A high-temperature lithium battery electrolyte based on a novel fluorine-containing oxalate additive and a preparation method thereof

Abstract

The application discloses a high-temperature lithium battery electrolyte based on a novel fluorine-containing oxalate additive and a preparation method thereof, and belongs to the technical field of lithium battery electrolytes. The electrolyte comprises an organic solvent, a lithium salt, a fluorine-containing oxalate additive and a basic additive; the fluorine-containing oxalate additive can be decomposed in the electrolyte in a battery formation stage, and a uniform and stable solid-phase interface is generated on a battery electrode / electrolyte interface layer, so that the excessive decomposition of the electrolyte at the electrode interface under a high-temperature environment is significantly inhibited, and the cycle life of the lithium battery under a high-temperature working condition is significantly improved. The application provides a synthesis of the above-mentioned electrolyte additive and a preparation method of the electrolyte. The electrolyte based on the additive disclosed by the application makes lithium ion batteries including a lithium iron phosphate / graphite system, a nickel-cobalt-manganese ternary positive electrode / graphite system and the like all exhibit excellent cycle stability under a high-temperature environment.

Description

Technical Field

[0001] This invention relates to the field of lithium battery electrolyte technology, and in particular to a high-temperature lithium battery electrolyte based on a novel fluorinated oxalate additive and its preparation method. Background Technology

[0002] Lithium-ion batteries are widely used in digital products, electric vehicles, and energy storage due to their advantages such as high specific energy, long lifespan, and lack of memory effect. However, with the increasing demands, existing lithium-ion batteries are gradually failing to meet the needs of different application scenarios, especially the specialized use cases of different products and fields. Developing lithium-ion batteries capable of stable operation in high-temperature environments (≥45℃) is crucial for deepening the application of lithium-ion batteries. However, under high-temperature conditions, the passivation effect of carbonate-based electrolytes with traditional additives on the electrode interface is unstable. High temperatures not only disrupt the original electrode interface environment but also induce excessive side reactions of the solvent and lithium salt in the electrolyte at the electrode interface. This rapidly depletes the electrolyte content in the cell while consuming active lithium, leading to significant irreversible capacity degradation and severely reduced battery life.

[0003] To address the aforementioned key scientific and technological challenges, introducing fluorinated oxalate esters with special structures into traditional electrolytes helps form a dense and uniform passivation layer at the electrode surface, suppressing the degree of side reactions inside the battery at high temperatures, thereby significantly improving the high-temperature performance of lithium-ion batteries. Currently, while some common additives such as FEC and VC provide some electrode protection, their actual performance at high temperatures remains unsatisfactory, and the effects of these traditional electrolyte additives vary considerably depending on the electrode material system.

[0004] Therefore, it is necessary to provide a high-temperature lithium battery electrolyte based on a novel fluorinated oxalate additive and its preparation method to solve the above problems. Summary of the Invention

[0005] This invention provides a high-temperature lithium battery electrolyte based on a novel fluorinated oxalate additive and its preparation method. By introducing fluorinated oxalate compounds with asymmetric structures as electrolyte additives, and utilizing the electrochemical film-forming properties of these compounds at the electrode interface, a passivation layer with high thermal stability and ion conductivity is constructed on the electrode surface, thereby suppressing side reactions of electrolyte components under high-temperature conditions.

[0006] In a first aspect, the present invention provides a high-temperature lithium battery electrolyte based on a novel fluorinated oxalate additive, the electrolyte comprising an organic solvent, a lithium salt, a fluorinated oxalate additive, and a basic additive.

[0007] According to the present invention, the fluorinated oxalate additive has the following basic structural formula:

[0008]

[0009] Wherein, R1 and R2 are alkyl groups having a carbon chain length of C1 to C6 and fluoroalkyl groups having a carbon chain length of C1 to C4. Specifically, R1 and R2 are at least one of -CH3, -CH2-CH3, -CH2-CH2-CH3, -CH(CH3)-CH3, -CH2-CH2-CH2-CH3, -C(CH3)3, -CH2F, -CHF2, -CF3, -CH2-CH2F, -CH2-CHF2, -CH2-CF3, -CH2-CH2-CH2-CH2F, -CH2-CH2-CH2-CHF2, and -CH2-CH2-CH2-CF3.

[0010] In a preferred embodiment of the present invention, the lithium salt comprises one or more combinations of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium perchlorate (LiClO4), and lithium perfluoroalkyl sulfonate; preferably, the lithium salt is LiPF6.

[0011] In a preferred embodiment of the present invention, the organic solvent is one or more of the following: ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), propylene carbonate (PC), ethyl acetate (EA), propyl acetate (PA), diethyl ether (DEE), ethylene glycol dimethyl ether (DME), tetrahydrofuran (THF), and dioxolane (DOL).

[0012] In a preferred embodiment of the present invention, the base additive is one or more of the following: fluorinated ethylene carbonate (FEC), 1,3-propanesulfonate lactone, vinylene carbonate (VC), lithium dioxalate borate (LiBOB), lithium difluorooxalate borate (LiDFOB), lithium nitrate, and lithium difluorophosphate.

[0013] In a preferred embodiment of the present invention, the concentration of the lithium salt in the lithium battery electrolyte is 0.5 mol / L to 3 mol / L; the mass fraction of the fluorinated oxalate additive in the lithium battery electrolyte is 0.1% to 4%; and the mass fraction of the base additive in the lithium battery electrolyte is 0.1% to 5%.

