanoethyl)borate cannot achieve the best effect. From the economic point of view, the amount of tris(2-cyanoethyl)borate controlled at 0.5%-2% and the amount of tris(2-cyanoethyl)borate controlled at 0.5%-2% are the best.

[0096] 4. From the comparison of example 1, example 7, examples 9-11, it can be found that the use of tetraethylenylsilane is the best, followed by methoxytrivinyldisilane, and the less the vinyl group, the worse the performance. At the same time, it also proves that the performance improvement advantage of the vinyl group is greater than that of the methoxy group, and the methoxy group is greater than the methyl group.

[0097] 5. From the comparison of example 1, example 6, examples 12, 13 and 15, it can be found that the combination of tris(2-cyanoethyl)borate, methoxytrivinyldisilane and tris(trimethylsilyl)borate is better than the combination of tris(2-cyanoethyl)borate, tetraethylenylsilane and tris(trimethylsilyl)borate, which has certain difference compared with the trend of example 1 and example 8.

[0098] The experiment of the present application can prove that the silane additive and the tris(2-cyanoethyl)borate have strong internal synergy, and the silane additive, the tris(2-cyanoethyl)borate and the tris(trimethylsilyl)borate have strong synergy.

[0099] The above examples are the preferred embodiments of the present application, but the embodiments of the present application are not limited to the above examples, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application should be equivalent replacement methods, and are all included in the protection scope of the present application.

Claims

1. A lithium-ion battery electrolyte, characterized in that, The electrolyte comprises an organic solvent, a lithium salt, a first additive, and a second additive. The first additive is tris(2-cyanoethyl)borate, and the second additive is a silane additive. The weight ratio of the first additive to the second additive is 0.1-10:

1. The silane additive has the following structural formula: Formula 1; At least one of R1, R2, R3, and R4 is a vinyl group, and the others are each independently selected from H, alkyl, alkoxy, or phenyl groups; The weight of the first additive is 0.1% to 10% of the total weight of the electrolyte; The weight of the second additive is 0.1% to 10% of the total weight of the electrolyte; The lithium-ion battery electrolyte is suitable for lithium-ion batteries with a maximum voltage of 4.5V.

2. The lithium-ion battery electrolyte according to claim 1, characterized in that, The weight of the first additive is 0.5% to 2% of the total weight of the electrolyte; The weight of the second additive is 0.5% to 2% of the total weight of the electrolyte.

3. The lithium-ion battery electrolyte according to claim 1, characterized in that, The second additive is tetraethylenesilane, methyltriethylenesilane, dimethyldivinylsilane, methoxytriethylenesilane, dimethoxydivinylsilane, vinyltrimethoxysilane, or vinylphenylmethylsilane.

4. The lithium-ion battery electrolyte according to claim 1, characterized in that, It also includes a third additive, which is tris(trimethylsilane)borate, and the amount of the third additive is 0.1% to 1% of the total weight of the electrolyte.

5. The lithium-ion battery electrolyte according to claim 1, characterized in that, The organic solvent is two or more of the following: ethylene carbonate, propylene carbonate, butyrate carbonate, fluoroethylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, and methyl propyl carbonate.

6. The lithium-ion battery electrolyte according to claim 1, characterized in that, The lithium salt is at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide, and the weight of the lithium salt is 10% to 20% of the total weight of the electrolyte.

7. A lithium-ion battery, characterized in that, The lithium-ion battery comprises a positive electrode, a negative electrode, a separator disposed between the positive and negative electrodes, and a lithium-ion battery electrolyte as described in any one of claims 1 to 6, wherein the maximum voltage of the lithium-ion battery is 4.5V.

8. The lithium-ion battery according to claim 7, characterized in that, The cathode material is selected from lithium transition metal oxides, wherein the lithium transition metal oxide is LiCoO2, LiMn2O4, LiMnO2, Li2MnO4, LiFePO4, Li 1+ a Mn 1-x M x O2, LiCo 1-x M x O2, LiFe 1-x M x PO4, Li2Mn 1-x O4, wherein M is selected from one or more of Ni, Co, Mn, Al, Cr, Mg, Zr, Mo, V, Ti, B, and F, 0≤a<0.2, 0≤x<1; the negative electrode material is at least one of graphite, soft carbon, hard carbon, silicon, silicon oxide, and silicon-carbon composite.

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