High-stability low-temperature lithium ion battery electrolyte, preparation method thereof and lithium ion battery

Abstract

This invention discloses a high-stability, low-temperature lithium-ion battery electrolyte, its preparation method, and a lithium-ion battery. The high-stability, low-temperature lithium-ion battery electrolyte of this invention comprises a solvent system and composite additives. By mass percentage, the solvent system comprises 94%–96% and the composite additives comprise 4%–6% of the liquid system. The solvent system includes ethylene carbonate, dimethyl sulfite, and carboxylic acid esters in a mass ratio of 35–45:30–40:10–30. The composite additives include siloxane additives and fluoroethylene carbonate in a mass ratio of 2–3:2–3. This invention, through the synergistic design of the solvent system, endows the electrolyte with excellent low-temperature ion conductivity. Simultaneously, by utilizing the synergistic effect of the siloxane composite additives and film-forming additives, it significantly improves the stability of the electrode / electrolyte interface, enabling the lithium-ion battery to exhibit good charge-discharge performance and cycle stability in the low-temperature range of -40℃ to 25℃.

Description

Technical Field

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

[0002] Lithium-ion batteries are widely used in consumer electronics, electric vehicles, and energy storage systems due to their high energy density, long cycle life, and lack of memory effect. However, in low-temperature environments (such as below -20°C), the viscosity of the electrolyte in lithium-ion batteries increases significantly, the ionic conductivity decreases sharply, and the charge transfer resistance at the electrode / electrolyte interface rises drastically. This leads to severe capacity decay, reduced charging and discharging efficiency, and even safety hazards such as lithium dendrite precipitation, severely limiting the application of lithium-ion batteries in low-temperature scenarios such as cold regions, high-altitude operations, and polar exploration.

[0003] To improve the low-temperature performance of lithium-ion batteries, existing technologies typically focus on two aspects: solvent system optimization and additive modification. Regarding solvent systems, cyclic carbonates (such as ethylene carbonate) have high dielectric constants and can effectively dissolve lithium salts, but their high melting point (around 36°C) makes them prone to crystallization at low temperatures, leading to poor electrolyte fluidity. Chain carbonates (such as dimethyl carbonate and ethyl methyl carbonate) have lower melting points, but their low dielectric constants limit lithium salt solubility. Sulfite solvents are considered potential low-temperature solvent components due to their low melting point and viscosity, and good solubility for lithium salts. However, sulfite solvents have poor chemical stability and are prone to decomposition at the electrode interface, leading to decreased interfacial stability.

[0004] Regarding additives, fluoroethylene carbonate, as a commonly used film-forming additive, can form a stable SEI film on the negative electrode surface, reducing electrolyte decomposition. However, films formed by fluoroethylene carbonate alone lack flexibility at low temperatures and are prone to cracking due to volume expansion and contraction. Siloxane compounds, due to their good chemical stability and low surface energy, have been explored as electrolyte additives to improve interfacial properties. However, current technologies mostly employ single siloxane additives, and their synergistic effect in improving interfacial stability and adapting to low-temperature performance still needs improvement. Furthermore, there is limited research on the compatibility of siloxane additives with low-temperature solvents such as sulfites.

[0005] Therefore, developing a lithium-ion battery electrolyte that combines excellent low-temperature ion conduction performance and high interface stability is of great significance for expanding the application scenarios of lithium-ion batteries.

