Electrolyte, preparation method thereof and lithium ion battery

Abstract

The invention belongs to the technical field of lithium ion batteries, and particularly relates to an electrolyte, a preparation method thereof 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, and the lithium salt comprises a lithium salt B; wherein the additive A is selected from at least one of compounds as shown in a formula I; and 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 difluoro (oxalato) phosphate, lithium trifluoromethanesulfonate and lithium 4, 5-dicyano-2-trifluoromethyl-imidazole. Wherein the additive A can be oxidized on the surface of a positive electrode to form a film, stabilize the crystal structure of the positive electrode and improve the high-temperature cycle performance, and compared with conventional lithium salt, the lithium salt B has the advantages that the radius of an anion group is larger, the binding capacity to lithium ions is weakened, the dissociation capacity of the lithium salt is improved, the conductivity of the electrolyte is improved, and the low-temperature discharge power performance is further improved.

Description

Technical Field

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

[0002] Lithium-ion batteries have been widely used in many fields due to their advantages such as high energy density, high cycle capacity retention rate and outstanding electrical performance, and will penetrate into more fields in the future. In the era of electrification, lithium-ion batteries play a pivotal role in the green energy family with their clean and environmentally friendly characteristics. Lithium-ion batteries are mainly composed of positive electrodes, negative electrodes, isolation membranes, electrolytes and outer packaging. The electrolyte, as the "blood" of lithium-ion batteries, is filled in various locations inside the lithium-ion battery and plays the role of transferring ions and conducting current inside the battery. The electrolyte plays a vital role in the electrical performance and safety performance of lithium-ion batteries.

[0003] However, as the application of lithium batteries becomes more widespread, the low-temperature performance of lithium-ion batteries is put to a more severe test in some low-temperature regions such as high latitudes and high altitudes. The existing electrolyte design has limited improvement in low-temperature discharge power, and usually adopts technical means such as adding high-kinetic solvents and low-impedance additives. After improving the low-temperature discharge, its high-temperature cycle performance also deteriorates.

[0004] Therefore, it is necessary to develop an electrolyte that has both high-temperature cycle performance and low-temperature discharge power performance. Summary of the invention

[0005] The purpose of the present invention is to provide an electrolyte having both high temperature cycle performance and low temperature discharge power performance and a preparation method thereof in view of the deficiencies of the prior art.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] An electrolyte, comprising an organic solvent, a lithium salt and a functional additive, wherein 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(oxalatoborate), lithium difluorooxalatoborate, lithium difluorobis(oxalatophosphate), lithium trifluoromethylsulfonate and lithium 4,5-dicyano-2-trifluoromethyl-imidazole;

[0009] Among them, the high temperature and cycle performance of lithium-ion batteries are improved by adding electrolyte additive A, and a certain concentration of lithium salt B is required. Compared with conventional lithium salts, lithium salt B has a larger anion group radius, is less bound to 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 further improving the low-temperature discharge power performance.

[0010] The content a of the additive A and the molar concentration b of the 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, the viscosity of the electrolyte is too large, the concentration of lithium salt B is relatively low, the degree of lithium ion dissociation is reduced, and the conductivity is reduced, which will lead to 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, the cycle performance of the electrolyte is poor, the concentration of lithium salt B is relatively high, the viscosity of the electrolyte is too large, and the low-temperature discharge power performance deteriorates. In addition, excessive concentration of lithium salt B will corrode the positive electrode collector and affect the normal use of the battery. Among them, both additives A and lithium salt B increase viscosity. When b is large and a is small, the value of 15.58*b / a is too large; when b is small and a is large, the value of 15.58*b / a is too small. Both results will lead to increased viscosity. Only when a and b are within a certain range can the viscosity not be too large to affect the electrolyte dynamics and deteriorate the low-temperature discharge power performance. Therefore, by adjusting 1.5≤15.58*b / a≤10, both the cycle performance and the low-temperature discharge performance of the battery can be taken into account.

