Electrolyte and lithium ion battery and method for improving performance of lithium ion battery

Abstract

The application belongs to the technical field of lithium ion batteries, and particularly relates to an electrolyte, which comprises a lithium salt, a non-aqueous organic solvent, a first auxiliary solvent, a second auxiliary solvent and a first additive; the first auxiliary solvent is sulfur dioxide; the second auxiliary solvent is a fluorine-containing solvent; and the first additive is lithium difluorobis(oxalato)phosphate. The electrolyte adopts high-concentration sulfur dioxide added into the electrolyte, and simultaneously adopts the fluorine-containing solvent. The fluorine-containing solvent can form a film on the negative electrode surface in priority, and on the other hand, can inhibit the reduction reaction of sulfur dioxide, thereby inhibiting the film formation of sulfur dioxide. Meanwhile, the fluorine-containing solvent and the additive are used in combination, so that the reduction reaction film formation of sulfur dioxide on the positive electrode surface is inhibited, so that the electrolyte has high electrical conductivity and oxidation resistance, and meanwhile, an integrated and high-strength interface protection film is formed on the negative electrode surface. Meanwhile, the application further discloses a lithium ion battery and a method for improving the battery performance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of lithium ion batteries, and particularly relates to an electrolyte and a lithium ion battery and a method for improving the performance of a lithium ion battery. BACKGROUND

[0002] CN114914536A, CN109962291B, CN112687953B, etc. all disclose that sulfur dioxide can exist as an additive of an electrolyte.

[0003] CN115172878A discloses an electrolyte, a preparation method thereof, and a secondary battery, which comprises a gas-liquid mixed additive, the mass fraction of the gas-liquid mixed additive in the electrolyte is 0.1-10wt%, and the mass fraction of sulfur dioxide in the gas-liquid mixed additive is 5-20wt%. The electrolyte contains a large amount of sulfur dioxide, and thus can effectively improve the cycle stability and electrochemical performance of the electrolyte.

[0004] The preparation method of the electrolyte is as follows: sulfur dioxide gas is mixed with carbonate organic solvents and carboxylate organic solvents to obtain a gas-liquid mixed additive, and then the gas-liquid mixed additive is added into a treatment liquid to obtain the electrolyte. The amount of dissolved sulfur dioxide gas in the electrolyte is greatly increased, and the sulfur dioxide gas is not easy to escape. Moreover, the amount of sulfur dioxide can be accurately controlled, the SEI film is not affected, the film is not thickened, the impedance is not increased, and the cycle and low-temperature performance of the lithium ion battery is not affected. Meanwhile, the amount of sulfur dioxide in the electrolyte is not too small, the film forming effect is not affected, the electrolyte consumption is not too much, the by-products are not increased, and the performance is not improved obviously.

[0005] However, it cannot be denied that, through experiments, it can be found that the conductivity of the battery is increased, the rate performance and the low-temperature performance are enhanced with the increase of the amount of sulfur dioxide.

[0006] Therefore, it is possible to significantly increase the amount of sulfur dioxide to improve the chemical performance of the battery, but the problem of the over-thick SEI film caused by the too high concentration of sulfur dioxide needs to be solved. SUMMARY

[0007] The purpose of the present application is to provide an electrolyte, which adds high-concentration sulfur dioxide into the electrolyte as part of the solvent, and uses a fluorine-containing solvent. The fluorine-containing solvent can form a film on the negative electrode surface in priority, inhibit the reduction reaction of sulfur dioxide, and further inhibit the film formation of sulfur dioxide. Meanwhile, the use of the fluorine-containing solvent and the additive in combination can inhibit the reduction reaction and film formation of sulfur dioxide on the positive electrode surface, so that the electrolyte has high conductivity and oxidation resistance, and an integrated and high-strength interface protection film is formed on the negative electrode surface.

[0008] Meanwhile, the application also discloses a lithium ion battery and a method for improving the performance of the lithium ion battery.

[0009] The technical scheme of the application is:

[0010] An electrolyte comprises a lithium salt, a non-aqueous organic solvent, a first auxiliary solvent, a second auxiliary solvent and a first additive; the first additive is lithium difluorobis(oxalato)phosphate;

[0011] The first auxiliary solvent is sulfur dioxide; the second auxiliary solvent is a fluorine-containing solvent; the amount of the first auxiliary solvent is 5-15wt% of the total amount of the electrolyte; and the amount of the second auxiliary solvent is 5-10wt% of the total amount of the electrolyte.

