Electrolyte and preparation method therefor, and lithium-ion battery

Abstract

An electrolyte for a lithium-ion battery and a preparation method therefor, and a lithium-ion battery. The electrolyte comprises an organic solvent, a lithium salt, and a functional additive. The functional additive comprises an additive A. The lithium salt comprises a lithium salt B. The lithium salt B is selected from one or more of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium difluorobis(oxalato)phosphate, lithium trifluoromethanesulfonate, and lithium 4,5-dicyano-2-(trifluoromethyl)imidazole. The additive A can be oxidized to form a film on a surface of the positive electrode, thereby stabilizing the crystal structure of the positive electrode and improving high-temperature cycling performance. Compared with conventional lithium salts, the lithium salt B has an anionic group with a larger radius, which weakens the binding capacity for lithium ions, improves the dissociation capacity of the lithium salt, and increases the conductivity of the electrolyte, thereby improving low-temperature discharge power performance.

Description

Electrolyte, preparation method thereof and lithium ion battery TECHNICAL FIELD

[0001] The present application relates to the field of lithium ion batteries, in particular to an electrolyte, a preparation method thereof and a lithium ion battery. BACKGROUND

[0002] Lithium ion batteries have a large energy density, a high cycle capacity retention rate, outstanding electrical performance and other advantages, and are currently used in many fields. In the electrification era, lithium ion batteries play an important role in the green energy family due to their clean and environmentally friendly characteristics. A lithium ion battery mainly comprises a positive electrode, a negative electrode, a separator, an electrolyte and an outer package. The electrolyte, as the "blood" of the lithium ion battery, is present in various positions inside the lithium ion battery and plays a role in transferring ions and conducting current in the battery. The electrolyte plays a crucial role in the electrical performance and safety performance of the lithium ion battery.

[0003] However, as the application of lithium batteries becomes more widespread, the low-temperature performance of lithium ion batteries is subjected to more severe tests in some low-temperature regions such as high latitudes and high altitudes. The existing electrolyte design has limited improvement in low-temperature discharge power. To improve the low-temperature discharge, techniques such as increasing high-kinetic solvents and low-impedance additives are usually used, but the high-temperature cycle performance is deteriorated after the low-temperature discharge is improved.

[0004] Therefore, it is necessary to develop an electrolyte for lithium ion batteries that has both high-temperature cycle performance and low-temperature discharge power performance. SUMMARY

[0005] The present application aims to provide an electrolyte and a preparation method thereof that have both high-temperature cycle performance and low-temperature discharge power performance, in view of the deficiencies of the prior art.

[0006] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0007] An electrolyte comprises an organic solvent, a lithium salt and a functional additive, the functional additive comprises an additive A, and the lithium salt comprises a lithium salt B;

[0008] The lithium salt B is selected from one or more of lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium difluorobis(oxalato)phosphate, lithium trifluoromethanesulfonate and lithium 4,5-dicyano-2-trifluoromethyl-imidazole;

[0009] Among them, the high temperature and cycle performance of lithium-ion batteries can be improved by adding electrolyte additive A, and it is necessary to match it with a certain concentration of lithium salt B. Compared with conventional lithium salt, lithium salt B has a larger anionic group radius, less binding of lithium ions, and is easier to dissociate. Under the same lithium salt concentration conditions, it can increase the lithium ion concentration, thereby improving the electrolyte conductivity and thus improving the low temperature discharge power performance.

