An electrolyte and a sodium-ion battery

Abstract

The present invention provides an electrolyte and a sodium-ion battery, belonging to the technical field of electrochemistry. The electrolyte includes a soluble sodium salt, water, and N,N-dimethylformamide; the volume ratio of water to N,N-dimethylformamide is 1:(3-9); the concentration of the soluble sodium salt in the electrolyte is 1-10 mol / kg. Water in the electrolyte provided by the present invention serves as a solvent. Adding a soluble sodium salt can inhibit the activity of water and broaden the electrochemical stability window of water. At the same time, N,N-dimethylformamide is added and its volume ratio to water is defined. N,N-dimethylformamide can reduce the hydrogen evolution and oxygen evolution reactions of the electrolyte, broaden the electrochemical window, and improve the performance of the electrolyte when the concentration of the soluble sodium salt is relatively low, thereby improving the performance of the sodium-ion battery assembled with the electrolyte.

Description

Technical Field

[0001] The present invention relates to the field of electrochemical technologies, and particularly to an electrolyte and a sodium-ion battery. Background Art

[0002] With the rapid development of renewable energy and the popularization of electric vehicles, the demand for efficient and reliable energy storage systems is increasing continuously. Traditional lithium-ion batteries are restricted due to problems such as limited and unevenly distributed lithium resources. Aqueous sodium-ion batteries have become an alternative attracting much attention because of their relatively abundant sodium resources and wide distribution.

[0003] The electrolyte of an aqueous sodium-ion battery is usually water and sodium salts. However, the electrochemical stability window of water is relatively narrow. In the prior art, highly concentrated (up to 63 mol / kg), highly soluble sodium imide salts are usually used to inhibit the activity of water, thereby broadening the electrochemical stability window of water. However, the price of this kind of salt is relatively high. Using highly concentrated salts will lead to an increase in the cost of the battery. At the same time, highly concentrated salts will cause the viscosity of the electrolyte to be too large, resulting in a decrease in the electrochemical performance of the battery, and further hindering the practical application of aqueous sodium-ion batteries. Therefore, how to reduce the cost of the electrolyte and ensure its electrochemical performance has become a difficult problem in this field. Summary of the Invention

[0004] The purpose of the present invention is to provide an electrolyte and a sodium-ion battery. The electrolyte provided by the present invention has a relatively low cost, and the assembled sodium-ion battery has good electrochemical performance.

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

[0006] The present invention provides an electrolyte, comprising a soluble sodium salt, water and N,N-dimethylformamide; the volume ratio of the water to the N,N-dimethylformamide is 1:(3 - 9); the concentration of the soluble sodium salt in the electrolyte is 1 - 10 mol / kg.

[0007] Preferably, the soluble sodium salt includes one or more of sodium perchlorate, sodium bis(trifluoromethylsulfonyl)imide and sodium trifluoromethanesulfonate.

[0008] Preferably, the electrolyte further comprises a hydrophobic substance.

[0009] Preferably, the hydrophobic substance includes tetraethylammonium trifluoromethanesulfonate.

[0010] Preferably, the mass concentration of the hydrophobic substance in the electrolyte is 1 - 15%.

[0011] Preferably, the electrolyte further comprises an insoluble sodium salt.

[0012] Preferably, the insoluble sodium salt includes sodium bis(oxalato)borate.

[0013] Preferably, the concentration of the insoluble sodium salt in the electrolyte is 3 to 10 mmol / L.

[0014] The present invention also provides a sodium ion battery, comprising a positive electrode, a negative electrode, and the electrolyte described in the above technical solution.