[0012] In the electrolyte, the amount of the first auxiliary solvent can be 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, 14wt% or 15wt%.

[0013] In the electrolyte, the amount of the second auxiliary solvent can be 5wt%, 6wt%, 7wt%, 8wt%, 9wt% or 10wt%.

[0014] In the above electrolyte, the amount of the first additive is 0.1-2wt% of the total amount of the electrolyte, preferably, the amount of the first additive is 0.2-1.5wt% of the total amount of the electrolyte, more preferably, the amount of the first additive is 0.2-1wt% of the total amount of the electrolyte.

[0015] In the electrolyte, the amount of the first additive is 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1.0wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, 1.5wt%, 1.6wt%, 1.7wt%, 1.8wt%, 1.9wt% or 2.0wt% of the total amount of the electrolyte.

[0016] In the above electrolyte, the amount of the first auxiliary solvent is 8-12wt% of the total amount of the electrolyte; and the amount of the second auxiliary solvent is 6-8wt% of the total amount of the electrolyte.

[0017] Preferably, the fluorine-containing solvent is fluoroethylene carbonate, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether or 4,4,4-trifluorobutyric acid ethyl ester.

[0018] The fluorine-containing solvent can be further selected from one or more of 2,2-difluoroethyl acetate, trifluoroethyl acetate, difluoroacetic acid ethyl ester, trifluoroacetic acid ethyl ester, and the like, in combination with the fluorinated ethylene carbonate, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, or 4,4,4-trifluorobutyric acid ethyl ester described above.

[0019] The fluorine-containing solvent can also be further selected from chain ethers having 3 to 10 carbon atoms substituted with fluorine, such as di(2-fluoroethyl) ether, di(2,2-difluoroethyl) ether, di(2,2,2-trifluoroethyl) ether, ethyl(2-fluoroethyl) ether, ethyl(2,2,2-trifluoroethyl) ether, ethyl(l,l,2,2-tetrafluoroethyl) ether, (2-fluoroethyl)(2,2,2-trifluoroethyl) ether, (2-fluoroethyl)(l,l,2,2-tetrafluoroethyl) ether, (2,2,2-trifluoroethyl)(l,l,2,2-tetrafluoroethyl) ether, ethyl-n-propyl ether, ethyl(3-fluoro-n-propyl) ether, ethyl(3,3,3-trifluoro-n-propyl) ether, ethyl(2,2,3,3-tetrafluoro-n-propyl) ether, ethyl(2,2,3,3,3-pentafluoro-n-propyl) ether, 2-fluoroethyl-n-propyl ether, (2-fluoroethyl)(3-fluoro-n-propyl) ether, (2-fluoroethyl)(3,3,3-trifluoro-n-propyl) ether, (2-fluoroethyl)(2,2,3,3-tetrafluoro-n-propyl) ether, (2-fluoroethyl)(2,2,3,3,3-pentafluoro-n-propyl) ether, 2,2,2-trifluoroethyl-n-propyl ether, (2,2,2-trifluoroethyl)(3-fluoro-n-propyl) ether, (2,2,2-trifluoroethyl)(3,3,3-trifluoro-n-propyl) ether, (2,2,2-trifluoroethyl)(2,2,3,3-tetrafluoro-n-propyl) ether, (2,2,2-trifluoroethyl)(2,2,3,3,3-pentafluoro-n-propyl) ether, l,l,2,2-tetrafluoroethyl-n-propyl ether, (l,l,2,2-tetrafluoroethyl)(3-fluoro-n-propyl) ether, (l,l,2,2-tetrafluoroethyl)(3,3,3-trifluoro-n-propyl) ether, (l,l,2,2-tetrafluoroethyl)(2,2,3,3-tetrafluoro-n-propyl) ether, (l,l,2,2-tetrafluoroethyl)(2,2,3,3,3-pentafluoro-n-propyl) ether, di-n-propyl ether, (n-propyl)(3-fluoro-n-propyl) ether, (n-propyl)(3,3,3-trifluoro-n-propyl) ether, (n-propyl)(2,2,3,3-tetrafluoro-n-propyl) ether, (n-propyl)(2,2,3,3,3-pentafluoro-n-propyl) ether, di(3-fluoro-n-propyl) ether, (3-fluoro-n-propyl)(3,3,3-trifluoro-n-propyl) ether, (3-fluoro-n-propyl)(2,2,3,3-tetrafluoro-n-propyl) ether, (3-fluoro-n-propyl)(2,2,3,3,3-pentafluoro-n-propyl) ether, di(3,3,3-trifluoro-n-propyl) ether, (3,3,3-trifluoro-n-propyl)(2,2,3,3-tetrafluoro-n-propyl) ether, (3,3,3-trifluoro-n-propyl)(2,2,3,3,3-pentafluoro-n-propyl) ether, di(2,2,3,3-tetrafluoro-n-propyl) ether, (2,2,3,3-tetrafluoro-n-propyl)(2,2,3,3,3-pentafluoro-n-propyl) ether, di(2,2,3,3,3-pentafluoro-n-propyl) ether;The chain ether is compounded with the above-mentioned fluorinated ethylene carbonate, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether or ethyl 4,4,4-trifluorobutyrate;