[0010] The content a of additive A and the molar concentration b of lithium salt B in the electrolyte satisfy the following relationship: 1.5≤15.58*b / a≤10;

[0011] When the value of 15.58*b / a is less than 1.5, the mass proportion of additive A is too large, resulting in high electrolyte viscosity. In this case, the concentration of lithium salt B is relatively low, leading to reduced lithium-ion dissociation and decreased conductivity, resulting in poor low-temperature discharge power performance. When the value of 15.58*b / a is greater than 10, the mass proportion of additive A is too small, resulting in poor electrolyte cycle performance. In this case, the concentration of lithium salt B is relatively high, leading to high electrolyte viscosity and deteriorating low-temperature discharge power performance. Furthermore, excessively high lithium salt B concentration can corrode the positive electrode current collector, affecting the normal use of the battery. Both additive A and lithium salt B increase viscosity. When b is large and / or a is small, the value of 15.58*b / a is too large; when b is small and / or a is large, the value of 15.58*b / a is too small, both of which lead to increased viscosity. Only when a and b are within a certain range can the viscosity be prevented from becoming too high, thus avoiding impacting electrolyte kinetics and deteriorating low-temperature discharge power performance. Therefore, adjusting 1.5≤15.58*b / a≤10 can simultaneously take into account the battery's cycle performance and low-temperature discharge performance.

[0012] Wherein, additive A is selected from at least one of the compounds shown in Formula I:

[0013] Among them, R1, R 13 Each is independently selected from any one of hydrogen or halogen; R2, R3, R4, R6, R7, R8, R 10 R 11 R 12 Each is independently selected from C1-C3 alkylene groups, C1-C3 alkoxy groups, oxygen atoms, Any one of R2, R3, R4, R6, R7, R8, R 10 R 11 R 12 At least one selected from Any one of the following: R2, R3, R4, R6, R7, R8, R 10 R 11 R 12At least one atom is selected from oxygen atoms; R5 and R9 are selected from structures of formula II:

[0014] Where q is an integer between 0 and 4, R 14 and R 16 Each is independently selected from a carbon-carbon single bond or a C1-C5 hydrocarbon group, R 15 Selected from carbon-carbon single bonds, ether bonds, Any one of them.

[0015] Preferably, the mass percentage a% of additive A in the electrolyte and the molar concentration b mol / L of lithium salt B in the electrolyte satisfy the following relationship: 1.5 ≤ 15.58 * b / a ≤ 5.

[0016] Preferably, the content a of additive A accounts for 0.5-3% of the total mass of the electrolyte.

[0017] Preferably, the molar concentration b of the lithium salt B in the electrolyte is 0.3-0.6 mol / L.

[0018] The additive A contains carbonate, sulfonate, and sulfate groups, which can be oxidized to form a dense and flexible CEI protective film when losing electrons on the positive electrode surface. This stabilizes the positive electrode crystal structure, inhibits the dissolution of transition metals and the release of active oxygen, thereby improving high-temperature performance and cycle performance. Furthermore, as the concentration of lithium salt B increases, the viscosity of the electrolyte also increases, which is detrimental to the transport of lithium ions in the electrolyte. Only when the mass content a of additive A is 0.5-4% and the molar concentration b of lithium salt B is 0.3-0.6 mol / L can the electrolyte possess both high-temperature cycling and low-temperature discharge power performance. Preferably, additive A is selected from at least one compound shown in Formulas I-1 to I-19.

[0019] Preferably, the organic solvent is selected from one or more of ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl propionate, propyl propionate, ethyl acetate, ethyl butyrate, and γ-butyrolactone, and the content of the organic solvent accounts for 20.0-70.0% of the total mass of the electrolyte.

[0020] Preferably, the functional additive further includes a film-forming additive; the film-forming additive includes one or more of fluoroethylene carbonate, vinylene carbonate, 1,3-propanesulfonate lactone, vinyl sulfate, methanedisulfonate, propylene sulfonate lactone, citrate anhydride, succinate, adiponitrile, ethylene glycol diether, and hexanetrionitrile, and the mass of the film-forming additive is 2.0-20.0% of the total mass of the electrolyte.

[0021] Preferably, the lithium salt further includes lithium salt C, which is selected from one or more of lithium hexafluorophosphate, lithium difluorophosphate, lithium tetrafluorooxalate phosphate, lithium oxalate phosphate, and lithium tetrafluoroborate, and the molar concentration of lithium salt C in the electrolyte is 0.5-1 mol / L.