[0014] In a preferred embodiment of the present invention, R1 is an alkyl group having a carbon chain length of C1 to C6, selected from at least one of -CH3, -CH2-CH3, -CH2-CH2-CH3, -CH(CH3)-CH3, -CH2-CH2-CH2-CH3, and -C(CH3)3;

[0015] R2 is a fluorinated alkyl group having a carbon chain length of C1 to C4, selected from at least one of -CH2F, -CHF2, -CF3, -CH2-CH2F, -CH2-CHF2, -CH2-CF3, -CH2-CH2-CH2-CH2F, -CH2-CH2-CH2-CHF2, and -CH2-CH2-CH2-CF3.

[0016] Secondly, the present invention provides a method for preparing the above-mentioned high-temperature lithium battery electrolyte, comprising the following steps:

[0017] S10. In an argon-protected glove box, the organic solvent is mixed evenly according to a preset ratio to obtain a mixed solvent.

[0018] S20. The lithium salt is slowly added to the mixed solvent and stirred to dissolve under the condition that the temperature is controlled below 40°C to obtain the basic electrolyte.

[0019] S30. Add the basic additive to the basic electrolyte, mix evenly, and then add the fluorinated oxalate additive. Continue stirring for 0.5-2 hours to obtain the high-temperature lithium battery electrolyte.

[0020] In a preferred embodiment of the present invention, in step S10, the volume ratio of each component in the mixed solvent is: ethylene carbonate: dimethyl carbonate: diethyl carbonate = 1:1:1.

[0021] In a preferred embodiment of the present invention, in step S20, the lithium salt addition rate is controlled at 0.5-2.0 kg / min, and the temperature inside the reactor is maintained at 25-35°C by external circulating cooling water.

[0022] Thirdly, the present invention provides a method for synthesizing a fluorinated oxalate additive, comprising the following steps:

[0023] M10. Dissolve fluorinated alcohol and triethylamine in dichloromethane solvent to prepare a first reaction solution; wherein, the triethylamine is used as an acid absorbent.

[0024] M20. Add the acyl chloride compound to the constant pressure dropping funnel and drop it into the first reaction solution over 5-10 minutes, controlling the molar ratio of fluorinated alcohol to acyl chloride compound to be 1:0.5-3.

[0025] M30, under nitrogen protection and ice-water bath conditions, control the temperature of the reaction system to not exceed 10℃, and continue stirring for 0.1-1h.

[0026] M40. Remove the ice-water bath and move the reaction system to room temperature (20-30℃) to continue the reaction for 10-24 hours.

[0027] After the reaction is completed (M50), the reaction solution is filtered to remove the byproduct triethylamine hydrochloride, and then the solvent and excess reactants are removed by vacuum distillation to obtain the fluorinated oxalate additive.

[0028] In a preferred embodiment of the present invention, in step M10, the fluorinated alcohol includes at least one of 2,2,2-trifluoroethanol, 3,3,3-trifluoropropanol, and 4,4,4-trifluorobutanol.

[0029] In a preferred embodiment of the present invention, in step M10, the mass ratio of dichloromethane to fluorinated alcohol is 5 to 15:1.

[0030] In a preferred embodiment of the present invention, in step M20, the acyl chloride compound includes at least one of oxaloyl chloride, ethyl oxaloyl chloride, methyl oxaloyl chloride, and propyl oxaloyl chloride.

[0031] In a preferred embodiment of the present invention, in step M30, the stirring speed is controlled at 200-500 rpm / min.

[0032] In a preferred embodiment of the present invention, in step M40, reflux is carried out through a condenser during the reaction process to prevent the loss of low-boiling-point components.

[0033] In a preferred embodiment of the present invention, in step M50, the pressure of vacuum distillation is controlled at 0.01-0.05 MPa and the temperature is controlled at 40-60°C.

[0034] In a preferred embodiment of the present invention, the applicable electrode materials for the electrolyte include lithium cobalt oxide, lithium iron phosphate, manganese nickel cobalt composite oxide, lithium nickel manganese oxide, lithium-rich manganese-based, nickel cobalt manganese ternary cathode, graphite anode, silicon anode, and lithium metal anode.

[0035] This invention addresses the shortcomings of the prior art and has the following beneficial effects:

[0036] (1) The electrolyte additive of the present invention adopts an asymmetric structure design, where R1 is a nonpolar or weakly polar alkyl group and R2 is a strongly electronegative fluorinated alkyl group. This asymmetric structure breaks the symmetry of the molecule and reduces crystallinity, thereby improving the solubility and dispersion uniformity of the additive in carbonate-based solvents. At the same time, the strong electron-withdrawing effect of the fluorinated group makes the molecule extremely prone to gaining and losing electrons. During the first charge-discharge (formation) stage of the battery, the molecule reacts preferentially on the electrode surface over the bulk organic solvent. Compared with the traditional symmetrical oxalate structure, which is prone to uneven distribution due to limited solubility, the fluorinated oxalate additive of the present invention can occupy the electrode active sites first to form a uniform film, avoiding the reduction of a large number of bulk solvent molecules to produce gas and form pores or cracks inside the interfacial film. This creates a highly dense and continuous passivation layer substrate for the battery in the initial stage of operation.

