High-voltage ternary soft package electrolyte and preparation method thereof

Abstract

The invention mainly relates to the technical field of electrolytes, in particular to a high-voltage ternary soft package electrolyte and a preparation method thereof.The high-voltage ternary soft package electrolyte is characterized in that a double-lithium salt composed of lithium hexafluorophosphate and lithium bis (fluorosulfonyl) imide and a three-solvent composed of ethylene carbonate, ethyl methyl carbonate and diethyl carbonate serve as a basic system, and a composite additive is added, 3-allylsultone and 1-ethyl-3-methylimidazolium bis (fluorosulfonyl) imide have a synergistic effect to form a compact film, decomposition of the electrolyte under high voltage can be inhibited, lithium difluorophosphate and lithium difluoro (oxalato) borate are small in film forming impedance, and meanwhile, the high-temperature storage and circulation capability of a battery cell can be effectively improved; vinylene carbonate and fluoroethylene carbonate are used as long-circulation film-forming additives, so that the long-circulation capability of the battery cell can be effectively improved; the high-voltage ternary soft package battery adopting the electrolyte can be stored at 60 DEG C for 70 days and circulated at 45 DEG C for 500 times, and the low-temperature-10 DEG C 0.2 C discharge retention rate is greater than 70%.

Description

Technical Field

[0001] This invention relates to the technical field of lithium battery electrolytes, specifically a high-voltage ternary soft-pack electrolyte and its preparation method. Background Technology

[0002] Lithium-ion battery electrolyte is the carrier of ion transport in the battery, mainly responsible for conducting conductive ions between the positive and negative electrodes. It is essential for lithium-ion batteries to achieve advantages such as high voltage and high specific energy. It plays a crucial role in the battery's energy density, cycle life, power density, safety performance, and wide-temperature application range, and is often referred to as the "blood of the battery." It is generally composed of lithium salts and organic solvents. Electrolytes are typically prepared by mixing high-purity organic solvents, lithium salt electrolytes, and necessary additives under specific conditions and in specific proportions.

[0003] High-voltage ternary soft-pack batteries using existing electrolytes have the following drawbacks: poor high-temperature storage performance: significant capacity decay and high swelling rate after storage at 60℃; insufficient cycle life: the number of cycles at 45℃ is generally less than 500; limited low-temperature performance: the discharge capacity retention rate is less than 70% at -10℃.

[0004] The present invention aims to develop a high-voltage ternary soft-pack electrolyte, which addresses the problems of high-temperature storage, insufficient cycle performance and limited low-temperature performance in existing battery technologies. It provides a high-voltage ternary soft-pack electrolyte and its preparation method, which can meet the requirements of high-end digital fields by taking into account both high and low temperature performance and cycle performance. Summary of the Invention

[0005] Based on this, the purpose of this invention is to provide a high-voltage ternary soft-pack electrolyte with good cycle performance and suitable for both high and low temperatures, as well as its preparation method, so as to solve the technical problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a high-voltage ternary soft-pack electrolyte, which is prepared by adding composite additives to a lithium salt and solvent-based system. The basic system consists of the following raw materials by weight percentage: Lithium salts 10-25% Solvent 75-90% The lithium salt is composed of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide in a mass ratio of (6-9):(4-1). The lithium salt uses a combination of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide, taking advantage of the excellent antioxidant properties, thermal stability and low film-forming resistance of lithium bis(fluorosulfonyl)imide. Lithium hexafluorophosphate, as the main lithium salt, can effectively inhibit the corrosion of aluminum foil by lithium bis(fluorosulfonyl)imide. The solvent is composed of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a mass ratio of (2-4):(4-6):(1-4). The use of a ternary solvent system can effectively improve its high-temperature boiling point and low-temperature freezing point, thus taking into account both high and low temperature performance. This invention utilizes a dual lithium salt system (lithium hexafluorophosphate and lithium difluorosulfonyl imide) and a three-solvent system (ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate) to positively promote the synergistic effect of EMIMFSI and PS / PST, resulting in a doubling of cycle life. With the conductive assistance of the dual lithium salt and the homogeneity of the solvent, the resulting composite CEI film is exceptionally robust and uniform, effectively suppressing side reactions during cycling and pushing cycle life to new heights. Extreme stability during high-temperature storage: In the corrosion-free environment created by LiPF6, EMIMFSI can fully exert its molecular barrier function, working with PS / PST to block the main pathways of gas generation and capacity decay at high temperatures. Significant reduction in interface impedance: Due to the high conductivity of LiFSI and the interface modification effect of EMIMFSI, even with the presence of a dense film formed by PS / PST, lithium-ion transport remains smooth, ensuring excellent low-temperature discharge performance.

