Low-temperature safe lithium iron phosphate battery electrolyte and preparation method thereof

Abstract

The invention discloses a low-temperature safe lithium iron phosphate battery electrolyte and a preparation method thereof, and belongs to the technical field of lithium battery electrolyte. The electrolyte is composed of three parts, namely a ternary composite solvent, a ternary composite lithium salt and a functional composite additive, wherein the ternary composite solvent is composed of an ether-based solvent, a phosphate solvent and a carbonate solvent, and the ternary composite lithium salt is composed of lithium hexafluorophosphate, lithium bis (fluorosulfonyl) imide and lithium difluoro (oxalato) borate. According to the preparation method, a step-by-step charging process is adopted, a solvent, a lithium salt and an additive are sequentially added according to a specific sequence, and a finished product is prepared through aging inspection, filtering and filling. According to the invention, low-temperature ion conduction and flame-retardant safety are synergistically improved through the ternary composite solvent, wide-temperature-range high conductivity and aluminum foil compatibility are considered through the ternary composite lithium salt, and a gradient double-layer interfacial film with inorganic inside and organic outside is constructed through reduction potential gradient difference of the functionalized additive so as to reduce low-temperature interface impedance; and a lithium ion solvation structure is regulated and controlled through a step-by-step preparation process, so that the low-temperature performance and the batch stability are improved.

Description

Technical Field

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

[0002] Lithium iron phosphate (LFP) batteries have been widely used in power batteries and energy storage systems due to their advantages such as good thermal stability, long cycle life, and low raw material cost. The electrolyte, as the ion transport medium connecting the positive and negative electrodes in a lithium battery, directly determines the battery's electrochemical performance under different operating conditions. Conventional commercial electrolytes typically use carbonate solvents as the main component and lithium hexafluorophosphate as the lithium salt. This system provides good ionic conductivity and electrochemical stability within a normal temperature range, meeting the needs of general applications. However, as new energy vehicles and energy storage technologies are increasingly applied to extreme low-temperature environments such as high latitudes and high altitudes, the insufficient performance of traditional carbonate-based electrolytes under low-temperature conditions is gradually becoming a core bottleneck restricting the application range of LFP batteries.

[0003] At low temperatures, the viscosity of carbonate solvents increases sharply, and the ion migration rate decreases significantly, leading to a substantial decline in the ionic conductivity of the electrolyte and severe polarization and capacity loss in the battery. To improve the low-temperature performance of the electrolyte, various improvement approaches have emerged in existing technologies. One approach is to introduce low-freezing-point ester or ether solvents into the carbonate system to reduce the electrolyte's viscosity and freezing point. However, these low-freezing-point solvents often have low flash points and insufficient oxidation stability, sacrificing electrolyte safety while improving low-temperature performance. Another approach is to use novel lithium salts to replace or partially replace traditional lithium hexafluorophosphate, such as lithium bis(fluorosulfonyl)imide to improve low-temperature ion conductivity. However, lithium bis(fluorosulfonyl)imide, when used in high proportions, can corrode the aluminum foil current collector, affecting the long-term reliability of the battery. Furthermore, some studies have optimized the solid electrolyte interface film structure on the negative electrode surface by adding film-forming additives to the electrolyte. However, single types or simple combinations of additives are insufficient to construct a high-quality interface film with rapid low-temperature ion transport capabilities on the negative electrode surface. In terms of electrolyte preparation processes, existing technologies generally employ a conventional method that involves mixing all solvents, lithium salts, and additives in a single step. This one-step preparation process ignores the differences in the exothermic dissolution of different lithium salts and the influence of the coordination competition between anions and lithium ions on the solvation structure. This results in an uneven solvation shell structure for lithium ions in the electrolyte, significant batch-to-batch performance fluctuations, and difficulty in consistently obtaining optimal low-temperature ion transport characteristics.

[0004] In summary, the existing technology lacks a lithium iron phosphate battery electrolyte solution that can simultaneously achieve low-temperature ion conduction performance, flame retardant safety, aluminum foil compatibility, and cycle stability. It also lacks an electrolyte preparation method that can precisely control the lithium-ion solvation structure to improve low-temperature performance and batch stability. Summary of the Invention

[0005] To address the problems existing in the background art, the present invention provides a low-temperature safe lithium iron phosphate battery electrolyte, which is made from the following components in parts by weight: 13-22 parts of 2-methyltetrahydrofuran, 8-15 parts of cyclopentylmethyl ether, 8-16 parts of triethyl phosphate, 16-27 parts of methyl ethyl carbonate, 16-27 parts of dimethyl carbonate, 5-8 parts of lithium hexafluorophosphate, 3-5 parts of lithium difluorosulfonyl imide, 1.1-2.3 parts of lithium difluorooxalate borate, 1.2-2.5 parts of vinylene carbonate, 0.5-1.5 parts of fluoroethylene carbonate, 0.4-1.0 parts of trimethylsilyl phosphate, and 1.0-3.0 parts of fluoromethyl ethyl carbonate.

[0006] In a preferred embodiment, the 2-methyltetrahydrofuran and cyclopentylmethyl ether are each prepared via the following steps: 2-methyltetrahydrofuran or cyclopentylmethyl ether is added to a sealed glass container pre-filled with activated 3Å molecular sieves, and the container is sealed and left to stand at 20–25°C for at least 16 hours under argon protection. The molecular sieve is removed by filtration through a polytetrafluoroethylene (PTFE) membrane, and the filtrate is collected. The filtrate is then transferred to a round-bottom flask equipped with a distillation apparatus and distilled under atmospheric pressure under argon protection. The fractions of 2-methyltetrahydrofuran with a boiling range of 79–81 °C and cyclopentylmethyl ether with a boiling range of 105–107 °C are collected, which are the prepared 2-methyltetrahydrofuran and cyclopentylmethyl ether.

[0007] In a preferred embodiment, the triethyl phosphate, methyl ethyl carbonate, and dimethyl carbonate are each prepared via the following steps: commercially available triethyl phosphate, methyl ethyl carbonate, or dimethyl carbonate are added to a sealed container pre-filled with activated 4Å molecular sieves, and the container is sealed and allowed to stand at 20–25°C for at least 12 hours under argon protection. The molecular sieve is removed by filtration through a polytetrafluoroethylene (PTFE) membrane, and the filtrate is collected, which is the prepared triethyl phosphate, methyl ethyl carbonate, and dimethyl carbonate.

[0008] In a preferred embodiment, the lithium hexafluorophosphate, lithium difluorosulfonylimide, and lithium difluorooxalate borate are all battery grade and are dried in a vacuum drying oven at 80–90°C for 10–12 hours before use.

[0009] This invention also provides a method for preparing a low-temperature safe lithium iron phosphate battery electrolyte, which is carried out in a closed inert atmosphere according to the following steps: S1: The dried and pretreated 2-methyltetrahydrofuran, cyclopentylmethyl ether, triethyl phosphate, methyl ethyl carbonate and dimethyl carbonate are added sequentially to a reaction vessel equipped with a seal and a stirring device according to the formula amount and mixed to obtain a clear and homogeneous mixed solvent. S2: Add the dried and pretreated lithium hexafluorophosphate, lithium difluorosulfonylimide, and lithium difluorooxalate borate as the main lithium salt to the mixed solvent obtained in S1 in sequence. After each lithium salt is added, stir thoroughly until completely dissolved to obtain a composite lithium salt solution. S3: To the composite lithium salt solution obtained in S2, lithium difluorooxalate borate, trimethylsilyl phosphate, fluoroethylene carbonate, vinylene carbonate and fluoroethyl carbonate are added in sequence as film-forming additives. After each component is added, the mixture is stirred until it is completely miscible with the solution to obtain crude electrolyte. S4: The crude electrolyte obtained in S3 is left to stand and age under a sealed inert atmosphere. After passing the tests for moisture and free acid content, it is filtered and filled through a microporous membrane to obtain the low-temperature safe lithium iron phosphate battery electrolyte.

