High-temperature electrolyte for sodium ion secondary battery as well as preparation method and application of high-temperature electrolyte

Abstract

The invention relates to a high-temperature electrolyte for a sodium ion secondary battery and application of the high-temperature electrolyte. The high-temperature electrolyte comprises a solvent and a sodium salt, the solvent comprises a high-boiling-point solvent and a weak-solvated fluorinated 2-alkoxy ethyoxyl silane solvent or a derivative thereof, and the concentration of the sodium salt is 0.2-4 mol / L. According to the electrolyte, the fluorinated siloxane solvent with weak solvation characteristics and high stability is introduced, so that the side reaction of an electrode interface at high temperature is effectively inhibited, and the working temperature range of the electrolyte is widened. Experimental results show that the NaNa3V2 (PO4) 2O2F battery using the electrolyte can still keep the average coulombic efficiency of 98.59% or above and excellent cycle stability at 90 DEG C. The electrolyte provided by the invention is simple in preparation method, the raw materials are easy to obtain, the high-temperature performance of the sodium-ion battery is remarkably improved, and the electrolyte has a wide application prospect.

Description

Technical Field

[0001] This invention relates to the field of electrochemical technology, specifically to a high-temperature electrolyte for sodium-ion secondary batteries, as well as the preparation method and application of the electrolyte. Background Technology

[0002] With the ever-increasing demand for energy storage, the development of battery technologies with high energy density, high power density, and excellent environmental adaptability has become increasingly urgent. Sodium-ion batteries, due to their abundant resources and low cost, have become a key research focus in the field of large-scale energy storage. Among them, sodium-ion secondary batteries using sodium vanadium oxyfluorophosphate (Na3V2(PO4)2O2F) as the cathode material have a high theoretical energy density (exceeding 494 Wh kg⁻¹). -1 With its advantages of high safety and low self-discharge rate, it has shown broad application prospects in high-end fields such as medical, military, and aerospace.

[0003] However, existing sodium-ion batteries face severe challenges in high-temperature environments (such as above 50 °C). High temperatures exacerbate electrolyte decomposition, narrow its electrochemical window, accelerate side reactions at the electrode / electrolyte interface, leading to a sharp decrease in battery coulombic efficiency, an increase in internal resistance, and rapid capacity decay, ultimately severely shortening battery life. For example, most batteries have a coulombic efficiency of less than 95% at 50 °C, and may even fail to function properly at higher temperatures. Therefore, developing an electrolyte that maintains stability over a wide temperature range (especially high temperatures) is of great significance for expanding the application scenarios of sodium-ion batteries. Summary of the Invention

[0004] This invention addresses the problem of poor high-temperature performance in existing sodium-ion batteries by providing a high-temperature electrolyte for sodium-ion secondary batteries. This electrolyte, through the use of a novel fluorinated siloxane solvent, significantly improves the battery's cycle stability and coulombic efficiency at high temperatures.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0006] A high-temperature electrolyte for sodium-ion secondary batteries comprises a solvent and a sodium salt, wherein the solvent comprises a high-boiling-point solvent and a weakly solvated fluorinated 2-alkoxyethoxysilane solvent or a derivative thereof; the concentration of the sodium salt is 0.2–4 mol / L.

[0007] Preferably, the structure of the fluorinated 2-alkoxyethoxysilane solvent is as shown in formula (I):

[0008] R f O–CH2CH2–O–SiR3 (I)

[0009] Among them, R fIt is a fluoroalkyl group, and SiR3 is a silyl group.

[0010] The fluoroalkyl R f Including but not limited to 2-fluoroethyl, 2,2-difluoroethyl, 2,2,2-trifluoroethyl, 2,2,3,3,3-pentafluoropropyl, 2,2,3,3-tetrafluoropropyl, 2,2,3,3,4,4,4-heptafluorobutyl, 2,2,3,3,4,4-hexafluorobutyl, 2,2,3,4,4,4-hexafluorobutyl, and perfluorotert-butyl; silyl SiR3 includes but is not limited to trimethylsilyl, triethylsilyl, triisopropylsilyl, tert-butyldimethylsilyl, and tert-butyldiphenylsilyl. Preferably, the fluorinated 2-alkoxyethoxysilane solvent is trimethyl(2-(2,2,2-trifluoroethoxy)ethoxy)silane.

