A high-concentration aqueous electrolyte with a wide electrochemical stability window and its application

Abstract

The present invention relates to the technical field of sodium secondary batteries, and particularly relates to a high-concentration aqueous sodium trifluoroacetate electrolyte with a wide electrochemical stability window. The electrolyte is composed of sodium trifluoroacetate, water and an organic solvent. In the high-concentration aqueous electrolyte, the molar ratio of sodium trifluoroacetate to (water + organic solvent) is higher than the molar ratio of sodium trifluoroacetate to water in a saturated aqueous solution of sodium trifluoroacetate. The electrolyte prepared by the present invention has a high electrochemical window and can inhibit the dissolution of electrode materials, thereby greatly improving the cycle stability of aqueous sodium secondary batteries.

Description

Technical Field

[0001] The present invention relates to the technical field of aqueous sodium ion batteries, and particularly to a high-concentration sodium trifluoroacetate aqueous electrolyte assisted by an organic solvent with a wide electrochemical stability window. Background Art

[0002] With the development of renewable energy sources such as wind energy and solar energy, large-scale stationary energy storage systems have become the focus of attention for power reliability and quality. Aqueous sodium ion batteries have the advantages of environmental friendliness, low cost, easy manufacturing, good safety, and easy recycling, meeting the requirements of large-scale energy storage systems. However, the electrochemical window of water is only 1.23 V, which greatly limits the output voltage of aqueous sodium ion batteries and the selection of electrode materials, resulting in a low energy density of aqueous sodium ion batteries. Moreover, due to the dissolution of inorganic electrode materials in water, the low cycle stability of the battery is caused. Therefore, seeking appropriate methods to broaden the electrochemical window of aqueous electrolytes and reduce the dissolution of electrode materials is the common goal of many scientific researchers.

[0003] Current research mainly improves the electrochemical performance of aqueous electrolytes by increasing the salt concentration. In 2015, the concept of "water in salt" was proposed and applied to aqueous sodium ion battery electrolytes in 2017 (Adv. Energy Mater., 2017, 7, 1701189). The sodium trifluoromethanesulfonate electrolyte with a concentration of 9.2 mol / kg elevates the stable voltage window of the electrolyte to 2.5 V by enabling all water molecules to participate in the solvation layer of Na + and forming an F-containing solid electrolyte interface layer (SEI) by anion decomposition on the surface of the negative electrode. However, due to the solubility limitation of common sodium salts, there is still a small amount of free water even in saturated high-concentration salts, which will erode the SEI of the negative electrode and the electrode material, leading to the occurrence of hydrogen evolution reaction and capacity decay of the electrode material. Summary of the Invention

[0004] In view of the deficiencies of the prior art, the purpose of the present invention is to provide a high-concentration aqueous sodium trifluoroacetate electrolyte with a wide electrochemical stability window.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0006] A high-concentration aqueous sodium trifluoroacetate electrolyte with a wide electrochemical stability window, the aqueous electrolyte comprising sodium trifluoroacetate, an organic solvent, and water. The electrolyte is obtained by mixing sodium trifluoroacetate, an organic solvent, and water in a certain proportion and stirring until a clear solution is formed, and the resulting solution is the high-concentration aqueous sodium trifluoroacetate electrolyte with a wide electrochemical stability window.

[0007] The organic solvent is selected from one or more of N-methylpyrrolidone, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, acetamide, N,N-dimethylacrylamide, urea, methylurea, tetramethylurea, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, propylene carbonate, vinylene carbonate, acetonitrile, succinonitrile, glutaronitrile, adiponitrile, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, methanol, ethanol, propanol, isopropanol, glycerol, 1,3-dioxolane, trimethyl phosphate, triethyl phosphate, etc.

[0008] In the high-concentration aqueous electrolyte, the molar ratio of sodium trifluoroacetate to (water + organic solvent) is higher than the molar ratio of sodium trifluoroacetate to water in a saturated aqueous solution of sodium trifluoroacetate.

[0009] The minimum addition amount of the organic solvent in the high-concentration aqueous electrolyte is for dissolving sodium trifluoroacetate, and the maximum addition amount is for making the molar ratio of sodium trifluoroacetate / (water + organic solvent) in the high-concentration aqueous electrolyte greater than the molar ratio of sodium trifluoroacetate to water in a saturated aqueous solution of sodium trifluoroacetate (about 1:2.13).

