Electrolyte and sodium-ion battery

Abstract

The application relates to the technical field of battery materials. Disclosed are an electrolyte and a sodium ion battery. The disclosed electrolyte comprises a sodium salt, a solvent and an additive; the additive comprises a main additive; the structural formula of the main additive is HS-R-COOH, wherein R is selected from an alkylene group, an alkyloxy group or a polyethylene glycol segment containing an ether bond; and the content of the main additive in the electrolyte is 0.5-2.0 wt%. The disclosed electrolyte can effectively solve the generation of gas in the battery cycle process when applied in a negative electrode-free or negative electrode-deficient sodium ion battery, and guarantees the long-life safe operation of the battery.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, and more specifically, to electrolytes and sodium-ion batteries. Background Technology

[0002] Sodium-ion batteries, due to abundant sodium resources, considerable cost-effectiveness, and similar working principles to lithium-ion batteries, are considered a potential alternative to current lithium-ion batteries. However, the peak energy density of sodium-ion batteries remains below 160 Wh / kg, significantly limiting their applicability in fields requiring higher energy density. Electrodeless batteries utilize current collectors as the negative electrode to replace excess metallic sodium, eliminating the need for negative electrode active materials and significantly reducing the overall thickness and weight of the battery, thereby increasing its volumetric and gravimetric energy density. Furthermore, the electrodeless design reduces the presence of metallic sodium on the negative electrode, greatly reducing battery safety risks. Although electrodeless batteries offer advantages such as good low-temperature performance, low cost, and high energy density, they face serious gas generation problems during cycling and storage. The electrolyte plays a crucial role in this problem; electrodeless batteries exhibit significant gas generation, resulting in poor high-temperature cycling and storage performance. This is primarily attributed to the high reactivity of metallic sodium on the negative electrode side, which readily reacts with the electrolyte, leading to an unstable and easily ruptured SEI film.

[0003] In view of this, the present invention is proposed. Summary of the Invention

[0004] The purpose of this invention is to provide an electrolyte and a sodium-ion battery, which aims to improve at least one of the problems mentioned in the background art.

[0005] This invention is implemented as follows: In a first aspect, the present invention provides an electrolyte comprising a sodium salt, a solvent, and additives; Additives include main additives; The main additive has the structural formula HS-R-COOH, where R is selected from alkylene, alkene, or polyethylene glycol segments containing ether bonds; The content of the main additive in the electrolyte is 0.5~2.0wt%.

[0006] In an optional embodiment, R in the structural formula of the main additive is -(CH2CH2O). n -CH2CH2-, n=2~10.

[0007] In an optional implementation, n is 4 to 8.

[0008] In an optional embodiment, the additive further includes an auxiliary additive selected from at least one of sodium difluorooxalate borate, sodium difluorosulfonamide and sodium tetrafluoroborate. The mass ratio of the main additive to the auxiliary additive is 1:1~2.

[0009] In an optional embodiment, the sodium salt is selected from at least one of sodium hexafluorophosphate (NaPF6), sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium perchlorate (NaClO4), sodium tetrafluoroborate (NaBF4), sodium trifluoromethanesulfonate (NaOTf / NaCF3SO3), and sodium dioxolane borate (NaBOB).

[0010] In an optional embodiment, the concentration of sodium salt in the electrolyte is 14-20 wt%.

[0011] In an optional embodiment, the solvent is selected from at least one of ethylene glycol dimethyl ether and diethylene glycol dimethyl ether.

[0012] In a second aspect, the present invention provides a sodium-ion battery, comprising a positive electrode, a negative electrode current collector, a separator, and an electrolyte as described in any of the foregoing embodiments; Optionally, the sodium-ion battery is a sodium-ion battery with no negative electrode or few negative electrodes.

[0013] In an optional embodiment, the positive active material in the positive active layer of the positive electrode sheet is sodium iron phosphate, Prussian blue, or layered oxide.

[0014] In an optional embodiment, the current collector of the positive electrode is aluminum foil, copper foil, carbon-coated aluminum foil, or carbon-coated copper foil.

[0015] The present invention has the following beneficial effects: The electrolyte provided in this invention includes HS-R-COOH, an anchoring agent with bifunctional end groups. Its -COOH end can undergo a condensation reaction with the intrinsic oxide / hydroxide layer (containing -NaO and -NaOH) on the sodium metal surface formed during the negative electrode deposition process, forming a stable -COO-Na covalent bond, isolating the sodium metal from the electrolyte. The -SH end is exposed, forming a hydrophobic layer that excludes water molecules, reducing side reactions. Furthermore, during the dehydration reaction (reaction of water molecules and carboxylate groups), after the bond between R and -COOH breaks, the additive molecule, driven by potential, will re-anchor to the exposed sodium site, completing the dynamic regeneration of the protective layer. This fundamentally solves the problem of gas generation during the cycling process of sodium-ion batteries with no or minimal negative electrodes, ensuring long-life and safe operation of the battery. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0017] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0018] This invention provides an electrolyte comprising a sodium salt, a solvent, and additives; Additives include main additives; The main additive has the structural formula HS-R-COOH, where R is selected from alkylene, alkene, or polyethylene glycol segments containing ether bonds; The content of the main additive in the electrolyte is 0.5~2.0wt%.