[0011] The present invention also provides a method for preparing the fluorinated 2-alkoxyethoxysilane solvent, comprising the following steps:

[0012] (1) A fluorinated 2-alkoxyethanol intermediate was synthesized by reacting a fluorinated alcohol with ethylene carbonate in a high-boiling solvent:

[0013] R f O−CH2CH2−OH (Ⅱ)

[0014] (2) The intermediate is reacted with a silyl ether protecting agent to obtain fluorinated 2-alkoxyethoxysilane.

[0015] Preferably, in step (1), the reaction temperature is 100-200 °C and the reaction time is 4-24 hours.

[0016] Preferably, in step (2), the reaction is carried out in the presence of an organic base at -5 to 30 °C.

[0017] Preferably, the high-boiling-point solvent is selected from one or more of propylene carbonate, ethylene carbonate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, tetraethanol dimethyl ether, dimethyl carbonate, diethyl carbonate, 1,3-dioxolane, and tetrahydrofuran.

[0018] Preferably, the volume ratio of the high-boiling-point solvent to the fluorinated 2-alkoxyethoxysilane solvent is (1-10):1.

[0019] Preferably, the sodium salt is selected from one or more of sodium hexafluorophosphate, sodium bis(trifluoromethanesulfonyl)imide, sodium bis(fluorosulfonyl)imide, sodium bis(oxalatoborate), sodium difluorooxalatoborate, sodium perchlorate, sodium tetrafluoroborate, and sodium nitrate.

[0020] The present invention further provides the application of the above-mentioned high-temperature electrolyte in sodium-ion secondary batteries.

[0021] Preferably, the positive electrode active material of the sodium-ion secondary battery is sodium vanadium fluorophosphate (Na3V2(PO4)2O2F).

[0022] The present invention also provides a sodium-ion secondary battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte as described above.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) The novel fluorinated siloxane solvent (such as trimethyl(2-(2,2,2-trifluoroethoxy)ethoxy)silane) provided by the present invention has an asymmetric molecular structure, which reduces volatility at high temperature; at the same time, the strong electron-withdrawing effect of trifluoromethyl and the steric hindrance effect of trimethylsilyl work together to precisely control the weak solvation ability of the solvent and effectively suppress the side reactions at the electrode interface at high temperature.

[0025] (2) The high-temperature electrolyte of the present invention can remain liquid at a high temperature of 90 °C and has good compatibility with high-voltage cathode materials, which can significantly improve the coulombic efficiency and cycle life of sodium-ion batteries at high temperatures. Experimental results show that the Na-Na3V2(PO4)2O2F battery using the electrolyte of the present invention still has an average coulombic efficiency of 98.59% after 400 cycles at 90 °C.

[0026] (3) The raw materials used in this invention are easy to obtain and the preparation method is simple, making it suitable for industrial production. Attached Figure Description

[0027] Figure 1 The ¹H NMR spectrum of trimethyl(2-(2,2,2-trifluoroethoxy)ethoxy)silane prepared in Example 1 of this invention;

[0028] Figure 2 The ¹³C NMR spectrum of trimethyl(2-(2,2,2-trifluoroethoxy)ethoxy)silane prepared in Example 1 of this invention;

[0029] Figure 3 The ¹ of trimethyl(2-(2,2,2-trifluoroethoxy)ethoxy)silane prepared in Example 1 of this invention 9 FNMR spectrum;

[0030] Figure 4 The graph shows the long-cycle performance of the Na||Na3V2(PO4)2O2F simulated battery in Example 2 of this invention at different temperatures.

[0031] Figure 5 The graph shows the long-cycle performance of the Na||Na3V2(PO4)2O2F simulated battery of Comparative Example 1 of this invention at 25 °C. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to specific embodiments. However, this invention is not limited to these embodiments.