[0007] Add the following composite additive raw materials to the base system in the following weight percentages: Vinyl carbonate (VC) 0.1%-0.5% Fluorinated ethylene carbonate (FEC) 0.5%-1% 1,3-Propanesulfonate lactone (PS) 2%-3% 1-Ethyl-3-methylimidazolium difluorosulfonylimide 1%-2% Lithium difluorophosphate (LiPO2F2) 0.2%-0.8% Lithium difluorooxalate borate (LiODFB) 0.2-0.5% 1,3-Propylenesulfonate lactone (PST) 0.2-0.5%.

[0008] Preferably, the following composite additive raw materials are added to the base system in the following weight percentages: Vinyl carbonate (VC) 0.1%-0.3% Fluorinated ethylene carbonate (FEC) 0.5%-0.8% 1,3-Propanesulfonate lactone (PS) 2%-2.5% 1-Ethyl-3-methylimidazolium difluorosulfonylimide 1%-2% Lithium difluorophosphate (LiPO2F2) 0.2%-0.6% Lithium difluorooxalate borate (LiODFB) 0.2-0.4% 1,3-Propylenesulfonate lactone (PST) 0.2-0.4%.

[0009] More preferably, a composite additive raw material is added to the base system in the following weight percentages: Vinyl carbonate (VC) 0.2% Fluorinated ethylene carbonate (FEC) 0.7% 1,3-Propanesulfonate lactone (PS) 2.4% 1-Ethyl-3-methylimidazolium difluorosulfonylimide 1.5% Lithium difluorophosphate (LiPO2F2) 0.4% Lithium difluorooxalate borate (LiODFB) 0.3% 1,3-Propylenesulfonate lactone (PST) 0.3%.

[0010] A method for preparing a high-voltage ternary soft-pack electrolyte includes the following preparation steps: Step A: Perform solvent purification and lithium salt pretreatment; Step B: First, add the prescribed amount of ethylene carbonate to the reaction vessel, and then add the prescribed amounts of methyl ethyl carbonate and diethyl carbonate under stirring; heat to 40±2℃ and continue stirring for 1-2 hours to obtain the composite solvent; Step C: At a temperature ≤45℃, LiPF6 and LiFSI are added to the composite solvent in batches and stirred until the conductivity of the solution is stable to obtain the basic electrolyte; Step D: Add the prescribed amount of 1-ethyl-3-methylimidazolium difluorosulfonylimide to the basic electrolyte and stir until homogeneous; Step E: Continue to add the formulated amounts of vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propanesulfonate lactone (PS), 1,3-propenesulfonate lactone (PST), lithium difluorophosphate (LiPO2F2), and lithium difluorooxalate borate (LiODFB) to the basic electrolyte obtained in step D in sequence. After standing and aging, filter to obtain the electrolyte.

[0011] In step C, the prescribed amount of LiPF6 is first added, the temperature is controlled at ≤10℃, and the mixture is stirred for 2-4 hours; then LiFSI is added, the temperature is raised to 25℃, and the mixture is kept at this temperature and stirred for 5-7 hours. LiPF6 preferentially dissolves to form an AlF3 protective layer, which blocks the corrosion of the aluminum current collector by LiFSI.

[0012] In step E, vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are added sequentially and stirred for 30-40 minutes; then 1,3-propanesulfonyl lactone (PS) and 1,3-propenesulfonyl lactone (PST) are added sequentially and stirred for 40-60 minutes; finally, lithium difluorophosphate (LiPO2F2) and lithium difluorooxalate borate (LiODFB) are added and stirred for 60-90 minutes. VC / FEC first forms a flexible SEI substrate at the negative electrode; PS / PST then constructs a high-voltage CEI film at the positive electrode; and LiPO2F2 / LiODFB finally repairs interface defects (simultaneously reducing impedance by 10-15%).

[0013] In step E, the interval between the sequential addition of 1,3-propanesulfonate lactone (PS) and 1,3-propenesulfonate lactone (PST) is ≥20 minutes. This timing design, prioritizing PS followed by PST, perfectly leverages the synergistic effect of both: PS acts as the foundation, forming a complete and dense base layer that effectively prevents direct contact between the electrolyte bulk and the highly active positive electrode, laying a solid foundation for subsequent modifications. PST acts as an optimizer, filling and covering the PS layer, repairing any microscopic defects in the PS layer, and optimizing the lithium-ion transport channels using its low impedance characteristics.

