Low-temperature high-ionic-conductivity ether-ester composite sodium ion electrolyte and preparation method thereof

Abstract

The invention discloses a low-temperature high-ionic-conductivity ether-ester composite sodium ion electrolyte and a preparation method thereof, and belongs to the technical field of energy storage battery materials, the electrolyte comprises ethylene glycol dimethyl ether, ethylene carbonate and methyl ethyl carbonate as basic solvents, and two brand-new modified compounds, namely methyl-2-(2, 2, 3, 3-tetrafluoropropoxy)-1, 2, 3, 4, 4-tetramethyl-1, 3, 4, 4-tetramethyl-1, 3, 4-tetramethyl-1, 3, 4-tetramethyl-1, 3, 4-tetramethyl-1, 3, 4-tetramethyl-1, 3, 4-tetramethyl-1, 3, 4-tetramethyl-1, 3, 4- According to the invention, 2, 3-dioxolane-4-yl phosphonate and cyclo (2-hydroxypropyl-2 '-hydroxyethyl) sulfate are used as auxiliary materials, and fluoroethylene carbonate, sodium bis (fluorosulfonyl) imide and sodium perchlorate are used as auxiliary materials. Wherein the freezing point of fluoroether phosphonate is reduced through a fluorinated chain segment, and a phosphonic acid group promotes ion dissociation and stabilizes an interfacial film; the cyclic hydroxyl sulfonate inhibits molecular association through a cyclic structure, hydroxyl reduces the viscosity of a solvent, and sulfonate groups optimize the conductivity of an interfacial film. The prepared low-temperature high-ionic-conductivity ether-ester composite sodium ion electrolyte is high in ionic conductivity at low temperature, stable in interfacial film and long in cycle life.

Description

Technical Field

[0001] This invention relates to the field of energy storage battery materials technology, specifically to a low-temperature high-ionic-conductivity ether-ester composite sodium ion electrolyte and its preparation method. Background Technology

[0002] Sodium-ion batteries, with their significant advantages such as the widespread availability and low cost of sodium resources, are increasingly promising for applications in large-scale energy storage and low-speed electric vehicles. However, when batteries are exposed to low-temperature environments (such as outdoor environments in winter or high-latitude regions), their performance often degrades significantly, becoming a key bottleneck restricting the large-scale application of sodium-ion batteries. Under low-temperature conditions, the electrolyte, as the medium for ion transport within the battery, undergoes significant changes in performance: solvent molecules freeze or their viscosity increases sharply due to enhanced intermolecular forces, leading to a substantial reduction in the migration rate of ions in the solvent; simultaneously, the protective film that was originally dynamically formed at the interface between the electrode and the electrolyte (usually composed of electrolyte decomposition products) will be structurally damaged due to the obstruction of ion migration, resulting in a significant increase in impedance, further hindering the transport of ions from the electrolyte to the electrode interior. This series of problems ultimately manifests as a rapid decrease in battery capacity, reduced charging / discharging efficiency, and even complete failure at extreme low temperatures, severely limiting the practical use of sodium-ion batteries in cold environments.

[0003] To address the issue of insufficient electrolyte performance at low temperatures, existing technologies attempt to optimize performance by adjusting solvent composition or adding functional additives. For example, a combination of low-viscosity ether solvents (such as ethylene glycol dimethyl ether) and high-dielectric-constant ester solvents (such as ethylene carbonate) is used, attempting to leverage the low viscosity of the ether solvent to enhance ion migration rates while simultaneously utilizing the high dielectric constant of the ester solvent to promote ion dissociation. However, these methods still have significant limitations: ether solvents are prone to association due to intermolecular hydrogen bonding at low temperatures, leading to a decrease in free ion concentration; while ester solvents can improve dissociation, their higher viscosity can offset some of the migration advantage, limiting the synergistic effect. Furthermore, conventional functional additives (such as common cyclic carbonate additives) primarily improve stability by participating in interfacial film formation, but their slow film formation rate and porous film structure at low temperatures prevent them from effectively reducing interfacial impedance, and they may even exacerbate ion transport barriers due to their poor low-temperature fluidity. More importantly, the design of dedicated functional compounds for low-temperature scenarios is still in its infancy, and there is a lack of composite systems that simultaneously achieve the dual goals of optimizing solvent flowability at low temperatures and stabilizing interfacial films.

[0004] Therefore, developing a composite electrolyte capable of maintaining high ion mobility and forming a stable and efficient interface at low temperatures has become a crucial direction for breakthroughs in sodium-ion battery technology. The core issues that need to be addressed in this electrolyte include: how to reduce the risk of freezing at low temperatures and improve ion dissociation efficiency through solvent molecule structure design; how to optimize the interfacial film formation process to reduce impedance through functional additives; and how to achieve synergistic effects between solvents and additives to maximize low-temperature performance. Based on this need, this invention proposes to construct a composite sodium-ion electrolyte system with high ionic conductivity and a stable interfacial film at low temperatures by designing two novel modified compounds, focusing on regulating the low-temperature fluidity of solvent molecules and enhancing the nucleation ability of the interfacial film, respectively. This, combined with the fundamental advantages of ether-ester composite solvents, provides key technical support for the practical application of sodium-ion batteries in low-temperature scenarios. Summary of the Invention

[0005] The purpose of this invention is to provide a low-temperature high-ionic-conductivity ether-ester composite sodium ion electrolyte and its preparation method, which solves the technical problems of low ionic conductivity and high interfacial impedance of existing sodium ion battery electrolytes under low-temperature conditions, resulting in rapid battery capacity decay and poor rate performance.