[0020] In the present application, the lithium salt is at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluorophosphate, lithium difluoro(oxalato)phosphate, lithium tetrafluoro(oxalato)phosphate and lithium bis(fluorosulfonyl)imide, the concentration of the lithium salt is 0.5-2M, preferably 0.5-1.5M; more preferably 0.6-1.2M.

[0021] The concentration of the optional lithium salt is 0.5M, 0.7M, 0.9M, 1.1M, 1.3M or 1.5M.

[0022] In the present application, the non-aqueous organic solvent is a cyclic organic solvent and / or a chain organic solvent;

[0023] The cyclic organic solvent is one or more combinations of propylene carbonate, ethylene carbonate and butylene carbonate;

[0024] The chain organic solvent is one or more combinations of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl formate, ethyl formate, methyl acetate and ethyl acetate.

[0025] As a further preferred embodiment of the present application, a second additive is further included, the amount of the second additive is 0.1-10wt% of the total amount of the electrolyte; more preferably, the amount of the second additive can be selected as 0.1-5wt%; more preferably 0.1-3wt%; more preferably 0.1-2wt%;

[0026] The amount of the second additive is 0.1wt%, 0.5wt%, 1.0wt%, 2.0wt%, 3.0wt%, 4.0wt%, 5.0wt%, 6.0wt%, 7.0wt%, 8.0wt%, 9.0wt% or 10.0wt% of the total amount of the electrolyte.

[0027] The second additive is vinylene carbonate, vinyl ethylene carbonate, vinyl sulfate, propylene carbonate, 1,3-propane sulfite, 1,3-propylene sulfite, 1,4-butane sulfite, 2,4-butane sulfite, succinic anhydride, maleic anhydride, 2-methyl maleic anhydride, methyl carbonic acid-2-propynyl ester, tetraethenesilane, triallyl isocyanurate, hexamethylene diisocyanate, o-phenanthroline, p-phenylene diisocyanate, 2,4-toluene diisocyanate, N-phenyl bis(trifluoromethane sulfonate) imide, bisvinyl sulfate, methyl sulfonic acid phenyl ester, bisvinyl sulfate, bispropylene sulfite, p-phenylenediamine difluorosulfonate, triallyl phosphate, tripropargyl phosphate, 2,4-butane sulfite, isocyanatoethyl methacrylate, methyl methane disulfonate, tris(trimethylsilyl) borate, tris(trimethylsilyl) phosphate, tris(vinyldimethylsilyl) phosphate, 4,4'-bi-1,3-dioxolan-2,2'-dione, propyl diprop-2-ynyl phosphate, ethyl diprop-2-ynyl phosphate, (2-allylphenoxy)trimethylsilane, tetramethyl methylene diphosphate, isocyanatoethyl methacrylate, or 2-fluoropyridine.

[0028] Meanwhile, the present application also discloses a lithium ion battery, which uses the electrolyte as described above.

