An aqueous electrolyte and its application in sodium-ion batteries

Abstract

The present invention relates to the field of electrochemical energy storage technologies, and particularly relates to an aqueous electrolyte based on dinitrile compounds and its application in sodium-ion batteries. The aqueous electrolyte comprises dinitrile compounds, water and sodium salts. By adding dinitrile compounds, the hydrogen bonds between water molecules are weakened and the O-H covalent bonds of water molecules are strengthened, and the hydrogen evolution potential and oxygen evolution potential of this aqueous electrolyte are significantly broadened relative to the corresponding saturated high-concentration brine solution. The sodium-ion battery composed of this aqueous electrolyte is both safe and capable of stable long-term cycling operation, while also taking into account excellent rate performance and energy density.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrochemical energy storage, and particularly relates to an aqueous electrolyte of dinitrile co-solvent and its application in secondary sodium ion batteries. Background Art

[0002] Advanced energy storage technologies are the internal driving force of modern life. Batteries have achieved great success in this field due to their low cost and convenience. Since the first battery was invented by Count Volta in the 19th century, a series of novel electrochemical energy storage devices such as lead-acid, nickel-cadmium, nickel-metal hydride and lithium-ion batteries have achieved great success in the commercial market. The wide application of these electrical energy storage systems, especially lithium-ion batteries, in cameras, mobile phones, laptop computers and electric vehicles has completely changed the communication and transportation modes of modern life. Although the sales of lithium-ion batteries have increased significantly, at present, large-scale energy storage of commercial batteries is still difficult to achieve worldwide. This is mainly due to the shortage of lithium resources. Due to the rich reserves of sodium resources, sodium ion batteries have attracted more and more attention in recent years and are considered as a beneficial supplement to lithium-ion batteries in the field of large-scale energy storage. In particular, aqueous sodium ion batteries can take into account the advantages of high safety and low cost and have great application value in large-scale energy storage. However, there is still a lack of aqueous electrolytes with excellent comprehensive performance to ensure the efficient and long-life operation of aqueous sodium ion batteries.

[0003] Free water exists in traditional aqueous electrolytes. Due to the narrow electrochemical stability window of water (only 1.23 V theoretically), the selection of electrode materials for aqueous batteries is limited. In 2017, researchers introduced the "water-in-salt" electrolyte (WiSE) concept into aqueous sodium-ion batteries by using 9.2 mol / kg sodium trifluoromethanesulfonate (Science, 2015, 350, 6263, 938; Adv. Energy Mater., 2017, 7, 1701189). This electrolyte broadens the electrochemical window of aqueous sodium-ion battery electrolytes to 2.5 V by converting free water into bound water and forming a passivation layer (SEI) on the negative electrode. However, the transformation from free water to bound water mainly inhibits the oxygen evolution activity of water, and due to the limited solubility of sodium salts, there is still a small amount of free water in the high-concentration salt, resulting in the inability of the SEI on the negative electrode to exist stably for a long time and the hydrogen evolution reaction cannot be avoided. The concept of molecular crowding proposed in 2020 effectively inhibits the hydrogen evolution reaction activity of water by adding a large amount of polyethylene glycol as a crowding agent, but still cannot simultaneously improve the oxygen evolution potential (Nat. Mater., 2020, 19, 9, 1006-1011), making some high-performance positive electrode materials for sodium-ion batteries unable to be applied. Obviously, there is currently no strategy that can simultaneously inhibit the hydrogen evolution and oxygen evolution activities. SUMMARY OF THE INVENTION

[0004] Aiming at the deficiencies of the above-mentioned existing technologies, the purpose of the present invention is to provide an aqueous electrolyte based on a dinitrile co-solvent and a sodium-ion battery that can simultaneously inhibit the hydrogen evolution and oxygen evolution activities of water.