[0015] The present invention provides an electrolyte, comprising a soluble sodium salt, water, and N,N-dimethylformamide; the volume ratio of water to N,N-dimethylformamide is 1:(3 to 9); the concentration of the soluble sodium salt in the electrolyte is 1 to 10 mol / kg. Water in the electrolyte provided by the present invention serves as a solvent. Adding the soluble sodium salt can inhibit the activity of water and broaden the electrochemical stability window of water. At the same time, N,N-dimethylformamide is added and its volume ratio to water is defined. N,N-dimethylformamide can reduce the hydrogen evolution and oxygen evolution reactions of the electrolyte, broaden the electrochemical window, and improve the performance of the electrolyte when the concentration of the soluble sodium salt is relatively low, thereby improving the performance of the sodium ion battery assembled with the electrolyte. Description of the Drawings

[0016] Figure 1 Linear voltammogram curves of the electrolytes of Example 1, Example 2, and Comparative Example 1;

[0017] Figure 2 Coulombic efficiency diagrams of the full cells (NVP / NTP) assembled with the electrolytes of Example 1, Example 2, and Comparative Example 1;

[0018] Figure 3 Cycling performance diagrams of the full cells (NVP / NVP) assembled with the electrolytes of Example 1, Example 3 to 5;

[0019] Figure 4 Cycling performance diagrams of the full cells (NVP / NVP) assembled with the electrolytes of Example 3, Example 6, and Example 7;

[0020] Figure 5 Raman spectra diagrams of the electrolytes of Example 1, Example 3, and Comparative Example 2. Detailed Embodiments

[0021] The present invention provides an electrolyte, comprising a soluble sodium salt, water, and N,N-dimethylformamide.

[0022] Unless otherwise specified, the present invention has no special limitation on the sources of the various components, and commercially available products well-known to those skilled in the art can be used.

[0023] In the present invention, the electrolyte comprises a soluble sodium salt.

[0024] In the present invention, the soluble sodium salt preferably includes one or more of sodium perchlorate, sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), and sodium trifluoromethanesulfonate.

[0025] In the present invention, the concentration of the soluble sodium salt in the electrolyte is 1 to 10 mol / kg, preferably 4 to 5 mol / kg. By limiting the type and concentration of the soluble sodium salt within the above ranges, the present invention can better inhibit the activity of water, thereby broadening the electrochemical stability window of water. At the same time, its concentration is not too high, which can reduce the cost of the electrolyte.

[0026] In the present invention, the electrolyte further includes water. In the present invention, the water serves as a solvent.

[0027] In the present invention, the electrolyte further includes N,N-dimethylformamide (DMF). In the present invention, the DMF can reduce the hydrogen evolution and oxygen evolution reactions of the electrolyte and broaden the electrochemical window.

[0028] In the present invention, the volume ratio of water to N,N-dimethylformamide is 1:(3 to 9), preferably 1:(4 to 8), and more preferably 1:6. By limiting the volume ratio of water to N,N-dimethylformamide within the above ranges, the present invention can further broaden the electrochemical window of the electrolyte while ensuring the safety performance of the electrolyte.

[0029] In the present invention, the electrolyte preferably further includes a hydrophobic substance; the hydrophobic substance preferably includes tetraethylammonium trifluoromethanesulfonate (TEAOTF). In the present invention, adding a hydrophobic substance can increase a hydrophobic cation layer on the electrode surface, thereby reducing the density of water molecules at the electrolysis interface, inhibiting the hydrogen evolution and oxygen evolution reactions, and improving the electrochemical performance of the electrolyte.

[0030] In the present invention, the mass concentration of the hydrophobic substance in the electrolyte is preferably 1 to 15%, more preferably 2 to 10%, and further preferably 2 to 5%. By limiting the concentration of the hydrophobic substance within the above ranges, the present invention can further improve the electrochemical performance of the electrolyte.

[0031] In the present invention, the electrolyte preferably further includes an insoluble sodium salt; the insoluble sodium salt preferably includes sodium bis(oxalato)borate (NaBOB). In the present invention, the insoluble sodium salt undergoes a redox reaction during charge and discharge, changing the composition of the SEI layer on the battery electrode surface and optimizing the SEI layer, thereby improving the electrochemical performance.

