Sodium-ion battery secondary liquid injection electrolyte and preparation method and liquid injection process thereof

Abstract

The invention belongs to the technical field of sodium-ion batteries, and particularly relates to a sodium-ion battery secondary liquid injection electrolyte and a preparation method and a liquid injection process thereof, and the sodium-ion battery secondary liquid injection electrolyte comprises a first electrolyte used for being added before formation and a second electrolyte used for being added after aging; the first electrolyte comprises sodium salt, a first non-aqueous organic solvent and a first functional additive; and the second electrolyte comprises sodium salt, a second non-aqueous organic solvent and a second functional additive. Compared with the prior art, the problem that in the prior art, a sodium-ion battery electrolyte system easily causes side reaction and thermal runaway under extreme working conditions, so that the long-term stability is rapidly degraded is solved. According to the scheme, through staged injection of the first electrolyte and the second electrolyte, the cycle life, the thermal stability and the safety performance of the sodium-ion battery are synergistically improved.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, specifically relating to a secondary electrolyte for sodium-ion batteries, its preparation method, and the electrolyte injection process. Background Technology

[0002] Sodium-ion batteries, due to their abundant sodium resources and low cost, show broad application prospects in smart grids, low-speed electric vehicles, and inexpensive electronic products. Compared to lithium-ion batteries, sodium-ion batteries have certain advantages in safety, such as the ability to use aluminum foil as the negative electrode current collector and the ability to be transported and stored at "zero charge," effectively reducing safety risks during transportation. However, sodium-ion batteries still have many safety hazards in practical applications, such as thermal runaway, volume expansion, gas production, and fire, which seriously affect their large-scale commercialization.

[0003] Current sodium-ion battery electrolyte systems primarily utilize carbonate-based organic solvents and sodium NaPF6. While these systems exhibit certain electrochemical performance, their thermal stability is insufficient, making them prone to side reactions and thermal runaway under extreme conditions such as high temperatures and overcharging. Specifically, sodium NaPF6 decomposes under high temperatures and pressures, generating corrosive byproducts (such as HF), leading to electrode interface deterioration, increased internal resistance, decreased cycle life, and even safety accidents. Furthermore, the instability of the SEI and CEI films causes rapid rupture and regeneration cycles during long-term cycling, resulting in continuously increasing battery impedance and a rapid decrease in capacity. Existing research has improved interfacial stability by introducing additives (such as fluoroethylene carbonate (FEC) and vinylene carbonate (VC), but the organic solvents in the electrolyte system remain robust and prone to volatility and combustion under high temperatures or extreme conditions, further compromising the high-temperature tolerance and ability to suppress side reactions of sodium-ion batteries.

[0004] In the prior art, CN116995303A discloses a sodium-ion battery electrolyte and a sodium-ion battery. The sodium-ion battery electrolyte includes a primary electrolyte and a secondary electrolyte. The primary electrolyte includes a first organic solvent, a first additive, and a sodium salt; the secondary electrolyte includes a second organic solvent, a second additive, and a sodium salt. The first additive includes vinylene carbonate, fluoroethylene carbonate, and sodium difluorosulfonylimide, but does not contain 1,3-propanesulfonate lactone; the second additive includes vinylene carbonate, fluoroethylene carbonate, and 1,3-propanesulfonate lactone. This approach focuses on forming a stable SEI film to improve the cycle stability of the sodium-ion battery, but its improvement effect is limited; after 600 cycles at 1C / 1C, the capacity retention rate is below 90%.

[0005] In the prior art, CN119009121A discloses a sodium-ion battery electrolyte, a secondary electrolyte-filled sodium-ion battery, and a method for preparing the same. The sodium-ion battery electrolyte includes a primary electrolyte and a secondary electrolyte. The primary electrolyte comprises a first organic solvent, a sodium salt, and a first additive. The first organic solvent is propylene carbonate, and the concentration of the sodium salt in the primary electrolyte is 2 wt.% to 6 wt.%. The second electrolyte comprises a second organic solvent, a sodium salt, and a second additive. The propylene carbonate content in the second organic solvent is not less than 70 wt.%, the concentration of the sodium salt in the secondary electrolyte is 30 wt.% to 60 wt.%, and the second additive contains fluoroethylene carbonate. However, this approach primarily improves battery cycle performance through the self-repair of the SEI film, and still suffers from the potential problems of continuously increasing battery impedance and rapid capacity decline.

