Electrolyte and lithium ion battery

Abstract

The invention relates to the technical field of secondary batteries, in particular to an electrolyte and a lithium ion battery. The electrolyte comprises a solvent, a lithium salt and an additive, the mass fraction of the solvent in the electrolyte is 75-90%, the solvent comprises a carboxylic ester solvent and a cyclic carbonate solvent, and the mass ratio m (CO): m (CC) of the carboxylic ester solvent to the cyclic carbonate solvent is greater than or equal to 5: 3; the lithium salt is dissolved in the solvent, the mass fraction of the lithium salt in the electrolyte is 10%-18%, the lithium salt comprises lithium bis (fluorosulfonyl) imide and lithium hexafluorophosphate, and the mass ratio m (LiFSI): m (LiPF6) of the lithium bis (fluorosulfonyl) imide to the lithium hexafluorophosphate is greater than or equal to 1: 3; the additive comprises tris (trimethylsilane) phosphate, and the mass fraction of the tris (trimethylsilane) phosphate in the electrolyte is 0.1%-0.5%.

Description

Technical Field

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

[0002] Lithium-ion batteries are widely used in the field of electric vehicles due to their high energy density and long cycle life. In order to pursue faster charging speeds and longer driving range for electric vehicles, lithium-ion batteries are required to have higher fast charging capabilities and higher energy density. The type of electrolyte will also affect the fast charging performance of lithium-ion batteries. In the prior art, in order to improve the fast charging capability, fast charging electrolytes often use carboxylic acid ester solvents such as ethyl acetate (EA) and ethyl propionate (EP) instead of traditional carbonate solvents. However, since the viscosity of this type of chain carboxylic acid ester is limited by the number of carbon atoms, it is difficult to take into account both the fast charging performance and high and low temperature performance of lithium-ion batteries. Summary of the invention

[0003] In view of the above problems, the present invention provides an electrolyte that can take into account both the fast charging performance and high and low temperature performance of lithium-ion batteries.

[0004] The first aspect of the present invention provides an electrolyte, comprising a solvent, a lithium salt and an additive, wherein the mass fraction of the solvent in the electrolyte is 75%-90%, the solvent comprises a carboxylate solvent and a cyclic carbonate solvent, and the mass ratio of the carboxylate solvent to the cyclic carbonate solvent is m(CO):m(CC)≥5:3; the lithium salt is dissolved in the solvent, and the mass fraction of the lithium salt in the electrolyte is 10%-18%, the lithium salt comprises lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate, and the mass ratio of lithium bis(fluorosulfonyl)imide to lithium hexafluorophosphate is m(LiFSI):m(LiPF6)≥1:3; the additive comprises tris(trimethylsilane) phosphate, and the mass fraction of tris(trimethylsilane) phosphate in the electrolyte is 0.1%-0.5%.

[0005] Optionally, the additive further includes triallyl isocyanurate, and the mass fraction of triallyl isocyanurate in the electrolyte is 0.1%-0.5%.

[0006] Optionally, the mass ratio of lithium hexafluorophosphate to tris(trimethylsilyl)phosphate is m(LiPF6):m(TMSP)=22:1-4:1.

[0007] Optionally, the mass ratio of lithium bis(fluorosulfonyl)imide to lithium hexafluorophosphate is m(LiFSI):m(LiPF6)≥7:1.

[0008] Optionally, the mass fraction of lithium bis(fluorosulfonyl)imide in the electrolyte is 10%-15%, and the mass fraction of lithium hexafluorophosphate in the electrolyte is 1%-3%.

[0009] Optionally, the additives also include methylene methanedisulfonate, vinylene carbonate and fluoroethylene carbonate, the mass fraction of methylene methanedisulfonate in the electrolyte is 0.3%-0.7%, the mass fraction of vinylene carbonate in the electrolyte is 1%-4%, and the mass fraction of fluoroethylene carbonate in the electrolyte is 0.5%-1.5%.