[0032] In the present invention, the concentration of the insoluble sodium salt in the electrolyte is preferably 3 to 10 mmol / L, more preferably 3 to 5 mmol / L. By limiting the concentration of the insoluble sodium salt in the electrolyte within the above ranges, the present invention can further improve the electrochemical performance of the electrolyte.

[0033] In the electrolyte provided by the present invention, water serves as a solvent. Adding soluble sodium salts can inhibit the activity of water. Meanwhile, N, N-dimethylformamide is added to reduce the hydrogen evolution and oxygen evolution reactions of the electrolyte, broaden the electrochemical window, add hydrophobic substances to improve the interfacial side reactions, and add insoluble sodium salts to optimize the components of the SEI layer. When the concentration of the soluble sodium salt is low, the performance of the electrolyte is improved, thereby improving the performance of the sodium-ion battery assembled with the electrolyte, and at the same time reducing the cost of the electrolyte.

[0034] In the present invention, preferably, an inert gas is introduced into the electrolyte for 2 h before use to remove the dissolved oxygen therein. In the present invention, the inert gas is preferably nitrogen. The present invention has no special limitation on the method and dosage of introducing the inert gas, as long as the dissolved oxygen in the electrolyte can be sufficiently removed.

[0035] In the present invention, the preparation method of the electrolyte is preferably: mixing water and N, N-dimethylformamide, and then adding soluble sodium salts to obtain the electrolyte.

[0036] In the present invention, when the electrolyte further includes hydrophobic substances and insoluble sodium salts, the hydrophobic substances and insoluble sodium salts are preferably added sequentially after adding the soluble sodium salts. By adopting the preparation method of the present invention, the mixing of each component can be made more sufficient.

[0037] The present invention also provides a sodium-ion battery, including a positive electrode, a negative electrode, and the electrolyte described in the above technical solution.

[0038] In the present invention, the active substance in the positive electrode is preferably sodium vanadium phosphate NVP; the active substance in the negative electrode is preferably sodium vanadium phosphate NVP or sodium titanium phosphate NTP.

[0039] In the present invention, the preparation method of the battery electrode sheet is preferably: mixing the active substance, conductive carbon black, and polyvinylidene fluoride PVDF in a mass ratio of 7:2:1, adding N-methylpyrrolidone, grinding and then ultrasonicating for 30 min to obtain a slurry; coating the slurry on a current collector, placing it in a blast drying oven, drying at 60 °C for 6 h, and then placing it in a vacuum drying oven, drying at 90 °C for 12 h; stamping the dried current collector into an electrode sheet with a diameter of 16 mm, and the loading amount is 1.5 ± 0.5 mg.

[0040] The present invention has no special limitation on the dosage of N-methylpyrrolidone, the grinding method and time, and the coating method, and the technical solutions well-known to those skilled in the art can be adopted.

[0041] The sodium-ion battery provided by the present invention has excellent electrochemical performance.

[0042] The technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0043] Example 1

[0044] An electrolyte is composed of sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), water, and N,N-dimethylformamide (DMF); the concentration of NaTFSI in the electrolyte is 4.5 mol / kg, and the volume ratio of water to DMF in the electrolyte is 1:6.

[0045] The preparation method of the electrolyte is as follows: Mix water and DMF to obtain a mixed solution, and then add NaTFSI and mix evenly to obtain the electrolyte.

[0046] Example 2

[0047] Replace the volume ratio of water to DMF in Example 1 with 1:4, and other parameters and preparation methods are the same as those in Example 1.

[0048] Example 3

[0049] An electrolyte is composed of sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), water, N,N-dimethylformamide (DMF), and tetraethylammonium trifluoromethanesulfonate (TEAOTF); the concentration of NaTFSI in the electrolyte is 4.5 mol / kg, the volume ratio of water to DMF in the electrolyte is 1:6, and the mass concentration of TEAOTF in the electrolyte is 2%.

[0050] The preparation method of the electrolyte is as follows: Mix water and DMF to obtain a mixed solution, then add NaTFSI and mix evenly, and then add TEAOTF and mix evenly to obtain the electrolyte.

