Ultralow-temperature electrolyte suitable for ternary high-specific-energy soft-package lithium ion battery system

Abstract

The invention belongs to the technical field of lithium battery electrolyte, and particularly relates to ultralow-temperature electrolyte suitable for a ternary high-specific-energy soft package lithium ion battery system. The invention provides an ultralow-temperature electrolyte suitable for a ternary high-specific-energy soft package lithium ion battery system. The ultralow-temperature electrolyte comprises a mixed organic solvent, a lithium salt and a functional additive, the mixed organic solvent contains linear carbonic ester with a low freezing point and linear carboxylic ester with a low freezing point; the mixed organic solvent also contains at least one of fluorinated carbonate and fluorinated carboxylic ester. Linear carbonic ester and linear carboxylic ester with low freezing points are adopted as solvents, carbonic ester solvents with high freezing points are replaced or partially replaced, the viscosity of the electrolyte is reduced, the Li < + > desolvation process is adjusted by adding fluorinated carbonic ester and fluorinated carboxylic ester, the oxidation resistance of the electrolyte is improved, and the SEI stability is considered. In addition, the low-temperature performance is improved in the aspects of improving the lithium ion conductivity, constructing a stable SEI film with high ion conductivity, accelerating interface reaction kinetics and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium battery electrolytes, and in particular relates to an ultra-low temperature electrolyte suitable for a ternary high-energy-density soft-pack lithium-ion battery system. Background Art

[0002] With the vigorous development of the new energy industry, people have increasingly stringent requirements on the use and performance of lithium-ion batteries. The operating temperature range of traditional lithium-ion batteries is -20 to +55°C. When the temperature is below -20°C, the cycle stability, service life and safety of lithium-ion batteries are seriously affected. It is difficult for lithium-ion batteries to work in low-temperature environments in winter in low-latitude areas, and they are even less capable of meeting the requirements for energy storage and release in ultra-low temperature areas such as polar regions and space.

[0003] In addition, as the market demands high energy density, excellent low temperature, rate performance and cycle life, and high safety performance of lithium-ion batteries, simply using ordinary electrolytes to adapt lithium iron phosphate batteries or ternary batteries cannot meet the above performance. In particular, it is difficult to prepare lithium-ion batteries that have high energy density, long cycle life and excellent safety performance under ultra-low temperature conditions.

[0004] The performance challenges faced by lithium-ion batteries in low temperature environments mainly stem from the lithium-ion transport kinetics, including the transport of lithium ions in the electrolyte, the desolvation of lithium ions at the interface, the passage of lithium ions through the SEI (solid electrolyte interface) membrane, and the diffusion of lithium ions in the electrode material. Compared with room temperature conditions, the discharge capacity and cycle life of lithium-ion batteries in low temperature environments (≤-20°C) will be significantly reduced. This phenomenon is mainly attributed to the following aspects: 1. The liquid phase ion conductivity of the electrolyte decreases; 2. The characteristics of the interface between the electrode and the electrolyte change; 3. The charge transfer rate slows down and the desolvation process of lithium ions is hindered; 4. The growth of lithium dendrites exists.

[0005] Specifically, the existing ternary high-energy-density soft-pack lithium-ion battery system has the following main problems under low temperature conditions:

[0006] 1. Insufficient battery life: In extremely cold environments, due to the decrease in energy density, the battery life of ternary high-energy-density batteries is shortened. This is a key obstacle for equipment that needs to work under extreme climatic conditions, such as polar exploration equipment, electric vehicles, and military equipment.

[0007] 2. Battery performance degradation: In low temperature environments, the viscosity of the electrolyte increases, making it difficult for lithium ions to migrate in the electrolyte, resulting in reduced ion conductivity. The internal resistance of the battery increases, ion transmission is blocked during the charge and discharge process, and the battery's charge and discharge performance deteriorates.

[0008] 3. Increased charge transfer impedance: At low temperatures, the activation energy of the charge transfer process increases, resulting in an increase in charge transfer impedance. This means that on the electrode surface, it becomes difficult for lithium ions to embed into or escape from the electrolyte into the electrode material, affecting the battery's charge and discharge efficiency and reversibility.

