Electrolyte additive, electrolyte and secondary battery

Abstract

The present invention belongs to the technical field of secondary batteries, and in particular to an electrolyte additive, comprising the following components in parts by weight: 1 to 5 parts of a first additive and 5 to 18 parts of a second additive, wherein the second additive is one or more of ethylene carbonate, fluoroethylene carbonate, vinyl ethylene carbonate, 1,3-propane sultone, propylene sulfite, vinyl sulfate, 4-methyl ethylene sulfate, succinic anhydride, succinonitrile, adiponitrile and 1,3,6-hexane trinitrile; the first additive comprises one or more of the first compound shown in formula I or the second compound shown in formula II. An electrolyte additive of the present invention can effectively improve the high temperature performance and cycle performance of the electrolyte.

Description

Technical Field

[0001] The invention belongs to the technical field of secondary batteries, and in particular relates to an electrolyte additive, an electrolyte and a secondary battery. Background Art

[0002] The reserves of traditional fossil energy are decreasing day by day, and the development and use of these energy sources have brought about a series of environmental problems, making the development of new renewable energy sources an inevitable trend. Lithium-ion batteries have the advantages of high energy density, long cycle life, and environmental friendliness, and have been widely used in digital consumer products, electric vehicles, and energy storage. However, lithium-ion batteries still have problems such as poor high-temperature performance and rate performance, so improving the high-temperature and rate performance of lithium-ion batteries has become a hot topic in research. Summary of the invention

[0003] One of the purposes of the present invention is to provide an electrolyte additive to improve the high temperature performance and rate performance of the electrolyte in view of the deficiencies in the prior art.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions:

[0005] An electrolyte additive comprises the following components in parts by weight: 1 to 5 parts of a first additive and 5 to 18 parts of a second additive, wherein the second additive is one or more of ethylene carbonate, fluoroethylene carbonate, vinyl ethylene carbonate, 1,3-propane sultone, propylene sulfite, vinyl sulfate, 4-methyl ethylene sulfate, succinic anhydride, succinonitrile, adiponitrile and 1,3,6-hexanetrinitrile; the first additive comprises one or more of a first compound shown in formula I or a second compound shown in formula II,

[0006]

[0007]

[0008] Wherein, R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11, R12 and R13 are independently selected from one of hydrogen, halogen atoms, alkyl groups having 1 to 10 carbon atoms, unsaturated hydrocarbon groups having 2 to 10 carbon atoms, alkoxy groups having 1 to 10 carbon atoms, alkanoyl groups having 2 to 10 carbon atoms and phenyl groups, and the hydrogen in the alkyl groups, the unsaturated hydrocarbon groups, the alkoxy groups and the alkanoyl groups may be partially or completely substituted by one or more of halogen atoms, cyano groups, carboxyl groups or sulfonic acid groups.

[0009] Among them, in the first compound represented by formula I, R1, R2 and R3 are alkane groups with 1 to 10 carbon atoms; in the second compound represented by formula II, R4, R5, R6, R7 are halogen atoms, R8 is a hydrogen group, R11 is a halogen atom, and R9, R10, R12 and R13 are hydrogen groups.

[0010] Among them, in the first compound represented by formula I, R1, R2 and R3 are methyl groups, and in the second compound, R4, R5, R6 are fluorine atoms, R7 is a chlorine atom, R8 is a hydrogen group, R11 is a chlorine atom, and R9, R10, R12 and R13 are hydrogen groups.

[0011] The first additive includes a first compound represented by Formula I and a second compound represented by Formula II, and the weight ratio of the first compound to the second compound is 1-3:1-3.

[0012] Wherein, the weight ratio of the first additive to the second additive is 1-5:6-15.

[0013] The second purpose of the present invention is to provide an electrolyte with good high temperature performance and rate performance in view of the deficiencies of the prior art.

[0014] In order to achieve the above object, the present invention adopts the following technical solutions:

[0015] An electrolyte comprises an organic solvent, a lithium salt and the electrolyte additive according to any one of claims 1 to 5, wherein the electrolyte additive accounts for 2% to 10% of the electrolyte weight.

[0016] The organic solvent includes a cyclic organic solvent and a chain organic solvent, the cyclic organic solvent is one or more of ethylene carbonate, propylene carbonate and butylene carbonate, and the chain organic solvent is one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl propionate and propyl propionate.

