Electrolyte, lithium secondary battery and method for improving performance of 4.55v high voltage lithium cobalt oxide cathode battery system

Abstract

The application belongs to the field of new energy, and discloses an electrolyte suitable for a 4.55V high-voltage lithium cobaltate positive electrode battery system, which comprises a lithium salt, a nonaqueous organic solvent, a first additive and a second additive; the first additive is tris(2-cyanoethyl)borate. The electrolyte contains the first additive and the second additive, the first additive and the second additive are both film-forming aids, mainly used for positive electrode film formation, the first additive contains boron atoms with lone pair electrons capable of combining with oxygen in the positive electrode to prevent oxygen precipitation, and it also contains three cyano groups capable of combining with cobalt in the positive electrode; the second additive contains four cyano groups to enhance the effect of the first additive on the combination with cobalt in the positive electrode and better prevent cobalt dissolution; meanwhile, the strength of the composite film formed by the first additive and the second additive is superior to that of the combination of other existing additives, and can effectively improve the electrical performance. Meanwhile, the application also discloses a lithium secondary battery and a method for improving the performance of the 4.55V high-voltage lithium cobaltate positive electrode battery system.

Description

Technical Field

[0001] This invention relates to the field of new energy, specifically to an electrolyte and lithium secondary battery suitable for a 4.55V high-voltage lithium cobalt oxide cathode battery system, as well as a method for improving the performance of the 4.55V high-voltage lithium cobalt oxide cathode battery system. Background Technology

[0002] CN108428940A discloses an electrolyte for lithium secondary batteries and a lithium secondary battery including the same. The electrolyte of the lithium secondary battery uses four cyano-based additives, which can improve DC-IR characteristics and battery storage characteristics, and can also improve high-temperature stability, low-temperature characteristics and lifespan characteristics, thus effectively being used to manufacture secondary batteries. In this scheme, a ternary system is used as the experimental object, and good results are obtained, with an experimental voltage of 4.2V.

[0003] CN112768772A discloses a tetracyano-containing nitrile ether electrolyte additive, comprising compounds having the structure shown in structural formula I: wherein A1 is selected from one of C1-20 alkylene or haloalkylene, or C2-20 alkenyl or haloalkenyl; A2, A3, A4, and A5 are each independently selected from C1-20 alkoxide or haloalkoxide, or C2-20 alkenyloxy or haloalkenyloxy. This additive exhibits good cycling performance under high voltage, as well as high-temperature storage and low-temperature performance when used in electrolytes.

[0004] Examples 8-12 in the instruction manual describe that this material can be used in lithium cobalt oxide cathode systems. Paragraph 42 of the instruction manual states: 25℃ 1.0C / 1.0C cycle test: Charge at 25℃ with a constant current of 1.0C to 4.5V, charge at a constant voltage of 4.5V to the cutoff current of 0.05C, and then discharge the battery with a constant current of 1.0C. The discharge capacity is recorded as C0. Repeat the charge-discharge cycle 1000 times to obtain the discharge capacity C of the 1000th cycle. 1000 The capacity retention rate is calculated as C1000 / C0*100%, indicating that the experimental voltage is 4.5V.

[0005] CN113161616A discloses a lithium-ion battery that exhibits both high energy density and excellent cycle life. The non-aqueous electrolyte of this lithium-ion battery contains a functional additive called tris(2-cyanoethyl)borate, which can be firmly adsorbed onto the surface of the positive electrode (especially positive electrodes with high effective compaction density), stabilizing the electrode / electrolyte interface, inhibiting the dissolution of transition metal ions and the oxidative decomposition of electrolyte components, thus achieving a stable battery system. This allows the lithium-ion battery to achieve both high energy density and excellent cycle life. Paragraph 25 of the specification states: The obtained battery is placed in a constant temperature environment of 45°C and subjected to charge-discharge tests at a rate of 0.7C / 0.5C, with a cutoff voltage range of 3.0V to 4.45V. After 500 and 800 charge-discharge cycles, the cycle discharge capacity is recorded and divided by the discharge capacity of the first cycle to obtain the high-temperature cycle capacity retention rate at 45°C. The cycle capacity retention rates for the 500th and 800th cycles are also recorded. It can be seen that the operating voltage of this system is 4.45V.

[0006] After applying the aforementioned substances or combinations to the lithium cobalt oxide system and increasing the voltage to 4.55V, we found that its high-temperature cycling performance and low-temperature discharge rate still need further improvement. The fundamental reason why the high-temperature cycling performance and low-temperature discharge rate of high-voltage lithium cobalt oxide are difficult to control is that, under high voltage, the electrolyte decomposes to produce HF, which promotes the dissolution of cobalt ions at the cathode. At the same time, under high voltage, oxygen evolution at the cathode is more severe, ultimately deteriorating the high-temperature cycling performance and low-temperature discharge rate. Therefore, this study attempts to improve the above-mentioned problems by optimizing the interface film.

[0007] The technical problem addressed in this case is: how to develop an electrolyte suitable for high-voltage lithium cobalt oxide systems to form a relatively stable and reliable interfacial film. Summary of the Invention

[0008] The purpose of this invention is to provide an electrolyte suitable for a 4.55V high-voltage lithium cobalt oxide cathode battery system. This electrolyte contains a first additive and a second additive, both of which are film-forming aids, mainly used for cathode film formation. The first additive contains boron atoms, which have lone pairs of electrons that can combine with oxygen in the cathode to prevent oxygen evolution. It also contains three cyano groups, which can combine with cobalt in the cathode. The second additive contains four cyano groups, which enhances the binding effect of the first additive with cobalt in the cathode and better prevents cobalt dissolution. At the same time, the composite film formed by the first and second additives has a higher strength than existing combinations of other additives, which can effectively improve electrical performance.

[0009] Meanwhile, the present invention also provides a lithium secondary battery and a method for improving the performance of a 4.55V high-voltage lithium cobalt oxide cathode battery system.

