Electrolyte of lithium-ion battery, and lithium-ion battery

Abstract

The present disclosure relates to the technical field of lithium-ion batteries, and specifically relates to an electrolyte of a lithium-ion battery, and a lithium-ion battery. The electrolyte of a lithium-ion battery comprises a fluorocarboxylate, a high-voltage additive and a flame-retardant additive, wherein the high-voltage additive comprises an unsaturated silane and / or an unsaturated siloxane, the unsaturated silane being an unsaturated silane containing a silicon-oxygen bond. In the present disclosure, a fluorocarboxylate solvent is used as a flame-retardant component, and therefore the flash point and flame retardance of the electrolyte are improved, the usage amount of the flame-retardant additive can be reduced, the compatibility between the electrolyte and positive / negative electrode materials is improved, and the electrolyte has a low viscosity and high conductivity, which is beneficial for improving the cycle performance and the rate capability; and the high-voltage additive containing an unsaturated bond and a silicon-oxygen bond can inhibit the oxygenolysis of the electrolyte, thereby enhancing the stability of a cathode interface and relieving the dissolution of transition metal ions. The lithium-ion battery prepared in the present disclosure has good flame retardance, high levels of safety, high energy density, good rate capability and cycling stability, and a low gas production amount.

Description

Lithium ion battery electrolyte and lithium ion battery

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to the Chinese patent application with application number 202311609904.X filed with the Chinese Patent Office on November 29, 2023, entitled “A Lithium-ion Battery Electrolyte and a Lithium-ion Battery”, the entire contents of which are incorporated by reference into this application. Technical Field

[0003] The present disclosure relates to the technical field of lithium-ion batteries, and in particular to a lithium-ion battery electrolyte and a lithium-ion battery. Background Art

[0004] Over the past few decades, lithium-ion batteries have occupied an irreplaceable position in the energy storage field due to their advantages such as high energy density, high power density and stability. However, the continuous expansion of the electric vehicle market around the world and the application of energy storage devices in fields such as deep sea and aviation have increased the requirements for energy storage devices with high energy density, high power density, long cycle life, wide temperature range application and good safety performance. In order to improve the energy density of batteries, researchers have continuously broadened the electrode potential of positive electrode materials, including lithium-rich layered oxides, nickel-rich layered oxides, spinel oxides and high-voltage polyanionic compounds. Carboxylate solvents have the advantages of low melting point, low viscosity, low density and high conductivity. Carboxylate-based electrolytes combined with high-voltage positive electrodes are expected to realize high-energy fast-charging lithium-ion batteries with wide temperature range applications. However, the electrochemical stability window of most carboxylate-based electrolytes is not sufficient to match the high-voltage positive electrode. The electrolyte undergoes oxidative decomposition at the positive electrode, resulting in a rapid decrease in the cycle life of the high-voltage battery. The high-voltage positive electrode and the electrolyte undergo side reactions, releasing a large amount of heat and flammable gases. At the same time, the boiling point and flash point of organic carboxylate solvents are low, and the film-forming properties with the positive and negative electrodes are poor, which exacerbates the potential combustion and explosion risks of lithium-ion batteries and affects the battery cycle performance.

[0005] To address the issues of electrolyte oxidative decomposition and thermal safety in high-voltage battery systems, current prevention and improvement efforts focus on both the cathode material and the electrolyte. For example, the cathode material can be coated or doped to isolate the cathode from the electrolyte and reduce the occurrence of side reactions. Alternatively, flammable organic electrolytes can be modified by adding high-voltage additives and flame-retardant additives. High-voltage additives such as boron compounds, organophosphorus compounds, fluorine-containing compounds, and organosilicon compounds form a good interface at the cathode, effectively improving the electrolyte's antioxidant properties. Flame-retardant additives such as phosphates, phosphites, organic halides, and phosphazenes can scavenge hydrogen and hydroxyl active radicals generated by side reactions, thereby reducing the risk of fire and explosion and improving the safety of high-voltage battery systems.

[0006] However, cathode material modification faces a complex coating process, which increases battery cost. Furthermore, once the artificially designed interface is damaged during cycling, it cannot be repaired in situ, deteriorating battery performance. Conventional flame retardant additives, such as phosphate esters and phosphazenes, have low flame retardant efficiency and generally require a content of over 20% to achieve a flame retardant effect. These additives generally have a large dielectric constant and undergo numerous side reactions with the positive and negative electrodes during charge and discharge. While improving the flame retardancy of the electrolyte, they can negatively impact the battery's cyclic reversibility. Conventional high-voltage additives, such as boron additives and sulfone solvents, can affect the viscosity of the electrolyte and reduce conductivity.