[0017] Wherein, the lithium salt is one or more of lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate and lithium difluorooxalatoborate.

[0018] The third object of the present invention is to provide a secondary battery with good high temperature performance and rate performance in view of the deficiencies of the prior art.

[0019] In order to achieve the above purpose, the present invention adopts the following technical documents:

[0020] A secondary battery comprises the above electrolyte.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: an electrolyte additive of the present invention includes a first additive and a second additive. The two additives are used in synergistic manner to significantly improve the high temperature performance and rate performance of the battery, and form a more uniform, dense and less resistive SEI film on the surface of the pole piece. Specifically, the first compound shown in Formula I has a silicon oxygen group (Si-O) and a carbon-carbon double bond, which can form a hydrophobic surface polymer around the lithium ion solvation, resisting the hydrolysis of LiPF6 while inhibiting the dissolution of positive electrode metal ions. The active groups work synergistically to generate a more stable electrolyte membrane, reducing the consumption of the electrolyte and improving the cycle and high temperature performance of the battery system. The second compound shown in Formula II has fluorine and chlorine atoms. The fluorine atoms and fluorine atoms form (LiF 1-x Cl x )SEI, Cl doping can endow LiF with a low ion migration barrier 1-x Cl x Higher Li conductivity, sufficient mechanical stability, and high surface energy, the double halide SEI is beneficial to Li + Fast transmission. At the same time (LiF 1-x Cl x )The compactness of SEI can prevent the solvent from continuously decomposing, and at the same time, enable the rapid diffusion of lithium ions in the electrolyte and electrode / electrolyte interface, reducing voltage polarization. In addition, the presence of the sulfonamide group in the additive formula synergistically establishes a stable interface between the positive and negative electrodes, maintains the structural stability of the high-voltage positive electrode, and improves the rate, cycle and high-temperature storage performance. DETAILED DESCRIPTION

[0022] The present invention is further described in detail below in conjunction with specific implementation examples and comparative examples, but the implementation examples of the present invention are not limited thereto.

[0023] An electrolyte additive comprises the following components in parts by weight: 1 to 5 parts of a first additive and 5 to 18 parts of a second additive, wherein the second additive is one or more of ethylene carbonate, fluoroethylene carbonate, vinyl ethylene carbonate, 1,3-propane sultone, propylene sulfite, vinyl sulfate, 4-methyl ethylene sulfate, succinic anhydride, succinonitrile, adiponitrile and 1,3,6-hexanetrinitrile; the first additive comprises one or more of a first compound shown in formula I or a second compound shown in formula II,

[0024]

[0025] Wherein, R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11, R12 and R13 are independently selected from one of hydrogen, halogen atoms, alkyl groups having 1 to 10 carbon atoms, unsaturated hydrocarbon groups having 2 to 10 carbon atoms, alkoxy groups having 1 to 10 carbon atoms, alkanoyl groups having 2 to 10 carbon atoms and phenyl groups, and the hydrogen in the alkyl groups, the unsaturated hydrocarbon groups, the alkoxy groups and the alkanoyl groups may be partially or completely substituted by one or more of halogen atoms, cyano groups, carboxyl groups or sulfonic acid groups.

[0026] In the first compound shown in Formula I, an active group C=C is introduced on the basis of the effective group Si-O (acid and water removal), and the carbon-carbon double bond is easy to produce a polymerization effect, forming a film on the surface of the positive and negative electrodes, and a hydrophobic surface polymer can be formed around the lithium ion solvation, inhibiting the hydrolysis of LiPF6 while inhibiting the dissolution of the positive electrode metal ions. The active groups work together to form a more stable electrolyte membrane, reducing the consumption of the electrolyte and improving the cycle and high temperature performance of the battery system. At the same time, the second compound shown in Formula II participates in the film formation to form a dihalide (LiF1-xClx) SEI formed by F and Cl atoms. Due to the lower ion migration energy barrier, Cl doping can give LiF1-xClx higher Li conductivity, sufficient mechanical stability and high surface energy, and the dihalide SEI is conducive to the rapid transmission of Li+. At the same time, the compactness of (LiF1-xClx) SEI can prevent the sustainable decomposition of the solvent, while realizing the rapid diffusion of lithium ions in the electrolyte and the electrode / electrolyte interface, reducing voltage polarization. Moreover, the presence of the sulfonamide group in the second compound shown in Formula II synergistically establishes a stable interface between the positive electrode and the negative electrode, maintains the structural stability of the high-voltage positive electrode, and improves the rate, cycle and high-temperature storage performance.