[0037] (2) The asymmetric fluorinated oxalate additive of the present invention imparts excellent mechanical flexibility and high-temperature physical barrier function to the electrode interface film through the directional film formation of the two end groups on the positive and negative electrodes. During the film formation process, the fluorinated functional groups induce the formation of a lithium fluoride-rich structure on the negative electrode side, and the fluorine atoms tend to align towards the electrolyte side, forming a hydrophobic layer with low surface energy; at the same time, the oxidative ring-opening polymerization of the oxalate skeleton on the positive electrode side forms a flexible organic chain segment rich in polyoxalate. This composite structure with both hydrophobic properties and high flexibility greatly hinders the penetration and erosion of free hydrofluoric acid and solvent molecules into the electrode from a kinetic perspective, reducing the continuous side reactions and gas production of the electrolyte at high temperatures (resulting in a thickness expansion rate reduction of more than 50%); on the other hand, its highly elastic polymer chain segment effectively alleviates the huge volume expansion stress caused by lithium insertion and extraction from the electrode material during high-temperature charging and discharging, preventing repeated rupture and regeneration of the interface passivation film. This fundamentally blocks the consumption of active lithium and significantly extends the high-temperature cycle life of the battery.

[0038] (3) The electrolyte preparation of the present invention adopts a specific sequential process of first dissolving lithium salt, then adding basic additives, and finally adding fluorinated oxalate additives. Utilizing the difference in dissolution kinetics between different components, the basic additives first occupy the solvation layer and form inorganic components in the early stage of formation. Subsequently, the added fluorinated oxalate is distributed on the periphery of the solvation shell to form organic components. The two are deposited alternately to construct a high-strength composite interface film. Unlike the inefficient patchwork schemes in the prior art that forcibly improve stability by introducing complex solid components such as glass powder, the pure liquid-phase homogeneous system of the present invention can more accurately exert a synergistic passivation effect on the electrode surface. This composite interface structure not only endows the electrode with extremely high mechanical strength and thermal stability, but also ensures excellent ion conduction capability, enabling the battery to still perform well even at high rates such as 1C and 2C, and the capacity recovery rate after being placed at 60°C for 30 days is as high as 92% or more.

[0039] (4) The lithium battery electrolyte of the present invention can ensure stable cycling of the battery under high temperature conditions. It can ensure that the lithium iron phosphate cathode can cycle stably for 400 cycles at a high temperature of 60°C and achieve a capacity retention rate of 95.5%; it can ensure that the nickel cobalt manganese ternary cathode battery can cycle stably for 400 cycles at a high temperature of 45°C and achieve a capacity retention rate of 98.9%.

[0040] (5) The preparation method of the lithium battery high-temperature electrolyte of the present invention is simple and low in cost; the amount of fluorinated oxalate additive is low, which can further control the cost; the high-temperature electrolyte significantly improves the high-temperature performance of various types of batteries; the electrolyte using fluorinated oxalate additive has repeatable performance and can stably achieve long cycle life of batteries under high-temperature conditions. Attached Figure Description

[0041] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0042] Figure 1 The battery capacity and coulombic efficiency of lithium iron phosphate / graphite batteries under 1C cycling conditions before and after the addition of fluorinated oxalate additives;

[0043] Figure 2 The battery capacity of a nickel-nickel ternary / graphite battery under 1C cycling conditions before and after the addition of fluorinated oxalate additive;

[0044] Figure 3 The values ​​represent the battery capacity and coulombic efficiency of high-nickel ternary / graphite batteries under 1C cycling conditions before and after the addition of fluorinated oxalate additives. Detailed Implementation

[0045] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0046] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein. Therefore, the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0047] This invention provides a high-temperature lithium battery electrolyte based on a novel fluorinated oxalate additive. This lithium battery electrolyte incorporates a fluorinated oxalate compound with a specific asymmetric structure as an electrolyte additive, and this compound has the following basic structural formula: .

[0048] In the formula, R1 and R2 are alkyl groups with carbon chain lengths of C1 to C6 and fluorinated alkyl groups with carbon chain lengths of C1 to C4, respectively.

[0049] In specific implementation, R1 and R2 are selected from at least one of -CH3, -CH2-CH3, -CH2-CH2-CH3, -CH(CH3)-CH3, -CH2-CH2-CH2-CH3, -C(CH3)3, -CH2F, -CHF2, -CF3, -CH2-CH2F, -CH2-CHF2, -CH2-CF3, -CH2-CH2-CH2-CH2F, -CH2-CH2-CH2-CHF2, and -CH2-CH2-CH2-CF3.

[0050] In a first aspect, the present invention provides a high-temperature lithium battery electrolyte based on a novel fluorinated oxalate additive, the electrolyte comprising an organic solvent, a lithium salt, a fluorinated oxalate additive, and a basic additive.

[0051] Specifically, the fluorinated oxalate additive has a mass fraction of 0.1% to 4% in the electrolyte. The lithium salt has a molar concentration of 0.5 mol / L to 3 mol / L in the electrolyte. The basic additive has a mass fraction of 0.1% to 5% in the electrolyte.

[0052] Further, the lithium salt includes one or more combinations of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium perchlorate, and lithium perfluoroalkyl sulfonate. The organic solvent includes one or more combinations of ethylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, propylene carbonate, ethyl acetate, propyl acetate, diethyl ether, ethylene glycol dimethyl ether, tetrahydrofuran, and dioxolane. The base additive includes one or more combinations of fluorinated ethylene carbonate, 1,3-propanesulfonyl lactone, vinylene carbonate, lithium dioxalate borate, lithium difluorooxalate borate, lithium nitrate, and lithium difluorophosphate.