[0007] In view of this, the present disclosure is proposed. Summary of the Invention

[0008] The first purpose of the present disclosure is to provide a lithium-ion battery electrolyte, which has good antioxidant properties and positive and negative electrode compatibility, and can improve the energy density, safety performance, cycle stability and fast charging performance of high-voltage batteries.

[0009] A second object of the present disclosure is to provide a lithium-ion battery comprising the lithium-ion battery electrolyte described above.

[0010] A lithium-ion battery electrolyte comprises a fluorocarboxylate, a high-voltage additive and a flame-retardant additive. The high-voltage additive comprises an unsaturated silane and / or an unsaturated siloxane. The unsaturated silane is an unsaturated silane containing a silicon-oxygen bond.

[0011] In some embodiments, in the solvent of the lithium-ion battery electrolyte, the volume percentage of the fluorocarboxylate is A%, and the degree of fluorination of the fluorocarboxylate is R a , and satisfy 10≤A≤50,0<R a ≤1.

[0012] In some embodiments, the mass percentage of the high-voltage additive in the lithium-ion battery electrolyte is B%, and the number of double bonds in the high-voltage additive molecule is recorded as the unsaturation R b , and satisfy 1≤B≤10, 1≤R b ≤6.

[0013] In some embodiments, the mass percentage of the flame retardant additive in the lithium-ion battery electrolyte is C%, and satisfies 1≤C≤5.

[0014] In some embodiments, the volume percentage A% and the degree of fluorination R of the fluorocarboxylate are a , the mass percentage B% and unsaturation R of the high pressure additive bThe mass percentage of the flame retardant additive C% satisfies the following relationship: Y1=(2C+0.4×A×R a ×B) / (100-A), 0.022<Y1≤4.2; Y2=(1.2×B×R b +0.3×A×R a ×C) / (100-A), 0.011<Y2≤2.7.

[0015] In some embodiments, the structural formula of the fluorocarboxylate is:

[0016] In the formula, R1 and R2 are independently selected from C1-C3 alkyl or C1-C3 fluoroalkyl, and at least one of R1 and R2 is selected from C1-C3 fluoroalkyl, and C1-C3 fluoroalkyl means that at least one hydrogen atom is replaced by fluorine.

[0017] In some further embodiments, the fluorocarboxylic acid ester comprises at least one of ethyl fluoroacetate, methyl fluoropropionate, ethyl fluoropropionate, propyl fluoropropionate, propyl fluoroacetate, and methyl fluoroacetate.

[0018] In some embodiments, the high-pressure additive is a compound represented by formula (I), formula (II) or (III):

[0019] In formula (I), R3, R4, R5, and R6 are independently selected from C1-C4 hydrocarbon groups, alkoxy groups, monohalogen-substituted or polyhalogen-substituted C1-C4 hydrocarbon groups, phenyl groups, halogen-substituted phenyl groups, cyano groups, or hydrocarbon groups containing cyano substituents, and at least one of R3, R4, R5, and R6 contains a carbon-carbon double bond;

[0020] In formula (II), R7, R8, R9, R 10 、R 11 、R 12 are independently selected from C1-C4 hydrocarbon groups, alkoxy groups, monohalogen-substituted or polyhalogen-substituted C1-C4 hydrocarbon groups, phenyl groups, halogen-substituted phenyl groups, cyano groups or hydrocarbon groups containing cyano substituents, and R7, R8, R9, R 10 、R 11 、R 12 At least one of them contains a carbon-carbon double bond;

[0021] In formula (III), R 13 、R 14 、R 15 、R 16 、R 17 、R 18are independently selected from C1-C4 hydrocarbon groups, alkoxy groups, monohalogen-substituted or polyhalogen-substituted C1-C4 hydrocarbon groups, phenyl groups, halogen-substituted phenyl groups, cyano groups or hydrocarbon groups containing cyano substituents, and R 13 、R 14 、R 15 、R 16 、R 17 、R 18 At least one of them contains a carbon-carbon double bond.

[0022] In some embodiments, the flame retardant additive includes at least one of a phosphate flame retardant, a phosphite flame retardant, and a phosphazene flame retardant.

[0023] In some embodiments, the lithium salt in the lithium-ion battery electrolyte includes at least one of LiF, Li2O, Li2O2, LiPF6, LiBF4, LiClO4, LiBOB, LiODFB, LiAsF6, LiCF3SO3, LiN(SO2CF3)2 and LiBF3C2F5.

[0024] In some embodiments, the solvent in the lithium-ion battery electrolyte further includes at least one of ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, propylene carbonate, ethyl propionate, propyl propionate, ethyl acetate, or propyl acetate.

[0025] A lithium ion battery comprises a positive electrode, a negative electrode, a separator and the lithium ion battery electrolyte as described above.

