Low-temperature high-voltage lithium ion battery electrolyte and preparation method thereof

Abstract

This invention discloses a low-temperature, high-voltage lithium-ion battery electrolyte, comprising a lithium salt, an organic solvent, a stabilizing agent, and a film-forming additive; the lithium salt includes sulfonylimide lithium salts and lithium borate first salts; the organic solvent includes cyclic carbonates, chain ester solvents, and fluorinated ether solvents; the stabilizing agent includes lithium borate second salts, and its mass accounts for 0.01% to 0.5% of the total mass of the electrolyte. The low-temperature, high-voltage lithium-ion battery electrolyte provided by this invention achieves Li + The dynamic reversible reconstruction of the solvation sheath significantly reduces the desolvation energy barrier and effectively inhibits the oxidative decomposition of organic solvents under high voltage. This is beneficial for the formation of a LiF-rich CEI / SEI film at the electrode interface, thereby improving the overall performance of the electrolyte under low temperature and high voltage conditions.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery material technology, specifically relating to a low-temperature high-voltage lithium-ion battery electrolyte and its preparation method. Background Technology

[0002] As electric vehicles and energy storage systems place increasingly higher demands on the energy density and environmental adaptability of lithium-ion batteries, the operating voltage of cathode materials is gradually increasing to above 4.5 V. Simultaneously, batteries need to maintain good discharge performance at ambient temperatures of -40°C or even lower. Traditional lithium-ion battery electrolytes exhibit a sharp decline in ionic conductivity at low temperatures and are prone to oxidative decomposition on the surface of high-voltage cathode materials, leading to rapid capacity decay and short cycle life. While existing technologies employ solutions to improve performance by introducing solvents with high dielectric constants or high-concentration lithium salts, these often sacrifice low-temperature fluidity or increase interfacial impedance, making it difficult to achieve synergistic compatibility between low temperatures and high voltages. Therefore, developing an electrolyte capable of maintaining high ionic conductivity at low temperatures while forming a stable interfacial film at high voltages is of significant practical importance. Summary of the Invention

[0003] To address the aforementioned technical problems, this invention provides a low-temperature, high-voltage lithium-ion battery electrolyte and its preparation method. The specific technical solution is as follows: A low-temperature, high-voltage lithium-ion battery electrolyte includes lithium salt, organic solvent, stabilizer, and film-forming additive; The lithium salts include sulfonylimide lithium salts and lithium first borate salts; The organic solvents include cyclic carbonates, chain ester solvents, and fluorinated ether solvents; The stabilizing agent includes a second lithium borate salt, and its mass accounts for 0.01% to 0.5% of the total mass of the electrolyte.

[0004] Preferably: The molar concentration of the sulfonamide lithium salt in the electrolyte is 1.0~2.0 mol / L; The molar concentration of the first lithium borate salt in the electrolyte is 0.2~0.5 mol / L.

[0005] Preferably: The sulfonylimide lithium salt is selected from at least one of lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide; One of the first lithium borate salt and the second lithium borate salt is lithium difluorooxalate borate, and the other is lithium dioxalate borate.

[0006] Preferably, the mass ratio of cyclic carbonates, chain esters, and fluorinated ethers in the organic solvent is (5~8):(7~10):(3~6); The cyclic carbonate is selected from at least one of fluoroethylene carbonate and ethylene carbonate; Chain ester solvents include ethyl methyl carbonate and ethyl propionate, with a mass ratio of 1:(1~3); The fluoroether solvent is 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0007] Preferably, the film-forming additive is selected from at least one of vinylene carbonate and 1,3-propanesulfonyl lactone, and its mass accounts for 0.5% to 2.5% of the total mass of the electrolyte.

[0008] Preferably, it further includes an interface-modifying monomer and a free radical initiator; The interface-modifying monomer is allyl 2,2,2-trifluoroethyl ether, which accounts for 0.5% to 2% of the total mass of the electrolyte. The free radical initiator is azobisisoheptanenitrile, which accounts for 0.01% to 0.03% of the total mass of the electrolyte.

[0009] Preferably, the electrolyte also includes a synergistic additive, which is a complex of tris(2,2,2-trifluoroethyl)borate and a phosphate ester compound, and its molar concentration in the electrolyte is 0.1~0.25 mol / L.

[0010] The present invention also provides a method for preparing a low-temperature high-voltage lithium-ion battery electrolyte as described in any one of the above claims, comprising the following steps: S1. At 0~15℃, dissolve lithium sulfonamide salts and lithium borate salts in a mixed solvent of cyclic carbonates and chain esters, and stir until homogeneous; S2. Add fluoroether solvent and lithium borate second salt at 15~30℃ and stir for 1~2 hours; S3. Add film-forming additives, mix well, and let stand to remove bubbles to obtain the final product.

[0011] Preferably, step S3 specifically includes the following sub-steps: S31. Add film-forming additives and interface-modifying monomers, stir and mix evenly, then heat to 45~55℃ at a heating rate of 0.4~0.6℃ / min. S32. Add the free radical initiator and stir for 2-4 hours to obtain the product.

[0012] Preferably, a synergistic additive is also added in step S2. The synergistic additive is prepared by the following steps: under an inert atmosphere, tris(2,2,2-trifluoroethyl)borate and triethyl phosphate are mixed evenly at a mass ratio of (1.1~1.7):1, and then stirred at 15~30°C for 1~2 hours to obtain the final product.