[0051] Example 4

[0052] Replace the mass concentration of TEAOTF in Example 3 with 4%, and other parameters are the same as those in Example 3.

[0053] Example 5

[0054] Replace the mass concentration of TEAOTF in Example 3 with 6%, and other parameters are the same as those in Example 3.

[0055] Example 6

[0056] An electrolyte is composed of sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), water, N,N-dimethylformamide (DMF), tetraethylammonium trifluoromethanesulfonate (TEAOTF), and sodium bis(oxalato)borate (NaBOB); the concentration of NaTFSI in the electrolyte is 4.5 mol / kg, the volume ratio of water to DMF in the electrolyte is 1:6, the mass concentration of TEAOTF in the electrolyte is 2%, and the concentration of NaBOB in the electrolyte is 5 mmol / L;

[0057] The preparation method of the electrolyte is as follows: Mix water and DMF to obtain a mixed solution, then add NaTFSI and mix evenly, then add TEAOTF and mix evenly, and finally add NaBOB and mix evenly to obtain the electrolyte.

[0058] Example 7

[0059] Replace the concentration of NaBOB in Example 6 with 3 mmol / L, and keep other parameters the same as those in Example 6.

[0060] Comparative Example 1

[0061] Replace the volume ratio of water to DMF in Example 1 with 1:2, and keep other parameters the same as those in Example 1.

[0062] Comparative Example 2

[0063] Replace the mass concentration of TEAOTF in Example 4 with 16%, and keep other parameters the same as those in Example 4.

[0064] Using a titanium foil as the working electrode, an Ag / AgCl electrode as the reference electrode, and a platinum electrode as the auxiliary electrode, a three-electrode system is formed, and the scanning rate is 0.1 mV s -1 , and the linear voltammetry curves of the electrolytes of Example 1, Example 2, and Comparative Example 1 are tested. The electrolytes are purged with nitrogen for 2 h before testing to remove the dissolved oxygen therein. The results are as Figure 1 shown. It can be seen from Figure 1 that when the volume ratio of H2O:DMF is 1:2, the electrochemical window is 2.9 V; when the volume ratio of H2O:DMF is 1:4, the electrochemical window is 3 V; when the volume ratio of H2O:DMF is 1:6, the electrochemical window is 3.4 V. When the volume ratio of H2O:DMF increases to 1:6, the electrochemical window expands significantly.

[0065] Mix NVP, conductive carbon black, and PVDF in a mass ratio of 7:2:1, add NMP, grind, and then ultrasonicate for 30 min to obtain a slurry. Coat the slurry on a current collector, then place it in a forced-air drying oven and dry at 60 °C for 6 h, and then place it in a vacuum drying oven and dry at 90 °C for 12 h. Finally, punch the current collector into a pole piece with a diameter of 16 mm and a loading of 1.5 mg to obtain a positive pole piece; the active material in the negative pole piece is NTP, and the preparation method is the same as that of the positive pole piece; assemble the positive pole piece, negative pole piece, and electrolyte (Examples 1-2, Comparative Example 1) into a full cell, place it in a constant-temperature oven at 25 °C, and perform charge-discharge tests with a voltage of 0-2 V and a current density of 1C (1C≈117 mAh g -1 ).

[0066] The Coulombic efficiencies of the full cells assembled with the electrolytes in Example 1, Example 2, and Comparative Example 1 are as Figure 2 shown. From Figure 2 it can be seen that when the volume ratio of H2O:DMF is 1:2, the average Coulombic efficiency is 97%, when the volume ratio of H2O:DMF is 1:4, the average Coulombic efficiency is 99.5%, and when the volume ratio of H2O:DMF is 1:6, the average Coulombic efficiency is the highest, at 99.9%. Considering Figure 1 and Figure 2 together, it can be seen that when the volume ratio of H2O:DMF is 1:6, the electrochemical performance of the full cell assembled with the electrolyte is the best.

