Electrolyte, secondary liquid injection formation method and lithium ion battery

Abstract

The invention relates to an electrolyte, a secondary liquid injection formation method and a lithium ion battery. The electrolyte comprises a first electrolyte and a second electrolyte, the first electrolyte comprises a first lithium salt, a first organic solvent and an additive, the additive is 1-fluoro-4-(trialkylsilyl) benzene, and the first organic solvent comprises propylene carbonate; the second electrolyte contains a second lithium salt and a second organic solvent. The additive 1-fluoro-4-(trialkylsilyl) benzene is introduced into the first electrolyte which is used for primary liquid injection and contains propylene carbonate, so that a solvation structure of lithium ions can be effectively adjusted, co-intercalation of propylene carbonate in a negative electrode is inhibited, formation of a thin and stable SEI film is facilitated, and the service life of the lithium ion battery is prolonged. The internal resistance of the battery is effectively reduced; the cycle life of the battery is prolonged; in addition, the 1-fluoro-4-(trialkylsilyl) benzene can reduce the surface tension and viscosity of the electrolyte, and is beneficial to improving the low-temperature capacity retention rate of the battery.

Description

Technical Field

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

[0002] Lithium-ion batteries have the advantages of high specific energy density, long cycle life and environmental friendliness, and are widely used in electric vehicles, energy storage power stations and portable electronic devices. As the scope of application expands, lithium-ion batteries need to meet the performance requirements of different application scenarios. For example, in winter or high-latitude areas, high requirements are placed on the low-temperature performance of batteries.

[0003] At present, conventional lithium-ion batteries use electrolytes with ethylene carbonate as the main solvent, but ethylene carbonate has a high melting point (37 °C), which leads to a high freezing point and a narrow liquid range for the electrolyte as a whole. For a low temperature environment of -40 °C, the capacity of conventional lithium-ion batteries drops sharply and the cycle is significantly deteriorated. In order to provide an electrolyte suitable for ultra-low temperature use scenarios, the key is to broaden the liquid range of the electrolyte and reduce the viscosity of the electrolyte. Among them, propylene carbonate has a similar structure to ethylene carbonate, and its performance is comparable to that of ethylene carbonate in many aspects such as the dissociation of lithium salts. At the same time, the melting point of propylene carbonate (-48.8 °C) is much lower than that of ethylene carbonate. Therefore, the freezing point of the electrolyte with propylene carbonate as the main solvent is much lower than that of ethylene carbonate, and it is more suitable for low-temperature electrolytes. However, the electrolytes using propylene carbonate as the main solvent currently face the following two problems: (1) Conventional lithium-ion batteries use artificial graphite as the negative electrode. During the formation stage of the solid electrolyte interface film (SEI film), propylene carbonate will co-intercalate with the artificial graphite, causing continuous damage to the negative electrode structure and making it impossible to form a stable SEI film; (2) The electrolyte using propylene carbonate as the main solvent has a high viscosity, which is not conducive to the flow of the electrolyte and the battery capacity at low temperatures.

[0004] Based on this, how to effectively improve the co-embedding and viscosity problems of low-temperature electrolytes with propylene carbonate as the main solvent is of great significance to improving the capacity retention rate and cycle life of batteries at low temperatures. Summary of the invention

[0005] To solve the above problems, the present invention provides an electrolyte, a secondary liquid injection formation method and a lithium ion battery. By introducing the additive 1-fluoro-4-(trialkylsilyl)benzene into the first electrolyte used for primary liquid injection and containing propylene carbonate, the solvation structure of lithium ions can be effectively adjusted, and the co-embedding of propylene carbonate in the negative electrode can be inhibited, which is conducive to the formation of a thin and stable SEI film, which can effectively reduce the internal resistance of the battery and improve the cycle life of the battery; in addition, 1-fluoro-4-(trialkylsilyl)benzene can reduce the surface tension and viscosity of the electrolyte, which is conducive to further improving the low-temperature capacity retention rate of the battery.

[0006] The present invention provides the following technical solutions: A first aspect of the present invention provides an electrolyte, wherein the electrolyte comprises a first electrolyte and a second electrolyte; The first electrolyte comprises a first lithium salt, a first organic solvent and an additive, the additive is 1-fluoro-4-(trialkylsilyl)benzene, and the first organic solvent comprises propylene carbonate; The second electrolyte includes a second lithium salt and a second organic solvent.

[0007] Furthermore, the 1-fluoro-4-(trialkylsilyl)benzene has the following general structural formula: , Wherein, R is a C1-C5 alkyl group, such as methyl, ethyl, propyl, isopropyl, butyl, pentyl, etc.

[0008] Furthermore, the mass percentage of the additive in the first electrolyte is preferably 0.5%-8%, for example 0.5%, 1%, 1.5%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, etc., including but not limited to the mass percentages listed above.