[0010] Due to the different solubilities of sodium trifluoroacetate in different organic solvents, the minimum addition amounts of the organic solvents are also different. Taking the organic solvent acetonitrile as an example, when the molar ratio of sodium trifluoroacetate to water is 1:1, the minimum addition amount of acetonitrile is 0.33 to ensure that sodium trifluoroacetate can dissolve in the mixed solvent of water and acetonitrile. When the organic solvent is propylene carbonate, when the molar ratio of sodium trifluoroacetate to water is 1:1, the minimum addition amount of propylene carbonate is 0.30 to ensure that sodium trifluoroacetate can dissolve in the mixed solvent of water and propylene carbonate.

[0011] Under the condition that the high-concentration aqueous electrolyte dissolves sodium trifluoroacetate and the molar ratio of sodium trifluoroacetate / (water + organic solvent) is greater than the molar ratio of sodium trifluoroacetate to water in a saturated aqueous solution of sodium trifluoroacetate (about 1:2.13), preferably, the molar ratio of sodium trifluoroacetate / water / solvent is 1:(0.2 - 2.12):(0.01 - 1.93), more preferably, the molar ratio of sodium trifluoroacetate, water, and organic solvent is 1:(0.5 - 2):(0.1 - 1.0), and further preferably, the molar ratio of sodium trifluoroacetate, water, and organic solvent is 1:(0.5 - 1):(0.3 - 1.0).

[0012] The electrochemical stability window of the aqueous electrolyte is 2.6 - 3.1 V, and the solubility ratio of the aqueous electrolyte to the electrode material is lower than that in a saturated solution of sodium trifluoroacetate.

[0013] Application of the high-concentration aqueous electrolyte with a wide electrochemical stability window in a sodium secondary battery system.

[0014] The sodium secondary battery includes a positive electrode sheet, a negative electrode sheet, a separator, and the aqueous electrolyte described above.

[0015] The positive electrode active material of the positive electrode sheet is one of Na3V2(PO4)3, Na2FeMn(CN)6, and NaNi 0.25 Mn 0.75 O2. The negative electrode active material of the negative electrode sheet is Na3V2(PO4)3 or NaTi2(PO4)3. The sodium secondary battery assembled with the electrolyte prepared in the present invention has long cycle stability. The separator is a commonly used separator for sodium secondary batteries and is not limited.

[0016] Advantages of the present invention: By introducing an organic solvent, the limit solubility of sodium trifluoroacetate in a pure aqueous phase (the molar ratio of sodium trifluoroacetate to water in a saturated aqueous solution of sodium trifluoroacetate is about 1:2.13) is broken through, the salt / water ratio of the aqueous solution is increased, water molecules enter the solvation layer of sodium, the activity of water is reduced, and a stable solid electrolyte interface layer containing F decomposed by anions is promoted to form on the surface of the negative electrode, so that the electrolyte has a wide electrochemical stability window (2.6 - 3.1 V). At the same time, the electrolyte can also inhibit the dissolution of electrode materials and improve the long cycle performance of the battery. Description of the Drawings

[0017] Figure 1 It is the cyclic voltammogram of the electrolyte provided in Example 1 of the present invention in a three - electrode system.

[0018] Figure 2 It is the cyclic voltammogram of the electrolyte provided in Example 2 of the present invention in a three - electrode system.

[0019] Figure 3 It is the cyclic voltammogram of the electrolyte provided in Example 3 of the present invention in a three - electrode system.

[0020] Figure 4 It is the cycling performance of a sodium vanadium phosphate / / sodium titanium phosphate battery assembled with the electrolyte provided in Example 3 of the present invention.

[0021] Figure 5 It is the charge - discharge curve of the 2nd cycle of a sodium vanadium phosphate / / sodium titanium phosphate battery assembled with the electrolyte provided in Example 3 of the present invention.

[0022] Figure 6 It is the F 1s spectrum of the X - ray photoelectron spectroscopy (XPS) of the surface of the negative electrode after cycling of a battery assembled with the electrolyte provided in Example 3 of the present invention.

[0023] Figure 7 It is the charge - discharge curve of a sodium vanadium phosphate / / sodium vanadium phosphate battery assembled with the electrolyte provided in Example 3 of the present invention.

[0024] Figure 8Cyclic voltammogram curve of the electrolyte provided in Comparative Example 1 of the present invention in a three-electrode system.

[0025] Figure 9 Cycling performance of the sodium vanadium phosphate / / sodium titanium phosphate battery assembled with the electrolyte provided in Comparative Example 1 of the present invention.

