Electrolyte and lithium ion battery

Abstract

The invention provides an electrolyte and a lithium ion battery, and relates to the technical field of lithium ion batteries. The electrolyte comprises a lithium salt, a non-aqueous solvent, a first additive and a second additive, the first additive is selected from one or more of a compound shown in a formula (I) and a compound shown in a formula (II). According to the electrolyte provided by the invention, the high-temperature circulation and storage performance of the lithium ion battery under a high-voltage condition can be remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to an electrolyte and a lithium ion battery. Background Art

[0002] Carbonate organic solvents are mainly used as electrolyte solvents in current commercial electrolytes. The electrolyte system composed of conventional carbonate solvents and lithium hexafluorophosphate is at 4.2V (Vs Li / Li + ) above, oxidative decomposition will occur, causing a decline in the performance of the entire lithium battery system. Therefore, developing electrolytes suitable for high voltage conditions is the only way to improve battery energy density. Summary of the Invention

[0003] Based on the technical problems existing in the background technology, the present invention proposes an electrolyte and a lithium-ion battery.

[0004] The present invention provides an electrolyte comprising a lithium salt, a non-aqueous solvent, a first additive, and a second additive; the first additive is selected from one or more compounds of formula (I) and formula (II):

[0005]

[0006] In the compound of formula (I), R1 to R2 are independently selected from hydrogen atoms, halogen atoms, phenyl groups, cyano groups, five-membered or six-membered heterocycles, substituted or unsubstituted C 1～6 One or more of alkyl, alkenyl, and alkynyl;

[0007]

[0008] In the compound of formula (II), R3 to R 11 are independently selected from hydrogen atoms, halogen atoms, phenyl groups, cyano groups, five-membered or six-membered heterocycles, substituted or unsubstituted C 1～5 One or more of alkyl, alkenyl and alkynyl.

[0009] The electrolyte provided by the present invention includes lithium salt, non-aqueous solvent, first additive and second additive. The raw materials act synergistically to obtain an electrolyte that can significantly improve the high-temperature cycle performance and storage performance of lithium-ion batteries under high voltage conditions.

[0010] Preferably, the halogen atom is selected from one or more of a fluorine atom, a bromine atom, and a chlorine atom.

[0011] Preferably, in the compound of formula (I), R1 to R2 are independently selected from hydrogen atoms, halogen atoms, phenyl groups, substituted or unsubstituted C 1～6 One or more of alkyl groups.

[0012] Preferably, in the compound of formula (II), R3 to R 11 are independently selected from hydrogen atoms, halogen atoms, phenyl groups, cyano groups, substituted or unsubstituted C 1～5 One or more of alkyl groups.

[0013] Preferably, the compound of formula (I) is selected from any one of Compounds 1 to 4:

[0014]

[0015]

[0016] Among them, the CAS number of compound 1 is 139555-01-4, the CAS number of compound 2 is 112678-64-5, the CAS number of compound 3 is 647-83-6, and the CAS number of compound 4 is 6167-35-7.

[0017] The sulfur element in the compound of formula (I) can increase the flexibility of the interface film, and elements such as fluorine and chlorine can enrich the interface film components of the electrode / electrolyte, thereby improving the structural stability of the interface film and enhancing the high-temperature storage performance of lithium-ion batteries under high voltage conditions.

[0018] When R1 and R2 in the compound of formula (I) are halogen atoms, the compound helps to improve the cycle performance of the lithium-ion battery at high voltage (2.8 to 4.5 V), reduce impedance, and improve the high-temperature performance of the battery.

[0019] Preferably, the compound of formula (II) is selected from any one of Compounds 5 to 8:

[0020]

[0021] Among them, the CAS number of compound 5 is 1910084-03-5, the CAS number of compound 6 is 871030-94-3, the CAS number of compound 7 is 73852-85-4, and the CAS number of compound 8 is 1204393-80-5.

[0022] The compound of formula (II) contains a benzene ring, which can undergo electropolymerization to form a film at the positive electrode, effectively protecting the positive electrode interface and inhibiting the migration of transition metal ions at the positive electrode and direct contact between the electrode interface and the electrolyte under high voltage conditions. The borate group can preferentially form a dense and stable passivation film at the negative electrode. In addition, the addition of functional groups such as cyano groups has strong coordination ability, which can bind to the active sites on the electrode surface, masking the active ions on the positive electrode surface, reducing the decomposition effect of the electrode on the electrolyte, and improving the cycle life of the battery at high voltage.