[0009] 4. Lithium deposition at the negative electrode: During low-temperature charging, even at low rates, the embedding speed of lithium ions in the negative electrode will slow down, and lithium metal deposition is likely to form on the surface of the negative electrode, which is also known as lithium deposition. The deposited lithium metal cannot be reversibly stripped off, forming "dead" lithium, resulting in reduced battery capacity, and lithium metal deposition may cause internal short circuits in the battery, posing a safety hazard.

[0010] 5. Electrolyte coagulation problem: The solvents used in ordinary electrolytes tend to coagulate at ultra-low temperatures. For example, the coagulation point of common solvents such as ethylene carbonate is relatively high. When the electrolyte coagulates, the battery cannot carry out normal ion transmission, causing the battery to not work properly.

[0011] 6. Battery safety issues: Low temperature conditions not only affect battery performance, but may also lead to safety risks such as thermal runaway due to improper charge and discharge management. If the battery is in an inappropriate charge and discharge state for a long time at low temperatures, it may cause safety issues such as thermal runaway.

[0012] Therefore, the development of ultra-low temperature electrolytes that are compatible with ternary high-energy-density soft-pack lithium-ion batteries is of great significance for opening up a wider market and more extensive applications. Summary of the invention

[0013] In view of the shortcomings of the prior art, the present invention provides an ultra-low temperature electrolyte suitable for a ternary high-energy-density soft-pack lithium-ion battery system. The purpose of the present invention is to provide an electrolyte suitable for a ternary high-energy-density soft-pack lithium-ion battery and capable of continuous rate discharge at -40°C or below, so that the lithium-ion battery can operate in an ultra-low temperature environment below -40°C.

[0014] The present invention not only uses linear carbonate and linear carboxylic acid ester with low freezing point as solvent to replace or partially replace carbonate solvent with high freezing point to reduce the viscosity of electrolyte, but also adjusts Li + The desolvation process improves the antioxidant properties of the electrolyte and optimizes the SEI stability. In addition, the lithium ion conductivity is improved by increasing the electrolyte salt content, and a multi-component film-forming additive is added to construct a stable SEI film with high ionic conductivity on the surface of the silicon negative electrode. The compatibility of the mixed solvent and the carbon-based negative electrode is improved, and the interfacial reaction kinetics are accelerated, thereby improving the low-temperature performance of lithium-ion batteries.

[0015] The ultra-low temperature electrolyte provided by the present invention is suitable for a ternary high specific energy soft-pack lithium ion battery system, comprising a mixed organic solvent, a lithium salt and a functional additive;

[0016] Wherein, the mixed organic solvent contains a linear carbonate with a low freezing point and a linear carboxylic acid ester with a low freezing point;

[0017] The mixed organic solvent further contains at least one of fluorinated carbonate ester and fluorinated carboxylic acid ester.

[0018] As a further optimization scheme for the above-mentioned ultra-low temperature electrolyte, the low-freezing-point linear carbonate is a linear carbonate having a freezing point below 4.6°C; the low-freezing-point linear carboxylate is a linear carboxylate having a freezing point below -73.5°C.

[0019] As a further optimization scheme for the ultra-low temperature electrolyte, the mixed organic solvent includes:

[0020] The cyclic carbonate accounts for 0-30% by mass of the mixed organic solvent;

[0021] A linear carbonate ester accounting for 20% to 70% by mass of the mixed organic solvent;

[0022] Fluorinated carbonate ester accounting for 6% to 13% by mass of the mixed organic solvent;

[0023] A linear carboxylic acid ester accounting for 10% to 40% by mass of the mixed organic solvent;

[0024] The mass percentage of the fluorinated carboxylic acid ester in the mixed organic solvent is 0-15%.

[0025] As a further optimization scheme for the above-mentioned ultra-low temperature electrolyte, the total mass molar ratio of lithium salt as electrolyte salt in the electrolyte is not less than 1M.