[0010] Optionally, the solvent consists of a carboxylate solvent and a cyclic carbonate solvent, the carboxylate solvent consists of ethyl acetate and ethyl propionate, the mass fraction of ethyl acetate in the electrolyte is 25%-40%, and the mass fraction of ethyl propionate in the electrolyte is 21%-30%; the cyclic carbonate solvent consists of ethylene carbonate and propylene carbonate, the mass fraction of ethylene carbonate in the electrolyte is 12%-20%, and the mass fraction of propylene carbonate in the electrolyte is 4%-10%.

[0011] A second aspect of the present invention provides a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet and the electrolyte provided by the first aspect of the present invention.

[0012] Optionally, the positive electrode plate includes a positive electrode current collector and a positive electrode active material, the positive electrode active material includes any one of lithium iron phosphate, lithium cobalt oxide, lithium nickel cobalt manganese oxide, or a combination of at least two thereof; the negative electrode plate includes a negative electrode current collector and a negative electrode active material, the negative electrode active material includes any one of artificial graphite, natural graphite, soft carbon, hard carbon, a mixture of graphite and silicon oxide, or a mixture of graphite and silicon, or a combination of at least two thereof.

[0013] The electrolyte provided in some examples of the present invention can take into account both the fast charging performance and high and low temperature performance of the electrolyte by using a specific proportion of carboxylic acid ester solvents and cyclic carbonate solvents in a mixed system of LiFSI and LiPF6, and adding a certain amount of additives for improving the high temperature stability of the electrolyte. DETAILED DESCRIPTION

[0014] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. 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.

[0015] Unless otherwise specified or incompatible herewith, the terms and phrases used herein shall have the following meanings:

[0016] Solid Electrolyte Interface (SEI): During the charging and discharging process of lithium-ion batteries, the electrode material and the electrolyte react at the solid-liquid interface to form a passivation layer covering the surface of the electrode material.

[0017] In this article, when it comes to numerical ranges, unless otherwise specified, the distribution of optional values ​​within the numerical range is considered continuous and includes the two numerical endpoints (i.e., the minimum value and the maximum value) of the numerical range, as well as every value between the two numerical endpoints. When multiple numerical ranges are provided to describe features or characteristics, these numerical ranges can be combined.

[0018] The electrolyte provided in some embodiments of the present invention includes a solvent, a lithium salt and an additive. Among them, the mass fraction of the solvent in the electrolyte is 75%-90%, the solvent includes a carboxylic acid ester solvent and a cyclic carbonate solvent, and the mass ratio of the carboxylic acid ester solvent and the cyclic carbonate solvent is m(CO):m(CC)≥5:3. The lithium salt is dissolved in the solvent, and the mass fraction of the lithium salt in the electrolyte is 10%-18%. The lithium salt includes lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate, and the mass ratio of lithium bis(fluorosulfonyl)imide to lithium hexafluorophosphate is m(LiFSI):m(LiPF6)≥1:3. Additives include additives for improving the stability of the SEI film, additives for improving high temperature stability, additives for improving low temperature and high voltage performance, additives for improving flame retardant properties, etc. Among the additives, the additive for improving high temperature stability includes tris(trimethylsilane) phosphate, and the mass fraction of tris(trimethylsilane) phosphate in the electrolyte is 0.1%-0.5%.

[0019] In some embodiments of the present invention, the lithium salt is composed of lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate. In other embodiments, the lithium salt may also include other types of lithium salts. Preferably, the mass fraction of lithium bis(fluorosulfonyl)imide in the electrolyte is 10%-15%, more preferably 14%; the mass fraction of lithium hexafluorophosphate in the electrolyte is 1%-3%, more preferably 2%. In order to further improve the kinetic properties of the electrolyte, preferably, the mass ratio of lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate is m(LiFSI):m(LiPF6)≥3:1, more preferably, m(LiFSI):m(LiPF6)≥7:1.