[0006] While existing technologies clearly define the advantages of various solvents, the lack of a proper formulation logic prevents the full realization of the synergistic effect of solvent blending. From the perspective of individual solvent characteristics, cyclic carbonates (such as ethylene carbonate) are key to lithium salt dissolution—their dielectric constant is as high as 89.6, enabling efficient dissociation of lithium salts such as lithium hexafluorophosphate. However, their melting point is 36°C, and they easily crystallize at low temperatures, causing the electrolyte to lose fluidity. Chain carbonates (such as dimethyl carbonate and ethyl methyl carbonate) have low melting points (below -45°C), improving low-temperature fluidity, but their dielectric constant is only 2.8~4.5, and their lithium salt solubility is less than 0.8 mol / L, making them unsuitable for ion conduction when used alone. Sulfate ester solvents (such as dimethyl sulfite) are key candidates for low-temperature performance optimization—they have a melting point as low as -64℃, a viscosity of only 1.5 mPa·s at 25℃, and a solubility for lithium salts of over 1.2 mol / L. When blended with ethylene carbonate, the high dielectric constant of ethylene carbonate can compensate for the insufficient dissociation ability of lithium salts in sulfite, while the low melting point and low viscosity of sulfite can inhibit the low-temperature crystallization of ethylene carbonate. Theoretically, this can significantly improve the low-temperature fluidity and ionic conductivity of the electrolyte. Carboxylic acid ester solvents (such as ethyl propionate) have a low melting point (-73℃) and moderate molecular polarity, which can further reduce the eutectic point of the solvent system and enhance the low-temperature homogeneous stability.

[0007] However, existing technologies have fatal flaws in solvent ratio and additive compatibility: First, the solvent ratio is unbalanced. Most solutions simply mix two types of solvents (such as ethylene carbonate + chain carbonate, ethylene carbonate + sulfite), without introducing carboxylic acid esters to form a ternary synergistic system. Moreover, the ratio of sulfite to ethylene carbonate is mostly below 20% or above 50%. If the ratio is too low, the improvement in low-temperature fluidity is limited; if the ratio is too high, the insufficient chemical stability of sulfite makes it easy to decompose at the electrode interface, causing capacity decay. Second, the compatibility between additives and solvent systems is poor. When fluoroethylene carbonate is used alone to form a film, the SEI film lacks flexibility at low temperatures and cannot adapt to the interfacial characteristics of the sulfite system. Although a single siloxane additive can improve interfacial stability, it cannot form a synergistic effect of "solvent fluidity-interfacial stability" with the sulfite-ethylene carbonate blended solvent. As a result, the capacity retention rate of existing sulfite-based electrolytes after 50 cycles at -40℃ is generally below 60%, which is far from meeting practical needs.

[0008] Therefore, precisely controlling the blending ratio of ethylene carbonate, sulfite, and carboxylic acid esters to leverage the synergistic effect of "dissolution-flow-stability," and matching it with a suitable composite additive system, to develop an electrolyte with both excellent low-temperature ion conductivity and high interfacial stability, has become the key to overcoming the bottleneck of low-temperature applications of lithium-ion batteries. Summary of the Invention

[0009] To address the issues of low ion conductivity and poor interface stability in existing lithium-ion battery electrolytes at low temperatures, this invention provides a highly stable low-temperature lithium-ion battery electrolyte. By optimizing the solvent system composition and employing the synergistic effect of siloxane composite additives and film-forming additives, the low-temperature performance and interface stability of the electrolyte are simultaneously improved.

[0010] This invention first provides a high-stability low-temperature lithium-ion battery electrolyte, comprising a solvent system and composite additives, and further comprising lithium salt. By mass percentage, in the liquid system composed of the solvent system and composite additives, the solvent system comprises 94%~96% and the composite additives comprise 4%~6%. The solvent system comprises ethylene carbonate, dimethyl sulfite and carboxylic acid ester, in a mass ratio of 35~45:30~40:10~30; The composite additive includes siloxane additives and fluoroethylene carbonate in a mass ratio of 2~3:2~3.

[0011] Preferably, the carboxylic acid ester is at least one selected from ethyl acetate, ethyl propionate, methyl butyrate, and ethyl butyrate. Ethyl propionate is most preferred, as it has a suitable dielectric constant and a low melting point (-73°C), which further improves the low-temperature fluidity of the solvent system.

[0012] Preferably, the siloxane additive comprises trimethylmethoxysilane, dimethyldimethoxysilane and methyltrimethoxysilane in a mass ratio of 1:1:1.

[0013] Preferably, the lithium salt is lithium hexafluorophosphate, and the concentration of the lithium salt in the solvent system is 1 mol / L.

[0014] This invention further provides a method for preparing the aforementioned high-stability low-temperature lithium-ion battery electrolyte, comprising the following steps: (1) The solvent system is obtained by uniformly mixing ethylene carbonate, dimethyl sulfite and carboxylic acid ester under an inert atmosphere; (2) Dissolve the lithium salt in the solvent system; (3) Add the composite additive and mix evenly to obtain the high-stability low-temperature lithium-ion battery electrolyte.