[0010] Furthermore, S1 includes the following operations: S11: Add 2-methyltetrahydrofuran and cyclopentylmethyl ether to the reactor at the same time, start stirring until the two are completely miscible, and visually observe that the liquid is clear and without layering to obtain an ether-based mixed solvent. S12: Slowly add the formulated amount of triethyl phosphate to the ether-based mixed solvent obtained in S11, and stir until the liquid is clear and homogeneous, without oil droplets or turbidity, to obtain an ether-phosphate mixture. S13: First, add the prescribed amount of methyl ethyl carbonate to the ether-phosphate mixture obtained in S12, stir until clear and homogeneous, then add the prescribed amount of dimethyl carbonate, and continue stirring until the liquid is completely mixed and there is no turbidity or separation visible to the naked eye, to obtain the mixed solvent.

[0011] Furthermore, S2 includes the following operations: S21: Divide the amount of lithium hexafluorophosphate in the formula into 3 to 5 equal parts. Add the first part to the mixed solvent obtained in S1 and stir until the solid particles disappear completely and the solution becomes clear. Then add the next part, and so on, until all the lithium hexafluorophosphate is added and completely dissolved to obtain a solution containing lithium hexafluorophosphate. S22: Add the prescribed amount of lithium difluorosulfonylimide to the solution obtained in S21 in one go, and stir until the solid completely disappears and the solution becomes clear to obtain a mixed lithium salt solution containing lithium difluorosulfonylimide and lithium hexafluorophosphate. S23: Add 0.8 to 1.5 parts of lithium difluorooxalate borate to the solution obtained in S22, and stir until the solid completely disappears and the solution is clear and slightly yellow to obtain the composite lithium salt solution.

[0012] Furthermore, S3 includes the following operations: S31: Add 0.3 to 0.8 parts of lithium difluorooxalate borate to the composite lithium salt solution obtained in S2, and stir until the solid is completely dissolved and the solution remains clear and transparent; S32: Add the prescribed amount of trimethylsilyl phosphate dropwise to the solution obtained in S31. After the addition is complete, stir until the solution is clear and homogeneous, without oil droplets or suspended matter. S33: First add the prescribed amount of fluoroethylene carbonate to the solution obtained in S32, stir until completely dissolved, then add the prescribed amount of vinylene carbonate, and stir until the solution is clear and homogeneous; S34: Add the prescribed amount of methyl ethyl fluorocarbonate to the solution obtained in S33, and stir until the liquid is completely miscible and the solution is clear to obtain the crude electrolyte.

[0013] Furthermore, S4 includes the following operations: S41: Transfer the crude electrolyte obtained in S3 into a sealed inert atmosphere container and let it stand for aging. The aging is considered complete when no solid precipitates at the bottom of the container, the liquid is clear and transparent, and the color of the upper and lower layers is uniform. S42: Take samples from the aged electrolyte to test the moisture content and free hydrogen fluoride content; S43: The qualified electrolyte is passed through a dry, inert gas atmosphere sequentially. and Filtration using polytetrafluoroethylene (PTFE) membranes with varying pore sizes, followed by filtrate filling into containers... The finished low-temperature safe lithium iron phosphate battery electrolyte is obtained by placing it in a sealed container after drying.

[0014] The beneficial effects achieved by this invention are as follows: This invention constructs a ternary composite solvent system by combining ether-based solvents, phosphate ester solvents, and carbonate solvents in a specific mass ratio. It fully utilizes the low freezing point and low viscosity at low temperatures of ether-based solvents to reduce the viscosity and freezing point of the electrolyte at low temperatures. The flame-retardant properties of the phosphorus-containing groups in the phosphate ester solvents endow the electrolyte with self-extinguishing ability. At the same time, the high dielectric constant of the carbonate solvents maintains the full dissociation of lithium salts and high ionic conductivity at room temperature. The synergistic effect of the three types of solvents enables the electrolyte to maintain high ionic conductivity even at extremely low temperatures and exhibits excellent flame-retardant and self-extinguishing properties in terms of safety. This overcomes the inherent defects of traditional carbonate single solvent systems, which have a sharp increase in viscosity and a sharp drop in ionic conductivity at low temperatures, and also solves the safety problems caused by simply introducing a low freezing point solvent.

[0015] This invention employs a ternary composite lithium salt system composed of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium difluorooxalate borate in a specific ratio. Lithium hexafluorophosphate forms a stable passivation protective layer on the surface of the aluminum foil current collector, effectively preventing corrosion during charging and discharging. Lithium bis(fluorosulfonyl)imide, with its weak anionic solvation characteristics, helps reduce the desolvation activation energy of lithium ions at low temperatures, thereby improving the low-temperature ion migration rate. Lithium difluorooxalate borate combines lithium salt functionality with film-forming capabilities; its anionic oxalate groups can complex trace amounts of water and hydrogen fluoride in the solution to maintain the chemical stability of the electrolyte. The three lithium salts synergistically regulate the lithium ion solvation shell structure through competitive coordination between anions, enabling the electrolyte to maintain good ion conductivity over a wide temperature range and effectively solving the corrosion problem of aluminum foil caused by a high proportion of lithium bis(fluorosulfonyl)imide.

[0016] This invention designs a functionalized composite additive system consisting of lithium difluorooxalate borate, trimethylsilyl phosphate, fluoroethylene carbonate, vinylene carbonate, and fluoromethyl ethyl carbonate. Each additive has a different reduction potential, and during battery formation, they can be reduced and decomposed sequentially on the negative electrode surface in order of decreasing reduction potential to form a film. This results in a gradient bilayer solid electrolyte interface film structure on the negative electrode surface, consisting of a dense inorganic inner layer and a flexible organic outer layer. The inner layer of this structure provides a low-impedance, fast transport channel for lithium ions and has high mechanical strength, while the outer organic layer has good elasticity to adapt to volume changes in the negative electrode material at low temperatures, thus preventing the interface film from cracking during cycling. Trimethylsilyl phosphate can also form a protective layer on the positive electrode surface and continuously remove hydrogen fluoride from the electrolyte to maintain the long-term stability of the electrolyte. The construction of the gradient bilayer interface film significantly reduces the interfacial migration resistance of lithium ions at low temperatures, resulting in a significant improvement in the battery's discharge capacity in extremely low-temperature environments. At the same time, it effectively improves the problem of capacity decay caused by continuous thickening of the interface film during room temperature cycling.