[0009] Further, the mass percentage of the first lithium salt in the first electrolyte is greater than the mass percentage of the second lithium salt in the second electrolyte; more preferably, the mass percentage of the first lithium salt in the first electrolyte is 15%-20%, for example, 15%, 16%, 17%, 18%, 19%, 20%, etc., and the mass percentage of the second lithium salt in the second electrolyte is 5%-12.5%, for example, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, etc., including but not limited to the mass percentages listed above.

[0010] Furthermore, the first lithium salt and the second lithium salt are respectively selected from one or more of lithium bis(trifluoromethanesulfonyl imide), lithium bis(fluorosulfonyl imide), lithium acetate, lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium trifluoroacetate, lithium fluoroalkyl phosphate, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate and lithium fluoride.

[0011] Furthermore, the first organic solvent also contains one or more of other cyclic carbonate solvents and chain carbonate solvents; the other cyclic carbonate solvents contain one or both of ethylene carbonate and butylene carbonate, and the chain carbonate solvent contains one or more of dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate.

[0012] Furthermore, the mass proportion of propylene carbonate in the first organic solvent is preferably 10%-70%, for example 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 60%, 65%, 70%, etc., including but not limited to the mass proportions listed above.

[0013] Furthermore, the second organic solvent comprises one or more of a cyclic carbonate solvent and a chain carbonate solvent; the cyclic carbonate solvent comprises one or more of propylene carbonate, ethylene carbonate, and butylene carbonate; the chain carbonate solvent comprises one or more of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0014] Further, the first organic solvent and the second organic solvent both contain cyclic carbonate solvents and chain carbonate solvents; more preferably, the mass ratio of cyclic carbonate solvents to chain carbonate solvents in the first organic solvent is (0.2-2):1, for example, 0.2:1, 0.5:1, 1:1, 1.5:1, 2:1, etc., including but not limited to the mass ratios listed above; the mass ratio of cyclic carbonate solvents to chain carbonate solvents in the second organic solvent is (0.2-2):1, for example, 0.2:1, 0.5:1, 1:1, 1.5:1, 2:1, etc., including but not limited to the mass ratios listed above.

[0015] Furthermore, the mass proportion of the propylene carbonate in the total organic solvent is 5%-70%, for example, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 60%, 65%, 70%, etc., including but not limited to the mass proportions listed above; wherein the mass of the total organic solvent is the sum of the mass of the first organic solvent in the first electrolyte added and the mass of the second organic solvent in the second electrolyte added.

[0016] Furthermore, the first electrolyte and the second electrolyte both contain other additives, and the other additives are selected from one or more of vinylene carbonate, vinylene carbonate, 1,3-propane sultone, vinyl sulfate, propylene sultone, methanedisulfonic acid methylene ester, tris(trimethylsilyl)borate, tris(trimethylsilyl)phosphate, adiponitrile, maleic anhydride, triallyl isocyanate and trimethylsilyl diethylamine.

[0017] The second aspect of the present invention provides a secondary liquid injection formation method, which uses the electrolyte described in the first aspect to perform secondary liquid injection formation, and the method comprises the following steps: S1, injecting the first electrolyte into the battery, sealing it, and letting it stand to obtain a primary injection battery; S2, forming the primary injection battery; S3, injecting the second electrolyte into the formed battery, sealing it, and completing the secondary electrolyte injection formation process.

[0018] Furthermore, in S1, the standing temperature is 30-60°C, for example, 45°C.

[0019] Further, in S2, the forming is a conventional forming operation.

[0020] Furthermore, the method further comprises a step of performing high temperature aging before injecting the second electrolyte, wherein the temperature of the high temperature aging is 30-60°C, for example, 45°C.

[0021] Furthermore, the total injection coefficient is controlled in the range of 1-6 g / Ah, more preferably 3-6 g / Ah, such as 3 g / Ah, 4 g / Ah, 5 g / Ah, 6 g / Ah, etc.

[0022] Furthermore, the injection volume of the first electrolyte is m1, and the injection volume of the second electrolyte is m2, satisfying 1≤m1 / m2≤19, for example, the ratio of m1 / m2 is 1, 1.5, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, etc., including but not limited to the ratios listed above.

[0023] The third aspect of the present invention provides a lithium ion battery, comprising the electrolyte described in the first aspect or prepared by the secondary liquid injection formation method described in the second aspect.

[0024] Compared with the prior art, the present invention has the following beneficial effects: The invention provides an electrolyte, using 1-fluoro-4-(trimethylsilyl)benzene as an additive for an electrolyte with propylene carbonate as a main solvent, utilizing 1-fluoro-4-(trimethylsilyl)benzene to adjust the solvation structure of lithium ions, effectively inhibiting the co-embedding of propylene carbonate, forming a thin and stable SEI film, thereby effectively reducing the internal resistance of a battery and improving the cycle life; meanwhile, the addition of 1-fluoro-4-(trimethylsilyl)benzene is beneficial to reducing the surface tension and viscosity of the electrolyte, and can further improve the capacity retention rate of the battery at low temperatures.