[0026] In some embodiments, the negative electrode includes at least one of a graphite negative electrode, a silicon-based negative electrode, and a tin-based negative electrode. DETAILED DESCRIPTION

[0027] The advantages of the embodiments in the application content will be explained in the embodiment section of the specification below, and some of them are obvious from the specification, or can be obtained through some embodiments of the embodiments of the present disclosure.

[0028] In order to make the purpose, technical solutions and advantages of the present disclosure more clear, the present disclosure is further described in detail below in conjunction with the embodiments. It should be understood that the embodiments described herein are only used to explain the present disclosure and are not used to limit the present disclosure. In addition, the technical features involved in the various embodiments of the present disclosure described below can be combined with each other as long as they do not conflict with each other. Without departing from the principles of the embodiments of the present disclosure, several improvements and modifications can be made, and these improvements and modifications are also considered to be within the scope of protection of the embodiments of the present disclosure.

[0029] A first aspect of the present disclosure provides a lithium-ion battery electrolyte, comprising a fluorocarboxylate, a high-voltage additive, and a flame retardant additive, wherein the high-voltage additive contains both a carbon-carbon double bond and a Si-O bond, and comprises an unsaturated silane and / or an unsaturated siloxane, wherein the unsaturated silane is an unsaturated silane containing a silicon-oxygen bond.

[0030] The present invention uses fluorocarboxylic acid ester as a high flash point solvent to replace carboxylic acid ester solvent to improve the flame retardancy of the electrolyte. The fluorine group forms a good interface between the positive and negative electrodes, improving the interfacial stability of the battery under high voltage. At the same time, a low content of conventional flame retardant additives is added as a vapor phase flame retardant to further enhance the non-flammable nature of the electrolyte. Silicon oxide compounds containing unsaturated double bonds and Si-O bonds are added as high-voltage additives. The carbon-carbon double bonds undergo polymerization reaction on the positive / negative electrode surface to form a strong interface containing Si-OR, while increasing the flash point of the electrolyte, which can significantly improve the safety performance, antioxidant performance, cycle performance and rate performance of the electrolyte.

[0031] This type of electrolyte has good antioxidant properties and positive and negative electrode compatibility. It forms a solid interface with the positive / negative electrode materials at high potential (4.3), reduces the side reactions between the electrode materials and the electrolyte, inhibits gas production, and improves the energy density and cycle stability of the high-nickel battery system; at the same time, this type of electrolyte has the characteristics of high flash point and non-flammability, which greatly improves the safety of high-voltage batteries without affecting the battery cycle performance; in addition, this type of electrolyte has the advantages of high conductivity and low viscosity, which accelerates the lithium ion transfer kinetics and improves the energy density, safety performance, stable cycle and fast charging performance of high-voltage batteries.

[0032] In some specific embodiments of the present disclosure, in the solvent of the lithium-ion battery electrolyte, the volume percentage of the fluorocarboxylate is A%, and the degree of fluorination of the fluorocarboxylate is R a , and satisfies 10≤A≤50, for example, A can be any point value among 10, 15, 20, 25, 30, 35, 40, 45, 50 or a range of values ​​consisting of any two point values; 0<R a ≤1, for example, R a It can be any point value among 0.25, 0.375, 0.5, 0.625, 1, or a range of values ​​consisting of any two points.

[0033] The degree of fluorination R in this disclosure a Refers to the ratio of the number of fluorine atoms to the total number of fluorine atoms and hydrogen atoms in a molecule, such as the R a 3 / 8; no fluorine atom replaces R a is 0, perfluorinated substituted R a is 1.

[0034] Fluorinated carboxylic acid esters are high-flash point solvents, and their content and degree of fluorination are positively correlated with the flame retardancy of the electrolyte. At the same time, they can preferentially form an interface layer containing LiF at the positive and negative electrodes, thereby improving the interface compatibility. However, if the proportion of fluorocarboxylic acid ester solvents is too high, the viscosity of the electrolyte will increase, the conductivity will decrease, the chemical instability will increase, the fluorinated molecules will generate HF, and the lithium source will be consumed to generate excessive LiF, which will have a negative impact on the battery rate performance, cycle performance and first efficiency. Therefore, the content of fluorocarboxylic acid esters and their degree of fluorination need to be controlled within an appropriate range.

[0035] In some specific embodiments of the present disclosure, the mass percentage of the high-voltage additive in the lithium-ion battery electrolyte is B%, and the number of double bonds in the high-voltage additive molecule is recorded as the unsaturation R b , and satisfies 1≤B≤10, for example, B can be any point value among 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or a range of values ​​consisting of any two point values; 1≤R b ≤6, for example, R b It can be any point value among 1, 2, 3, 4, 5, 6, or a range value consisting of any two point values.