[0027] Preferably, the CAS number of the first compound represented by formula I is 111918-90-2, and the CAS number of the second compound represented by formula II is 26574-59-4.

[0028] Among them, in the first compound represented by formula I, R1, R2 and R3 are alkane groups with 1 to 10 carbon atoms; in the second compound represented by formula II, R4, R5, R6, R7 are halogen atoms, R8 is a hydrogen group, R11 is a halogen atom, and R9, R10, R12 and R13 are hydrogen groups.

[0029] Among them, in the first compound represented by formula I, R1, R2 and R3 are methyl groups, and in the second compound, R4, R5, R6 are fluorine atoms, R7 is a chlorine atom, R8 is a hydrogen group, R11 is a chlorine atom, and R9, R10, R12 and R13 are hydrogen groups.

[0030] The first additive includes a first compound represented by Formula I and a second compound represented by Formula II, and the weight ratio of the first compound to the second compound is 1 to 3: 1 to 3. The weight ratio of the first compound to the second compound can be 1: 1, 1: 2, 1: 3, 2: 1, 2: 3.

[0031] Wherein, the weight ratio of the first additive to the second additive is 1-5:6-15. The weight ratio of the first additive to the second additive is 1:6, 1:8, 1:10, 1:12, 1:15, 2:7, 2:9, 2:11, 2:13, 2:15, 3:2, 3:5, 3:7, 3:9, 3:11, 3:15, 4:7, 4:10, 4:13, 4:15.

[0032] The electrolyte of the present invention has good high temperature performance and rate performance.

[0033] Specifically, an electrolyte of the present invention comprises an organic solvent, a lithium salt and the electrolyte additive, wherein the electrolyte additive accounts for 2% to 10% by weight of the electrolyte. The electrolyte additive may account for 2%, 4%, 6%, 8% or 10% by weight of the electrolyte.

[0034] The organic solvent includes a cyclic organic solvent and a chain organic solvent, the cyclic organic solvent is one or more of ethylene carbonate, propylene carbonate and butylene carbonate, and the chain organic solvent is one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl propionate and propyl propionate.

[0035] Wherein, the lithium salt is one or more of lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate and lithium difluorooxalatoborate.

[0036] A secondary battery of the present invention has good high temperature performance and rate performance. A secondary battery of the present invention comprises a positive electrode sheet, a negative electrode sheet, a separator, a shell and the above-mentioned electrolyte, wherein the separator is used to separate the positive electrode sheet from the negative electrode sheet, and the shell is used to install and package the positive electrode sheet, the negative electrode sheet, the separator and the electrolyte.

[0037] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material. The positive electrode active material may include but is not limited to a chemical formula such as Li a Ni x Co y M z O 2-b N b(wherein 0.95≤a≤1.2, x>0, y≥0, z≥0, and x+y+z=1, 0≤b≤1, M is selected from a combination of one or more of Mn and Al, and N is selected from a combination of one or more of F, P, and S) The positive electrode active material may also be, but is not limited to, LiCoO2, LiNiO2, LiVO2, LiCrO2, LiMn2O4, LiCoMnO4, Li2NiMn3O8, LiNi 0.5 Mn 1.5 O4, LiCoPO4, LiMnPO4, LiFePO4, LiNiPO4, LiCoFSO4, CuS2, FeS2, MoS2, NiS, TiS2, or a combination thereof. The positive active material may also be subjected to a modification treatment, and the method for modifying the positive active material should be known to those skilled in the art. For example, the positive active material may be modified by coating, doping, or the like, and the material used for the modification treatment may include, but is not limited to, a combination of one or more of Al, B, P, Zr, Si, Ti, Ge, Sn, Mg, Ce, W, or the like. The positive current collector is usually a structure or part for collecting current, and the positive current collector may be any material suitable for use as a positive current collector for lithium-ion batteries in the art. For example, the positive current collector may include, but is not limited to, metal foil, and more specifically, may include, but is not limited to, aluminum foil, or the like.