[0013] The low-temperature, high-voltage lithium-ion battery electrolyte provided by this invention achieves Li + The dynamic reversible reconstruction of the solvation sheath significantly reduces the desolvation energy barrier and effectively inhibits the oxidative decomposition of organic solvents under high voltage. This is beneficial for the formation of a LiF-rich CEI / SEI film at the electrode interface, thereby improving the overall performance of the electrolyte under low temperature and high voltage conditions. Detailed Implementation

[0014] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be described in detail below. The description in this part is only exemplary and explanatory, and should not be used to limit the scope of protection of the present invention in any way.

[0015] This embodiment provides a low-temperature, high-voltage lithium-ion battery electrolyte, comprising lithium salt, organic solvent, stabilizer, and film-forming additive.

[0016] Lithium salts include sulfonylimide lithium salts and lithium first borate salts.

[0017] Organic solvents include cyclic carbonates, chain esters, and fluorinated ethers.

[0018] The stabilizing agent includes lithium borate second salt, and its mass accounts for 0.01% to 0.5% of the total mass of the electrolyte.

[0019] Specifically, the stabilizing agent is a second lithium borate salt. Its boron center constructs an acid-radical double buffer network through Lewis acid-base coordination, which can effectively capture trace amounts of acid / HF and free radical precursors in the system, thereby preventing the main lithium salt from undergoing decomposition chain reactions and the ring-opening side reactions of organic solvents. At the same time, it acts as an interface enhancer to assist the film formation process.

[0020] The second lithium borate salt accounts for 0.01% to 0.5% of the total mass of the electrolyte, and its contribution is independent of the molar concentration of the main lithium salt. This stabilizing agent can be replaced by other lithium salts or borate esters with similar weakly coordinated boron centers within a reasonable range, as long as its core functions as an acid-radical double buffer stabilizer and interface enhancer are maintained.

[0021] The low-temperature, high-voltage lithium-ion battery electrolyte provided in this embodiment achieves Li + The dynamic reversible reconstruction of the solvation sheath significantly reduces the desolvation energy barrier and effectively inhibits the oxidative decomposition of organic solvents under high voltage. This is beneficial for the formation of a LiF-rich CEI / SEI film at the electrode interface, thereby improving the overall performance of the electrolyte under low temperature and high voltage conditions.

[0022] Furthermore: The molar concentration of sulfonylimide lithium salts in the electrolyte is 1.0~2.0 mol / L.

[0023] The molar concentration of the first lithium borate salt in the electrolyte is 0.2~0.5 mol / L.

[0024] Among them, lithium sulfonamide salts, at a molar concentration of 1.0–2.0 mol / L in the electrolyte, serve as the main conductive salt, providing a high ionic conductivity carrier. Meanwhile, lithium borate salts, at a lower molar concentration of 0.2–0.4 mol / L, participate in the construction of the solvated outer sheath through their weakly coordinating anions, forming a tightly-sliding, dynamically reversible solvation structure with the cyclic carbonate of the inner sheath, thereby effectively reducing Li... + Energy barriers in the desolvation process.

[0025] The molar concentration ranges of sulfonylimide lithium salts and lithium borate first salts can be reasonably replaced by similar high-concentration and low-concentration ranges, as long as the concentration gradient relationship of sulfonylimide lithium salt as the main salt and lithium borate first salt as the auxiliary salt is maintained, so as to maintain the internal tightness and external sliding characteristics of the solvated sheath.

[0026] This concentration gradient configuration allows the electrolyte to maintain a high ion transport number at low temperatures, while significantly reducing the interfacial impedance at high voltages. This is beneficial for improving the capacity retention rate of the full cell during low-temperature high-rate discharge and the capacity retention capacity during high-voltage cycling.

[0027] Furthermore: The sulfonylimide lithium salt is selected from at least one of lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide.

[0028] One of the first and second lithium borate salts is lithium difluorooxalate borate, and the other is lithium dioxalate borate.

[0029] Specifically, sulfonylimide lithium salts are selected from at least one of lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide, which can provide high thermal stability and ionic conductivity; while lithium difluorooxalateborate combines the film-forming properties of LiF and the stability of oxalic acid groups, playing a dominant role in low-temperature performance; lithium dioxalateborate has a very strong ability to form a film (CEI) on the positive electrode surface and has excellent acid suppression (HF) effect.

[0030] Beneficially, this combination of lithium salts synergistically constructs a dynamically reversible solvated sheath layer, effectively suppressing the sharp decline in ionic conductivity at low temperatures, while significantly reducing the tendency of organic solvents to oxidize and decompose at high voltages, which is beneficial to extending the cycle life of the battery under low temperature and high voltage conditions.

[0031] Furthermore, in the organic solvent, the mass ratio of cyclic carbonates, chain esters and fluorinated ethers is (5~8):(7~10):(3~6).

[0032] The cyclic carbonate is selected from at least one of fluoroethylene carbonate and ethylene carbonate.

[0033] Chain ester solvents include methyl ethyl carbonate and ethyl propionate, with a mass ratio of 1:(1~3).