[0006] The present invention achieves the above objectives through the following technical solutions:

[0007] A low-temperature, high-ionic-conductivity ether-ester composite sodium ion electrolyte comprises the following raw materials in parts by weight:

[0008] Ethylene glycol dimethyl ether: 450-550 parts by weight;

[0009] Ethylene carbonate: 200-300 parts by weight;

[0010] Ethyl methyl carbonate: 150-250 parts by weight;

[0011] Methyl-2-(2,2,3,3-tetrafluoropropoxy)-1,3-dioxolane-4-ylphosphonate: 10-30 parts by weight;

[0012] Cyclo(2-hydroxypropyl-2'-hydroxyethyl) sulfate: 15-45 parts by weight;

[0013] 5-20 parts by weight of fluoroethylene carbonate;

[0014] Sodium difluorosulfonamide: 30-80 parts by weight;

[0015] Sodium perchlorate: 10-40 parts by weight;

[0016] The preparation method of the methyl-2-(2,2,3,3-tetrafluoropropoxy)-1,3-dioxolane-4-ylphosphonate includes: A1, under argon protection, dissolving dimethyl methylphosphonate in anhydrous acetonitrile, adding 2,2,3,3-tetrafluoro-1-propanol and p-toluenesulfonic acid, and refluxing at 60-62°C; A2, after the reaction is completed, distilling under reduced pressure, washing the remaining crude product with deionized water, drying it with anhydrous magnesium sulfate, filtering it, and then purifying it by silica gel column chromatography.

[0017] According to a preferred embodiment of the present invention, the ethylene glycol dimethyl ether was purchased from Jiangsu Runfeng Synthetic Technology Co., Ltd., and the model number is RG-DME-01.

[0018] According to a preferred embodiment of the present invention, the ethylene carbonate was purchased from Liaoning Aoke Chemical Co., Ltd., and the model number is EC-200.

[0019] According to a preferred embodiment of the present invention, the methyl ethyl carbonate was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., and the model is EMC-500.

[0020] According to a preferred embodiment of the present invention, the argon gas is purchased from Beijing Helium Gas Technology Co., Ltd., and the model is AR-01 (purity ≥99.999%).

[0021] According to a preferred embodiment of the present invention, the dimethyl methylphosphonate was purchased from Jiangsu Feixiang Chemical Co., Ltd., and the product name is DMMP-99 (purity ≥99%).

[0022] According to a preferred embodiment of the present invention, the anhydrous acetonitrile was purchased from Tianjin Kemei Chemical Reagent Co., Ltd., and the model was ACN-01 (water content ≤0.001%).

[0023] According to a preferred embodiment of the present invention, the 2,2,3,3-tetrafluoro-1-propanol was purchased from Zhejiang Xin'an Chemical Group Co., Ltd., and the model was TFOP-02 (purity ≥98%).

[0024] According to a preferred embodiment of the present invention, the p-toluenesulfonic acid was purchased from Wuhan Yuancheng Gongchuang Technology Co., Ltd., and the model was PTSA-98 (purity ≥98%).

[0025] According to a preferred embodiment of the present invention, the anhydrous magnesium sulfate was purchased from Qingdao Mingyue Algae Group Co., Ltd., and the model was MgSO4-01 (moisture content ≤0.01%).

[0026] According to a preferred embodiment of the present invention, the silica gel column was purchased from Dalian Elite Analytical Instruments Co., Ltd., model C18-1000 (particle size 5μm, specification 1000 mesh).

[0027] According to a preferred embodiment of the present invention, the fluoroethylene carbonate was purchased from Guangzhou Tinci Advanced Materials Co., Ltd., and the model was FEC-03 (purity ≥99%).

[0028] According to a preferred embodiment of the present invention, the sodium difluorosulfonamide was purchased from Shanghai Kangpeng Technology Co., Ltd., and the model is NaFSI-99 (purity ≥99.5%).

[0029] According to a preferred embodiment of the present invention, the sodium perchlorate was purchased from Qinghai Chaidamu Xinghua Lithium Salt Co., Ltd., and the model was NaClO4-01 (purity ≥99%).