[0036] High-voltage additives can change the LUMO / HOMO value of the electrolyte. During the charge and discharge process, the carbon-carbon double bonds undergo polymerization reaction at the positive and negative electrodes to form an interface layer containing Si-OR, which has a strong and stable characteristic and improves the high-voltage resistance and interface compatibility of the electrolyte. However, as the content of siloxane additives increases, the viscosity of the electrolyte increases, the conductivity decreases, and the fast charging advantage of the carboxylate-based electrolyte is affected. b If it is too large, the additive will be more likely to undergo polymerization reaction, which may cause the interface layer to thicken and increase the battery impedance. Therefore, the content and unsaturation of the high-voltage additive need to be controlled within an appropriate range.

[0037] In some specific embodiments of the present disclosure, the mass percentage of the flame retardant additive in the lithium ion battery electrolyte is C%, and satisfies 1≤C≤5, for example, C can be any point value among 1, 2, 3, 4, 5 or a range value consisting of any two point values.

[0038] The addition of conventional phosphate flame retardants, phosphite flame retardants and / or phosphazene flame retardants disclosed herein can prevent combustion reactions between gas phases, but will have a negative impact on the rate performance and interfacial stability of the battery. Therefore, it is necessary to control the content of the flame retardant additives within an appropriate range to improve flame retardancy while ensuring battery performance.

[0039] In some embodiments of the present disclosure, the volume percentage A% and the fluorination degree R of the fluorocarboxylate are a , the mass percentage B% and unsaturation R of the high pressure additive bAnd the mass percentage C% of the flame retardant additive satisfies the following relationship:

[0040] Y1=(2C+0.4×A×R a ×B) / (100-A), 0.022<Y1≤4.2, for example, Y1 can be any value among 0.0711, 0.0833, 0.2029, 0.2500, 0.3382, 0.3824, 0.4643, or a range consisting of any two values;

[0041] Y2=(1.2×B×R b +0.3×A×R a ×C) / (100-A), 0.011<Y2≤2.7, for example, Y2 can be any point value among 0.0430, 0.0642, 0.1301, 0.2404, 0.3772, 0.3816, 0.4125, or a range value consisting of any two point values.

[0042] Y1 represents the flame retardant performance of the electrolyte, and Y2 represents the high-voltage resistance of the electrolyte. The content of the three substances and the degree of fluorination of the fluorocarboxylic acid ester are positively correlated with the flame retardant and high-voltage resistance of the electrolyte. The unsaturation of the high-voltage additive is positively correlated with the high-voltage resistance of the electrolyte. However, if the A and Ra values ​​are too high, the viscosity of the electrolyte will increase, the conductivity will decrease, and more lithium sources will be consumed to produce fluorine-containing compounds. If the B and Rb values ​​are too high, the viscosity and rate performance of the electrolyte will be affected, and more polymerization reactions will occur to produce a thick interface layer, increasing the battery impedance. Conventional flame retardant additives have high polarity, and excessively large C values ​​will cause side reactions on the positive and negative electrode surfaces, affecting the battery interface compatibility. Different components and compositions simultaneously affect the flame retardant, high-voltage resistance, interface compatibility and rate performance of the electrolyte, and different properties restrict each other. For example, pursuing extreme flame retardant performance will inevitably have negative consequences on rate performance and interface compatibility. Reasonable regulation of the range of various electrolyte parameters and limiting the values ​​of Y1 and Y2 can pursue a battery system with optimal overall performance.

[0043] In some specific embodiments of the present disclosure, the structural formula of the fluorocarboxylate is:

[0044] In the formula, R1 and R2 are independently selected from C1-C3 alkyl or C1-C3 fluoroalkyl, and at least one of R1 and R2 is selected from C1-C3 fluoroalkyl, and C1-C3 fluoroalkyl means that at least one hydrogen atom is replaced by fluorine.

[0045] In some preferred embodiments of the present disclosure, the fluorocarboxylic acid ester includes at least one of ethyl fluoroacetate, methyl fluoropropionate, ethyl fluoropropionate, propyl fluoropropionate, propyl fluoroacetate and methyl fluoroacetate; as a specific example, the fluorocarboxylic acid ester can be ethyl difluoropropionate, propyl difluoropropionate, ethyl trifluoroacetate, propyl trifluoroacetate or methyl tetrafluoroacetate, etc.