[0026] As a further optimization scheme of the ultra-low temperature electrolyte, lithium salt is used as electrolyte salt, including:

[0027] Lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide occupy a total mass molar ratio of 1-1.6M in the electrolyte;

[0028] Lithium difluorophosphate accounting for 0.5%-1% by mass of the electrolyte;

[0029] Lithium tetrafluoroborate accounting for 0-0.5% by mass of the electrolyte;

[0030] Lithium difluorobis(oxalate)phosphate accounting for 0-0.2% by mass of the electrolyte;

[0031] The lithium difluorooxalate borate accounts for 0-0.5% by mass of the electrolyte.

[0032] As a further optimization scheme for the above-mentioned ultra-low temperature electrolyte, the functional additives are selected from one or more of low-temperature additives, positive and negative electrode film-forming additives, and water-removing and acid-inhibiting additives.

[0033] As a further optimization solution for the above ultra-low temperature electrolyte, the functional additives include:

[0034] 0-1% by mass of tris(trimethylsilyl)phosphate in the electrolyte;

[0035] 1% to 2% by mass of vinyl sulfate in the electrolyte;

[0036] Tripropynyl phosphate accounting for 0-0.2% by mass of the electrolyte;

[0037] Citraconic anhydride accounting for 0-0.2% by mass of the electrolyte;

[0038] 0-1% by mass of vinylene carbonate in the electrolyte;

[0039] 1,3-propane sultone accounting for 0.5%-2% by mass of the electrolyte;

[0040] The mass percentage of p-toluenesulfonyl isocyanate is 0-0.5% of the electrolyte.

[0041] As a further optimization solution for the above-mentioned ultra-low temperature electrolyte, the ternary high-energy-density lithium-ion battery is a battery system in which the silicon content of the negative electrode material system exceeds 20% and the nickel content of the positive electrode material system exceeds 92%.

[0042] As a further optimization scheme for the above-mentioned ultra-low temperature electrolyte, the mixed organic solvent accounts for 75%-80% of the total mass of the electrolyte, and is mixed with ethylene carbonate, propylene carbonate, ethyl methyl carbonate, fluoroethylene carbonate, and dimethyl carbonate in a mass ratio of 12:13:32.5:12.5:30.

[0043] As a further optimization scheme of the ultra-low temperature electrolyte, the mixed organic solvent accounts for 77.7% of the total mass of the electrolyte; the electrolyte also includes:

[0044] Lithium difluorophosphate accounting for 0.8% of the total mass of the electrolyte;

[0045] 1,3-propane sultone accounting for 2% of the total mass of the electrolyte;

[0046] 1.5% of the total mass of the electrolyte is vinyl sulfate;

[0047] Lithium bis(fluorosulfonyl)imide accounting for 6% of the total mass of the electrolyte;

[0048] Lithium hexafluorophosphate accounting for 11% of the total mass of the electrolyte;

[0049] 0.5% of tris(trimethylsilyl) phosphate by weight of the total electrolyte;

[0050] Lithium difluorooxalatoborate accounts for 0.5% of the total mass of the electrolyte.

[0051] Beneficial Effects

[0052] The electrolyte provided by the present invention is used for a ternary high-energy-density soft-pack lithium-ion battery, which can perform continuous rate discharge in an ultra-low temperature environment, thereby ensuring that the lithium-ion battery can continue to work stably in the ultra-low temperature environment.

[0053] The invention uses a high-content mixed novel electrolyte salt containing bisfluorosulfonyl imide lithium salt, phosphate lithium salt and borate lithium salt, which has high dissociation characteristics and can improve the ion conductivity of the electrolyte.

[0054] The solvent used in the present invention enables the electrolyte to have a wide electrochemical stability window and low desolvation energy. The electrolyte adds carboxylic acid esters, fluorinated carboxylic acid esters and fluorinated carbonates to cyclic carbonates and linear carbonates, which can increase the oxidation and reduction potentials of the solvent, improve its anti-oxidation stability, and provide a certain degree of flame retardancy, while retaining its low freezing point and low viscosity characteristics. In addition, since the dielectric constants of linear carboxylic acid ester solvents and fluorinated carboxylic acid esters are lower than those of commercial carbonate solvents such as EC and PC, when the lithium salt is sufficiently dissolved, the interaction between the solvent and lithium ions is greatly reduced, and the desolvation energy is reduced.