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

[0013]

[0014] Among them, R 1 , R 13 Each is independently selected from any one of hydrogen and halogen; R 2 , R 3 , R 4 , R 6 , R 7 , R 8 , R 10 , R 11 , R 12 Each independently selected from C1-C3 alkylene, C1-C3 alkoxy, oxygen atom, Any one of the above, and R 2 , R 3 , R 4 , R 6 , R 7 , R8 , R 10 , R 11 , R 12 At least one selected from Any one of R 2 , R 3 , R 4 , R 6 , R 7 , R 8 , R 10 , R 11 , R 12 At least one selected from oxygen atom; R 5 , R 9 Selected from the structure of formula II:

[0015]

[0016] Where q is an integer from 0 to 4, R 14 and R 16 Each independently selected from a carbon-carbon single bond or a C1-C5 alkylene group, R 15 Selected from carbon-carbon single bonds, ether bonds, Any one of .

[0017] Preferably, the mass percentage a% of the additive A in the electrolyte and the molar concentration b mol / L of the lithium salt B in the electrolyte satisfy the following relationship: 1.5≤15.58*b / a≤5.

[0018] Preferably, the content a of the additive A accounts for 0.5-3% of the total mass of the electrolyte.

[0019] Preferably, the molar concentration b of the lithium salt B in the electrolyte is 0.3-0.6 mol / L.

[0020] Among them, the carbonate groups, sulfonate groups, sulfate groups and other groups contained in additive A can be oxidized to form dense and flexible CEI protective film groups when losing electrons on the positive electrode surface, thereby stabilizing the positive electrode crystal structure, inhibiting the dissolution of positive electrode transition metals and the release of active oxygen, and thus achieving the purpose of improving high temperature performance and cycle performance. And as the concentration of lithium salt B increases, the viscosity of the electrolyte will also increase, which is not conducive to the transmission of lithium ions in the electrolyte. 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, the electrolyte can have both high temperature cycle and low temperature discharge power performance.

[0021] Preferably, the additive A is selected from at least one of the compounds represented by formula I-1 to formula I-19;

[0022]

[0023]

[0024] 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.

[0025] Preferably, the functional additives also include film-forming additives; the film-forming additives include one or more of fluoroethylene carbonate, vinylene carbonate, 1,3-propane sultone, vinyl sulfate, methylene disulfonate, propene sultone, citraconic anhydride, succinonitrile, adiponitrile, ethylene glycol diether and hexane trinitrile, and the mass of the film-forming additive is 2.0-20.0% of the total mass of the electrolyte.

[0026] Preferably, the lithium salt further comprises a lithium salt C, and the lithium salt C 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 the lithium salt C in the electrolyte is 0.5-1 mol / L.

[0027] In addition, the present invention also provides a method for preparing an electrolyte, comprising the following steps:

[0028] S1, mixing organic solvents, stirring to obtain solvent A, and cooling for standby use;

[0029] S2, adding lithium salt C to solution A in step S1, stirring evenly to obtain solution B;

[0030] S3, adding additive A, lithium salt B and film-forming additive to solution B in step S2, stirring evenly, to obtain the electrolyte; in step S1, the cooling temperature is lower than 10°C.

[0031] When lithium salt is dissolved in the solvent, it releases heat. Lithium hexafluorophosphate is easily decomposed by heat (about 70°C). When the lithium salt concentration decreases, hydrofluoric acid is generated, which deteriorates the battery performance. Lowering the solvent temperature and then adding lithium salt can effectively control the maximum temperature of the electrolyte to be lower than the decomposition temperature of lithium hexafluorophosphate.

[0032] In addition, the present invention also provides a lithium-ion battery, comprising a battery core wound with a negative electrode sheet, a positive electrode sheet and a separator, an electrolyte, and a shell encapsulating the battery core and the electrolyte, wherein the electrolyte is the above-mentioned electrolyte.