[0034] The fluoroether solvent is 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0035] Among them, cyclic carbonates provide a higher dielectric constant for the formation of Li. + The inner coordination sheath layer and the chain ester solvents ensure the low viscosity and good flowability of the system, while the fluorinated ether solvents (especially 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether) can form a sliding outer sheath due to their low melting point and weakly coordinated oxygen atoms, thus jointly achieving the dynamic reversible reconstruction of the solvated sheath layer.

[0036] Furthermore, the film-forming additive is selected from at least one of vinylene carbonate and 1,3-propanesulfonyl lactone, and its mass accounts for 0.5% to 2.5% of the total mass of the electrolyte. It undergoes a reduction or oxidative polymerization reaction at the electrode interface to form a dense CEI / SEI film rich in inorganic components, thereby effectively passivating the active sites on the positive and negative electrode surfaces.

[0037] The synergistic effect of film-forming additives and stabilizing agents further enhances the LiF enrichment of the interfacial film, significantly reduces interfacial impedance, and effectively inhibits the continuous decomposition of organic solvents under high voltage, which is beneficial to improving the capacity retention of the electrolyte during high-voltage cycling.

[0038] Furthermore, it also includes interface-modifying monomers and free radical initiators.

[0039] The interface-modifying monomer is allyl 2,2,2-trifluoroethyl ether, which accounts for 0.5% to 2% of the total mass of the electrolyte. The free radical initiator is azobisisoheptanenitrile, which accounts for 0.01% to 0.03% of the total mass of the electrolyte.

[0040] In this process, the interface-modified monomer and the free radical initiator undergo a selective free radical copolymerization reaction under mild conditions. Driven by the electric field and temperature gradient at the electrode interface, a fluorinated-carbonate hybrid gradient shell is formed, which further modifies the outer structure of the solvated sheath.

[0041] Furthermore, it also includes a synergistic additive, which is a complex of tri(2,2,2-trifluoroethyl)borate and phosphate ester compounds, with a molar concentration of 0.1~0.25 mol / L in the electrolyte.

[0042] Specifically, the synergistic additive undergoes controlled micro-decomposition under high pressure or overcharge conditions, releasing trace amounts of Lewis acid. This acid is neutralized and passivated in situ by the buffering effect of the stabilizing agent in the system, forming a composite CEI film. Simultaneously, it reduces local Li+ levels through an anion bridging network. + Concentration gradient.

[0043] This embodiment also provides a method for preparing a low-temperature high-voltage lithium-ion battery electrolyte as described in any of the above embodiments, comprising the following steps: S1. At 0~15℃, dissolve sulfonylimide lithium salt and lithium borate first salt in a mixed solvent of cyclic carbonate and chain ester solvent, and stir to mix evenly.

[0044] S2. Add fluoroether solvent and lithium borate second salt at 15~30℃ and stir for 1~2 hours.

[0045] S3. Add film-forming additives, mix well, and let stand to remove bubbles to obtain the final product.

[0046] The preparation method provided in this embodiment employs a multi-stage temperature-controlled gradient dissolution process to ensure the orderly coordination of lithium salt, solvent, and stabilizing agents, thus avoiding early HF formation and side reactions. The entire preparation process is preferably carried out in an argon-atmosphere glove box to strictly control moisture and oxygen content; simultaneously, HF content is monitored in real time at critical stages to ensure it remains below 10 ppm and the system remains clear and colorless.

[0047] This embodiment reveals that when the first lithium borate salt (LiDFOB) and the second lithium borate salt (LiBOB) are used in combination at specific stages, a significant synergistic effect is observed. In stage S1, LiDFOB constructs a low-impedance initial interfacial film rich in LiF; while the trace amount of LiBOB introduced in stage S2, due to its higher film-forming potential, preferentially undergoes a reduction reaction at weak points in the interface, thus playing a role in interfacial repair.

[0048] Furthermore, step S3 specifically includes the following sub-steps: S31. Add film-forming additives and interface-modifying monomers, stir and mix evenly, then heat to 45~55℃ at a heating rate of 0.4~0.6℃ / min.

[0049] S32. Add the free radical initiator and stir for 2-4 hours to obtain the product.

[0050] Specifically, this step, through controlled temperature rise and stirring, facilitates the selective copolymerization of film-forming additives and interface-modifying monomers with low activation energy under the Lewis acid-base coordination assistance of an excess of lithium second borate salt, forming a fluorinated-carbonate hybrid gradient shell. The entire process strictly controls the temperature to not exceed 55°C, and combines this with real-time monitoring of HF content (ensuring <10ppm) and maintaining the system in a clear, colorless state to avoid side reactions.

[0051] Furthermore, a synergistic additive is added in step S2. This synergistic additive is prepared by mixing tris(2,2,2-trifluoroethyl)borate and triethyl phosphate at a mass ratio of (1.1~1.7):1 under an inert atmosphere, followed by stirring at 15~30°C for 1~2 hours. After the pre-complex is added in step S2, it participates in the decomposition-buffering-passivation dynamic equilibrium network together with the second lithium borate salt. Under high pressure or overcharge, trace amounts of Lewis acid are released and immediately neutralized, forming a ceramic-like composite CEI membrane, while simultaneously constructing an anion bridging network.