[0046] In some preferred embodiments of the present disclosure, the high-pressure additive is a compound represented by formula (I), formula (II) or (III):

[0047] In formula (I), R3, R4, R5, and R6 are independently selected from C1-C4 hydrocarbon groups, alkoxy groups, monohalogen-substituted or polyhalogen-substituted C1-C4 hydrocarbon groups, phenyl groups, halogen-substituted phenyl groups, cyano groups, or hydrocarbon groups containing cyano substituents, and at least one of R3, R4, R5, and R6 contains a carbon-carbon double bond;

[0048] In formula (II), R7, R8, R9, R 10 、R 11 、R 12 are independently selected from C1-C4 hydrocarbon groups, alkoxy groups, monohalogen-substituted or polyhalogen-substituted C1-C4 hydrocarbon groups, phenyl groups, halogen-substituted phenyl groups, cyano groups or hydrocarbon groups containing cyano substituents, and R7, R8, R9, R 10 、R 11 、R 12 At least one of them contains a carbon-carbon double bond;

[0049] In formula (III), R 13 、R 14 、R 15 、R 16 、R 17 、R 18 are independently selected from C1-C4 hydrocarbon groups, alkoxy groups, monohalogen-substituted or polyhalogen-substituted C1-C4 hydrocarbon groups, phenyl groups, halogen-substituted phenyl groups, cyano groups or hydrocarbon groups containing cyano substituents, and R 13 、R 14 、R 15 、R 16 、R 17 、R 18 At least one of them contains a carbon-carbon double bond.

[0050] As a specific example, the high-pressure additive may be vinyltrimethoxysilane, 1,3-divinyltetramethyldisiloxane, or 1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane.

[0051] In some specific embodiments of the present disclosure, the flame retardant additive includes at least one of a phosphate flame retardant, a phosphite flame retardant and a phosphazene flame retardant; as a specific example, the flame retardant additive can be ethoxypentafluorocyclotriphosphazene, ethoxytetrafluoromethylcyclotriphosphazene, methylpentafluorocyclotriphosphazene, trimethyltrifluorocyclotriphosphazene, methoxytrifluoromethylethylcyclotriphosphazene and methoxydifluorotriethylcyclotriphosphazene, etc.

[0052] The purpose of adding a small amount of conventional flame retardants such as phosphonate flame retardants, phosphite flame retardants and / or phosphazene flame retardants in the present disclosure is to prevent the combustion reaction between the gas phase. However, in order to avoid the negative impact of conventional flame retardants on battery rate performance and interface stability, their dosage needs to be controlled within a certain range.

[0053] In some specific embodiments of the present disclosure, the lithium salt in the lithium-ion battery electrolyte includes at least one of LiF, Li2O, Li2O2, LiPF6, LiBF4, LiClO4, LiBOB, LiODFB, LiAsF6, LiCF3SO3, LiN(SO2CF3)2 and LiBF3C2F5.

[0054] In some specific embodiments of the present disclosure, the solvent in the lithium-ion battery electrolyte further includes at least one of ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, propylene carbonate, ethyl propionate, propyl propionate, ethyl acetate or propyl acetate.

[0055] A second aspect of the present disclosure provides a lithium ion battery comprising a positive electrode, a negative electrode, a separator, and the lithium ion battery electrolyte as described above.

[0056] The lithium-ion battery provided by the present disclosure has good flame retardancy, high safety, high energy density, good rate performance and cycle stability, and low gas production.

[0057] In some specific embodiments of the present disclosure, the negative electrode includes at least one of a graphite negative electrode, a silicon-based negative electrode, and a tin-based negative electrode.

[0058] Silicon-based negative electrodes face huge volume expansion during the cycle, causing the active particles to break and pulverize. The electrolyte in this application contains a fluorocarboxylate solvent, which forms a SEI containing LiF on the surface of the silicon negative electrode. Studies have shown that LiF has an expressed surface energy on the silicon negative electrode and does not adhere to silicon particles, which can better inhibit the expansion of silicon particles. At the same time, the high-voltage additive forms an interface containing Si-O bonds on the outer layer of the SEI, further alleviating the huge stress caused by the expansion of silicon particles and improving the cycle stability of the Si / NCM811 battery system.

[0059] The following describes some embodiments of the present disclosure in detail with reference to specific application examples. The raw materials used in the examples, unless otherwise specified, can be purchased commercially.

[0060] Example 1

[0061] (1) Preparation of electrolyte:

[0062] In an argon glove box, ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / ethyl acetate (EA) / ethyl 1-trifluoroacetate (ETFA) were mixed at a volume ratio of 25:25:35:15. a is 0.375, the molar concentration of lithium salt LiPF6 in the electrolyte is 1.2 mol / L, the added high-voltage additive is vinyltrimethoxysilane, the mass percentage is 5%, R b is 1, and the molecular structure is The flame retardant additive is ethoxy pentafluorocyclotriphosphazene, with a mass percentage of 3%.