[0023] Preferably, the mass ratio of the compound of formula (I) to the compound of formula (II) in the first additive is (1-2):(1-2).

[0024] The mass ratio of the compound of formula (I) to the compound of formula (II) within a certain range helps the two compounds to act synergistically, further improving the cycle performance and high temperature performance of the battery under high voltage conditions.

[0025] Preferably, the mass of the first additive accounts for 0.1 to 10 wt % of the total mass of the electrolyte.

[0026] More preferably, the mass of the first additive accounts for 0.5-4 wt % of the total mass of the electrolyte.

[0027] The mass proportion of the first additive within a certain range helps to improve the high-temperature performance of the electrolyte under high voltage conditions in the battery.

[0028] Preferably, the second additive is selected from at least one of vinylene carbonate (VC), fluoroethylene carbonate (FEC), diethylene sulfate (DTD), tris(trimethylsilyl) phosphate (TMSP), tris(trimethylsilyl) borate (TMSB), lithium difluorophosphate (LiDFP), and 1,3-propane sultone (PS).

[0029] Preferably, the mass of the second additive accounts for 1-5 wt% of the total mass of the electrolyte.

[0030] The role of the second additive is to assist in optimizing battery performance, such as promoting the formation of SEI film, reducing electrolyte viscosity, and improving ionic conductivity.

[0031] Preferably, the lithium salt is selected from at least two of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiPF4), lithium dioxalatoborate (LiBOB), lithium difluorooxalatoborate (LiODFB), lithium difluorooxalatophosphate (LiODFP), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).

[0032] More preferably, the lithium salt is a combination of LiPF6 and LiFSI.

[0033] More preferably, the mass ratio of LiPF6 to LiFSI is 10:3.

[0034] The synergistic effect of multiple lithium salts is used to effectively improve the battery coulombic efficiency and achieve uniform deposition of metallic lithium. By optimizing the lithium salt ratio, the ionic conductivity is improved and the stable deposition of metallic lithium is achieved.

[0035] Preferably, the mass of the lithium salt accounts for 10-20 wt% of the total mass of the electrolyte.

[0036] More preferably, the mass of the lithium salt accounts for 13 wt % of the total mass of the electrolyte.

[0037] Preferably, the non-aqueous solvent includes at least two of a chain carbonate solvent, a cyclic carbonate solvent, and a carboxylate solvent.

[0038] More preferably, the cyclic carbonate solvent is selected from at least one of ethylene carbonate (EC) and propylene carbonate (PC); the chain carbonate solvent is selected from at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC); and the carboxylate solvent is selected from at least one of propyl acetate (PA), ethyl acetate (EA), and propyl propionate (PP).

[0039] Cyclic carbonates have a high dielectric constant but a high viscosity; chain carbonates have a relatively low viscosity but a lower dielectric constant than cyclic carbonates; and carboxylates have an average freezing point 20-30°C lower than carbonates and a lower viscosity, significantly improving the low-temperature characteristics of lithium-ion batteries, but their overall stability is lower than that of carbonates. Taking all these factors into consideration, the non-aqueous solvent is a mixed solvent of at least two of the following: cyclic carbonates, chain carbonates, and carboxylates, taking into account the requirements for dielectric constant, viscosity, and temperature.

[0040] The present invention adopts a mixture of carbonate solvents and carboxylate solvents to replace conventional carbonate organic solvents, which can effectively reduce the oxidation and decomposition of electrolyte solvents under high voltage conditions.

[0041] A lithium-ion battery comprises a positive electrode, a negative electrode, a separator and the above-mentioned electrolyte.

[0042] Preferably, the active material of the positive electrode is selected from nickel-cobalt-manganese ternary material or nickel-cobalt-aluminum ternary material, and the active material of the negative electrode is selected from graphite or silicon-carbon material.