[0022] In addition, this application also provides a method for preparing an electrolyte for lithium-ion batteries, comprising the following steps:

[0023] S1. Mix the organic solvents and stir to obtain solvent A, then cool and set aside.

[0024] S2. Add lithium salt C to solution A from step S1 and stir until homogeneous to obtain solution B;

[0025] S3. Add additive A, lithium salt B and film-forming additive to solution B in step S2, and stir evenly to obtain the electrolyte; in step S1, the cooling temperature is below 10°C.

[0026] When lithium salts dissolve in a solvent, they release heat. Lithium hexafluorophosphate is prone to decomposition when heated (around 70°C), leading to a decrease in lithium salt concentration and the production of hydrofluoric acid, which degrades battery performance. Lowering the solvent temperature before adding lithium salts can effectively control the maximum electrolyte preparation temperature to be lower than the decomposition temperature of lithium hexafluorophosphate.

[0027] Furthermore, this application also provides a lithium-ion battery, including a cell wound with a negative electrode, a positive electrode, and a separator, an electrolyte, and a casing for encapsulating the cell and electrolyte. The electrolyte is the aforementioned lithium-ion battery electrolyte.

[0028] Compared to existing technologies, the advantages of this application are as follows: This application obtains a lithium-ion battery electrolyte that combines high-temperature cycling performance and low-temperature discharge power performance by adding additive A and lithium salt B to the electrolyte. Additive A can be oxidized to form a film on the positive electrode surface, stabilizing the positive electrode crystal structure and improving high-temperature cycling performance. Compared to conventional lithium salts, lithium salt B has a larger anionic group radius, weakening its binding ability to lithium ions, increasing the dissociation ability of the lithium salt, and raising the electrolyte conductivity, thereby improving low-temperature discharge power performance. Detailed Implementation

[0029] To make the technical solutions and advantages of this application clearer, the technical solutions of this application will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0030] According to a first aspect of this application, this application provides an electrolyte for lithium-ion batteries, comprising an organic solvent, a lithium salt, and a functional additive, wherein the functional additive includes additive A, and the lithium salt includes lithium salt B.

[0031] Lithium salt B is selected from one or more of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium bis(oxalato)borate, lithium difluorooxalato)borate, lithium difluorobis(oxalato)phosphate, lithium trifluoromethanesulfonate, and lithium 4,5-dicyano-2-trifluoromethyl-imidazolium.

[0032] The mass percentage a% of additive A in the electrolyte and the molar concentration b mol / L of lithium salt B in the electrolyte satisfy the following relationship: 1.5≤15.58*b / a≤10;

[0033] Wherein, additive A is selected from at least one of the compounds shown in Formula I:

[0034] Among them, R1, R 13 Each is independently selected from any one of hydrogen or halogen; R2, R3, R4, R6, R7, R8, R 10 R 11 R 12 Each is independently selected from C1-C3 alkylene groups, C1-C3 alkoxy groups, oxygen atoms, Any one of R2, R3, R4, R6, R7, R8, R 10 R 11 R 12 At least one selected from Any one of the following: R2, R3, R4, R6, R7, R8, R 10 R 11 R 12 At least one atom is selected from oxygen atoms; R5 and R9 are selected from structures of formula II:

[0035] Where q is an integer between 0 and 4, R 14 and R 16 Each is independently selected from a carbon-carbon single bond or a C1-C5 hydrocarbon group, R 15 Selected from carbon-carbon single bonds, ether bonds, Any one of them.

[0036] In some embodiments, the mass percentage a% of additive A in the electrolyte and the molar concentration b mol / L of lithium salt B in the electrolyte satisfy the following relationship: 1.5 ≤ 15.58 * b / a ≤ 5, for example, it can be 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5.