[0038] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer arranged on the surface of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material. The negative electrode active material may include but is not limited to one or more of graphite, soft carbon, hard carbon, carbon fiber, mesophase carbon microspheres, silicon-based materials, tin-based materials, lithium titanate or other metals that can form alloys with lithium. Among them, the graphite can be selected from one or more of artificial graphite, natural graphite and modified graphite; the silicon-based material can be selected from one or more of elemental silicon, silicon oxide compounds, silicon carbon composites, and silicon alloys; the tin-based material can be selected from one or more of elemental tin, tin oxide compounds, and tin alloys. The negative electrode current collector is usually a structure or part that collects current. The negative electrode current collector can be various materials in the art that are suitable for use as a negative electrode current collector for lithium-ion batteries. For example, the negative electrode current collector can include but is not limited to metal foil, and more specifically can include but is not limited to copper foil.

[0039] The separator can be any material suitable for lithium-ion battery separators in the art, for example, it can be a combination of one or more materials including but not limited to polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester and natural fibers.

[0040] The shell is made of stainless steel or aluminum-plastic film.

[0041] Example 1

[0042] Positive electrode preparation: The positive electrode material 4.5V high voltage LiCoO2, binder PVDF (polyvinylidene fluoride), and conductive agent Super-P are dispersed in NMP (N-methylpyrrolidone) organic solvent at a mass ratio of 98:1:1, stirred until stable and uniform under the action of a vacuum mixer, and evenly coated on an aluminum foil with a thickness of 10μm. After the aluminum foil is dried at room temperature, it is transferred to a 120℃ forced air oven for drying for 1h, and then cold pressed and die-cut to make a positive electrode sheet.

[0043] Negative electrode preparation: Commercial graphite, binder PVDF, and conductive agent Super-P were mixed together in a mass ratio of 97:2:1, dispersed in NMP organic solvent, and evenly coated on a copper foil with a thickness of 8μm. The aluminum foil was dried at room temperature and transferred to a 120℃ forced air oven for drying for 1h, and then cold pressed and die-cut to form a negative electrode sheet.

[0044] Electrolyte preparation:

[0045] 1) In a glove box filled with nitrogen (O2 < 2ppm, H2O < 3ppm), an organic solvent was prepared, wherein the organic solvent consisted of EC (ethylene carbonate), PC (propylene carbonate), EP (ethyl propionate), and PP (propyl propionate) in a weight ratio of 20:5:10:65;

[0046] 2) Slowly add LiPF6 to the organic solvent to prepare a lithium salt solution with a concentration of 1.12 mol / L;

[0047] 3) mixing a lithium salt solution, an organic solvent, a first additive and a second additive in 14 parts, 70 parts, 1 part and 15 parts by weight to prepare an electrolyte;

[0048] The first additive is a mixture of a compound of Formula 1 and a compound of Formula 2, wherein the weight ratio of the compound of Formula 1 to the compound of Formula 2 is 2:1, and its structure is shown below:

[0049]

[0050] Preparation of lithium-ion batteries: stack the positive electrode, separator, and negative electrode in order, wind them to get a bare battery cell, and then package them with aluminum-plastic film, bake them again, inject liquid, let them stand, form them, shape them with a fixture, seal them again, and test their capacity to complete the preparation of lithium-ion batteries.

[0051] Example 2

[0052] The difference from Example 1 is that the weight parts of the lithium salt solution, the organic solvent, the first additive and the second additive are 14 parts, 68 parts, 3 parts and 15 parts respectively.

[0053] The rest is the same as Example 1 and will not be described again here.

[0054] Example 3

[0055] The difference from Example 1 is that the lithium salt solution, the organic solvent, the first additive and the second additive are 14 parts, 66 parts, 5 parts and 15 parts by weight.

[0056] The rest is the same as Example 1 and will not be described again here.

[0057] Example 4

[0058] The difference from Example 1 is that the first additive only includes the first compound of Formula 3, and the structure is shown below:

[0059]

[0060]

[0061] The rest is the same as Example 1 and will not be described again here.

[0062] Example 5

[0063] The difference from Example 1 is that the first additive only includes the second compound of Formula 4, and the structural formula is as follows:

[0064]

[0065] The rest is the same as Example 1 and will not be described again here.