[0043] The beneficial effects of the present invention are:

[0044] The electrolyte provided by the present invention includes lithium salt, non-aqueous solvent, first additive and second additive. The raw materials act synergistically to obtain an electrolyte that can significantly improve the high-temperature cycle performance and storage performance of lithium-ion batteries under high voltage conditions.

[0045] The present invention adopts a mixture of carbonate solvents and carboxylate solvents to replace conventional carbonate organic solvents, which can effectively reduce the oxidation and decomposition of electrolyte solvents under high voltage conditions.

[0046] The synergistic effect of multiple lithium salts is used to effectively improve the coulombic efficiency of batteries under high voltage and achieve uniform deposition of metallic lithium. By optimizing the lithium salt ratio, the ionic conductivity is improved and the stable deposition of metallic lithium is achieved.

[0047] The additives in the present invention contain benzene rings, which can undergo electropolymerization film formation at the positive electrode, effectively protect the positive electrode interface, inhibit the migration of positive electrode transition metal ions and the direct contact between the electrode interface and the electrolyte under high voltage conditions, and the borate group can preferentially generate a dense and stable passivation film at the negative electrode. In addition, the additives containing cyclic sulfates in the present invention are prone to undergo reduction film formation reaction at the negative electrode interface, forming a stable SEI film at the negative electrode interface, isolating the direct contact between the electrolyte and the electrode interface, alleviating the consumption of the electrolyte and reducing the irreversible loss of active lithium at the negative electrode interface under high temperature conditions, thereby improving the high temperature performance of the battery. The addition of fluorine atoms can combine with the lithium in the electrolyte to form LiF to increase the stability of the interface film. The synergistic interaction of the three is beneficial to the protection of the positive and negative electrode interfaces, thereby effectively isolating the direct contact between the electrolyte and the electrode interface, and improving the electrochemical performance of the battery under different working conditions. Therefore, the electrolyte of the present invention is applied to lithium ion batteries, which can significantly improve the high temperature cycle performance and storage performance of lithium ion batteries under high voltage conditions. DETAILED DESCRIPTION

[0048] The technical solution of the present invention is described in detail through specific embodiments.

[0049] Unless otherwise specified, the materials, reagents, etc. used in the following examples and comparative examples can be obtained from commercial sources.

[0050] Example 1

[0051] Prepare the electrolyte:

[0052] In an argon-filled glove box (water <0.1 ppm, oxygen <0.1 ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), propylene carbonate (PC) and ethyl acetate (EA) were mixed in a mass ratio of EC:EMC:PC:EA=20:45:5:30 to obtain a solvent; lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) were slowly added to the solvent; after LiPF6 and LiFSI were completely dissolved, vinylene carbonate (VC), tris(trimethylsilyl) phosphate (TMSP), fluoroethylene carbonate (FEC) and compound 3 were added to obtain;

[0053] Among them, the raw materials of the electrolyte are composed of the following components in mass percentage: 10% lithium hexafluorophosphate (LiPF6), 3% lithium bis(fluorosulfonyl)imide (LiFSI), 0.5% vinylene carbonate (VC), 1.0% tris(trimethylsilyl) phosphate (TMSP), 1.0% fluoroethylene carbonate (FEC) and 0.5% compound 3, and the solvent is supplemented to 100%.

[0054] Preparation of positive electrode:

[0055] The active material lithium nickel cobalt manganese oxide (NCM811), the conductive agent super conductive carbon (SP), and the binder polyvinylidene fluoride (PVDF) were fully stirred and mixed in the solvent N-methyl-2-pyrrolidone at a mass ratio of 97:1:2. The obtained slurry was coated on the current collector Al foil, and the positive electrode sheet was obtained by slitting, die-cutting, and rolling. Its compaction density is 3.45g / cm 3 .

[0056] Preparation of negative electrode sheet:

[0057] Silicon carbon anode (SiOC600), conductive agent super conductive carbon (SP), binder styrene butadiene rubber (SBR), thickener sodium carboxymethyl cellulose (CMC) were mixed thoroughly in deionized water at a mass ratio of 95:1:2:2. The obtained slurry was coated on the current collector Cu foil. After slitting, rolling and die-cutting, the negative electrode sheet was obtained. Its compaction density was 1.6 g / cm 3 .