[0020] In this embodiment, the solvent includes a carboxylate solvent and a cyclic carbonate solvent, wherein the mass ratio of the carboxylate solvent to the cyclic carbonate solvent is m(CO):m(CC)≥5:3. Using the solvent with the above ratio, the two-component lithium salt provided in the embodiment of the present invention can be efficiently dissolved and ionized, and the overall viscosity of the electrolyte is still low, taking into account the conductivity, fast charging performance and low temperature performance of the electrolyte. In addition, in the mixed system of carboxylate solvents and cyclic carbonate solvents, carboxylate solvents can use their good solubility to increase the dissolution amount of electrolyte, and cyclic carbonate solvents can provide a wider electrochemical stability window, thereby improving the comprehensive performance of the electrolyte, and further enhancing the low temperature performance and fast charging capability of lithium-ion batteries.

[0021] In an embodiment of the present invention, by adjusting the mass ratio of the carboxylate solvent and the cyclic carbonate solvent in the electrolyte, the kinetic performance of the overall electrolyte is brought to a better level. According to the performance test results of lithium-ion batteries in some embodiments of the present invention, more preferably, the mass ratio of the carboxylate solvent and the cyclic carbonate solvent is m(CO):m(CC)≥3:1.

[0022] In some embodiments of the present invention, the carboxylate solvent in the electrolyte is one or more of ethyl acetate (EA), propyl acetate (PA), and ethyl propionate (EP), and the cyclic carbonate solvent is one or both of ethylene carbonate (EC) and propylene carbonate (PC). Among them, the mass fraction of ethyl acetate in the electrolyte is preferably 25%-40%, more preferably 35%-40%. The mass fraction of propylene carbonate in the electrolyte is preferably 4%-10%, more preferably 3%-5%. In order to further improve the low temperature performance and fast charging capability of lithium-ion batteries, it is more preferred that the mass ratio of ethyl acetate and propylene carbonate in the electrolyte is not less than 8:1 and not more than 10.

[0023] However, under certain conditions, such as high temperature and high voltage, carboxylic acid ester solvents may decompose in the electrolyte due to their low overall stability, thus affecting their performance and use as solvents. Taking lithium-ion batteries as an example, during the battery charging and discharging process, if the working environment temperature is high, the carboxylic acid ester solvents may not be able to withstand the high voltage environment, decompose to produce gases and other substances, and affect the lithium-ion battery.

[0024] In the embodiment of the present invention, in order to balance the high temperature stability of the lithium-ion battery and improve the cycle stability of the lithium-ion battery in a high temperature environment, tris(trimethylsilyl)phosphate (TMSP) is also added to the electrolyte. Tris(trimethylsilyl)phosphate can significantly reduce the room temperature DCR of the lithium-ion battery, while maintaining the high temperature cycle performance of the lithium-ion battery at a high level.

[0025] The preferred addition amount of tris(trimethylsilyl)phosphate in the electrolyte is 0.1%-0.5%wt. More preferably, in some embodiments, the mass ratio of lithium hexafluorophosphate to tris(trimethylsilyl)phosphate is m(LiPF6):m(TMSP)=22:1-4:1.

[0026] In some embodiments of the present invention, the additive used further includes triallyl isocyanurate (TAIC), whose chemical structure is represented by formula (1):

[0027]

[0028] Since TAIC contains more unsaturated bonds, it is preferentially oxidized to form a film at the positive electrode, which mainly improves the stability of the positive electrode material during cycling and high-temperature storage. On the other hand, when testing the performance of lithium-ion batteries with TAIC added to the electrolyte, it was found that excessive TAIC may lead to a deterioration of the DCR value of the lithium-ion battery, so preferably, the mass fraction of TAIC in the electrolyte is 0.1-0.5%.