[0053] Secondly, the present invention provides a method for preparing a lithium battery electrolyte, comprising the following steps:

[0054] Step S10: In an argon-protected glove box, with the moisture content controlled below 10 ppm and the acidity controlled below 20 ppm, the organic solvent is mixed evenly according to a preset ratio to obtain a mixed solvent. In a specific embodiment, the volume ratio of each component in the mixed solvent is: ethylene carbonate (EC): dimethyl carbonate (DMC): diethyl carbonate (DEC) = 1:1:1.

[0055] Step S20: Slowly add lithium salt to the mixed solvent. The lithium salt addition rate is controlled at 0.5-2.0 kg / min. The temperature inside the reactor is maintained at 25-35℃ and not exceeding 40℃ by external circulating cooling water. Stir to dissolve and obtain the basic electrolyte.

[0056] Step S30: Add basic additives to the basic electrolyte, mix evenly, then add fluorinated oxalate additives and stir continuously for 0.5-2 hours to obtain lithium battery electrolyte.

[0057] Thirdly, the present invention provides a method for synthesizing fluorinated oxalate additives, comprising the following steps:

[0058] Step M10: Dissolve the fluorinated alcohol and triethylamine in dichloromethane solvent to prepare a first reaction solution. The fluorinated alcohol includes at least one selected from 2,2,2-trifluoroethanol, 3,3,3-trifluoropropanol, and 4,4,4-trifluorobutanol. The mass ratio of dichloromethane to the fluorinated alcohol is 5:1 to 15:1. The amount of triethylamine added is 5% to 10% of the mass of the fluorinated alcohol.

[0059] Step M20: Add the acyl chloride compound to the constant pressure dropping funnel and drop it into the first reaction solution over 5-10 minutes, controlling the molar ratio of the fluorinated alcohol to the acyl chloride compound to be 1:0.5-3. The acyl chloride compound includes at least one of oxaloyl chloride, ethyl oxaloyl chloride, methyl oxaloyl chloride, and propyl oxaloyl chloride.

[0060] Step M30: Under nitrogen protection and ice-water bath conditions, control the temperature of the reaction system to not exceed 10℃, control the stirring speed to 200-500 rpm / min, and continue stirring for 0.1-1h.

[0061] M40: Remove the ice-water bath and move the system to room temperature (20-30℃) to continue the reaction for 10-24 hours. Reflux the reaction through a condenser during the process.

[0062] M50: After the reaction is complete, the reaction solution is filtered to remove the byproduct triethylamine hydrochloride, and then the solvent and excess reactants are removed by vacuum distillation. The pressure of vacuum distillation is controlled at 0.01-0.05 MPa and the temperature is controlled at 40-60℃ to obtain the fluorinated oxalate additive.

[0063] More specifically, in the fluorinated oxalate additive structure, R1 is an alkyl group having a carbon chain length of C1 to C6, selected from at least one of -CH3, -CH2-CH3, -CH2-CH2-CH3, -CH(CH3)-CH3, -CH2-CH2-CH2-CH3, and -C(CH3)3; and R2 is a fluorinated alkyl group having a carbon chain length of C1 to C4, selected from at least one of -CH2F, -CHF2, -CF3, -CH2-CH2F, -CH2-CHF2, -CH2-CF3, -CH2-CH2-CH2-CH2F, -CH2-CH2-CH2-CHF2, and -CH2-CH2-CH2-CF3.

[0064] In this invention, the physicochemical properties of the additive can be altered by adjusting the carbon chain lengths of the R1 and R2 groups. For example, when R1 is propyl (C3) and R2 is 2,2,2-trifluoroethyl, the resulting compound is propyl oxalate 2,2,2-trifluoroethyl. In applications targeting high-nickel ternary cathodes (such as NCM811), the number of fluorine atoms substituted in the R2 group is increased; in applications targeting silicon-carbon anodes, R1 groups with longer chain segments (such as C4–C6) are selected.

[0065] The technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0066] Example 1:

[0067] The synthesis of fluorinated oxalate additives specifically includes:

[0068] M10: Add 10g of trifluoroethanol and 5mL of NEt3 to a 250mL three-necked flask, and dissolve in 100mL of dichloromethane.

[0069] M20: Add 13.7g of oxaloyl chloride to a constant pressure dropping funnel and drop it into a three-necked flask over 10 minutes. The molar ratio of trifluoroethanol to oxaloyl chloride is 1:1.

[0070] M30: Under nitrogen protection and ice-water bath conditions, control the temperature of the reaction system to not exceed 10℃, stir at 300rpm, and stir for 0.5h.

[0071] M40: Move to 25°C and react at room temperature for 18 hours, then turn on reflux condenser.

[0072] M50: After filtration, the sample was distilled under reduced pressure at 0.03 MPa and 50 °C to obtain sample 1.

[0073] The preparation of electrolytes specifically includes:

[0074] S10: In an argon atmosphere glove box, take 400 mL each of dehydrated EC, DMC and DEC, add them to beakers and mix well, then put them into aluminum bottles and let stand for 8 hours to obtain carbonate mixed solvent (EC:DMC:DEC=1:1:1).

[0075] S20: In an argon atmosphere glove box, take out 50 mL of mixed solvent and put it into a beaker. Measure 5.064 mL of LiPF6 and slowly add it to the carbonate mixed solvent. During the process, control the temperature to not exceed 40℃ and mix evenly.

[0076] S30: Add 2 wt% of the basic additive LiDFOB to the above mixed electrolyte, mix thoroughly, and then add 1 wt% of the fluorinated oxalate additive sample. After thorough mixing, the electrolyte is obtained; the lithium salt concentration in this electrolyte is 1.0 M.