[0066] Example 6

[0067] The difference from Example 1 is that the weight ratio of the first compound of Formula 1 to the second compound of Formula 2 is 1:1.

[0068] The rest is the same as Example 1 and will not be described again here.

[0069] Example 7

[0070] The difference from Example 1 is that the weight ratio of the first compound of Formula 1 to the second compound of Formula 2 is 1:2.

[0071] The rest is the same as Example 1 and will not be described again here.

[0072] Example 8

[0073] The difference from Example 1 is that the weight ratio of the first compound of Formula 1 to the second compound of Formula 2 is 1:3.

[0074] The rest is the same as Example 1 and will not be described again here.

[0075] Example 9

[0076] The difference from Example 1 is that the weight ratio of the first compound of Formula 1 to the second compound of Formula 2 is 3:1.

[0077] The rest is the same as Example 1 and will not be described again here.

[0078] Comparative Example 1

[0079] The difference from Example 1 is that the lithium salt solution, the organic solvent and the second additive are 14 parts, 71 parts and 15 parts by weight.

[0080] The rest is the same as in Example 1 and will not be described again here.

[0081] Comparative Example 2

[0082] The difference from Example 1 is that the lithium salt solution, the organic solvent and the first additive are 14 parts, 85 parts and 1 part by weight.

[0083] The rest is the same as in Example 1 and will not be described again here.

[0084] The above-mentioned Examples 1 to 9 and Comparative Examples 1 and 2 were subjected to performance tests, and the test results are recorded in Table 2.

[0085] Normal temperature / high temperature cycle test:

[0086] The battery was placed in a constant temperature oven at 25℃ / 45℃ for 4 hours, then charged to 4.5V at a constant current of 0.5C, then charged at a constant voltage until the current dropped to 0.05C, then discharged to 3.0V at a constant current of 1C, and so on, recording the initial capacity of the battery and the discharge capacity of the last cycle (800 / 500th cycle). Capacity retention rate = discharge capacity of the last cycle (800 / 500th cycle) / initial capacity × 100%.

[0087] 60℃ high temperature storage:

[0088] The battery was charged to 4.5V at a constant current of 0.5C in a 25°C environment, charged at a constant voltage until the current dropped to 0.02C, and then discharged to 3.0V at a constant current of 0.2C, and recorded as the cell thickness, internal resistance, and initial capacity; charged to 4.5V in the same way, and then placed in an oven at a constant temperature of 60°C for 7 days. The cell thickness, internal resistance, and capacity discharged to 3.0V at a current of 0.2C were measured for 7 days and recorded as the residual capacity. The stored cell was charged to 4.5V at a constant current of 0.5C, charged at a constant voltage until the current dropped to 0.02C, and then discharged to 3.0V at a constant current of 0.2C, and recorded as the recovery capacity.

[0089] Residual value = residual capacity / initial capacity × 100%,

[0090] Recovery value = recovery capacity / initial capacity × 100%,

[0091] Thickness expansion rate = (thickness after storage - thickness before storage) / thickness before storage × 100%,

[0092] Internal resistance growth rate=(internal resistance after storage-internal resistance before storage) / internal resistance before storage×100%.

[0093] Rate performance:

[0094] The battery was charged to 4.5V at a constant current of 0.5C at 25°C, charged at a constant voltage until the current dropped to 0.02C, and then discharged to 3.0V at constant currents of 0.2C / 1C / 2C / 3C / 5C, respectively, which was recorded as the discharge capacity of the battery cell; capacity retention rate at different rates = discharge capacity at different rates / initial capacity at 0.2C × 100%.