[0037] In some embodiments, the content of additive A is 0.5-3% of the total mass of the electrolyte, for example, it can be 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, or 3.0%.

[0038] In some embodiments, the molar concentration b of lithium salt B in the electrolyte is 0.3-0.6 mol / L, for example, it can be 0.30 mol / L, 0.35 mol / L, 0.40 mol / L, 0.45 mol / L, 0.5 mol / L, or 0.6 mol / L.

[0039] In some embodiments, additive A is selected from at least one of the compounds shown in Formula I-1 to Formula I-19;

[0040] In some embodiments, compound I-6 can be obtained by the following preparation process: after reacting diacetone-D-mannitol, dioxane, potassium carbonate, methanol, dimethyl carbonate, etc. under heating and stirring conditions for several hours, a certain amount of oxalic acid is added to adjust the pH of the solution to neutral. After filtration and concentration, intermediate product 1 is obtained. An appropriate amount of carbonate, acid, pure water, etc. are added to intermediate product 1 to carry out a hydrolysis reaction to obtain intermediate product 2. Intermediate product 2, carbonate solvent, and thionyl chloride are heated to obtain intermediate product 3. Finally, intermediate product 3 is oxidized using an oxidizing agent such as potassium periodate to obtain compound I-6.

[0041] In some embodiments, compound I-12 can be obtained by the following preparation process: methanol and sorbitol are added to organic solvents such as dimethyl carbonate and DMF, potassium hydroxide is added to the reactor as an alkaline catalyst, and the mixture is heated at 45-55°C for 6-8 hours. Then, a certain amount of oxalic acid is added to adjust the pH to neutral. After filtration and recrystallization, intermediate product 4 is obtained. Intermediate product 1, thionyl chloride, and carbonate are subjected to esterification reaction at 60-70°C to obtain intermediate product 5. After oxidation by oxidizing agents such as potassium periodate, compound I-12 can be obtained.

[0042] Those skilled in the art, knowing the structural formula of the compound of structural formula I, can deduce the preparation method of the above-mentioned compound based on common knowledge in the field of chemical synthesis.

[0043] In some embodiments, the organic solvent is selected from one or more of ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl propionate, propyl propionate, ethyl acetate, ethyl butyrate, and γ-butyrolactone, and the content of the organic solvent accounts for 20.0-70.0% of the total mass of the electrolyte.

[0044] In some embodiments, the functional additive further includes a film-forming additive; the film-forming additive includes one or more of fluoroethylene carbonate, vinylene carbonate, 1,3-propanesulfonate lactone, vinyl sulfate, methanedisulfonate, propylene sulfonate lactone, citrate anhydride, succinate, adiponitrile, ethylene glycol dipropionitrile ether, and hexanetrionitrile, and the mass of the film-forming additive is 2.0-20.0% of the total mass of the electrolyte.

[0045] In some embodiments, the lithium salt further includes lithium salt C, which is selected from one or more of lithium hexafluorophosphate, lithium difluorophosphate, lithium tetrafluorooxalate phosphate, lithium oxalate phosphate, and lithium tetrafluoroborate, and the molar concentration of lithium salt C in the electrolyte is 0.5-1 mol / L.

[0046] According to a second aspect of this application, this application provides a method for preparing an electrolyte for lithium-ion batteries, comprising the following steps:

[0047] S1. Mix the organic solvents and stir to obtain solvent A, then cool and set aside.

[0048] S2. Add lithium salt C to solution A from step S1 and stir until homogeneous to obtain solution B;

[0049] S3. Add additive A, lithium salt B and film-forming additive to solution B in step S2, stir evenly to obtain the electrolyte.

[0050] In step S1, the cooling temperature is below 10°C.

[0051] According to a third aspect of this application, this application provides a lithium-ion battery, including a cell wound with a negative electrode, a positive electrode and a separator, an electrolyte, and a casing for encapsulating the cell and the electrolyte, wherein the electrolyte is the aforementioned electrolyte for lithium-ion batteries.