[0029] In the above lithium ion battery, the positive electrode of the lithium ion battery is selected from lithium transition metal oxides, wherein the lithium transition metal oxides are LiCoO2, LiMn2O4, LiMnO2, Li2MnO4, LiFePO4, Li 1+a Mn 1-x MxO2, LiCo 1-x M x O2, LiFe 1-x M x PO4, Li2Mn 1-x O4, wherein M is one or more selected from Ni, Co, Mn, Al, Cr, Mg, Zr, Mo, V, Ti, B, F, 0≤a<0.2, 0≤x<1; in addition, as long as it is a substance capable of electrochemically intercalating and deintercalating alkali metal ions, other than lithium, it can be used without limitation, and examples include sodium and the like. Two or more positive electrode active materials can also be used in combination;

[0030] In the present application, the lithium ion battery should include a separator and a negative electrode, which are well known;

[0031] The negative electrode is selected from at least one of graphite, silicon-carbon composite material, and lithium titanate.

[0032] The negative active material in the negative electrode includes at least one of carbonaceous material, silicon-carbon material, alloy material, lithium metal-containing composite oxide material, but is not limited thereto, and the negative active material can be selected from various conventionally known materials capable of electrochemically intercalating and deintercalating active ions, which can be used as a negative active material of an electrochemical device;

[0033] The method for preparing the negative electrode sheet is a conventionally known method for preparing a negative electrode sheet that can be used in an electrochemical device; the negative active material layer further includes a binder and a solvent. The negative active material is added to the binder and the solvent, and a thickening agent, a conductive agent, a filler, etc. are added as necessary to prepare a negative electrode slurry, and then the negative electrode slurry is coated on a negative electrode current collector, dried, and pressed to prepare the negative electrode sheet. The negative electrode slurry forms the negative active material layer after drying and cold pressing. Similarly, in the preparation of the negative electrode slurry, a solvent is generally added. The solvent is removed during the drying process. The binder is a conventionally known binder that can be used as a binder of the negative active material layer, and the binder is, for example, but not limited to, styrene-butadiene rubber. The solvent is a conventionally known solvent that can be used as a solvent of the negative active material layer, and the solvent is, for example, but not limited to, water. The thickening agent is a conventionally known thickening agent that can be used as a thickening agent of the negative active material layer, and the thickening agent is, for example, but not limited to, carboxymethyl cellulose. In some embodiments, when the negative active material includes an alloy material, a vapor deposition method, a sputtering method, a plating method, etc. can be used to form the negative active material layer;

[0034] The separator is a conventionally known separator that can be used in an electrochemical device and is stable to the electrolyte used, and is, for example, but not limited to, a resin, glass fiber, inorganic matter.

[0035] For example, the separator includes at least one of polyolefin, aramid, polytetrafluoroethylene, polyethersulfone. Preferably, the polyolefin includes at least one of polyethylene, polypropylene. Preferably, the polyolefin includes polypropylene. Preferably, the separator is laminated by multiple layers of material, for example, the separator is a three-layer separator laminated in the order of polypropylene, polyethylene, polypropylene.

[0036] Finally, the present application also discloses a method for improving the performance of a lithium ion battery, using the electrolyte as described above as the electrolyte of the lithium ion battery.

[0037] The beneficial effects of the present application are as follows:

[0038] The electrolyte uses high-concentration sulfur dioxide added to the electrolyte as part of the solvent. As a solvent, it can effectively improve the conductivity of the electrolyte, but due to its strong reducibility, it is easy to deposit on the surface of the graphite negative electrode during charging, forming a high-impedance and unstable interface film.

[0039] The fluorine-containing solvent can form a stable interface film on the surface of the negative electrode and is resistant to oxidation itself. The fluorine-containing solvent mainly acts on the negative electrode and can inhibit the reduction reaction of sulfur dioxide on the negative electrode to form an unstable interface film.

[0040] The lithium difluorobis(oxalato)phosphate can form a stable interface film on the surface of the positive electrode and the negative electrode. The synergistic effect of the lithium difluorobis(oxalato)phosphate and the fluorine-containing solvent is excellent, and a more stable interface film is formed on the negative electrode, thereby inhibiting the decomposition of the electrolyte on the negative electrode interface. Since the lithium difluorobis(oxalato)phosphate also forms a film on the positive electrode, the corrosion of the positive electrode by HF is inhibited.