[0095] Table 1

[0096] serial number Lithium salt mass fraction Solvent mass fraction Weight of the first additive Second additive weight Example 1 14 70 1 15 Example 2 14 68 3 15 Example 3 14 66 5 15 Example 4 14 64 1 15 Example 5 14 70 1 15 Example 6 14 70 1 15 Example 7 14 70 1 15 Example 8 14 70 1 15 Example 9 14 70 1 15 Comparative Example 1 14 70 1 15 Comparative Example 2 14 85 1 0

[0097] Table 2

[0098]

[0099] It can be concluded from Table 2 above that the electrolyte to which the electrolyte additive of the present invention is added has better high temperature performance and cycle performance than Comparative Examples 1 and 2. By comparing Example 1, Comparative Examples 1 and 2, it can be concluded that when the first additive and the second additive are used in combination in the electrolyte additive, the high temperature storage performance and cycle performance of the battery can be significantly improved. By comparing Examples 1-3, it can be concluded that when the weight ratio of the lithium salt solution, the organic solvent, and the first additive and the second additive in the electrolyte is set to 14:70:1:15, the high temperature performance and cycle performance of the electrolyte are better. By comparing Examples 1, 4, and 5, it can be concluded that when the first additive in the electrolyte solution includes the first compound and the second compound, the prepared electrolyte has better performance, and the two can work synergistically to establish a stable interface between the positive electrode and the negative electrode, maintain the structural stability of the high-voltage positive electrode, and improve the rate and cycle and high temperature storage performance. By comparing Examples 1, 6-9, it can be concluded that when the weight ratio of the first compound and the second compound is set to 2:1, the prepared electrolyte has better performance and good high temperature performance and cycle performance.

[0100] According to the disclosure and teaching of the above description, those skilled in the art to which the present invention belongs can also change and modify the above embodiment. Therefore, the present invention is not limited to the above specific embodiment, and any obvious improvement, replacement or modification made by those skilled in the art on the basis of the present invention belongs to the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for the convenience of description and do not constitute any limitation to the present invention.

Claims

1. An electrolyte additive, characterized in that: The invention comprises the following components in parts by weight: 1 to 5 parts of a first additive and 5 to 18 parts of a second additive, wherein the second additive is one or more of ethylene carbonate, fluoroethylene carbonate, vinyl ethylene carbonate, 1,3-propane sultone, propylene sulfite, vinyl sulfate, 4-methyl ethylene sulfate, succinic anhydride, succinonitrile, adiponitrile and 1,3,6-hexanetrinitrile; the first additive comprises a first compound shown in formula I and a second compound shown in formula II, wherein R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11, R12 and R13 are independently selected from one of hydrogen, halogen atoms, alkyl groups having 1 to 10 carbon atoms, unsaturated hydrocarbon groups having 2 to 10 carbon atoms, alkoxy groups having 1 to 10 carbon atoms, alkanoyl groups having 2 to 10 carbon atoms and phenyl groups, and part or all of the hydrogen in the alkyl groups, the unsaturated hydrocarbon groups, the alkoxy groups and the alkanoyl groups are substituted with one or more of halogen atoms, cyano groups, carboxyl groups or sulfonic acid groups; In the second compound, R4, R5, and R6 are fluorine atoms, R7 is a chlorine atom, R8 is a hydrogen group, R11 is a chlorine atom, and R9, R10, R12, and R13 are hydrogen groups.

2. The electrolyte additive according to claim 1, characterized in that: In the first compound represented by formula I, R1, R2 and R3 are alkane groups with 1 to 10 carbon atoms; in the second compound represented by formula II, R4, R5, R6, R7 are halogen atoms, R8 is a hydrogen group, R11 is a halogen atom, and R9, R10, R12 and R13 are hydrogen groups.

3. The electrolyte additive according to claim 1, characterized in that: In the first compound represented by formula I, R1, R2 and R3 are methyl groups.

4. The electrolyte additive according to claim 1, characterized in that: The weight ratio of the first compound to the second compound is 1-3:1-3.

5. The electrolyte additive according to claim 1, characterized in that: The weight ratio of the first additive to the second additive is 1-5:6-15.

6. An electrolyte, characterized in that: The invention comprises an organic solvent, a lithium salt and the electrolyte additive according to any one of claims 1 to 5, wherein the electrolyte additive accounts for 2% to 10% of the electrolyte weight.

7. The electrolyte according to claim 6, characterized in that The organic solvent includes a cyclic organic solvent and a chain organic solvent. The cyclic organic solvent is one or more of ethylene carbonate, propylene carbonate and butylene carbonate. The chain organic solvent is one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl propionate and propyl propionate.

8. The electrolyte according to claim 6, characterized in that The lithium salt is one or more of lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate and lithium difluorooxalatoborate.

9. A secondary battery, characterized in that: An electrolyte comprising the electrolyte according to any one of claims 6 to 8.

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