WO2025098091A1 - Electrolyte and sodium-ion battery - Google Patents

Abstract

The present application comprises an electrolyte and a sodium-ion battery. The electrolyte comprises a sodium salt main salt, a sodium salt auxiliary salt, an organic solvent, and an additive. The sodium salt auxiliary salt comprises sodium difluorophosphate and the additive comprises vinylene carbonate, the mass percentage of the sodium salt auxiliary salt in the electrolyte being 0.01%-1%, and the mass percentage of the vinylene carbonate in the electrolyte being 0.5%-3%. The electrolyte provided in the present application enables batteries to exhibit excellent low-temperature cycling performance, rate performance, and initial coulombic efficiency.

Description

Electrolyte and sodium ion battery

This application claims the priority of the Chinese patent application filed with the China Patent Office on November 10, 2023, with application number CN202311497398.X and application name “A Electrolyte and Sodium Ion Battery”, the entire contents of which are incorporated by reference in this application.

Technical Field

The present application relates to an electrolyte, and in particular to an electrolyte and a sodium ion battery, belonging to the technical field of sodium ion batteries.

Background Art

Lithium-ion batteries are widely used in consumer electronics, electric vehicles, energy storage and other fields due to their high energy density and cycle performance. However, the rising price of lithium salts has led to lithium resources limiting the development of lithium-ion batteries. Sodium is an element in the same main group as lithium, and has very similar physical and chemical properties to lithium. In addition, sodium is more abundant on Earth than lithium and has a lower cost, so the development of sodium-ion batteries as large-scale energy storage devices has become a better choice. To date, the relevant technical research on sodium-ion batteries has attracted widespread attention in academia and industry.

However, there are still some problems and challenges in the practical application of sodium-ion batteries, such as low cycle life and low rate performance. Therefore, it is particularly important to improve the performance of sodium-ion batteries. In the sodium-ion battery system, vinylene carbonate in the electrolyte has a certain degree of improvement on the first coulombic efficiency, 1C capacity and initial DCR of the battery, but it will lead to deterioration of cycle performance at low temperatures and poor rate performance.

Based on the above shortcomings, it is necessary to develop an electrolyte that can significantly improve the electrical performance of sodium ion batteries.

Summary of the invention

The present application provides an electrolyte, which enables a battery to have excellent low-temperature cycle performance, rate performance and first coulombic efficiency.

The present application provides a sodium ion battery, which includes the above electrolyte and has a low temperature cycle life. It has the advantages of long life, high rate performance and high first coulombic efficiency.

The present application provides an electrolyte, comprising a sodium salt main salt, a sodium salt auxiliary salt, an organic solvent and a first additive; the sodium salt auxiliary salt comprises sodium difluorophosphate; the first additive comprises vinylene carbonate;

The mass percentage of the sodium salt auxiliary salt in the electrolyte is 0.01% to 1%; the mass percentage of the vinylene carbonate in the electrolyte is 0.5% to 3%.

As mentioned above, the sodium salt main salt includes sodium hexafluorophosphate and sodium bis(fluorosulfonyl)imide.

As mentioned above, wherein the mass percentage of the sodium hexafluorophosphate in the electrolyte is 2% to 15%; and/or,

The mass percentage of the sodium bis(fluorosulfonyl)imide in the electrolyte is 1% to 12%.

As described above, the organic solvent includes at least one of carbonate, fluorocarbonate, carboxylate, fluorocarboxylate, ether and fluoroether.

As mentioned above, the mass percentage of the organic solvent in the electrolyte is 10% to 90%.

As described above, at 0°C, the viscosity of the electrolyte is 7-12 mPa·s, and the ion conductivity is 4-7 mS/cm; at -40-45°C, the turbidity of the electrolyte is not higher than 5 NTU.

As described above, the electrolyte further comprises a second additive, which comprises at least one of 1,3-propane sultone, 1,3-propylene sultone and vinyl sulfate; the mass percentage of the second additive in the electrolyte is 0.1% to 3%.

As described above, the third additive is also included, and the third additive includes a silane phosphate additive, and the silane phosphate additive includes tri(trimethylsilane) phosphate and/or tri(vinyl-dimethylsilane) phosphate; the mass percentage of the third additive in the electrolyte is 0.1% to 2%.