[0005] To achieve the above purpose, the present invention provides an aqueous electrolyte on the one hand, which comprises a dinitrile compound, water and a sodium salt. The dinitrile compound is one or more of succinonitrile, glutarodinitrile, adiponitrile, pimelonitrile, suberonitrile, azelaonitrile and sebaconitrile. The sodium salt is not limited and is a commonly used sodium salt for sodium-ion batteries. Preferably, the sodium salt is one or more of sodium perchlorate, sodium perchlorate monohydrate, sodium bis(trifluoromethanesulfonyl)imide, sodium sulfate, sodium trifluoromethanesulfonate, sodium fluoroacetate and sodium acetate.

[0006] The present invention adds a dinitrile compound with an oxygen atom whose Lewis basicity is weaker than that of water and has a strong coordination effect with metal Na + ions to water. The dinitrile compound and water jointly participate in Na +Coordination of ions. Dinitrile compounds form weak hydrogen bonds with water, replacing the strong hydrogen bonds between water molecules. There is a competitive relationship between the hydrogen bonds of water and the covalent O-H bonds. When the hydrogen bonds become weaker, the O-H bonds become stronger. The addition of dinitrile compounds weakens the hydrogen bonds of water and enhances the strength of the O-H bonds. The enhancement of the O-H bonds inhibits the activity of water, achieving the purpose of simultaneously inhibiting the hydrogen evolution reaction and oxygen evolution reaction of water.

[0007] To achieve the above effects, the molar ratio of the water, dinitrile compounds, and sodium salt should not only dissolve the sodium salt but also enable both the water and dinitrile compounds to participate in the coordination of sodium ions, with no free solvent present. Water is a monodentate ligand, dinitrile compounds are bidentate ligands, and sodium ions are hexacoordinated. That is, the ratio of the sum of the amount of water and twice the amount of dinitrile compounds to the amount of sodium ions should be less than or equal to 6:1, which can ensure the absence of free solvent and improve the stability of the aqueous electrolyte.

[0008] Under the condition that the molar ratio of water, dinitrile compounds, and sodium salt not only dissolves the sodium salt but also enables both the water and dinitrile compounds to participate in the coordination of sodium ions, with no free solvent present, the molar ratio of the sodium salt, water, and dinitrile compounds is 1:(0.2 - 3):(0.1 - 2.9). Preferably, the molar ratio of the sodium salt, water, and dinitrile compounds is 1:(0.2 - 2):(1 - 2.9). More preferably, the molar ratio of the sodium salt, water, and dinitrile compounds is 1:(0.2 - 2):(0.8 - 1.5). The hydrogen evolution potential and oxygen evolution potential of this aqueous electrolyte are significantly broadened relative to the corresponding saturated high-concentration brine solution.

[0009] In another aspect of the present invention, the present invention provides a sodium-ion battery, including a positive electrode sheet, a negative electrode sheet, a separator, and the above-mentioned aqueous electrolyte.

[0010] The positive electrode sheet includes a positive electrode current collector and a positive electrode film. The positive electrode film includes a positive electrode active material, a conductive agent, and a binder. Calculated by mass fraction, the positive electrode active material accounts for 30% - 95% of the positive electrode film, the conductive agent accounts for 3% - 50% of the positive electrode film, and the binder accounts for 1% - 30% of the positive electrode film. The positive electrode active material is one of sodium vanadium phosphate, fluorinated sodium vanadium phosphate, Prussian blue analogs, V2O5, and Mo6S8.

[0011] The negative electrode sheet includes a negative electrode current collector and a negative electrode film. The negative electrode film includes a negative electrode active material, a conductive agent, and a binder. Calculated by mass fraction, the negative electrode active material accounts for 30% - 95% of the negative electrode film, the conductive agent accounts for 3% - 50% of the negative electrode film, and the binder accounts for 1% - 30% of the negative electrode film. The negative electrode active material is sodium titanium phosphate or sodium vanadium phosphate.

[0012] There are no restrictions on the positive electrode, negative electrode current collectors, and separator, which are common materials for sodium-ion batteries.