[0029] In order to improve film-forming performance, cycle performance, etc., electrolyte additives may also specifically include methylene methanedisulfonate (MMDS), vinylene carbonate (VC), and fluoroethylene carbonate (FEC). Among them, when MMDS is added to the electrolyte, on the one hand, during the formation and charging of the lithium-ion battery, the positive electrode surface will first polymerize to form a uniform and dense SEI film, preventing the electrolyte from further decomposing on the negative electrode surface, and can effectively block the solvent molecules from embedding into the negative electrode material, thereby improving the cycle life of the battery.

[0030] During the first charging process of lithium ions, VC and FEC in the electrolyte will undergo reduction reaction on the surface of the negative electrode, decomposing on the electrode surface before other components in the electrolyte to form a SEI film, which prevents further decomposition of the electrolyte and excessive reaction of the electrode material, thereby extending the cycle life of the battery.

[0031] In an embodiment of the present invention, preferably, the mass fraction of MMDS in the electrolyte is 0.3%-0.7%, the mass fraction of VC in the electrolyte is 1%-4%, and the mass fraction of FEC in the electrolyte is 0.5%-1.5%. More preferably, the mass fraction of MMDS in the electrolyte is 0.5%, the mass fraction of VC in the electrolyte is 1.5%, and the mass fraction of FEC in the electrolyte is 0.7%.

[0032] In order to further improve the cycle life of the lithium-ion battery, in some embodiments of the present invention, the electrolyte additive further includes lithium difluorophosphate (LiPO2F2), and the mass fraction of lithium difluorophosphate (LiPO2F2) in the electrolyte is preferably 0.5%.

[0033] The second aspect of the present invention also provides a lithium-ion battery, which includes a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte in the above embodiment. During the battery charging and discharging process, lithium ions are embedded and extracted back and forth between the positive electrode sheet and the negative electrode sheet, the separator is arranged between the positive electrode sheet and the negative electrode sheet to play a role of isolation, and the electrolyte plays a role of conducting ions between the positive electrode sheet and the negative electrode sheet.

[0034] The positive electrode plate of the lithium-ion battery provided in the embodiment of the present invention includes a positive electrode current collector and a positive electrode active material. The positive electrode current collector is usually made of a material with good conductivity and mechanical strength, which is not limited here. The positive electrode active material may include lithium iron phosphate (LiFePO4), lithium cobalt oxide (LiCoO2, LCO), lithium nickel cobalt manganese oxide (LiNi x Co y Mn z O2(x+y+z=1)), any one or a combination of at least two of them. In other embodiments, the positive electrode active material may also be selected from other materials, which are not specifically limited here.

[0035] The negative electrode plate of the lithium-ion battery includes a negative electrode current collector and a negative electrode active material. The negative electrode current collector can be made of a material with good conductivity and mechanical strength, which is not limited here. The negative electrode active material can include any one of artificial graphite, natural graphite, soft carbon, hard carbon, a mixture of graphite and silicon oxide, and a mixture of graphite and silicon, or a combination of at least two of them.

[0036] The isolation membrane may be any conventional type in the art, for example, a polyethylene (PE) or polypropylene (PP) porous membrane may be used as the isolation membrane.

[0037] <Preparation of Electrolyte>

[0038] First, the solvents are mixed and stirred, then the lithium salt is added to the mixed solvent under stirring conditions, and stirring is continued until the lithium salt is completely dissolved, and finally the additive is added to the solution and stirred evenly.

[0039] The types of solvents, lithium salts, and additives used in the electrolytes of Examples 1-12 of the present invention and Comparative Examples 1-3 and the mass fractions based on the total weight of the electrolytes are all based on the settings in Table 1. The above components are mixed to obtain an electrolyte. The electrolytes in different embodiments and comparative examples are prepared in the same manner except that the component ratios are specified in the table.

[0040] The electrolyte compositions of different groups of embodiments and comparative examples are specifically referred to Table 1.