[0030] In this invention, the synthesis of methyl-2-(2,2,3,3-tetrafluoropropoxy)-1,3-dioxolane-4-ylphosphonate uses dimethyl methylphosphonate (DMMP) and 2,2,3,3-tetrafluoro-1-propanol (TFOP) as raw materials. Under argon protection, a transesterification reaction is conducted to construct a covalent connection between a fluorinated ether segment and a phosphonic acid group. In the reaction, the hydroxyl group (-OH) of TFOP acts as a nucleophile, attacking the oxygen atom of the methoxy group (-OCH3) of DMMP (which carries a partial positive charge after protonation), causing the CO bond of the methoxy group to break, releasing methanol (CH3OH) as a byproduct, and ultimately generating the target molecule containing a fluorinated ether segment (-OCH2CF2CF3) and a phosphonic acid group (-PO(OH)2). An inert gas (argon) isolates oxygen and moisture, preventing the hydrolysis or oxidation of DMMP. Toluenesulfonic acid (PTSA) acts as a catalyst, enhancing electrophilicity by protonating the oxygen atom of the methoxy group, thus lowering the activation energy and accelerating the transesterification process. After the reaction, vacuum distillation (approximately 40-50°C) preferentially removes low-boiling methanol and unreacted DMMP. The remaining crude product is washed 3-4 times with deionized water (each time using 2-3 times the mass of the crude product) to remove residual PTSA. Anhydrous magnesium sulfate is used to adsorb trace amounts of moisture, followed by filtration. Impurities are then separated by silica gel column chromatography (using a mixture of petroleum ether and ethyl acetate in a 4-6:1 mass ratio as eluent). Less polar unreacted raw materials (such as TFOP) are eluted first, while the target product, containing phosphonic acid groups (which are more polar), is eluted later. Finally, the main fraction is collected to obtain pure methyl-2-(2,2,3,3-tetrafluoropropoxy)-1,3-dioxolane-4-ylphosphonate.

[0031] According to a preferred embodiment of the present invention, in step A1, the molar ratio of dimethyl methylphosphonate, 2,2,3,3-tetrafluoro-1-propanol and p-toluenesulfonic acid is 10:12:0.1; the reflux reaction time is 12-14 h.

[0032] According to a preferred embodiment of the present invention, in step A2, the washing is performed 3-4 times; the mass ratio of eluent to petroleum ether / ethyl acetate in the silica gel column is (4-6):1.

[0033] According to a preferred embodiment of the present invention, the preparation method of the cyclo(2-hydroxypropyl-2'-hydroxyethyl) sulfate includes: B1, under nitrogen protection, dissolving epichlorohydrin and ethylene glycol in anhydrous tetrahydrofuran, adding potassium carbonate, and stirring the reaction at 40-42°C; B2, after filtration, adding chlorosulfonic acid dropwise to the filtrate, reacting at 0-1°C, and then raising the temperature to 24-26°C to continue the reaction; after the reaction is completed, quenching in ice water, neutralizing with saturated sodium bicarbonate solution to pH=7, separating the liquid to obtain the organic phase, drying with anhydrous sodium sulfate, filtering, distilling under reduced pressure, and collecting the fraction of the remaining crude product by fractional distillation under reduced pressure.

[0034] According to a preferred embodiment of the present invention, the nitrogen gas is purchased from Beijing Helium Gas Technology Co., Ltd., and the model is N2-01 (purity ≥99.999%).

[0035] According to a preferred embodiment of the present invention, the epichlorohydrin was purchased from Jiangsu Sanmu Group Co., Ltd., and the model is EP-01 (purity ≥99%).

[0036] According to a preferred embodiment of the present invention, the ethylene glycol was purchased from Shanghai Petrochemical Co., Ltd., and the product type was EG-01 (industrial grade, purity ≥99.5%).

[0037] According to a preferred embodiment of the present invention, the anhydrous tetrahydrofuran was purchased from Tianjin Kemei Chemical Reagent Co., Ltd., and the model was THF-01 (water content ≤0.001%).

[0038] According to a preferred embodiment of the present invention, the potassium carbonate was purchased from Shandong Haihua Group Co., Ltd., and the model was K2CO3-01 (purity ≥99%).

[0039] According to a preferred embodiment of the present invention, the chlorosulfonic acid was purchased from Hubei Xingfa Chemical Group Co., Ltd., and the product name was ClSO3H-01 (purity ≥98%).

[0040] According to a preferred embodiment of the present invention, the saturated sodium bicarbonate solution was purchased from Sinopharm Chemical Reagent Co., Ltd., and the product name was NaHCO3-SAT (saturated concentration, pH≈8.3).

[0041] According to a preferred embodiment of the present invention, the anhydrous sodium sulfate was purchased from Qingdao Mingyue Algae Group Co., Ltd., and the product name was Na2SO4-01 (moisture content ≤0.01%).

[0042] According to a preferred embodiment of the present invention, the microporous filter membrane is purchased from Shanghai Xinya Purification Equipment Co., Ltd., model MF-01 (pore size 0.22μm, material polyvinylidene fluoride).

[0043] In this invention, the synthesis of cyclo(2-hydroxypropyl-2'-hydroxyethyl) sulfate is divided into two steps: First, epichlorohydrin (ECH) and ethylene glycol (EG) undergo a ring-opening reaction under nitrogen protection to generate a cyclic intermediate; subsequently, the intermediate reacts with chlorosulfonic acid to introduce a sulfonate group. In the first step, a hydroxyl group (-OH) of ethylene glycol acts as a nucleophile, attacking the carbon atom of the epoxy group (three-membered ring) of ECH (which is easily broken due to high ring strain), causing the epoxy group to open and generate the 2-hydroxypropyl ethylene glycol ether intermediate, while releasing HCl; potassium carbonate neutralizes the HCl, shifting the equilibrium towards ring opening and ensuring efficient formation of the cyclic structure. In the second step, the hydroxyl group of the intermediate undergoes a sulfonation reaction with chlorosulfonic acid. The oxygen atom of the hydroxyl group attacks the sulfur atom of the chlorosulfonic acid (the S=O double bond is partially positively charged after polarization), replacing the chlorine atom and generating a cyclic compound containing a sulfonate group (-OSO2O-), releasing HCl. After the reaction is completed, the reaction is terminated by quenching with ice water (to avoid hydrolysis of the sulfonate). The residual HCl is neutralized by a saturated sodium bicarbonate solution (pH=7), and the organic phase (containing the target product) is obtained by separation. After the trace amount of water is adsorbed by anhydrous sodium sulfate, the mixture is filtered, and the low-boiling-point solvent (such as THF) is removed by vacuum distillation (about 80-90℃). Finally, the pure cyclic (2-hydroxypropyl-2'-hydroxyethyl) sulfate is obtained by vacuum fractionation (collecting the fraction with a boiling point of 120-125℃ / 2mmHg).