[0017] The preparation method provided by this invention employs a stepwise feeding process. Five solvents are mixed sequentially in the order of ether-based solvent, phosphate ester solvent, and carbonate solvent to avoid transient phase separation caused by rapid mixing. Three lithium salts are added stepwise in order of decreasing heat of dissolution to control the temperature rise of the system, suppress the hydrolysis side reaction of lithium hexafluorophosphate, and thus reduce the generation of hydrogen fluoride. A uniform and thin weakly solvated structure is constructed by utilizing the gradual competitive coordination of the lithium-ion solvated shell after the dissolution of each lithium salt anion. Functional additives are added sequentially in order of decreasing reduction potential to ensure that each component can form a film in a predetermined order during the subsequent battery formation process. The entire stepwise preparation process achieves effective regulation of the lithium-ion solvated structure and the interfacial film formation process through precise control of the feeding sequence, improving the ion conduction performance and batch stability of the electrolyte under low temperature conditions. It overcomes the problems of uneven solvated structure and batch-to-batch performance fluctuations caused by neglecting the heat of dissolution and coordination competition in the traditional one-step preparation process. Attached Figure Description

[0018] Figure 1 This is a flowchart of a low-temperature safe lithium iron phosphate battery electrolyte and its preparation method according to the present invention. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. In addition, the forms of the various structures described in the following embodiments are merely illustrative. The present invention is not limited to the structures described in the following embodiments. All other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] Example 1: This example provides a low-temperature safe lithium iron phosphate battery electrolyte, made from the following components in parts by weight: 2-methyltetrahydrofuran (2-MeTHF), a five-membered ring ether solvent with a melting point of approximately -137°C and a boiling point of approximately 80°C (20 parts); cyclopentyl methyl ether (CPME), a low-polarity ether solvent with a boiling point of approximately 106°C (14 parts); triethyl phosphate (TEP), a phosphorus-containing organic ester solvent with flame-retardant properties (12 parts); ethyl methyl carbonate (EMC), a linear carbonate solvent with low viscosity and moderate dielectric constant (27 parts); dimethyl carbonate (DMC), a linear carbonate solvent with a melting point of approximately 4.6°C and low viscosity (27 parts); lithium hexafluorophosphate (LiPF6), with the molecular formula LiPF6 and a molecular weight of 151.91 g / mol, the most commonly used main salt in lithium batteries (7.2 parts); lithium bis(fluorosulfonyl)imide (LiFSI), with the molecular formula LiN(SO2F)2 and a molecular weight of 187.07 g / mol, a... Novel lithium salts with excellent low-temperature ionic conductivity and high thermal decomposition temperature (3.6 parts); Lithium difluorooxalate borate (LiODFB), molecular formula LiBF2(C2O4), molecular weight 143.77 g / mol, with dual functions as lithium salt and film-forming additive (1.7 parts in total, of which 1.2 parts are used as the main lithium salt and 0.5 parts are used as film-forming additive); Vinylene carbonate (VC), an unsaturated cyclic carbonate, is a widely used negative electrode film-forming additive in lithium batteries (2.0 parts); Fluorinated ethylene carbonate (FEC), a fluorinated cyclic carbonate, can decompose on the negative electrode surface to generate a solid electrolyte interface film rich in lithium fluoride (1.0 part); Trimethylsilyl phosphate (TMSP), a silicon-containing organophosphate, can effectively capture hydrogen fluoride in the electrolyte and form a protective layer on the positive electrode surface (0.8 parts); and Fluorinated methyl ethyl carbonate (FEMC), a fluorinated linear carbonate with high flash point, with dual functions of flame retardancy and film formation (2.0 parts).

[0021] Reference Figure 1 The preparation method of the electrolyte described above is carried out in a glove box with a closed inert atmosphere and argon purity ≥99.999% and dew point ≤-70℃, and is implemented according to the following steps.

[0022] First, the solvent was pretreated by drying. For ether-based solvents, 20 parts of 2-methyltetrahydrofuran were added to a pre-filled container with activated 3Å molecular sieves (3Å, or approximately 0.3 nm pore size), which selectively adsorbs water molecules but not organic solvent molecules. Activation conditions were: activation in a muffle furnace at 300℃ for 4 hours, followed by cooling to room temperature in a sealed glass container. The mass ratio of molecular sieve to solvent was approximately 1:5. The container was sealed and allowed to stand at 22℃ for 18 hours under argon protection. The molecular sieve particles were removed by filtration through a 0.45 μm PTFE membrane, and the filtrate was collected. The filtrate was transferred to a round-bottom flask equipped with a distillation apparatus and distilled at atmospheric pressure under argon protection. The fraction with a boiling range of 79–81℃ was collected, which was the purified 2-methyltetrahydrofuran. 14 parts of cyclopentylmethyl ether were treated in the same way, and the fraction with a boiling range of 105–107℃ was collected. After the above treatment, the water content in the ether-based solvent can be reduced to below 3 ppm, and the impurity content is significantly reduced, thus laying the foundation for inhibiting the hydrolysis of lithium hexafluorophosphate and reducing the generation of hydrogen fluoride in the subsequent preparation process.

[0023] For the drying pretreatment of phosphate ester and carbonate solvents, 12 parts of commercially available battery-grade triethyl phosphate with a purity ≥99.5% were added to a sealed container pre-loaded with activated 4Å molecular sieves (pore size approximately 0.4 nm), suitable for dehydrating highly polar ester solvents. The container was sealed and allowed to stand for 14 hours at 22°C under argon protection. The molecular sieve was then removed by filtration through a 0.45 μm polytetrafluoroethylene (PTFE) membrane, and the filtrate was collected. The same method was used to pretreat 27 parts of methyl ethyl carbonate and 27 parts of dimethyl carbonate, respectively. After treatment, the moisture content of each solvent was below 5 ppm.

[0024] For the drying pretreatment of lithium salts, 7.2 parts of lithium hexafluorophosphate, 3.6 parts of lithium difluorosulfonylimide, and 1.7 parts of lithium difluorooxalate borate, all battery-grade with a purity ≥99.9%, were placed in a vacuum drying oven and dried at 85°C and a vacuum degree ≤-0.09MPa for 12 hours. After drying, they were immediately transferred to a glove box for later use. This step can remove adsorbed moisture from the surface of the lithium salts and avoid side reactions caused by moisture during the dissolution process.

[0025] S1: Preparation of the mixed solvent. This step involves mixing five pre-treated solvents in a specific order to obtain a clear and homogeneous mixed solvent system.

[0026] S11: Add 20 parts of pretreated 2-methyltetrahydrofuran and 14 parts of cyclopentylmethyl ether to a 500mL glass reactor equipped with a sealed lid and magnetic stirrer. Start the stirrer at 200rpm and stir for about 15 minutes until the two are completely miscible. Visually observe that the liquid is clear and without layering, thus obtaining an ether-based mixed solvent. Both 2-methyltetrahydrofuran and cyclopentylmethyl ether are low-viscosity ether solvents with good miscibility. Mixing them can significantly lower the freezing point of the system because the intermolecular forces of ether solvents are weak, making it difficult for them to form an ordered lattice at low temperatures.

[0027] S12: Slowly add 12 parts of pretreated triethyl phosphate to the ether-based mixed solvent obtained in S11, stirring at 200 rpm for about 20 minutes until the liquid is clear and homogeneous, without oil droplets or turbidity, to obtain an ether-phosphate mixture. The introduction of triethyl phosphate not only provides flame retardant properties—phosphorus can generate phosphate compounds during combustion that cover the material surface, isolating oxygen and providing dual flame retardant effects in both the gas and condensed phases—but its moderate dielectric constant also facilitates the dissolution of lithium salts.