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] 1. Due to the special weak hydrogen bond interaction between dinitrile compounds and water, the O-H covalent bond strength of water molecules can be effectively enhanced, while the hydrogen evolution reaction and oxygen evolution reaction of water are inhibited, the oxidation and reduction stability of the aqueous electrolyte is improved, the electrochemical stability window is broadened, and the cycle stability of the battery is enhanced. The electrochemical stability window of the aqueous electrolyte even exceeds that of the corresponding saturated sodium salt solution.

[0015] 2. Since there is no free solvent in the aqueous electrolyte, the dissolution problems of the positive and negative electrode materials of the sodium-ion battery during electrochemical cycling can be well inhibited, the cycle stability of the battery is improved, and the range of positive electrode materials for aqueous sodium-ion batteries is broadened.

[0016] 3. The aqueous electrolyte contains a certain amount of water molecules, which can improve the safety of the battery.

[0017] 4. The obtained aqueous sodium-ion battery is both safe and capable of long-term stable cycling operation, while also taking into account excellent rate performance and energy density. Description of the Drawings

[0018] Figure 1 is the electrochemical stability window of the aqueous electrolyte of Example 1 measured by linear sweep voltammetry in a three-electrode system.

[0019] Figure 2 is the cycling performance of the Na3V2(PO4)3 / / NaTi2(PO4)3 full battery assembled with the aqueous electrolyte of Example 1 at a current density of 100 mA / g (calculated based on the mass of the Na3V2(PO4)3 active material).

[0020] Figure 3 is the electrochemical stability window of the aqueous electrolyte of Example 2 measured by linear sweep voltammetry in a three-electrode system.

[0021] Figure 4 is the cycling performance of the Na3V2(PO4)3 / / NaTi2(PO4)3 full battery assembled with the aqueous electrolyte of Example 2 at a current density of 100 mA / g (calculated based on the mass of the Na3V2(PO4)3 active material).

[0022] Figure 5 is the electrochemical stability window of the aqueous electrolyte of Example 3 measured by linear sweep voltammetry in a three-electrode system.

[0023] Figure 6Cycling performance of the Na3V2(PO4)3 / / Na3V2(PO4)3 full cell assembled with the aqueous electrolyte in Example 3 at 5 C (calculated based on the mass of the active material of the negative electrode Na3V2(PO4)3, 1C = 58.5 mA / g).

[0024] Figure 7 Electrochemical stability window of the aqueous electrolyte of Comparative Example 1 measured by linear sweep voltammetry in a three-electrode system.

[0025] Figure 8 Electrochemical stability window of the aqueous electrolyte of Comparative Example 2 measured by linear sweep voltammetry in a three-electrode system.

[0026] Figure 9 Cycling performance of the Na3V2(PO4)3 / / NaTi2(PO4)3 full cell assembled with the aqueous electrolyte of Comparative Example 2 at a current density of 100 mA / g (calculated based on the mass of the active material of Na3V2(PO4)3).

[0027] Figure 10 Cycling performance of the Na3V2(PO4)3 / / Na3V2(PO4)3 full cell assembled with the aqueous electrolyte of Comparative Example 3 at 5 C (calculated based on the mass of the active material of the negative electrode Na3V2(PO4)3, 1C = 58.5 mA / g). Detailed implementation mode

[0028] Example 1

[0029] Sodium bis(trifluoromethanesulfonyl)imide, water, and adiponitrile were mixed at a molar ratio of 1:1:2, and a clear and transparent electrolyte solution was prepared after heating. A platinum sheet and an aluminum sheet were used as the working electrodes for measuring the oxidation stability window and the reduction stability window, respectively. A platinum sheet was used as the counter electrode, and Ag / AgCl was used as the reference electrode. The electrochemical stability window of this aqueous electrolyte was measured by linear sweep voltammetry at a scanning rate of 1 mV / s, and the electrochemical stability window of this aqueous electrolyte was obtained as 3.2 V (1.2 - 4.4 V) (see Figure 1 ). Subsequently, a battery was assembled in the order of sodium titanium phosphate negative electrode, glass fiber separator, electrolyte, and sodium vanadium phosphate positive electrode, and the battery was stamped into a whole by a stamping machine to make the sodium titanium phosphate negative electrode, electrolyte, and sodium intercalated positive electrode in close contact, thus assembling a sodium ion battery.