[0015] Preferably, the inert atmosphere is an argon atmosphere or a nitrogen atmosphere.

[0016] The present invention also provides a lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that the electrolyte is the high-stability low-temperature lithium-ion battery electrolyte.

[0017] Preferably, the positive electrode is made of lithium iron phosphate or ternary lithium material, the negative electrode is made of graphite, silicon-based negative electrode or lithium titanate, and the separator is made of polyethylene, polypropylene or composite separator.

[0018] Key to this invention: 1. Synergistic formulation design of ternary solvent systems: Key components: a specific range of proportions of 35%~45% cyclic carbonates + 30%~40% chain sulfites + 10%~30% carboxylic acid esters.

[0019] Technical logic: Cyclic carbonates provide high dielectric constant to ensure lithium salt dissolution, dimethyl sulfite improves low-temperature fluidity with low melting point / low viscosity, and carboxylic acid esters lower the eutectic point to enhance homogeneous stability. The three form a "dissolution-flow-stability" triangular synergy, breaking through the bottleneck of existing binary solvents where "dissolution capacity and low-temperature fluidity cannot be taken into account".

[0020] 2. Synergistic film-forming system of siloxane tricomponent compound + fluoroethylene carbonate: Key components: 2%~3% siloxane complex (trimethylmethoxysilane: dimethyldimethoxysilane: methyltrimethoxysilane = 1:1:1, 2%~3% fluoroethylene carbonate).

[0021] Technical logic: Fluorinated ethylene carbonate preferentially forms a fluorinated basic SEI film. The three siloxane components synergistically modify the SEI film through "low surface energy adsorption (trimethylmethoxysilane) + flexible chain buffer (dimethyldimethoxysilane) + multifunctional group bonding (methyltrimethoxysilane)," thus solving the defects of single additives such as "easy breakage of SEI film at low temperature and easy decomposition of solvent."

[0022] 3. Systematic adaptation design of solvent-additive combination: Key element: directional compatibility between sulfite-based solvents (dimethyl sulfite) and siloxane compound additives.

[0023] Technical Logic: To address the inherent defect of insufficient chemical stability of dimethyl sulfite, a siloxane modification layer is used to suppress its oxidative decomposition at the electrode interface. At the same time, the low viscosity of dimethyl sulfite ensures the rapid adsorption of siloxane molecules at the interface, forming a positive synergy of "low-temperature fluidity-interface stability".

[0024] This invention endows the electrolyte with excellent low-temperature ion conduction performance through the synergistic design of the solvent system. At the same time, it utilizes the synergistic effect of siloxane composite additives and film-forming additives to significantly improve the stability of the electrode / electrolyte interface, enabling the lithium-ion battery to have good charge-discharge performance and cycle stability in the low-temperature range of -40℃ to 25℃.

[0025] Excellent low-temperature performance: The solvent system of this invention adopts a synergistic ratio of 35%~45% ethylene carbonate, 30%~40% dimethyl sulfite, and 15%~25% carboxylic acid ester. Ethyl carbonate provides a high dielectric constant to ensure the full dissolution of lithium hexafluorophosphate. Dimethyl sulfite, as a chain sulfite, has an extremely low melting point (-64℃) and viscosity (approximately 1.5 mPa·s at 25℃), which can significantly reduce the freezing point and viscosity of the solvent system. The carboxylic acid ester further improves the low-temperature fluidity of the solvent system. The synergistic effect of these three components ensures that the viscosity of the electrolyte remains below 10 mPa·s at -40℃, and the ionic conductivity is higher than 1×10⁻⁶. -4 S / cm ensures rapid lithium-ion transport at low temperatures.