[0028] S13: Add 27 parts of pretreated methyl ethyl carbonate to the ether-phosphate mixture obtained in S12, stir for about 15 minutes until the liquid is clear and homogeneous, then add 27 parts of dimethyl carbonate, and continue stirring for about 20 minutes until the liquid is completely miscible and there is no turbidity or layering visible to the naked eye, thus obtaining a mixed solvent. The order of adding the carbonate solvent is methyl ethyl carbonate first, then dimethyl carbonate. This is because the molecular asymmetry of methyl ethyl carbonate makes it slightly more compatible with ether solvents than dimethyl carbonate. Adding methyl ethyl carbonate first can avoid transient phase separation in the ether-carbonate system during rapid mixing. The final mixed solvent is a colorless and transparent liquid with good fluidity at room temperature.

[0029] S2: Preparation of the composite lithium salt solution. In this step, three lithium salts are sequentially dissolved in the mixed solvent obtained in S1 to form a ternary composite lithium salt solution. The order of addition of the lithium salts was optimized: lithium hexafluorophosphate, which has the greatest exothermic reaction at dissolution, is added first, followed by lithium bis(fluorosulfonyl)imide, and finally lithium difluorooxalateborate. This order is beneficial for the orderly competitive coordination of each anion in the lithium-ion solvation shell, thus constructing a weakly solvation structure.

[0030] S21: Divide 7.2 parts of dried lithium hexafluorophosphate into 4 equal portions, each approximately 1.8 parts. Add the first portion to the mixed solvent obtained in S1 and stir at 150 rpm. Observe the solid particles gradually dissolve, and the solution changes from colorless to slightly yellow. After the solid completely disappears and the solution becomes clear, about 8-12 minutes later, add the second portion, and so on, until all the lithium hexafluorophosphate has been added and completely dissolved. The dissolution process of lithium hexafluorophosphate is an exothermic reaction. Adding it in batches can effectively control the temperature rise of the system and avoid excessively high local temperatures that could lead to the hydrolysis side reaction of lithium hexafluorophosphate: LiPF6 + H2O → LiF↓ + POF3↑ + 2HF. This side reaction is significantly accelerated at temperatures above 40℃. By using the batch addition method, the measured system temperature was always controlled below 30℃, thereby controlling the free hydrogen fluoride content to below 15 ppm, resulting in a solution containing lithium hexafluorophosphate.

[0031] S22: Add 3.6 parts of lithium difluorosulfonylimide to the solution obtained in S21 all at once, and stir at 150 rpm for about 25 minutes until the solid completely disappears and the solution becomes clear. The heat of dissolution of lithium difluorosulfonylimide is much lower than that of lithium hexafluorophosphate, so it can be added all at once. FSI of lithium difluorosulfonylimide - The anion has weak Lewis basicity and reacts with the already formed PF6 after entering the solution. - -Li + The solvation shell undergoes ligand exchange competition, partially replacing solvent molecules in the shell and reducing the thickness of the lithium-ion solvation layer. This is beneficial for lowering the desolvation activation energy of lithium ions at the electrode / electrolyte interface under low-temperature conditions. A mixed lithium salt solution containing lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate is obtained.

[0032] S23: Add 1.2 parts of lithium difluorooxalate borate to the solution obtained in S22, and stir at 150 rpm for about 30 minutes until the solid completely disappears and the solution is clear and slightly yellow. ODFB of lithium difluorooxalate borate. - The anion contains an oxalate group (-C2O4) and a BF bond. After entering the solution, it can further participate in regulating the lithium-ion coordination environment, causing the average coordination number of lithium ions to tend towards 4–5, close to the theoretical optimum. Simultaneously, ODFB… - The oxalate group can complex trace amounts of water and hydrogen fluoride in the solution, thus stabilizing the chemical properties of the electrolyte. A composite lithium salt solution is obtained.

[0033] S3: Addition of functionalized composite additives. This step involves adding the additives in descending order of their reduction potential during battery formation to ensure that during subsequent battery charging and formation, the additives can be reduced and formed into films on the negative electrode surface in a predetermined order, thus constructing a gradient bilayer solid electrolyte interlayer (SEI) film structure with an inner inorganic layer and an outer organic layer.

[0034] S31: Add 0.5 parts of lithium difluorooxalate borate, used as a film-forming additive, to the composite lithium salt solution obtained in S2. Stir at 150 rpm for approximately 15 minutes until the solid is completely dissolved, and the solution remains clear and transparent. This portion of lithium difluorooxalate borate, used as an additive component, has a reduction potential of approximately 2.2V relative to Li / Li + The reference electrode has the highest concentration of all additives. Therefore, during the battery formation process, it will first be reduced and decomposed on the surface of the negative electrode to generate a dense inorganic underlayer containing LiF, Li2B4O7 and Li-BOF compounds, which will provide mechanical support for the subsequent deposition of organic layers.

[0035] S32: Add 0.8 parts of trimethylsilyl phosphate dropwise to the solution obtained in S31. After the addition is complete, stir at 150 rpm for about 15 minutes until the solution is clear, homogeneous, and free of oil droplets or suspended matter. The Si-O bonds in trimethylsilyl phosphate can form a Si-OC film on the positive electrode surface, which can effectively inhibit the dissolution of the positive electrode active material at high temperatures or high potentials. In addition, trimethylsilyl phosphate can react with trace amounts of hydrogen fluoride in the electrolyte to form trimethylfluorosilane (boiling point 16.4℃), which easily escapes from the system, thereby continuously removing hydrogen fluoride from the system and maintaining the chemical stability of lithium hexafluorophosphate.

[0036] S33: Add 1.0 part of fluoroethylene carbonate to the solution obtained in S32, and stir at 150 rpm for about 10 minutes until completely dissolved. Then add 2.0 parts of vinylene carbonate and continue stirring for about 10 minutes until the solution is clear and homogeneous. During battery formation, fluoroethylene carbonate is reduced and decomposed on the inorganic layer to replenish LiF, enhancing the uniformity and density of the inorganic layer. Vinylene carbonate is reduced and polymerized at a lower potential to generate a polycarbonate organic layer. This organic layer has good elasticity and can adapt to the volume expansion and contraction caused by the change in the interlayer spacing of the graphite anode at low temperatures, preventing the SEI film from cracking during low-temperature cycling. Thus, a gradient bilayer SEI film structure with an inner inorganic thickness of about 3-8 nm and an outer organic thickness of about 5-15 nm is self-assembled on the anode surface.

[0037] S34: Add 2.0 parts of methyl ethyl fluorocarbonate to the solution obtained in S33, and stir at 100 rpm for about 10 minutes until the liquid is completely mixed and the solution is clear. methyl ethyl fluorocarbonate has both flame-retardant and film-forming functions. The CF bond in its molecule has a high bond energy of about 485 kJ / mol, making it difficult to break. During combustion, it releases fluorine-containing free radicals, which can effectively inhibit the propagation of chain combustion reactions. The crude electrolyte is obtained.

[0038] S4: Aging, Inspection and Filling.

[0039] S41: Transfer the crude electrolyte obtained in S3 into a sealed, inert atmosphere container and allow it to stand at room temperature for 24 hours. The purpose of aging is to allow the solvation structure of each component in the solution to reach thermodynamic equilibrium, and to stabilize the coordination relationship between lithium ions, anions, and solvent molecules. The criteria for judging the completion of aging are: no solid precipitation at the bottom of the container, clear and transparent liquid, and uniform color between the upper and lower layers.