[0026] Figure 10 Charge-discharge curve of the sodium vanadium phosphate / / sodium vanadium phosphate battery assembled with the electrolyte provided in Comparative Example 1 of the present invention. Detailed implementation manners

[0027] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0028] Measurement of the electrochemical window: Using an Ag / AgCl / saturated KCl standard electrode (0.198 V vs. SHE) as the reference electrode, a platinum sheet as the counter electrode, a titanium mesh as the working electrode, and performing cyclic voltammetry (CV) tests on a VMP-300 type electrochemical workstation with a scanning rate of 1 mV / s using a three-electrode system.

[0029] Preparation of the electrode sheet: Using N-methylpyrrolidone (NMP) solvent, mixing the active material: Super P: polyvinylidene fluoride (PVDF) = 8:1:1 (mass ratio) with the solvent evenly to form a slurry, coating it on a titanium foil with a 100 µm doctor blade, and cutting it into an electrode sheet with a diameter of 12 mm after drying. The positive electrode active material is one of Na3V2(PO4)3, Na2FeMn(CN)6 and NaNi 0.25 Mn 0.75 O2, and the negative electrode active material is Na3V2(PO4)3 or NaTi2(PO4)3.

[0030] Characterization of the electrochemical performance: The electrochemical performance was tested for charge-discharge performance using a CT2001A type Blue Power battery test system.

[0031] Example 1

[0032] Sodium trifluoroacetate, water and acetonitrile were mixed at a molar ratio of 1:1:1 and stirred evenly to obtain a transparent electrolyte. The electrolyte was subjected to CV tests in a three-electrode system, and its electrochemical stability window was 2.7 V (see Figure 1 ), indicating that the electrolyte has a wide electrochemical stability window.

[0033] Example 2

[0034] Sodium trifluoroacetate, water and acetamide were mixed at a molar ratio of 1:0.2:1.0, and stirred well to obtain a transparent electrolyte. The electrolyte was subjected to CV test in a three-electrode system, and its electrochemical stability window was 3.1 V (see Figure 2 ), indicating that the electrolyte has a wide electrochemical stability window.

[0035] Example 3

[0036] Sodium trifluoroacetate, water and propylene carbonate (PC) were mixed at a molar ratio of 1:1:0.5, and stirred well at 30 °C to obtain a transparent electrolyte. The electrolyte was subjected to CV test in a three-electrode system, and its electrochemical stability window was 2.9 V (see Figure 3 ).

[0037] The sodium vanadium phosphate / / sodium titanium phosphate full cell (P / N = 1.2, capacity ratio) was assembled using the electrolyte prepared in this example, and tested under the conditions of 25 °C, 2 C (sodium titanium phosphate, 1 C = 133 mA / g), 0.5 - 2.0 V. The battery capacity was 122 mAh / g, and the capacity did not decay after 100 cycles (see Figure 4 ), the average Coulombic efficiency was 99.2%, and the charge-discharge curve of the second cycle is shown in Figure 5 , indicating that the electrolyte has low solubility for the electrode material, greatly improving the cycle stability of the aqueous sodium-ion battery.

[0038] The sodium vanadium phosphate positive electrode charged in the sodium vanadium phosphate / / sodium titanium phosphate battery was removed, and the electrode was immersed in the electrolyte prepared in this example and a saturated aqueous solution of sodium trifluoroacetate for 30 days respectively. After 30 days, the saturated aqueous solution of sodium trifluoroacetate changed from colorless to light yellow, indicating that a large amount of vanadium dissolved in the solution, while the electrolyte prepared in this example remained colorless. The immersion experiment proved that the solubility of the sodium vanadium phosphate positive electrode material in this electrolyte is lower than that in the saturated aqueous solution of sodium trifluoroacetate, which is beneficial to the stable operation of the sodium vanadium phosphate positive electrode material.

[0039] The sodium vanadium phosphate / / sodium vanadium phosphate full cell (P / N = 1.1, capacity ratio) was assembled using the electrolyte prepared in this example. After cycling 20 times at a rate of 5 C (sodium vanadium phosphate, 1 C = 58.5 mA / g), the components of the solid electrolyte interface layer (SEI) on the surface of the negative electrode were tested by X-ray photoelectron spectroscopy (XPS). The test results showed that the electrolyte can form an SEI containing NaF on the negative electrode (see Figure 6 ).

