Sodium-ion battery and nitrile-based electrolyte thereof

Abstract

The invention relates to a sodium ion battery and a nitrile-based electrolyte thereof. The electrolyte comprises: a solvent formed by fluoroethylene carbonate and at least one selected from the group consisting of dinitrile, 3-methoxypropionitrile, isobutyronitrile, valeronitrile, ethylene glycol bis (propionitrile) ether and 4-cyano tetrahydropyran, wherein the volume fraction of fluoroethylene carbonate is 5-20%; and at least one sodium salt selected from the group consisting of sodium hexafluorophosphate, sodium trifluoromethanesulfonate, bis (trifluoromethylsulfonyl) imide sodium and bis (fluorosulfonyl) imide sodium, with a concentration of 2.0-4.0 mol L <-1 >. The electrolyte provided by the invention can simultaneously stabilize an interface between a positive electrode and a negative electrode, and has reasonable ionic conductivity and high electrochemical stability.

Description

Technical Field

[0001] The present invention generally relates to sodium-ion batteries, and more particularly to their electrolyte compositions. Background Art

[0002] Since sodium has a larger ionic radius and a heavier atomic mass than lithium, sodium-ion batteries have lower mass and volume energy densities compared to lithium-ion batteries. The above characteristics make sodium-ion batteries more suitable for fields such as large-scale energy storage where the requirement for energy density is not high, but the requirements for battery cost and resource abundance are relatively high. To reduce the application cost and increase the service life of the battery, it is crucial to develop sodium-ion batteries with excellent long-cycle performance. However, commonly used ester-based electrolytes and ether-based electrolytes have a relatively low electrochemical stability window and are prone to continuous oxidative decomposition at high voltages, resulting in the inability to stably form an electrode-electrolyte interface film and continuous decomposition of the electrolyte, thereby leading to a rapid decay of capacity. In addition, transition metals in commonly used layered cathode materials also have high activity at high voltages and are prone to the dissolution of transition metals (such as nickel and manganese) and cation mixing. The dissolved transition metals not only reduce the capacity but also further catalyze the decomposition of the electrolyte, all of which lead to a decrease in the battery cycle life.

[0003] CN117832629A, CN118472393A, CN118993960A, CN119153782A, etc. disclose that nitrile compounds can be used as electrolyte additives or co-solvents. However, due to the strong reactivity of nitrile with the negative electrode, it is prone to reduction polymerization at low voltages, and the reaction products cannot form a stable electrode-electrolyte interface film. It is difficult to perform normal charge and discharge if used as the main solvent in sodium-ion batteries.

[0004] Therefore, there is an urgent need to develop a long-life sodium-ion electrolyte system with high oxidation stability and capable of effectively inhibiting the dissolution of transition metal cations in the positive electrode. Summary of the Invention

[0005] The object of the present invention is to provide a sodium-ion battery electrolyte with stable performance.

[0006] According to one aspect of the present invention, there is provided an electrolyte for a sodium-ion battery, comprising:

[0007] A solvent formed by fluoroethylene carbonate and at least one selected from adiponitrile, 3-methoxypropionitrile, isobutyronitrile, valeronitrile, ethylene glycol bis(propionitrile) ether, and 4-cyanotetrahydropyran, wherein the volume fraction of fluoroethylene carbonate is 5% to 20%; and

[0008] At least one sodium salt selected from sodium hexafluorophosphate (NaPF6), sodium trifluoromethanesulfonate (NaOTf), sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), and sodium bis(fluorosulfonyl)imide (NaFSI), with a concentration of 2.0 - 4.0 mol L -1 .

[0009] For the electrolyte according to the present invention, the volume fraction of fluoroethylene carbonate in the formed solvent is preferably 5% - 10%.

[0010] For the electrolyte according to the present invention, the sodium salt is preferably NaTFSI, and the concentration is preferably 3.0 - 4.0 mol L -1 .

[0011] For the electrolyte according to the present invention, the solvent is preferably formed by fluoroethylene carbonate and adiponitrile.

[0012] According to another aspect of the present invention, a sodium-ion battery is further provided, including a positive electrode and the above electrolyte, wherein the positive electrode is made of layered transition metal oxide NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2.