[0010] To achieve the above objectives, the present invention provides the following technical solution: an electrolyte suitable for high-voltage lithium cobalt oxide cathode systems, characterized in that it comprises lithium salt, non-aqueous organic solvent, a first additive, and a second additive;

[0011] The first additive is tris(2-cyanoethyl)boronic acid ester;

[0012] The second additive has the structure shown in Formula 1:

[0013]

[0014] A1 is one of C1-20 alkylene or haloalkylene, C2-20 alkenyl or haloalkenyl; A2, A3, A4, and A5 are each independently selected from C1-20 alkoxide or haloalkoxide, C2-20 alkenyloxy or haloalkenyloxy.

[0015] In Formula 1 above, A1 refers to an alkylene group having 1 to 20 carbon atoms. The alkylene group can be a chain alkyl group or a cycloalkyl group. The hydrogen atom on the ring of the cycloalkyl group can be replaced by an alkyl group. Preferably, the alkyl group has 1 to 12 carbon atoms. More preferably, it has 1 to 6 carbon atoms and 3 to 8 carbon atoms. Even more preferably, it has 2 to 6 carbon atoms and 4 to 7 carbon atoms. Even more preferably, it has 3 to 6 carbon atoms and 4 to 7 carbon atoms. Even more preferably, it has 4 to 5 carbon atoms and 4 to 7 carbon atoms.

[0016] Examples of alkyl groups include: methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, isopentyl, neopentyl, hexyl, 2-methylpentyl, 3-methylpentyl, 1,1,2-trimethyl-propyl, 3,3'-dimethyl-butyl, heptyl, 2-heptyl, 3-heptyl, 2-methylhexyl, 3-methylhexyl, isoheptyl, octyl, nonyl, and decyl.

[0017] The halogen atom in a haloalkylene group having 1 to 20 carbon atoms can be substituted at any position of the aforementioned alkylene group. Specifically, for chain alkyl groups, the substitution position can be any C atom; for cycloalkyl groups, hydrogen can be substituted on the cyclic structure or on a carbon atom outside the cyclic structure.

[0018] The alkylene groups (A2, A3, A4, A5) referred to as alkeneoxy or haloalkeneoxy groups can be chain alkyl groups or cycloalkyl groups. The hydrogen atom on the ring of the cycloalkyl group can be replaced by an alkyl group. The alkyl group is selected from 1 to 12 carbon atoms. More preferably, it is selected from 1 to 6 chain alkyl groups and 4 to 7 cycloalkyl groups. Even more preferably, it is selected from 2 to 6 chain alkyl groups and 3 to 7 cycloalkyl groups. Even more preferably, it is selected from 3 to 6 chain alkyl groups and 4 to 7 cycloalkyl groups. Even more preferably, it is selected from 4 to 5 chain alkyl groups and 4 to 7 cycloalkyl groups. Examples of alkyl groups include: methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, isopentyl, neopentyl, hexyl, 2-methylpentyl, 3-methylpentyl, 1,1,2-trimethyl-propyl, 3,3'-dimethyl-butyl, heptyl, 2-heptyl, 3-heptyl, 2-methylhexyl, 3-methylhexyl, isoheptyl, octyl, nonyl, and decyl.

[0019] When A2, A3, A4, and A5 are olefinic or haloolefinic, the olefinic bond can be located at any position on the carbon chain, preferably near the nitrile.

[0020] More preferably, the second additive is n-butanol-1,2,3,4-tetrapropionitrile ether and / or pentaerythritol tetrapropionitrile ether.

[0021] In this invention, the amount of the first additive is equivalent to 0.1%-3.0% of the total weight of the electrolyte, and the amount of the second additive is equivalent to 0.1%-3.0% of the total weight of the electrolyte. Preferably, the amount of the first additive is equivalent to 0.2%-2.5% of the total weight of the electrolyte, and the amount of the second additive is equivalent to 0.2%-2.5% of the total weight of the electrolyte; preferably, the amount of the first additive is equivalent to 0.5%-2% of the total weight of the electrolyte, and the amount of the second additive is equivalent to 0.5%-2% of the total weight of the electrolyte; preferably, the amount of the first additive is equivalent to 0.5%-1.5% of the total weight of the electrolyte, and the amount of the second additive is equivalent to 0.5%-1.5% of the total weight of the electrolyte; preferably, the amount of the first additive is equivalent to 0.2%-1.0% of the total weight of the electrolyte, and the amount of the second additive is equivalent to 1.0%-3.0% of the total weight of the electrolyte.

[0022] As an example of a practically selectable dosage, the dosage of the first additive may be 0.1%, 0.5%, 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, 2.5%, or 3.0% of the total weight of the electrolyte.

[0023] The amount of the second additive may be 0.1%, 0.5%, 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, 2.5% or 3.0% of the total weight of the electrolyte.

[0024] To further improve the stability of the interfacial film, a third additive is also included, which is lithium difluorooxalate phosphate; the amount of the third additive is equivalent to 0.1%-3.0% of the total weight of the electrolyte; preferably, the amount of the third additive is equivalent to 0.2%-2.5% of the total weight of the electrolyte; preferably, the amount of the third additive is equivalent to 0.5%-2% of the total weight of the electrolyte; preferably, the amount of the third additive is equivalent to 0.5%-1.5% of the total weight of the electrolyte.

[0025] As an example of a practically optional amount, the amount of the third additive may be 0.1%, 0.5%, 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, 2.5%, or 3.0% of the total weight of the electrolyte.

[0026] The total amount of the first additive, the second additive, and the third additive does not exceed 6% of the total weight of the electrolyte; preferably, the total amount of the first additive, the second additive, and the third additive does not exceed 5% of the total weight of the electrolyte; preferably, the total amount of the first additive, the second additive, and the third additive does not exceed 4% of the total weight of the electrolyte; preferably, the total amount of the first additive, the second additive, and the third additive is equivalent to 4-6% of the total weight of the electrolyte; preferably, the total amount of the first additive, the second additive, and the third additive is equivalent to 4.5-5.5% of the total weight of the electrolyte.