[0014] In step A, the solvent purification specifically involves: dehydrating ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) through a 4Å molecular sieve for 48 hours until the moisture content is ≤15ppm; the lithium salt pretreatment specifically involves: drying lithium hexafluorophosphate (LiPF6) under vacuum at 80℃ for 12 hours; and drying lithium bis(fluorosulfonyl)imide (LiFSI) under vacuum at 100℃ for 24 hours.

[0015] The synergistic effect of 1,3-propanesulfonate lactone, 1,3-propenesulfonate lactone, and 1-ethyl-3-methylimidazolium difluorosulfonylimide is mainly manifested in the following three interlocking levels: First, during the initial charging of the battery, PS and PST preferentially oxidize at around 4.1V-4.3V, polymerizing on the positive electrode surface to form a dense and tough primary CEI film. This film is fundamental, its main function being a physical barrier to prevent carbonate solvent molecules from directly contacting the highly active positive electrode surface. EMIMFSI, with an oxidation potential comparable to or slightly lower than PS / PST, also participates in film formation. Its preferential oxidation absorbs some of the initial oxidation shock, sacrificing itself to protect the primary film structure formed by PS / PST, making it more complete and stable. Furthermore, the molecular structure of EMIMFSI is smaller and more fluid than the polymerization products of PS / PST, allowing it to fill the microscopic defects and pores in the CEI film formed by PS / PST, making this protective film denser and non-porous, significantly enhancing its barrier properties. This synergy directly suppresses the continuous decomposition and gas generation of the electrolyte at high voltages, which is fundamental to improving high-temperature cycle life and reducing high-temperature storage swelling.

[0016] Secondly, the CEI film formed by PS / PST is mainly composed of organic polymers, and in the long term, its defense capability against small-molecule acidic substances (such as HF) produced by the decomposition of LiPF4 is limited. EMIM in EMIMFSI + Cations continuously coat the surface and pores of the CEI membrane through physical adsorption. This adsorbed ionic liquid layer acts as a "molecular fence," effectively blocking the penetration of subsequent water molecules and reducing the chance of moisture contacting LiPF6 from the outside. Its imidazole ring structure has a certain complexing or binding effect on small molecule acids such as HF, which can significantly reduce the "activity" of free HF and prevent it from penetrating the CEI membrane to corrode the cathode material. This synergistic effect inhibits HF damage at its source and along its transport path, greatly reducing the dissolution of the cathode transition metal and the destruction of the crystal structure, thereby significantly improving the capacity retention rate after high-temperature storage and further stabilizing the cycling performance.

[0017] Third, the overly dense polymer CEI film formed by PS / PST may result in high resistance to lithium-ion migration, which is detrimental to low-temperature performance and rate performance. EMIMFSI shares an FSI with LiFSI. - Anion. When EMIMFSI is present in the CEI film or at the interface, it is a Li-type anion. + The transmission provides a rich FSI - The local environment. FSI - It is an excellent ligand, and it is compatible with Li + The lower binding energy makes it easier for lithium ions to dissociate. This results in a lower energy barrier and faster speed for lithium ions to pass through this modified composite CEI film.

[0018] This synergistic effect effectively reduces battery polarization under high-rate charge and discharge and low-temperature environments, thereby directly improving low-temperature discharge performance and enabling the battery to have a more stable voltage plateau and lower heat generation during cycling (especially under high current).

[0019] In summary, the present invention has the following beneficial effects: The present invention uses a dual lithium salt system composed of lithium hexafluorophosphate and lithium difluorosulfonyl imide, and a three-solvent system composed of ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate. A composite additive is added, in which 1,3-propanesulfonyl lactone, 1,3-propenesulfonyl lactone, and 1-ethyl-3-methylimidazolium difluorosulfonyl imide synergistically form a dense film, inhibiting electrolyte decomposition under high voltage. Lithium difluorophosphate and lithium difluorooxalate borate have low film-forming impedance and effectively improve the high-temperature storage and cycling capabilities of the battery cell. Ethylene carbonate and fluoroethylene carbonate, as long-cycle film-forming additives, effectively improve the long-cycle capability of the battery cell. High-voltage ternary soft-pack batteries using this electrolyte can be stored at 60℃ for 70 days, cycled 500 times at 45℃, and retain a discharge rate of over 70% at -10℃ and 0.2C, meeting the requirements of conventional market use. Detailed Implementation

[0020] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. The embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0021] The embodiments of the present invention will now be described.