[0026] High interfacial stability: The 2%~3% siloxane complex system in the composite additive works synergistically with 2%~3% fluoroethylene carbonate. Fluoroethylene carbonate preferentially decomposes on the negative electrode surface to form a basic SEI film. Trimethylmethoxysilane, dimethyldimethoxysilane and methyltrimethoxysilane in the siloxane complex system form hydrogen bonds with the hydroxyl groups on the electrode surface through the methoxy groups in the molecules, and are directionally adsorbed on the SEI film surface and the electrode / electrolyte interface to form a dense siloxane modification layer. This modification layer can inhibit the decomposition of solvent molecules such as dimethyl sulfite at the interface. On the other hand, the Si-O backbone of the siloxane molecules has excellent flexibility and chemical stability, which can alleviate the SEI film rupture caused by electrode volume changes during low-temperature charge and discharge, and improve interfacial stability.

[0027] Excellent cycle performance: After 50 cycles at -40℃, the capacity retention rate of the electrolyte of this invention is still higher than 80%, which is much higher than that of traditional electrolytes (usually lower than 60%); after 1000 cycles at room temperature of 25℃, the capacity retention rate is higher than 90%, achieving a balance between low-temperature performance and room-temperature cycle performance. Attached Figure Description

[0028] Figure 1 Capacity retention curve after 50 cycles at -40℃.

[0029] Figure 2 Photograph of the negative electrode interface after being fully charged and circulated at -40℃ for 50 cycles in Example 1.

[0030] Figure 3 Photo of the negative electrode interface after being fully charged following a -40℃ cycle of 50 times in Comparative Example 1.

[0031] Figure 4 Photo of the negative electrode interface after being fully charged following a -40℃ cycle of 50 times in Comparative Example 2.

[0032] Figure 5 Photo of the negative electrode interface after being fully charged following a -40℃ cycle of 50 times in Comparative Example 3. Detailed Implementation

[0033] Example 1 A highly stable low-temperature lithium-ion battery electrolyte is prepared by the following steps: In an argon-protected glove box (water content ≤10ppm, oxygen content ≤10ppm), weigh out 40% ethylene carbonate, 35% dimethyl sulfite, and 20% ethyl propionate by mass percentage, stir and mix evenly to obtain the solvent system. Slowly add 1 mol / L lithium hexafluorophosphate to the above solvent system and stir continuously for 2 hours until the lithium hexafluorophosphate is completely dissolved; Add 2.5% siloxane additive (by mass ratio, trimethylmethoxysilane: dimethyldimethoxysilane: methyltrimethoxysilane = 1:1:1) and 2.5% fluoroethylene carbonate, and continue stirring for 1 hour until the mixture is homogeneous to obtain the target electrolyte.

[0034] Example 2 A highly stable low-temperature lithium-ion battery electrolyte is prepared by the following steps: In an argon-protected glove box (water content ≤10ppm, oxygen content ≤10ppm), weigh out 45% ethylene carbonate, 40% dimethyl sulfite, and 10% ethyl propionate by mass percentage, stir and mix evenly to obtain the solvent system. Slowly add 1 mol / L lithium hexafluorophosphate to the above solvent system and stir continuously for 2 hours until the lithium hexafluorophosphate is completely dissolved; Add 2.5% siloxane additive (by mass ratio, trimethylmethoxysilane: dimethyldimethoxysilane: methyltrimethoxysilane = 1:1:1) and 2.5% fluoroethylene carbonate, and continue stirring for 1 hour until the mixture is homogeneous to obtain the target electrolyte.

[0035] Example 3 A highly stable low-temperature lithium-ion battery electrolyte is prepared by the following steps: In an argon-protected glove box (water content ≤10ppm, oxygen content ≤10ppm), weigh out 35% ethylene carbonate, 30% dimethyl sulfite, and 30% ethyl propionate by mass percentage, stir and mix evenly to obtain the solvent system. Slowly add 1 mol / L lithium hexafluorophosphate to the above solvent system and stir continuously for 2 hours until the lithium hexafluorophosphate is completely dissolved; Add 2.5% siloxane additive (by mass ratio, trimethylmethoxysilane: dimethyldimethoxysilane: methyltrimethoxysilane = 1:1:1) and 2.5% fluoroethylene carbonate, and continue stirring for 1 hour until the mixture is homogeneous to obtain the target electrolyte.