[0025] The present invention adopts a method of injecting a high lithium salt concentration electrolyte containing 1-fluoro-4-(trimethylsilyl)benzene and propylene carbonate and injecting a low lithium salt concentration electrolyte, utilizing the solvation structure change of lithium ions under high lithium salt concentration and coordinating the adjustment effect of 1-fluoro-4-(trimethylsilyl)benzene on the solvation structure of lithium ions, thereby further optimizing the system's inhibitory effect on the co-embedding of propylene carbonate in the negative electrode, improving the structural stability of the SEI film, and improving the cycle performance of the battery. DETAILED DESCRIPTION

[0026] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present invention belongs. The terms used herein in the specification of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items. The "include" or "comprising" described in the present invention means that in addition to the components described, other components may also be included or contained. The "include" or "comprising" described in the present invention may also be replaced by the closed "for" or "consisting of...".

[0027] As described in the background art, low-temperature electrolytes using propylene carbonate as the main solvent have the following problems: (1) Conventional lithium-ion batteries use artificial graphite as the negative electrode. During the formation of the solid electrolyte interface film (SEI film), propylene carbonate will co-embed with the artificial graphite, causing continuous damage to the negative electrode structure and failing to form a stable SEI film; (2) The electrolyte using propylene carbonate as the main solvent has a high viscosity, which is not conducive to the flow of the electrolyte and the battery capacity at low temperatures. These problems limit the practical application of this type of low-temperature electrolyte.

[0028] At present, it is proposed that adding N-methylpyrrolidone (NMP) to the electrolyte with propylene carbonate as the main solvent can improve the co-embedding problem of propylene carbonate in the SEI film formation process to a certain extent, but the viscosity of NMP is large, which will increase the viscosity of the electrolyte, and the presence of NMP will reduce the stability of the SEI film and cause the decomposition of the lithium salt LiPF6, which will deteriorate the battery impedance, low temperature capacity retention rate and cycle performance. In addition, some researchers have proposed using low-viscosity hexafluorobenzene as an additive to introduce it into the electrolyte with propylene carbonate as the main solvent, which can not only reduce the surface tension of the electrolyte and improve the viscosity of the electrolyte, but also change the lithium ion solvation structure, which plays a role in inhibiting the co-embedding of propylene carbonate, but hexafluorobenzene will produce a byproduct HF during the battery charge and discharge process, which seriously affects the cycle life of the battery.

[0029] To solve the above problems, the present invention provides an electrolyte, which includes a first electrolyte and a second electrolyte; The first electrolyte comprises a first lithium salt, a first organic solvent and an additive, the additive is 1-fluoro-4-(trialkylsilyl)benzene, and the first organic solvent comprises propylene carbonate; The second electrolyte includes a second lithium salt and a second organic solvent.

[0030] The present invention is to solve the co-embedding problem of the low-temperature electrolyte with propylene carbonate as the main solvent during the formation of the SEI film and the problem of high adhesion of the electrolyte. After a large number of experimental studies, it was unexpectedly found that by introducing the additive 1-fluoro-4-(trimethylsilyl)benzene into the electrolyte with propylene carbonate as the main solvent, the negative electrode co-embedding problem of propylene carbonate in the formation of the SEI film can be effectively suppressed, and a thin and stable SEI film can be formed, thereby effectively reducing the internal resistance of the battery and improving the cycle performance of the battery; and the introduction of 1-fluoro-4-(trimethylsilyl)benzene can reduce the surface tension and viscosity of the electrolyte and improve the capacity retention rate of the battery at low temperatures. In addition, unlike hexafluorobenzene, 1-fluoro-4-(trimethylsilyl)benzene does not produce HF-type byproducts during the battery cycle, so it does not affect the cycle performance of the battery, so that the battery containing the above electrolyte not only has a lower internal resistance and a higher discharge capacity retention rate at low temperatures, but also exhibits better low-temperature cycle performance.

[0031] In the present invention, 1-fluoro-4-(trialkylsilyl)benzene has the following general structural formula: , Wherein, R is a C1-C5 alkyl group, such as methyl, ethyl, propyl, isopropyl, butyl, pentyl, etc.

[0032] Specifically, 1-fluoro-4-(trialkylsilyl)benzene can be 1-fluoro-4-(trimethylsilyl)benzene (TMSiFB, CAS No.: 455-17-4) or 1-fluoro-4-(triethylsilyl)benzene (TESiFB, CAS No.: 2129647-70-5); TMSiFB is more preferably used as an additive. Compared with TESiFB, TMSiFB has a more suitable molecular structure size and is more conducive to regulating Li + solvation structure, thereby effectively inhibiting co-embedding.