[0041] In addition, we believe that the lithium difluorobis(oxalato)phosphate can synergistically inhibit the unstable interface film formed by the reduction reaction of sulfur dioxide near the positive electrode with the fluorine-containing solvent.

[0042] Therefore, in the present application, sulfur dioxide mainly exists as an additive for increasing conductivity. The fluorine-containing solvent and the lithium difluorobis(oxalato)phosphate not only exist as film-forming additives, but also inhibit the reduction film-forming reaction of sulfur dioxide on the positive electrode and the negative electrode, thereby significantly enhancing the performance of the battery system. DETAILED DESCRIPTION

[0043] The technical solutions of the present application will be further described in detail below in combination with specific embodiments, but do not constitute any limitation on the present application.

[0044] Example 1

[0045] 1. Preparation of the electrolyte: EC and EMC were used as the main solvent and mixed in a volume ratio of 1:1. The second auxiliary solvent, fluoroethylene carbonate, was added to the main solvent and mixed uniformly. Then, sulfur dioxide was injected into the main solvent. Then, lithium salt LiPF6 was added to adjust the concentration of lithium salt in the system to 1.0 M. The amount of sulfur dioxide added was equivalent to 10% of the total weight of the electrolyte. The amount of fluoroethylene carbonate added was equivalent to 7.5% of the total weight of the electrolyte. Lithium difluorobis(oxalato)phosphate was added in an amount equivalent to 0.5% of the total weight of the electrolyte.

[0046] 2. Preparation of the positive electrode sheet: The positive electrode material (LiNi 0.8 Mn 0.1 Co 0.1 O4), conductive agent SuperP, adhesive PVDF, and carbon nanotubes (CNT) were mixed in a mass ratio of 95:2.3:2:0.7 to form a lithium ion battery positive electrode slurry with a certain viscosity, which was coated on both sides of the current collector aluminum foil at a coating amount of 35 g / m 2 After drying at 85°C, cold pressing was performed. Then, edge cutting, sheet cutting, and striping were performed. After striping, the strips were dried at 85°C under vacuum for 4 hours, and the tabs were welded to form a lithium ion battery positive electrode sheet that meets the requirements.

[0047] 3. Preparation of the negative electrode sheet: graphite, conductive agent Super P, thickening agent CMC, and adhesive SBR (styrene-butadiene rubber emulsion) were mixed in a ratio of 95:1.5:1.0:2.5 to form a slurry, which was uniformly mixed. The mixed slurry was coated on both sides of a copper foil, and then dried and rolled to obtain a negative electrode sheet. A lithium ion battery negative electrode sheet meeting the requirements was prepared.

[0048] 4. Preparation of the lithium ion battery: the positive electrode sheet, the negative electrode sheet, and the separator prepared according to the above process were stacked to form a lithium ion battery with a thickness of 4.7 mm, a width of 55 mm, and a length of 60 mm. The battery was vacuum baked at 75°C for 10 hours, and then the above electrolyte was injected. After standing for 24 hours, the battery was placed in an environment at 45°C, and a pressure of 3 kg was applied. The battery was charged to 4.0 V at 0.1C (180 mA), and then stood for 2 days to fully activate the battery. Then, the performance of the battery was tested.

[0049] The preparation processes of Examples 2-10 and Comparative Examples 1-5 were the same as those of Example 1; the formulations were as shown in Table 1, as follows:

[0050] Table 1 Formulation table of Examples 1-10 and Comparative Examples 1-7

[0051]

[0052]

[0053] Performance test

[0054] Test item 1: normal temperature performance test

[0055] The lithium ion batteries in Examples 1-9 and Comparative Examples 1-7 were subjected to normal temperature cycle performance tests, and the test method was as follows:

[0056] The lithium ion battery was first charged at 1.0C to a voltage of 4.3V, and then discharged at 1.0C to 2.75V at 25°C. The discharge capacity of the first cycle was recorded. The battery was subjected to 300 cycles of charge and discharge tests, and the discharge capacity of the 300th cycle was recorded.

[0057] Capacity retention rate = (discharge capacity of the 300th cycle / discharge capacity of the first cycle) x 100%.