[0052] The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer includes a positive active material, which may be, but is not limited to, a chemical formula such as Li. a Ni x Co y M z O 2-b N b(where 0.95≤a≤1.2, x>0, y≥0, z≥0, and x+y+z=1, 0≤b≤1, M is selected from one or more combinations of Mn and Al, and N is selected from one or more combinations 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 The positive electrode active material can be one or more combinations thereof, including O4, LiCoPO4, LiMnPO4, LiFePO4, LiNiPO4, LiCoFSO4, CuS2, FeS2, MoS2, NiS, and TiS2. The positive electrode active material can also be modified. Methods for modifying the positive electrode active material are known to those skilled in the art. For example, coating, doping, and other methods can be used to modify the positive electrode active material. The materials used for modification can be one or more combinations thereof, including but not limited to Al, B, P, Zr, Si, Ti, Ge, Sn, Mg, Ce, and W. The positive electrode current collector is typically a structure or component that collects current. The positive electrode current collector can be any material suitable for use as a positive electrode current collector in lithium-ion batteries. For example, the positive electrode current collector can be, but is not limited to, metal foil, and more specifically, aluminum foil.

[0053] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the surface of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, which may be one or more of the following: 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. Specifically, the graphite may be selected from one or more of artificial graphite, natural graphite, and modified graphite; the silicon-based material may be selected from one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, and silicon alloys; and the tin-based material may be selected from one or more of elemental tin, tin oxide compounds, and tin alloys. The negative electrode current collector is typically a structure or component that collects current. The negative electrode current collector may be any material suitable for use as a negative electrode current collector in lithium-ion batteries, for example, the negative electrode current collector may be, but is not limited to, metal foil, and more specifically, may be, but is not limited to, copper foil.

[0054] Example 1

[0055] Electrolyte preparation:

[0056] S1. In an argon-filled glove box, EC, PC, EMC, and DEC are added to a PP bottle in a ratio of 20:10:20:50 and stirred until homogeneous to obtain solvent A. After sealing, the solvent A is cooled thoroughly in a refrigerator below 10°C for later use.

[0057] S2. Add lithium hexafluorophosphate (lithium salt C) and lithium difluorosulfonyl imide (lithium salt B) to solution A, and stir until completely dissolved to obtain solution B;

[0058] S3. Based on the mass of the electrolyte, add 3% PS (1,3-propanesulfonate lactone), 2% FEC (fluoroethylene carbonate), 1% DTD (ethylene sulfate) and 0.5% of compound I-12 (additive A) to solution B, and stir until homogeneous to obtain the electrolyte.

[0059] The content of additive A accounts for 0.5% of the total mass of the electrolyte, the molar concentration b of lithium salt B in the electrolyte is 0.3 mol / L, and the molar concentration of lithium salt C in the electrolyte is 0.8 mol / L.

[0060] Preparation of positive electrode:

[0061] By weight percentage, 97.4% lithium nickel cobalt manganese oxide, 0.8% conductive carbon, 0.8% carbon nanotubes, and 1.0% polyvinylidene fluoride are evenly dispersed in N-methylpyrrolidone using a dual planetary mixer, and then uniformly coated onto 9μm aluminum foil using an extrusion coater. After drying, rolling to a certain thickness, and slitting to a certain width, a positive electrode sheet is obtained.

[0062] Preparation of negative electrode:

[0063] By weight percentage, 97.7% graphite, 1.2% polystyrene-butadiene copolymer, and 1.1% lithium carboxymethyl cellulose are evenly dispersed in deionized water using a double planetary mixer, and then evenly coated onto 5μm copper foil using an extrusion coater. After drying, rolling to a certain thickness, and slitting to a certain width, the negative electrode sheet is obtained.