[0033] In the present invention, the amount of the additive 1-fluoro-4-(trialkylsilyl)benzene added to the first electrolyte will affect the electrochemical performance of the battery. For example, when the amount of 1-fluoro-4-(trialkylsilyl)benzene added to the first electrolyte is too small, it is difficult to effectively regulate the lithium ion solvation structure in the electrolyte, thereby being unable to effectively inhibit the co-embedding problem of propylene carbonate in the SEI film formation process, and having a poor effect on reducing the surface tension of the electrolyte, and being unable to effectively reduce the viscosity of the electrolyte, thereby affecting the cycle performance and capacity of the battery; however, if the amount of 1-fluoro-4-(trialkylsilyl)benzene added to the first electrolyte is too large, the cycle performance of the battery will also deteriorate. This is because as the content of 1-fluoro-4-(trialkylsilyl)benzene in the electrolyte increases, the side reactions related to the benzene ring will increase during the battery cycle, thereby increasing the risk of battery self-discharge and reducing the reversible capacity retention rate of the battery. Preferably, the mass percentage of the additive in the first electrolyte is controlled in the range of 0.5%-8%, for example 0.5%, 1%, 1.5%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, etc., including but not limited to the mass percentages listed above, more preferably 0.5%-5%.

[0034] In the present invention, the mass percentage of the first lithium salt in the first electrolyte is greater than the mass percentage of the second lithium salt in the second electrolyte. Without changing the lithium salt concentration in the total electrolyte, the lithium salt concentration in the first electrolyte used for the first injection is higher than the lithium salt concentration in the second electrolyte used for the second injection. By utilizing the change in the solvation structure of lithium ions under high lithium salt concentration and the adjusting effect of the additive 1-fluoro-4-(trimethylsilyl)benzene on the solvation structure of lithium ions, the system can further optimize the inhibitory effect of propylene carbonate co-embedded in the negative electrode and improve the structural stability of the SEI film.

[0035] More preferably, the mass percentage of the first lithium salt in the first electrolyte is 15%-20%, for example 15%, 16%, 17%, 18%, 19%, 20%, etc., and the mass percentage of the second lithium salt in the second electrolyte is 5%-12.5%, for example 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, etc., including but not limited to the mass percentages listed above.

[0036] In the present invention, the first lithium salt and the second lithium salt can be selected from one or more of lithium bis(trifluoromethanesulfonyl imide), lithium bis(fluorosulfonyl imide), lithium acetate, lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium trifluoroacetate, lithium fluoroalkyl phosphate, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate and lithium fluoride.

[0037] In the present invention, the first organic solvent further comprises one or more of other cyclic carbonate solvents and chain carbonate solvents; wherein, the other cyclic carbonate solvents comprise one or two of ethylene carbonate and butylene carbonate, and the chain carbonate solvent comprises one or more of dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate; more preferably, the first organic solvent comprises a cyclic carbonate solvent and a chain carbonate solvent.

[0038] In some preferred embodiments, the mass ratio of the cyclic carbonate solvent to the linear carbonate solvent in the first organic solvent is (0.2-2):1, for example 0.2:1, 0.5:1, 1:1, 1.5:1, 2:1, etc., including but not limited to the mass ratios listed above.

[0039] In some preferred embodiments, the mass proportion of propylene carbonate in the first organic solvent is preferably 10%-70%, for example 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 60%, 65%, 70%, etc., including but not limited to the mass proportions listed above.

[0040] In the present invention, the second organic solvent comprises one or more of a cyclic carbonate solvent and a chain carbonate solvent; wherein the cyclic carbonate solvent comprises one or more of propylene carbonate, ethylene carbonate, and butylene carbonate, and the chain carbonate solvent comprises one or more of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; more preferably, the second organic solvent comprises a cyclic carbonate solvent and a chain carbonate solvent.

[0041] In some preferred embodiments, the mass ratio of the cyclic carbonate solvent to the linear carbonate solvent in the second organic solvent is (0.2-2):1, for example 0.2:1, 0.5:1, 1:1, 1.5:1, 2:1, etc., including but not limited to the mass ratios listed above.

[0042] In the present invention, the mass proportion of propylene carbonate in the total organic solvent is 5%-70%, for example 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 60%, 65%, 70%, etc., including but not limited to the mass proportions listed above; wherein the mass of the total organic solvent is the sum of the mass of the first organic solvent in the first electrolyte added and the mass of the second organic solvent in the second electrolyte added.

[0043] The embodiment of the present invention also provides a secondary liquid injection formation method, which uses the above electrolyte to perform secondary liquid injection formation, specifically comprising the following steps: S1, injecting the first electrolyte into the battery, sealing it, and letting it stand to obtain a primary injection battery; S2, forming the primary injection battery; S3, injecting the second electrolyte into the formed battery, sealing it, and completing the secondary electrolyte injection formation process.

[0044] In step S1 of the present invention, the standing temperature is 30-60° C., for example, 45° C., to accelerate the rapid infiltration of the electrolyte.

[0045] In step S2 of the present invention, the formation is a conventional formation operation. In some preferred embodiments, the formation step is specifically: charging at a constant current of 0.02 C at 45° C. for 10 h.

[0046] In the present invention, the method further comprises a step of high temperature aging before injecting the second electrolyte, wherein the temperature of the high temperature aging is 30-60°C, for example, 45°C.

[0047] In some preferred embodiments, the total injection coefficient is controlled in the range of 1-6 g / Ah, more preferably 3-6 g / Ah, for example 3 g / Ah, 4 g / Ah, 5 g / Ah, 6 g / Ah, etc.; wherein the injection amount of the first electrolyte is m1, and the injection amount of the second electrolyte is m2, satisfying 1≤m1 / m2≤19, for example, the ratio of m1 / m2 is 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, etc., including but not limited to the ratios listed above.