[0058] Test item 2: low temperature performance test

[0059] The lithium ion batteries in Examples 1-9 and Comparative Examples 1-7 were subjected to low temperature discharge performance tests, and the test method was as follows:

[0060] The lithium ion battery was charged at 1C constant current to 4.3V, charged at 4.3V constant voltage to 0.05C, then discharged at 1C to 2.75V at room temperature, and recorded as room temperature discharge capacity. Then the battery was charged at 1C constant current to 4.3V, charged at 4.3V constant voltage to 0.05C, and placed in a -20℃ low temperature cabinet for >4h, until the battery temperature dropped to -20℃, then discharged at 0.5C to 2.75V, and recorded as -20℃ 0.5C discharge capacity.

[0061] -20℃ discharge capacity retention rate = (-20℃ 0.5C discharge capacity / room temperature discharge capacity) x 100%

[0062] Test item 3: high temperature performance test

[0063] The lithium ion batteries in examples 1-9 and comparative examples 1-7 were subjected to high temperature storage performance test, and the test method was as follows:

[0064] The lithium ion battery was charged at 1C constant current to 4.3V, charged at 4.3V constant voltage to 0.05C at room temperature, then discharged at 1C to 2.75V, and recorded as pre-storage discharge capacity. The lithium ion battery was charged at 1C constant current to 4.3V, charged at 4.3V constant voltage to 0.05C, and at this time the thickness of the lithium ion battery was tested and recorded as H0; then it was placed in a 60℃ oven for 14 days, then taken out and cooled to room temperature, and the thickness was first tested and recorded as H1, then discharged at 1C to 2.75V, and the post-storage discharge capacity was recorded.

[0065] High temperature storage capacity retention rate = (post-storage discharge capacity / pre-storage discharge capacity) x 100%;

[0066] High temperature storage expansion rate = (H1 / H0) x 100%.

[0067] Test item 4: conductivity test

[0068] The lithium ion batteries in examples 1-9 and comparative examples 1-7 were subjected to high temperature storage performance test, and the test method was as follows:

[0069] The test electrolyte was placed in a thermostat at 25℃ for 15 minutes or more to ensure that the electrolyte temperature was 25±0.3℃ (measured with a thermometer); the electrodes of the conductivity meter were rinsed with deionized water, the water on the electrodes was shaken off gently, and the electrodes were dried with filter paper; the electrodes were inserted into the electrolyte, and the reading was taken to obtain the conductivity data of the electrolyte.

[0070] The test results are shown in Table 2 below:

[0071] Table 2: Test results of lithium ion batteries

[0072]

[0073] Result analysis

[0074] 1. From examples 1-3, the optimal amount of sulfur dioxide is 10%, too little sulfur dioxide will affect its low temperature performance, too much sulfur dioxide will affect its high temperature performance; from examples 2, 4, 5, the optimal amount of fluorine-containing solvent (fluorinated ethylene carbonate) is 7.5%, too little fluorine-containing solvent will affect its cycle performance and high temperature storage performance, too much fluorine-containing solvent will affect its low temperature performance and high temperature storage performance; from examples 2, 6, 7, the optimal amount of lithium difluorobis (oxalate) phosphate is 0.5%, too much or too little will affect the comprehensive performance of the battery. From examples 2, 8, 9, the addition of the second additive improves the battery performance, but the improvement space is not big.

[0075] 2. From comparative examples 1, 2, 7 and example 2, the main role of SO2 is to improve the conductivity of the electrolyte as a solvent, thereby improving the low temperature performance of the battery.

[0076] 3. From comparative examples 1, 3, 6 and example 2, the main role of the fluorinated solvent is to act as an additive and a solvent, the role of the additive is to form a film on the negative electrode and inhibit the decomposition of the electrolyte on the surface of the negative electrode; the role of the solvent is to improve the high voltage resistance of the electrolyte, thereby improving the cycle performance and low temperature performance of the battery.

[0077] 4. From comparative examples 1, 4, 5 and example 2, the main role of lithium difluorobis (oxalate) phosphate is to form a stable low impedance interface film on the surface of the positive and negative electrodes as an additive, to inhibit the decomposition of the electrolyte on the surface of the positive and negative electrodes, thereby improving the cycle stability, low temperature performance and high temperature storage performance of the battery.