[0063] (2) Preparation of positive electrode sheet:

[0064] LiNi 0.8 Co 0.1 Mn 0.1 (NCM811), conductive carbon black (SP), carbon nanotubes (CNTs), and polyvinylidene fluoride (PVDF) were mixed in a solvent, N-methylpyrrolidone, at a weight ratio of 96:1:0.5:2.5 and stirred evenly to produce a positive electrode slurry. Aluminum foil was used as the positive electrode current collector, and the positive electrode slurry was coated on the positive electrode current collector. The cathode was then baked at 120°C for 1 hour, followed by cold pressing, cutting, and slitting to produce the positive electrode sheets.

[0065] (3) Preparation of negative electrode sheet:

[0066] The negative electrode active materials, silicon-carbon anode, polyacrylonitrile (PAN), and Ketjen Black (KB), were mixed in deionized water at a weight ratio of approximately 96:2:2 and stirred evenly to produce a negative electrode slurry. Copper foil was used as the negative electrode current collector, and the negative electrode slurry was coated on the negative electrode current collector. The negative electrode was then baked at 120°C for 1 hour, followed by cold pressing, cutting, and slitting to produce the negative electrode sheets.

[0067] (4) Battery assembly:

[0068] The positive electrode sheet, separator, and negative electrode sheet are arranged in order, and the electrode group is obtained by using the rapid stacking technology. The electrode group is welded into the shell (aluminum-plastic film) through the positive and negative electrode ears, the side is sealed and dried, and then the electrolyte (4Ah, injection coefficient 3.3g / Ah) is injected. After standing, pre-charging, secondary injection, and formation processes, a lithium-ion secondary battery is obtained.

[0069] Example 2

[0070] Example 2 is similar to Example 1, except that the high-pressure additive is 1,3-divinyltetramethyldisiloxane, whose molecular structure is R b is 2, the mass percentage of the flame retardant additive ethoxypentafluorocyclotriphosphazene is 5%; the other conditions are the same as those in Example 1.

[0071] Example 3

[0072] Example 3 is similar to Example 1, except that the fluorocarboxylic acid ester is 1,2-pentafluoromethyl propionate with a higher degree of fluorination, R a is 0.625, and the high-pressure additive is 1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane with a higher degree of unsaturation, and the molecular structure is R b is 3, the mass percentage of the flame retardant additive ethoxypentafluorocyclotriphosphazene is 5%; the other conditions are the same as those in Example 1.

[0073] Example 4

[0074] Example 4 is similar to Example 2, except that the volume ratio of the fluorocarboxylate solvent is changed to ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / ethyl acetate (EA) / ethyl 1-trifluoroacetate (ETFA) at a volume ratio of 25:25:40:10, the mass percentage of the high-pressure additive is 1%, and the mass percentage of the flame retardant additive ethoxypentafluorocyclotriphosphazene is 3%; the other conditions are the same as in Example 2.

[0075] Example 5

[0076] Example 5 is similar to Example 2, except that the mass percentage of the high-pressure additive is 10%, and the other conditions are the same as Example 2.

[0077] Example 6

[0078] Example 6 is similar to Example 4, except that the volume ratio of the fluorocarboxylate solvent is changed to ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / ethyl acetate (EA) / ethyl 1-trifluoroacetate (ETFA) at a volume ratio of 25:25:45:5, and the other conditions are the same as those in Example 4.

[0079] Example 7

[0080] Example 7 is similar to Example 2, except that the volume ratio of the fluorocarboxylate solvent is changed to ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / ethyl acetate (EA) / ethyl 1-trifluoroacetate (ETFA) at a volume ratio of 25:25:20:30, and the other conditions are the same as in Example 2.

[0081] Comparative Example 1

[0082] Comparative Example 1 is similar to Example 1, except that no fluorocarboxylate, high-pressure additive, or flame retardant additive is added, and the volume ratio of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / ethyl acetate (EA) is 25:25:50; the other conditions are the same as in Example 1.

[0083] Comparative Example 2

[0084] Comparative Example 2 is similar to Example 1, except that no high-pressure additive is added, the volume ratio of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / ethyl acetate (EA) / ethyl 1-trifluoroacetate (ETFA) is 25:25:45:5; and the other conditions are the same as in Example 1.

[0085] Comparative Example 3

[0086] Comparative Example 3 is similar to Example 2, except that no fluorocarboxylate and flame retardant additive are added, the volume ratio of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / ethyl acetate (EA) is 25:25:50, and the other conditions are the same as Example 2.

[0087] Experimental example

[0088] Basic battery performance verification:

[0089] (1) Physical and chemical properties test and safety performance test of electrolyte at room temperature (25°C)

[0090] In a glove box, samples of the electrolyte prepared in each Example and Comparative Example were dripped onto a glass fiber bowl and ignited in a fume hood. The time from flame burning to extinguishing was recorded. The oxidation potential and conductivity (25°C) of all electrolytes were measured using an electrochemical workstation (VMP300) and a Mettler-Toledo Fe38 conductivity meter. Thermal runaway testing was performed on the prepared lithium-ion batteries using an accelerating rate calorimeter (NETZSCH ARC254). The raw material parameters for the Examples and Comparative Examples are shown in Table 1, and the test results are shown in Table 2.