[0033] Compared with the prior art, the beneficial effect of the present invention is that the present invention obtains an electrolyte having both high-temperature cycle 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 surface of the positive electrode, stabilize the positive electrode crystal structure, and improve the high-temperature cycle performance. Compared with conventional lithium salts, lithium salt B has a larger anion group radius, weakens the binding ability of lithium ions, improves the dissociation ability of lithium salts, and increases the conductivity of the electrolyte, thereby improving the low-temperature discharge power performance. DETAILED DESCRIPTION

[0034] In order to make the technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] According to a first aspect of the present application, the present application provides an electrolyte, including an organic solvent, a lithium salt and a functional additive, wherein the functional additive includes an additive A, and the lithium salt includes a lithium salt B;

[0036] The lithium salt B is selected from one or more of lithium bis(trifluoromethylsulfonyl imide), lithium bis(fluorosulfonyl imide), lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium difluorobis(oxalatophosphate), lithium trifluoromethylsulfonate and lithium 4,5-dicyano-2-trifluoromethyl-imidazole;

[0037] The mass percentage a% of the additive A in the electrolyte and the molar concentration b mol / L of the lithium salt B in the electrolyte satisfy the following relationship: 1.5≤15.58*b / a≤10;

[0038] Wherein, the additive A is selected from at least one of the compounds shown in Formula I:

[0039]

[0040] Among them, R 1 , R 13 Each is independently selected from any one of hydrogen and halogen; R 2 , R 3 , R 4 , R 6 , R 7 , R 8 , R 10 , R 11 , R 12 Each independently selected from C1-C3 alkylene, C1-C3 alkoxy, oxygen atom, Any one of the above, and R 2 , R3 , R 4 , R 6 , R 7 , R 8 , R 10 , R 11 , R 12 At least one selected from Any one of R 2 , R 3 , R 4 , R 6 , R 7 , R 8 , R 10 , R 11 , R 12 At least one selected from oxygen atom; R 5 , R 9 Selected from the structure of formula II:

[0041]

[0042] Where q is an integer from 0 to 4, R 14 and R 16 Each independently selected from a carbon-carbon single bond or a C1-C5 alkylene group, R 15 Selected from carbon-carbon single bonds, ether bonds,

[0043] Any one of .

[0044] In some embodiments, the mass percentage a% of the additive A in the electrolyte and the molar concentration b mol / L of the 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.

[0045] In some embodiments, the content of additive A accounts for 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%.

[0046] In some embodiments, the molar concentration b of the lithium salt B in the electrolyte is 0.3-0.6 mol / L, for example, 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.

[0047] In some embodiments, the additive A is selected from at least one of the compounds represented by Formula I-1 to Formula I-19;

[0048]

[0049]

[0050]

[0051] In some embodiments, compound I-6 can be obtained by the following preparation process: diacetone-D-mannitol, dioxane, potassium carbonate, methanol, dimethyl carbonate, etc. are reacted under heating and stirring conditions for several hours, and then a certain amount of oxalic acid is added to adjust the pH of the solution to neutral, and the intermediate product 1 is obtained after filtering and concentrating. An appropriate amount of carbonate, acid, pure water, etc. are added to the intermediate product 1 for hydrolysis reaction to obtain intermediate product 2. Intermediate product 2, carbonate solvent, and dichlorothionyl are heated to obtain intermediate product 3. Finally, intermediate product 3 is oxidized using an oxidant such as potassium periodate to obtain compound I-6.

[0052] In some embodiments, compound I-12 can be obtained by the following preparation process: methanol and sorbitol are added to an organic solvent such as dimethyl carbonate and DMF, potassium hydroxide is added to the reactor as an alkaline catalyst, and after heating at 45-55°C for 6-8 hours, a certain amount of oxalic acid is added to adjust the pH to neutral, and the intermediate product 4 is obtained by filtering and recrystallizing. The intermediate product 1, dichlorothionyl and carbonate are esterified at 60-70°C to obtain intermediate product 5, and then oxidized with an oxidant such as potassium periodate to obtain compound I-12.

[0053] A person skilled in the art, knowing the structural formula of the compound of structural formula I, can deduce the preparation method of the above compound according to the common knowledge in the field of chemical synthesis.

[0054] 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.