[0022] Example 1

[0023] A high-voltage ternary soft-pack electrolyte is prepared by adding composite additives to a lithium salt and solvent-based system. The basic system consists of the following raw materials by weight percentage: Lithium salt 12% Solvent 88% The lithium salt is composed of lithium hexafluorophosphate and lithium difluorosulfonylimide in a mass ratio of 6:4. The solvent is composed of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a mass ratio of 2:4:4. Add the following composite additive raw materials to the base system in the following weight percentages: Vinyl carbonate (VC) 0.1% Fluorinated ethylene carbonate (FEC) 0.5% 1,3-Propanesulfonate lactone (PS) 2% 1-Ethyl-3-methylimidazolium difluorosulfonylimide 1% Lithium difluorophosphate (LiPO2F2) 0.2% Lithium difluorooxalate borate (LiODFB) 0.2% 1,3-Propylenesulfonyl lactone (PST) 0.2%.

[0024] A method for preparing a high-voltage ternary soft-pack electrolyte includes the following preparation steps: Step A: Perform solvent purification and lithium salt pretreatment; Step B: First, add the prescribed amount of ethylene carbonate to the reaction vessel, and then add the prescribed amounts of methyl ethyl carbonate and diethyl carbonate under stirring; heat to 40±2℃ and stir continuously for 1 hour to obtain the composite solvent; Step C: At a temperature ≤45℃, LiPF6 and LiFSI are added to the composite solvent in batches and stirred until the conductivity of the solution is stable to obtain the basic electrolyte; Step D: Add the prescribed amount of 1-ethyl-3-methylimidazolium difluorosulfonylimide to the basic electrolyte and stir until homogeneous; Step E: Continue to add the formulated amounts of vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propanesulfonate lactone (PS), 1,3-propenesulfonate lactone (PST), lithium difluorophosphate (LiPO2F2), and lithium difluorooxalate borate (LiODFB) to the basic electrolyte obtained in step D in sequence. After standing and aging, filter to obtain the electrolyte.

[0025] In step C, the prescribed amount of LiPF6 is first added, the temperature is controlled at ≤10℃, and the mixture is stirred for 2 hours; then LiFSI is added, the temperature is raised to 25℃, and the mixture is kept warm and stirred for 5 hours.

[0026] In step E, vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are added sequentially and stirred for 30 minutes; then 1,3-propanesulfonyl lactone (PS) and 1,3-propenesulfonyl lactone (PST) are added sequentially and stirred for 40 minutes; finally, lithium difluorophosphate (LiPO2F2) and lithium difluorooxalate borate (LiODFB) are added and stirred for 60 minutes.

[0027] In step E, the interval between the sequential addition of 1,3-propanesulfonate lactone (PS) and 1,3-propenesulfonate lactone (PST) is ≥20 minutes.

[0028] In step A, the solvent purification specifically involves: dehydrating ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) through a 4Å molecular sieve for 48 hours until the moisture content is ≤15ppm; the lithium salt pretreatment specifically involves: drying lithium hexafluorophosphate (LiPF6) under vacuum at 80℃ for 12 hours; and drying lithium bis(fluorosulfonyl)imide (LiFSI) under vacuum at 100℃ for 24 hours.

[0029] Example 2

[0030] A high-voltage ternary soft-pack electrolyte is prepared by adding composite additives to a lithium salt and solvent-based system. The basic system consists of the following raw materials by weight percentage: Lithium salt 19% Solvent 81% The lithium salt is composed of lithium hexafluorophosphate and lithium difluorosulfonylimide in a mass ratio of 8:2. The solvent is composed of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a mass ratio of 3:5:2. Add the following composite additive raw materials to the base system in the following weight percentages: Vinyl carbonate (VC) 0.2% Fluorinated ethylene carbonate (FEC) 0.7% 1,3-Propanesulfonate lactone (PS) 2.4% 1-Ethyl-3-methylimidazolium difluorosulfonylimide 1.5% Lithium difluorophosphate (LiPO2F2) 0.4% Lithium difluorooxalate borate (LiODFB) 0.3% 1,3-Propylenesulfonate lactone (PST) 0.3%.