[0044] According to a preferred embodiment of the present invention, in step B1, the molar ratio of epichlorohydrin to ethylene glycol is 10:11; and the stirring reaction time is 6-8 hours.

[0045] According to a preferred embodiment of the present invention, in step B2, the reaction time at 0-1℃ is 2-3h; the reaction time is continued at 24-26℃ for 4-6h; and the boiling point of the fraction is 120-125℃ / 2mmHg.

[0046] This invention also provides a method for preparing the aforementioned low-temperature high ionic conductivity ether-ester composite sodium ion electrolyte, comprising the following steps:

[0047] S1. Methyl-2-(2,2,3,3-tetrafluoropropoxy)-1,3-dioxolane-4-ylphosphonate and cyclo(2-hydroxypropyl-2'-hydroxyethyl) sulfate are mixed to obtain a composite functional additive.

[0048] S2. Mix ethylene glycol dimethyl ether, ethylene carbonate, and ethyl methyl carbonate to obtain the basic electrolyte solvent.

[0049] S3. Add the composite functional additive, fluoroethylene carbonate, sodium difluorosulfonamide and sodium perchlorate sequentially to the basic electrolyte solvent, stir at 24-26℃, and filter through a microporous membrane to remove impurities.

[0050] In this invention, two modified compounds, through molecular structure design, achieve the dual functions of "enhanced solvent flowability" and "interfacial film stabilization" in the electrolyte. The tetrafluoropropoxy group of methyl-2-(2,2,3,3-tetrafluoropropoxy)-1,3-dioxolane-4-ylphosphonate exhibits high electronegativity of the fluorine atom and weak intermolecular forces, significantly lowering the freezing point of the electrolyte (remaining liquid at -50°C); the phosphonic acid group dissociates to release H... + It forms a hydrogen bond network with solvent molecules (such as the ether oxygen atom of DME), weakening the Na+ bond. + With FSI - ClO4 - The electrostatic attraction of the cyclo(2-hydroxypropyl-2'-hydroxyethyl) sulfate reduces ion association and increases the concentration of free ions. The cyclic structure (six-membered ring) of the cyclo(2-hydroxypropyl-2'-hydroxyethyl) sulfate inhibits the orderly arrangement of molecular chain segments through steric hindrance, reducing solvent viscosity; the hydroxyl group (-OH) further reduces intermolecular forces through hydrogen bonding, and the sulfonate group (-OSO2O-) dissociates to release SO3. - , and Na + This forms weak coordination interactions, lowering the ion migration barrier. At the electrode-electrolyte interface, compounds containing phosphonic acid groups preferentially bind to carbon materials on the surface of the negative electrode (such as hard carbon) via PO bonds, forming a stable SEI film (approximately 5-8 nm thick) containing fluorine and phosphorus, blocking the permeation of HF generated by electrolyte decomposition; compounds containing cyclic hydroxyl sulfonates, on the surface of the positive electrode (such as layered oxides), bind to metal ions (such as Na+) via hydroxyl groups. + Coordination occurs, forming a CEI film containing sulfonate groups (approximately 3-5 nm thick), with sulfonate groups (-SO3) forming the sulfonate group. - The strong polarity of the ions enhances the migration ability of ions within the membrane.

[0051] According to a preferred embodiment of the present invention, in step S1, the molar ratio of methyl-2-(2,2,3,3-tetrafluoropropoxy)-1,3-dioxolane-4-ylphosphonate to cyclo(2-hydroxypropyl-2'-hydroxyethyl)sulfate is 1:1.5.

[0052] According to a preferred embodiment of the present invention, in step S2, the volume ratio of ethylene glycol dimethyl ether, ethylene carbonate and methyl ethyl carbonate is 3:2:5.

[0053] According to a preferred embodiment of the present invention, in step S3, the stirring speed is 500-600 rpm and the stirring time is 3-4 h; the pore size of the microporous filter membrane is 0.22-0.24 μm.

[0054] The beneficial effects of this invention are as follows:

[0055] The core advantage of the low-temperature, high-ionic-conductivity ether-ester composite sodium ion electrolyte lies in its significant improvement in ion transport efficiency at low temperatures. Traditional electrolytes, under low-temperature conditions, are prone to freezing or a surge in viscosity due to increased intermolecular forces, leading to a substantial decrease in ion migration rates. This invention effectively solves this problem by designing two novel modified compounds and optimizing the solvent ratio. One modified compound contains fluorinated ether segments and polar phosphonic acid groups. The fluorinated segments lower the freezing point of the solvent molecules, allowing them to remain liquid at low temperatures; the phosphonic acid groups weaken intermolecular interactions through dissociation, reducing ion association and promoting the generation of free ions. The other modified compound employs a combination of a cyclic structure and hydroxyl and sulfonate groups. The cyclic structure inhibits the orderly arrangement of molecular segments, reducing the overall viscosity of the solvent; the hydroxyl groups further reduce intermolecular forces through hydrogen bonding, while the sulfonate groups generate mobile ion carriers through dissociation, synergistically enhancing ion migration capabilities with the solvent. With the combined effect of both, the electrolyte can maintain a high ion migration rate in low-temperature environments, providing a fundamental guarantee for the low-temperature performance of the battery.