[0048] The embodiments of the present invention also provide a lithium-ion battery prepared by the above-mentioned electrolyte and the above-mentioned secondary liquid injection formation method. The lithium-ion battery exhibits low battery internal resistance, high capacity retention rate and better cycle performance under low temperature conditions.

[0049] In the present invention, the above-mentioned lithium ion battery can be a cylindrical lithium ion battery, a soft-pack lithium ion battery or a square lithium ion battery; and the types of positive electrode active materials and negative electrode active materials used in the lithium ion battery are not limited, wherein the positive electrode active material can be selected from one or more of lithium iron phosphate, lithium iron manganese phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium cobalt oxide, lithium manganese oxide and lithium-rich lithium manganese oxide, and other types of positive electrode active materials can also be used, and the negative electrode active material can be selected from one or more of lithium metal, natural graphite, artificial graphite, mesophase carbon balls, hard carbon, soft carbon, silicon, silicon-carbon composites, Li-Sn alloys, Li-Sn-O alloys, and spinel structured Li-Al alloys.

[0050] In the lithium-ion battery of the present invention, there is no limitation on the specific materials and structures of the positive electrode sheet, the separator and the negative electrode sheet, and any components and structures known in the art that can be used in secondary batteries can be selected as long as the purpose of the present invention can be achieved.

[0051] The present invention will be further described below in conjunction with specific examples so that those skilled in the art can better understand the present invention and implement it, but the examples are not intended to limit the present invention. If no specific techniques or conditions are specified in the examples, the techniques or conditions described in the literature in this area or the product instructions are used. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be obtained commercially. Example 1

[0052] This embodiment relates to an electrolyte and a lithium ion battery, and the specific preparation process is as follows: (1) Preparation of electrolyte: First electrolyte: In an argon atmosphere glove box with a water content of <10 ppm, propylene carbonate and ethyl methyl carbonate are uniformly mixed in a mass ratio of 3:7, and then additives of vinylene carbonate, fluoroethylene carbonate and vinyl sulfate are added to make the concentrations of vinyl carbonate, fluoroethylene carbonate and vinyl sulfate in the electrolyte 2%, 0.5% and 2%, respectively, to obtain a basic electrolyte; then additives TMSiFB and lithium salt LiPF6 are added to the electrolyte, wherein the mass proportion of the additive TMSiFB in the electrolyte is 3%, and the mass proportion of the lithium salt LiPF6 in the electrolyte is 18%, to obtain a first electrolyte.

[0053] Second electrolyte: In an argon atmosphere glove box with a water content of <10 ppm, propylene carbonate and ethyl methyl carbonate were uniformly mixed in a mass ratio of 3:7, and then additives of vinylene carbonate, fluoroethylene carbonate and vinyl sulfate were added to make the concentrations of vinyl carbonate, fluoroethylene carbonate and vinyl sulfate in the electrolyte 2%, 0.5% and 2%, respectively, to obtain a basic electrolyte; and then lithium salt LiPF6 was added to make its mass proportion in the electrolyte 8%, to obtain a second electrolyte.

[0054] (2) Preparation of lithium-ion batteries Positive electrode sheet: The positive electrode active material lithium iron phosphate, the conductive agent SP (manufacturer: TIMCAL, model: KS-6), and the binder polyvinylidene fluoride (manufacturer: Arkema, model: HSV900) are fully stirred and mixed in a proper amount of NMP solvent in a mass ratio of 96.7:1.5:1.8 to form a uniform positive electrode slurry; the slurry is coated on the positive electrode current collector Al foil, dried, and cold pressed to obtain a positive electrode sheet.

[0055] Negative electrode sheet: The negative electrode active material artificial graphite (manufacturer: B&T, model: GSN-17), conductive agent SP (manufacturer: TIMCAL, model: KS-6), binder styrene-butadiene rubber (manufacturer: Zeon, model: BM451B), and thickener sodium carboxymethyl cellulose are fully stirred and mixed in a proper amount of deionized water solvent in a weight ratio of 95.7:1:1.5:1.8 to form a uniform negative electrode slurry; the slurry is coated on the negative electrode current collector Cu foil, dried, and cold pressed to obtain a negative electrode sheet.

[0056] The positive electrode sheet, separator (PE separator), and negative electrode sheet are stacked in order, so that the separator is placed between the positive electrode sheet and the negative electrode sheet to play an isolating role, and then wound and placed in an outer packaging aluminum-plastic foil. After baking, the moisture content of the positive and negative electrodes is less than 200 ppm, and a 3 Ah battery cell is obtained for standby use.