[0030] The battery was charged and discharged at a current density of 100 mA / g (based on the mass of the active material of the positive electrode), and the cycling performance of the battery was obtained. After 50 cycles, the capacity retention rate of the battery was 77.8%, and the Coulomb efficiency could reach 98.5% (see Figure 2 ).

[0031] Example 2

[0032] Mix water, sodium perchlorate, and succinonitrile in a molar ratio of 1:1:2.5, and prepare a clear and transparent solution after heating. Use a platinum sheet and an aluminum sheet as the working electrodes for measuring the oxidation stability window and the reduction stability window respectively, a platinum sheet as the counter electrode, and Ag / AgCl as the reference electrode. Measure the electrochemical stability window of this electrolyte using linear sweep voltammetry at a scanning rate of 1 mV / s, and obtain the electrochemical stability window of the electrolyte as 3.2 V (1.4 - 4.6 V) (see Figure 3 ).

[0033] Subsequently, assemble a battery in the order of sodium titanium phosphate negative electrode, glass fiber separator, electrolyte, and sodium vanadium phosphate positive electrode, and use a stamping machine to stamp the battery into a whole, so that the sodium titanium phosphate negative electrode, the electrolyte, and the sodium intercalated positive electrode are in close contact to assemble a sodium ion battery.

[0034] The battery is charged and discharged at a current density of 100 mA / g (based on the mass of the active material of the positive electrode) to obtain the cycling performance of the battery. After 50 cycles, the capacity retention rate of the battery is 90%, and the Coulomb efficiency can reach 99.5% (see Figure 4 ).

[0035] Example 3

[0036] Mix sodium trifluoroacetate, water, and adiponitrile in a molar ratio of 1:0.2:1.5, and prepare a clear and transparent solution after heating. Use a platinum sheet and an aluminum sheet as the working electrodes for measuring the oxidation stability window and the reduction stability window respectively, a platinum sheet as the counter electrode, and Ag / AgCl as the reference electrode. Measure the electrochemical stability window of this electrolyte using linear sweep voltammetry at a scanning rate of 1 mV / s, and obtain the electrochemical stability window of the electrolyte as 3.2 V (1.2 - 4.4 V) (see Figure 5 ).

[0037] Subsequently, assemble a battery in the order of sodium vanadium phosphate negative electrode sheet, glass fiber separator, electrolyte, and sodium vanadium phosphate positive electrode sheet, and use a stamping machine to stamp the battery into a whole, so that the sodium vanadium phosphate negative electrode sheet, the electrolyte, and the sodium vanadium phosphate positive electrode sheet are in close contact.

[0038] The battery is charged and discharged at a current density of 5 C (1 C = 58.5 mA / g) (based on the mass of the active material of the negative electrode) to obtain the cycling performance of the battery. After 50 cycles, the capacity retention rate of the battery is as high as 100%, and the Coulomb efficiency can reach 99.1% (see Figure 6 ).

[0039] Comparative Example 1

[0040] An aqueous solution of a saturated solution of sodium perchlorate (17 mol / L), using a platinum sheet and an aluminum sheet as working electrodes for measuring the oxidation stability window and the reduction stability window respectively, a platinum sheet as the counter electrode, and Ag / AgCl as the reference electrode, at a scanning rate of 1 mV / s, using linear sweep voltammetry to measure the electrochemical stability window of this electrolyte, and the electrochemical stability window of the electrolyte is 2.3 V (2.0 - 4.3 V) (see Figure 7 ).

[0041] Comparative Example 2

[0042] A saturated aqueous solution of sodium bis(trifluoromethanesulfonyl)imide (9 mol / L), using a platinum sheet and an aluminum sheet as working electrodes for measuring the oxidation stability window and the reduction stability window respectively, and the electrochemical stability window of the electrolyte is 2.6 V (1.4 - 4.0 V) (see Figure 8 ).