[0013] For the sodium-ion battery according to the present invention, its working voltage is preferably 2.0V - 4.0V, and the working current is preferably 0.13Ag -1 or so.

[0014] The sodium-ion battery according to the present invention may further include, for example, a separator made of glass fiber and a negative electrode made of sodium sheet, etc.

[0015] In the present invention, by introducing a specific (small amount) of fluoroethylene carbonate as an additive into a nitrile compound main solvent such as adiponitrile and using a high-concentration sodium salt to form a sodium-ion electrolyte, the reactivity of the nitrile-based electrolyte with the negative electrode is synergistically and effectively reduced, and the battery performance is significantly improved.

[0016] The lower lowest unoccupied molecular orbital (LUMO) of fluoroethylene carbonate in the electrolyte of the present invention enables it to preferentially reduce on the surface of the negative electrode to form a stable and strong fluorine-containing inorganic solid electrolyte interface film (SEI), thereby inhibiting the contact between the electrolyte and the negative electrode and enhancing the mechanical strength and electronic insulation of the SEI film. At the same time, in the high-concentration salt electrolyte of the present invention, the proportion of aggregates (AGG) increases while the proportion of solvent-separated ion pairs (SSIP) decreases. More solvent molecules participate in sodium ion coordination, and the free solvent molecules in the electrolyte decrease accordingly, synergistically weakening the reactivity of the electrolyte at the negative electrode; at the same time, more TFSI -Anions also enter the solvation shell and participate in the formation process of the SEI on the negative electrode surface, making the SEI film rich in inorganic substances, more robust and stable, and conducive to ion transport. Additionally, in the present invention, by using nitrile compounds such as adiponitrile as the main solvent, the strongly electronegative cyano functional group can form strong coordination with transition metals on the positive electrode surface, inhibiting the dissolution of transition metal ions, thereby preventing the contact between electrolyte molecules and transition metal ions; on the one hand, this alleviates the structural collapse of the electrode, and on the other hand, it inhibits the decomposition of the electrolyte catalyzed by transition metals. Moreover, the nitrile solvent can also participate in constructing a stable and robust nitrogen-containing inorganic cathode-electrolyte interface film (CEI), improving the interfacial ion transport rate and inhibiting side reactions of the electrolyte.

[0017] In summary, the electrolyte of the present invention can simultaneously stabilize the interfaces of both the positive and negative electrodes, and has reasonable ionic conductivity and high electrochemical stability. Compared with the commonly used ester-based electrolytes at present, it can effectively improve the long-cycle performance of sodium-ion batteries (able to stably cycle nearly 1000 times with a high capacity retention rate). Description of the Drawings

[0018] Figure 1 It is the electrochemical window of the electrolytes obtained in Example 1 and Comparative Example 1.

[0019] Figure 2 It is the cycle performance diagram of the sodium-ion batteries obtained in Example 1 and Comparative Example 1.

[0020] Figure 3 It is the cycle performance diagram of the sodium-ion batteries obtained in Example 1 and Comparative Examples 2 and 3.

[0021] Figure 4 It is the cycle performance diagram of the sodium-ion batteries obtained in Example 1 and Comparative Example 4. Detailed Embodiments

[0022] The present invention will be described in detail below with reference to examples and comparative examples. Those skilled in the art should understand that these examples and comparative examples are only for explanation and not for limiting the present invention. Unless otherwise specified, all raw materials used can be obtained commercially.

[0023] Example 1

[0024] (1) Preparation of the positive electrode sheet

[0025] The layered transition metal oxide positive electrode material NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3O2, conductive agent Super-P, and binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 7:2:1, dispersed in an organic solvent N-methylpyrrolidone (NMP), and stirred until stable and uniform to form a positive electrode slurry. The positive electrode slurry was coated on an aluminum current collector and placed in a vacuum drying oven to dry at 80 °C for 12 hours, and then roll-pressed and cut to obtain a positive electrode plate.

[0026] (2) Preparation of electrolyte

[0027] In a glove box under an argon atmosphere, 1.8188 g of NaTFSI was weighed and completely dissolved in a mixed solution of 1.90 mL of adiponitrile and 0.10 mL of fluoroethylene carbonate to make the electrolyte salt concentration 3.0 mol L -1 , and the electrolyte of the sodium-ion battery of the present invention was obtained after mixing evenly.