[0027] As the electrolyte in the non-aqueous electrolyte of the present invention, there are no particular restrictions on any known lithium salt used in this application, and any lithium salt can be used arbitrarily. The following lithium salts are examples.

[0028] Examples can be given as follows:

[0029] Inorganic lithium salts such as LiPF6, LiBF4, LiClO4, LiAlF4, LiSbF6, LiTaF6, and LiWF7; and lithium tungstates such as LiWOF5.

[0030] Lithium carboxylate salts such as HCO2Li, CH3CO2Li, CH2FCO2Li, CHF2CO2Li, CF3CO2Li, CF3CH2CO2Li, CF3CF2CO2Li, CF3CF2CF2CO2Li, CF3CF2CF2CF2CO2Li, etc.

[0031] Lithium sulfonate salts such as FSO3Li, CH3SO3Li, CH2FSO3Li, CHF2SO3Li, CF3SO3Li, CF3CF2SO3Li, CF3CF2CF2SO3Li, CF3CF2CF2CF2SO3Li, etc.

[0032] LiN(FCO)2, LiN(FCO)(FSO2), LiN(FSO2)2, LiN(FSO2)(CF3SO2), LiN(CF3SO2)2, LiN(C2F5SO2)2, cyclic 1,2-perfluoroethane disulfonylimide lithium, cyclic 1,3-perfluoropropane disulfonylimide lithium, LiN(CF3SO2)(C4F9SO2) and other lithium imide salts;

[0033] Methylated lithium salts such as LiC(FSO2)3, LiC(CF3SO2)3, and LiC(C2F5SO2)3;

[0034] Lithium difluorooxalate borate, lithium bis(oxalate)borate, and other lithium oxalate borate salts;

[0035] Lithium oxalate phosphate salts such as lithium tetrafluorooxalate phosphate, lithium difluorobis(oxalate) phosphate, and lithium tri(oxalate) phosphate.

[0036] And fluorinated organic lithium salts such as LiPF4(CF3)2, LiPF4(C2F5)2, LiPF4(CF3SO2)2, LiPF4(C2F5SO2)2, LiBF3CF3, LiBF3C2F5, LiBF3C3F7, LiBF2(CF3)2, LiBF2(C2F5)2, LiBF2(CF3SO2)2, and LiBF2(C2F5SO2)2; etc.

[0037] These lithium salts can be used alone or in combination of two or more.

[0038] More preferably, the lithium salt is at least one selected from lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalate)borate, lithium difluorophosphate, lithium difluorooxalate phosphate, lithium tetrafluorooxalate phosphate, lithium difluorobis(oxalate) phosphate, and lithium difluorosulfonylimide, and the concentration of the lithium salt is 0.5-2M, and the concentration of the lithium salt is further preferably 0.5-1.5M.

[0039] As an example of the present invention, the concentration of the lithium salt can be selected as 0.5M, 0.6M, 0.7M, 0.8M, 0.9M, 1.0M, 1.1M, 1.2M, 1.3M, 1.4M, 1.5M, 1.6M, 1.7M, 1.8M, 1.9M or 2.0M.

[0040] The non-aqueous organic solvent is a cyclic organic solvent and / or a chain organic solvent; the cyclic organic solvent is one or more combinations of propylene carbonate, ethylene carbonate, and butene carbonate; the chain organic solvent is one or more combinations of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl formate, ethyl formate, methyl acetate, and ethyl acetate.

[0041] The following are examples of the proportions of the available solvent combinations:

[0042] The preferred ratio of ethylene carbonate (EC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) is 1:0.1-10:0.1-10; more preferably, the preferred ratio is 1:0.2-5:0.2-5; and more preferably, the preferred ratio is 1:0.5-2:0.5-2.

[0043] The preferred ratio of ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC) is 1:0.1-10:0.1-10; the preferred ratio of ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC) is 1:0.2-5:0.2-5; more preferably, the preferred ratio of ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC) is 1:0.5-2:0.5-2.

[0044] The preferred ratio of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) is 1:0.1-10:0.1-10; the preferred ratio of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) is 1:0.2-5:0.2-5; more preferably, the preferred ratio of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) is 1:0.5-2:0.5-2.

[0045] The above description of organic solvents does not mean that the above solvent system cannot contain other types of solvents. Commonly used solvents that can be selected for this lithium salt, such as cyclic carboxylic esters, chain carboxylic esters, ether compounds, and sulfone compounds, can be added, and the amount is not limited. In this embodiment, it is recommended that the amount of the above-mentioned cyclic carboxylic esters, chain carboxylic esters, ether compounds, and sulfone compounds added exceeds 30% of the solvent weight.

[0046] The specific substances of cyclic carboxylic acid esters can be selected from γ-butyrolactone, γ-valerolactone, γ-caprolactone, ε-caprolactone, etc.; they can avoid the decrease of conductivity, suppress the increase of negative electrode resistance, and make it easy for the high current discharge characteristics of non-aqueous electrolyte secondary batteries to reach a good range.

[0047] Chain carboxylic acid esters are preferably those with 3 to 7 carbon atoms. Specific examples include: methyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, n-propyl propionate, isopropyl propionate, n-butyl propionate, isobutyl propionate, tert-butyl propionate, methyl butyrate, ethyl butyrate, n-propyl butyrate, isopropyl butyrate, methyl isobutyrate, ethyl isobutyrate, n-propyl isobutyrate, isopropyl isobutyrate, etc. Chain carboxylic acid esters can suppress the increase of negative electrode resistance and enable non-aqueous electrolyte batteries to achieve good high-current discharge characteristics and cycle characteristics.