[0077] Example 2:

[0078] The preparation of electrolytes specifically includes:

[0079] S10: In an argon atmosphere glove box, take 400 mL each of dehydrated EC, DMC and DEC, add them to beakers and mix well, then put them into aluminum bottles and let stand for 8 hours to obtain carbonate mixed solvent (EC:DMC:DEC=1:1:1).

[0080] S20: In an argon atmosphere glove box, take out 50 mL of mixed solvent and put it into a beaker. Measure 5.570 mL of LiPF6 and slowly add it to the carbonate mixed solvent. During the process, control the temperature to not exceed 40℃ and mix evenly.

[0081] S30: Add 2 wt% of the basic additive VC to the above mixed electrolyte, mix thoroughly, and then add 2 wt% of the fluorinated oxalate additive sample 1. After thorough mixing, the electrolyte is obtained; the lithium salt concentration in this electrolyte is 1.1 M.

[0082] Example 3:

[0083] The synthesis of fluorinated oxalate additives specifically includes:

[0084] M10: Add 11.4 g of 3,3,3-trifluoro-1-propanol and 5 mL of NEt3 to a 250 mL three-necked flask, and dissolve in 150 mL of dichloromethane.

[0085] M20: Add 19.1g of oxalyl chloride to a constant pressure dropping funnel and drop it into a three-necked flask over 10 minutes. The molar ratio of 3,3,3-trifluoro-1-propanol to oxalyl chloride is 1:1.5.

[0086] M30: Under nitrogen protection and ice-water bath conditions, control the temperature of the reaction system to not exceed 10℃, stir at 300rpm, and stir for 0.5h.

[0087] M40: Move to 25°C and react at room temperature for 12 hours, then turn on reflux condenser.

[0088] M50: After filtration, the sample was distilled under reduced pressure at 0.03 MPa and 50 °C to obtain sample 2.

[0089] The preparation of electrolytes specifically includes:

[0090] S10: In an argon atmosphere glove box, take 400 mL each of dehydrated EC, DMC and DEC, add them to beakers and mix well, then put them into aluminum bottles and let stand for 8 hours to obtain carbonate mixed solvent (EC:DMC:DEC=1:1:1).

[0091] S20: In an argon atmosphere glove box, take out 50 mL of mixed solvent and put it into a beaker. Measure 5.570 mL of LiPF6 and slowly add it to the carbonate mixed solvent. During the process, control the temperature to not exceed 40℃ and mix evenly.

[0092] S30: Add 2 wt% of the basic additive VC to the above mixed electrolyte, mix thoroughly, and then add 2 wt% of the fluorinated oxalate additive sample 2. After thorough mixing, the electrolyte is obtained; the lithium salt concentration in this electrolyte is 1.1M.

[0093] Comparative Example 1:

[0094] This comparative example is basically the same as Example 1, except that: no fluorinated oxalate additive sample 1 was added to this electrolyte.

[0095] Comparative Example 2:

[0096] This comparative example is basically the same as Example 2, except that: no fluorinated oxalate additive sample 1 was added to this electrolyte.

[0097] Comparative Example 3:

[0098] This comparative example is basically the same as Example 3, except that: no fluorinated oxalate additive sample 2 was added to this electrolyte.

[0099] Comparative Example 4:

[0100] This comparative example is basically the same as Example 1, except that the fluorinated oxalate additive sample 1 in step S30 is replaced with an equal mass of conventional non-fluorinated symmetrical additive - diethyl oxalate.

[0101] Comparative Example 5:

[0102] This comparative example is basically the same as Example 1, except that in step S30, the basic additive LiDFOB is not added, and only 1 wt% of the fluorinated oxalate additive sample 1 is added.

[0103] Comparative Example 6:

[0104] This comparative example is basically the same as Example 1, except that the novel fluorinated oxalate additive in step S30 is replaced with an equal mass of symmetrical fluorinated additive—bis(2,2,2-trifluoroethyl) oxalate.

[0105] All examples and comparative examples used positive and negative electrode sheets for the batteries prepared in a dry, cleanroom. The following is the battery preparation process:

[0106] T10: After the corresponding electrode is mixed with slurry, it is coated on the surface of the foil by a coating machine and dried. Then, it is sliced, rolled, cut and other processes are carried out to stack the corresponding electrode with the diaphragm to make the electrode used in the examples and comparative examples.

[0107] T20: The electrodes are encapsulated using an aluminum-plastic film and dried in an 80℃ vacuum oven for 24 hours.

[0108] T30: After removing the corresponding battery, use the electrolytes of Examples 1-3 and Comparative Examples 1-3 to inject into the thoroughly dried battery in an argon atmosphere glove box, with an injection volume of 15g per battery, and pre-seal under negative pressure conditions.

[0109] T40: After high-temperature resting, high-temperature fixture formation, and secondary sealing, high-temperature capacity testing and high-temperature cycling are performed. Specifically, the secondary-sealed batteries are placed in the corresponding high-temperature test chamber (60℃ for lithium iron phosphate batteries, 45℃ for medium-nickel and high-nickel ternary batteries) and left to stand for 2 hours to reach thermal equilibrium. Then, they are charged at a constant current of 0.1C to the corresponding cutoff voltage (3.6V for lithium iron phosphate, 4.2V for medium-nickel ternary, and 4.4V for high-nickel ternary), switched to constant voltage charging until the current drops to 0.05C, and left to stand for 10 minutes; then, they are discharged at a constant current of 0.1C to the corresponding discharge cutoff voltage (2.5V for lithium iron phosphate, 3.2V for medium-nickel ternary, and 2.8V for high-nickel ternary), and left to stand for 10 minutes. This charge-discharge process is repeated twice to complete the capacity testing.