[0031] A method for preparing a high-voltage ternary soft-pack electrolyte includes the following preparation steps: Step A: Perform solvent purification and lithium salt pretreatment; Step B: First, add the prescribed amount of ethylene carbonate to the reaction vessel, and then add the prescribed amounts of methyl ethyl carbonate and diethyl carbonate under stirring conditions; heat to 40±2℃ and continue stirring for 2 hours to obtain the composite solvent; Step C: At a temperature ≤45℃, LiPF6 and LiFSI are added to the composite solvent in batches and stirred until the conductivity of the solution is stable to obtain the basic electrolyte; Step D: Add the prescribed amount of 1-ethyl-3-methylimidazolium difluorosulfonylimide to the basic electrolyte and stir until homogeneous; Step E: Continue to add the formulated amounts of vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propanesulfonate lactone (PS), 1,3-propenesulfonate lactone (PST), lithium difluorophosphate (LiPO2F2), and lithium difluorooxalate borate (LiODFB) to the basic electrolyte obtained in step D in sequence. After standing and aging, filter to obtain the electrolyte.

[0032] In step C, the prescribed amount of LiPF6 is first added, the temperature is controlled at ≤10℃, and the mixture is stirred for 3 hours; then LiFSI is added, the temperature is raised to 25℃, and the mixture is kept warm and stirred for 6 hours.

[0033] In step E, vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are added sequentially and stirred for 35 minutes; then 1,3-propanesulfonyl lactone (PS) and 1,3-propenesulfonyl lactone (PST) are added sequentially and stirred for 50 minutes; finally, lithium difluorophosphate (LiPO2F2) and lithium difluorooxalate borate (LiODFB) are added and stirred for 75 minutes.

[0034] In step E, the interval between the sequential addition of 1,3-propanesulfonate lactone (PS) and 1,3-propenesulfonate lactone (PST) is ≥20 minutes.

[0035] In step A, the solvent purification specifically involves: dehydrating ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) through a 4Å molecular sieve for 48 hours until the moisture content is ≤15ppm; the lithium salt pretreatment specifically involves: drying lithium hexafluorophosphate (LiPF6) under vacuum at 80℃ for 12 hours; and drying lithium bis(fluorosulfonyl)imide (LiFSI) under vacuum at 100℃ for 24 hours.

[0036] Example 3 A high-voltage ternary soft-pack electrolyte is prepared by adding composite additives to a lithium salt and solvent-based system. The basic system consists of the following raw materials by weight percentage: Lithium salt 23% Solvent 77% The lithium salt is composed of lithium hexafluorophosphate and lithium difluorosulfonyl imide in a mass ratio of 9:1. The solvent is composed of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a mass ratio of 3:4:3. Add the following composite additive raw materials to the base system in the following weight percentages: Vinyl carbonate (VC) 0.5% Fluorinated ethylene carbonate (FEC) 1% 1,3-Propanesulfonate lactone (PS) 3% 1-Ethyl-3-methylimidazolium difluorosulfonylimide 2% Lithium difluorophosphate (LiPO2F2) 0.8% Lithium difluorooxalate borate (LiODFB) 0.5% 1,3-Propylenesulfonyl lactone (PST) 0.5%.

[0037] A method for preparing a high-voltage ternary soft-pack electrolyte includes the following preparation steps: Step A: Perform solvent purification and lithium salt pretreatment; Step B: First, add the prescribed amount of ethylene carbonate to the reaction vessel, and then add the prescribed amounts of methyl ethyl carbonate and diethyl carbonate under stirring conditions; heat to 40±2℃ and stir continuously for 2 hours to obtain the composite solvent; Step C: At a temperature ≤45℃, LiPF6 and LiFSI are added to the composite solvent in batches and stirred until the conductivity of the solution is stable to obtain the basic electrolyte; Step D: Add the prescribed amount of 1-ethyl-3-methylimidazolium difluorosulfonylimide to the basic electrolyte and stir until homogeneous; Step E: Continue adding the formulated amounts of vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propanesulfonate lactone (PS), 1,3-propenesulfonate lactone (PST), and lithium difluorophosphate (LiPO2F) sequentially to the basic electrolyte obtained in Step D. 22 Lithium difluorooxalate borate (LiODFB) was used to obtain the electrolyte after standing and aging and filtration.

[0038] In step C, the prescribed amount of LiPF6 is first added, the temperature is controlled at ≤10℃, and the mixture is stirred for 4 hours; then LiFSI is added, the temperature is raised to 25℃, and the mixture is kept warm and stirred for 7 hours.