[0019] The electrolyte provided in this invention includes HS-R-COOH, an anchoring agent with bifunctional end groups. Its -COOH end can undergo a condensation reaction with the intrinsic oxide / hydroxide layer (containing -NaO and -NaOH) on the sodium metal surface formed during the negative electrode deposition process, forming a stable -COO-Na covalent bond, isolating the sodium metal from the electrolyte. The -SH end is exposed, forming a hydrophobic layer that excludes water molecules, reducing side reactions. Furthermore, during the dehydration reaction (reaction of water molecules and carboxylate groups), after the bond between R and -COOH breaks, the additive molecule, driven by potential, will re-anchor to the exposed sodium site, completing the dynamic regeneration of the protective layer. This fundamentally solves the problem of gas generation during the cycling process of sodium-ion batteries with no or minimal negative electrodes, ensuring long-life and safe operation of the battery.

[0020] Optionally, R in the structural formula of the main additive is -(CH2CH2O). n -CH2CH2-, n=2~10. When R has this structure, hydrophobic shielding and Na+ can be achieved simultaneously. + It possesses a triple function of ion conduction assistance and interfacial volume buffering, and its amphiphilic nature endows the additive with good solubility in electrolytes and the ability to self-assemble in an ordered manner at the sodium interface. Preferably, to ensure better electrochemical performance of the battery, the main additive is a polyethylene glycol segment with ether bonds, and n is preferably 4 to 8.

[0021] Optionally, the additive also includes auxiliary additives, which are selected from at least one of sodium difluorooxalate borate (NaDFOB), sodium difluorosulfonamide (NaFSI), and sodium tetrafluoroborate (NaBF4). The mass ratio of the main additive to the auxiliary additive is 1:1 to 2.

[0022] Additives can be reduced / oxidized on the sodium anode surface before solvent molecules, participating in the construction of an initial solid electrolyte interphase (SEI) film rich in inorganic substances (such as boron and ferrous elements). This reduces the direct contact between solvents and impurities (such as water) and sodium metal at both the bulk and interfacial levels, suppressing side reactions and gas generation at the source. Using appropriate amounts of additives in conjunction with the main additive can form a dual gas generation suppression mechanism of "source reduction + interfacial interception," synergistically inhibiting battery gas production.

[0023] It should be noted that the ratio of auxiliary additives to main additives should be within the range required by this invention. If there are too few auxiliary additives, their effect will be insignificant. If there are too many, it means that the content in the electrolyte is too high, which will form an excessively thick SEI film and increase the interfacial impedance.

[0024] Optionally, the sodium salt is selected from at least one of sodium hexafluorophosphate (NaPF6), sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium perchlorate (NaClO4), sodium tetrafluoroborate (NaBF4), sodium trifluoromethanesulfonate (NaOTf / NaCF3SO3), and sodium dioxolane borate (NaBOB).

[0025] Optionally, the concentration of sodium salt in the electrolyte is 14-20 wt%.

[0026] Optionally, the solvent can be any commonly used organic solvent for sodium-ion batteries. Preferably, in some embodiments of the present invention, at least one selected from diethylene glycol dimethyl ether (DME) and diethylene glycol dimethyl ether (DEGDME) can be used. These solvents can spontaneously form an ordered self-assembled monolayer at the sodium interface without requiring additional treatment of the sodium surface.

[0027] The present invention provides a sodium-ion battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte provided in the present invention.

[0028] The sodium-ion battery provided by this invention, because it includes the electrolyte provided in the embodiments of this invention, produces less gas and has a longer battery life.

[0029] Optionally, the sodium-ion battery is a sodium-ion battery with no negative electrode or few negative electrodes.

[0030] Optionally, the positive active material in the positive active layer of the positive electrode sheet is sodium iron phosphate, Prussian blue, or layered oxide.

[0031] Optionally, the current collector of the positive electrode is aluminum foil, copper foil, carbon-coated aluminum foil, or carbon-coated copper foil.