[0064] The fabrication of lithium-ion batteries:

[0065] The positive and negative electrode sheets obtained in the above steps are welded to a separator of a certain width on a winding machine, followed by tab welding, adhesive application, winding, cutting, and hot pressing to obtain a bare battery cell. A suitably sized aluminum-plastic film is then stamped, and the bare battery cell is encapsulated in the stamped aluminum-plastic film. The encapsulated battery cell is injected with the electrolyte obtained in the above steps and pre-sealed. After being allowed to stand at high temperature and then at room temperature for a certain period of time, it undergoes hot pressing formation. After formation, it is degassed and resealed.

[0066] Example 2

[0067] Unlike Example 1, in the preparation of the electrolyte in this example, the content of additive A accounts for 1% of the total mass of the electrolyte.

[0068] The rest is the same as in Example 1, and will not be repeated here.

[0069] Example 3

[0070] Unlike Example 1, in the preparation of the electrolyte in this example, the content of additive A accounts for 2% of the total mass of the electrolyte.

[0071] The rest is the same as in Example 1, and will not be repeated here.

[0072] Example 4

[0073] Unlike Example 1, in the preparation of the electrolyte in this example, the content of additive A accounts for 3% of the total mass of the electrolyte.

[0074] The rest is the same as in Example 1, and will not be repeated here.

[0075] Example 5

[0076] Unlike Example 1, in the preparation of the electrolyte in this example, the molar concentration b of lithium salt B in the electrolyte is 0.4 mol / L, and the content of additive A accounts for 3% of the total mass of the electrolyte.

[0077] The rest is the same as in Example 1, and will not be repeated here.

[0078] Example 6

[0079] Unlike Example 1, in the preparation of the electrolyte in this example, the molar concentration b of lithium salt B in the electrolyte is 0.5 mol / L, and the content of additive A accounts for 3% of the total mass of the electrolyte.

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

[0081] Example 7

[0082] Unlike Example 1, in the preparation of the electrolyte in this example, the molar concentration b of lithium salt B in the electrolyte is 0.6 mol / L, and the content of additive A accounts for 3% of the total mass of the electrolyte.

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

[0084] Example 8

[0085] Unlike Example 1, in this example, additive A is Formula I-6.

[0086] The rest is the same as in Example 1, and will not be repeated here.

[0087] Example 9

[0088] Unlike Example 1, in this example, lithium salt B is lithium bis(trifluoromethanesulfonyl)imide.

[0089] The rest is the same as in Example 1, and will not be repeated here.

[0090] Comparative Example 1

[0091] Unlike Example 1, no additive A or lithium salt B was added in the preparation of this comparative electrolyte. The rest is the same as in Example 1 and will not be repeated here.

[0092] Comparative Example 2

[0093] Unlike Example 1, lithium salt B was not added in the preparation of the electrolyte in this comparative example.

[0094] The rest is the same as in Example 1, and will not be repeated here.

[0095] Comparative Example 3

[0096] Unlike Example 1, no additive A was added in the preparation of this comparative electrolyte.

[0097] The rest is the same as in Example 1, and will not be repeated here.

[0098] Comparative Example 4

[0099] Unlike Example 1, in the preparation of this comparative electrolyte, the molar concentration b of lithium salt B in the electrolyte is 0.7 mol / L, and the content of additive A accounts for 3% of the total mass of the electrolyte.

[0100] The rest is the same as in Example 1, and will not be repeated here.

[0101] Comparative Example 5

[0102] Unlike Example 1, in the preparation of this comparative electrolyte, the molar concentration b of lithium salt B in the electrolyte is 0.8 mol / L, and the content of additive A accounts for 3% of the total mass of the electrolyte.

[0103] The rest is the same as in Example 1, and will not be repeated here.

[0104] Comparative Example 6

[0105] Unlike Example 1, in the preparation of this comparative electrolyte, the content of additive A accounts for 3.5% of the total mass of the electrolyte.

[0106] The rest is the same as in Example 1, and will not be repeated here.