[0067] Mix NVP, conductive carbon black, and PVDF in a mass ratio of 7:2:1, add NMP, grind, and then ultrasonicate for 30 min to obtain a slurry. Coat the slurry on a current collector, then place it in a forced-air drying oven and dry at 60 °C for 6 h, and then place it in a vacuum drying oven and dry at 90 °C for 12 h. Finally, punch the current collector into a pole piece with a diameter of 16 mm and a loading of 1.5 mg to obtain a positive pole piece; the active material in the negative pole piece is also NVP, and the preparation method is the same as that of the positive pole piece; assemble the positive pole piece, negative pole piece, and electrolyte (Examples 1-7) into a full cell, place it in a constant-temperature oven at 25 °C, and perform charge-discharge tests with a voltage of 0-2 V and a current density of 1C (1C≈117 mAh g -1 ).

[0068] The cycle performance of the full cells assembled with the electrolytes in Example 1, Examples 3-5 is as Figure 3 shown. From Figure 3 it can be seen that the capacity retention rate of the cell assembled with the electrolyte without adding TEAOTF decreases significantly after 100 cycles. After activation, the specific capacity of the cell after 100 cycles decreases from 66.94 mAh g -1 to 33.87 mAh g -1, the capacity retention rate was 50.5%. When 2% TEAOTF was added, the cycle stability of the battery was significantly improved, and its specific capacity decreased from 60.7 mAh g -1 to 57.8 mAh g -1 after 100 cycles, and the capacity retention rate was 95.2%. When 4% TEAOTF was added, the specific capacity after 100 cycles was 57.3 mAh g -1 and the capacity retention rate was 91.3%. When 6% TEAOTF was added, the specific capacity after 100 cycles was 56.1 mAh g -1 and the capacity retention rate was 91.3%.

[0069] The cycle performance of the full batteries assembled with the electrolytes in Examples 3, 6, and 7 is as shown in Figure 4 . It can be seen from Figure 4 that the Coulomb efficiency of the battery assembled with the electrolyte containing 3 mmol / L NaBOB increased to 99.8% after 100 cycles. By comparing different addition amounts, the optimal addition amount of NaBOB was determined to be 3 mmol / L. Considering comprehensively Figure 3 and Figure 4 , based on its electrochemical performance and cost reduction, 2 wt% TEAOTF is the optimal addition amount.

[0070] The Raman spectra of the electrolytes in Example 1, Example 3, and Comparative Example 2 are as shown in Figure 5 . It can be seen from Figure 5 that after adding 2 wt% TEAOTF, the S-N-S bending vibration mode of the electrolyte hardly changed. However, when the TEAOTF increased to 16 wt%, the S-N-S bending vibration mode showed a weak red shift. These results indicate that adding 2 wt% TEAOTF does not change the chemical environment of the anion, but as the TEAOTF increases from 2 wt% to 16 wt%, the interaction between the cation and the anion begins to weaken. This may be because OTF - shows a stronger interaction with Na + than TFSI - .

[0071] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can still be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. An electrolyte solution, comprising soluble sodium salt, water and N,N-dimethylformamide; the volume ratio of the water to the N,N-dimethylformamide is 1:(3-9); the concentration of the soluble sodium salt in the electrolyte solution is 1-10 mol / kg; The electrolyte solution further comprises a hydrophobic substance; the hydrophobic substance is tetraethylammonium trifluoromethanesulfonate; the mass concentration of the hydrophobic substance in the electrolyte solution is 1-15%; The electrolyte solution further comprises an insoluble sodium salt; the insoluble sodium salt is sodium bis(oxalato)borate; the concentration of the insoluble sodium salt in the electrolyte solution is 3-10 mmol / L; The soluble sodium salt includes one or more of sodium perchlorate, sodium bis(trifluoromethylsulfonyl)imide and sodium trifluoromethanesulfonate.

2. A sodium-ion battery, comprising a positive electrode, a negative electrode and the electrolyte solution according to claim 1.

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