[0040] S42: Take samples from the aged electrolyte and determine the moisture content using the Karl Fischer coulometric titration method. The measured value is 12 ppm, and the requirement is <15 ppm. Determine the free hydrogen fluoride content using acid-base titration or ion chromatography. The measured value is 18 ppm, and the requirement is <30 ppm according to the process control standard. Both tests are qualified.

[0041] S43: The qualified electrolyte is filtered sequentially through a series of polytetrafluoroethylene (PTFE) membranes with 5 μm pore size and a PTFE membrane with 0.2 μm pore size under dry argon protection, first coarse filtration followed by fine filtration to remove any trace amounts of insoluble particulate matter. The filtrate is then filled into a sealed stainless steel container that has been dried at 120°C for at least 2 hours to obtain the low-temperature safe lithium iron phosphate battery electrolyte product of this embodiment. The resulting electrolyte is a pale yellow, clear, and transparent liquid with no odor.

[0042] Example 2 provides a low-temperature safe lithium iron phosphate battery electrolyte, prepared using the same method as in Example 1, except for the different mass proportions of each component. This example uses a lower proportion of ether-based solvent and a higher proportion of carbonate solvent. The specific formulation is as follows: 15 parts 2-methyltetrahydrofuran, 10 parts cyclopentylmethyl ether, 10 parts triethyl phosphate, 25 parts methyl ethyl carbonate, 25 parts dimethyl carbonate, 6.0 parts lithium hexafluorophosphate, 3.2 parts lithium difluorosulfonylimide, 1.3 parts lithium difluorooxalate borate (1.0 part added as the main lithium salt in step S23, 0.3 parts added as a film-forming additive in step S31), 1.5 parts vinylene carbonate, 0.8 parts fluoroethylene carbonate, 0.5 parts trimethylsilyl phosphate, and 1.5 parts fluoromethyl ethyl carbonate. All operating conditions, stirring rates, and judgment criteria in the preparation steps are consistent with those in Example 1. In this embodiment, the total amount of carbonate solvent is 50 parts, which accounts for a relatively high proportion of the total solvent. This is beneficial for obtaining a higher dielectric constant at room temperature and improving the degree of lithium salt dissociation.

[0043] Example 3 provides a low-temperature safe lithium iron phosphate battery electrolyte, which is prepared in the same way as in Example 1, except that a higher proportion of ether-based solvent and a higher amount of flame retardant additive are used. The specific formula is as follows: 22 parts of 2-methyltetrahydrofuran, 15 parts of cyclopentyl methyl ether, 16 parts of triethyl phosphate, 18 parts of methyl ethyl carbonate, 18 parts of dimethyl carbonate, 6.5 parts of lithium hexafluorophosphate, 3.5 parts of lithium difluorosulfonyl imide, 2.0 parts of lithium difluorooxalate borate (1.2 parts of which are added as the main lithium salt in step S23 and 0.8 parts of which are added as a film-forming additive in step S31), 2.5 parts of vinylene carbonate, 1.5 parts of fluoroethylene carbonate, 1.0 part of trimethylsilyl phosphate, and 3.0 parts of fluoromethyl ethyl carbonate. In this embodiment, the total mass ratio of ether-based solvent is 37 parts, triethyl phosphate is 16 parts, and methyl ethyl fluorocarbonate is 3.0 parts. The total mass ratio of flame retardant components is relatively high, which enhances the low-temperature performance and further improves the flame retardant and self-extinguishing ability of the electrolyte.

[0044] Example 4 provides a low-temperature safe lithium iron phosphate battery electrolyte. The preparation method is the same as in Example 1, except that the amounts of each component are taken near the lower limit of the scope of the claims. The specific formulation is as follows: 13 parts 2-methyltetrahydrofuran, 8 parts cyclopentyl methyl ether, 8 parts triethyl phosphate, 24 parts methyl ethyl carbonate, 24 parts dimethyl carbonate, 5.0 parts lithium hexafluorophosphate, 3.0 parts lithium difluorosulfonyl imide, 1.1 parts lithium difluorooxalate borate (0.8 parts added as the main lithium salt in step S23, 0.3 parts added as a film-forming additive in step S31), 1.2 parts vinylene carbonate, 0.5 parts fluoroethylene carbonate, 0.4 parts trimethylsilyl phosphate, and 1.0 part fluoromethyl ethyl carbonate. In this example, the amounts of each component are close to the lower boundary of the scope of the claims, and the amounts of ether solvent and functional additives are relatively small. The purpose is to verify that the electrolyte can still meet the basic requirements for low-temperature performance and safety under the minimum dosage conditions.

[0045] Example 5: This example provides a low-temperature safe lithium iron phosphate battery electrolyte. Its preparation method is the same as that of Example 1, except that the amount of each component is taken near the upper limit of the claims. The specific formula is as follows: 22 parts of 2-methyltetrahydrofuran, 15 parts of cyclopentyl methyl ether, 16 parts of triethyl phosphate, 16 parts of methyl ethyl carbonate, 16 parts of dimethyl carbonate, 8.0 parts of lithium hexafluorophosphate, 5.0 parts of lithium difluorosulfonyl imide, 2.3 parts of lithium difluorooxalate borate (1.5 parts of which are added as the main lithium salt in step S23 and 0.8 parts of which are added as a film-forming additive in step S31), 2.5 parts of vinylene carbonate, 1.5 parts of fluoroethylene carbonate, 1.0 part of trimethylsilyl phosphate, and 3.0 parts of fluoromethyl ethyl carbonate. In this embodiment, the total amount of lithium salt and the total amount of additives are close to the upper limit. A higher lithium salt concentration is beneficial to improving ionic conductivity, and a higher amount of additives is beneficial to forming a denser SEI film. However, an excessively high lithium salt concentration will also lead to an increase in viscosity. Therefore, the low-temperature viscosity of this embodiment is slightly higher than that of Example 1.

[0046] Comparative Example 1 provides a conventional carbonate-based electrolyte as a benchmark for performance comparison with the embodiments of the present invention. The formulation is: 50 parts methyl ethyl carbonate, 50 parts dimethyl carbonate, and 8.0 parts lithium hexafluorophosphate. It does not contain ether-based solvents, phosphate solvents, lithium difluorosulfonylimide, lithium difluorooxalate borate, or any functional additives. The preparation method is a conventional one-step process: methyl ethyl carbonate and dimethyl carbonate are mixed evenly in a glove box, and then all the lithium hexafluorophosphate is added at once, stirred until completely dissolved.

[0047] Comparative Example 2 provides a binary lithium salt electrolyte system without lithium difluorooxalate borate, using the same solvent system as in Example 1. The formulation is: 20 parts 2-methyltetrahydrofuran, 14 parts cyclopentyl methyl ether, 12 parts triethyl phosphate, 27 parts methyl ethyl carbonate, 27 parts dimethyl carbonate, 8.5 parts lithium hexafluorophosphate, 4.5 parts lithium difluorosulfonyl imide, 2.0 parts vinylene carbonate, 1.0 part fluoroethylene carbonate, 0.8 parts trimethylsilyl phosphate, and 2.0 parts fluoromethyl ethyl carbonate. It does not contain lithium difluorooxalate borate. The preparation method is a one-step process: after mixing all solvents, two lithium salts are added sequentially and stirred until dissolved, then all additives are added and stirred until homogeneous. This comparative example is used to verify the contribution of lithium difluorooxalate borate as a third salt and film-forming additive to the electrolyte performance.