The present application also includes a battery with the electrolyte as described above.

As mentioned above, the positive electrode active material of the positive electrode sheet in the sodium ion battery is a ternary layered sodium-based material.

The electrolyte provided in the present application limits the selection of the sodium salt auxiliary salt and the first additive in the electrolyte, and at the same time limits the mass percentage of each of the sodium salt auxiliary salt and the first additive, so that a SEI film with high stability and a specific content of sodium ions and phosphorus elements is formed on the surface of the negative electrode of the battery, thereby improving the mobility of sodium ions and reducing the active site free radicals in the electrolyte, while optimizing the viscosity of the electrolyte, improving the adsorption and separation of sodium ions in the negative electrode of the battery, and thus improving the low-temperature cycle performance and rate capability of the battery. Performance and first coulombic efficiency.

The sodium ion battery of the present application is prepared based on the electrolyte as described above. The SEI film formed after the battery undergoes the formation stage has high stability and low impedance, so that the battery has excellent low-temperature cycle performance, rate performance and first coulombic efficiency.

DETAILED DESCRIPTION

In order to make the purpose, technical solutions and advantages of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below in combination with the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, not all of them. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

In a first aspect, the present application provides an electrolyte solution, comprising a sodium salt main salt, a sodium salt auxiliary salt, an organic solvent and a first additive; the sodium salt auxiliary salt comprises sodium difluorophosphate; the first additive comprises vinylene carbonate;

The mass percentage of the sodium salt auxiliary salt in the electrolyte is 0.01% to 1%; the mass percentage of the vinylene carbonate in the electrolyte is 0.5% to 3%.

The electrolyte of the present application includes a sodium salt main salt, a sodium salt auxiliary salt and an organic solvent, wherein the organic solvent is used to dissolve the sodium salt to prepare an electrolyte with high conductivity. The sodium salt main salt can be a common sodium salt in the art, such as sodium hexafluorophosphate, sodium bisfluorosulfonyl imide, sodium bisoxalate borate and sodium difluorooxalate borate, etc., and the present application does not make too many choices.

It is worth noting that the working mechanism of sodium ion batteries is different from that of lithium ion batteries. The working mechanism of sodium ion batteries is: during the charge and discharge process, sodium ions are divided into two parts to work, one part of the sodium ions are embedded-desorbed on the positive and negative electrodes, and the other part of the sodium ions are adsorbed-separated on the negative electrode. In the electrolyte of the present application, vinylene carbonate is a film-forming additive, which can form a passivation film on the surface of the negative electrode of the battery, reduce the consumption of irreversible sodium ions in the battery, and thus improve the first coulomb efficiency of the battery. However, the applicant found that under low temperature conditions, the SEI film formed by vinylene carbonate on the surface of the negative electrode will hinder the migration of sodium ions, resulting in an increase in the internal resistance of the sodium ion battery, thereby reducing the low-temperature cycle performance and rate performance of the sodium ion battery. At the same time, the addition of vinylene carbonate will increase the viscosity of the electrolyte. Under low temperature conditions, the viscosity of the electrolyte is further increased, which has a stronger blocking effect on sodium ions with a large radius, which is not conducive to the adsorption and separation of sodium ions in the negative electrode of the battery, thereby deteriorating the low-temperature cycle performance and rate performance of the battery.

According to the above scheme provided by the present application, after the electrolyte is applied to the sodium ion battery, the sodium ion battery has excellent low temperature cycle performance, rate performance and first coulomb efficiency compared to only adding vinylene carbonate. The applicant analyzed this principle and believed that the reason may be that when the mass percentage of vinylene carbonate and sodium difluorophosphate is within the above range, on the one hand, sodium difluorophosphate can form a sufficiently stable SEI film with suitable sodium ion and phosphorus content on the surface of the negative electrode of the battery together with vinylene carbonate, which can effectively reduce the migration of irreversible sodium ions in the battery, and effectively reduce the transmission resistance of sodium ions in a low temperature environment, increase the migration rate of sodium ions in the SEI film, and significantly reduce the active site free radicals in the electrolyte, reduce the negative impact caused by the active site free radicals, thereby obtaining a battery with excellent low temperature cycle performance, rate performance and first coulomb efficiency; on the other hand, the viscosity of the electrolyte can be optimized, and the adsorption and separation of sodium ions in the negative electrode of the battery can be improved, thereby improving the low temperature cycle performance of the battery.