[0041] The technical scheme of the present invention is described in detail below through specific examples and comparative examples. Unless otherwise specified, the raw materials and reagents used in the following examples are all commercially available products, or can be prepared by conventional methods in the art, and the instruments used in the examples are all commercially available.

[0042] Example 1

[0043] The present embodiment provides a lithium ion battery electrolyte, in which the solvent is composed of ethylene carbonate (EC), ethyl acetate (EA), ethyl propionate (EP) and propylene carbonate (PC), and the mass fractions of EC, EA, EP and PC in the electrolyte are 15.1%, 36.1%, 25.0% and 4.5%, respectively; the selected lithium salts are lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI), and the mass fractions of LiPF6 and LiFSI in the electrolyte are 2.0% and 14.0%, respectively; the selected additives include fluoroethylene carbonate (FEC), vinylene carbonate (VC), trimethylsilyl phosphate (TMSP), methylene methanedisulfonate (MMDS), and lithium difluorophosphate (LiPO2F2), and the mass fractions of the above additives in the electrolyte are 0.7%, 1.5%, 0.1%, 0.5% and 0.5%, respectively.

[0044] Example 2

[0045] The system is the same as that in Example 1, but the mass fractions of EC, EA and PC in the electrolyte are 14.7%, 34.0% and 7.0%, respectively, and the rest is the same as that in Example 1.

[0046] Example 3

[0047] The system is the same as that in Example 1, but the mass fractions of EC, EA and PC in the electrolyte are 19.9%, 25.0% and 10.0% respectively, the mass fraction of FEC in the electrolyte is 1.5%, and the rest is the same as that in Example 1.

[0048] Example 4

[0049] The system is the same as that in Example 1, but the mass fraction of EA in the electrolyte is 38.0%; the mass fractions of LiPF6 and LiFSI in the electrolyte are 12.0% and 4.0%, respectively; the rest is the same as in Example 1.

[0050] Example 5

[0051] The system is the same as that in Example 1, but the mass fraction of EA in the electrolyte is 38.0%; the mass fractions of LiPF6 and LiFSI in the electrolyte are 8.0% and 8.0% respectively; the rest is the same as in Example 1.

[0052] Example 6

[0053] The system is the same as that in Example 1, but the mass fraction of EA in the electrolyte is 38.0%; the mass fractions of LiPF6 and LiFSI in the electrolyte are 4.0% and 12.0%, respectively; the rest is the same as in Example 1.

[0054] Example 7

[0055] The system is the same as that in Example 1, but the mass fraction of EA in the electrolyte is 38.0%; the mass fraction of TMSP in the electrolyte is 0.2%; the rest is the same as that in Example 1.

[0056] Example 8

[0057] The system is the same as that in Example 1, but the mass fractions of EC and EA in the electrolyte are 14.9% and 38.0% respectively; the mass fraction of TMSP in the electrolyte is 0.3%; the rest is the same as that in Example 1.

[0058] Example 9

[0059] The system is the same as that in Example 1, but the mass fractions of EC and EA in the electrolyte are 14.7% and 38.0% respectively; the mass fraction of TMSP in the electrolyte is 0.5%; the rest is the same as that in Example 1.

[0060] Example 10

[0061] The system is the same as that in Example 1, but the mass fraction of EA in the electrolyte is 38.0%; the mass fraction of VC in the electrolyte is 1.4%; triallyl isocyanurate (TAIC) is also added to the electrolyte, and its mass fraction in the electrolyte is 0.1%, and the rest is the same as in Example 1.

[0062] Embodiment 11

[0063] The system is the same as that in Example 1, but the mass fractions of EC and EA in the electrolyte are 14.9% and 37.8% respectively; the mass fraction of VC in the electrolyte is 1.4%; triallyl isocyanurate (TAIC) is also added to the electrolyte, and its mass fraction in the electrolyte is 0.3%, and the rest is the same as in Example 1.