[0048] Ether compounds are preferably chain ethers with 3 to 10 carbon atoms in which some hydrogen atoms are optionally substituted with fluorine, and cyclic ethers with 3 to 6 carbon atoms; examples of chain ethers with 3 to 10 carbon atoms include: 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(1,1,2,2-tetrafluoroethyl) ether, (2-fluoroethyl)(2,2,2-trifluoroethyl) ether, (2-fluoroethyl)(1,1,2,2-tetrafluoroethyl) ether, (2,2,2-trifluoroethyl)(1,1,2,2-tetrafluoroethyl) ether, ethyl n-propyl ether, ethyl(3-fluoron-propyl) ether, etc. 3,3,3-trifluoropropyl ether, ethyl (2,2,3,3-tetrafluoropropyl) ether, ethyl (2,2,3,3,3-pentafluoropropyl) ether, 2-fluoroethyl propyl ether, (2-fluoroethyl)(3-fluoropropyl) ether, (2-fluoroethyl)(3,3,3-trifluoropropyl) ether, (2-fluoroethyl)(2,2,3,3-tetrafluoropropyl) ether, (2-fluoroethyl)(2,2,3,3,3-pentafluoropropyl) ether, 2,2,2-trifluoroethyl propyl ether, (2,2,2-trifluoroethyl)(3-fluoropropyl) ether, (2,2,2-trifluoroethyl)(3,3,3-trifluoropropyl) ether, (2,2,2-trifluoroethyl)(2,2,3,3-tetrafluoropropyl) ether, (2,2,2-trifluoroethyl)(2,2,3,3-tetrafluoropropyl) ether (n-propyl) ether, (2,2,2-trifluoroethyl)(2,2,3,3,3-pentafluoropropyl) ether, 1,1,2,2-tetrafluoroethyl n-propyl ether, (1,1,2,2-tetrafluoroethyl)(3-fluoropropyl) ether, (1,1,2,2-tetrafluoroethyl)(3,3,3-trifluoropropyl) ether, (1,1,2,2-tetrafluoroethyl)(2,2,3,3-tetrafluoropropyl) ether, (1,1,2,2-tetrafluoroethyl)(2,2,3,3,3-pentafluoropropyl) ether, di-n-propyl ether, (n-propyl)(3-fluoropropyl) ether, (n-propyl)(3,3,3-trifluoropropyl) ether, (n-propyl)(2,2,3,3-tetrafluoropropyl) ether, (n-propyl)(2, 2,3,3,3-Pentafluoropropyl) ether, di(3-fluoropropyl) ether, (3-fluoropropyl)(3,3,3-trifluoropropyl) ether, (3-fluoropropyl)(2,2,3,3-tetrafluoropropyl) ether, (3-fluoropropyl)(2,2,3,3,3-pentafluoropropyl) ether, di(3,3,3-trifluoropropyl) ether, (3,3,3-trifluoropropyl)(2,2,3,3-tetrafluoropropyl) ether, (3,3,3-trifluoropropyl)(2,2,3,3,3-pentafluoropropyl) ether, di(2,2,3,3-tetrafluoropropyl) ether, (2,2,3,3-tetrafluoropropyl)(2,2,3,3,3-pentafluoropropyl) ether, di(2,2,3,3,3-Pentafluoropropyl) ether, di-n-butyl ether, dimethoxymethane, methoxyethoxymethane, methoxy(2-fluoroethoxy)methane, methoxy(2,2,2-trifluoroethoxy)methane, methoxy(1,1,2,2-tetrafluoroethoxy)methane, diethoxymethane, ethoxy(2-fluoroethoxy)methane, ethoxy(2,2,2-trifluoroethoxy)methane, ethoxy(1,1,2,2-tetrafluoroethoxy)methane, di(2-fluoroethoxy)methane, (2-fluoroethoxy) (2,2,2-trifluoroethoxy)methane, (2-fluoroethoxy)(1,1,2,2-tetrafluoroethoxy)methane, di(2,2,2-trifluoroethoxy)methane, (2,2,2-trifluoroethoxy)(1,1,2,2-tetrafluoroethoxy)methane, di(1,1,2,2-tetrafluoroethoxy)methane, dimethoxyethane, methoxyethoxyethane, methoxy(2-fluoroethoxy)ethane, methoxy(2,2,2-trifluoroethoxy)ethane, methoxy(1,1,2,2-tetrafluoroethoxy)methane, dimethoxyethane, methoxyethoxyethane, methoxy(2-fluoroethoxy)ethane, methoxy(2,2,2-trifluoroethoxy)ethane, methoxy(1,1, 2,2-Tetrafluoroethoxy)ethane, diethoxyethane, ethoxy(2-fluoroethoxy)ethane, ethoxy(2,2,2-trifluoroethoxy)ethane, ethoxy(1,1,2,2-tetrafluoroethoxy)ethane, di(2-fluoroethoxy)ethane, (2-fluoroethoxy)(2,2,2-trifluoroethoxy)ethane, (2-fluoroethoxy)(1,1,2,2-tetrafluoroethoxy)ethane, di(2,2,2-trifluoroethoxy)ethane, (2,2,2-trifluoroethoxy)( 1,1,2,2-Tetrafluoroethoxy)ethane, di(1,1,2,2-tetrafluoroethoxy)ethane, ethylene glycol di-n-propyl ether, ethylene glycol di-n-butyl ether, diethylene glycol dimethyl ether, etc.; cyclic ethers with 3 to 6 carbon atoms include: tetrahydrofuran, 2-methyltetrahydrofuran, 3-methyltetrahydrofuran, 1,3-dioxapentane, 2-methyl-1,3-dioxapentane, 4-methyl-1,3-dioxapentane, 1,4-dioxapentane, etc., and their fluorinated compounds;

[0049] When ether compounds are used as auxiliary solvents, and the negative electrode active material is carbonaceous, it is easy to avoid the problem of capacity reduction caused by co-intercalation of ether compounds with lithium ions.