[0107] Comparative Example 7

[0108] Unlike Example 1, in the preparation of this comparative electrolyte, the content of additive A accounts for 0.3% of the total mass of the electrolyte.

[0109] The rest is the same as in Example 1, and will not be repeated here.

[0110] The data for Examples 1-9 and Comparative Examples 1-7 are shown in Table 1 below.

[0111] Table 1

[0112] The lithium-ion batteries prepared in the examples and comparative examples were subjected to the following performance tests:

[0113] (1) High-temperature cycle life test: 1. Place the battery cell in a 45℃ forced-air drying oven for 2 hours; 2. Charge the battery cell at 1C constant current and constant voltage to 4.45V, with a cutoff rate of 0.05C; 3. Let it rest for 10 minutes; 4. Discharge the battery cell at 1C constant current to 3.0V; 5. Let it rest for 10 minutes; 6. Repeat steps 2-4 to 500 cycles. Calculation: Capacity retention rate = Discharge capacity on the 500th cycle / Discharge capacity on the first cycle.

[0114] (2) Low-temperature discharge power performance test: 1. Charge the battery to 4.45V at 0.5C constant current and constant voltage in a constant temperature environment of 25℃, with a cutoff rate of 0.02C; 2. Let it rest for 10 minutes; 3. Discharge the battery to 3.0V at 0.2C constant current; 4. Let it rest for 10 minutes; 5. Charge the battery to 4.45V at 0.5C constant current and constant voltage, with a cutoff rate of 0.02C, and test the cell thickness; 7. Place the cell in a 0℃ forced-air drying oven for 2 hours and discharge it at a constant power of 22W. Start timing from the beginning of the discharge and stop timing when the discharge ends, and record the cell discharge time.

[0115] The performance test results are shown in Table 2 below.

[0116] Table 2

[0117] The results above show that when 15.58*b / a is less than 1.5, the mass proportion of additive A is too high, resulting in high electrolyte viscosity. At this point, the concentration of lithium salt B is relatively low, leading to reduced lithium-ion dissociation and decreased conductivity, resulting in poor low-temperature discharge power performance. Conversely, when 15.58*b / a is greater than 10, the mass proportion of additive A is too low, resulting in poor electrolyte cycle performance. At this point, the concentration of lithium salt B is relatively high, leading to high electrolyte viscosity and deteriorating low-temperature discharge power performance. Furthermore, excessively high lithium salt B concentration can corrode the positive electrode current collector, affecting the normal operation of the battery. Both additive A and lithium salt B increase viscosity. When b is large and / or a is small, the value of 15.58*b / a is too large; when b is small and / or a is large, the value of 15.58*b / a is too small, both of which lead to increased viscosity. Only when a and b are within a certain range can the viscosity be kept within acceptable limits, thus avoiding impact on electrolyte kinetics and deterioration of low-temperature discharge power performance. Therefore, adjusting 1.5≤15.58*b / a≤10 can simultaneously take into account the battery's cycle performance and low-temperature discharge performance.

[0118] In summary, this application provides an electrolyte for lithium-ion batteries. By adding electrolyte additive A and lithium salt B, an electrolyte for lithium-ion batteries with both high-temperature cycling performance and low-temperature discharge power performance is obtained. Additive A can be oxidized to form a film on the positive electrode surface, stabilizing the positive electrode crystal structure and improving high-temperature cycling performance. Compared with conventional lithium salts, lithium salt B has a larger anionic group radius, weakening its binding ability to lithium ions and improving the dissociation ability of the lithium salt. This increases the electrolyte conductivity, thereby improving low-temperature discharge power performance.

[0119] Based on the disclosure and teachings of the foregoing specification, those skilled in the art can make changes and modifications to the above embodiments. Therefore, this application is not limited to the specific embodiments described above, and any obvious improvements, substitutions, or modifications made by those skilled in the art based on this application are within the scope of protection of this application. Furthermore, although some specific terms are used in this specification, these terms are only for convenience of explanation and do not constitute any limitation on this application.