[0055] The electrolyte of the present invention is used in conjunction with a ternary high-energy-density soft-pack lithium-ion battery, which is not only suitable for special application scenarios with high requirements for driving range, but also shows good compatibility with electrode materials in terms of performance, can form a good interface contact with the surface of the high-nickel positive electrode, promote the insertion and extraction of lithium ions, and can effectively inhibit the expansion of the high-silicon negative electrode and the lithium precipitation phenomenon at low temperatures, reducing the potential safety hazards of the battery. The electrolyte is mixed with a new electrolyte salt and a film-forming additive, and a low-impedance and stable SEI film with a higher inorganic content is constructed on the surface of the negative electrode, and the interface reaction kinetics are accelerated, which has a significant improvement effect on the low-temperature charge and discharge performance of the lithium-ion battery. DETAILED DESCRIPTION

[0056] The present invention is further illustrated by specific examples below. These examples are exemplary and are intended to illustrate the problem and explain the present invention, but are not intended to be limiting.

[0057] Example 1

[0058] The preparation method of the ultra-low temperature electrolyte of this embodiment is as follows: in an inert atmosphere glove box with a water content and an oxygen content both lower than 1 ppm, ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC), and dimethyl carbonate (DMC) are mixed in a mass ratio of 12:13:32.5:12.5:30 to prepare an organic solvent accounting for 77.7% of the total mass of the electrolyte; and lithium difluorophosphate (LiPO4) accounting for 0.8% of the total mass of the electrolyte is added. 2 F 2 ), 1,3-propane sultone (PS) accounting for 2% of the total mass of the electrolyte, ethylene sulfate (DTD) accounting for 1.5% of the total mass of the electrolyte, and lithium bis(fluorosulfonyl)imide (LiFSI) accounting for 6% of the total mass of the electrolyte; freeze at -20℃ for 2h, and quickly add lithium hexafluorophosphate (LiPF 6 ); stirring and shaking, testing the water content and free acid density of the electrolyte, after the water content and free acid test are qualified (water content ≤ 20ppm, free acid ≤ 30ppm), adding 0.5% of tris(trimethylsilyl) phosphate (TMSP) and 0.5% of lithium difluorooxalatoborate (LiODFB) accounting for the total mass of the electrolyte, stirring and shaking to obtain an ultra-low temperature electrolyte.

[0059] Example 2

[0060] This embodiment adopts the same implementation as that of embodiment 1, and the only difference from embodiment 1 is the organic solvent. In this embodiment, the organic solvent is prepared by mixing ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC), and ethyl propionate (EP) in a mass ratio of 12:13:32.5:12.5:30.

[0061] Example 3

[0062] This embodiment adopts the same implementation as that of embodiment 1, and the only difference from embodiment 1 is the organic solvent. In this embodiment, the organic solvent is prepared by mixing ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC), and ethyl propionate (EP) in a mass ratio of 12:13:22.5:12.5:40.

[0063] Example 4

[0064] This embodiment adopts the same implementation as that of embodiment 1, and the only difference from embodiment 1 is the organic solvent. In this embodiment, the organic solvent is prepared by mixing ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC), ethyl propionate (EP), and ethyl trifluoroacetate (ETFA) in a mass ratio of 12:13:32.5:12.5:15:15.

[0065] Comparative Example 1

[0066] The electrolyte used in this comparative example is a commercial electrolyte purchased from the market. The commercial electrolyte is composed of a mixed organic solvent of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and fluoroethylene carbonate (FEC) in a mass ratio of 5:20:32.5:12.5:30, which accounts for 80.3% of the total mass of the electrolyte, and lithium hexafluorophosphate (LiPF 6 ), lithium difluorophosphate (LiPO 2 F 2 ), 1% of the total mass of the electrolyte was 1,3-propane sultone (PS), 1.5% of the total mass of the electrolyte was diethylene sulfate (DTD), 4% of the total mass of the electrolyte was lithium bis(fluorosulfonyl)imide (LiFSI), 0.5% of the total mass of the electrolyte was lithium difluorooxalatoborate (LiODFB), 0.2% of the total mass of the electrolyte was citraconic anhydride (CA), and 0.2% of the total mass of the electrolyte was tripropargyl phosphate (TPP).