[0032] Example 1 This embodiment provides an electrolyte comprising sodium salt (NaPF6), solvent (DME), and main additive (HS-R-COOH, where R is -(CH2CH2O)). n -CH2CH2-, n is 6), and auxiliary additives (NaDFOB); The concentration of sodium salt in the electrolyte is 18 wt%; the concentration of the main additive in the electrolyte is 1 wt%; and the mass ratio of the main additive to the auxiliary additive is 1:1.5.

[0033] This embodiment also provides a sodium-ion battery with a low negative electrode, which includes a positive electrode, a negative electrode, a separator, and an electrolyte provided in this embodiment.

[0034] The current collector of the positive electrode is aluminum foil, and the composition of the positive electrode active layer includes 95.0% sodium iron phosphate, 2.5% conductive agent (conductive carbon black) and 2.5% binder (PVDF) by mass. The negative electrode includes a carbon-coated aluminum foil and an active layer disposed on the surface of the carbon-coated aluminum foil. The active layer includes 94.5 wt% hard carbon, 2.5 wt% conductive agent (SP) and 3% binder (PAA). The thickness of the negative electrode is 60 µm. The diaphragm is a PP membrane; The electrolyte injection coefficient is 3.8 g / Ah.

[0035] Example 2 This embodiment provides an electrolyte comprising sodium salt (NaPF6), solvent (DEGDME), main additive (HS-R-COOH, where R is an alkylene group with 2 carbon atoms) and auxiliary additive (NaFSI). The concentration of sodium salt in the electrolyte is 14 wt%; the concentration of the main additive in the electrolyte is 0.5 wt%; and the mass ratio of the main additive to the auxiliary additive is 1:2.

[0036] This embodiment also provides a negative electrode-free sodium-ion battery, whose positive electrode sheet, negative electrode current collector, separator, and electrolyte injection volume are the same as those in Embodiment 1. The only difference is that the electrolyte injected into the negative electrode-free sodium-ion battery provided in this embodiment is the electrolyte provided in this embodiment.

[0037] Example 3 This embodiment provides an electrolyte comprising sodium salt (NaPF6), solvent (DME), main additive (HS-R-COOH, where R is an alkeneoxy group with 3 carbon atoms) and auxiliary additive (NaBF4). The concentration of sodium salt in the electrolyte is 20 wt%; the concentration of the main additive in the electrolyte is 2 wt%; and the mass ratio of the main additive to the auxiliary additive is 1:1.

[0038] This embodiment also provides a negative electrode-free sodium-ion battery, whose positive electrode sheet, negative electrode current collector, separator, and electrolyte injection volume are the same as those in Embodiment 1. The only difference is that the electrolyte injected into the negative electrode-free sodium-ion battery provided in this embodiment is the electrolyte provided in this embodiment.

[0039] Example 4 This embodiment is basically the same as Embodiment 1, except that: The main additive is HS-R-COOH, where R is -(CH2CH2O). n -CH2CH2-, n=2.

[0040] Example 5 This embodiment is basically the same as Embodiment 1, except that: The main additive is HS-R-COOH, where R is -(CH2CH2O). n -CH2CH2-, n=10.

[0041] Example 6 This embodiment is basically the same as Embodiment 1, except that the electrolyte does not include auxiliary additives.

[0042] Example 7 This embodiment is basically the same as embodiment 1, except that: the amount of auxiliary additives is too large, and the mass ratio of main additives to auxiliary additives is 1:3.

[0043] Comparative Example 1 This comparative example is basically the same as Example 6, except that the electrolyte does not include the main additive.

[0044] Comparative Example 2 This comparative example is basically the same as Example 6, except that the amount of the main additive is too large, and its content in the electrolyte is 4 wt%.

[0045] Comparative Example 3 This comparative example is basically the same as Example 6, except that: The battery provided in this comparative example is a conventional sodium-ion battery. The only difference between the battery in this comparative example and the battery in Example 6 is that the negative electrode current collector is replaced with a negative electrode sheet. The negative electrode sheet uses copper foil as the current collector and a negative electrode active layer is loaded on the copper foil. The negative electrode active layer includes 94.5 wt% hard carbon, 2.5 wt% conductive agent (SP) and 3% binder (PAA). The thickness of the negative electrode sheet is 120 µm (conventional sodium battery).

[0046] Comparative Example 4 This comparative example is basically the same as Comparative Example 3, except that: The electrolyte does not contain any main additives.