[0006] Therefore, developing a sodium-ion battery electrolyte with high thermal stability, high interfacial stability, and excellent ability to suppress side reactions has become an urgent need in the industry. Summary of the Invention

[0007] The purpose of this invention is to provide a secondary electrolyte for sodium-ion batteries, its preparation method, and the electrolyte injection process, in order to solve at least one of the aforementioned problems. This addresses the issue that existing sodium-ion battery electrolyte systems are prone to side reactions and thermal runaway under extreme conditions, leading to rapid long-term stability degradation. This solution, through the staged injection of a first electrolyte and a second electrolyte, synergistically improves the cycle life, thermal stability, and safety performance of sodium-ion batteries.

[0008] The objective of this invention is achieved through the following technical solution: The first aspect of the present invention discloses a secondary electrolyte for sodium-ion batteries, comprising: a first electrolyte for addition before formation, and a second electrolyte for addition after aging; The first electrolyte comprises: Sodium salt, first non-aqueous organic solvent, and first functional additive; The second electrolyte comprises: Sodium salt, second non-aqueous organic solvent, and second functional additive; in, The sodium salt is sodium difluorosulfonamide (NaFSi); the first non-aqueous organic solvent is an organic carbonate solvent; the first functional additive is used to promote the formation of SEI during the formation process; the second non-aqueous organic solvent is a solvent system compatible with the first electrolyte; and the second functional additive is a flame retardant additive.

[0009] Preferred, In the first electrolyte: The concentration range of sodium salt is 6 to 15 wt%, the concentration range of the first functional additive is 1.2 to 16 wt%, and the balance is the first non-aqueous organic solvent; In the second electrolyte: The concentration range of the sodium salt is 6 to 15 wt%, the concentration range of the second functional additive is 2 to 10 wt%, and the balance is the second non-aqueous organic solvent.

[0010] Preferred, The first non-aqueous organic solvent is an electrolyte solvent system formed by mixing propylene carbonate (PC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) in a mass ratio of 3:4:3. The second non-aqueous organic solvent is an electrolyte solvent system formed by mixing ethylene carbonate (EC), 2,2,2-trifluoroethyl phosphate (TFEP), and methyl ethyl carbonate in a mass ratio of 3:4:3.

[0011] Preferred, The first functional additive includes one or more of fluoroethylene carbonate (FEC), difluoroacetic anhydride, sulfonate additives, and sulfate additives; The second functional additive includes one or more of phosphate ester flame retardant additives and fluorinated solvent flame retardant additives.

[0012] Preferred, The first functional additive is a mixture of fluoroethylene carbonate, ethylene sulfate (DTD), and difluoroacetic anhydride having any one of the structures shown in formulas (I) to (IV). Formula (I), Formula (II) Formula (III) Formula (IV); The second functional additive is pentafluoro(phenoxy)cyclotriphosphazene.

[0013] Preferred, In the first functional additive: the amount of fluoroethylene carbonate added to the first electrolyte is 5 wt%, the amount of ethylene sulfate added to the first electrolyte is 1 wt%, and the amount of difluoroacetic anhydride added to the first electrolyte is 2 wt%. In the second functional additive, the amount of pentafluoro(phenoxy)cyclotriphosphazene added to the second electrolyte is 1 wt.

[0014] More preferably, In the first electrolyte: The concentration of sodium salt is 1.2 mol / L; the first non-aqueous organic solvent is propylene carbonate, diethyl carbonate and methyl ethyl carbonate in a mass ratio of 3:4:3; the first functional additive is 5 wt% fluoroethylene carbonate, 1 wt% ethylene sulfate and 2 wt% difluoroacetic anhydride. In the second electrolyte: The concentration of sodium salt is 1 mol / L; the second non-aqueous organic solvent is ethylene carbonate, 2,2,2-trifluoroethyl phosphate and methyl ethyl carbonate in a mass ratio of 3:4:3; the second functional additive is pentafluoro(phenoxy)cyclotriphosphazene at 1 wt%.