[0067] Comparative Example 2

[0068] This comparative example adopts the same implementation method as comparative example 1, and uses the same electrolyte as comparative example 1. The only difference from comparative example 1 is that it is implemented on soft-pack lithium-ion batteries of different capacities.

[0069] Electrochemical performance evaluation of electrolyte

[0070] Take 50 soft-pack lithium-ion batteries of the same specification and capacity of 1.5Ah to be filled with liquid. The main material of the positive electrode of the soft-pack lithium-ion battery is Ni93 ternary material, and the main material of the negative electrode is artificial graphite and silicon oxide material. At the same time, the positive and negative electrodes also contain auxiliary materials such as conductive agents and binders. Inject the same amount of low-temperature electrolyte prepared in Examples 1 to 4 and Comparative Example 2 into each of the above 10 soft-pack lithium-ion batteries to make finished batteries. At the same time, take 10 soft-pack lithium-ion batteries of the same specification and capacity of 23Ah to be filled with liquid. The main material of the positive electrode of the soft-pack lithium-ion battery is Ni93 ternary material, and the main material of the negative electrode is artificial graphite and silicon oxide material. At the same time, the positive and negative electrodes also contain auxiliary materials such as conductive agents and binders. Inject a specified amount of low-temperature electrolyte of Comparative Example 1 into the above 10 soft-pack lithium-ion batteries to make finished batteries. The electrochemical properties of the above batteries were tested under the same conditions. The test items included: battery capacity and initial efficiency test, low temperature discharge test, rate charge test, rate discharge test and 55°C storage test.

[0071] Battery capacity and first efficiency test: After the battery is formed, it is aged at 45℃ for 24h, then fully charged at 0.33C constant current and constant voltage, the charging upper limit voltage is 4.25V, the cut-off current is 0.05C, and after being left for 10min, it is discharged at 0.33C constant current to 2.5V, and cycled for 2 times. The first discharge capacity and first efficiency are calculated. The test results are shown in Table 1.

[0072] Low temperature discharge test: First, at 25°C, use a current of 0.33C / 1C and a voltage range of 2.5-4.25V to set the battery capacity. Then fully charge it, place it in a -20°C constant temperature box for 7 hours, and then discharge it at a constant current of 1C to 2.0V to obtain the -20°C discharge capacity. The -40°C discharge capacity was tested using the same method, and the test results are shown in Table 1.

[0073] Rate charging test: at 25°C, fully charged with 0.33C constant current and constant voltage, the charging upper limit voltage is 4.25V, the cut-off current is 0.05C, and it is left for 10 minutes. The battery is discharged at 0.33C constant current to 2.5V, and it is left for 10 minutes, and it is cycled for 2 circles; fully charged with 1C constant current and constant voltage, the charging upper limit voltage is 4.25V, the cut-off current is 0.05C, and it is left for 10 minutes. The battery is discharged at 0.33C constant current to 2.5V, and it is left for 10 minutes, and it is cycled for 2 circles; the 1C constant current charging ratio is calculated, and the results are shown in Table 1.