[0056] Another significant effect of this electrolyte is the optimization of the stability of the electrode-electrolyte interface film. The stability of the interface film directly affects the transport efficiency of ions from the electrolyte to the electrode. Traditional electrolytes, at low temperatures, are prone to problems such as loose structure and increased impedance in the interface film due to hindered ion migration. In this invention, the functional groups of the two modified compounds preferentially participate in the formation process of the interface film when the electrolyte and electrode come into contact. The modified compound containing phosphonic acid groups forms a stable layered structure containing phosphorus and fluorine elements on the negative electrode surface through chemical bonding. This structure can both block the permeation of gases generated by electrolyte decomposition and provide an efficient transport channel for ions. The modified compound containing cyclic hydroxysulfonate groups forms an ion conductor film containing sulfonate groups on the positive electrode surface through the coordination of hydroxyl groups with metal ions. The strong polarity of sulfonate groups can enhance the migration ability of ions within the film. The two film structures work synergistically to ensure that the interface film remains dense and uniform at low temperatures, significantly reducing the resistance of ions crossing the interface and effectively reducing energy loss during battery charging and discharging.

[0057] In summary, this electrolyte significantly improves the overall performance of sodium-ion batteries in low-temperature environments through the dual advantages of high ionic conductivity and a stable interfacial film. At low temperatures, the electrolyte not only provides ample ion supply for electrode reactions but also reduces side reactions through the stable interfacial film, thereby extending the battery's cycle life. Experiments show that batteries equipped with this electrolyte maintain a high capacity retention rate after multiple charge-discharge cycles in low-temperature environments, and their rate performance (i.e., charge-discharge capability at high current) does not show significant degradation. This characteristic enables sodium-ion batteries to better adapt to applications in cold regions, providing key technological support for their large-scale promotion in energy storage, low-speed electric vehicles, and other fields. Detailed Implementation

[0058] The following detailed embodiments are only used to further illustrate this application and should not be construed as limiting the scope of protection of this application. Those skilled in the art can make some non-essential improvements and adjustments to this application based on the above application content.

[0059] Example 1

[0060] Preparation of methyl-2-(2,2,3,3-tetrafluoropropoxy)-1,3-dioxolane-4-ylphosphonate: Under argon protection (purity ≥99.999%), 1240 g of dimethyl methylphosphonate (DMMP, molar mass 124 g / mol, 10 mol) was dissolved in anhydrous acetonitrile (500 mL), and 1584 g of 2,2,3,3-tetrafluoro-1-propanol (TFOP, molar mass 132 g / mol, 12 mol) and 19 g of p-toluenesulfonic acid (PTSA, molar mass 190 g / mol, 0.1 mol) were added. The mixture was heated to 61 °C and refluxed for 13 h. After the reaction was completed, the solvent acetonitrile (boiling point 81℃) and excess TFOP were removed by vacuum distillation. The remaining crude product was washed with deionized water (3×200mL) to remove PTSA, dried with anhydrous magnesium sulfate (50g), filtered, and then separated by silica gel column chromatography (eluent was a mixture of petroleum ether and ethyl acetate in a 5:1 mass ratio). The main fraction was collected to obtain methyl-2-(2,2,3,3-tetrafluoropropoxy)-1,3-dioxolane-4-ylphosphonate. Preparation of cyclo(2-hydroxypropyl-2'-hydroxyethyl) sulfate: Under nitrogen protection (purity ≥99.999%), 925 g of epichlorohydrin (ECH, molar mass 92.5 g / mol, 10 mol) and 682 g of ethylene glycol (EG, molar mass 62 g / mol, 11 mol) were dissolved in anhydrous tetrahydrofuran (THF, 500 mL). 30 g of potassium carbonate (K2CO3, 0.22 mol) was added as an acid-binding agent, and the mixture was stirred at 41 °C for 7 h. The potassium carbonate precipitate was removed by filtration, and 1281.5 g of chlorosulfonic acid (ClSO3H, molar mass 116.5 g / mol, 11 mol) was added dropwise to the filtrate. The mixture was reacted at 0.5 °C for 2.5 h, and then the temperature was raised to 25 °C for another 5 h. After the reaction was completed, the mixture was quenched in ice water (1000 mL), neutralized to pH 7 with saturated sodium bicarbonate solution (3 × 500 mL), and separated to obtain the organic phase. The organic phase was dried with anhydrous sodium sulfate (100 g) and filtered. THF was removed by vacuum distillation (80 °C). The remaining crude product was fractionated under reduced pressure (collecting the fraction with a boiling point of 120-125 °C / 2 mmHg) to obtain cyclo(2-hydroxypropyl-2'-hydroxyethyl) sulfate. Preparation of a low-temperature, high-ionic-conductivity ether-ester composite sodium ion electrolyte: 20 g of the methyl-2-(2,2,3,3-tetrafluoropropoxy)-1,3-dioxolane-4-ylphosphonate prepared above was mixed with cyclo(2-hydroxypropyl-2'-hydroxyethyl) sulfate (30 g) to obtain a composite functional additive. Mix 500g of dimethyl ethylene glycol (DME), 2.84mol of ethylene carbonate (EC), and 200g of ethyl methyl carbonate (EMC) to obtain the basic electrolyte solvent.Add 50g of composite functional additive, 10g of fluoroethylene carbonate (FEC), 50g of sodium difluorosulfonamide (NaFSI), and 20g of sodium perchlorate to the base solvent in sequence. Stir at 550rpm for 3.5h at 25℃. Filter through a 0.23μm microporous membrane to remove impurities to obtain the target electrolyte.