[0015] A second aspect of this invention discloses a method for preparing a secondary electrolyte for a sodium-ion battery as described in any of the above-mentioned methods, comprising the following steps: The preparation of the first electrolyte was carried out entirely in a glove box environment: S1: Prepare the first non-aqueous organic solvent, mix it evenly, and then freeze it; S2: Add sodium salt to the first non-aqueous organic solvent after freezing and stir until completely dissolved; S3: Add the first functional additive to the first non-aqueous organic solvent containing dissolved sodium salt, and stir continuously to obtain the first electrolyte; The preparation of the second electrolyte was carried out entirely in a glove box environment. T1: Prepare a second non-aqueous organic solvent, mix thoroughly, and then freeze; T2: Add sodium salt to the frozen second non-aqueous organic solvent and stir until completely dissolved; T3: Add the second functional additive to the second non-aqueous organic solvent containing dissolved sodium salt, and stir continuously to obtain the second electrolyte.

[0016] Preferably, the glove box environment is an argon atmosphere with an H2O content of less than 10 ppm; the freezing is carried out at -20°C for 12 hours; and the continuous stirring time is 2 hours.

[0017] A third aspect of this invention discloses an injection process for a secondary electrolyte for a sodium-ion battery as described in any of the preceding embodiments, comprising the following steps: The first electrolyte is preferentially injected during the electrolyte injection stage before the formation of sodium-ion batteries; A second electrolyte is injected after the sodium-ion battery has aged.

[0018] Preferably, the mass ratio of the first electrolyte to the second electrolyte is 8:2 to 7:3.

[0019] The working principle of this invention is as follows: The first electrolyte uses a highly stable sodium salt system (sodium bis(fluorosulfonyl)imide, NaFSi), which effectively avoids the decomposition of NaPF6 under high temperature and high pressure, thus improving the stability of the electrolyte.

[0020] The first electrolyte, through a combination of solvent, sodium salt, and functional additives, can form a dense solid electrolyte film during the formation stage, thereby improving the interfacial stability and cycle performance of sodium-ion batteries.

[0021] The second electrolyte, through a combination of highly stable solvents, sodium salt (NaFSi), and flame-retardant additives, can effectively improve the high-temperature stability and safety performance of sodium-ion batteries.

[0022] By using a secondary electrolyte injection process, the density of the solid electrolyte membrane during the formation process is optimized while flame-retardant electrolyte is injected a second time, ensuring the stability of the battery interface and further improving its safety.

[0023] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention relates to a sodium-ion battery electrolyte with secondary electrolyte injection, comprising a first electrolyte and a second electrolyte. The first electrolyte is injected before the formation stage, effectively improving the density and stability of the solid electrolyte film formed during the cell formation stage. Subsequently, the second electrolyte is injected a second time to further enhance the high-temperature stability and safety performance of the electrolyte. This results in a sodium-ion battery electrolyte suitable for high safety, high thermal stability, high interfacial stability, and excellent suppression of side reactions.

[0024] 2. The sodium salt system of the electrolyte is replaced by NaFSI instead of the traditional NaPF6, which significantly improves thermal stability and reduces harmful side reactions. FEC, DTD and difluoroacetic anhydride are used together as the first electrolyte additive. Through interfacial film formation, solvation regulation and synergistic effect, the ion transport path is optimized, so that the solid electrolyte membrane of the cell has higher stability during the formation stage.

[0025] 3. In the second electrolyte solvent, EC, TFEP and EMC are used in combination to design a phosphate ester / carbonate electrolyte with high flame retardant efficiency and good compatibility. The designed phosphate ester / carbonate electrolyte has the advantages of low cost, good electrochemical compatibility and excellent flame retardant performance. Sodium-ion batteries using this electrolyte can still maintain a safe state under extreme conditions.

[0026] 4. The second electrolyte uses pentafluoro(phenoxy)cyclotriphosphazene additive. Through the synergistic effect of phosphorus and fluorine, the flammability of the electrolyte is significantly reduced. The phosphorus-containing free radicals and flame-retardant gases generated by its decomposition can dilute oxygen and block the combustion chain reaction at high temperatures. The pentafluoro(phenoxy)cyclotriphosphazene works together with the FEC of the first electrolyte. The pentafluoro(phenoxy)cyclotriphosphazene provides a rigid inorganic layer, while the FEC provides a flexible organic layer. Together, they optimize the interfacial membrane structure, reduce polarization, and improve cycle stability. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the electrolyte injection process for a sodium-ion battery.

[0028] Figure 2 The results show the cycle performance test results of the sodium-ion battery secondary electrolyte prepared in Example 1. Detailed Implementation

[0029] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0030] Unless otherwise specified, the reagents used in the following description are conventional commercial products, the methods used are common knowledge in the field, and other matters not covered herein are existing technologies.