[0064] Example 12

[0065] The system is the same as that in Example 1, but the mass fractions of EC and EA in the electrolyte are 14.7% and 37.6% respectively; the mass fraction of VC in the electrolyte is 1.4%; triallyl isocyanurate (TAIC) is also added to the electrolyte, and its mass fraction in the electrolyte is 0.5%, and the rest is the same as in Example 1.

[0066] Comparative Example 1

[0067] The system is the same as that in Example 1, but the mass fraction of EA in the electrolyte is 38.0%; the only lithium salt selected is lithium hexafluorophosphate (LiPF6), and the mass fraction of LiPF6 in the electrolyte is 16.0%; VC and TMSP additives are not added to the electrolyte; the rest is the same as in Example 1.

[0068] Comparative Example 2

[0069] The system is the same as that in Example 1, but the mass fractions of EC, EA and EP in the electrolyte are 10.0%, 50.8% and 20.1% respectively, and PC is not added to the electrolyte; the mass fraction of VC in the electrolyte is 1.4%; no TMSP additive is added to the electrolyte; the rest is the same as that in Example 1.

[0070] Comparative Example 3

[0071] The system is the same as that in Example 1, but the mass fractions of EC and EA in the electrolyte are 14.9% and 37.8% respectively; the mass fraction of VC in the electrolyte is 2.2%; FEC, TMSP, MMDS and LiPO2F2 additives are not added to the electrolyte; the rest is the same as in Example 1.

[0072] Table 1. Electrolyte compositions of different groups of examples and comparative examples

[0073]

[0074]

[0075] The electrolytes of Examples 1 to 12 and Comparative Examples 1 to 3 were respectively used in the preparation of lithium ion batteries.

[0076] In the embodiment of the present invention, the electrochemical device is a lithium ion battery, and the lithium ion battery is a primary lithium ion battery or a secondary lithium ion battery, comprising: a positive electrode sheet, a negative electrode sheet, a separator between the positive electrode and the negative electrode, and an electrolyte. The preparation method of the secondary lithium ion battery in the embodiment of the present invention is as follows:

[0077] The negative electrode sheet, the separator, and the positive electrode sheet are stacked in sequence and placed in an aluminum-plastic film to obtain a dry cell, and the dry cell is baked at 80° C. to remove water. The electrolyte is injected into the dry cell and packaged to obtain a finished lithium-ion battery.

[0078] The performance of lithium-ion batteries assembled from the electrolytes in Examples 1-12 and Comparative Examples 1-3 was tested. The test results are shown in Table 2. The test method is as follows:

[0079] (1) 25℃ DCR ​​test, the test method is as follows:

[0080] At 25°C, charge the lithium-ion battery at 1 / 3C constant current to 4.35V, then charge at constant voltage to a current of 0.05C. Then discharge the battery at 1 / 3C constant current to 2.5V. Repeat the above charging steps and record the charging capacity as C2. Discharge the battery at 1 / 3C constant current to (50%*C2), record the initial voltage as V2. Discharge the battery at 1C2 constant current for 30s, record the terminal voltage as V3. DCR = (V2-V3) / (C2*1).

[0081] (2) High temperature storage test, the test method is as follows:

[0082] The lithium-ion battery was stored at 60°C for 27 days. The battery was then placed at 25°C and discharged at a constant current of 1 / 3C to 2.5V. The battery was then charged at a constant current of 1 / 3C to 4.35V, and then charged at a constant voltage to a current of 0.05C. The battery was then discharged at a constant current of 1 / 3C to 2.5V, and the discharge capacity was recorded as C3. The capacity recovery rate is (C3 / C2)*100%.