[0055] In some embodiments, the functional additives also include film-forming additives; the film-forming additives include one or more of fluoroethylene carbonate, vinylene carbonate, 1,3-propane sultone, vinyl sulfate, methylene disulfonate, propene sultone, citraconic anhydride, succinonitrile, adiponitrile, ethylene glycol dipropylene glycol ether and hexane trinitrile, and the mass of the film-forming additive is 2.0-20.0% of the total mass of the electrolyte.

[0056] 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.

[0057] According to a second aspect of the present application, the present application provides a method for preparing an electrolyte, comprising the following steps:

[0058] S1, mixing organic solvents, stirring to obtain solvent A, and cooling for standby use;

[0059] S2, adding lithium salt C to solution A in step S1, stirring evenly to obtain solution B;

[0060] S3, adding additive A, lithium salt B and film-forming additive to solution B in step S2, stirring evenly, to obtain the electrolyte;

[0061] In step S1, the cooling temperature is lower than 10°C.

[0062] According to the third aspect of the present application, the present application provides a lithium-ion battery, comprising a battery cell formed by winding a negative electrode sheet, a positive electrode sheet and a separator, an electrolyte, and a shell encapsulating the battery cell and the electrolyte, wherein the electrolyte is the above-mentioned electrolyte.

[0063] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material. The positive electrode active material may include but is not limited to a chemical formula such as Li a Ni x Co y M z O 2-b N b (wherein 0.95≤a≤1.2, x>0, y≥0, z≥0, and x+y+z=1, 0≤b≤1, M is selected from a combination of one or more of Mn and A1, and N is selected from a combination of one or more of F, P, and S) The positive electrode active material may also be, but is not limited to, LiCoO 2 、LiNiO 2 、LiVO 2 、LiCrO 2 、LiMn 2 O 4 、LiCoMnO 4 , Li 2 NiMn 3 O 8 、LiNi 0.5 Mn 1.5 O 4 、LiCoPO 4 、LiMnPO 4 、LiFePO 4 、LiNiPO 4 、LiCoFSO 4 , CuS2 , FeS 2 、MoS 2 、NiS、TiS 2 The positive electrode active material may also be subjected to a modification treatment. The method for modifying the positive electrode active material should be known to those skilled in the art. For example, the positive electrode active material may be modified by coating, doping, etc. The material used for the modification treatment may include but is not limited to a combination of one or more of Al, B, P, Zr, Si, Ti, Ge, Sn, Mg, Ce, W, etc. The positive electrode current collector is usually a structure or part for collecting current. The positive electrode current collector may be any material suitable for use as a positive electrode current collector for lithium-ion batteries in the art. For example, the positive electrode current collector may include but is not limited to metal foil, etc., and more specifically may include but is not limited to aluminum foil, etc.

[0064] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer arranged on the surface of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material. The negative electrode active material may include but is not limited to graphite, soft carbon, hard carbon, carbon fiber, mesophase carbon microspheres, silicon-based materials, tin-based materials, lithium titanate or other metals that can form alloys with lithium. Among them, the graphite can be selected from one or more of artificial graphite, natural graphite and modified graphite; the silicon-based material can be selected from one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, and silicon alloys; the tin-based material can be selected from one or more of elemental tin, tin oxide compounds, and tin alloys. The negative electrode current collector is usually a structure or part that collects current. The negative electrode current collector can be various materials in the art that are suitable for use as a negative electrode current collector for lithium-ion batteries. For example, the negative electrode current collector may include but is not limited to metal foil, and more specifically may include but is not limited to copper foil.

[0065] Example 1

[0066] Preparation of electrolyte:

[0067] S1. In a glove box filled with argon, EC, PC, EMC and DEC were added into a PP bottle in a ratio of 20:10:20:50 and stirred evenly to obtain solvent A. The mixture was sealed and fully cooled in a refrigerator below 10°C for later use;

[0068] S2, adding 0.8 mol of lithium hexafluorophosphate (lithium salt C) and lithium bis(fluorosulfonyl)imide (lithium salt B) to solution A, stirring until completely dissolved to obtain solution B;

[0069] S3, based on the mass of the electrolyte, add 3% PS (1,3-propane sultone), 2% FEC (fluoroethylene carbonate), 1% DTD (vinyl sulfate) and 0.5% of the compound of formula I-12 (additive A) to solution B, and stir evenly to obtain the electrolyte;

[0070] 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.