[0078] 5. From all examples and comparative examples, when the sulfur dioxide is 10%, the fluorinated ethylene carbonate is 7.5%, and the lithium difluorobis (oxalate) phosphate is 0.5%, the synergistic effect is optimal.

[0079] The above examples are the preferred embodiments of the present application, but the embodiments of the present application are not limited by the above examples, any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application are equivalent replacement ways, all of which are included in the protection scope of the present application.

Claims

1. An electrolyte, characterized in that, It includes lithium salt, non-aqueous organic solvent, first auxiliary solvent, second auxiliary solvent, and first additive; the first additive is lithium difluorobis(oxalate)phosphate; the first auxiliary solvent is sulfur dioxide; and the second auxiliary solvent is a fluorinated solvent. The amount of the first auxiliary solvent is equivalent to 6-15 wt% of the total electrolyte; the amount of the second auxiliary solvent is equivalent to 5-10 wt% of the total electrolyte; and the amount of the first additive is equivalent to 0.1-2 wt% of the total electrolyte.

2. The electrolyte according to claim 1, characterized in that, The amount of the first additive is equivalent to 0.2-1.5 wt% of the total electrolyte.

3. The electrolyte according to claim 2, characterized in that, The amount of the first additive is equivalent to 0.2-1 wt% of the total electrolyte.

4. The electrolyte according to claim 1, characterized in that, The amount of the first auxiliary solvent is equivalent to 8-12 wt% of the total electrolyte; the amount of the second auxiliary solvent is equivalent to 6-8 wt% of the total electrolyte.

5. The electrolyte according to claim 1, characterized in that, The fluorinated solvent is fluoroethylene carbonate, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, or ethyl 4,4,4-trifluorobutyrate.

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

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

8. The electrolyte according to claim 1, characterized in that, It also includes a second additive, the amount of which is equivalent to 0.1-10 wt% of the total electrolyte. The second additive is vinylene carbonate, ethylene carbonate, vinyl sulfate, propylene carbonate, 1,3-propanesulfonate lactone, 1,3-propenesulfonate lactone, 1,4-butanesulfonate lactone, 2,4-butanesulfonate lactone, succinic anhydride, maleic anhydride, 2-methylmaleic anhydride, methyl carbonate-2-propynyl ester, tetraethylenesilane, triallyl isocyanurate, hexamethylene diisocyanate, o-phenanthroline, terephthalic diisocyanate, 2,4-toluene diisocyanate, N-phenylbis(trifluoromethanesulfonyl)imide, vinyl disulfate, phenyl methanesulfonate, vinyl disulfate. Bis-spiropropyl sulfate, hydroquinone difluorosulfonate, triallyl phosphate, triargyl phosphate, 2,4-butane sulpholol, isocyanoethyl methacrylate, methylene disulfonate, tri(trimethylsilane)borate, tri(trimethylsilane) phosphate, tri(vinyl dimethylsilane) phosphate, 4,4'-bi-1,3-dioxolane-2,2'-dione, propyl dipropyl-2-alkynyl phosphate, ethyl dipropyl-2-alkynyl phosphate, (2-allylphenoxy)trimethylsilane, tetramethylmethylene diphosphate, isocyanoethyl methacrylate, or 2-fluoropyridine.

9. A lithium-ion battery, characterized in that, The electrolyte used is as described in any one of claims 1-8.

10. The lithium-ion battery according to claim 9, characterized in that, The positive electrode of the lithium-ion battery is selected from lithium transition metal oxides, wherein the lithium transition metal oxide is LiCoO2, LiMn2O4, LiMnO2, Li2MnO4, LiFePO4, Li 1+a Mn 1-x MxO2, LiCo 1-x M x O2, LiFe 1-x M x PO4, Li2Mn 1-x O4, wherein M is selected from one or more of Ni, Co, Mn, Al, Cr, Mg, Zr, Mo, V, Ti, B, and F, 0≤a<0.2, 0≤x<1; the negative electrode is selected from at least one of graphite, silicon-carbon composite material, and lithium titanate.

11. A method for improving the performance of lithium-ion batteries, characterized in that, The electrolyte as described in any one of claims 1-8 is used as the electrolyte for the lithium-ion battery.

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