[0057] Secondary liquid injection formation: add the first electrolyte into the battery cell, the amount of the first electrolyte added is 7.2 g, the injection coefficient is 2.4 g / Ah, after vacuum sealing, place it at 45 ℃ for 12 hours; perform formation, specifically: charge at 45 ℃ at a constant current of 0.02 C for 10 hours; the formed battery is placed at 45 ℃ for high temperature aging for 8 hours. Then cut the seal, add the second electrolyte into the battery cell, the amount of the second electrolyte added is 4.8 g, the injection coefficient is 1.6 g / Ah, seal it, and complete the secondary liquid injection formation process.

[0058] The battery was then placed at 45 °C for 8 h for degassing, secondary sealing and subsequent capacity separation to prepare a lithium-ion battery. Example 2

[0059] This embodiment relates to an electrolyte and a lithium-ion battery, and the only difference from Example 1 is that: the mass proportion of the additive TMSiFB in the first electrolyte is 5%, and the mass proportion of the lithium salt is 20%; the mass proportion of the lithium salt in the second electrolyte is 5%; the remaining operations are the same, and the corresponding lithium-ion battery is obtained, and the mass concentration of lithium salt in the total electrolyte in the lithium-ion battery is consistent with that in Example 1. Example 3

[0060] This embodiment relates to an electrolyte and a lithium-ion battery, which differs from Embodiment 1 only in that: the mass proportion of the additive TMSiFB in the first electrolyte is 2%, and the mass proportion of the lithium salt is 16%; the mass proportion of the lithium salt in the second electrolyte is 11%; the remaining operations are the same, and the corresponding lithium-ion battery is obtained, and the mass concentration of lithium salt in the total electrolyte in the lithium-ion battery is consistent with that in Embodiment 1. Example 4

[0061] This embodiment relates to an electrolyte and a lithium-ion battery, which differs from Embodiment 1 only in that: the mass proportion of the additive TMSiFB in the first electrolyte is 0.5%, and the mass proportion of the lithium salt is 15%; the mass proportion of the lithium salt in the second electrolyte is 12.5%; the remaining operations are the same, and the corresponding lithium-ion battery is obtained, and the mass concentration of lithium salt in the total electrolyte in the lithium-ion battery is consistent with that in Embodiment 1. Example 5

[0062] This embodiment relates to an electrolyte and a lithium ion battery, which differs from Embodiment 1 only in that: the mass proportion of the additive TMSiFB in the first electrolyte is 7%, and the mass proportion of the lithium salt is 18%; the mass proportion of the lithium salt in the second electrolyte is 8%; the remaining operations are the same, and the corresponding lithium ion battery is obtained, and the mass concentration of lithium salt in the total electrolyte in the lithium ion battery is consistent with that in Embodiment 1. Example 6

[0063] This embodiment relates to an electrolyte and a lithium ion battery, and the only difference from Example 1 is that: the mass proportion of the additive TMSiFB in the first electrolyte is 8%, and the mass proportion of the lithium salt is 18%; the mass proportion of the lithium salt in the second electrolyte is 8%; the remaining operations are the same, and the corresponding lithium ion battery is obtained, and the mass concentration of lithium salt in the total electrolyte in the lithium ion battery is consistent with that in Example 1. Example 7

[0064] This embodiment relates to an electrolyte and a lithium ion battery, and the only difference from Example 1 is that: the mass proportion of the additive TMSiFB in the first electrolyte is 0.4%, and the mass proportion of the lithium salt is 18%; the mass proportion of the lithium salt in the second electrolyte is 8%; the remaining operations are the same, and the corresponding lithium ion battery is obtained, and the mass concentration of lithium salt in the total electrolyte in the lithium ion battery is consistent with that in Example 1. Example 8

[0065] This embodiment relates to an electrolyte and a lithium ion battery, and the only difference from Example 1 is that: the mass proportion of the additive TMSiFB in the first electrolyte is 3%, and the mass proportion of the lithium salt is 14%; the mass proportion of the lithium salt in the second electrolyte is 14%; the remaining operations are the same, and the corresponding lithium ion battery is obtained, and the mass concentration of lithium salt in the total electrolyte in the lithium ion battery is consistent with that in Example 1. Example 9

[0066] This embodiment relates to an electrolyte and a lithium-ion battery, which differs from Embodiment 1 only in that an equal amount of TESiFB is used to replace the additive TMSiFB in the first electrolyte, and the remaining operations are the same to obtain a corresponding lithium-ion battery, and the lithium salt mass concentration of the total electrolyte in the lithium-ion battery is consistent with that in Embodiment 1. Comparative Example 1

[0067] This comparative example relates to an electrolyte and a lithium ion battery, which differs from Example 1 only in that the first electrolyte does not contain the additive TMSiFB, and the other operations are the same to obtain a corresponding lithium ion battery, and the lithium salt mass concentration of the total electrolyte in the lithium ion battery is consistent with that in Example 1. Comparative Example 2

[0068] This comparative example relates to an electrolyte and a lithium ion battery, which differs from Example 1 only in that an equal amount of hexafluorobenzene is used to replace the additive TMSiFB in the first electrolyte, and the other operations are the same to obtain a corresponding lithium ion battery, and the lithium salt mass concentration of the total electrolyte in the lithium ion battery is consistent with that in Example 1. Performance Testing