Claims

1. An electrolyte comprising an organic solvent, a lithium salt, and a functional additive, wherein the functional additive comprises additive A, and the lithium salt comprises lithium salt B; The lithium salt B is selected from one or more of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium bis(oxalato)borate, lithium difluorooxalato)borate, lithium difluorobis(oxalato)phosphate, lithium trifluoromethanesulfonate, and lithium 4,5-dicyano-2-trifluoromethyl-imidazolium. The mass percentage a% of additive A in the electrolyte and the molar concentration bmol / L of lithium salt B in the electrolyte satisfy the following relationship: 1.5≤15.58*b / a≤10; in, The additive A is selected from at least one of the compounds shown in Formula I: Among them, R1, R 13 Each is independently selected from any one of hydrogen or halogen; R2, R3, R4, R6, R7, R8, R 10 R 11 R 12 Each is independently selected from C1-C3 alkylene groups, C1-C3 alkoxy groups, oxygen atoms, Any one of R2, R3, R4, R6, R7, R8, R 10 R 11 R 12 At least one selected from Any one of the following: R2, R3, R4, R6, R7, R8, R 10 R 11 R 12 At least one atom is selected from oxygen atoms; R5 and R9 are selected from structures of formula II: Where q is an integer between 0 and 4, R 14 and R 16 Each is independently selected from a carbon-carbon single bond or a C1-C5 hydrocarbon group, R 15 Selected from carbon-carbon single bonds, ether bonds, Any one of them.

2. The electrolyte according to claim 1, wherein, The mass percentage a% of additive A in the electrolyte and the molar concentration b mol / L of lithium salt B in the electrolyte satisfy the following relationship: 1.5≤15.58*b / a≤5.

3. The electrolyte according to claim 1, wherein, The mass percentage (a%) of additive A in the electrolyte is 0.5-3%.

4. The electrolyte according to claim 1, wherein, The molar concentration b mol / L of the lithium salt B in the electrolyte is 0.3-0.6 mol / L.

5. The electrolyte according to claim 1, wherein, The additive A is selected from at least one of the compounds shown in Formula I-1 to Formula I-19; 6. The electrolyte according to claim 1, wherein, The organic solvent is selected from one or more of ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl propionate, propyl propionate, ethyl acetate, ethyl butyrate, and γ-butyrolactone, and the content of the organic solvent accounts for 20.0-70.0% of the total mass of the electrolyte.

7. The electrolyte according to claim 1, wherein, The functional additives also include film-forming additives; the film-forming additives include one or more of the following: fluoroethylene carbonate, vinylene carbonate, 1,3-propanesulfonate lactone, vinyl sulfate, methanedisulfonate, propylene sulfonate lactone, citrate anhydride, succinate, adiponitrile, ethylene glycol diether, and hexanetrionitrile, and the mass of the film-forming additives is 2.0-20.0% of the total mass of the electrolyte.

8. The electrolyte according to claim 1, wherein, The lithium salt further includes lithium salt C, which is selected from one or more of lithium hexafluorophosphate, lithium difluorophosphate, lithium tetrafluorooxalate phosphate, lithium oxalate phosphate, and lithium tetrafluoroborate, and the molar concentration of lithium salt C in the electrolyte is 0.5-1 mol / L.

9. A method for preparing an electrolyte according to any one of claims 1-8, comprising the following steps; S1. Mix the organic solvents and stir to obtain solvent A, then cool and set aside. S2. Add lithium salt C to solution A from step S1 and stir until homogeneous to obtain solution B; S3. Add additive A, lithium salt B and film-forming additive to solution B in step S2, stir evenly to obtain the electrolyte. in, In step S1, the cooling temperature is below 10°C.

10. A lithium-ion battery comprising the electrolyte according to any one of claims 1-8.

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