[0052] Specific embodiments are provided below. These embodiments are intended to enable those skilled in the art to more fully understand the present invention, but do not limit the present invention in any way. Example 1

[0053] Take 30 g of fluoroethylene carbonate, 10 g of methyl ethyl carbonate, and 30 g of ethyl propionate, and place them in an argon-atmospheric glove box to cool to 5°C. Slowly add 36.5 g of lithium bis(fluorosulfonyl)imide and 6.8 g of lithium difluorooxalate borate while stirring, and continue stirring for 90 minutes until the solids are completely dissolved and the solution is clear.

[0054] The system was heated to 20°C, and 25 g of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 0.15 g of lithium bis(oxalato)borate were added. The mixture was stirred for another 90 minutes.

[0055] Then add 1.5 g of vinylene carbonate, stir for 30 minutes to mix thoroughly, and allow to stand to remove bubbles. Throughout the process, continuously monitor the HF content to ensure it is below 8 ppm and the system remains clear and colorless. A clear, colorless electrolyte is obtained with a water content below 5 ppm.

[0056] The obtained electrolyte was assembled into a pouch cell (the positive electrode used high-voltage NCM811 material, and the negative electrode used graphite), and performance tests were conducted: the ionic conductivity at 25℃ and -40℃ was measured using a conductivity meter; discharge tests were performed at 0.5C rate at -40℃ and -50℃, and the capacity retention rate (relative to the capacity at 25℃) was calculated; the capacity retention rate (relative to the initial discharge capacity) was recorded after 500 cycles of 1C constant current charge-discharge at a cutoff voltage of 4.5 V using a charge-discharge tester.

[0057] The performance test data is shown in Table 1 below: The electrolyte prepared in this embodiment exhibits high ionic conductivity and discharge capacity retention at low temperatures. After 500 cycles at a high voltage of 4.5 V, it still maintains good capacity, which fully demonstrates that the combination of sulfonylimide lithium salt, first lithium borate salt, second lithium borate salt (stabilizing agent), a specific ternary organic solvent system, and film-forming additives, along with a multi-stage temperature-controlled mixing process, can effectively achieve dynamic reconstruction of the solvated sheath layer, thereby significantly improving the low-temperature and high-voltage performance of the electrolyte. Example 2

[0058] Take 25 g of fluoroethylene carbonate, 15 g of methyl ethyl carbonate, and 30 g of ethyl propionate, and place them in an argon-atmospheric glove box to cool to 8°C. Slowly add 36.5 g of lithium bis(fluorosulfonyl)imide and 6.8 g of lithium difluorooxalate borate while stirring, and continue stirring for 90 minutes until the solids are completely dissolved and the solution is clear.

[0059] The system was heated to 22°C, and 25 g of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 0.15 g of lithium bis(oxalato)borate were added. The mixture was stirred for another 90 minutes.

[0060] Then add 1.5 g of vinylene carbonate, stir for 30 minutes to mix thoroughly, and allow to stand to remove bubbles. Throughout the process, continuously monitor the HF content to ensure it is below 8 ppm and the system remains clear and colorless. A clear, colorless electrolyte is obtained with a water content below 5 ppm.

[0061] The obtained electrolyte was assembled into a pouch cell (the positive electrode used high-voltage NCM811 material, and the negative electrode used graphite), and performance tests were conducted: the ionic conductivity at 25℃ and -40℃ was measured using a conductivity meter; discharge tests were performed at 0.5C rate at -40℃ and -50℃, and the capacity retention rate (relative to the capacity at 25℃) was calculated; the capacity retention rate (relative to the initial discharge capacity) was recorded after 500 cycles of 1C constant current charge-discharge at a cutoff voltage of 4.5 V using a charge-discharge tester.

[0062] The performance test data is shown in Table 2 below: This embodiment, by adjusting the mass ratio of fluoroethylene carbonate, methyl ethyl carbonate, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, achieved good low-temperature ionic conductivity, discharge capacity retention, and high voltage cycling stability while maintaining the lithium salt concentration and the amount of stabilizing agent. This indicates that the mass ratio range of the organic solvent is reasonable, and the electrolyte can still achieve the effect of dynamic reconstruction of the solvated sheath when varying within this range. Example 3

[0063] Take 30 g of fluoroethylene carbonate, 10 g of methyl ethyl carbonate, and 30 g of ethyl propionate, and place them in an argon-atmospheric glove box to cool to 6°C. Add 36.5 g of lithium bis(fluorosulfonyl)imide and 6.8 g of lithium difluorooxalate borate while stirring, and continue stirring for 90 minutes until the solids are completely dissolved and the solution is clear.

[0064] The system was heated to 21°C, and 25 g of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 0.4 g of lithium bis(oxalato)borate were slowly added. The mixture was stirred for another 90 minutes.

[0065] Then add 1.5 g of vinylene carbonate, stir for 30 minutes to mix thoroughly, and allow to stand to remove bubbles. Throughout the process, continuously monitor the HF content to ensure it is below 8 ppm and the system remains clear and colorless. A clear, colorless electrolyte is obtained with a water content below 5 ppm.

[0066] The obtained electrolyte was assembled into a pouch cell (the positive electrode used high-voltage NCM811 material, and the negative electrode used graphite), and performance tests were conducted: the ionic conductivity at 25℃ and -40℃ was measured using a conductivity meter; discharge tests were performed at 0.5C rate at -40℃ and -50℃, and the capacity retention rate (relative to the capacity at 25℃) was calculated; the capacity retention rate (relative to the initial discharge capacity) was recorded after 500 cycles of 1C constant current charge-discharge at a cutoff voltage of 4.5 V using a charge-discharge tester.