[0058] Preparation of lithium-ion batteries:

[0059] The positive electrode sheet, the separator (the separator used is a polypropylene PP film coated with a nano-aluminum oxide coating, wherein the thickness of the PP film is 12 μm and the thickness of the nano-aluminum oxide coating is 4 μm), and the negative electrode sheet are stacked in sequence according to the process of stacked batteries to obtain a bare battery cell, and then the bare battery cell is pre-packaged in an aluminum-plastic film, injected with the prepared electrolyte, and then packaged to obtain the required soft-pack battery.

[0060] Example 2

[0061] Prepare the electrolyte:

[0062] In an argon-filled glove box (water <0.1 ppm, oxygen <0.1 ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), propylene carbonate (PC) and ethyl acetate (EA) were mixed in a mass ratio of EC:EMC:PC:EA=20:45:5:30 to obtain a solvent; lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) were slowly added to the solvent; after LiPF6 and LiFSI were completely dissolved, vinylene carbonate (VC), tris(trimethylsilyl) phosphate (TMSP), fluoroethylene carbonate (FEC) and compound 3 were added to obtain;

[0063] Among them, the raw materials of the electrolyte are composed of the following components in mass percentage: 10% lithium hexafluorophosphate (LiPF6), 3% lithium bis(fluorosulfonyl)imide (LiFSI), 0.5% vinylene carbonate (VC), 1.0% tris(trimethylsilyl) phosphate (TMSP), 1.0% fluoroethylene carbonate (FEC) and 1% compound 3, and the solvent is supplemented to 100%.

[0064] The preparation methods of the positive electrode sheet, negative electrode sheet and lithium ion battery are the same as those in Example 1.

[0065] Example 3

[0066] Prepare the electrolyte:

[0067] In an argon-filled glove box (water <0.1 ppm, oxygen <0.1 ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), propylene carbonate (PC) and ethyl acetate (EA) were mixed in a mass ratio of EC:EMC:PC:EA=20:45:5:30 to obtain a solvent; lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) were slowly added to the solvent; after LiPF6 and LiFSI were completely dissolved, vinylene carbonate (VC), tris(trimethylsilyl) phosphate (TMSP), fluoroethylene carbonate (FEC) and compound 3 were added to obtain;

[0068] Among them, the raw materials of the electrolyte are composed of the following components in mass percentage: 10% lithium hexafluorophosphate (LiPF6), 3% lithium bis(fluorosulfonyl)imide (LiFSI), 0.5% vinylene carbonate (VC), 1.0% tris(trimethylsilyl) phosphate (TMSP), 1.0% fluoroethylene carbonate (FEC) and 3% compound 3, and the solvent is supplemented to 100%.

[0069] The preparation methods of the positive electrode sheet, negative electrode sheet and battery are the same as those in Example 1.

[0070] Example 4

[0071] Prepare the electrolyte:

[0072] In an argon-filled glove box (water <0.1 ppm, oxygen <0.1 ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), propylene carbonate (PC) and ethyl acetate (EA) were mixed in a mass ratio of EC:EMC:PC:EA=20:45:5:30 to obtain a solvent; lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) were slowly added to the solvent, and after LiPF6 and LiFSI were completely dissolved, vinylene carbonate (VC), tris(trimethylsilyl) phosphate (TMSP), fluoroethylene carbonate (FEC) and compound 6 were added to obtain;

[0073] Among them, the raw materials of the electrolyte are composed of the following components in the following mass percentages: 10% lithium hexafluorophosphate (LiPF6), 3% lithium bis(fluorosulfonyl)imide (LiFSI), 0.5% vinylene carbonate (VC), 1.0% tris(trimethylsilyl) phosphate (TMSP), 1.0% fluoroethylene carbonate (FEC) and 0.5% compound 6, and the solvent is supplemented to 100%.

[0074] The preparation methods of the positive electrode sheet, negative electrode sheet and battery are the same as those in Example 1.