[0033] Example 1: Synthesis of trimethyl(2-(2,2,2-trifluoroethoxy)ethoxy)silane

[0034] In a 250 mL flask equipped with a magnetic stirrer, ethylene carbonate (49 g, 554 mmol), 2,2,2-trifluoroethanol (43 mL, 600 mmol), sodium hydroxide (2 g, 50 mmol), and 80 mL of tetraethylene glycol dimethyl ether were added. The mixture was stirred at room temperature for 30 minutes until the ethylene carbonate was completely dissolved, then heated to 150 °C and refluxed for 8 hours. After the reaction was complete, the mixture was cooled to room temperature and distilled under reduced pressure. The fraction distilled at 80–130 °C was collected. This fraction was then subjected to a second distillation under reduced pressure, collecting the fraction distilled at 90–92 °C to obtain a colorless, transparent liquid intermediate.

[0035] In a 500 mL flask equipped with a magnetic stirrer, the above intermediate (36 g, 250 mmol), triethylamine (42 mL, 300 mmol), and 150 mL of dichloromethane were added. Trimethylchlorosilane (35 mL, 275 mmol) was added dropwise under stirring at 0–5 °C. After the addition was complete, the mixture was allowed to cool to room temperature, and the reaction was continued with stirring for 8 hours. After the reaction was complete, the mixture was filtered under reduced pressure. The filtrate was washed three times with water, and the organic phase was dried over anhydrous sodium sulfate and concentrated under reduced pressure. The residue was distilled under reduced pressure, and the fraction collected at 83–85 °C yielded a colorless, transparent liquid product, trimethyl(2-(2,2,2-trifluoroethoxy)ethoxy)silane, in 87% yield.

[0036] Product NMR data: ¹H NMR (600 MHz, DMSO-d6) δ 4.03 (q, J = 9.3 Hz, 2H), 3.70 (t, J = 5.0 Hz, 2H), 3.64 (t, 2H), 0.08 (s, 9H); ¹³C NMR (151 MHz, CDCl3) δ 124.61 (q, J = 279.35 Hz), 74.50, 69.44 (q, J = 33.7 Hz), 62.63; ¹ 9 F NMR (565 MHz, CDCl3) δ -74.39 (t, J = 9.04 Hz). The spectrum is shown below. Figure 1-3 As shown.

[0037] Example 2: Preparation of high-temperature electrolyte and battery performance testing

[0038] Under anhydrous and oxygen-free conditions, sodium hexafluorophosphate (NaPF6, concentration 1 mol / L) and sodium difluorooxalate borate (NaDFOB, mass 2% of the total electrolyte) were dissolved in a mixed solvent of propylene carbonate and trimethyl(2-(2,2,2-trifluoroethoxy)ethoxy)silane prepared in Example 1 at a volume ratio of 4:1 to obtain a high-temperature electrolyte.

[0039] A coin cell battery was assembled using sodium vanadium fluorophosphate (Na3V2(PO4)2O2F) as the positive electrode material and sodium metal as the negative electrode material. Charge-discharge tests were conducted at 25 ℃, 70 ℃, 80 ℃, and 90 ℃ within a voltage range of 2.0–4.3 V. The results show that the battery exhibits excellent cycle stability at different temperatures; specific data are shown in Table 1 and [Table data would be inserted here]. Figure 4 .

[0040] Comparative Example 1

[0041] Under anhydrous and oxygen-free conditions, sodium hexafluorophosphate (NaPF6, 1 mol / L) and sodium difluorooxalate borate (NaDFOB, 2% of the total electrolyte mass) were dissolved in propylene carbonate as the sole solvent to obtain a control electrolyte. Batteries were assembled under the same conditions, and tests were conducted only at 25°C.