[0050] Sulfone compounds can be selected from: dimethyl sulfone, ethyl methyl sulfone, diethyl sulfone, n-propyl methyl sulfone, isopropyl methyl sulfone, n-butyl methyl sulfone, tert-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 tert-butyl sulfone, trifluoromethyl n-butyl sulfone, trifluoromethyl tert-butyl sulfone, etc. When sulfone compounds are present as auxiliary solvents, they can improve the cycle performance and cycle retention performance of the battery, reduce the solution viscosity, and improve the electrochemical performance.

[0051] Furthermore, the present invention does not exclude the addition of other additives, and a suitable amount of a fourth additive may be added. The fourth additive is preferably controlled to be less than 1% of the total weight of the electrolyte; as follows: one or more of the following: nitrile additives, aromatic additives, isocyanate additives, other additives containing triple bonds, additives containing S=O groups, cyclic acetal additives, other additives containing P, cyclic acid anhydride additives, cyclic phosphazene additives, and fluorine-containing additives.

[0052] More specifically, for example:

[0053] One or more nitriles selected from acetonitrile, propionitrile, butadiene nitrile, glutaronitrile, adiponitrile, heptacyanide, octadiene nitrile, and sebaconitrile; cyclohexylbenzene, fluorocyclohexylbenzene compounds (1-fluoro-2-cyclohexylbenzene, 1-fluoro-3-cyclohexylbenzene, 1-fluoro-4-cyclohexylbenzene), tert-butylbenzene, tert-pentylbenzene, 1-fluoro-4-tert-butylbenzene and other branched alkyl aromatic compounds, biphenyl, terphenyl (ortho, meta, para), diphenyl ether, fluorobenzene, difluorobenzene (ortho, meta, para), anisole, 2,4-difluoroanisole, and partially hydrides of terphenyl (1,2-dicyclohexylbenzene, 2-phenylbicyclohexyl, 1,2-diphenylcyclohexane, ... Aromatic compounds such as o-cyclohexylbiphenyl; isocyanates selected from one or more of the following: methyl isocyanate, ethyl isocyanate, butyl isocyanate, phenyl isocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, octamethylene diisocyanate, 1,4-phenylene diisocyanate, ethyl 2-isocyanate acrylate, and ethyl 2-isocyanate methacrylate; and isocyanates selected from 2-propynyl methyl carbonate, 2-propynyl acetate, 2-propynyl formate, 2-propynyl methacrylate, 2-propynyl methanesulfonate, 2-propynyl vinylsulfonate, 2-propynyl 2-(methanesulfonyloxy)propionate, di(2-propynyl)oxalate, 2... -Methyl propyne oxalate, ethyl 2-propyne oxalate, di(2-propyne) glutarate, 2-butyn-1,4-dimethyl dimethyl sulfonate, 2-butyn-1,4-dimethyl dicarboxylate, and 2,4-hexadiyne-1,6-dimethyl dimethyl sulfonate, or one or more compounds containing a triple bond; selected from 1,3-propanesulfonolactone, 1,3-butanesulfonolactone, 2,4-butanesulfonolactone, 1,4-butanesulfonolactone, 1,3-propenesulfonolactone, 2,2-dioxide-1,2-oxothiacyclopentan-4-ylacetate, 5,5-dimethyl-1,2-oxothiacyclopentan-4-one 2,2-dioxide, etc. sulfonolactones, ethylene sulfite, hexamethyl sulfite, etc. One or more S=O group compounds selected from cyclic sulfites such as benzo[1,3,2]dioxane-2-oxide (also known as 1,2-cyclohexanediol cyclic sulfite), 5-vinyl-hexahydro-1,3,2-benzodioxane-2-oxide, butane-2,3-dimethyl dimethyl sulfonate, butane-1,4-dimethyl dimethyl sulfonate, methylene methane disulfonate, divinyl sulfone, 1,2-bis(vinylsulfonyl)ethane, bis(2-vinylsulfonylethyl) ether, etc.; cyclic acetal compounds selected from 1,3-dioxane, 1,3-dioxane, 1,3,5-trioxane, etc.Selected from trimethyl phosphate, tributyl phosphate, trioctyl phosphate, tris(2,2,2-trifluoroethyl) phosphate, bis(2,2,2-trifluoroethyl) phosphate, bis(2,2,2-trifluoroethyl) 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 Ethyl fluoroethyl ester, methyl (2,2,2-trifluoroethyl)(2,2,3,3-tetrafluoropropyl) phosphate, tris(1,1,1,3,3,3-hexafluoropropane-2-yl) phosphate, methyl methylene bisphosphonate, ethyl methylene bisphosphonate, methyl ethyl ethylene bisphosphonate, ethyl ethylene bisphosphonate, methyl butyl bisphosphonate, ethyl butyl bisphosphonate, methyl 2-(dimethylphosphoryl)acetate, ethyl 2-(dimethylphosphoryl)acetate, methyl 2-(diethylphosphoryl)acetate, ethyl 2-(diethylphosphoryl)acetate One or more phosphorus-containing compounds selected from esters, 2-(dimethylphosphoryl)acetic acid 2-propynyl ester, 2-(diethylphosphoryl)acetic acid 2-propynyl ester, 2-(dimethoxyphosphoryl)acetic acid methyl ester, 2-(dimethoxyphosphoryl)acetic acid methyl ester, 2-(diethoxyphosphoryl)acetic acid ethyl ester, 2-(dimethoxyphosphoryl)acetic acid 2-propynyl ester, 2-(diethoxyphosphoryl)acetic acid 2-propynyl ester, methyl pyrophosphate, and ethyl pyrophosphate; acetic anhydride, propionic acid Chain-like carboxylic anhydrides such as anhydrides, or cyclic anhydrides such as succinic anhydride, maleic anhydride, 2-allyl succinic anhydride, glutaric anhydride, itaconic anhydride, 3-sulfonyl-propionic anhydride, etc.; cyclic phosphazene compounds such as methoxypentafluorocyclotriphosphazene, ethoxypentafluorocyclotriphosphazene, phenoxypentafluorocyclotriphosphazene, or ethoxyheptafluorocyclotetraphosphazene; fluorinated compounds such as methyl fluorocarbonate, dimethyl fluorocarbonate, diethyl fluorocarbonate, ethyl fluoropropionate, propyl fluoropropionate, methyl fluoropropionate, ethyl fluoroacetate, methyl fluoroacetate, or propyl fluoroacetate;

[0054] Meanwhile, the present invention also discloses a lithium secondary battery, including a positive electrode, a negative electrode, a separator, and an electrolyte as described above.