The main sodium salt of the present application includes sodium hexafluorophosphate and sodium bis(fluorosulfonyl)imide. Sodium hexafluorophosphate, as an ion transport medium, can help sodium ion batteries to carry out charge and discharge reactions, and sodium hexafluorophosphate has the function of stabilizing the battery electrolyte, so that the sodium ion battery has excellent electrochemical properties. Sodium bis(fluorosulfonyl)imide has high solubility and high dissociation constant, which can improve the kinetic properties of the electrolyte, thereby improving the electrochemical properties of the battery, and sodium bis(fluorosulfonyl)imide in the electrolyte can be partially decomposed, thereby introducing components such as sodium nitride, sodium sulfate and sodium fluoride into the SEI membrane or CEI membrane, reducing electrode interface polarization, improving the stability of the SEI membrane or CEI membrane, and making the sodium ion battery exhibit excellent high-temperature cycle performance.

In a specific embodiment, the mass percentage of sodium hexafluorophosphate in the electrolyte is 2% to 15%; and/or the mass percentage of sodium bisfluorosulfonyl imide in the electrolyte is 1% to 12%, that is, every 100 grams of electrolyte includes 2g to 15g of sodium hexafluorophosphate and/or 1g to 12g of sodium bisfluorosulfonyl imide, which can be specifically selected from 2% sodium hexafluorophosphate and/or 1% sodium bisfluorosulfonyl imide, 5% sodium hexafluorophosphate and/or 4% sodium bisfluorosulfonyl imide, 10% sodium hexafluorophosphate and/or 8% sodium bisfluorosulfonyl imide, 15% sodium hexafluorophosphate and/or 12% sodium bisfluorosulfonyl imide or a range consisting of any two of them. When the mass percentages of sodium hexafluorophosphate and sodium bis(fluorosulfonyl)imide are respectively within the above ranges, sodium hexafluorophosphate can be fully dissolved in an organic solvent to form an electrolyte with more stable chemical properties and higher electrical conductivity, thereby making the sodium ion battery exhibit better electrochemical performance; sodium bis(fluorosulfonyl)imide can improve the kinetic properties of the electrolyte and reduce electrode interface polarization to a greater extent, thereby obtaining a sodium ion battery with higher high-temperature cycle performance.

The organic solvents of the present application include carbonates, fluorocarbons, carboxylates, fluorocarboxylates, ethers, These solvents can dissolve sodium salts to prepare electrolytes with stable chemical properties and high conductivity, so that sodium ion batteries exhibit excellent electrochemical performance.

In a specific embodiment, the mass percentage of the organic solvent in the electrolyte is 10% to 90%, that is, 10g to 90g of organic solvent is included in every 100g of electrolyte, which can be specifically selected from the range of 10%, 30%, 60%, 90% or any two thereof. The mass percentage of the organic solvent is limited to the above range, which not only avoids the reduction of the migration ability of sodium ions due to excessive viscosity of the electrolyte, but also ensures a higher conductivity of the electrolyte, so that the battery has better low-temperature cycle performance and rate performance.

In a specific embodiment, at 0°C, the viscosity of the electrolyte is 7 to 12 mPa·s, and the ionic conductivity is 4 to 7 mS/cm; at -40 to 45°C, the turbidity of the electrolyte is not higher than 5NTU. Specifically, the electrolyte in the present application is subjected to viscosity test and conductivity test at 0°C, and the viscosity of the electrolyte is 7 to 12 mPa·s, and the ionic conductivity is 4 to 7 mS/cm; the scattering method turbidity test is performed at -40 to 45°C, and the turbidity of the electrolyte is not higher than 5NTU. When the viscosity and ionic conductivity of the electrolyte at 0°C are respectively within the above ranges, and the turbidity of the electrolyte at -40 to 45°C is not higher than 5NTU, the transmission performance of sodium ions in the electrolyte in a low temperature environment is ensured to be better, so that the low temperature cycle performance and rate performance of the battery are better. The viscosity, ionic conductivity and stable state of the electrolyte in the present application can be achieved according to the scheme provided in the present application. Specifically, the electrolyte with specific viscosity, ionic conductivity and stable state is obtained by the combined action of the above-mentioned specific mass percentages of sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, sodium difluorophosphate, vinylene carbonate and organic solvent.