[0067] The performance test data is shown in Table 3 below: In this embodiment, the amount of the second lithium borate salt (stabilizing agent) is slightly increased to 0.4 wt% based on that in Example 1. The system remains clear and colorless with low HF content. The low-temperature discharge capacity retention and high-voltage cycling performance of the resulting electrolyte are similar to those in Example 1. This indicates that the amount of the second lithium borate salt in the range of 0.01% to 0.5% can effectively exert the acid-free radical double buffer and interface enhancement effect, further verifying the independent contribution and beneficial effect of the stabilizing agent. Example 4

[0068] Take 30 g of fluoroethylene carbonate, 10 g of methyl ethyl carbonate, and 30 g of ethyl propionate, and place them in an argon-atmospheric glove box to cool to 5°C. Slowly add 36.5 g of lithium bis(fluorosulfonyl)imide and 6.8 g of lithium difluorooxalate borate while stirring, and continue stirring for 90 minutes until the solids are completely dissolved and the solution is clear.

[0069] The system was heated to 21°C, and 25 g of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 0.15 g of lithium bis(oxalato)borate were added. The mixture was stirred for another 90 minutes.

[0070] Then, 1.5 g of vinylene carbonate and 1.2 g of allyl 2,2,2-trifluoroethyl ether were added, and the mixture was stirred for 30 minutes to ensure homogeneity. The temperature was slowly increased to 51°C at a rate of 0.5°C / min, and 0.02 g of azobisisobutyronitrile was added, with stirring continued for 3 hours. Throughout the process, the HF content was continuously monitored to ensure that the HF was below 9 ppm and that the system remained clear and colorless. After cooling to room temperature, the solution was allowed to stand and degas, yielding a clear and colorless electrolyte with a water content below 5 ppm.

[0071] The obtained electrolyte was assembled into a pouch cell (the positive electrode used high-voltage NCM811 material, and the negative electrode used graphite), and performance tests were conducted: the ionic conductivity at 25℃ and -40℃ was measured using a conductivity meter; discharge tests were performed at 0.5C rate at -40℃ and -50℃, and the capacity retention rate (relative to the capacity at 25℃) was calculated; the capacity retention rate (relative to the initial discharge capacity) was recorded after 500 cycles of 1C constant current charge-discharge at a cutoff voltage of 4.5 V using a charge-discharge tester.

[0072] The performance test data is shown in Table 4 below: This embodiment introduces allyl 2,2,2-trifluoroethyl ether and azobisisoheptanenitrile based on Example 1, and stirs the mixture under mild heating conditions. The resulting electrolyte exhibits further improved low-temperature ionic conductivity and discharge capacity retention, as well as higher high-voltage cycling capacity retention. This indicates that the interface-modified monomer, free radical initiator, and corresponding preparation steps can effectively reduce the desolvation energy barrier and improve oxidative stability, demonstrating a significant synergistic effect. Example 5

[0073] In an argon-atmospheric glove box, 4.2 g of tri(2,2,2-trifluoroethyl)borate and 3.0 g of triethyl phosphate were stirred at 15-25°C for 2 hours to obtain a clear boron-phosphorus complex.

[0074] Take 30 g of fluoroethylene carbonate, 10 g of methyl ethyl carbonate, and 30 g of ethyl propionate, and place them in an argon-atmospheric glove box to cool to 6°C. Add 36.5 g of lithium bis(fluorosulfonyl)imide and 6.8 g of lithium difluorooxalate borate while stirring, and continue stirring for 90 minutes until the solids are completely dissolved and the solution is clear.

[0075] The system was heated to 20°C, and 25 g of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 0.15 g of lithium bis(oxalato)borate, and 7.2 g of the boron-phosphorus complex prepared above were added. The mixture was stirred for another 90 minutes.

[0076] Then add 1.5 g of vinylene carbonate, stir for 30 minutes to mix thoroughly, and allow to stand to remove bubbles. Throughout the process, continuously monitor the HF content to ensure it is below 8 ppm and the system remains clear and colorless. A clear, colorless electrolyte is obtained with a water content below 5 ppm.

[0077] The obtained electrolyte was assembled into a pouch cell (the positive electrode used high-voltage NCM811 material, and the negative electrode used graphite), and performance tests were conducted: the ionic conductivity at 25℃ and -40℃ was measured using a conductivity meter; discharge tests were performed at 0.5C rate at -40℃ and -50℃, and the capacity retention rate (relative to the capacity at 25℃) was calculated; the capacity retention rate (relative to the initial discharge capacity) was recorded after 500 cycles of 1C constant current charge-discharge at a cutoff voltage of 4.5 V using a charge-discharge tester.

[0078] The performance test data is shown in Table 5 below: This embodiment introduces a pre-complex of tris(2,2,2-trifluoroethyl)borate and triethyl phosphate as a synergistic additive, based on Example 1. The resulting electrolyte not only maintains excellent low-temperature and high-voltage performance, but also exhibits a higher discharge capacity retention rate at -50°C, indicating that the synergistic additive can effectively suppress low-temperature concentration polarization and lithium plating, and improve high-voltage withstand capability. Example 6

[0079] In an argon-atmospheric glove box, 4.8 g of tri(2,2,2-trifluoroethyl)borate and 3.4 g of triethyl phosphate were stirred at 18°C ​​for 2 hours to obtain a clear boron-phosphorus complex.