[0042] Table 1 Comparison of battery performance between Example 2 and Comparative Example 1

[0043]

[0044] From Table 1 and Figure 4 , Figure 5 As can be seen, the high-temperature electrolyte provided by this invention (Example 2) maintains extremely high coulombic efficiency (greater than 98.5%) over a wide temperature range of 25 °C to 90 °C and achieves long cycle life at high temperatures, while the electrolyte of Comparative Example 1 cannot function properly at high temperatures. This indicates that this invention significantly improves the high-temperature performance of sodium-ion batteries by introducing a novel fluorinated siloxane solvent.

[0045] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A high-temperature electrolyte for sodium-ion secondary batteries, characterized in that, It comprises a solvent and a sodium salt, wherein the solvent comprises a high-boiling-point solvent and a weakly solvated fluorinated 2-alkoxyethoxysilane solvent or a derivative thereof; the concentration of the sodium salt is 0.2–4 mol / L.

2. The high-temperature electrolyte according to claim 1, characterized in that, The structure of the fluorinated 2-alkoxyethoxysilane solvent is shown in formula (I): R f O–CH2CH2–O–SiR3 (I) Among them, R f It is a fluoroalkyl group, including 2-fluoroethyl, 2,2-difluoroethyl, 2,2,2-trifluoroethyl, 2,2,3,3,3-pentafluoropropyl, 2,2,3,3-tetrafluoropropyl, 2,2,3,3,4,4,4-heptafluorobutyl, 2,2,3,3,4,4-hexafluorobutyl, 2,2,3,4,4,4-hexafluorobutyl, and perfluorotert-butyl; SiR3 is a silane, including trimethylsilyl, triethylsilyl, triisopropylsilyl, tert-butyldimethylsilyl, and tert-butyldiphenylsilyl.

3. The high-temperature electrolyte according to claim 2, characterized in that, The fluorinated 2-alkoxyethoxysilane solvent is trimethyl(2-(2,2,2-trifluoroethoxy)ethoxy)silane.

4. The high-temperature electrolyte according to claim 1, characterized in that, The high-boiling-point solvent is selected from one or more of propylene carbonate, ethylene carbonate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, tetraethanol dimethyl ether, dimethyl carbonate, diethyl carbonate, 1,3-dioxolane, and tetrahydrofuran.

5. The high-temperature electrolyte according to claim 1, characterized in that, The volume ratio of the high-boiling-point solvent to the fluorinated 2-alkoxyethoxysilane solvent is (1-10):

1.

6. The high-temperature electrolyte according to claim 1, characterized in that, The sodium salt is selected from one or more of sodium hexafluorophosphate, sodium bis(trifluoromethylsulfonyl)imide, sodium bis(fluorosulfonyl)imide, sodium bis(oxalatoborate), sodium difluorooxalatoborate, sodium perchlorate, sodium tetrafluoroborate, and sodium nitrate.

7. A method for preparing a high-temperature electrolyte according to any one of claims 1 to 6, characterized in that, Under anhydrous and oxygen-free conditions, a high-boiling-point solvent is mixed with a fluorinated 2-alkoxyethoxysilane solvent or its derivative, and then a sodium salt is added to dissolve it to obtain the high-temperature electrolyte; the volume ratio of the high-boiling-point solvent to the fluorinated 2-alkoxyethoxysilane solvent or its derivative is (1-10):1; the concentration of the sodium salt is 0.2-4 mol / L.

8. A method for preparing a fluorinated 2-alkoxyethoxysilane solvent, characterized in that, Includes the following steps: (a) A fluorinated alcohol reacts with ethylene carbonate in a high-boiling ether solvent to give a fluorinated 2-alkoxyethanol intermediate: R f O−CH2CH2−OH (Ⅱ) (b) The intermediate is reacted with a silyl ether protecting agent to give fluorinated 2-alkoxyethoxysilane.

9. A sodium-ion secondary battery, characterized in that, It includes a positive electrode, a negative electrode, a separator, and a high-temperature electrolyte as described in any one of claims 1 to 6.

10. The sodium-ion secondary battery according to claim 9, characterized in that, The positive electrode active material in the positive electrode sheet is sodium vanadium fluorophosphate Na3V2(PO4)2O2F.

Images