[0048] Comparative Example 3 uses the exact same formulation as Example 1, with identical component types and amounts. The only difference is the one-step preparation method. Specifically, all five solvents are mixed at once in a glove box, then three lithium salts are added simultaneously and stirred until dissolved. Finally, all additives are added simultaneously and stirred until homogeneous, without stepwise addition or solvent pre-distillation purification. This comparative example is used to verify the effect of the specific addition sequence of each sub-step (S1-S4) in the stepwise preparation process of the present invention on the final performance of the electrolyte.

[0049] Comparative Example 4 provides an electrolyte without flame-retardant components to verify the contribution of triethyl phosphate and methyl fluorocarbonate to the flame-retardant properties of the electrolyte. The formulation is as follows: 20 parts 2-methyltetrahydrofuran, 14 parts cyclopentyl methyl ether, 33 parts methyl fluorocarbonate, 33 parts dimethyl carbonate, 7.2 parts lithium hexafluorophosphate, 3.6 parts lithium difluorosulfonyl imide, 1.7 parts lithium difluorooxalate borate (1.2 parts as the main lithium salt, 0.5 parts as a film-forming additive), 2.0 parts vinylene carbonate, 1.0 part fluoroethylene carbonate, and 0.8 parts trimethylsilyl phosphate. It does not contain triethyl phosphate or methyl fluorocarbonate. The preparation method is consistent with steps S1-S4 of Example 1, but the addition of triethyl phosphate and methyl fluorocarbonate is skipped.

[0050] Experimental Example 1: Basic physicochemical properties test of electrolyte; Electrolyte samples prepared in Examples 1-5 and Comparative Examples 1-4 were taken respectively, and their ionic conductivity and dynamic viscosity were tested under different temperature conditions. At the same time, the freezing point was determined to evaluate the basic physicochemical properties of the electrolyte in a wide temperature range.

[0051] Ionic conductivity was measured using a conductivity meter. Before testing, the sensor was calibrated with a standard conductivity solution. The conductivity sensor was immersed in 10 mL of electrolyte sample and equilibrated for 30 min in constant temperature chambers at 25℃ and -30℃, respectively, before reading the stable value. Dynamic viscosity was measured using a rotational rheometer at -30℃. The freezing point was determined by differential scanning calorimetry (DSC) at a cooling rate of 2℃ / min from 25℃ to -80℃, with the initial temperature corresponding to the exothermic peak on the heat flow curve taken as the freezing point. The experimental results are shown in Table 1.

[0052] Table 1. Basic physicochemical properties of the electrolytes in each example and comparative example.

[0053] As shown in Table 1, the ionic conductivity of the electrolytes in Examples 1-5 at -30℃ is all in the range of 3.8-5.2 mS / cm, which is much higher than the 0.22 mS / cm of the conventional carbonate system in Comparative Example 1. This indicates that the introduction of ether-based solvents and the ternary composite lithium salt system can significantly improve the low-temperature ionic conductivity of the electrolytes. Example 1 has the highest conductivity at -30℃ (5.2 mS / cm) and the lowest viscosity at -30℃ (8.5 mPa·s). This is because the ratio of 20 parts 2-methyltetrahydrofuran to 14 parts cyclopentylmethyl ether achieves the optimal balance of intermolecular interactions in the ether-carbonate system. Example 3 has the lowest freezing point (-62℃) because it uses the highest amount of ether-based solvent (37 parts in total). The low freezing point of ether solvents further lowers the overall freezing point of the system. Comparative Example 2, without lithium difluorooxalate borate, exhibited a conductivity of 4.1 mS / cm at -30°C, slightly lower than that of Example 1, indicating that lithium difluorooxalate borate, as the third salt, positively contributes to ion conductivity. Comparative Example 3, prepared by a one-step method, showed a conductivity of 4.9 mS / cm at -30°C, slightly lower than that of Example 1. This is because the one-step method lacks stepwise feeding and sequence optimization, resulting in less uniform and optimal lithium-ion solvation structure compared to the stepwise method. Comparative Example 4, without flame-retardant components, showed conductivity and viscosity similar to Example 1, indicating that the introduction of triethyl phosphate and methyl fluorocarbonate had minimal impact on the ion conductivity of the electrolyte.

[0054] Experimental Example 2: Low-Temperature Discharge Performance Test; 18650 cylindrical lithium iron phosphate batteries were assembled using the electrolytes prepared in Examples 1-5 and Comparative Examples 1-4. The positive electrode active material was lithium iron phosphate (LiFePO4) with a median particle size (D50) of 3 μm. The positive electrode formulation was lithium iron phosphate: conductive carbon black (SuperP): polyvinylidene fluoride (PVDF) in a mass ratio of 95:3:2. A slurry was prepared using N-methylpyrrolidone (NMP) as solvent, coated onto an aluminum foil current collector, and then dried, rolled, and cut. The negative electrode active material was artificial graphite with a median particle size (D50) of 5 μm. The negative electrode formulation was artificial graphite: conductive carbon black: sodium carboxymethyl cellulose (CMC): styrene-butadiene rubber (SBR) in a mass ratio of 96:1.2:1.3:1.5. A slurry was prepared using deionized water as solvent and coated onto a copper foil current collector. The separator was a 25 μm thick polypropylene microporous membrane with a porosity of 45%. After battery assembly, it was formed by constant current charging at 0.05C to 3.65V, left to stand for 12 hours, and then charged and discharged three times at 0.1C. Discharge performance was then tested under different low-temperature conditions. The low-temperature discharge efficiency was calculated as follows: C is denoted as the discharge capacity at 25℃ and 0.5C. 25 The batteries were placed in constant temperature chambers at -20℃, -30℃, and -40℃ for 4 hours, respectively, and then discharged at 0.5C to 2.0V. The discharge capacity CT was recorded, and the low-temperature discharge efficiency was calculated as CT / C25×100%. The experimental results are shown in Table 2.

[0055] Table 2 Low-temperature discharge efficiency of batteries in various embodiments and comparative examples

[0056] As shown in Table 2, the discharge efficiency of Examples 1-5 of the present invention all reached over 72% at -40℃, with Example 1 reaching 80%, significantly better than the conventional carbonate system of Comparative Example 1 (<25%). This result indicates that the synergistic effect of the ether-phosphate-carbonate ternary composite solvent and the ternary composite lithium salt system can significantly improve the ultra-low temperature discharge capability of lithium iron phosphate batteries. Comparative Example 2, without lithium difluorooxalate-borate, had a discharge efficiency of 70% at -40℃, lower than Example 1 (using the same solvent system) at 180%, a difference of 10 percentage points. This indicates that lithium difluorooxalate-borate, as the third salt and film-forming additive, significantly reduced the interfacial migration resistance of lithium ions at low temperatures by constructing a low-resistance inorganic SEI inner layer containing BOC and LiF on the negative electrode surface. Comparative Example 3, using a one-step method, had a discharge efficiency of 76% at -40℃, lower than Example 1 (with the same formulation but using a step-by-step preparation process) at 180%, indicating that the step-by-step feeding sequence effectively controls the lithium-ion solvation structure and improves low-temperature discharge performance. The low-temperature discharge efficiency of Comparative Example 4, without flame retardant components, is close to that of Example 1, indicating that the flame retardant components have a relatively small impact on the low-temperature electrochemical performance. The lower limit formulation of Example 4 has a discharge efficiency of 72% at -40°C, which is slightly lower than that of Example 1, but still meets the design target of ≥75% performance boundary, verifying the feasibility of the lower limit of the formulation range.