[0028] (3) Preparation of sodium-ion battery

[0029] A positive electrode plate with a diameter of 11 mm, uniform and complete, was selected, weighed using a precision balance with an accuracy of 0.01 mg, and the mass of the active material was calculated ((total mass - aluminum foil mass) * 0.7). In a glove box under an argon atmosphere, it was assembled with a positive electrode case, a negative electrode case, a glass fiber separator, a sodium sheet (diameter 14 mm, thickness 0.5 mm), and the above-mentioned electrolyte according to the correct operation steps, and pressed at 800 kPa for 5 s to obtain a CR2025 type button battery.

[0030] Example 2

[0031] The difference from Example 1 is only that 1.2125 g of NaTFSI was added to the electrolyte in this example, and the volumes of the other components remained unchanged, making the electrolyte salt concentration 2.0 mol L -1 .

[0032] Example 3

[0033] The difference from Example 1 is only that 2.4251 g of NaTFSI was added to the electrolyte in this example, and the volumes of the other components remained unchanged, making the electrolyte salt concentration 4.0 mol L -1 .

[0034] Example 4

[0035] The difference from Example 1 is only that 1.80 mL of adiponitrile and 0.20 mL of fluoroethylene carbonate were added to the electrolyte in this example, and the type and concentration of the sodium salt remained unchanged.

[0036] Example 5

[0037] The difference from Example 1 is only that 1.60 mL of adiponitrile and 0.40 mL of fluoroethylene carbonate are added to the electrolyte in this example, and the type and concentration of the sodium salt remain unchanged.

[0038] Example 6

[0039] The difference from Example 1 is only that the solvent in the electrolyte in this example is 3-methoxypropionitrile, and the type, concentration and volume of each component of the sodium salt remain unchanged.

[0040] Example 7

[0041] The difference from Example 1 is only that the solvent in the electrolyte in this example is 0.95 mL of adiponitrile and 0.95 mL of 3-methoxypropionitrile, and the type, concentration and volume of fluoroethylene carbonate of the sodium salt remain unchanged.

[0042] Example 8

[0043] The difference from Example 1 is only that the sodium salt in the electrolyte in this example is NaPF6, and the concentration of the sodium salt and the volume of the remaining components remain unchanged.

[0044] Example 9

[0045] The difference from Example 1 is only that the sodium salt in the electrolyte in this example is NaOTf, and the concentration of the sodium salt and the volume of the remaining components remain unchanged.

[0046] Example 10

[0047] The difference from Example 1 is only that the sodium salt in the electrolyte in this example is NaFSI, and the concentration of the sodium salt and the volume of the remaining components remain unchanged.

[0048] Comparative Example 1

[0049] Different from Example 1, the electrolyte in this example is ethylene carbonate:propylene carbonate:fluoroethylene carbonate = 47.5:47.5:5 vt%, 1 mol L -1 NaClO4 electrolyte.

[0050] Comparative Example 2

[0051] The difference from Example 1 is only that 2.0 mL of adiponitrile is added to the electrolyte in this example, fluoroethylene carbonate is not added, and the type and concentration of the sodium salt remain unchanged.

[0052] Comparative Example 3

[0053] The difference from Example 1 is only that 1.96 mL of adiponitrile and 0.04 mL of fluoroethylene carbonate are added to the electrolyte in this example, and the type and concentration of the sodium salt remain unchanged.

[0054] Comparative Example 4

[0055] The difference from Example 1 is only that 0.6063 g of NaTFSI is added to the electrolyte in this example, and the volumes of the remaining components remain unchanged, so that the concentration of the electrolyte salt is 1.0 mol L -1 .

[0056] Performance Test

[0057] The electrolytes obtained from the above Example 1 and Comparative Example 1 were subjected to anodic potential tests. Linear sweep voltammetry was used to detect the high-voltage resistance ability of the electrolytes. Under an argon atmosphere, a button cell was formed with the sample electrolyte, a stainless steel sheet, and a sodium sheet (where the stainless steel sheet was used as the working electrode, and the sodium sheet was used as the reference electrode and the counter electrode), and its electrochemical window was tested on an electrochemical workstation (CHI660E), with a test range of 1 V to 6 V and a scan rate of 5 mV s -1 , and the test results are as Figure 1 shown. It can be seen that the electrolyte provided by the present invention has a wider electrochemical window compared with the ester-based electrolytes in the comparative examples, and has a lower current under high voltage, indicating that the oxidation and decomposition reaction of the electrolyte under high voltage is effectively inhibited by using the nitrile-based solvent of the present invention.