[0074] Rate discharge test: At 25°C, fully charged at 0.33C constant current and constant voltage, the upper limit voltage of charging is 4.25V, the cut-off current is 0.05C, and it is left for 10 minutes. Then, it is discharged at 0.33C constant current to 2.5V, and it is left for 10 minutes. This cycle is repeated 2 times. Fully charged at 0.33C constant current and constant voltage, the upper limit voltage of charging is 4.25V, the cut-off current is 0.05C, and it is left for 10 minutes. Then, it is discharged at 1C constant current to 2.5V, and it is left for 10 minutes. This cycle is repeated 2 times. Fully charged at 0.33C constant current and constant voltage, the upper limit voltage of charging is 4.25V, the cut-off current is 0.05C, and it is left for 10 minutes. It is discharged at 2C constant current to 2.5V, and it is left for 10 minutes, and it is cycled for 2 circles. Fully charged at 0.33C constant current and constant voltage, the upper limit voltage of charging is 4.25V, the cut-off current is 0.05C, and it is left for 10 minutes. It is discharged at 3C constant current to 2.5V, and it is left for 10 minutes, and it is cycled for 2 circles. The 3C capacity retention rate is calculated, and the results are shown in Table 1.

[0075] 55℃ storage test: at 25℃, fully charged with 1C constant current and constant voltage, the charging upper limit voltage is 4.25V, the cut-off current is 0.05C, and it is left for 10min. Then, it is discharged with 1C constant current to 2.5V, and it is left for 10min, and it is cycled for 3 times. After that, it is placed in a 55℃ blast drying oven, left for 7 days, and it is taken out and cooled to room temperature. Then, at 25℃, it is discharged with 1C constant current to 2.5V, and it is left for 10min. Then, it is fully charged with 1C constant current and constant voltage, the charging upper limit voltage is 4.25V, and the cut-off current is 0.05C, and it is left for 10min, and it is cycled for 3 times. The discharge capacity retention rate and the discharge capacity recovery rate are calculated. The test results are shown in Table 1.

[0076] Table 1 Electrochemical properties of low-temperature electrolytes of Examples and Comparative Examples

[0077]

[0078] Table 1 shows that the electrolyte prepared in the embodiment is suitable for ternary high-energy-density soft-pack lithium-ion batteries, can be continuously discharged at an ultra-low temperature of -40°C, and has good comprehensive performance in high temperature and cycle.

[0079] Further comparison of the electrical performance test data in Table 1 shows that the addition of co-solvents DMC and carboxylic acid esters to conventional electrolytes has no significant adverse effect on battery capacity and first efficiency, and an appropriate increase in the amount of carboxylic acid ester has no significant effect on battery capacity and first efficiency. Comparison of Comparative Example 1 with Comparative Example 2 shows that the battery capacity has a greater impact on low-temperature discharge capacity and high-temperature storage performance. Comparison of Examples 2 to 3 with Comparative Example 1 shows that the addition of carboxylic acid esters to the electrolyte can further improve the low-temperature discharge capacity retention rate of the battery. Comparison of Example 2 and Example 3 shows that increasing the carboxylic acid ester content in the electrolyte improves the low-temperature discharge capacity retention rate of the battery, and has little effect on high-temperature storage performance. Comparison of Example 1 with Comparative Example 1 shows that the addition of DMC to the electrolyte improves the low-temperature discharge capacity retention rate of the battery at -20°C, reduces the low-temperature discharge capacity retention rate of the battery at -40°C, and reduces the high-temperature storage performance. Comparison of Example 2 and Example 4 shows that the low-temperature discharge capacity retention rate of the fluorinated carboxylic acid ester battery with a solvent ratio of 15% is reduced, which has little effect on high-temperature storage performance.

[0080] The above embodiments are exemplary, and their purpose is to illustrate the technical concept and features of the present invention so that people familiar with the technology in this field can understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. An ultra-low temperature electrolyte suitable for a ternary high specific energy soft pack lithium ion battery system, characterized in that: Including mixed organic solvent, lithium salt and functional additive; The mixed organic solvent contains a linear carbonate with a low freezing point and a linear carboxylic acid ester with a low freezing point; The mixed organic solvent further contains at least one of fluorinated carbonate ester and fluorinated carboxylic acid ester.

2. The ultra-low temperature electrolyte suitable for a ternary high specific energy soft pack lithium ion battery system according to claim 1, characterized in that: The linear carbonate with a low freezing point is a linear carbonate with a freezing point lower than 4.6°C; the linear carboxylic acid ester with a low freezing point is a linear carboxylic acid ester with a freezing point lower than -73.5°C.