[0031] This invention provides a sodium-ion battery electrolyte system with high safety and long cycle performance. Its core lies in using two different electrolyte formulations in combination with a secondary electrolyte injection process to optimize the formation process of sodium-ion batteries. The electrolyte system is specifically composed of a first electrolyte and a second electrolyte, which are injected at different manufacturing stages of sodium-ion batteries to synergistically improve the cycle life, thermal stability and safety performance of the battery.

[0032] 1. Structure and function of the first electrolyte: Sodium salt component: Sodium difluorosulfonamide (NaFSi) was selected to replace the traditional sodium hexafluorophosphate (NaPF6), which solved the problem of its easy decomposition under high temperature and high pressure environment and significantly improved the chemical and thermal stability of the electrolyte.

[0033] Solvent system: Select organic carbonate solvents with high polarity, low viscosity and strong thermal stability, such as propylene carbonate (PC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), ethylene carbonate (EC), etc., to ensure good sodium ion conductivity.

[0034] Functional additives, such as FEC (fluoroethylene carbonate), difluoroacetic anhydride (any one of formulas (I) to (IV)), and DTD (sulfate esters), can promote the formation of the interfacial solid electrolyte membrane (SEI) during the formation process, making it more dense and stable, thereby improving the stability of the electrode interface and inhibiting side reactions.

[0035] Formula (I), Formula (II) Formula (III) Formula (IV).

[0036] The first electrolyte formulation is as follows: The concentration range of sodium salt is 6-15 wt%, the concentration range of functional additives is 1.2-16 wt%, and the balance is solvent.

[0037] 2. Structure and function of the second electrolyte: Sodium salt: Sodium difluorosulfonamide (NaFSi) is selected.

[0038] Solvent system: A highly stable solvent system compatible with the first electrolyte is adopted, such as ethylene carbonate (EC), 2,2,2-trifluoroethyl phosphate (TFEP), methyl ethyl carbonate, etc., to further ensure the stability of ion conduction during long-term cycling.

[0039] Functional additives: The addition of flame retardant additives such as phosphate esters and fluorinated solvents significantly improves the overall thermal stability and flame resistance of the electrolyte, preventing thermal runaway under high temperature or short circuit conditions.

[0040] The formula for the second electrolyte is as follows: The concentration range of sodium salt is 6-15 wt%, the concentration range of additives is 2-10 wt%, and the balance is solvent.

[0041] 3. Secondary injection process, such as... Figure 1 As shown: First electrolyte injection: The first electrolyte is injected before formation to form a high-quality SEI film during the formation process.

[0042] Secondary electrolyte injection: After aging, a second electrolyte is injected to repair electrolyte loss that may have been caused by the previous reaction. At the same time, flame retardant additives are injected to improve overall thermal stability and safety.

[0043] Through the above-described combined structure and process, the present invention can ensure that the SEI film is dense and stable, and that the electrolyte maintains good physicochemical properties under high temperature and long-term cycling, effectively extending the battery life and enhancing safety performance. The sodium-ion battery prepared based on the present invention has excellent thermal stability, interface stability and cycle life, and is suitable for high-requirement scenarios such as energy storage power stations and power batteries.

[0044] Compared with traditional sodium-ion battery electrolyte technology, this invention achieves significant improvements in the following key aspects: 1. Solving the problem of sodium salt decomposition: Existing technologies use NaPF6 as the main sodium salt, which is prone to decomposition under high temperature and high pressure conditions to generate corrosive byproducts such as PF5 and HF, causing SEI film instability, electrode interface damage, and ultimately reducing battery cycle life.

[0045] This invention uses NaFSi, which has better thermal stability, higher decomposition temperature, and less tendency to generate by-products, thus significantly improving the stability of the electrolyte.

[0046] 2. Improve battery safety: The organic solvents used in traditional electrolytes are highly volatile and flammable at high temperatures, which can easily lead to thermal runaway.

[0047] This invention introduces a second electrolyte containing flame retardants after the aging stage through a secondary electrolyte injection method, which can significantly reduce the flammability of the electrolyte and improve the safety performance of the battery under extreme operating conditions.

[0048] 3. Stable electrode interface structure: In existing technologies, SEI and CEI films are often unstable and prone to rupture and regeneration during cycling, leading to increased interfacial impedance and rapid capacity decay of the battery.