[0055] In the above-mentioned lithium secondary battery, the positive electrode is selected from lithium-containing transition metal oxides, wherein the lithium transition metal oxide is LiCoO2, LiMn2O4, LiMnO2, Li2MnO4, LiFePO4, Li1+aMn1-xMxO2, LiCo1-xMxO2, LiFe1-xMxPO4, Li2Mn1-xO4, where M is selected from one or more of Ni, Co, Mn, Al, Cr, Mg, Zr, Mo, V, Ti, B, and F, and 0≤a<0.2, 0≤x<1.

[0056] In the aforementioned lithium secondary battery, the active material in the positive electrode is LiCoO2; the operating voltage of the lithium secondary battery is 4.55V.

[0057] In this invention, the negative electrode active material in the negative electrode includes at least one of carbonaceous materials, silicon-carbon materials, alloy materials, and lithium-containing metal composite oxide materials, but is not limited thereto. The negative electrode active material may be any conventionally known material that can be used as a negative electrode active material in an electrochemical device and is capable of electrochemically inserting and de-inserting active ions.

[0058] The method for preparing the negative electrode sheet is a well-known method in the art and can be used in electrochemical devices. The negative electrode active material layer also includes a binder and a solvent. A negative electrode active material is mixed with a binder and solvent, and thickeners, conductive agents, fillers, etc., are added as needed to prepare a negative electrode slurry. The negative electrode slurry is then coated onto a negative electrode current collector, dried, and pressed to obtain a negative electrode sheet. The negative electrode slurry forms a negative electrode active material layer after drying and cold pressing. Similarly, a solvent is typically added in the preparation of the negative electrode slurry. The solvent is removed during the drying process. The binder is a well-known binder that can be used as a negative electrode active material layer, such as, but not limited to, styrene-butadiene rubber. The solvent is a well-known solvent that can be used as a negative electrode active material layer, such as, but not limited to, water. The thickener is a well-known thickener that can be used as a negative electrode active material layer, such as, but not limited to, carboxymethyl cellulose. In some embodiments, when the negative electrode active material contains an alloy material, the negative electrode active material layer can be formed using methods such as vapor deposition, sputtering, or plating.

[0059] The separator is a separator known in the art that can be used in electrochemical devices and is stable to the electrolyte used, such as, but not limited to, resin, glass fiber, and inorganic materials.

[0060] For example, the separator membrane contains at least one of polyolefin, aromatic polyamide, polytetrafluoroethylene, and polyethersulfone. Preferably, the polyolefin contains at least one of polyethylene and polypropylene. Preferably, the polyolefin contains polypropylene. Preferably, the separator membrane is formed by laminating multiple layers of materials, for example, the separator membrane is a three-layer separator membrane formed by laminating polypropylene, polyethylene, and polypropylene in that order.

[0061] Finally, the present invention also provides a method for improving the performance of a lithium cobalt oxide cathode battery system, wherein the electrolyte described above is used as the electrolyte of the lithium cobalt oxide cathode battery system; the operating voltage of the lithium cobalt oxide cathode battery system is 4.55V.

[0062] Compared with the prior art, the beneficial effects of the present invention are:

[0063] The electrolyte of the present invention contains a first additive and a second additive, both of which are film-forming aids, mainly used for film formation at the positive electrode. The first additive contains boron atoms, which have lone pairs of electrons that can combine with oxygen in the positive electrode to prevent oxygen evolution. The second additive contains tetrapropionitrile groups, which can combine with cobalt at the positive electrode to prevent cobalt dissolution. At the same time, the composite film formed by the first and second additives has a higher strength than existing combinations of other additives, and can effectively improve electrical performance. Detailed Implementation

[0064] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0065] Example 1

[0066] 1. Preparation of electrolyte: Ethyl carbonate (EC), diethyl carbonate (DEC) and methyl ethyl carbonate (EMC) are mixed in a mass ratio of EC:DEC:EMC = 1:1:1. After mixing, 1 mol of lithium hexafluorophosphate (LiPF6) is added. After the lithium salt is completely dissolved, 0.5% of tris(2-cyanoethyl)borate and 0.5% of pentaerythritol tetrapropionitrile ether are added as a second additive.

[0067] 2. Preparation of the positive electrode sheet: Lithium cobalt oxide (LiCoO2), conductive carbon black (SuperP), polyvinylidene fluoride (PVDF), and carbon nanotubes (CNT) are mixed evenly at a mass ratio of 95:2.3:2:0.7 to prepare a lithium-ion battery positive electrode slurry of a certain viscosity. The slurry is coated on both sides of the current collector aluminum foil, dried at 85°C, and then cold-pressed. The slurry is then trimmed, cut into sheets, and slit. After slitting, the sheets are dried at 95°C under vacuum for 12 hours. The tabs are then welded to produce a lithium-ion battery positive electrode sheet that meets the requirements, with an areal density of 33 mg / cm2.