[0043] Assemble a full cell with sodium vanadium phosphate as the positive electrode and sodium titanium phosphate as the negative electrode. After 40 cycles, the capacity retention rate of the battery is only 55%, and the Coulomb efficiency is 97% (see Figure 9 ).

[0044] Comparative Example 3

[0045] Assemble a full cell with sodium vanadium phosphate as the positive and negative electrodes and a saturated aqueous solution of sodium trifluoroacetate (26 mol / L) as the electrolyte. Charge and discharge at a current density of 5 C (1 C = 58.5 mA / g) (based on the mass of the active material of the negative electrode), and the obtained cycling performance is that after 50 cycles, the capacity retention rate of the battery is only 58.5%, and the Coulomb efficiency is 90% (see Figure 10 ).

[0046] From the above-mentioned examples and comparative examples, it can be seen that compared with the comparative examples of the corresponding saturated aqueous solutions, the oxidation and reduction stabilities of the aqueous electrolyte of the present invention are improved, the electrochemical stability window is broadened, and the cycling performance of the sodium-ion battery is significantly improved.

[0047] 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 inventive concept of the present invention, several improvements and transformations can still be made, and these all belong to the protection scope of the present invention.

Claims

1. An aqueous electrolyte, characterized in that, The aqueous electrolyte comprises a dinitrile compound, water and a sodium salt. The molar ratio of water and the dinitrile compound to the sodium salt should not only dissolve the sodium salt, but also enable both water and the dinitrile compound to participate in the coordination of sodium ions, with no free solvent present. The ratio of the sum of the amount of water and twice the amount of the dinitrile compound to the amount of sodium ions should be less than or equal to 6:

1.

2. The aqueous electrolyte according to claim 1, wherein Under the condition that the molar ratio of water and the dinitrile compound to the sodium salt not only dissolves the sodium salt, but also enables both water and the dinitrile compound to participate in the coordination of sodium ions, with no free solvent present, the molar ratio of the sodium salt, water and the dinitrile compound is 1:(0.2 - 3):(0.1 - 2.9).

3. Aqueous electrolyte according to claim 1, characterized in that, The dinitrile compound is one or more of succinonitrile, glutaronitrile, adiponitrile, pimelonitrile, suberonitrile, azelaonitrile and sebaconitrile.

4. A water-based electrolyte according to claim 1, characterized in that, The sodium salt is one or more of sodium perchlorate, sodium perchlorate monohydrate, sodium bis(trifluoromethylsulfonyl)imide, sodium sulfate, sodium trifluoromethanesulfonate, sodium fluoroacetate and sodium acetate.

5. A sodium-ion battery, characterized in that, It includes a positive electrode sheet, a negative electrode sheet, a separator and the aqueous electrolyte according to any one of claims 1 - 4.

6. A sodium-ion battery according to claim 5, characterized in that, The positive electrode sheet comprises a current collector and a positive electrode film. The positive electrode film comprises a positive electrode active material, a conductive agent and a binder. By mass fraction, the positive electrode active material accounts for 30% - 95% of the positive electrode film, the conductive agent accounts for 3% - 50% of the positive electrode film, and the binder accounts for 1% - 30% of the positive electrode film.

7. A sodium-ion battery according to claim 6, characterized in that, The positive electrode active material is one of sodium vanadium phosphate, sodium fluorovanadium phosphate, Prussian blue analogues, V2O5, Mo6S8.

8. A sodium ion battery according to claim 5, characterized in that, The negative electrode sheet comprises a current collector and a negative electrode film. The negative electrode film comprises a negative electrode active material, a conductive agent and a binder. By mass fraction, the negative electrode active material accounts for 30% - 95% of the negative electrode film, the conductive agent accounts for 3% - 50% of the negative electrode film, and the binder accounts for 1% - 30% of the negative electrode film.

9. A sodium ion battery according to claim 8, characterized in that, The negative electrode active material is sodium titanium phosphate or sodium vanadium phosphate.

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