[0075] Example 5

[0076] Prepare the electrolyte:

[0077] In an argon-filled glove box (water <0.1 ppm, oxygen <0.1 ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), propylene carbonate (PC) and ethyl acetate (EA) were mixed in a mass ratio of EC:EMC:PC:EA=20:45:5:30 to obtain a solvent; lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) were slowly added to the solvent, and after LiPF6 and LiFSI were completely dissolved, vinylene carbonate (VC), tris(trimethylsilyl) phosphate (TMSP), fluoroethylene carbonate (FEC) and compound 6 were added to obtain;

[0078] Among them, the raw materials of the electrolyte are composed of the following components in the following mass percentages: 10% lithium hexafluorophosphate (LiPF6), 3% lithium bis(fluorosulfonyl)imide (LiFSI), 0.5% vinylene carbonate (VC), 1.0% tris(trimethylsilyl) phosphate (TMSP), 1.0% fluoroethylene carbonate (FEC) and 2% compound 6, and the solvent is supplemented to 100%.

[0079] The preparation methods of the positive electrode sheet, negative electrode sheet and battery are the same as those in Example 1.

[0080] Example 6

[0081] Prepare the electrolyte:

[0082] In an argon-filled glove box (water <0.1 ppm, oxygen <0.1 ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), propylene carbonate (PC) and ethyl acetate (EA) were mixed in a mass ratio of EC:EMC:PC:EA=20:45:5:30 to obtain a solvent; lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) were slowly added to the solvent, and after LiPF6 and LiFSI were completely dissolved, vinylene carbonate (VC), tris(trimethylsilyl) phosphate (TMSP), fluoroethylene carbonate (FEC) and compound 6 were added to obtain;

[0083] Among them, the raw materials of the electrolyte are composed of the following components in the following mass percentages: 10% lithium hexafluorophosphate (LiPF6), 3% lithium bis(fluorosulfonyl)imide (LiFSI), 0.5% vinylene carbonate (VC), 1.0% tris(trimethylsilyl) phosphate (TMSP), 1.0% fluoroethylene carbonate (FEC) and 4% compound 6, and the solvent is supplemented to 100%.

[0084] The preparation methods of the positive electrode sheet, negative electrode sheet and battery are the same as those in Example 1.

[0085] Example 7

[0086] Prepare the electrolyte:

[0087] In an argon-filled glove box (water <0.1 ppm, oxygen <0.1 ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), propylene carbonate (PC) and ethyl acetate (EA) were mixed in a mass ratio of EC:EMC:PC:EA=20:45:5:30 to obtain a solvent; lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) were slowly added to the solvent, and after LiPF6 and LiFSI were completely dissolved, vinylene carbonate (VC), tris(trimethylsilyl) phosphate (TMSP), fluoroethylene carbonate (FEC), compound 3 and compound 6 were added to obtain;

[0088] Among them, the raw materials of the electrolyte are composed of the following components in mass percentage: 10% lithium hexafluorophosphate (LiPF6), 3% lithium bis(fluorosulfonyl)imide (LiFSI), 0.5% vinylene carbonate (VC), 1.0% tris(trimethylsilyl) phosphate (TMSP), 1.0% fluoroethylene carbonate (FEC), 1% compound 3 and 2% compound 6, and the solvent is supplemented to 100%.

[0089] The preparation methods of the positive electrode sheet, negative electrode sheet and battery are the same as those in Example 1.

[0090] Example 8

[0091] Prepare the electrolyte:

[0092] In an argon-filled glove box (water <0.1 ppm, oxygen <0.1 ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), propylene carbonate (PC) and ethyl acetate (EA) were mixed in a mass ratio of EC:EMC:PC:EA=20:45:5:30 to obtain a solvent; lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) were slowly added to the solvent, and after LiPF6 and LiFSI were completely dissolved, vinylene carbonate (VC), tris(trimethylsilyl) phosphate (TMSP), fluoroethylene carbonate (FEC), compound 3 and compound 6 were added to obtain;

[0093] Among them, the raw materials of the electrolyte are composed of the following components in mass percentage: 10% lithium hexafluorophosphate (LiPF6), 3% lithium bis(fluorosulfonyl)imide (LiFSI), 0.5% vinylene carbonate (VC), 1.0% tris(trimethylsilyl) phosphate (TMSP), 1.0% fluoroethylene carbonate (FEC), 1.5% compound 3 and 1.5% compound 6, and the solvent is supplemented to 100%.