[0069] The lithium ion batteries prepared in the above Examples 1-9 and Comparative Examples 1 and 2 were subjected to the following electrochemical performance tests: -20 ℃ low temperature DCR performance test: Place the lithium-ion battery in a 25 ℃ constant temperature box and let it stand for 30 minutes to allow the lithium-ion battery to reach a constant temperature; discharge at a constant current of 1 C to 2.0 V and let it stand for 30 minutes; charge at a constant current of 1 C to 3.65 V, charge at a constant voltage to a current of 0.05 C, and then discharge at a constant current of 1 C for 30 min; transfer the lithium-ion battery to a -20 ℃ constant temperature box and let it stand for 120 minutes to allow the lithium-ion battery to reach a constant temperature; then discharge at 0.36 C for 30 s, and record the initial discharge voltage U1 and the end voltage U2 of this step. -20 ℃ low temperature DCR=(U1-U2) / 0.36C.

[0070] -40 ℃ low temperature discharge capacity retention test: Place the lithium-ion battery in a 25 ℃ constant temperature box and let it stand for 30 minutes to allow the lithium-ion battery to reach a constant temperature; discharge it to 2.0 V at a constant current of 1 C and let it stand for 30 minutes; charge it to 3.65 V at a constant current of 1 C, charge it at a constant voltage until the current is 0.05 C, and then discharge it to 2.0 V at a constant current of 0.5 C. Record the discharge capacity of this step as C1. After standing for 30 minutes, charge the battery to 3.65 V at a constant current of 1 C, and charge it at a constant voltage until the current is 0.05 C; then transfer the lithium-ion battery to a -40 ℃ constant temperature box and let it stand for 120 minutes to allow the lithium-ion battery to reach a constant temperature; then discharge it to 2.0 V at a constant current of 0.5 C, and record the discharge capacity of this step as C2. -40 ℃ low temperature discharge capacity retention = C2 / C1×100%.

[0071] 0 ℃ low temperature cycle performance test: Place the lithium-ion battery in a 45 ℃ constant temperature box and let it stand for 60 minutes to allow the lithium-ion battery to reach a constant temperature; discharge it at a constant current of 1 C to 2.0 V and let it stand for 30 minutes; charge it at a constant current of 0.2 C to 3.65 V, and charge it at a constant voltage to a current of 0.05 C; let it stand for 30 minutes; then discharge it at 0.2 C to 2.0 V. The capacity at this step is used as the benchmark and recorded as the first cycle discharge capacity Q 初始 This step is repeated for 1000 cycles, and the discharge capacity Q of the 1000th cycle is recorded. 1000圈 , calculate the capacity retention rate, the capacity retention rate after 1000 cycles = Q 1000圈 / Q 初始 ×100%.

[0072] The above performance test results are shown in Table 1 below: Table 1

[0073] As shown in Table 1, compared with the lithium battery prepared in Comparative Example 1 which cannot operate normally under low temperature conditions, Examples 1-9 can effectively improve the electrochemical performance of the battery under low temperature conditions by introducing TMSiFB or TESiFB into the first electrolyte, so that the battery has a relatively low DCR at a low temperature of -20 °C, and can still maintain a high discharge capacity at a lower temperature (e.g., -40 °C), and exhibits better low-temperature cycle performance; among them, TMSiFB has a more suitable molecular structure size than TESiFB, which is more conducive to regulating Li + solvation structure, thereby effectively inhibiting co-embedding.

[0074] It can be seen from Example 1 and Comparative Example 2 that compared with hexafluorobenzene, the performance of the lithium battery containing TMSiFB in the electrolyte is better. Not only is the DCR smaller at low temperature, but the capacity at -40 °C and the capacity retention rate after 1000 cycles at 0 °C are significantly improved compared with Comparative Example 2. This is because the appropriate molecular structure size of TMSiFB can more effectively adjust the Li + At the same time, hexafluorobenzene in the electrolyte will produce HF as a byproduct during the battery cycle, thereby deteriorating the battery performance. TMSiFB does not produce HF during the battery cycle, so it is more conducive to the cycle stability of the battery at low temperatures.

[0075] It can be seen from Examples 1-7 that the content of TMSiFB in the first electrolyte affects the low temperature performance of the lithium-ion battery, wherein the DCR of the battery at low temperature decreases with the increase of the TMSiFB content, and the discharge capacity retention rate and cycle capacity retention rate of the battery at low temperature both increase with the increase of the TMSiFB content, and the overall trend of first increasing and then decreasing is presented. This is because the higher the TMSiFB content in the electrolyte, the more effectively it can inhibit the co-embedding problem of propylene carbonate in the SEI film formation process, forming a thin and stable SEI film, which is beneficial to reducing the internal resistance of the battery, and the high content of TMSiFB is more conducive to reducing the surface tension of the electrolyte and improving the kinetic performance of the battery. Therefore, the DCR value of the battery shows an overall downward trend with the increase of the TMSiFB content in the electrolyte; however, with the increase of the TMSiFB content in the electrolyte, the side reactions related to the benzene ring in the cycle process increase, increasing the risk of self-discharge, reducing the reversible capacity retention rate, and deteriorating the cycle life. Therefore, in order to make the lithium battery have low DCR at low temperature while having high discharge capacity retention rate and excellent cycle performance, preferably, the mass proportion of TMSiFB in the first electrolyte is controlled in the range of 0.5%-8%, more preferably in the range of 0.5%-5%, so that the capacity retention rate of the lithium-ion battery after 1000 cycles of charge and discharge at 0°C can still reach more than 80%.