[0068] 3. Preparation of negative electrode sheet: Graphite, conductive agent SuperP, thickener sodium carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber latex (SBR) are mixed in a mass ratio of 95:1.5:1.0:2.5 to form a slurry. The slurry is coated on both sides of copper foil, dried at 85°C, and then cold-pressed. Then, the edges are trimmed, cut into sheets, and slit. After slitting, the sheets are dried at 85°C under vacuum for 12 hours. The tabs are then welded to produce a lithium-ion battery negative electrode sheet that meets the requirements, with an areal density of 21.1 mg / cm2.

[0069] 4. Preparation of Lithium-ion Batteries: The positive electrode, negative electrode, and separator prepared according to the above process are stacked to form a lithium-ion battery with a thickness of 4.7 mm, a width of 55 mm, and a length of 60 mm, with a theoretical capacity of 1800 mAh (1C). The battery is then vacuum-baked at 75°C for 10 hours, followed by injection of the above-mentioned electrolyte. After standing for 24 hours, the battery is placed in an environment of 45°C, subjected to a pressure of 3 kg, and charged to 4.0V at 0.1C (180 mA). It is then left to stand for 2 days to fully activate the battery, completing the battery fabrication.

[0070] Examples 2-10 and Comparative Examples 1-5 are largely the same as Example 1, except that the electrolyte formulation has been adjusted.

[0071] Comparative Example 6 is largely the same as Comparative Example 3, except that the charging cutoff voltage is 4.5V (as described in the performance test below).

[0072] Specifically, the electrolyte formulations and positive electrode materials for Examples 1-10 and Comparative Examples 1-6 are shown in Table 1 below.

[0073] Table 1. Electrolyte formulations and battery systems for the examples and comparative examples.

[0074]

[0075]

[0076]

[0077] Lithium-ion battery performance testing

[0078] The lithium-ion batteries in Examples 1-10 and Comparative Examples 1-5 were tested for room temperature, high temperature cycle performance, high temperature storage performance, and low temperature discharge performance. The test methods are as follows.

[0079] 25℃ 1.0C / 1.0C Cycling Test: At 25℃, charge at a constant current of 1.0C to 4.55V, then charge at a constant voltage of 4.55V to the cutoff current of 0.05C. Then discharge the battery at a constant current of 1.0C. The discharge capacity is recorded as C1. Repeat the charge-discharge cycle for 500 cycles to obtain the discharge capacity C500 on the 500th cycle. Capacity retention rate = C500 / C1*100%.

[0080] 45℃ 1.0C / 1.0C Cycling Test: Charge the battery at 45℃ with a constant current of 1.0C to 4.55V, then charge it with a constant voltage to the cutoff current of 0.05C. Then discharge the battery with a constant current of 1.0C. The discharge capacity is recorded as C1. Repeat the charge and discharge cycle for 500 cycles to obtain the discharge capacity C500 on the 500th cycle. The capacity retention rate is C500 / C1*100%.

[0081] Capacity retention test after 14 days of storage at 60℃: At 25℃, the battery was charged at a constant current of 1.0C to 4.55V, then charged at a constant voltage of 4.55V until the cutoff current reached 0.05C. The battery was then discharged at a constant current of 1.0C, and the discharge capacity was recorded as C0. At 25℃, the battery was charged at a constant current of 1.0C to 4.55V, then charged at a constant voltage of 4.55V until the cutoff current reached 0.05C. The battery was then transferred to 45℃ and stored for 14 days. The battery was then discharged at a constant current of 1.0C, and the discharge capacity was recorded as C1. The capacity retention rate after 14 days of storage at 60℃ = C1 / C0 * 100%.

[0082] -20℃ Low Temperature Discharge Test: At 25℃, the battery is charged at a constant current of 1.0C to 4.55V, then charged at a constant voltage of 4.55V until the cutoff current is 0.05C. The battery is then discharged at a constant current of 1.0C, and the discharge capacity is recorded as C0. At 25℃, the battery is charged at a constant current of 1.0C to 4.55V, then charged at a constant voltage of 4.55V until the cutoff current is 0.05C. The battery is then transferred to -20℃ and left to stand for 240 minutes. The battery is then discharged at a constant current of 0.5C, and the discharge capacity is recorded as C1. The -20℃ discharge rate = C1 / C0 * 100%.

[0083] The test method for Comparative Example 6 is as follows:

[0084] 25℃ 1.0C / 1.0C Cycling Test: At 25℃, charge at a constant current of 1.0C to 4.50V, then charge at a constant voltage of 4.50V to the cutoff current of 0.05C. Then discharge the battery at a constant current of 1.0C. The discharge capacity is recorded as C1. Repeat the charge-discharge cycle for 500 cycles to obtain the discharge capacity C500 on the 500th cycle. Capacity retention rate = C500 / C1*100%.

[0085] 45℃ 1.0C / 1.0C Cycling Test: Charge the battery at 45℃ with a constant current of 1.0C to 4.50V, then charge it at a constant voltage to the cutoff current of 0.05C. Then discharge the battery with a constant current of 1.0C. The discharge capacity is recorded as C1. Repeat the charge and discharge cycle for 500 cycles to obtain the discharge capacity C500 on the 500th cycle. The capacity retention rate is C500 / C1*100%.

[0086] Capacity retention test after 14 days of storage at 60℃: At 25℃, the battery was charged at a constant current of 1.0C to 4.50V, then charged at a constant voltage of 4.50V to a cutoff current of 0.05C. The battery was then discharged at a constant current of 1.0C, and the discharge capacity was recorded as C0. At 25℃, the battery was charged at a constant current of 1.0C to 4.50V, then charged at a constant voltage of 4.50V to a cutoff current of 0.05C. The battery was then transferred to 45℃ and stored for 14 days. The battery was then discharged at a constant current of 1.0C, and the discharge capacity was recorded as C1. The capacity retention rate after 14 days of storage at 60℃ = C1 / C0 * 100%.