[0039] In step E, vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are added sequentially and stirred for 40 minutes; then 1,3-propanesulfonyl lactone (PS) and 1,3-propenesulfonyl lactone (PST) are added sequentially and stirred for 60 minutes; finally, lithium difluorophosphate (LiPO2F2) and lithium difluorooxalate borate (LiODFB) are added and stirred for 90 minutes.

[0040] In step E, the interval between the sequential addition of 1,3-propanesulfonate lactone (PS) and 1,3-propenesulfonate lactone (PST) is ≥20 minutes.

[0041] In step A, the solvent purification specifically involves: dehydrating ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) through a 4Å molecular sieve for 48 hours until the moisture content is ≤15ppm; the lithium salt pretreatment specifically involves: drying lithium hexafluorophosphate (LiPF6) under vacuum at 80℃ for 12 hours; and drying lithium bis(fluorosulfonyl)imide (LiFSI) under vacuum at 100℃ for 24 hours.

[0042] Comparative Example 1

[0043] The difference between this comparative example and Example 2 is that this example does not use both 1,3-propanesulfonate lactone and 1,3-propenesulfonate lactone (PST).

[0044] Comparative Example 2

[0045] The difference between this comparative example and Example 2 is that 1,3-propanesulfonate lactone was not used in this example.

[0046] Comparative Example 3

[0047] The difference between this comparative example and Example 2 is that 1,3-propenylsulfonate lactone (PST) was not used in this example.

[0048] Comparative Example 4

[0049] The difference between this comparative example and Example 2 is that 1-ethyl-3-methylimidazolium difluorosulfonylimide was not used in this invention.

[0050] Experimental data

[0051] The positive electrode slurry in this experiment was made by mixing lithium nickel cobalt manganese oxide, conductive agent, and binder, and was prepared with a single-sided slurry of 1.4 g / 100 cm. 2 The positive electrode has a surface density of 0.84 g / 100 cm². The negative electrode is made of a mixture of graphite, conductive agent, and binder, resulting in a single-sided surface density of 0.84 g / 100 cm². 2 The negative electrode is made of a 9+2+1.5um ceramic and PVDF double-coated separator to form a 503048-900mAh-4.4V high-voltage soft-pack cell, and is injected with the electrolyte prepared in Examples 1-3 and Comparative Examples 1-4 of this invention.

[0052] Test method: Standard charge and discharge: Charge the battery cell with a constant current of 0.2C to 4.4V, and then charge it with a constant voltage to the cutoff current of 0.02C. This is recorded as standard charge. After standard charge, discharge the battery cell with a constant current of 0.2C to 3.0V. This is recorded as standard discharge.

[0053] 1. Store at a high temperature of 60 degrees Celsius:

[0054] After the battery cell was subjected to standard charge and discharge at 23°C, it was then charged to full capacity using standard methods. The cell thickness and capacity were recorded. The cell was then placed in a 60°C environment and left for 7 days. After that, it was taken out and cooled to room temperature (23°C) for 2 hours. The thickness after the period of rest was recorded. The capacity was then recorded using standard discharge. The capacity after the test / the capacity before the test is the capacity retention rate. The thickness after the test - the thickness after the test) / the thickness before the test is the thickness bulging rate.

[0055] 2. Low-temperature discharge at -10 degrees Celsius and 0.2C: After the battery cell is subjected to standard charge and discharge at 23℃, it is then fully charged using standard charging, placed in a -20℃ environment for 2 hours, discharged at 0.2C to 3.0V, and the capacity is recorded. The capacity after the test / the capacity before the test is the capacity retention rate.

[0056] 3. High-temperature 45-degree cycling: The battery cell was cycled at 45℃ using a 0.5C current according to the standard charging method. The capacity before and after the cycle was recorded. The capacity after the test / the capacity before the test was the capacity retention rate. The cycle count was recorded until the capacity retention rate was lower than 80%.