[0076] In addition, it can be seen from Examples 1 and 8 that, compared with the lithium-ion battery (Example 8) prepared by using the first electrolyte and the second electrolyte with the same lithium salt concentration for secondary injection, the lithium-ion battery (Example 1) prepared by using the first electrolyte with a higher lithium salt concentration for primary injection and the second electrolyte with a low lithium salt concentration for secondary injection has a lower DCR under low temperature conditions, and can maintain a higher discharge capacity at low temperatures, and exhibits better low-temperature cycle performance. This is because the solvation structure formed by the high-concentration lithium salt is more conducive to inhibiting the co-embedding of propylene carbonate in the negative electrode, thereby forming a thin and stable SEI film.

[0077] The above-described embodiments are only preferred embodiments for fully illustrating the present invention, and the protection scope of the present invention is not limited thereto. Equivalent substitutions or changes made by those skilled in the art based on the present invention are within the protection scope of the present invention. The protection scope of the present invention shall be subject to the claims.

Claims

1. An electrolyte, characterized in that: The electrolyte includes a first electrolyte and a second electrolyte; The first electrolyte comprises a first lithium salt, a first organic solvent and an additive, the additive is 1-fluoro-4-(trialkylsilyl)benzene, and the first organic solvent comprises propylene carbonate; The second electrolyte includes a second lithium salt and a second organic solvent.

2. The electrolyte according to claim 1, characterized in that The 1-fluoro-4-(trialkylsilyl)benzene has the following general structural formula: ， Wherein, R is a C1-C5 alkyl group.

3. The electrolyte according to claim 1 or 2, characterized in that The mass percentage of the additive in the first electrolyte is 0.5%-8%.

4. The electrolyte according to claim 1, characterized in that The mass percentage of the first lithium salt in the first electrolyte is greater than the mass percentage of the second lithium salt in the second electrolyte; The mass percentage of the first lithium salt in the first electrolyte is 15%-20%; The mass percentage of the second lithium salt in the second electrolyte is 5%-12.5%; The first lithium salt and the second lithium salt are respectively selected from one or more of lithium bis(trifluoromethanesulfonyl imide), lithium bis(fluorosulfonyl imide), lithium acetate, lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium trifluoroacetate, lithium fluoroalkyl phosphate, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate and lithium fluoride.

5. The electrolyte according to claim 1, characterized in that The first organic solvent further comprises one or more of other cyclic carbonate solvents and chain carbonate solvents; the second organic solvent comprises one or more of cyclic carbonate solvents and chain carbonate solvents; The other cyclic carbonate solvents include one or two of ethylene carbonate and butylene carbonate; The linear carbonate solvent comprises one or more of dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate; The cyclic carbonate solvent includes one or more of propylene carbonate, ethylene carbonate and butylene carbonate.

6. The electrolyte according to claim 1 or 5, characterized in that The first organic solvent and the second organic solvent both include a cyclic carbonate solvent and a chain carbonate solvent; The mass ratio of the cyclic carbonate solvent to the chain carbonate solvent in the first organic solvent is (0.2-2):1; The mass ratio of the cyclic carbonate solvent to the chain carbonate solvent in the second organic solvent is (0.2-2):

1.

7. The electrolyte according to claim 1, characterized in that The first electrolyte and the second electrolyte both contain other additives, and the other additives are selected from one or more of vinylene carbonate, vinylene carbonate, 1,3-propane sultone, vinyl sulfate, propylene sultone, methylene disulfonate, tris(trimethylsilyl)borate, tris(trimethylsilyl)phosphate, adiponitrile, maleic anhydride, triallyl isocyanate and trimethylsilyl diethylamine.

8. A secondary liquid injection formation method, characterized in that: The electrolyte according to any one of claims 1 to 7 is used for secondary liquid injection formation, and the method comprises the following steps: S1, injecting the first electrolyte into the battery, sealing it, and letting it stand to obtain a primary injection battery; S2, forming the primary injection battery; S3, injecting the second electrolyte into the formed battery, sealing it, and completing the secondary electrolyte injection formation process.

9. The secondary liquid injection formation method according to claim 8, characterized in that: In S1, the standing temperature is 30-60°C; The injection amount of the first electrolyte is m1, and the injection amount of the second electrolyte is m2, satisfying 1≤m1 / m2≤19.

10. A lithium ion battery, characterized in that: The electrolyte comprises the electrolyte described in any one of claims 1 to 7 or is prepared by the secondary liquid injection formation method described in claim 8 or 9.

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