The electrolyte of the present application also includes a second additive, which includes at least one of 1,3-propane sultone, 1,3-propylene sultone and vinyl sulfate. In a specific embodiment, the mass percentage of the second additive in the electrolyte is 0.1% to 3%, that is, 0.1g to 3g of the second additive is included in every 100 grams of electrolyte, and can be specifically selected from 0.1%, 1%, 3% or any two of them. Because 1,3-propane sultone and propenyl-1,3 sultone carry sulfonic acid groups, they can be decomposed to obtain acidic decomposition products, appropriately neutralize the weak alkalinity brought by the bisfluorosulfonyl imide group in the electrolyte, reduce the degradation effect of weak alkalinity on the battery during high-temperature storage, and form a stable passivation film on the surface of the positive electrode material, prevent the oxidative decomposition of the electrolyte on the positive electrode surface, inhibit the dissolution of transition metal ions in the positive electrode material, and improve the stability of the structure and interface of the positive electrode material, thereby improving the high-temperature cycle performance and high-temperature storage performance of the battery. Vinyl sulfate can be decomposed on the surface of graphite negative electrode to generate organic sulfonate with good ion conductivity, thereby reducing interface impedance, inhibiting the decrease of initial capacity of the battery, increasing initial discharge capacity, reducing battery expansion after high temperature storage, and thus improving the rate performance and high temperature performance of the battery. When the mass percentage of the additive is within the above range, the gas production problem of the battery can be suppressed to a greater extent, thereby obtaining a battery with better high temperature performance.

The electrolyte of the present application also includes a third additive, which includes a silane phosphate additive, which can be specifically selected from tris(trimethylsilane) phosphate and/or tris(vinyl-dimethylsilane) phosphate. In a specific embodiment, the mass percentage of the third additive in the electrolyte is 0.1% to 2%, that is, every 100 grams of electrolyte includes 0.1g to 2g of the third additive, which can be specifically selected from 0.1%, 1%, 2% or any two of them. These silane phosphate additives are improved high-temperature additives, which can be preferentially reduced to form a film on the surface of the negative electrode before the solvent is reduced, effectively preventing the reduction and decomposition of the electrolyte on the surface of the negative electrode, thereby effectively suppressing the storage and gas production problem of the battery at high temperature, and can reduce the transmission impedance of sodium ions in the SEI film during the charge and discharge process, forming a low-impedance interface film, thereby improving the high-temperature cycle performance and high-temperature storage performance of the battery. When the mass percentage of the third additive is within the above range, the third additive can suppress the gas production problem during high-temperature storage of the battery to a greater extent, and further reduce the impedance of the SEI film, thereby further improving the high-temperature cycle performance and high-temperature storage performance of the battery.

The present application does not limit the preparation method of the electrolyte. In a specific implementation, it is sufficient to mix the sodium salt main salt, sodium difluorophosphate, organic solvent and vinylene carbonate in a prescribed ratio.

Furthermore, when the electrolyte further includes a second additive and a third additive, sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, sodium difluorophosphate, an organic solvent, vinylene carbonate, the second additive and the third additive are mixed in a prescribed ratio.

The second aspect of the present application provides a sodium ion battery, the battery comprising the aforementioned electrolyte. Based on the electrolyte provided by the present application, the battery provided by the present application has excellent low temperature cycle performance, rate performance, first coulombic efficiency, high temperature cycle performance and high temperature storage performance.