[0091] (2) Battery cycle performance test

[0092] At 25°C, the lithium-ion batteries prepared in each Example and Comparative Example were charged at a constant current to 4.2V. After the voltage reached 4.3V, they were charged at a constant voltage with a cutoff current of 0.05C and left for 10 minutes. The batteries were then discharged at a constant current to 2.75V and left for 10 minutes. The above steps were repeated N times to obtain the capacity of the battery after the Nth cycle when discharged to 2.75V. The initial discharge capacity of the battery at 25°C was recorded, and the capacity retention rate was calculated. The capacity retention rate and gas production after 500 cycles are shown in Table 3.

[0093] Table 1

[0094] Table 2

[0095] Table 3

[0096] It can be seen from the data in Table 1 and Table 2 that, compared with Comparative Example 1, the introduction of fluorinated carboxylate solvents, high-voltage additives, and low-content conventional flame retardant additives into the carboxylate electrolyte in Examples 1-7 significantly improves the high-voltage resistance and safety performance of the electrolyte, with the oxidation potential being raised to 5.3V and the self-extinguishing time being reduced to 0s. When Y1>0.2, the electrolyte has good flame retardant properties, but after Y1>0.33, as Y1 increases, the cycle stability of the battery decreases, the gas production increases, and the rate performance and cycle stability of the battery are negatively affected. This is because the fluorinated components, high-voltage additives, and flame retardant additives reduce the conductivity of the electrolyte, and conventional flame retardants affect interface stability. When Y2>0.13, the oxidation potential of the electrolyte is increased to above 4.7V, and the composite high-voltage NCM811 does not decompose, but when Y2>0.37, the high-viscosity additives affect the rate performance and cycle performance of the battery. From the above data, it can be seen that when 0.2<Y1<0.47, 0.13<Y2<0.42, the battery safety performance, high voltage performance, rate performance and cycle performance are better. Optimally, the volume percentage of fluorocarboxylate is 15%, partial trifluoro substitution, R a When the mass percentage of the high-pressure additive with an unsaturation degree of 2 is 0.375, the mass percentage of the phosphazene flame retardant additive is 5%, and the mass percentage of the phosphazene flame retardant additive is 5%, the capacity retention rate after 500 cycles at 0.3C is 91%, and the capacity retention rate after 500 cycles at 1.6C is 89%. The gas production is the least and the overall performance of the battery is the best.

[0097] Compared with Examples 1, 2, and 3, increasing the unsaturation of the high-voltage additive is positively correlated with the oxidation potential of the electrolyte, but will deteriorate the electrolyte conductivity to a certain extent.

[0098] Compared with Examples 2, 4, and 5 and Comparative Example 2, the content of the high-voltage additive at 1% has little effect on improving the antioxidant capacity of the electrolyte. The content of 5% and 10% can increase the oxidation potential to above 5V, but the increase in content is negatively correlated with the conductivity.

[0099] Compared with Examples 2, 6, and 7 and Comparative Example 3, the volume percentage of the fluorocarboxylic acid ester is positively correlated with the flame retardancy of the electrolyte. However, too high a volume percentage will lead to reduced conductivity, increased electrolyte instability, and worse circulation and gas production. Industrial Applicability

[0100] In summary, the present disclosure provides a lithium-ion battery electrolyte and a lithium-ion battery. The present disclosure uses a fluorocarboxylate solvent as a flame retardant component to improve the flash point and flame retardancy of the electrolyte, which can reduce the amount of conventional flame retardant additives, circumvent the disadvantages of high-content conventional flame retardant additives, and improve the compatibility of the electrolyte with the positive / negative electrode materials, so that the electrolyte has low viscosity and high conductivity, which is beneficial to improving the cycle performance and rate performance. A compound containing an unsaturated double bond and a Si-O bond is used as a high-voltage additive. The Si-O functional group can capture HF in the electrolyte. The unsaturated double bond has high activity and strong polymerization ability. It is preferentially oxidized during the charging process to form a strong and uniform passivation layer, thereby inhibiting the oxidative decomposition of the electrolyte, enhancing the stability of the cathode interface, and alleviating the dissolution of transition metal ions. By regulating the degree of fluorination of the fluorocarboxylate solvent and the unsaturation of the unsaturated silicon oxide compound, the overall performance of the battery is improved.