[0058] The sodium ion batteries obtained from the above Example 1 and Comparative Example 1 were subjected to long cycle tests, with a charge-discharge cut-off voltage range of 2.0 V to 4.0 V and a current density of 0.13 A g -1 , and the test results are as Figure 2 shown. It can be seen that the electrolyte of the present invention has significantly improved long cycle performance compared with commercial ester-based electrolytes.

[0059] The sodium ion batteries obtained from the above Example 1 and Comparative Examples 2 and 3 were subjected to long cycle tests, with a charge-discharge cut-off voltage range of 2.0 V to 4.0 V and a current density of 0.13 A g -1 , and the test results are as Figure 3 shown. It can be seen that compared with Comparative Examples 2 and 3 with a lower content of fluoroethylene carbonate additive (volume fraction 0%, 2%), Example 1 has significantly better long cycle stability, with a capacity retention rate of 84.9% after 500 cycles and still 71.9% after 900 cycles. This shows that fluoroethylene carbonate has an important synergistic effect on stabilizing the nitrile-based solvent interface and improving the battery cycle performance, and can significantly improve the compatibility between the nitrile-based solvent and the negative electrode of the sodium ion battery.

[0060] The sodium ion batteries obtained from the above Example 1 and Comparative Example 4 were subjected to long cycle tests, with a charge-discharge cut-off voltage range of 2.0 V to 4.0 V and a current density of 0.13 A g -1 , and the test results are as Figure 4As shown. It can be seen that after increasing the electrolyte concentration, the long-cycle performance of the battery is significantly improved, indicating the role of the high-concentration salt electrolyte design in enhancing the battery cycle life.

[0061] The electrolytes obtained from the above Examples 1 to 7 and Comparative Examples 1 to 4 were subjected to ion conductivity tests. The resistance R between two parallel plates in the conductance electrode was measured by the alternating current impedance method, and the reciprocal of it multiplied by the electrode constant Q of the conductance electrode was used to obtain the ion conductivity of the electrolyte. In addition, the long-cycle performance of the above sodium-ion batteries was tested. The charge-discharge cut-off voltage range was 2.0 V to 4.0 V, and the current density was 0.13 Ag -1 , and the initial discharge capacity and the discharge capacity after 300 charge-discharge cycles were tested respectively. The test results are shown in Table 1.

[0062] Table 1

[0063]

[0064] As can be seen from Table 1, the electrolytes of the embodiments of the present invention still have relatively high ion conductivity. Fluoroethylene carbonate and high-concentration salts can effectively cooperate to improve the stability of the negative electrode interface, and the modified nitrile solvent can also effectively inhibit the dissolution of transition metal ions on the positive electrode side, enabling the sodium-ion battery to have excellent cycle stability.

Claims

1. An electrolyte for a sodium-ion battery, comprising: A solvent formed by fluoroethylene carbonate and at least one selected from adiponitrile, 3-methoxypropionitrile, isobutyronitrile, valeronitrile, ethylene glycol bis(propionitrile) ether, and 4-cyanotetrahydropyran, wherein the volume fraction of fluoroethylene carbonate is 5% to 20%; and At least one sodium salt selected from sodium hexafluorophosphate (NaPF6), sodium trifluoromethanesulfonate (NaOTf), sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), and sodium bis(fluorosulfonyl)imide (NaFSI), with a concentration of 2.0 to 4.0 mol L -1 .

2. The electrolyte according to claim 1, wherein the volume fraction of fluoroethylene carbonate in the formed solvent is 5% to 10%.

3. The electrolyte according to claim 1, wherein the concentration of the sodium salt is 3.0 to 4.0 mol / L -1 .

4. A sodium-ion battery, comprising a positive electrode and an electrolyte according to any one of claims 1-3, wherein the positive electrode is made of layered transition metal oxide NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2.

5. The sodium-ion battery according to claim 4 has an operating voltage of 2.0 V to 4.0 V and an operating current of 0.13 A / g -1 .

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