In a specific embodiment, the positive electrode active material of the positive electrode sheet in the sodium ion battery is a ternary layered sodium-based material. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer arranged on the surface of the positive electrode current collector, and the positive electrode active material layer includes a positive electrode active material, a conductive agent and a binder, wherein the positive electrode current collector is generally an aluminum foil, and the positive electrode active material is a ternary layered sodium-based material. The present application does not specifically limit the ternary layered sodium-based material, such as one of NaNi _1/3 Fe _1/3 Mn _1/3 O ₂ , NaNi _1/3 Mn _1/3 Co _1/3 O ₂ (NMC), Na ₃ V ₂ (PO ₄ ) ₃ (NMN), Na _0.9 Mn _0.6 Fe _0.4 PO ₄ (NMFP), sodium ferric phosphate pyrophosphate Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) and sodium ferrocyanide. It can be understood that the above electrolyte is applied to a sodium ion battery including a positive electrode active material as a ternary layered sodium-based material, so that each of the electrolyte The components work synergistically with the ternary layered sodium-based materials, improving the stability of the ternary layered sodium-based materials, inhibiting the dissolution of transition metals and oxygen precipitation in the sodium-based materials, while also inhibiting the acid corrosion of the positive electrode oxide material by HF produced by the decomposition of the electrolyte, thereby improving the crystal structure stability of the ternary layered sodium-based materials, and enabling the battery to have better low-temperature cycle performance, rate performance, first coulombic efficiency, high-temperature cycle performance and high-temperature storage performance.

In a specific embodiment, in addition to the electrolyte provided in the first aspect of the present application and the ternary layered sodium-based material provided in the second aspect, the electrolyte further includes a negative electrode sheet and a separator, specifically:

The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer arranged on the surface of the negative electrode current collector, the negative electrode active material layer includes a negative electrode active material, a conductive agent and a binder, wherein the negative electrode current collector is generally a copper foil, and the negative electrode active material is selected from one or more of carbonaceous materials, silicon-carbon materials, alloy materials, and sodium-containing metal composite oxides.

The conductive agent and the binder in the positive electrode active material layer and the negative electrode active material layer may be conventional materials in the art.

The separator is a separator known in the art that can be used in batteries and is stable to the electrolyte used, and can include one or more of polyolefin, aromatic polyamide, polytetrafluoroethylene, and polyethersulfone, and can be specifically configured as needed.

Hereinafter, the present application will be further described in detail through specific embodiments.

Example 1

The electrolyte provided in this embodiment includes, in terms of mass percentage, sodium salt, organic solvent and additives, wherein the sodium salt includes 10% sodium hexafluorophosphate (NaPF ₆ ), 8% sodium bis(fluorosulfonyl)imide (NaFSI) and 0.01% sodium difluorophosphate (NaPOF ₂ ), the organic solvent includes 13.42% ethylene carbonate (EC), 13.42% propylene carbonate (PC) and 53.66% ethyl methyl carbonate (EMC), and the additive includes 1.49% vinylene carbonate (VC).

In a glove box filled with argon, the organic solvents were mixed, the additives were added in sequence, and then the sodium salt was added and stirred to obtain the sodium ion battery electrolyte. The viscosity and conductivity of the electrolyte were tested, and the viscosity of the electrolyte was 9mPa·s and the conductivity was 5.5mS/cm.

The positive electrode active material NaNi _1/3 Fe _1/3 Mn _1/3 O ₂ powder, the conductive agent acetylene black, the carbon nanotubes and the binder (polyvinylidene fluoride) PVDF are dispersed in the solvent NMP according to the mass ratio of 95:2:1:2 to obtain a positive electrode active material layer slurry; the positive electrode active material layer slurry is evenly coated on the surface of the positive electrode current collector aluminum foil. The positive electrode sheet is obtained after drying, rolling, baking, slitting and spot welding of the electrode ears. The total thickness of the positive electrode sheet is 134μm.

The negative electrode active material hard carbon, the conductive agent conductive carbon black super-p, the binder SBR, and the thickener hydroxymethyl cellulose (CMC) are dispersed in deionized water in a mass ratio of 95:1.5:2:1.5, and stirred evenly to obtain the negative electrode active material layer slurry; the negative electrode active material layer slurry is evenly coated on the surface of the negative electrode collector aluminum foil, and the negative electrode sheet is obtained after drying, rolling, baking, slitting and spot welding of the electrode ears. The total thickness of the negative electrode sheet is 150μm.

The prepared positive electrode sheet, negative electrode sheet and 16μm three-layer separator are stacked in order, the separator is placed between the positive electrode sheet and the negative electrode sheet, and the wound body is flattened and placed in an aluminum-plastic film packaging bag after winding. It is vacuum-baked at 75°C for 48h to obtain a battery cell to be filled with liquid, and then the above-mentioned electrolyte is injected into the battery cell in a glove box. After packaging, formation, aging and capacity separation, the preparation of the sodium ion battery is completed.