[0071] Preparation of positive electrode:

[0072] 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 by a double planetary mixer, and evenly coated on a 9μm aluminum foil on an extrusion coater. After drying, rolling to a certain thickness, and slitting to a certain width, the positive electrode sheet is obtained.

[0073] Preparation of negative electrode:

[0074] By weight percentage, 97.7% graphite, 1.2% polystyrene butadiene copolymer, and 1.1% lithium carboxymethyl cellulose are evenly dispersed in deionized water by a double planetary mixer, and evenly coated on a 5μm copper foil on an extrusion coater. After drying, rolling to a certain thickness, and slitting to a certain width, a negative electrode sheet is obtained.

[0075] Preparation of lithium-ion batteries:

[0076] The positive electrode sheet and negative electrode sheet prepared in the above steps and the separator of a certain width are welded to the pole ears, glued, wound, cut, hot pressed, etc. on a winding machine to obtain a bare battery cell, and the aluminum-plastic film of suitable size is punched, and the bare battery cell is encapsulated in the punched aluminum-plastic film. The encapsulated battery cell is injected with the electrolyte prepared in the above steps and pre-sealed, and then hot-pressed after standing at high temperature and room temperature for a certain period of time, and then degassing and re-sealing are performed.

[0077] Example 2

[0078] Different from Example 1, in the preparation of the electrolyte of this example, the content of additive A accounts for 1% of the total mass of the electrolyte.

[0079] The rest is the same as Example 1 and will not be described again here.

[0080] Example 3

[0081] Different from Example 1, in the preparation of the electrolyte of this example, the content of additive A accounts for 2% of the total mass of the electrolyte.

[0082] The rest is the same as Example 1 and will not be described again here.

[0083] Example 4

[0084] Different from Example 1, in the preparation of the electrolyte of this example, the content of additive A accounts for 3% of the total mass of the electrolyte.

[0085] The rest is the same as Example 1 and will not be described again here.

[0086] Example 5

[0087] Different from Example 1, in the preparation of the electrolyte of this example, the molar concentration b of the lithium salt B in the electrolyte is 0.4 mol / L, and the content of the additive A accounts for 3% of the total mass of the electrolyte.

[0088] The rest is the same as Example 1 and will not be described again here.

[0089] Example 6

[0090] Different from Example 1, in the preparation of the electrolyte of this example, the molar concentration b of the lithium salt B in the electrolyte is 0.5 mol / L, and the content of the additive A accounts for 3% of the total mass of the electrolyte.

[0091] The rest is the same as Example 1 and will not be described again here.

[0092] Example 7

[0093] Different from Example 1, in the preparation of the electrolyte of this example, the molar concentration b of the lithium salt B in the electrolyte is 0.6 mol / L, and the content of the additive A accounts for 3% of the total mass of the electrolyte.

[0094] The rest is the same as Example 1 and will not be described again here.

[0095] Example 8

[0096] Different from Example 1, the additive A in this example is Formula I-6.

[0097] The rest is the same as Example 1 and will not be described again here.

[0098] Example 9

[0099] Different from Example 1, the lithium salt B in this example is lithium bis(trifluoromethylsulfonyl)imide.

[0100] The rest is the same as Example 1 and will not be described again here.

[0101] Comparative Example 1

[0102] Different from Example 1, in the preparation of the electrolyte of this comparative example, no additive A and no lithium salt B are added. The rest is the same as Example 1 and will not be described again.

[0103] Comparative Example 2

[0104] Different from Example 1, lithium salt B is not added in the preparation of the electrolyte of this comparative example.

[0105] The rest is the same as Example 1 and will not be described again here.

[0106] Comparative Example 3

[0107] Different from Example 1, additive A was not added in the preparation of the electrolyte of this comparative example.