[0110] After capacity determination, the battery is charged at a constant current and constant voltage of 1C at the corresponding high temperature until fully charged. After resting for 10 minutes, it is discharged at a constant current of 1C until the cutoff voltage, and the initial discharge capacity is recorded. This 1C / 1C charge-discharge cycle is repeated for 400 cycles (or until the battery capacity is severely degraded), and the discharge capacity is recorded. The capacity retention rate after the cycle is calculated (capacity retention rate = discharge capacity after cycle / initial discharge capacity × 100%).

[0111] Further, batteries from the same batch after capacity testing were charged to full capacity at 1C constant current and constant voltage at room temperature. The initial center thickness of the battery was measured and recorded using a micrometer, and the initial discharge capacity was also recorded. The fully charged batteries were then placed in a 60℃ constant temperature chamber for 30 days. After the period, the batteries were removed, allowed to cool at room temperature, and the center thickness was measured again. The thickness expansion rate was calculated (thickness expansion rate = (thickness after cooling - initial thickness) / initial thickness × 100%). Subsequently, the batteries were charged and discharged at 1C, and the recovered discharge capacity was recorded. The capacity recovery rate was calculated (capacity recovery rate = recovered capacity / initial discharge capacity × 100%). Specific test data are shown in Table 1.

[0112] Table 1. High-temperature test data of lithium batteries based on the electrolytes of the examples and comparative examples.

[0113]

[0114] Depend on Figure 1 It can be seen that the electrolyte prepared by the present invention can greatly improve the capacity retention rate of lithium iron phosphate batteries at high temperatures; Figure 2 and Figure 3 This indicates that nickel-cobalt-manganese ternary cathode batteries exhibit higher cycle stability and better coulombic efficiency when fluorinated oxalate additives are added.

[0115] Based on the comprehensive test results in Table 1, and through a horizontal comparison between Example 1 and the comparative examples, it can be seen that:

[0116] At an extreme high temperature of 60°C, Example 1 maintained a capacity retention of 95.5% after 400 cycles; while Comparative Example 4, using conventional diethyl oxalate, only experienced a capacity reduction to 80.5% after 200 cycles, with significant thickness expansion. This indicates that the simple oxalate ester framework cannot effectively block solvent side reactions at high temperatures. Furthermore, even with the introduction of fluorine atoms (Comparative Example 6), its symmetrical structure leads to higher crystallinity, limiting its dispersibility and film density in carbonate solvents, resulting in a capacity retention and gas production suppression effect far inferior to Example 1. This further strongly demonstrates the unique design of the asymmetric fluorinated oxalate ester structure of this invention, achieving remarkable progress in improving the liquid-repellent barrier properties and flexibility of the interfacial film.

[0117] Comparative Example 1 failed rapidly at high temperatures, and although Comparative Example 5 improved the capacity retention to 88.2% thanks to the preferential film-forming effect of the novel additive, it still did not reach the optimal state. However, in Example 1, after combining the two, the capacity retention jumped to 95.5%, and the capacity recovery rate after resting reached as high as 92.4%. This indicates that the inorganic components formed in the early stages of LiDFOB formation provide a solid framework, while the organic fluorinated network formed by the polymerization of the novel asymmetric fluorinated oxalate provides excellent flexibility and hydrophobicity. The high-strength composite passivation layer constructed by these two components solves the problem of interfacial failure under high-temperature conditions.

[0118] In addition, during the research and development process, it was unexpectedly discovered that when the R1 group of the fluorinated oxalate additive is designed with specific steric hindrance (such as isopropyl or tert-butyl) and used in conjunction with lithium bis(fluorosulfonyl)imide (LiFSI) and low-viscosity carboxylic acid ester solvents (such as ethyl acetate or propyl acetate), it is possible to break the conventional understanding in the field that "high-temperature interface films inevitably lead to a surge in low-temperature impedance" and achieve performance balance in a wide temperature range of -40°C to 60°C.

[0119] Specifically, traditional high-temperature film-forming additives often form an excessively dense and thick polymer passivation layer on the electrode surface, resulting in extremely high lithium-ion insertion / extraction impedance at low temperatures, making the battery almost unable to discharge at -40°C. However, this invention unexpectedly discovered that specific asymmetric molecules with large steric hindrance branches (R1) and strongly electronegative fluorine-containing groups (R2), when participating in electrochemical polymerization, exhibit a strong steric hindrance effect due to the large volume of the branched groups. This steric hindrance effect effectively limits the excessive extension polymerization of the oxalate ester skeleton on the electrode surface, forcing it to crosslink with the inorganic-rich substances (LiF, Li2S) generated by LiFSI reduction, thereby constructing an extremely thin but highly resilient ultrathin composite passivation film. This ultrathin film not only perfectly inherits the antioxidant and solvent-resistant capabilities of the fluorine-containing groups at 60°C, but also, due to its nanoscale thickness and inorganic-rich properties, completely opens the lithium-ion transport channel at low temperatures. Even under extremely cold conditions of -40°C, it can still maintain more than 75% of its room-temperature discharge capacity, providing a new solution for efficient thermal management and all-weather operation of power batteries in high-altitude and cold regions.