[0036] Example 4 A highly stable low-temperature lithium-ion battery electrolyte is prepared by the following steps: In an argon-protected glove box (water content ≤10ppm, oxygen content ≤10ppm), weigh out 40% ethylene carbonate, 35% dimethyl sulfite, and 20% ethyl propionate by mass percentage, stir and mix evenly to obtain the solvent system. Slowly add 1 mol / L lithium hexafluorophosphate to the above solvent system and stir continuously for 2 hours until the lithium hexafluorophosphate is completely dissolved; Add 3% siloxane additive (by mass ratio, trimethylmethoxysilane: dimethyldimethoxysilane: methyltrimethoxysilane = 1:1:1) and 2% fluoroethylene carbonate, and continue stirring for 1 hour until the mixture is homogeneous to obtain the target electrolyte.

[0037] Example 5 A highly stable low-temperature lithium-ion battery electrolyte is prepared by the following steps: In an argon-protected glove box (water content ≤10ppm, oxygen content ≤10ppm), weigh out 40% ethylene carbonate, 35% dimethyl sulfite, and 20% ethyl propionate by mass percentage, stir and mix evenly to obtain the solvent system. Slowly add 1 mol / L lithium hexafluorophosphate to the above solvent system and stir continuously for 2 hours until the lithium hexafluorophosphate is completely dissolved; Add 2% siloxane additive (by mass ratio, trimethylmethoxysilane: dimethyldimethoxysilane: methyltrimethoxysilane = 1:1:1) and 3% fluoroethylene carbonate, and continue stirring for 1 hour until the mixture is homogeneous to obtain the target electrolyte.

[0038] Example 6 A highly stable low-temperature lithium-ion battery electrolyte is prepared by the following steps: In an argon-protected glove box (water content ≤10ppm, oxygen content ≤10ppm), weigh out 40% ethylene carbonate, 35% dimethyl sulfite, and 20% ethyl acetate by mass percentage, stir and mix evenly to obtain the solvent system; Slowly add 1 mol / L lithium hexafluorophosphate to the above solvent system and stir continuously for 2 hours until the lithium hexafluorophosphate is completely dissolved; Add 2.5% siloxane additive (by mass ratio, trimethylmethoxysilane: dimethyldimethoxysilane: methyltrimethoxysilane = 1:1:1) and 2.5% fluoroethylene carbonate, and continue stirring for 1 hour until the mixture is homogeneous to obtain the target electrolyte.

[0039] Example 7 A highly stable low-temperature lithium-ion battery electrolyte is prepared by the following steps: In an argon-protected glove box (water content ≤10ppm, oxygen content ≤10ppm), weigh out 40% ethylene carbonate, 35% dimethyl sulfite, and 20% ethyl butyrate by mass percentage, stir and mix evenly to obtain the solvent system. Slowly add 1 mol / L lithium hexafluorophosphate to the above solvent system and stir continuously for 2 hours until the lithium hexafluorophosphate is completely dissolved; Add 2.5% siloxane additive (by mass ratio, trimethylmethoxysilane: dimethyldimethoxysilane: methyltrimethoxysilane = 1:1:1) and 2.5% fluoroethylene carbonate, and continue stirring for 1 hour until the mixture is homogeneous to obtain the target electrolyte.

[0040] Comparative Example 1 A lithium-ion battery electrolyte is prepared in a manner that is basically the same as in Example 1, except that the solvent system does not contain dimethyl sulfite, but is replaced with 35% dimethyl carbonate.

[0041] Comparative Example 2 A lithium-ion battery electrolyte is prepared in a manner that is basically the same as in Example 1, except that it does not contain siloxane additives and the composite additive is 5% fluoroethylene carbonate.

[0042] Comparative Example 3 A lithium-ion battery electrolyte is prepared using the same steps as in Example 1, except that the siloxane additive is a single trimethylmethoxysilane.

[0043] Detection Example 1 The electrolytes from Examples 1-7 and Comparative Examples 1-3 were injected into lithium iron phosphate / graphite pouch batteries (5Ah capacity) of the same specifications, and the following performance tests were performed: (1) Low temperature viscosity and ionic conductivity test: The viscosity of the electrolyte at -40℃ was tested by a rotational viscometer, and the ionic conductivity of the electrolyte at -40℃ was tested by AC impedance method.