The electrolytes and sodium ion batteries provided in Examples 2 to 21 and Comparative Examples 1 to 6 are substantially the same as those in Example 1, with the differences being shown in Table 1.

Table 1 Composition of electrolytes and positive electrode active materials provided in Examples 1 to 21 and Comparative Examples 1 to 6

The sodium ion batteries prepared in Examples 1 to 21 and Comparative Examples 1 to 6 were subjected to the following battery performance tests, and the test method was as follows:

First coulombic efficiency: the sodium ion battery after injection is charged at 0.1C constant current for 390min at room temperature, and the charging capacity C ₁ is recorded. After aging for 24h, the battery is sealed again. The sealed battery is charged to 3.8V at 0.2C constant current and constant voltage, with a cut-off current of 0.05C, and the charging capacity is recorded as C ₂ . Then, it is discharged to 2.0V at 0.2C constant current and discharged to 2.0V at 0.5C constant current and constant voltage, with a cut-off current of _0.05C , and discharged to 2.0V at 0.5C constant current. Then, it is charged to 3.8V at 1C constant current and constant voltage, with a cut-off current of 0.05C, and discharged to 2.0V at 1C constant current and constant voltage, with a cut-off current of 0.05C, and discharged to 2.0V at 1C constant current and constant voltage. The 1C charge and discharge cycle is repeated 5 times, and the last discharge capacity is taken to obtain the 1C capacity utilization value C ₄ . The first coulombic efficiency η ₁ =C ₃ /(C ₁ +C ₂ )*100% is recorded.

Rate performance test: At 25°C, the battery was charged to 3.8V at 2C constant current, and charged to a cut-off current of 0.05C at 3.8V constant voltage, then left for 10 minutes, and then discharged to 2.0V at 2C constant current. The discharge capacity is recorded as C ₅ . After standing for 10 minutes, the charge and discharge steps are repeated, and the capacity of the battery discharged with a current of 5C is recorded as C ₆ , and the 2C and 5C rate discharge retention rates η ₂ =C ₅ /C ₄ *100% and η ₃ =C ₆ /C ₄ *100% are recorded.

Low temperature cycle performance test: at an ambient temperature of 0°C, after standing for 4 hours, the battery is charged to 3.8V at a constant current and constant voltage of 0.2C, with a cut-off current of 0.05C. After the battery is left for 10 minutes, it is discharged to 2.0V at a constant current of 0.5C, and the discharge capacity is recorded as _C7. The charge and discharge steps are repeated until the cycle is 200 cycles, and the discharge capacity after 200 cycles is recorded as _C8 . The corresponding cycle retention rate _η4 = _C8 / _C7 *100%, see Table 2.

High temperature cycle performance test: At 45°C, the battery was subjected to a 1C/1C cycle test: At an ambient temperature of 45°C, after standing for 4 hours, the battery was charged to 3.8V at 1C constant current and constant voltage, with a cut-off current of 0.05C. After the battery was left for 10 minutes, it was discharged to 2.0V at 1C constant current, and the discharge capacity was recorded as C ₉ . The charge and discharge steps were repeated until 500 cycles were completed, and the discharge capacity was recorded as C ₁₀ . The high temperature cycle capacity retention rate η ₅ =C ₁₀ /C ₉ *100%, see Table 2.

High temperature storage performance test: The battery was transferred to a high temperature of 60°C and placed for 14 days. After 14 days, it was discharged at a constant current of 1C at room temperature of 25°C, and the discharge capacity was recorded as C ₁₁ . The 60°C capacity retention rate η ₆ =C ₁₁ /C ₄ *100%, see Table 2. The test results are shown in Table 2.