3. The ultra-low temperature electrolyte suitable for a ternary high specific energy soft-pack lithium-ion battery system according to claim 2, characterized in that: The mixed organic solvent includes: The cyclic carbonate accounts for 0-30% by mass of the mixed organic solvent; A linear carbonate ester accounting for 20% to 70% by mass of the mixed organic solvent; Fluorinated carbonate esters accounting for 6% to 13% by mass of the mixed organic solvent; A linear carboxylic acid ester accounting for 10% to 40% by mass of the mixed organic solvent; The mass percentage of the fluorinated carboxylic acid ester in the mixed organic solvent is 0-15%.

4. The ultra-low temperature electrolyte suitable for a ternary high specific energy soft pack lithium ion battery system according to claim 2, characterized in that: The lithium salt is used as an electrolyte salt, and the total mass molar ratio of the lithium salt in the electrolyte is not less than 1M.

5. The ultra-low temperature electrolyte suitable for a ternary high specific energy soft-pack lithium-ion battery system according to claim 4, characterized in that: The lithium salt as the electrolyte salt includes: Lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide occupy a total mass molar ratio of 1-1.6M in the electrolyte; Lithium difluorophosphate accounting for 0.5%-1% by mass of the electrolyte; Lithium tetrafluoroborate accounting for 0-0.5% by mass of the electrolyte; Lithium difluorobis(oxalate)phosphate accounting for 0-0.2% by mass of the electrolyte; Lithium difluorooxalate borate accounts for 0-0.5% of the electrolyte mass percentage.

6. The ultra-low temperature electrolyte suitable for a ternary high specific energy soft pack lithium ion battery system according to claim 2, characterized in that: The functional additive is selected from one or more of low-temperature additives, positive and negative electrode film-forming additives, and water-removing and acid-suppressing additives.

7. The ultra-low temperature electrolyte suitable for a ternary high specific energy soft-pack lithium-ion battery system according to claim 6, characterized in that: The functional additives include: 0-1% by mass of tris(trimethylsilyl)phosphate in the electrolyte; 1% to 2% by mass of vinyl sulfate in the electrolyte; Tripropynyl phosphate accounting for 0-0.2% by mass of the electrolyte; Citraconic anhydride accounting for 0-0.2% by mass of the electrolyte; 0-1% by mass of vinylene carbonate in the electrolyte; 1,3-propane sultone accounting for 0.5%-2% by mass of the electrolyte; The mass percentage of p-toluenesulfonyl isocyanate is 0-0.5% of the electrolyte.

8. The ultra-low temperature electrolyte suitable for a ternary high specific energy soft-pack lithium-ion battery system according to claim 2, characterized in that: The ternary high-energy-density lithium-ion battery is a battery system in which the silicon content of the negative electrode material system exceeds 20% and the nickel content of the positive electrode material system exceeds 92%.

9. The ultra-low temperature electrolyte suitable for a ternary high specific energy soft-pack lithium-ion battery system according to claim 2, characterized in that: The mixed organic solvent accounts for 75%-80% of the total mass of the electrolyte, and is prepared by mixing ethylene carbonate, propylene carbonate, ethyl methyl carbonate, fluoroethylene carbonate, and dimethyl carbonate in a mass ratio of 12:13:32.5:12.5:

30.

10. The ultra-low temperature electrolyte suitable for a ternary high specific energy soft-pack lithium-ion battery system according to claim 9, characterized in that: The mixed organic solvent accounts for 77.7% of the total mass of the electrolyte; the electrolyte also includes: Lithium difluorophosphate accounting for 0.8% of the total mass of the electrolyte; 1,3-propane sultone accounting for 2% of the total mass of the electrolyte; 1.5% of the total mass of the electrolyte is vinyl sulfate; Lithium bis(fluorosulfonyl)imide accounting for 6% of the total mass of the electrolyte; Lithium hexafluorophosphate accounting for 11% of the total mass of the electrolyte; 0.5% tris(trimethylsilyl) phosphate by weight of the total electrolyte; Lithium difluorooxalate borate accounts for 0.5% of the total mass of the electrolyte.

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