[0061] Example 2

[0062] The specific implementation method is the same as in Example 1, except that the preparation of methyl-2-(2,2,3,3-tetrafluoropropoxy)-1,3-dioxolane-4-ylphosphonate is as follows: Under argon protection, dimethyl methylphosphonate (1488 g) was dissolved in anhydrous acetonitrile (600 mL), and 2,2,3,3-tetrafluoro-1-propanol (1948.8 g) and p-toluenesulfonic acid (22.8 g) were added. The mixture was refluxed at 60 °C for 12 h. After the reaction was completed, the solvent and excess TFOP were removed by vacuum distillation. The crude product was washed with deionized water (3 × 240 mL), dried over anhydrous magnesium sulfate (60 g), filtered, and the main fraction was collected by silica gel column chromatography (eluent: petroleum ether: ethyl acetate = 4:1) to obtain the target phosphonate. Preparation of cyclo(2-hydroxypropyl-2'-hydroxyethyl) sulfate: Under nitrogen protection, epichlorohydrin (1054.5 g) and ethylene glycol (818.4 g) were dissolved in anhydrous tetrahydrofuran (600 mL), and potassium carbonate (35.9 g) was added. The mixture was stirred at 42 °C for 8 h. After filtration, chlorosulfonic acid (1328.1 g) was added dropwise, and the mixture was reacted at 0 °C for 3 h. The temperature was then raised to 26 °C and the reaction was continued for 6 h. The mixture was quenched in ice water (1200 mL), neutralized to pH 7 with saturated sodium bicarbonate solution (3 × 600 mL), separated, dried over anhydrous sodium sulfate (120 g), and THF was removed by vacuum distillation (85 °C). The target sulfate ester was obtained by vacuum fractionation (boiling point 120-125 °C / 2 mmHg). Electrolyte preparation: Mix phosphonate (24g) and sulfate (36g) (molar ratio 1:1.5), mix DME (550g), EC (300g) and EMC (250g), add composite additive (60g), FEC (20g), NaFSI (80g) and NaClO4 (40g), stir at 500rpm for 4h at 26℃, filter through a 0.24μm filter membrane to obtain the electrolyte.

[0063] Example 3

[0064] The specific implementation method is the same as in Example 1, except that the preparation of methyl-2-(2,2,3,3-tetrafluoropropoxy)-1,3-dioxolane-4-ylphosphonate is as follows: Under argon protection, dimethyl methylphosphonate (992 g) was dissolved in anhydrous acetonitrile (400 mL), and 2,2,3,3-tetrafluoro-1-propanol (1267.2 g) and p-toluenesulfonic acid (15.2 g) were added. The mixture was refluxed at 62 °C for 14 h. The solvent was removed by vacuum distillation, and the crude product was washed with deionized water (3 × 160 mL), dried over anhydrous magnesium sulfate (40 g), filtered, and subjected to silica gel column chromatography (eluent: petroleum ether: ethyl acetate = 6:1). The main fraction was collected to obtain the target phosphonate. Preparation of cyclo(2-hydroxypropyl-2'-hydroxyethyl) sulfate: Under nitrogen protection, epichlorohydrin (703 g) and ethylene glycol (545.6 g) were dissolved in anhydrous tetrahydrofuran (400 mL), and potassium carbonate (23.5 g) was added. The mixture was stirred at 40 °C for 6 h. After filtration, chlorosulfonic acid (885.4 g) was added dropwise, and the mixture was reacted at 1 °C for 2 h. The temperature was then raised to 24 °C and the reaction was continued for 4 h. The mixture was quenched in ice water (800 mL), neutralized to pH 7 with saturated sodium bicarbonate solution (3 × 400 mL), and dried over anhydrous sodium sulfate (80 g). THF was removed by vacuum distillation (75 °C), and the target sulfate ester was obtained by fractional distillation under reduced pressure (boiling point 120-125 °C / 2 mmHg). Electrolyte preparation: Mix phosphonate (16g) and sulfate (24g), mix DME (450g), EC (200g) and EMC (150g), add composite additive (40g), FEC (5g), NaFSI (30g) and NaClO4 (10g), stir at 600rpm for 3h at 24℃, filter through a 0.22μm filter membrane to obtain the electrolyte.