[0080] Take 28 g of fluoroethylene carbonate, 12 g of methyl ethyl carbonate, and 30 g of ethyl propionate, and cool them to 4°C in an argon-atmosphere glove box. Add 39.4 g of lithium bis(fluorosulfonyl)imide and 7.9 g of lithium difluorooxalate borate while stirring, and continue stirring for 90 minutes until the solids are completely dissolved and the solution is clear.

[0081] The system was heated to 21°C, and 22 g of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 0.25 g of lithium bis(oxalato)borate, and 8.2 g of the boron-phosphorus complex prepared above were slowly added. The mixture was stirred for another 90 minutes.

[0082] Then, 1.8 g of vinylene carbonate and 1.5 g of allyl 2,2,2-trifluoroethyl ether were added, and the mixture was stirred for 30 minutes to ensure homogeneity. The temperature was slowly increased to 51°C at a rate of 0.5°C / min, and 0.025 g of azobisisobutyronitrile was added, with stirring continued for 3 hours. Throughout the process, the HF content was continuously monitored to ensure that the HF was below 7 ppm and that the system remained clear and colorless. After cooling to room temperature, the solution was allowed to stand and degas, yielding a clear and colorless electrolyte with a water content below 5 ppm.

[0083] The obtained electrolyte was assembled into a pouch cell (the positive electrode used high-voltage NCM811 material, and the negative electrode used graphite), and performance tests were conducted: the ionic conductivity at 25℃ and -40℃ was measured using a conductivity meter; discharge tests were performed at 0.5C rate at -40℃, -50℃, and -50℃, and the capacity retention rate (relative to the capacity at 25℃) was calculated; the capacity retention rate (relative to the initial discharge capacity) was recorded after 500 cycles of 1C constant current charge-discharge at a cutoff voltage of 4.5 V using a charge-discharge tester.

[0084] The performance test data is shown in Table 6 below: This embodiment employs optimized lithium salt concentration, organic solvent ratio, stabilizing agent dosage, a mild copolymerization step involving interface-modified monomers and free radical initiators, and boron-phosphorus complex synergistic additives, through multi-stage temperature-controlled mixing and double buffering-catalysis. The resulting electrolyte maintains a high discharge capacity retention rate at -50℃, and the capacity retention rate significantly improves after 500 cycles at a high voltage of 4.5 V, fully demonstrating the breakthrough effect of low-temperature and high-voltage synergy.

[0085] Comparative Example 1 Take 30 g of fluoroethylene carbonate, 10 g of methyl ethyl carbonate, and 30 g of ethyl propionate, and place them in an argon-atmospheric glove box to cool to 5°C. Add 36.5 g of lithium bis(fluorosulfonyl)imide and 6.8 g of lithium difluorooxalate borate while stirring, and continue stirring for 90 minutes until the solids are completely dissolved and the solution is clear.

[0086] Heat the system to 21°C, add 25 g of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and continue stirring for 90 minutes.

[0087] Then, 1.5 g of vinylene carbonate was added, and the mixture was stirred for 30 minutes to ensure homogeneity. The mixture was then allowed to stand to remove air bubbles. The HF content was monitored throughout the process; eventually, the HF level rose to 15 ppm, and the system turned slightly yellow. A slightly yellow electrolyte was obtained, with a water content below 5 ppm.

[0088] The obtained electrolyte was assembled into a pouch cell (the positive electrode used high-voltage NCM811 material, and the negative electrode used graphite), and performance tests were conducted: the ionic conductivity at 25℃ and -40℃ was measured using a conductivity meter; discharge tests were performed at 0.5C rate at -40℃ and -50℃, and the capacity retention rate (relative to the capacity at 25℃) was calculated; the capacity retention rate (relative to the initial discharge capacity) was recorded after 500 cycles of 1C constant current charge-discharge at a cutoff voltage of 4.5 V using a charge-discharge tester.

[0089] The performance test data is shown in Table 7 below: The comparative example did not include a second lithium borate salt (stabilizing agent), resulting in discoloration of the system. The ionic conductivity and discharge capacity retention decreased significantly at low temperatures, and the capacity decay accelerated during high-voltage cycling, indicating that the second lithium borate salt plays a crucial role in achieving synergistic performance at low temperatures and high voltage.

[0090] Comparative Example 2 Take 30 g of fluoroethylene carbonate and 65 g of methyl ethyl carbonate, place them in an argon-atmospheric glove box and cool them to 5°C. Add 36.5 g of lithium bis(fluorosulfonyl)imide and 6.8 g of lithium difluorooxalate borate while stirring, and continue stirring for 90 minutes until the solids are completely dissolved and the solution is clear.

[0091] Heat the system to 21°C, add 0.15 g of lithium bis(oxalate-borate), and continue stirring for 90 minutes.

[0092] Then, 1.5 g of vinylene carbonate was added, and the mixture was stirred for 30 minutes to ensure homogeneity. The mixture was then allowed to stand to remove air bubbles. The HF content was monitored throughout the process, and the final HF level was 12 ppm. The system remained clear and colorless. A clear, colorless electrolyte was obtained, with a water content of less than 5 ppm.