[0049] This invention utilizes the synergistic effect of functional additives in the first electrolyte to form a dense and stable SEI film in the early stages of formation. At the same time, the second electrolyte replenishes interfacial components and provides thermal stability, effectively preventing interfacial structure degradation.

[0050] 4. Process optimization and improved cycle performance: Traditional processes involve a single electrolyte injection, making it difficult to control the quality of the SEI membrane, and the electrolyte is prone to performance degradation due to reaction losses.

[0051] The secondary electrolyte injection process of this invention can effectively control the formation quality of the SEI film and maintain the stability of the system by replenishing the electrolyte in the later stage, thus ensuring the high capacity retention rate of the battery during long-term use.

[0052] Example 1 1) Preparation of the first electrolyte: Non-aqueous organic solvents: propylene carbonate (PC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) are mixed in a mass ratio of 3:4:3 to provide an electrolyte solvent system with high dielectric constant and appropriate viscosity.

[0053] Sodium salt: Sodium difluorosulfonamide (NaFSi), with a concentration of 1.2 mol / L, is used as the main salt of the first electrolyte.

[0054] additive: Additive A: Fluorinated ethylene carbonate (FEC), 5 wt%. FEC preferentially forms a NaF-rich solid electrolyte membrane on the negative electrode surface during the formation stage, improving the stability of the solid electrolyte membrane.

[0055] Additive B: Vinyl sulfate (DTD), 1 wt%. DTD reacts with Na... + Strong coordination effect, preferentially enters Na + The solvation of the sheath weakens Na + The interaction with highly polar solvents lowers the desolvation energy barrier. This modulation allows Na... + It is easier to extract from the solvent and embed into the electrode material, suppressing solvent co-intercalation. DTD and FEC synergistically optimize the interface film in sodium-ion batteries. FEC provides a flexible organic layer, while DTD contributes a rigid inorganic layer. The combination of the two reduces polarization and improves cycle stability.

[0056] Additive C: difluoroacetic anhydride, CAS: 401-67-2, added at 2wt%. The high bond energy of the CF bond endows the compound with stronger antioxidant and thermal stability. When used in conjunction with FEC and DTD, it optimizes the ion transport pathway, resulting in higher stability of the solid electrolyte membrane in the cell during the formation stage.

[0057] Preparation method of the first electrolyte: This electrolyte needs to be prepared in an argon glove box atmosphere (H2O < 10 ppm). The main steps are as follows: 1. First, mix the organic solvent evenly and then freeze at -20°C for 12 hours.

[0058] 2. Slowly add NaFSi and stir until completely dissolved.

[0059] 3. Add additives A, B and C in sequence, and continue stirring for 2 hours to obtain a uniform electrolyte.

[0060] 4. This electrolyte needs to be stored in a sealed environment at a temperature below 10℃.

[0061] 2) Preparation of the second electrolyte: Non-aqueous organic solvents: Ethylene carbonate (EC), 2,2,2-trifluoroethyl phosphate (TFEP), and ethyl methyl carbonate (EMC) are mixed in a mass ratio of 3:4:3. Using TFEP as a co-solvent, the directional binding effect of anions and cations is enhanced under lower salt concentration conditions by regulating the interaction between them in the electrolyte, thereby improving the high-temperature stability and safety of the electrolyte.

[0062] Sodium salt: Sodium difluorosulfonamide (NaFSi), with a concentration of 1 mol / L, is used as the main salt of the second electrolyte.

[0063] additive: Additive D: Pentafluoro(phenoxy)cyclotriphosphazene, 1 wt%. On the current collector surface, pentafluoro(phenoxy)cyclotriphosphazene interacts with the FEC of the first electrolyte. Pentafluoro(phenoxy)cyclotriphosphazene provides a rigid inorganic layer, while the FEC provides a flexible organic layer, jointly optimizing the interfacial film structure, reducing polarization, and improving cycle stability. Furthermore, this additive complexes free aluminum ions through fluorine groups, preventing their damage to the negative electrode SEI film, thereby improving battery safety. Pentafluoro(phenoxy)cyclotriphosphazene, through flame retardancy, interface optimization, and solvation regulation, has become a key additive for improving battery energy density, safety, and cycle life.

[0064] Since the second electrolyte does not participate in the reaction during the formation stage, its components will not be present in the SEI during the formation stage, thus maximizing its flame-retardant properties.

[0065] The second electrolyte preparation method: This electrolyte needs to be prepared in an argon glove box atmosphere (H2O < 10 ppm). The main steps are as follows: 1. First, mix the solvent evenly and then freeze at -20°C for 12 hours.