[0057] Experiment Example 3: Flame Retardant Performance Test; The self-extinguishing time (SET) was used to evaluate the flame retardant performance of each electrolyte. The test method followed the widely used glass fiber cotton core method in the literature: 0.3g of degreased glass fiber cotton was immersed in the electrolyte to be tested to ensure full absorption of liquid. After removal, it was suspended and fixed, ignited with an alcohol lamp, and the time (s) from the removal of the ignition source to the complete extinguishing of the flame was recorded. This time was divided by the mass (g) of electrolyte absorbed by the glass fiber cotton to obtain the self-extinguishing time (SETs / g). The smaller the SET value, the better the flame retardant performance. SET=0 indicates that the sample self-extinguishes immediately after ignition. Simultaneously, 18650 lithium iron phosphate batteries assembled using the above electrolytes were subjected to a puncture safety test at a fully charged state of 3.65V. A 3mm diameter steel needle was used to vertically puncture the center of the battery at a speed of 25mm / s, and the occurrence of fire or explosion was observed. The experimental results are shown in Table 3.

[0058] Table 3 Flame retardant properties of electrolytes in each embodiment and comparative example

[0059] As shown in Table 3, the self-extinguishing time of Examples 1-5 does not exceed 2.8 s / g, with Examples 1, 3, and 5 having a SET of 0 s / g, meaning they self-extinguish immediately after ignition, exhibiting excellent flame-retardant self-extinguishing characteristics. This is because the phosphorus element in triethyl phosphate (TEP) and the fluorine element in fluoroethyl methyl carbonate (FEMC) achieve a dual flame-retardant mechanism of gas-phase-condensed phase through the phosphate ester covering effect and the fluorine-containing free radical capture effect, respectively, during combustion. The SET of Example 4 is 2.8 s / g, which is slightly higher than the other examples, but still much lower than the comparative example of 175 s / g. This is because Example 4 contains only 8 parts of triethyl phosphate and only 1.0 part of fluoroethyl methyl carbonate, with the amount of flame-retardant component close to the lower limit, but still meeting the design target of ≤3 s / g. Comparative Examples 1 and 4, which contain no flame-retardant component at all, have SETs as high as 75 s / g and 68 s / g, respectively, and both ignited in the needle penetration test. Although Comparative Example 2 does not contain lithium difluorooxalate borate, it still contains triethyl phosphate and methyl fluorocarbonate, thus exhibiting good flame retardant properties (SET=0.5s / g). This further confirms that the flame retardant properties are mainly contributed by triethyl phosphate and methyl fluorocarbonate.

[0060] Experiment Example 4: Room Temperature Cycling Performance and Aluminum Foil Compatibility Test; 18650 lithium iron phosphate batteries assembled using the electrolytes of Examples 1-5 and Comparative Examples 1-4 were subjected to constant current and constant voltage charge-discharge cycle tests at a constant temperature of 30℃ and a rate of 0.5C (0.5 times the nominal capacity of the battery). The charging cut-off voltage was 3.65V and the discharging cut-off voltage was 2.0V. A total of 300 charge-discharge cycles were performed. The discharge capacity of the 1st and 300th cycles was recorded, and the capacity retention rate was calculated as (300th discharge capacity / 1st discharge capacity) × 100%. Simultaneously, electrochemical impedance spectroscopy (EIS) was used to measure the interfacial impedance (RSEI) of the battery at -40℃. The test frequency range was 100kHz to 0.01Hz, and the perturbation voltage amplitude was 5mV. The diameter of the semicircle in the high-frequency region was taken as the RSEI value. The aluminum foil compatibility test was performed using linear sweep voltammetry (LSV), with the aluminum foil as the working electrode and the lithium sheet as the counter and reference electrodes. The scan rate was 1 mV / s from the open circuit potential to 5.5 V vs. Li / Li. + During the scanning process, we observed whether any abnormal oxidation current peaks corresponding to aluminum foil corrosion appeared. The experimental results are shown in Table 4.

[0061] Table 4 Cycle performance, interface impedance, and aluminum foil compatibility of batteries in each embodiment and comparative example.

[0062] Table 4 shows that the capacity retention rates of Examples 1-5 after 300 cycles at 30°C were all above 91.5%, with Example 1 achieving the highest at 94.5%. This indicates that the ternary composite lithium salt and gradient bilayer SEI film system of the present invention can effectively suppress the continuous thickening of the SEI film and the irreversible consumption of active lithium during cycling. Comparative Example 1, using a traditional carbonate single electrolyte, showed a capacity retention rate of only 39% after 300 cycles, indicating severe degradation. This is mainly due to the loose structure and poor mechanical stability of the SEI film formed on the negative electrode surface by the pure carbonate system, which continuously grows and consumes electrolyte and active lithium during repeated charge-discharge cycles. The capacity retention rate of Comparative Example 2 was only 82%, significantly lower than the 94.5% of Example 1, and a significant aluminum foil corrosion oxidation current peak appeared near 3.8V in the LSV test. This is because Comparative Example 2 did not contain lithium difluorooxalate borate, thus lacking ODFB. - It plays a protective role in forming the BOC passivation layer on the aluminum foil surface, while the higher proportion of lithium bis(fluorosulfonyl)imide contains FSI. - Anions are corrosive to aluminum foil. This invention effectively solves this problem by introducing a synergistic passivation mechanism of lithium hexafluorophosphate and lithium difluorooxalate borate into a ternary salt system. The capacity retention of the one-step method in Comparative Example 3 was 90.5%, slightly lower than that of Example 1, while the interfacial impedance at -40°C was 155Ω, higher than the 115Ω of Example 1. This indicates that the stepwise preparation process effectively reduced interfacial migration resistance and improved cycle stability by optimizing the lithium-ion solvation structure.

[0063] Based on the test results of Experiments 1-4, the low-temperature safe lithium iron phosphate battery electrolyte provided by this invention exhibits an ionic conductivity ≥3.8 mS / cm at -30℃, with Example 1 reaching 5.2 mS / cm. The self-extinguishing time is ≤2.8 s / g. In most examples, the capacity retention rate after 300 cycles at 30℃ is ≥91.5%, with Example 1 reaching 94.5%. The electrolyte also shows good compatibility with aluminum foil. This demonstrates the synergistic effect of the ether-based solvent in the ternary composite solvent system (lowering freezing point and viscosity), the phosphate ester solvent (providing flame retardancy), and the carbonate solvent (maintaining high dielectric constant). Furthermore, the ternary composite lithium salt system utilizes lithium hexafluorophosphate for aluminum foil passivation protection, lithium difluorosulfonylimide for high low-temperature conductivity, and lithium difluorooxalate-borate for both film formation and chemical stabilization. Finally, the functionalized composite additives achieve layered, ordered film formation through reduction potential gradient differences, resulting in a comprehensive effect of constructing an internal inorganic and external organic gradient double-layer SEI film structure.