[0120] The following detailed description of the low-temperature solution for electrolytes is provided in conjunction with specific embodiments.

[0121] To verify the performance over a wide temperature range (-40℃ to 60℃), the following three structures of fluorinated oxalate additives were synthesized in advance as raw materials for subsequent electrolyte formulation:

[0122] 1. The synthesis of additive sample 3, specifically including:

[0123] 10 g of 2,2,2-trifluoroethanol and 5 mL of triethylamine were added to a 250 mL three-necked flask and dissolved in 100 mL of anhydrous dichloromethane. 15.2 g of isopropyl oxalyl chloride was added to a constant-pressure dropping funnel and added dropwise into the flask over 10 min. After stirring for 0.5 h under nitrogen protection and an ice-water bath (≤10 °C), the mixture was moved to 25 °C and reacted at room temperature for 18 h, followed by reflux. The precipitate was removed by suction filtration, and the mixture was distilled under reduced pressure at 0.03 MPa and 45 °C to obtain additive sample 3 (R1 being sterically hindered isopropyl).

[0124] 2. The synthesis of additive sample 4 specifically includes:

[0125] The synthesis steps were the same as for additive sample 3, the only difference being that isopropyl oxaloyl chloride was replaced with an equimolar amount of tert-butyl oxaloyl chloride. This resulted in additive sample 4 (R1 being a sterically hindered tert-butyl group).

[0126] 3. The synthesis of additive sample 5 was compared, specifically including:

[0127] The synthesis steps were the same as for additive sample 3, the only difference being that isopropyl oxaloyl chloride was replaced with an equimolar amount of n-propyl oxaloyl chloride. Finally, comparative additive sample 5 (R1 being linear n-propyl) was obtained.

[0128] Example 4:

[0129] The preparation of electrolytes specifically includes:

[0130] S10: In an argon atmosphere glove box, take 400 mL each of dehydrated EC, DMC and EA, add them to beakers and mix well, then put them into aluminum bottles and let stand for 8 hours to obtain carbonate mixed solvent (EC:DMC:EA=1:1:1).

[0131] S20: In an argon atmosphere glove box, take out 50 mL of mixed solvent and put it into a beaker. Slowly add LiFSI, keeping the temperature below 40°C during the process. Stir to dissolve, so that the lithium salt concentration reaches 1.2 mol / L.

[0132] S30: Add 2 wt% of the basic additive FEC to the above mixed electrolyte, mix thoroughly, and then add 2 wt% of additive sample 3. After thorough mixing, the electrolyte is obtained.

[0133] Example 5:

[0134] The only difference between this embodiment and Embodiment 4 is step S30: replacing 2 wt% of additive sample 3 with an equal mass of additive sample 4. The solvent, lithium salt, and base additives are completely identical.

[0135] Comparative Example 7:

[0136] The only difference between this comparative example and Example 4 is step S30: 2 wt% of additive sample 3 is replaced with an equal mass of comparative additive sample 5. The solvent, lithium salt, and base additives are completely identical.

[0137] Comparative Example 8:

[0138] The only difference between this comparative example and Example 4 is step S20: 1.2 mol / L LiFSI is replaced with an equimolar concentration of LiPF6. The solvent, additive sample 3, and base additive are completely identical.

[0139] Comparative Example 9:

[0140] The only difference between this comparative example and Example 4 is step S10: the mixed solvent (EC:DMC:EA=1:1:1) is replaced with a conventional solvent (EC:DMC:DEC=1:1:1) that does not contain low-viscosity esters. The lithium salt, additive sample 3, and basic additives are completely identical.

[0141] Using the electrolytes prepared in Examples 4 and 5, and Comparative Examples 7 to 9, pouch cells were fabricated using a high-nickel ternary cathode (NCM811) and a silicon-carbon anode (SiC) (the cell fabrication and electrolyte injection formation process is the same as described in T10-T30 above). The formed cells underwent the following targeted wide-temperature-range performance tests:

[0142] The testing method is the same as the T40 step described above. The cyclic testing conditions are: 400 cycles of 1C / 1C charge and discharge at 45℃, and the capacity retention rate is recorded; the high-temperature storage conditions are: at room temperature and fully charged, placed in a 60℃ constant temperature chamber for 30 days, the thickness change before and after storage is measured, and the thickness expansion rate is calculated.

[0143] In addition, after capacity testing, each group of batteries was charged to full capacity (4.2V) at 1C constant current and constant voltage at room temperature (25℃), and then placed in an ultra-low temperature environment test chamber at -40℃ for 12 hours to ensure that the batteries reached thermal equilibrium. Subsequently, they were discharged at a constant current of 0.2C to the discharge cutoff voltage (2.8V) at -40℃, and the discharge capacity under extreme cold conditions was recorded. The calculation formula is: Extreme low temperature discharge capacity retention rate = (0.2C discharge capacity at -40℃ / full charge discharge capacity at 25℃) × 100%. The test results are detailed in Table 2.

[0144] Table 2. Comprehensive performance test data of wide-temperature-range specialized electrolytes

[0145]

[0146] As shown in Table 2, when the solvent and lithium salt are completely identical, the linear additive (Comparative Example 7) readily forms a thick passivation film, resulting in a halving of the capacity retention rate to 42.3% at -40°C. However, the introduction of sterically hindered side chains (Examples 4 and 5) effectively prevents excessive polymerization of molecules on the electrode surface due to the strong steric hindrance effect. This ultrathin and resilient composite passivation film, while inheriting high-temperature oxidation resistance, also opens up lithium-ion transport channels under extremely cold conditions.