[0094] The preparation methods of the positive electrode sheet, negative electrode sheet and battery are the same as those in Example 1.

[0095] Example 9

[0096] Prepare the electrolyte:

[0097] In an argon-filled glove box (water <0.1 ppm, oxygen <0.1 ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), propylene carbonate (PC) and ethyl acetate (EA) were mixed in a mass ratio of EC:EMC:PC:EA=20:45:5:30 to obtain a solvent; lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) were slowly added to the solvent, and after LiPF6 and LiFSI were completely dissolved, vinylene carbonate (VC), tris(trimethylsilyl) phosphate (TMSP), fluoroethylene carbonate (FEC), compound 3 and compound 6 were added to obtain;

[0098] Among them, the raw materials of the electrolyte are composed of the following components in mass percentage: 10% lithium hexafluorophosphate (LiPF6), 3% lithium bis(fluorosulfonyl)imide (LiFSI), 0.5% vinylene carbonate (VC), 1.0% tris(trimethylsilyl) phosphate (TMSP), 1.0% fluoroethylene carbonate (FEC), 2% compound 3 and 1% compound 6, and the solvent is supplemented to 100%.

[0099] The preparation methods of the positive electrode sheet, negative electrode sheet and battery are the same as those in Example 1.

[0100] Comparative Example 1

[0101] The electrolyte of Comparative Example 1 differs from that of Example 1 only in that PC and EA are not added, the mass ratio of EC to EMC is 30:70, and Compound 3 is not contained. The remaining raw materials and preparation methods are the same as those of Example 1.

[0102] Comparative Example 2

[0103] The electrolyte of Comparative Example 2 differs from that of Example 1 only in that it does not contain Compound 3. The remaining raw materials and preparation methods are the same as those of Example 1.

[0104] Comparative Example 3

[0105] The only difference between the electrolyte of Comparative Example 3 and Example 1 is that the mass ratio of LiPF6 to LiFSI is adjusted to 9:4. The rest of the raw materials and preparation method are the same as those of Example 1.

[0106] Comparative Example 4

[0107] The only difference between the electrolyte of Comparative Example 4 and Example 1 is that the mass ratio of LiPF6 to LiFSI is adjusted to 8:5. The remaining raw materials and preparation method are the same as those of Example 1.

[0108] Test example

[0109] (1) 60℃ high temperature shelf test

[0110] The experimental batteries in Examples 1 to 9 and Comparative Example 1 were divided into different capacities and then fixed in capacity, and the discharge capacity at the fixed capacity was used as the initial capacity. The lower voltage limit of the battery is 2.8V, and the upper voltage limit is 4.5V. In a fully charged state, the battery was placed in a constant temperature oven at 60°C for 28 days, and the discharge capacity of the battery after shelving was tested at 0.33C, which was used as the retention capacity. The 0.33C charge and discharge was continued for 1 week, and the discharge capacity was used as the recovery capacity. The capacity retention rate and recovery rate after 28 days were calculated according to the following formula.

[0111] Capacity retention rate = retained capacity / initial capacity*100%

[0112] Capacity recovery rate = recovery capacity / initial capacity*100%

[0113] (2) 45℃ cycle test

[0114] The experimental batteries in Examples 1 to 9 and Comparative Example 1 were divided into different capacities and then fixed in capacity. The discharge capacity at the fixed capacity was used as the initial capacity. The batteries were then placed in a constant temperature cabinet at 45°C, charged to full charge at 0.5C, and discharged to empty at 1C for 500 cycles. The capacity retention rate after 500 cycles was calculated.

[0115] The test results are shown in Table 1:

[0116] Table 1 Comparison of technical solutions and results of examples and comparative examples

[0117]

[0118]

[0119] From the results of Comparative Examples 1 to 4 and Example 1, it can be seen that by optimizing the solvent composition and the lithium salt ratio, the requirements of dielectric constant and viscosity can be taken into account, thereby improving the overall performance of the battery.

[0120] The results of Comparative Example 2 and Examples 1-3 show that the addition of Compound 3 significantly improves the battery's capacity retention and recovery rate after extended shelf life, as well as its capacity retention after 500 cycles. This is because sulfur increases the flexibility of the interfacial film, while elements like boron and oxygen enrich the electrode / electrolyte interfacial film components, thereby improving the structural stability of the interfacial film and enhancing the high-temperature storage performance of lithium-ion batteries.