[0044] (2) Low temperature charge and discharge performance test: At -40℃, charge to 3.65V at 0.2C rate, let stand for 30 minutes, and then discharge to 2.5V at 0.2C rate. Record the first discharge capacity; then cycle 50 times at the same rate, record the discharge capacity after the cycle and calculate the capacity retention rate.

[0045] (3) Interface impedance test: The electrode / electrolyte interface impedance of the battery after 50 cycles at -40℃ was tested by AC impedance method.

[0046] Table 1 Test Results

[0047] The test results are shown in Table 1 and Figure 1 As shown. In Examples 1 and Comparative Examples 1 to 3, the negative electrode interface was disassembled after being cyclically charged at -40℃ for 50 cycles and fully charged. Figures 2-5 As shown.

[0048] In Examples 1-7, the viscosity at -40℃ was controlled at 170~215 mPa·s, the ionic conductivity reached 0.18~0.24 mS / cm, the initial discharge capacity was significantly improved, and the capacity retention rate remained stable at 80.8%~82.2% after 50 cycles. The interfacial impedance was only 330~390 Ω, which is far superior to Comparative Example 1 (conventional carbonate system, viscosity 520 mPa·s, conductivity 0.08 mS / cm, retention rate 47.0%) and Comparative Example 2 (fluorine-free ethylene carbonate, retention rate 47.0%). The results of comparison with the control (20.6% retention rate, 950Ω impedance) and Comparative Example 3 (without siloxane, 16.8% retention rate, 1100Ω impedance) fully demonstrate that the triangular synergy of "high dielectric constant-low viscosity-low eutectic point" can overcome the bottlenecks of solubility and low-temperature fluidity. The siloxane-fluoroethylene carbonate compound can form a dense, low-temperature resistant interface film, and the directional adaptation of dimethyl sulfite and siloxane can suppress interfacial side reactions while ensuring low-temperature fluidity. The overall performance is significantly better than that of conventional carbonate systems and single additive schemes.

Claims

1. A high-stability low-temperature lithium-ion battery electrolyte, comprising a solvent system and composite additives, and further comprising a lithium salt, characterized in that, By mass percentage, in the liquid system composed of solvent system and composite additive, solvent system accounts for 94%~96% and composite additive 4%~6%; The solvent system comprises ethylene carbonate, dimethyl sulfite and carboxylic acid ester, in a mass ratio of 35~45:30~40:10~30; The composite additive includes siloxane additives and fluoroethylene carbonate in a mass ratio of 2~3:2~3.

2. The high-stability low-temperature lithium-ion battery electrolyte according to claim 1, characterized in that, The carboxylic acid ester is at least one of ethyl acetate, ethyl propionate, methyl butyrate, and ethyl butyrate.

3. The high-stability low-temperature lithium-ion battery electrolyte according to claim 1, characterized in that, The siloxane additive includes trimethylmethoxysilane, dimethyldimethoxysilane and methyltrimethoxysilane in a mass ratio of 1:1:

1.

4. The high-stability low-temperature lithium-ion battery electrolyte according to claim 1, characterized in that, The lithium salt is lithium hexafluorophosphate, and the concentration of the lithium salt in the solvent system is 1 mol / L.

5. The method for preparing the high-stability low-temperature lithium-ion battery electrolyte according to any one of claims 1 to 4, characterized in that, Includes the following steps: (1) The solvent system is obtained by uniformly mixing ethylene carbonate, dimethyl sulfite and carboxylic acid ester under an inert atmosphere; (2) Dissolve the lithium salt in the solvent system; (3) Add the composite additive and mix evenly to obtain the high-stability low-temperature lithium-ion battery electrolyte.

6. The preparation method according to claim 5, characterized in that, The inert atmosphere is an argon atmosphere or a nitrogen atmosphere.

7. A lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The electrolyte is the high-stability low-temperature lithium-ion battery electrolyte according to any one of claims 1 to 4.

8. The lithium-ion battery according to claim 7, characterized in that, The positive electrode uses lithium iron phosphate or ternary lithium materials, the negative electrode uses graphite, silicon-based negative electrode or lithium titanate, and the separator uses polyethylene, polypropylene or composite separator.

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