[0047] Experimental Example The electrochemical performance of the negative electrode-free sodium-ion batteries prepared in each embodiment and comparative example was measured. The specific test methods included: Battery first-week coulomb efficiency test: After the assembled battery was left to rest for 10 hours, it was charged to 3.5V at a constant current and constant voltage of 0.2C, then left to rest for another 0.5 hours, and then discharged to 2.0V at a constant current of 0.5C. The ratio of the battery's discharge capacity to its charge capacity was calculated as CE. Cycle life test: After the assembled battery is left to rest for 10 hours, it is charged to 3.5V at 0.5C constant current and constant voltage at room temperature of 25℃. After resting for 0.5 hours, it is discharged to 2.0V at 1C constant current. The cycle is repeated 100 times and the capacity retention rate is recorded. Volume change test: After the battery was formed and capacity tested, it was placed at room temperature (25°C) for 10 hours. The initial volume of the battery was measured by the water displacement method and recorded as V1. Then it was placed in a high-temperature room at 45°C for 7 days. After that, it was taken out and cooled to room temperature (25°C). The volume after storage was measured by the water displacement method and recorded as V2. The volume change rate after 7 days of storage at 45°C is ΔV=(V2-V1) / V1×100%.

[0048] Record the test results in Table 1.

[0049] Table 1. Electrochemical performance of the negative electrode-free sodium-ion batteries prepared in each embodiment and comparative example.

[0050] As can be seen from Table 1, the electrolytes provided in the various embodiments of the present invention can significantly improve the electrochemical performance of the battery when applied to sodium-ion batteries with few negative electrodes compared to ordinary electrolytes (Comparative Example 1). Comparing Example 6 with Example 1, the performance of Example 6 is worse in all aspects, indicating that adding auxiliary additives helps to improve the cycle stability of the battery. Comparing Example 7 with Example 1, the performance of Example 7 is worse, indicating that the amount of additives should not be too much. Too much additives will lead to an increase in interfacial impedance and reduce cycle stability. Comparing Comparative Example 2 with Example 6, the performance of Comparative Example 2 is significantly worse, indicating that the amount of main additive should not be too large. Too much additive will lead to interface disorder, hinder the pathway of sodium ions, and reduce cycle stability. Comparing Comparative Example 4 and Comparative Example 3, Comparative Example 4 performs slightly better than Comparative Example 3, with the initial coulombic efficiency of Comparative Example 3 increasing by approximately 4.07% compared to Comparative Example 4. Comparing Comparative Example 1 and Example 6, Example 6 performs significantly better than Comparative Example 1, with the initial coulombic efficiency of Example 6 increasing by approximately 13% compared to Comparative Example 1. This 13% is significantly greater than 4.07%, indicating that the electrolyte provided by this invention, when applied to a low-negative-electrode sodium-ion battery (Example 6), has a more significant effect on improving electrochemical performance compared to its application in a conventional sodium-ion battery (Comparative Example 3). This further demonstrates that the electrolyte provided by the embodiments of this invention is more suitable for application in low-negative-electrode sodium-ion batteries.

[0051] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An electrolyte, characterized in that, Includes sodium salts, solvents, and additives; The additives include main additives; The main additive has the structural formula HS-R-COOH, where R is selected from alkylene, alkeneoxy, or polyethylene glycol segments containing ether bonds; The main additive is present in the electrolyte at a concentration of 0.5 to 2.0 wt%.

2. The electrolyte according to claim 1, characterized in that, In the structural formula of the main additive, R is -(CH2CH2O). n -CH2CH2-, n=2~10.

3. The electrolyte according to claim 2, characterized in that, n=4~8。 4. The electrolyte according to claim 1, characterized in that, The additive also includes auxiliary additives, which are selected from at least one of sodium difluorooxalate borate, sodium difluorosulfonamide and sodium tetrafluoroborate. The mass ratio of the main additive to the auxiliary additive is 1:1~2.

5. The electrolyte according to any one of claims 1 to 4, characterized in that, The sodium salt is selected from at least one of sodium hexafluorophosphate, sodium difluorosulfonamide, sodium difluoromethanesulfonamide, sodium perchlorate, sodium tetrafluoroborate, sodium trifluoromethanesulfonate, and sodium dioxolaneborate.

6. The electrolyte according to any one of claims 1 to 4, characterized in that, The concentration of the sodium salt in the electrolyte is 14~20wt%.

7. The electrolyte according to any one of claims 1 to 4, characterized in that, The solvent is selected from at least one of ethylene glycol dimethyl ether and diethylene glycol dimethyl ether.

8. A sodium-ion battery, characterized in that, Includes a positive electrode, a negative current collector, a separator, and an electrolyte as described in any one of claims 1 to 7; Optionally, the sodium-ion battery is a sodium-ion battery with no negative electrode or few negative electrodes.

9. The sodium-ion battery according to claim 8, characterized in that, The positive active material in the positive active layer of the positive electrode sheet is sodium iron phosphate, Prussian blue, or layered oxide.

10. The sodium-ion battery according to claim 8, characterized in that, The current collector of the positive electrode is aluminum foil, copper foil, carbon-coated aluminum foil, or carbon-coated copper foil.

Images