[0093] The obtained electrolyte was assembled into a pouch cell (the positive electrode used high-voltage NCM811 material, and the negative electrode used graphite), and performance tests were conducted: the ionic conductivity at 25℃ and -40℃ was measured using a conductivity meter; discharge tests were performed at 0.5C rate at -40℃ and -50℃, and the capacity retention rate (relative to the capacity at 25℃) was calculated; the capacity retention rate (relative to the initial discharge capacity) was recorded after 500 cycles of 1C constant current charge-discharge at a cutoff voltage of 4.5 V using a charge-discharge tester.

[0094] The performance test data is shown in Table 8 below: No fluorinated ether solvents were added in this comparative example. The ionic conductivity and discharge capacity retention decreased significantly at low temperatures. Although the high-voltage cycling performance was better than Comparative Example 1, it was still significantly worse than the example, indicating that fluorinated ether solvents and their synergistic effect with cyclic carbonates and chain ester solvents played a crucial role in achieving breakthroughs in low-temperature performance.

[0095] Comparative Example 3 Take 33 g of ethylene carbonate, 33 g of dimethyl carbonate, and 34 g of methyl ethyl carbonate, place them in an argon atmosphere glove box and cool them to 10°C. Add 15.2 g of lithium hexafluorophosphate while stirring, and continue stirring for 120 minutes until the solids are completely dissolved and the solution is clear.

[0096] Then add 1.5 g of vinylene carbonate, stir for 30 minutes to mix evenly, allow to stand to remove bubbles, and keep the system clear and colorless. A clear, colorless electrolyte with a water content of less than 5 ppm is obtained.

[0097] The obtained electrolyte was assembled into a pouch cell (the positive electrode used high-voltage NCM811 material, and the negative electrode used graphite), and performance tests were conducted: the ionic conductivity at 25℃ and -40℃ was measured using a conductivity meter; discharge tests were performed at 0.5C rate at -40℃ and -50℃, and the capacity retention rate (relative to the capacity at 25℃) was calculated; the capacity retention rate (relative to the initial discharge capacity) was recorded after 500 cycles of 1C constant current charge-discharge at a cutoff voltage of 4.5 V using a charge-discharge tester.

[0098] The performance test data is shown in Table 9 below: This comparative example uses a conventional commercial electrolyte system and does not contain sulfonylimide lithium salts, lithium borate first salts, lithium borate second salts, or fluorinated ether solvents. At low temperatures, the ionic conductivity decreases sharply, and the discharge capacity retention is significantly worse than in the example. Capacity decay is most severe during high-voltage cycling, fully verifying the synergistic advantage of low-temperature and high-voltage cycling provided by this embodiment.

[0099] Comparative Example 4 Take 30 g of fluoroethylene carbonate, 10 g of methyl ethyl carbonate, and 30 g of ethyl propionate, and place them in an argon-atmospheric glove box to cool to 5°C. Slowly add 36.5 g of lithium bis(fluorosulfonyl)imide and 6.8 g of lithium difluorooxalate borate while stirring, and continue stirring for 90 minutes until the solids are completely dissolved and the solution is clear.

[0100] The system was heated to 21°C, and 25 g of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 0.15 g of lithium bis(oxalato)borate were added. The mixture was stirred for another 90 minutes.

[0101] Then, 1.5 g of vinylene carbonate and 1.2 g of allyl 2,2,2-trifluoroethyl ether were added, and the mixture was stirred for 30 minutes to ensure homogeneity. At 21°C, 0.02 g of azobisisoheptanenitrile was added, and stirring continued for 3 hours. The HF content was continuously monitored throughout the process, and the final HF concentration was 11 ppm. The system remained clear and colorless. After cooling to room temperature, the mixture was allowed to stand to remove bubbles, yielding the electrolyte.

[0102] The obtained electrolyte was assembled into a pouch cell (the positive electrode used high-voltage NCM811 material, and the negative electrode used graphite), and performance tests were conducted: the ionic conductivity at 25℃ and -40℃ was measured using a conductivity meter; discharge tests were performed at 0.5C rate at -40℃ and -50℃, and the capacity retention rate (relative to the capacity at 25℃) was calculated; the capacity retention rate (relative to the initial discharge capacity) was recorded after 500 cycles of 1C constant current charge-discharge at a cutoff voltage of 4.5 V using a charge-discharge tester.

[0103] The performance test data is shown in Table 10 below: Comparative Example 4, which contains the same type and proportion of interface-modified monomers and free radical initiators as Example 4, shows a significant decrease in capacity retention after 500 cycles at 4.5 V compared to Example 4 because it does not employ a gradient heating process of 0.4~0.6℃ / min.

[0104] Comparative Example 5 Take 30 g of fluoroethylene carbonate, 10 g of methyl ethyl carbonate, and 30 g of ethyl propionate, and place them in an argon-atmospheric glove box to cool to 5°C. Slowly add 36.5 g of lithium bis(fluorosulfonyl)imide and 6.8 g of lithium difluorooxalate borate while stirring, and continue stirring for 90 minutes until the solids are completely dissolved and the solution is clear.

[0105] The system was heated to 21°C, and 25 g of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 0.15 g of lithium bis(oxalato)borate were added. The mixture was stirred for another 90 minutes.