[0066] 2. Slowly add NaFSi and stir until completely dissolved.

[0067] 3. Add additive D and continue stirring for 2 hours to obtain a uniform electrolyte.

[0068] 4. This electrolyte needs to be stored in a sealed environment at a temperature below 10℃.

[0069] Test Example 1 The first electrolyte and the second electrolyte prepared in Example 1 were injected sequentially at a mass ratio of 80:20, wherein the first electrolyte was injected before formation and the second electrolyte was injected after aging.

[0070] The battery cell used was a 4A·h soft-pack battery cell, with an electrolyte injection volume of 30g. The test environment was 25℃, with 1C / 1C cycles. The test results are shown in Table 1.

[0071] Furthermore, the battery constructed in this test case was subjected to a needle penetration thermal runaway test (8mm steel needle) when fully charged, and it did not catch fire, demonstrating extremely high safety.

[0072] Test Example 2 The first electrolyte and the second electrolyte prepared in Example 1 were injected sequentially at a mass ratio of 70:30, wherein the first electrolyte was injected before formation and the second electrolyte was injected after aging.

[0073] The battery cell used was a 4A·h soft-pack battery cell, with an electrolyte injection volume of 30g. The test environment was 25℃, with 1C / 1C cycles. The test results are shown in Table 1.

[0074] Comparative Test Example 1 The first electrolyte and the second electrolyte prepared in Example 1 were injected sequentially at a mass ratio of 90:10, wherein the first electrolyte was injected before formation and the second electrolyte was injected after aging.

[0075] The battery cell used was a 4A·h soft-pack battery cell, with an electrolyte injection volume of 30g. The test environment was 25℃, with 1C / 1C cycles. The test results are shown in Table 1.

[0076] Comparative Test Example 2 Only the first electrolyte prepared in Example 1 is injected, and the first electrolyte is injected before formation.

[0077] The battery cell used was a 4A·h soft-pack battery cell, with an electrolyte injection volume of 30g. The test environment was 25℃, with 1C / 1C cycles. The test results are shown in Table 1.

[0078] Comparative Test Case 3 Only the second electrolyte prepared in Example 1 was injected, and the second electrolyte was injected after aging.

[0079] The battery cell used was a 4A·h soft-pack battery cell, with an electrolyte injection volume of 30g. The test environment was 25℃, with 1C / 1C cycles. The test results are shown in Table 1.

[0080] Table 1. Summary of results for Test Cases 1 and 2 and Comparative Test Cases 1-3 Comparative Test Example 4 The first electrolyte and the second electrolyte prepared in Example 1 were injected together in a single batch before formation at a mass ratio of 80:20.

[0081] The battery cell used was a 4A·h soft-pack battery cell, with an electrolyte injection volume of 30g. The test environment was 25℃, with 1C / 1C cycles. The test results are shown in Table 2.

[0082] Comparative Test Example 5 The first electrolyte and the second electrolyte prepared in Example 1 were injected together in a 70:30 mass ratio before formation.

[0083] The battery cell used was a 4A·h soft-pack battery cell, with an electrolyte injection volume of 30g. The test environment was 25℃, with 1C / 1C cycles. The test results are shown in Table 2.

[0084] Comparative Test Case 6 The first electrolyte and the second electrolyte prepared in Example 1 were injected together in a 90:10 mass ratio before formation.

[0085] The battery cell used was a 4A·h soft-pack battery cell, with an electrolyte injection volume of 30g. The test environment was 25℃, with 1C / 1C cycles. The test results are shown in Table 2.

[0086] Table 2 Summary of results from comparative tests 4-6 Comparative Test Example 7 The first electrolyte and the second electrolyte prepared in Example 1 were injected sequentially at a mass ratio of 80:20, wherein the second electrolyte was injected before formation and the first electrolyte was injected after aging.

[0087] The battery cell used was a 4A·h soft-pack battery cell, with an electrolyte injection volume of 30g. The test environment was 25℃, with 1C / 1C cycles. The test results are shown in Table 3.

[0088] Comparative Test Example 8 The first electrolyte and the second electrolyte prepared in Example 1 were injected sequentially at a mass ratio of 70:30, wherein the second electrolyte was injected before formation and the first electrolyte was injected after aging.

[0089] The battery cell used was a 4A·h soft-pack battery cell, with an electrolyte injection volume of 30g. The test environment was 25℃, with 1C / 1C cycles. The test results are shown in Table 3.