[0108] The rest is the same as Example 1 and will not be described again here.

[0109] Comparative Example 4

[0110] Different from Example 1, in the preparation of the electrolyte of this comparative example, the molar concentration b of the lithium salt B in the electrolyte is 0.7 mol / L, and the content of the additive A accounts for 3% of the total mass of the electrolyte.

[0111] The rest is the same as Example 1 and will not be described again here.

[0112] Comparative Example 5

[0113] Different from Example 1, in the preparation of the electrolyte of this comparative example, the molar concentration b of the lithium salt B in the electrolyte is 0.8 mol / L, and the content of the additive A accounts for 3% of the total mass of the electrolyte.

[0114] The rest is the same as Example 1 and will not be described again here.

[0115] Comparative Example 6

[0116] Different from Example 1, in the preparation of the electrolyte of this comparative example, the content of additive A accounts for 3.5% of the total mass of the electrolyte.

[0117] The rest is the same as Example 1 and will not be described again here.

[0118] Comparative Example 7

[0119] Different from Example 1, in the preparation of the electrolyte of this comparative example, the content of additive A accounts for 4% of the total mass of the electrolyte.

[0120] The rest is the same as Example 1 and will not be described again. The data of Examples 1-9 and Comparative Examples 1-7 are shown in Table 1 below.

[0121] Group a(wt%) b(mol / L) 15.58*b / a Example 1 0.5 0.3 9.35 Example 2 1.0 0.3 4.67 Example 3 2.0 0.3 2.34 Example 4 3.0 0.3 1.56 Example 5 3.0 0.4 2.08 Example 6 3.0 0.5 2.60 Example 7 3.0 0.6 3.12 Example 8 0.5 0.3 9.35 Example 9 0.5 0.3 9.35 Comparative Example 1 / / / Comparative Example 2 0.5 / / Comparative Example 3 / 0.3 / Comparative Example 4 3.0 0.7 3.64 Comparative Example 5 3.0 0.8 4.16 Comparative Example 6 3.5 0.3 1.34 Comparative Example 7 0.3 0.3 15.58

[0122] The following performance tests were performed on the lithium ion batteries prepared in the examples and comparative examples:

[0123] (1) High temperature cycle life test: 1. Place the battery cell in a 45℃ forced air drying oven for 2 hours; 2. Charge to 4.45V at 1C constant current and constant voltage, with a cut-off rate of 0.05C; 3. Place for 10 minutes; 4. Discharge to 3.0V at 1C constant current; 5. Place for 10 minutes; 6. Repeat steps 2-4 for 500 cycles. Calculation: Capacity retention rate = discharge capacity at the 500th cycle ÷ discharge capacity at the first cycle.

[0124] (2) Low temperature discharge power performance test: 1. In a constant temperature environment of 25℃, charge to 4.45V with 0.5C constant current and constant voltage, and the cut-off rate is 0.02C; 2. Leave for 10 minutes; 3. Discharge to 3.0V with 0.2C constant current; 4. Leave for 10 minutes; 5. Charge to 4.45V with 0.5C constant current and constant voltage, and the cut-off rate is 0.02C, and test the thickness of the battery cell; 7. Place the battery cell in a 0℃ forced air drying oven for 2 hours, discharge at a constant power of 22W, start timing from the start of discharge, stop timing when the discharge ends, and record the discharge time of the battery cell.

[0125] Among them, the above performance test results are shown in Table 2 below.

[0126] Table 2

[0127]

[0128] From the above results, it can be seen that: when the value of 15.58*b / a is less than 1.5, the mass proportion of additive A is too large, the viscosity of the electrolyte is too large, the concentration of lithium salt B is relatively low, the dissociation degree of lithium ions is reduced, and the conductivity is reduced, which will lead to 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, the cycle performance of the electrolyte is poor, the concentration of lithium salt B is relatively high, the viscosity of the electrolyte is too large, and the low-temperature discharge power performance deteriorates. In addition, excessive concentration of lithium salt B will corrode the positive electrode collector and affect the normal use of the battery. Among them, both additives A and lithium salt B increase viscosity. When b is large and a is small, the value of 15.58*b / a is too large; when b is small and a is large, the value of 15.58*b / a is too small. Both results will lead to increased viscosity. Only when a and b are within a certain range can the viscosity not be too large to affect the electrolyte dynamics and deteriorate the low-temperature discharge power performance. Therefore, by adjusting 1.5≤15.58*b / a≤10, both the cycle performance and the low-temperature discharge performance of the battery can be taken into account.