[0121] Compared with Comparative Example 2, Examples 4 to 6 show that when the additive content of Compound 6 is 2%, the capacity retention rate after 28 days of high storage is most significantly improved. Compound 6 contains a benzene ring, which can undergo electropolymerization to form a film at the positive electrode, effectively protecting the positive electrode interface, inhibiting the migration of positive electrode transition metal ions under high voltage conditions and direct contact between the electrode interface and the electrolyte, and the borate group can preferentially form a dense and stable passivation film at the negative electrode. In addition, after the addition of the cyano functional group, it is not easily oxidized, so it has good stability at the positive electrode and a relatively wide electrochemical window.

[0122] Examples 7 to 9 are the combined use of two additives. It can be seen that the improvement is further enhanced, indicating that the two additives have a synergistic effect. After the two compounds in different proportions are combined, the improvement effects of the capacity retention rate, recovery rate and cycle retention rate after 28 days of high storage are better than those of a single compound, and the performance is optimal when the mass ratio of compound 3 and compound 6 is 1:1. Example 8 improves the capacity retention rate by 3.2% and the recovery rate by 4% after 28 days compared with Example 5, while further suppressing volume expansion. In addition, the capacity retention rate is increased by 4.2% after 500 cycles. It can be seen that the synergistic effect can be maximized only by continuously optimizing the content and ratio between the two types of additives.

[0123] In summary, the electrolyte provided by the present invention can significantly improve the high-temperature storage and cycle performance of lithium-ion batteries under high voltage conditions.

[0124] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. An electrolyte, characterized in that: The invention comprises a lithium salt, a non-aqueous solvent, a first additive, and a second additive; the first additive is selected from one or more compounds of formula (I) and formula (II): In the compound of formula (I), R1 to R2 are independently selected from hydrogen atoms, halogen atoms, phenyl groups, cyano groups, five-membered or six-membered heterocycles, substituted or unsubstituted C 1～6 One or more of alkyl, alkenyl, and alkynyl; In the compound of formula (II), R3 to R 11 are independently selected from hydrogen atoms, halogen atoms, phenyl groups, cyano groups, five-membered or six-membered heterocycles, substituted or unsubstituted C 1～5 One or more of alkyl, alkenyl and alkynyl.

2. The electrolyte according to claim 1, characterized in that The compound of formula (I) is selected from any one of Compounds 1 to 4:

3. The electrolyte according to claim 1, characterized in that The compound of formula (II) is selected from any one of Compounds 5 to 8:

4. The electrolyte according to claim 1, characterized in that The mass ratio of the compound of formula (I) to the compound of formula (II) in the first additive is (1-2):(1-2).

5. The electrolyte according to claim 1, characterized in that The mass of the first additive accounts for 0.1 to 10 wt % of the total mass of the electrolyte.

6. The electrolyte according to claim 1, characterized in that The second additive is selected from at least one of vinylene carbonate, fluoroethylene carbonate, vinyl sulfate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)borate, lithium difluorophosphate, and 1,3-propane sultone; the mass of the second additive accounts for 1 to 5 wt% of the total mass of the electrolyte.

7. The electrolyte according to claim 1, characterized in that The lithium salt is selected from at least two of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium dioxalatoborate, lithium difluorooxalatoborate, lithium difluorooxalatophosphate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide; the mass of the lithium salt accounts for 10-20 wt% of the total mass of the electrolyte.

8. The electrolyte according to claim 1, characterized in that The non-aqueous solvent includes at least two of a chain carbonate solvent, a cyclic carbonate solvent, and a carboxylate solvent; the cyclic carbonate solvent is selected from at least one of ethylene carbonate and propylene carbonate; the chain carbonate solvent is selected from at least one of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; and the carboxylate solvent is selected from at least one of propyl acetate, ethyl acetate, and propyl propionate.

9. A lithium-ion battery, characterized in that: The electrolyte comprises a positive electrode, a negative electrode, a separator and the electrolyte according to any one of claims 1 to 8.

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