[0090] Comparative Test Example 9 The first electrolyte and the second electrolyte prepared in Example 1 were injected sequentially at a mass ratio of 90:10, wherein the second electrolyte was injected before formation and the first electrolyte was injected after aging.

[0091] The battery cell used was a 4A·h soft-pack battery cell, with an electrolyte injection volume of 30g. The test environment was 25℃, with 1C / 1C cycles. The test results are shown in Table 3.

[0092] Table 3 Summary of results from comparative test cases 7-9 In summary, the first electrolyte and second electrolyte configured in this scheme, along with a staged electrolyte injection process—adding the first electrolyte before formation and the second electrolyte after aging—can create a sodium-ion battery electrolyte suitable for high safety, high thermal stability, high interfacial stability, and excellent suppression of side reactions. Specifically, formation after a single electrolyte injection effectively improves the density and stability of the solid electrolyte film formed during the cell formation stage; the second electrolyte injection further enhances the high-temperature stability and safety performance of the electrolyte. Since the first and second electrolytes have different functions, their injection order cannot be interchanged.

[0093] 1. This invention innovatively introduces additive C (difluoroacetic anhydride) into the first electrolyte, and works in conjunction with other additives to improve the high-temperature and high-voltage stability of the solid electrolyte membrane of the electrolyte.

[0094] If additive C is lacking in the first electrolyte, the stability of the SEI formed during the formation stage will be reduced; and it will be difficult to work together with the TFEP solvent and additive D (pentafluoro(phenoxy)cyclotriphosphazene) in the second electrolyte, increasing the risk of thermal runaway of the battery.

[0095] 2. The second electrolyte of this invention uses TFEP as a solvent, which minimizes charge transfer resistance and, to some extent, inhibits the decomposition of salt and solvent, reducing battery polarization. From the perspectives of solid electrolyte membrane and solvent, this invention comprehensively addresses the risk of thermal runaway in batteries.

[0096] 3. In this invention, the additive D introduced into the second electrolyte can regulate the flammability of the electrolyte and can immediately reduce the internal short-circuit current of the battery during processes such as puncture and compression. Through its combined action with the additive C in the first electrolyte and TFEP in the second electrolyte, the thermal stability of the battery is improved and the risk of thermal runaway of the battery is reduced.

[0097] If additive D is used in the first electrolyte, it will participate in the formation stage after the first electrolyte is injected. After the battery is formed, the impedance is likely to be too high, which will reduce the cycle performance of the battery.

[0098] 4. Sodium-ion batteries prepared using this electrolyte and the secondary electrolyte injection process exhibit excellent cycle performance and high-temperature stability. After 1500 cycles at 25°C, the capacity retention exceeds 90%, and after 1500 cycles at 45°C, the capacity retention exceeds 80%. Figure 2 As shown, the test conditions are the same as in Test Example 1; this provides key technical support for the development of a new generation of high-performance sodium-ion batteries.

[0099] Therefore, this invention provides a highly safe sodium-ion battery electrolyte with long-term cycling capability, the main contents of which are as follows: 1. By inventing a sodium-ion battery electrolyte for secondary electrolyte injection, the long-cycle performance and high-temperature stability of sodium-ion batteries are improved. This electrolyte comprises a first electrolyte and a second electrolyte, wherein the first electrolyte is injected during the formation stage, and the second electrolyte is injected after aging.

[0100] 2. The first electrolyte uses a highly stable sodium salt system (sodium bis(fluorosulfonyl)imide, NaFSi), which effectively avoids the decomposition defect of NaPF6 under high temperature and high pressure, and improves the stability of the electrolyte.

[0101] 3. The first electrolyte, through the combination of solvent, sodium salt and functional additives, can form a dense solid electrolyte film during the formation stage, thereby improving the interfacial stability and cycle performance of sodium-ion batteries.

[0102] 4. The second electrolyte, through a combination of highly stable solvents, sodium salt (NaFSi), and flame-retardant additives, can effectively improve the high-temperature stability and safety performance of sodium-ion batteries.

[0103] 5. By using a secondary electrolyte injection process, the density of the solid electrolyte membrane during the formation process is optimized, and flame-retardant electrolyte is injected a second time, which ensures the stability of the battery interface and further improves its safety.