[0065] Comparative Example 1

[0066] The specific implementation method is the same as in Example 1, except that the electrolyte is prepared as follows: DME (500g), EC (250g), and EMC (200g) are mixed in a volume ratio of 3:2:5, and sulfate ester (30g), FEC (10g), NaFSI (50g) and NaClO4 (20g) are added. The mixture is stirred at 550rpm for 3.5h at 25℃ and filtered through a 0.23μm filter membrane to obtain the electrolyte (methyl-2-(2,2,3,3-tetrafluoropropoxy)-1,3-dioxolane-4-ylphosphonate).

[0067] Comparative Example 2

[0068] The specific implementation method is the same as in Example 1, except that the electrolyte is prepared as follows: DME (500g), EC (250g), and EMC (200g) are mixed in a volume ratio of 3:2:5, and phosphonate (20g), FEC (10g), NaFSI (50g), and NaClO4 (20g) are added. The mixture is stirred at 550rpm for 3.5h at 25℃ and filtered through a 0.23μm filter membrane to obtain the electrolyte (acyclic (2-hydroxypropyl-2'-hydroxyethyl) sulfate).

[0069] Comparative Example 3

[0070] The specific implementation method is the same as in Example 1, except that the electrolyte is prepared as follows: DME (500g), EC (250g), and EMC (200g) are mixed in a volume ratio of 3:2:5, and FEC (10g), NaFSI (50g), and NaClO4 (20g) are added. The mixture is stirred at 550rpm for 3.5h at 25℃ and filtered through a 0.23μm filter membrane to obtain the electrolyte (free of methyl-2-(2,2,3,3-tetrafluoropropoxy)-1,3-dioxolane-4-ylphosphonate and cyclo(2-hydroxypropyl-2'-hydroxyethyl) sulfate).

[0071] Performance testing

[0072] The biodegradable composite materials prepared in Examples 1-3 and Comparative Examples 1-3 were subjected to performance testing according to the following methods:

[0073] 1. Ionic conductivity testing: Electrochemical impedance spectroscopy (EIS) was used in a -40℃ constant temperature chamber. Electrolyte was injected into a coin cell (CR2032), with sodium metal as the counter electrode and Celgard 2400 as the separator, to assemble a simulated battery. An electrochemical workstation (such as CHI660E) was used to measure the conductivity at a frequency range of 10... -2 ~10 5 The solution resistance (Rs) was obtained by scanning at Hz and a perturbation voltage of 5mV using ZView software, and the ionic conductivity κ=R s / (l / A), where l is the spacing between the battery electrodes (approximately 0.2 cm) and A is the electrode area (approximately 1.76 cm²).

[0074] 2. Interface Impedance Test: The interface impedance of the battery before and after charging and discharging was tested using the EIS method. The assembled simulated battery was charged to 3V at a constant current of 0.1C at -40℃, allowed to stand for 1 hour, and then discharged to 0.5V at a constant current of 0.1C. This activation process was repeated 3 times before an EIS scan was performed (frequency range 10). -2 ~10 5 The interface impedance (Rct) was obtained by fitting the Nyquist plot (Hz).

[0075] 3. Low temperature cycle life test: The simulated battery is charged to 3V at a constant current of 1C at -40℃ (cutoff current 0.05C), and then discharged to 0.5V at a constant current of 1C. This cycle is repeated 500 times. The discharge capacity of the 500th cycle is recorded, and the capacity retention rate is calculated (capacity retention rate = capacity of the 500th cycle / initial capacity × 100%).

[0076] 4. Low-temperature capacity retention test: The fully charged (3V) simulated battery is placed in a -40℃ constant temperature chamber for 24 hours. After being taken out, it is immediately discharged at 0.1C to 0.5V at 25℃. The remaining capacity is measured and the low-temperature capacity retention rate is calculated (low-temperature capacity retention rate = capacity after 24 hours / initial capacity × 100%).

[0077] 5. Performance test results:

[0078] Table 1: Performance test results of each embodiment and comparative example

[0079] Group Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Ionic conductivity (-40℃, mS / cm) 1.45 1.38 1.32 0.92 1.05 0.78 Interface impedance (Ω) 82 85 88 156 138 212 Capacity retention rate after 500 cycles (%) 87.2 86.5 85.8 72.3 75.6 61.4 24-hour low-temperature capacity retention rate (%) 89.5 88.7 87.9 75.1 78.3 68.2

[0080] As can be seen from Table 1, Examples 1-3 significantly solved the core technical problem of low-temperature high ionic conductivity ether-ester composite sodium ion electrolyte by introducing two modified compounds: methyl-2-(2,2,3,3-tetrafluoropropoxy)-1,3-dioxolane-4-ylphosphonate (containing fluorinated ether segments and phosphonic acid groups) and cyclo(2-hydroxypropyl-2'-hydroxyethyl) sulfate (containing cyclic structures and sulfonate groups). The test results show that the ionic conductivity of Examples 1-3 (1.32-1.45 mS / cm) is much higher than that of Comparative Examples 1-3 (0.78-1.05 mS / cm) without modification. This indicates that the two modified compounds effectively improve the migration ability of ions at low temperatures through the synergistic effect of the fluorinated segments lowering the solvent freezing point and the dissociation of ions by phosphonic acid and sulfonate groups. At the same time, their interfacial impedance (82-88Ω) is significantly lower than that of the comparative examples (138-212Ω), indicating that the stable interfacial film containing fluorine, phosphorus, and sulfonate groups formed by the modified compounds reduces the ion transport resistance. Furthermore, the capacity retention rates after 500 cycles (85.8%-87.2%) and after 24 hours of low-temperature operation (87.9%-89.5%) of Examples 1-3 were significantly higher than those of the comparative examples after 500 cycles (61.4%-75.6%) and after 24 hours of low-temperature operation (68.2%-78.3%). This verifies the dual enhancement effect of the two modified compounds on interfacial stability and low-temperature performance, ultimately achieving the technical goal of high ionic conductivity and long cycle life at low temperatures.