[0083] (3) High temperature cycle test, the test method is as follows:

[0084] At 45°C, charge the lithium-ion battery to 4.35V at 1 / 3C constant current, then charge at constant voltage to a current of 0.05C. Then discharge the battery to 2.5V at 1 / 3C constant current. Record the capacity C0, repeat the above charge and discharge steps 1200 times, and record the discharge capacity C1 of 1200 cycles. The capacity retention rate of the battery = C1 / C0*100%.

[0085] The test results are shown in Table 2.

[0086] Table 2. Lithium ion battery performance test results of Examples 1-12 and Comparative Examples 1-3

[0087]

[0088] Analyzing the above data, we can draw the following conclusions:

[0089] (1) By comparing Examples 1-3 and Comparative Example 2, it can be seen that when the EA content in the electrolyte increases and the PC content decreases, the DCR value of the battery at room temperature is low, indicating that the kinetic performance of the electrolyte is good; when EA / PC ≥ 2.5, the overall performance of the battery is good; when the EA / PC value in the electrolyte increases from 2.5:1 (Example 3) to 8:1 (Example 1), the DCR value of the battery at room temperature decreases, and at the same time, the capacity recovery rate of 90 days of storage at high temperature of 60°C is improved to a certain extent, and the capacity retention rate of 1200cls of high-temperature cycling has an optimized performance. It shows that within a reasonable range, increasing the EA content in the electrolyte can keep the deterioration of the high-temperature performance of the lithium-ion battery within a reasonable range, and at the same time, due to the improvement of the electrolyte kinetics, the cycle capacity and 25°C DCR value of the lithium-ion battery are significantly optimized. However, when the EA / PC ratio in the electrolyte is too large (Comparative Example 2), the 25°C DCR value of the battery deteriorates, and the high-temperature storage and cycle times also tend to deteriorate seriously. This indicates that when the EA content in the electrolyte is too high, EA will decompose under high temperature conditions, resulting in deterioration of the performance of the lithium-ion battery.

[0090] (2) By comparing Examples 1 and 4-6, and Comparative Example 1, it can be seen that when the content of LiFSI in the electrolyte increases and the content of LiPF6 decreases, the 25°C DCR value of the battery is low, indicating that the kinetic performance of the electrolyte is good. When LiFSI / LiPF6≥7:1, the overall performance of the battery is good. By setting three groups of gradients for the value of LiFSI / LiPF6 in the electrolyte, namely 1:3 (Example 4), 1:1 (Example 5) and 3:1 (Example 6), the results show that when the value of LiFSI increases, the 25°C DCR value of the battery decreases synchronously, while the high-temperature storage capacity recovery rate and high-temperature cycle performance remain at the same level. It shows that within a reasonable range, the deterioration of high-temperature performance by increasing the content of LiFSI can be kept within a reasonable range, and at the same time, due to the improvement of electrolyte kinetics, the 25°C DCR has a significant optimization performance. By comparing Example 6 with Example 1, it can be found that after further increasing the LiFSI content to LiFSI / LiPF6=7:1, the 25°C DCR is slightly reduced. When only LiPF6 is used as a single salt in the electrolyte (Comparative Example 1), the 25°C DCR of the lithium-ion battery deteriorates, and its high-temperature storage capacity recovery rate and 45°C cycle performance are seriously deteriorated, indicating that the use of LiPF6 alone will lead to deterioration of the performance of the lithium-ion battery.

[0091] (3) By comparing Example 1 and Examples 7-9, it can be seen that the TMSP content in the electrolyte has a great influence on the battery performance. The mass fraction of TMSP in the electrolyte is preferably 0.1-0.5%. By setting the mass fraction of TMSP in the electrolyte to 0.1% (Example 1), 0.2% (Example 7), 0.3% (Example 8) and 0.5% (Example 9), respectively. By comparing the DCR test results, it is found that when the TMSP content increases within this range, the 25°C DCR value of Example 9 is the lowest, and the high temperature storage performance and high temperature cycle performance are the best, indicating that the mass fraction of TMSP in the electrolyte is preferably 0.5%.