[0129] In summary, the present invention provides an electrolyte, and an electrolyte having both high-temperature cycle performance and low-temperature discharge power performance is obtained by adding electrolyte additive A and lithium salt B. Among them, additive A can be oxidized to form a film on the surface of the positive electrode, stabilize the crystal structure of the positive electrode, and improve the high-temperature cycle performance. Compared with conventional lithium salts, the radius of the anion group of lithium salt B is larger, the binding ability of lithium ions is weakened, the dissociation ability of lithium salt is improved, and the conductivity of the electrolyte is increased, thereby improving the low-temperature discharge power performance.

[0130] According to the disclosure and teaching of the above description, those skilled in the art to which the present invention belongs can also change and modify the above embodiment. Therefore, the present invention is not limited to the above specific embodiment, and any obvious improvement, replacement or modification made by those skilled in the art on the basis of the present invention belongs to the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for the convenience of description and do not constitute any limitation to the present invention.

Claims

1. An electrolyte, characterized in that: It includes 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; The lithium salt B is selected from one or more of lithium bis(trifluoromethylsulfonyl imide), lithium bis(fluorosulfonyl imide), lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium difluorobis(oxalatophosphate), lithium trifluoromethylsulfonate and lithium 4,5-dicyano-2-trifluoromethyl-imidazole; The mass percentage a% of the additive A in the electrolyte and the molar concentration bmol / L of the lithium salt B in the electrolyte satisfy the following relationship: 1.5≤15.58*b / a≤10; Wherein, 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 and halogen; R2, R3, R4, R6, R7, R8, R 10 , R 11 , R 12 Each independently selected from C1-C3 alkylene, C1-C3 alkoxy, oxygen atom, Any one of, and R2, R3, R4, R6, R7, R8, R 10 , R 11 , R 12 At least one selected from Any one of R2, R3, R4, R6, R7, R8, R 10 , R 11 , R 12 At least one is selected from oxygen atom; R5 and R9 are selected from the structure of formula II: Where q is an integer from 0 to 4, R 14 and R 16 Each independently selected from a carbon-carbon single bond or a C1-C5 alkylene group, R 15 Selected from carbon-carbon single bonds, ether bonds, Any one of .

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

3. An electrolyte according to claim 1, characterized in that: The mass percentage a% of the additive A in the electrolyte is 0.5-3%.

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

5. An electrolyte according to claim 1, characterized in that: The additive A is selected from at least one of the compounds represented by formula I-1 to formula I-19; 6. An electrolyte according to claim 1, characterized in that: 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. An electrolyte according to claim 1, characterized in that: The functional additives also include film-forming additives; the film-forming additives include one or more of fluoroethylene carbonate, vinylene carbonate, 1,3-propane sultone, vinyl sulfate, methylene disulfonate, propene sultone, citraconic anhydride, succinonitrile, adiponitrile, ethylene glycol diether and hexane trinitrile, and the mass of the film-forming additive is 2.0-20.0% of the total mass of the electrolyte.

8. An electrolyte according to claim 1, characterized in that: The lithium salt also 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 the 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 to 8, characterized in that: The steps include: S1, mixing organic solvents, stirring to obtain solvent A, and cooling for standby use; S2, adding lithium salt C to solution A in step S1, stirring evenly to obtain solution B; S3, adding additive A, lithium salt B and film-forming additive to solution B in step S2, stirring evenly, to obtain the electrolyte; In step S1, the cooling temperature is lower than 10°C.

10. A lithium ion battery, characterized in that: The electrolyte comprising the electrolyte according to any one of claims 1 to 8.

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