[0106] Then, 1.5 g of vinylene carbonate and 1.2 g of allyl 2,2,2-trifluoroethyl ether were added, and the mixture was stirred for 30 minutes to ensure homogeneity. The temperature was slowly increased to 51°C at a rate of 0.5°C / min, and stirring was maintained for 3 hours. The HF content was continuously monitored throughout the process, and the final HF content was 10 ppm, with the system remaining clear and colorless. After cooling to room temperature, the mixture was allowed to stand to remove bubbles, yielding the electrolyte.

[0107] The obtained electrolyte was assembled into a pouch cell (the positive electrode used high-voltage NCM811 material, and the negative electrode used graphite), and performance tests were conducted: the ionic conductivity at 25℃ and -40℃ was measured using a conductivity meter; discharge tests were performed at 0.5C rate at -40℃ and -50℃, and the capacity retention rate (relative to the capacity at 25℃) was calculated; the capacity retention rate (relative to the initial discharge capacity) was recorded after 500 cycles of 1C constant current charge-discharge at a cutoff voltage of 4.5 V using a charge-discharge tester.

[0108] The performance test data is shown in Table 11 below: Although this comparative example followed the same gradient heating process as Example 4, its 4.5 V cycle retention rate was not only lower than that of Example 4, but also lower than that of the comparative example without monomer, due to the absence of a free radical initiator. Furthermore, its discharge capacity retention rate at low temperatures also showed a significant decrease.

[0109] This article uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the technical scope disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be within the scope of protection of the present invention.

Claims

1. A low-temperature, high-voltage lithium-ion battery electrolyte, characterized in that, Including lithium salts, organic solvents, stabilizers, and film-forming additives; The lithium salts include sulfonylimide lithium salts and lithium first borate salts; The organic solvents include cyclic carbonates, chain ester solvents, and fluorinated ether solvents; The stabilizing agent includes a second lithium borate salt, and its mass accounts for 0.01% to 0.5% of the total mass of the electrolyte.

2. The low-temperature high-voltage lithium-ion battery electrolyte according to claim 1, characterized in that: The molar concentration of the sulfonamide lithium salt in the electrolyte is 1.0~2.0 mol / L; The molar concentration of the first lithium borate salt in the electrolyte is 0.2~0.5 mol / L.

3. The low-temperature high-voltage lithium-ion battery electrolyte according to claim 1 or 2, characterized in that: The sulfonylimide lithium salt is selected from at least one of lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide; One of the first lithium borate salt and the second lithium borate salt is lithium difluorooxalate borate, and the other is lithium dioxalate borate.

4. The low-temperature high-voltage lithium-ion battery electrolyte according to claim 1, characterized in that, In the organic solvent, the mass ratio of cyclic carbonate, chain ester solvent and fluorinated ether solvent is (5~8):(7~10):(3~6); The cyclic carbonate is selected from at least one of fluoroethylene carbonate and ethylene carbonate; Chain ester solvents include ethyl methyl carbonate and ethyl propionate, with a mass ratio of 1:(1~3); The fluoroether solvent is 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

5. The low-temperature high-voltage lithium-ion battery electrolyte according to claim 1, characterized in that, The film-forming additive is selected from at least one of vinylene carbonate and 1,3-propanesulfonyl lactone, and its mass accounts for 0.5% to 2.5% of the total mass of the electrolyte.

6. The low-temperature high-voltage lithium-ion battery electrolyte according to claim 1, characterized in that, It also includes interface-modifying monomers and free radical initiators; The interface-modifying monomer is allyl 2,2,2-trifluoroethyl ether, which accounts for 0.5% to 2% of the total mass of the electrolyte. The free radical initiator is azobisisoheptanenitrile, which accounts for 0.01% to 0.03% of the total mass of the electrolyte.

7. The low-temperature high-voltage lithium-ion battery electrolyte according to claim 1, characterized in that, It also includes a synergistic additive, which is a complex of tris(2,2,2-trifluoroethyl)borate and a phosphate ester compound, with a molar concentration of 0.1~0.25 mol / L in the electrolyte.

8. A method for preparing a low-temperature, high-voltage lithium-ion battery electrolyte as described in any one of claims 1 to 8, characterized in that, Includes the following steps: S1. At 0~15℃, dissolve lithium sulfonamide salts and lithium borate salts in a mixed solvent of cyclic carbonates and chain esters, and stir until homogeneous; S2. Add fluoroether solvent and lithium borate second salt at 15~30℃ and stir for 1~2 hours; S3. Add film-forming additives, mix well, and let stand to remove bubbles to obtain the final product.

9. The preparation method according to claim 8, characterized in that, Step S3 specifically includes the following sub-steps: S31. Add film-forming additives and interface-modifying monomers, stir and mix evenly, then heat to 45~55℃ at a heating rate of 0.4~0.6℃ / min. S32. Add the free radical initiator and stir for 2-4 hours to obtain the product.

10. The preparation method according to claim 8, characterized in that, In step S2, a synergistic additive is also added. The synergistic additive is prepared by the following steps: under an inert atmosphere, tris(2,2,2-trifluoroethyl)borate and triethyl phosphate are mixed evenly at a mass ratio of (1.1~1.7):1, and then stirred at 15~30°C for 1~2 hours to obtain the final product.

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