[0147] Furthermore, if LiFSI is simply replaced with the conventional high-temperature lithium salt LiPF6, the -40℃ retention rate drops sharply to 28.7%, demonstrating that the inorganic nanocrystalline domains (LiF, Li2S) generated by LiFSI co-reduction are crucial for reducing the interfacial charge transfer impedance at extremely low temperatures. Similarly, if only the low-viscosity solvent EA is removed, the -40℃ retention rate is only 35.6%, proving that EA plays an irreplaceable role in overcoming the surge in viscosity of conventional solvents under extremely cold conditions and maintaining the bulk ionic conductivity.

[0148] In summary, this invention achieves a leap in performance over a wide temperature range (-40℃ to 60℃) through the deep synergy of steric confinement film formation, LiFSI inorganic modification, and EA low viscosity transport, which is difficult to predict in existing technologies.

[0149] Based on the preferred embodiments of the present invention described above, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A lithium battery electrolyte, characterized in that, The electrolyte includes organic solvents, lithium salts, fluorinated oxalate additives, and basic additives; The characteristic structure of the fluorinated oxalate additive is as follows: Wherein, R1 and R2 are alkyl groups having a carbon chain length of C1 to C6 and fluorinated alkyl groups having a carbon chain length of C1 to C4; The fluorinated oxalate additive in the electrolyte has a mass fraction of 0.1% to 4%. The basic additive has a mass fraction of 0.1% to 5% in the electrolyte.

2. The lithium battery electrolyte according to claim 1, characterized in that, R1 and R2 are at least one of -CH3, -CH2-CH3, -CH2-CH2-CH3, -CH(CH3)-CH3, -CH2-CH2-CH2-CH3, -C(CH3)3, -CH2F, -CHF2, -CF3, -CH2-CH2F, -CH2-CHF2, -CH2-CF3, -CH2-CH2-CH2-CH2F, -CH2-CH2-CH2-CHF2, and -CH2-CH2-CH2-CF3.

3. The lithium battery electrolyte according to claim 1, characterized in that, The lithium salt includes one or more combinations of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium perchlorate, and lithium perfluoroalkyl sulfonate; the molar concentration of the lithium salt in the electrolyte is from 0.5 mol / L to 3 mol / L.

4. A lithium battery electrolyte according to claim 1, characterized in that, The organic solvent includes one or more combinations of ethylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, propylene carbonate, ethyl acetate, propyl acetate, diethyl ether, ethylene glycol dimethyl ether, tetrahydrofuran, and dioxolane.

5. A lithium battery electrolyte according to claim 1, characterized in that, The basic additives include one or more combinations of fluorinated ethylene carbonate, 1,3-propanesulfonate lactone, vinylene carbonate, lithium dioxalate borate, lithium difluorooxalate borate, lithium nitrate, and lithium difluorophosphate.

6. A lithium battery electrolyte according to claim 1, characterized in that, R1 is an alkyl group having a carbon chain length of C1 to C6, selected from at least one of -CH3, -CH2-CH3, -CH2-CH2-CH3, -CH(CH3)-CH3, -CH2-CH2-CH2-CH3, and -C(CH3)3; R2 is a fluorinated alkyl group having a carbon chain length of C1 to C4, selected from at least one of -CH2F, -CHF2, -CF3, -CH2-CH2F, -CH2-CHF2, -CH2-CF3, -CH2-CH2-CH2-CH2F, -CH2-CH2-CH2-CHF2, and -CH2-CH2-CH2-CF3.

7. A method for preparing a lithium battery electrolyte as described in any one of claims 1-6, characterized in that, Includes the following steps: S10. Under an argon-protected environment, the organic solvent is mixed evenly to obtain a mixed solvent; S20. The lithium salt is added to the mixed solvent and stirred to dissolve, thereby obtaining the basic electrolyte; S30. Add the basic additive to the basic electrolyte, mix well, then add the fluorinated oxalate additive, mix well, and obtain the electrolyte.

8. The method for preparing a lithium battery electrolyte according to claim 7, characterized in that, In S20, the lithium salt is added at a rate of 0.5-2.0 kg / min, and the temperature inside the reactor is maintained at 25-35°C by external circulating cooling water.

9. A method for synthesizing the fluorinated oxalate additive as described in claim 1, characterized in that, Includes the following steps: M10. Dissolve the fluorinated alcohol and triethylamine in dichloromethane solvent to prepare the first reaction solution; M20. Add the acyl chloride compound to the constant pressure dropping funnel and drop it into the first reaction solution over 5-10 minutes, controlling the molar ratio of fluorinated alcohol to acyl chloride compound to be 1:0.5-3. M30, under nitrogen protection and ice-water bath conditions, control the temperature of the reaction system to not exceed 10℃, and continue stirring for 0.1-1h; M40, remove the ice-water bath and continue the reaction at room temperature for 10-24 hours; After the reaction is complete (M50), the reaction solution is filtered to obtain the fluorinated oxalate additive.

10. A lithium battery electrolyte according to claim 1, characterized in that, Suitable electrode materials for electrolytes include lithium cobalt oxide, lithium iron phosphate, manganese nickel cobalt composite oxide, lithium nickel manganese oxide, lithium-rich manganese-based, nickel cobalt manganese ternary cathode, graphite anode, silicon anode, and lithium metal anode.

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