[0040] The sodium vanadium phosphate / / sodium vanadium phosphate full cell assembled with the electrolyte prepared in this example (P / N = 1.1, capacity ratio) was tested under the conditions of 5 C and 0.8 - 1.9 V (sodium vanadium phosphate negative electrode, 1C = 58.5 mA / g). The initial and 100th discharge capacities of the battery were 52 mAh / g and 48 mAh / g respectively (see Figure 7 ), and the capacity retention rate was as high as 92.3%.

[0041] Comparative Example 1

[0042] Prepare a saturated aqueous solution of sodium trifluoroacetate, and perform CV test on this electrolyte in a three - electrode system. Its electrochemical stability window is only 2.5 V (see Figure 8 ). It can be seen that the present invention greatly broadens the electrochemical stability window of the aqueous solution.

[0043] The sodium vanadium phosphate / / sodium titanium phosphate full cell assembled with the electrolyte prepared in Comparative Example 1 (P / N = 1.2, capacity ratio) was tested under the conditions of 25 °C, 2 C (sodium titanium phosphate, 1 C = 133 mA / g), and 0.5 - 2.0 V. The initial and 100th discharge capacity densities of the battery were 113 mAh / g and 37 mAh / g respectively (see Figure 9 ), and the significant attenuation of the capacity indicates the dissolution of the electrode material.

[0044] The sodium vanadium phosphate / / sodium vanadium phosphate full cell assembled with the electrolyte prepared in Comparative Example 1 (P / N = 1.1, capacity ratio) was tested under the conditions of 5 C (sodium vanadium phosphate negative electrode, 1C = 58.5 mA / g) and 0.8 - 1.9 V. The initial and 100th discharge capacities of the battery were 57 mAh / g and 23 mAh / g respectively (see Figure 10 ), and the capacity retention rate was only 40.3%. It can be seen from the comparative examples and examples that compared with the saturated aqueous solution of sodium trifluoroacetate, the potential window and cycling performance of this high - concentration aqueous electrolyte have been greatly improved.

Claims

1. A high-concentration aqueous electrolyte with a wide electrochemical stability window, characterized in that: The aqueous electrolyte comprises sodium trifluoroacetate, an organic solvent and water, and the molar ratio of sodium trifluoroacetate to (water + organic solvent) in the high-concentration aqueous electrolyte is higher than the molar ratio of sodium trifluoroacetate to water in a saturated aqueous solution of sodium trifluoroacetate.

2. The high-concentration aqueous electrolyte with a wide electrochemical stability window according to claim 1, characterized in that: Under the condition that the high-concentration aqueous electrolyte dissolves sodium trifluoroacetate and the molar ratio of sodium trifluoroacetate / (water + organic solvent) is greater than the molar ratio of sodium trifluoroacetate to water in a saturated aqueous solution of sodium trifluoroacetate, the molar ratio of sodium trifluoroacetate / water / organic solvent is 1: (0.2~2.12): (0.01~1.93).

3. A high-concentration aqueous electrolyte with a wide electrochemical stability window according to claim 1, characterized in that: The organic solvent is selected from one or more of N-methylpyrrolidone, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, acetamide, N,N-dimethylacrylamide, urea, methylurea, tetramethylurea, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, propylene carbonate, vinylene carbonate, acetonitrile, succinonitrile, glutaronitrile, adiponitrile, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, methanol, ethanol, propanol, isopropanol, glycerol, 1,3-dioxolane, trimethyl phosphate, and triethyl phosphate.

4. A high-concentration aqueous electrolyte with a wide electrochemical stability window according to claim 1, characterized in that: The electrochemical stability window of the high-concentration aqueous electrolyte is 2.6∽3.1 V.

5. Application of a high-concentration aqueous electrolyte having a wide electrochemical stability window according to any one of claims 1∽4 in a sodium secondary battery system.

6. A sodium secondary battery, characterized in that: The sodium secondary battery comprises a positive electrode sheet, a negative electrode sheet, a separator and a high-concentration aqueous electrolyte having a wide electrochemical stability window according to any one of claims 1∽4.

7. The sodium secondary battery according to claim 6, characterized in that: The active material of the positive electrode sheet of the sodium secondary battery is one of Na3V2(PO4)3, Na2FeMn(CN)6, and NaNi 0.25 Mn 0.75 O2, and the active material of the negative electrode sheet is Na3V2(PO4)3 or NaTi2(PO4)3.

Images