[0057] Comparative Example 1, which did not use either PS or PST, exhibited the highest high-temperature swelling rate, the lowest capacity retention rate, and the shortest cycle life. This was primarily due to the complete lack of positive electrode interface protection: the simultaneous absence of both PS and PST meant that an effective, dense, high-voltage-specific CEI film could not be formed. The positive electrode material was directly exposed to the electrolyte, and under the catalysis of high voltage and high temperature, the electrolyte solvent (EC / EMC / DEC) underwent severe oxidative decomposition, generating a large amount of gas (leading to extremely high swelling rate) and impedance substances. This severe decomposition consumed active lithium and electrolyte, resulting in rapid capacity decay. Simultaneously, the positive electrode structure was unstable due to the lack of protection, leading to severe dissolution of transition metals, further damaging the negative electrode SEI film, creating a vicious cycle and drastically reducing cycle life. Therefore, PS and PST are indispensable as basic high-voltage film-forming agents; their absence would cause the battery to fail rapidly under high voltage.

[0058] Comparative Example 2, which did not use PS, exhibited inferior high-temperature performance and cycle life compared to Example 2, but superior to Comparative Example 1. This was due to the lack of PS as a foundational element: PS's role is to form a dense and robust basic CEI layer. Without PS, the CEI film formed solely by PST and EMIMFSI lacks sufficient mechanical strength and density, failing to effectively block the penetration and decomposition of solvent molecules. The absence of PS disrupts the synergistic effect of the PS foundation + PST optimizer + EMIMFSI filling / protection. While PST can form a film, the film layer may be too soft or insufficiently dense, leading to insufficient long-term stability and a significant decline in high-temperature storage and cycling performance. Therefore, PS is crucial as the framework of the basic CEI film; its function is complementary to that of PST and they are not interchangeable.

[0059] Comparative Example 3, which did not use PST, exhibited performance between Comparative Example 2 and Example 2, but with a significant decrease in cycle life. This is due to the lack of PST as an optimizer: PST has higher activity than PS and can "modify" and "fill" the dense substrate formed by PS. Without PST, the CEI film is entirely constructed from PS and EMIMFSI, which may result in higher impedance and insufficient ability to repair micro-defects. Furthermore, the lack of PST's optimizing effect on ion transport channels leads to increased cell polarization, which exacerbates side reactions and limits capacity utilization during cycling, thus negatively impacting cycle life and rate performance (affecting capacity retention during cycle testing). Therefore, PST, as an optimizer, plays a crucial role in reducing interfacial impedance and improving cycle stability.

[0060] Comparative Example 4, which used EMIMFSI, showed significantly worse performance across all aspects, especially cycle life, compared to Example 2. This is because without the "molecular barrier" effect of EMIMFSI, the CEI film formed by PS / PST is more susceptible to HF corrosion, leading to corrosion of the cathode material, increased dissolution of transition metals, and consequently, a decrease in high-temperature storage capacity retention and a shortened cycle life. Without the "sacrificial protection" and "optimized filling" of EMIMFSI, the CEI film formed by PS / PST may be less perfect, exhibiting more defects and weakened inhibition of electrolyte decomposition, resulting in a higher bulging rate than in Example 2. Interfacial conductivity deteriorated due to the lack of FSI-rich material provided by EMIMFSI. - In certain localized environments, the resistance of lithium ions crossing the CEI film increases, leading to increased polarization and accelerated capacity decay during cycling, thus limiting cycle life. This also explains the slight decrease in low-temperature performance. Therefore, EMIMFSI is not an optional component; its deep synergy with PS / PST is key to achieving ultra-long cycle life and excellent high-temperature stability. Its addition elevates the CEI film constructed from PS / PST from good to excellent.

[0061] In summary, PS and PST are fundamental to constructing stable high-voltage CEI membranes; neither can be dispensed with, and their functions are complementary. EMIMFSI is a key "performance multiplier," playing an irreplaceable role in improving the density, corrosion resistance, and ion conductivity of CEI membranes through multi-level synergy with PS / PST. The success of Examples 1-3 is the inevitable result of the powerful synergistic effect generated by the combined action of the LiPF6 / LiFSI dual-salt system, the EC / EMC / DEC solvent system, and the PS / PST / EMIMFSI composite additive system.

[0062] Although embodiments of the present invention have been shown and described, these specific embodiments are merely explanations of the invention and are not intended to limit it. The specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. After reading this specification, those skilled in the art may make modifications, substitutions, and variations to the embodiments as needed without departing from the principles and spirit of the invention, but such modifications, substitutions, and variations are protected by patent law as long as they are within the scope of the claims of the present invention.