Claims

1. A lithium ion battery electrolyte, characterized in that: The invention comprises fluorocarboxylate, high-pressure additive and flame retardant additive. The high-pressure additive comprises unsaturated silane and / or unsaturated siloxane. The unsaturated silane is an unsaturated silane containing silicon-oxygen bond.

2. The lithium ion battery electrolyte according to claim 1, characterized in that: The invention comprises at least one of the following features (1) to (3): (1) In the solvent of the lithium ion battery electrolyte, the volume percentage of the fluorocarboxylic acid ester is A%, and the degree of fluorination of the fluorocarboxylic acid ester is R a , and satisfy 10≤A≤50, 0<R a ≤1; (2) The mass percentage of the high-voltage additive in the lithium-ion battery electrolyte is B%, and the number of double bonds in the high-voltage additive is recorded as the unsaturation R b , and satisfy 1≤B≤10, 1≤R b ≤6; (3) The mass percentage of the flame retardant additive in the lithium ion battery electrolyte is C%, and satisfies 1≤C≤5.

3. The lithium ion battery electrolyte according to claim 2, characterized in that: The volume percentage A% and the degree of fluorination R of the fluorocarboxylic acid ester are a , the mass percentage content B% and the unsaturation R of the high pressure additive b The mass percentage C% of the flame retardant additive satisfies the following relationship: Y1=(2C+0.4×A×R a ×B) / (100-A), 0.022<Y1≤4.2; Y2=(1.2×B×R b +0.3×A×R a ×C) / (100-A), 0.011<Y2≤2.

7.

4. The lithium ion battery electrolyte according to claim 1, characterized in that: The structural formula of the fluorocarboxylate is: In the formula, R1 and R2 are independently selected from C1-C3 alkyl or C1-C3 fluoroalkyl, and at least one of R1 and R2 is selected from C1-C3 fluoroalkyl.

5. The lithium ion battery electrolyte according to claim 4, characterized in that: The fluorocarboxylic acid ester includes at least one of ethyl fluoroacetate, methyl fluoropropionate, ethyl fluoropropionate, propyl fluoropropionate, propyl fluoroacetate and methyl fluoroacetate.

6. The lithium ion battery electrolyte according to claim 1, characterized in that: The high pressure additive is a compound represented by formula (I), formula (II) or formula (III): In formula (I), R3, R4, R5, and R6 are independently selected from C1-C4 hydrocarbon groups, alkoxy groups, monohalogen-substituted or polyhalogen-substituted C1-C4 hydrocarbon groups, phenyl groups, halogen-substituted phenyl groups, cyano groups, or hydrocarbon groups containing cyano substituents, and at least one of R3, R4, R5, and R6 contains a carbon-carbon double bond; In formula (II), R7, R8, R9, R 10 , R 11 , R 12 are independently selected from C1-C4 hydrocarbon groups, alkoxy groups, monohalogen-substituted or polyhalogen-substituted C1-C4 hydrocarbon groups, phenyl groups, halogen-substituted phenyl groups, cyano groups or hydrocarbon groups containing cyano substituents, and R7, R8, R9, R 10 , R 11 , R 12 At least one of them contains a carbon-carbon double bond; In formula (III), R 13 , R 14 , R 15 , R 16 , R 17 , R 18 are independently selected from C1-C4 hydrocarbon groups, alkoxy groups, monohalogen-substituted or polyhalogen-substituted C1-C4 hydrocarbon groups, phenyl groups, halogen-substituted phenyl groups, cyano groups, or hydrocarbon groups containing cyano substituents, and R 13 , R 14 , R 15 , R 16 , R 17 , R 18 At least one of them contains a carbon-carbon double bond.

7. The lithium ion battery electrolyte according to claim 1, characterized in that: The flame retardant additive includes at least one of a phosphate flame retardant, a phosphite flame retardant and a phosphazene flame retardant.

8. The lithium ion battery electrolyte according to claim 1, characterized in that: The lithium salt in the lithium-ion battery electrolyte includes at least one of LiF, Li2O, Li2O2, LiPF6, LiBF4, LiClO4, LiBOB, LiODFB, LiAsF6, LiCF3SO3, LiN(SO2CF3)2 and LiBF3C2F5.

9. The lithium ion battery electrolyte according to claim 1, characterized in that: The solvent in the lithium ion battery electrolyte further includes at least one of ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, propylene carbonate, ethyl propionate, propyl propionate, ethyl acetate or propyl acetate.

10. A lithium ion battery, characterized in that: The invention comprises a positive electrode, a negative electrode, a separator and a lithium ion battery electrolyte as claimed in any one of claims 1 to 9.

11. The lithium ion battery according to claim 10, characterized in that: The negative electrode includes at least one of a graphite negative electrode, a silicon-based negative electrode and a tin-based negative electrode.

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