Table 2 Performance test results of sodium ion batteries prepared in Examples 1 to 21 and Comparative Examples 1 to 6

As shown in Table 2, according to the comparison between Examples 1 to 21 and Comparative Examples 1 to 6, it is found that when the sodium salt auxiliary salt includes sodium difluorophosphate, the first additive includes vinylene carbonate, the mass percentage of the sodium salt auxiliary salt in the electrolyte is 0.01% to 1%, and the mass percentage of vinylene carbonate is 0.5% to 3%, it is helpful to improve the low temperature cycle performance, rate performance and first coulomb efficiency of the battery; according to the comparison between Examples 2 and 5 to 7, when the mass percentage of sodium hexafluorophosphate in the electrolyte is 2% to 15%, it is beneficial to The low-temperature cycle performance, rate performance and first coulombic efficiency of the battery are improved; according to the comparison of Examples 2, 8 to 10, when the mass percentage of sodium bis(fluorosulfonyl)imide in the electrolyte is 1% to 12%, the low-temperature cycle performance, rate performance, first coulombic efficiency, high-temperature cycle performance and high-temperature storage performance of the battery are improved; according to Example 11, when the viscosity of the electrolyte is 7 to 12 mPa·s at 0°C, the low-temperature cycle performance, rate performance, first coulombic efficiency, high-temperature cycle performance and high-temperature storage performance of the battery are improved. The storage performance is improved; according to Example 12, when the ionic conductivity of the electrolyte is 4-7mS/cm at 0°C, the low-temperature cycle performance, rate performance, first coulombic efficiency, high-temperature cycle performance and high-temperature storage performance of the battery are better; according to Examples 2, 13-16, when the mass percentage of the second additive in the electrolyte is 0.1%-3%, the low-temperature cycle performance, rate performance, first coulombic efficiency, high-temperature cycle performance and high-temperature storage performance of the battery are better; according to the comparison of Examples 14, 17-20, when the mass percentage of the third additive in the electrolyte is 0.1%-2%, it is helpful to improve the low-temperature cycle performance, rate performance, first coulombic efficiency, high-temperature cycle performance and high-temperature storage performance of the battery; according to the comparison of Examples 18 and 21, when the positive electrode active material in the sodium ion battery is a ternary layered sodium-based material, the low-temperature cycle performance, rate performance, first coulombic efficiency, high-temperature cycle performance and high-temperature storage performance of the battery are better. In summary, the electrolyte provided in the present application can significantly improve the low-temperature cycle performance, rate performance, first coulombic efficiency, high-temperature cycle performance and high-temperature storage performance of sodium-ion batteries.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit it. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present application.

Claims (13)

An electrolyte, comprising a sodium salt main salt, a sodium salt auxiliary salt, an organic solvent and a first additive; the sodium salt auxiliary salt comprises sodium difluorophosphate; the first additive comprises vinylene carbonate; The mass percentage of the sodium salt auxiliary salt in the electrolyte is 0.01% to 1%; the mass percentage of the vinylene carbonate in the electrolyte is 0.5% to 3%. The electrolyte according to claim 1, wherein the sodium salt main salt comprises sodium hexafluorophosphate and sodium bis(fluorosulfonyl)imide. The electrolyte according to claim 2, wherein the mass percentage of the sodium hexafluorophosphate in the electrolyte is 2% to 15%. The electrolyte according to claim 2, wherein the mass percentage of the sodium bis(fluorosulfonyl)imide in the electrolyte is 1% to 12%. The electrolyte according to claim 1, wherein the organic solvent comprises at least one of carbonate, fluorocarbon, carboxylate, fluorocarboxylate, ether and fluoroether. The electrolyte according to claim 5, wherein the mass percentage of the organic solvent in the electrolyte is 10% to 90%. The electrolyte according to claim 6, wherein at 0°C, the viscosity of the electrolyte is 7 to 12 mPa·s, and the ionic conductivity is 4 to 7 mS/cm; at -40 to 45°C, the turbidity of the electrolyte is not higher than 5 NTU. The electrolyte according to claim 1, further comprising a second additive, wherein the second additive comprises at least one of 1,3-propane sultone, 1,3-propylene sultone and vinyl sulfate. The electrolyte according to claim 8, wherein the mass percentage of the second additive in the electrolyte is 0.1% to 3%. The electrolyte according to claim 1, further comprising a third additive, wherein the third additive comprises a silane phosphate additive, and the silane phosphate additive comprises tris(trimethylsilane) phosphate and/or tris(vinyl-dimethylsilane) phosphate. The electrolyte according to claim 10, wherein the mass percentage of the third additive in the electrolyte is 0.1% to 2%. A sodium ion battery, comprising the electrolyte according to any one of claims 1 to 11. The sodium ion battery according to claim 12, wherein the positive electrode active material of the positive electrode sheet in the sodium ion battery is a ternary layered sodium-based material.