[0064] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A low-temperature safe lithium iron phosphate battery electrolyte, characterized in that, It is made from the following components in parts by weight: 13-22 parts of 2-methyltetrahydrofuran, 8-15 parts of cyclopentylmethyl ether, 8-16 parts of triethyl phosphate, 16-27 parts of methyl ethyl carbonate, 16-27 parts of dimethyl carbonate, 5-8 parts of lithium hexafluorophosphate, 3-5 parts of lithium difluorosulfonyl imide, 1.1-2.3 parts of lithium difluorooxalate borate, 1.2-2.5 parts of vinylene carbonate, 0.5-1.5 parts of fluoroethylene carbonate, 0.4-1.0 parts of trimethylsilyl phosphate, and 1.0-3.0 parts of fluoromethyl ethyl carbonate.

2. The low-temperature safe lithium iron phosphate battery electrolyte as described in claim 1, characterized in that, The 2-methyltetrahydrofuran and cyclopentylmethyl ether are each prepared via the following steps: 2-methyltetrahydrofuran or cyclopentylmethyl ether is added to a sealed glass container pre-filled with activated 3Å molecular sieves, and the container is sealed and allowed to stand at 20–25°C for at least 16 hours under argon protection. The molecular sieve is removed by filtration through a polytetrafluoroethylene (PTFE) membrane, and the filtrate is collected. The filtrate is then transferred to a round-bottom flask equipped with a distillation apparatus and distilled under atmospheric pressure under argon protection. The fractions of 2-methyltetrahydrofuran with a boiling range of 79–81 °C and cyclopentylmethyl ether with a boiling range of 105–107 °C are collected, which are the prepared 2-methyltetrahydrofuran and cyclopentylmethyl ether.

3. The low-temperature safe lithium iron phosphate battery electrolyte as described in claim 1, characterized in that, The triethyl phosphate, methyl ethyl carbonate, and dimethyl carbonate are each prepared via the following steps: Triethyl phosphate, methyl ethyl carbonate, or dimethyl carbonate are respectively added to a sealed container pre-filled with activated 4Å molecular sieves, and the container is sealed and allowed to stand at 20–25°C for at least 12 hours under argon protection. The molecular sieve is removed by filtration through a polytetrafluoroethylene (PTFE) membrane, and the filtrate is collected, which is the prepared triethyl phosphate, methyl ethyl carbonate, and dimethyl carbonate.

4. The low-temperature safe lithium iron phosphate battery electrolyte as described in claim 1, characterized in that, Before use, the lithium hexafluorophosphate, lithium difluorosulfonylimide, and lithium difluorooxalate borate are dried in a vacuum drying oven at 80–90°C for 10–12 hours.

5. A method for preparing a low-temperature safe lithium iron phosphate battery electrolyte as described in any one of claims 1 to 4, characterized in that, In a closed, inert atmosphere environment, proceed as follows: S1: The dried and pretreated 2-methyltetrahydrofuran, cyclopentylmethyl ether, triethyl phosphate, methyl ethyl carbonate and dimethyl carbonate are added sequentially to a reaction vessel equipped with a seal and a stirring device according to the formula amount and mixed to obtain a clear and homogeneous mixed solvent. S2: Add the dried and pretreated lithium hexafluorophosphate, lithium difluorosulfonylimide, and lithium difluorooxalate borate as the main lithium salt to the mixed solvent obtained in S1 in sequence. After each lithium salt is added, stir thoroughly until completely dissolved to obtain a composite lithium salt solution. S3: To the composite lithium salt solution obtained in S2, lithium difluorooxalate borate, trimethylsilyl phosphate, fluoroethylene carbonate, vinylene carbonate and fluoroethyl carbonate are added in sequence as film-forming additives. After each component is added, the mixture is stirred until it is completely miscible with the solution to obtain crude electrolyte. S4: The crude electrolyte obtained in S3 is left to stand and age under a sealed inert atmosphere. After passing the tests for moisture and free acid content, it is filtered and filled through a microporous membrane to obtain the low-temperature safe lithium iron phosphate battery electrolyte.

6. The preparation method according to claim 5, characterized in that, S1 includes the following operations: S11: Add 2-methyltetrahydrofuran and cyclopentylmethyl ether to the reactor at the same time, start stirring until the two are completely miscible, and visually observe that the liquid is clear and without layering to obtain an ether-based mixed solvent. S12: Slowly add the formulated amount of triethyl phosphate to the ether-based mixed solvent obtained in S11, and stir until the liquid is clear and homogeneous, without oil droplets or turbidity, to obtain an ether-phosphate mixture. S13: First, add the prescribed amount of methyl ethyl carbonate to the ether-phosphate mixture obtained in S12, stir until clear and homogeneous, then add the prescribed amount of dimethyl carbonate, and continue stirring until the liquid is completely mixed and there is no turbidity or separation visible to the naked eye, to obtain the mixed solvent.

7. The preparation method according to claim 5, characterized in that, S2 includes the following operations: S21: Divide the amount of lithium hexafluorophosphate in the formula into 3 to 5 equal parts. Add the first part to the mixed solvent obtained in S1 and stir until the solid particles disappear completely and the solution becomes clear. Then add the next part, and so on, until all the lithium hexafluorophosphate is added and completely dissolved to obtain a solution containing lithium hexafluorophosphate. S22: Add the prescribed amount of lithium difluorosulfonylimide to the solution obtained in S21 in one go, and stir until the solid completely disappears and the solution becomes clear to obtain a mixed lithium salt solution containing lithium difluorosulfonylimide and lithium hexafluorophosphate. S23: Add 0.8 to 1.5 parts of lithium difluorooxalate borate to the solution obtained in S22, and stir until the solid completely disappears and the solution is clear and slightly yellow to obtain the composite lithium salt solution.

8. The preparation method according to claim 5, characterized in that, S3 includes the following operations: S31: Add 0.3 to 0.8 parts of lithium difluorooxalate borate to the composite lithium salt solution obtained in S2, and stir until the solid is completely dissolved and the solution remains clear and transparent; S32: Add the prescribed amount of trimethylsilyl phosphate dropwise to the solution obtained in S31. After the addition is complete, stir until the solution is clear and homogeneous, without oil droplets or suspended matter. S33: First add the prescribed amount of fluoroethylene carbonate to the solution obtained in S32, stir until completely dissolved, then add the prescribed amount of vinylene carbonate, and stir until the solution is clear and homogeneous; S34: Add the prescribed amount of methyl ethyl fluorocarbonate to the solution obtained in S33, and stir until the liquid is completely miscible and the solution is clear to obtain the crude electrolyte.

9. The preparation method according to claim 5, characterized in that, S4 includes the following operations: S41: Transfer the crude electrolyte obtained in S3 into a sealed inert atmosphere container and let it stand for aging. The aging is considered complete when no solid precipitates at the bottom of the container, the liquid is clear and transparent, and the color of the upper and lower layers is uniform. S42: Take samples from the aged electrolyte to test the moisture content and free hydrogen fluoride content; S43: The qualified electrolyte is passed through a dry, inert gas atmosphere sequentially. and Filtration using polytetrafluoroethylene (PTFE) membranes with varying pore sizes, followed by filtrate filling into containers... The finished low-temperature safe lithium iron phosphate battery electrolyte is obtained by placing it in a sealed container after drying.

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