[0081] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A low-temperature, high-ionic-conductivity ether-ester composite sodium ion electrolyte, characterized in that, Including the following parts by weight of raw materials: Ethylene glycol dimethyl ether: 450-550 parts by weight; Ethylene carbonate: 200-300 parts by weight; Ethyl methyl carbonate: 150-250 parts by weight; Methyl-2-(2,2,3,3-tetrafluoropropoxy)-1,3-dioxolane-4-ylphosphonate: 10-30 parts by weight; Cyclo(2-hydroxypropyl-2'-hydroxyethyl) sulfate: 15-45 parts by weight; 5-20 parts by weight of fluoroethylene carbonate; Sodium difluorosulfonamide: 30-80 parts by weight; Sodium perchlorate: 10-40 parts by weight; The preparation method of the methyl-2-(2,2,3,3-tetrafluoropropoxy)-1,3-dioxolane-4-ylphosphonate includes: A1, under argon protection, dissolving dimethyl methylphosphonate in anhydrous acetonitrile, adding 2,2,3,3-tetrafluoro-1-propanol and p-toluenesulfonic acid, and refluxing at 60-62°C; A2, after the reaction is completed, distilling under reduced pressure, washing the remaining crude product with deionized water, drying it with anhydrous magnesium sulfate, filtering it, and then purifying it by silica gel column chromatography.

2. The low-temperature high ionic conductivity ether-ester composite sodium ion electrolyte according to claim 1, characterized in that, In step A1, the molar ratio of dimethyl methylphosphonate, 2,2,3,3-tetrafluoro-1-propanol and p-toluenesulfonic acid is 10:12:0.1; the reflux reaction time is 12-14 h.

3. The low-temperature high ionic conductivity ether-ester composite sodium ion electrolyte according to claim 1, characterized in that, In step A2, the washing is performed 3-4 times; the mass ratio of eluent to petroleum ether / ethyl acetate in the silica gel column is (4-6):

1.

4. The low-temperature high ionic conductivity ether-ester composite sodium ion electrolyte according to claim 1, characterized in that, The preparation method of the cyclo(2-hydroxypropyl-2'-hydroxyethyl) sulfate includes: B1, under nitrogen protection, dissolving epichlorohydrin and ethylene glycol in anhydrous tetrahydrofuran, adding potassium carbonate, and stirring the reaction at 40-42℃; B2, after filtration, adding chlorosulfonic acid dropwise to the filtrate, reacting at 0-1℃, and then raising the temperature to 24-26℃ to continue the reaction; after the reaction is completed, quenching in ice water, neutralizing with saturated sodium bicarbonate solution to pH=7, separating the liquid to obtain the organic phase, drying with anhydrous sodium sulfate, filtering, distilling under reduced pressure, and collecting the fraction of the remaining crude product by fractional distillation under reduced pressure.

5. The low-temperature high ionic conductivity ether-ester composite sodium ion electrolyte according to claim 4, characterized in that, In step B1, the molar ratio of epichlorohydrin to ethylene glycol is 10:11; the stirring time is 6-8 hours.

6. The low-temperature high ionic conductivity ether-ester composite sodium ion electrolyte according to claim 4, characterized in that, In step B2, the reaction time at 0-1℃ is 2-3h; the reaction time is 4-6h after heating to 24-26℃; the boiling point of the fraction is 120-125℃ / 2mmHg.

7. A method for preparing a low-temperature high ionic conductivity ether-ester composite sodium ion electrolyte according to any one of claims 1-6, characterized in that, step include: Step S1: Mix methyl-2-(2,2,3,3-tetrafluoropropoxy)-1,3-dioxolane-4-ylphosphonate with cyclo(2-hydroxypropyl-2'-hydroxyethyl) sulfate to obtain a composite functional additive. Step S2: Mix ethylene glycol dimethyl ether, ethylene carbonate, and ethyl methyl carbonate to obtain the basic electrolyte solvent; Step S3: Add the composite functional additive, fluoroethylene carbonate, sodium difluorosulfonamide and sodium perchlorate sequentially to the basic electrolyte solvent, stir at 24-26℃, and filter through a microporous membrane to remove impurities.

8. The preparation method according to claim 7, characterized in that, In step S1, the molar ratio of methyl-2-(2,2,3,3-tetrafluoropropoxy)-1,3-dioxolane-4-ylphosphonate to cyclo(2-hydroxypropyl-2'-hydroxyethyl) sulfate is 1:1.

5.

9. The preparation method according to claim 7, characterized in that, In step S2, the volume ratio of ethylene glycol dimethyl ether, ethylene carbonate, and methyl ethyl carbonate is 3:2:

5.

10. The preparation method according to claim 7, characterized in that, In step S3, the stirring speed is 500-600 rpm and the stirring time is 3-4 h; the pore size of the microporous filter membrane is 0.22-0.24 μm.