[0092] (4) By comparing the performance test results of Examples 1 and 10-12, it is shown that the addition of triallyl isocyanurate (TAIC) significantly deteriorates the DCR at room temperature, but at the same time significantly improves the high temperature storage and cycle performance. Preferably, the mass fraction of TAIC in the electrolyte is in the range of 0.1 to 0.5%, and more preferably, the mass fraction is 0.1%.

[0093] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. An electrolyte, characterized in that: include: A solvent, wherein the mass fraction of the solvent in the electrolyte is 75%-90%, the solvent comprises a carboxylate solvent and a cyclic carbonate solvent, and the mass ratio of the carboxylate solvent to the cyclic carbonate solvent is m(CO):m(CC)≥5:3; A lithium salt is dissolved in the solvent, wherein the mass fraction of the lithium salt in the electrolyte is 10%-18%, the lithium salt comprises lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate, and the mass ratio of the lithium bis(fluorosulfonyl)imide to the lithium hexafluorophosphate is m(LiFSI):m(LiPF6)≥1:3; and The additive comprises tris(trimethylsilane)phosphate, and the mass fraction of the tris(trimethylsilane)phosphate in the electrolyte is 0.1%-0.5%.

2. The electrolyte according to claim 1, characterized in that The additive further comprises triallyl isocyanurate, and the mass fraction of the triallyl isocyanurate in the electrolyte is 0.1%-0.5%.

3. The electrolyte according to claim 1, characterized in that The mass ratio of the lithium hexafluorophosphate to the tris(trimethylsilyl)phosphate is m(LiPF6):m(TMSP)=22:1-4:

1.

4. The electrolyte according to claim 1, characterized in that The mass ratio of the lithium bis(fluorosulfonyl)imide to the lithium hexafluorophosphate is m(LiFSI):m(LiPF6)≥7:

1.

5. The electrolyte according to claim 1, characterized in that The mass fraction of the lithium bis(fluorosulfonyl)imide in the electrolyte is 10%-15%, and the mass fraction of the lithium hexafluorophosphate in the electrolyte is 1%-3%.

6. The electrolyte according to claim 1, characterized in that The additives also include methylene methanedisulfonate, vinylene carbonate and fluoroethylene carbonate. The mass fraction of methylene methanedisulfonate in the electrolyte is 0.3%-0.7%, the mass fraction of vinylene carbonate in the electrolyte is 1%-4%, and the mass fraction of fluoroethylene carbonate in the electrolyte is 0.5%-1.5%.

7. The electrolyte according to claim 1, characterized in that The solvent consists of the carboxylate solvent and the cyclic carbonate solvent, The carboxylate solvent is composed of ethyl acetate and ethyl propionate, the mass fraction of ethyl acetate in the electrolyte is 25%-40%, and the mass fraction of ethyl propionate in the electrolyte is 21%-30%; The cyclic carbonate solvent consists of ethylene carbonate and propylene carbonate. The mass fraction of the ethylene carbonate in the electrolyte is 12%-20%, and the mass fraction of the propylene carbonate in the electrolyte is 4%-10%.

8. A lithium ion battery, characterized in that: The lithium-ion battery comprises a positive electrode sheet, a negative electrode sheet and an electrolyte according to any one of claims 1 to 7.

9. The lithium ion battery according to claim 8, characterized in that The positive electrode plate comprises a positive electrode current collector and a positive electrode active material, wherein the positive electrode active material comprises any one of lithium iron phosphate, lithium cobalt oxide, and lithium nickel cobalt manganese oxide, or a combination of at least two thereof; The negative electrode plate comprises a negative electrode current collector and a negative electrode active material, wherein the negative electrode active material comprises any one of artificial graphite, natural graphite, soft carbon, hard carbon, a mixture of graphite and silicon oxide, and a mixture of graphite and silicon, or a combination of at least two thereof.

Images