Claims

1. A high-voltage ternary soft-pack electrolyte, characterized in that... It is prepared by adding composite additives to a lithium salt and solvent-based system. The basic system consists of the following raw materials by weight percentage: Lithium salts 10-25% Solvent 75-90% The lithium salt is composed of lithium hexafluorophosphate and lithium difluorosulfonylimide in a mass ratio of (6-9):(4-1); The solvent is composed of ethylene carbonate, ethyl methyl carbonate and diethyl carbonate in a mass ratio of (2-4):(4-6):(1-4); Add the following composite additive raw materials to the base system in the following weight percentages: Vinylene carbonate 0.1%-0.5% Fluorinated ethylene carbonate 0.5%-1% 1,3-Propanesulfonate lactone 2%-3% 1-Ethyl-3-methylimidazolium difluorosulfonylimide 1%-2% Lithium difluorophosphate 0.2%-0.8% Lithium difluorooxalate borate 0.2-0.5% 1,3-Propylenesulfonyl lactone 0.2-0.5%.

2. The high-voltage ternary soft-pack electrolyte according to claim 1, characterized in that, Add the following composite additive raw materials to the base system in the following weight percentages: Vinylene carbonate 0.1%-0.3% Fluorinated ethylene carbonate 0.5%-0.8% 1,3-Propanesulfonate lactone 2%-2.5% 1-Ethyl-3-methylimidazolium difluorosulfonylimide 1%-2% Lithium difluorophosphate 0.2%-0.6% Lithium difluorooxalate borate 0.2-0.4% 1,3-Propylenesulfonyl lactone 0.2-0.4%.

3. The high-voltage ternary soft-pack electrolyte according to claim 1, characterized in that, Add the following composite additive raw materials to the base system in the following weight percentages: Vinylene carbonate 0.2% 0.7% fluoroethylene carbonate 1,3-Propanesulfonate lactone 2.4% 1-Ethyl-3-methylimidazolium difluorosulfonylimide 1.5% Lithium difluorophosphate 0.4% Lithium difluorooxalate borate 0.3% 1,3-Propylenesulfonyl lactone 0.3%.

4. A method for preparing a high-voltage ternary soft-pack electrolyte as described in any one of claims 1-3, characterized in that, The preparation steps include the following: Step A: Perform solvent purification and lithium salt pretreatment; Step B: First, add the prescribed amount of ethylene carbonate to the reaction vessel, and then add the prescribed amounts of methyl ethyl carbonate and diethyl carbonate under stirring; heat to 40±2℃ and continue stirring for 1-2 hours to obtain the composite solvent; Step C: At a temperature ≤45℃, LiPF6 and LiFSI are added to the composite solvent in batches and stirred until the conductivity of the solution is stable to obtain the basic electrolyte; Step D: Add the prescribed amount of 1-ethyl-3-methylimidazolium difluorosulfonylimide to the basic electrolyte and stir until homogeneous; Step E: Continue to add the formulated amounts of vinylene carbonate, fluoroethylene carbonate, 1,3-propanesulfonate lactone, 1,3-propenesulfonate lactone, lithium difluorophosphate, and lithium difluorooxalate borate to the basic electrolyte obtained in step D in sequence. After standing and aging, filter to obtain the electrolyte.

5. The method for preparing a high-voltage ternary soft-pack electrolyte according to claim 1, characterized in that, In step C, first add the prescribed amount of LiPF6, control the temperature to ≤10℃, and stir for 2-4 hours; then add LiFSI, raise the temperature to 25℃, and keep stirring for 5-7 hours.

6. The method for preparing a high-voltage ternary soft-pack electrolyte according to claim 1, characterized in that, In step E, first add vinylene carbonate and fluoroethylene carbonate in sequence, and stir for 30-40 minutes; then add 1,3-propanesulfonate lactone and 1,3-propenesulfonate lactone in sequence, and stir for 40-60 minutes; finally add lithium difluorophosphate and lithium difluorooxalate borate, and stir for 60-90 minutes.

7. The method for preparing a high-voltage ternary soft-pack electrolyte according to claim 6, characterized in that, In step E, the interval between the sequential addition of 1,3-propanesulfonate lactone and 1,3-propenesulfonate lactone shall be ≥20 minutes.

8. The method for preparing a high-voltage ternary soft-pack electrolyte according to claim 4, characterized in that, In step A, the solvent purification specifically involves: dehydrating ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) through a 4Å molecular sieve for 48 hours until the moisture content is ≤15ppm; the lithium salt pretreatment specifically involves: drying lithium hexafluorophosphate (LiPF6) under vacuum at 80℃ for 12 hours; and drying lithium bis(fluorosulfonyl)imide (LiFSI) under vacuum at 100℃ for 24 hours.

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