[0087] -20℃ Low Temperature Discharge Test: At 25℃, the battery is charged at a constant current of 1.0C to 4.50V, then charged at a constant voltage of 4.50V to a cutoff current of 0.05C. The battery is then discharged at a constant current of 1.0C, and the discharge capacity is recorded as C0. At 25℃, the battery is charged at a constant current of 1.0C to 4.5V, then charged at a constant voltage of 4.5V to a cutoff current of 0.05C. The battery is then transferred to -20℃ and left to stand for 240 minutes. The battery is then discharged at a constant current of 0.5C, and the discharge capacity is recorded as C1. The -20℃ discharge rate = C1 / C0 * 100%.

[0088] The test results are shown in Table 2 below:

[0089] Table 2 Performance test results of lithium-ion batteries with different electrolyte formulations

[0090]

[0091]

[0092] Results analysis:

[0093] 1. As can be seen from Examples 1-3, the optimal amount of the tetracyanoether additive n-butanol-1,2,3,4-tetrapropionitrile ether is 2%.

[0094] 2. As can be seen from Examples 4-6, the optimal amount of tris(2-cyanoethyl)borate is 2%;

[0095] 3. As can be seen from Examples 2, 6 and 7, the tetracyano ether additives n-butanol-1,2,3,4-tetrapropionitrile ether and pentaerythritol tetrapropionitrile ether have comparable effects, and the effect is better when both are added at the same time than when they are added alone.

[0096] 4. As can be seen from Examples 7, 8, 9, and 10, the addition of lithium difluorooxalate phosphate can significantly improve battery performance, and its optimal addition amount is 1%.

[0097] 5. As can be seen from Examples 1-10 and Comparative Examples 1-5, the addition of tris(2-cyanoethyl)borate, n-butanol-1,2,3,4-tetrapropionitrile ether, pentaerythritol tetrapropionitrile ether, and lithium difluorooxalate phosphate individually all improve battery performance. The synergistic effect is best when all four are added simultaneously in appropriate proportions.

[0098] 6. Comparative Examples 3 and 6 show that even with only 2% tetracyano-based additives, the batteries perform well at a charging voltage of 4.5V. Comparing this to the poorer battery performance at 4.55V, it's clear that adding tetracyano-based additives alone at 4.55V is insufficient to effectively suppress oxygen evolution, electrolyte decomposition, and cobalt ion dissolution. Therefore, it's necessary to add tri(2-cyanoethyl)borate containing empty boron orbitals to bind active oxygen, thereby suppressing oxygen evolution. Furthermore, its polycyano structure synergistically interacts with tetracyano-based additives, further enhancing cobalt ion dissolution. Lithium difluorooxalate phosphate exhibits film-forming characteristics at both positive and negative electrodes. It enhances the stability of the positive electrode interface film while also improving the stability of the negative electrode interface film, thus better suppressing electrolyte decomposition on the surface of the positive and negative electrode interfaces.

[0099] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. An electrolyte suitable for a 4.55V high-voltage lithium cobalt oxide cathode battery system, characterized in that, Including lithium salts, non-aqueous organic solvents, first additive, and second additive; The first additive is tris(2-cyanoethyl)boronic acid ester; The second additive has the structure shown in Formula 1: Formula 1; A1 is one of C1-20 alkylene or haloalkylene, C2-20 alkenyl or haloalkenyl; A2, A3, A4, and A5 are each independently selected from C1-20 alkoxide or haloalkoxide, C2-20 alkenyloxy or haloalkenyloxy.

2. The electrolyte according to claim 1, characterized in that, The second additive is n-butanol-1,2,3,4-tetrapropionitrile ether and / or pentaerythritol tetrapropionitrile ether.

3. The electrolyte according to claim 1, characterized in that, The amount of the first additive is equivalent to 0.1%-3.0% of the total weight of the electrolyte, and the amount of the second additive is equivalent to 0.1%-3.0% of the total weight of the electrolyte.

4. The electrolyte according to claim 1, characterized in that, The amount of the first additive is equivalent to 0.2%-1.0% of the total weight of the electrolyte, and the amount of the second additive is equivalent to 1.0%-3.0% of the total weight of the electrolyte.

5. The electrolyte according to claim 1, characterized in that, It also includes a third additive, which is lithium difluorooxalate phosphate; the amount of the third additive is equivalent to 0.1%-3.0% of the total weight of the electrolyte; the total amount of the first additive, the second additive and the third additive does not exceed 6% of the total weight of the electrolyte.

6. The electrolyte according to claim 1, characterized in that, The lithium salt is at least one selected from lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalate)borate, lithium difluorophosphate, lithium di(oxalate)phosphate, lithium tetrafluorooxalate phosphate, lithium di(oxalate)phosphate, and lithium difluorosulfonylimide, and the concentration of the lithium salt is 0.5-2M.

7. The electrolyte according to claim 1, characterized in that, The non-aqueous organic solvent is a cyclic organic solvent and / or a chain organic solvent; the cyclic organic solvent is one or more combinations of propylene carbonate, ethylene carbonate, and butene carbonate; the chain organic solvent is one or more combinations of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl formate, ethyl formate, methyl acetate, and ethyl acetate.

8. A lithium secondary battery, characterized in that, It includes a positive electrode, a negative electrode, a separator, and an electrolyte as described in any one of claims 1-7.

9. The lithium secondary battery according to claim 8, characterized in that, The positive electrode is selected from lithium-containing transition metal oxides, wherein the lithium transition metal oxide is LiCoO2, LiMn2O4, LiMnO2, Li2MnO4, LiFePO4, Li 1+a Mn 1- x MxO2, 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, and 0≤x<1.

10. The lithium secondary battery according to claim 9, characterized in that, The active material in the positive electrode is LiCoO2; the operating voltage of the lithium secondary battery is 4.55V.

11. A method for improving the performance of a lithium cobalt oxide cathode battery system, characterized in that, The electrolyte as described in any one of claims 1-7 is used as the electrolyte in the lithium cobalt oxide cathode battery system; the operating voltage of the lithium cobalt oxide cathode battery system is 4.55V.

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