[0104] Based on this main content, the present invention has the following main technical effects: 1. This electrolyte has good stability and its structure is more stable at high temperatures, avoiding the formation of highly corrosive byproducts such as hydrofluoric acid, which would lead to deterioration of the electrode interface.

[0105] 2. The sodium-ion battery prepared using this electrolyte and the secondary electrolyte injection process has excellent cycle performance and high temperature stability. After 1,500 cycles at room temperature of 25°C, the capacity retention rate exceeds 90%, and after 1,500 cycles at high temperature of 45°C, the capacity retention rate exceeds 80%.

[0106] 3. The second electrolyte solvent system is adjusted to improve the overall stability of the electrolyte, which can be used in high-temperature environments up to 80°C. When used together with the second electrolyte additive, it can prevent a fully charged battery from catching fire when passing a nail penetration test, thus significantly improving safety.

[0107] 4. The combined action of FEC, DTD, and additive C results in a dense and uniform solid electrolyte membrane that effectively inhibits electrolyte decomposition and electrode corrosion, with an impedance increase of less than 20% during long-term cycling.

[0108] 5. The second electrolyte uses pentafluoro(phenoxy)cyclotriphosphazene as an additive, which significantly reduces the flammability of the electrolyte. The phosphorus-containing free radicals and flammable gases produced by its decomposition can dilute oxygen and block the combustion chain reaction at high temperatures. In addition, the additive can work together with the FEC of the first electrolyte to optimize the interfacial film structure, reduce polarization and improve cycle stability.

[0109] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A secondary liquid injection electrolyte for a sodium-ion battery, characterized by, Comprising: a first electrolyte for pre-formation, and a second electrolyte for post-aging; The first electrolyte comprises: a sodium salt, a first non-aqueous organic solvent, and a first functional additive; The second electrolyte comprises: a sodium salt, a second non-aqueous organic solvent, and a second functional additive; Among them, the sodium salt is sodium bisfluorosulfonylimide; the first non-aqueous organic solvent is an organic carbonate solvent, the first functional additive is used to promote the formation of SEI in the formation process; the second non-aqueous organic solvent is a solvent system compatible with the first electrolyte, and the second functional additive is a flame-retardant additive.

2. The secondary injection electrolyte of the sodium ion battery according to claim 1, wherein In the first electrolyte: the concentration of the sodium salt is 6-15 wt%, the concentration of the first functional additive is 1.2-16 wt%, and the balance is the first non-aqueous organic solvent; In the second electrolyte: the concentration of the sodium salt is 6-15 wt%, the concentration of the second functional additive is 2-10 wt%, and the balance is the second non-aqueous organic solvent.

3. The secondary injection electrolyte of the sodium ion battery according to claim 1, wherein The first non-aqueous organic solvent is an electrolyte solvent system formed by mixing propylene carbonate, diethyl carbonate and methyl ethyl carbonate in a mass ratio of 3:4:3; The second non-aqueous organic solvent is an electrolyte solvent system formed by mixing ethylene carbonate, 2,2,2-trifluoroethyl phosphate and methyl ethyl carbonate in a mass ratio of 3:4:

3.

4. The secondary injection electrolyte of the sodium ion battery according to claim 1, wherein The first functional additive comprises one or more of fluoroethylene carbonate, difluoroacetic anhydride, sulfonate additive and sulfatide additive; The second functional additive comprises one or more of phosphoric acid ester flame-retardant additive and fluorinated solvent flame-retardant additive.

5. The secondary injection electrolyte of the sodium ion battery according to claim 1, wherein The first functional additive is a mixed additive of fluoroethylene carbonate, ethylene sulfate and difluoroacetic anhydride having any one of the structures shown in formula (I) to formula (IV); formula (I), formula (II), formula (III), formula (IV); The second functional additive is pentafluoro(phenoxy)cyclotriphosphazene.

6. The secondary injection electrolyte of the sodium ion battery according to claim 5, wherein In the first functional additive: the addition amount of fluoroethylene carbonate in the first electrolyte is 5 wt%, the addition amount of ethylene sulfate in the first electrolyte is 1 wt%, and the addition amount of difluoroacetic anhydride in the first electrolyte is 2 wt%; In the second functional additive: the addition amount of pentafluoro(phenoxy)cyclotriphosphazene in the second electrolyte is 1 wt%.

7. A method for preparing a secondary injection electrolyte for a sodium-ion battery according to any one of claims 1 to 6, characterized in that, The preparation of the first electrolyte is carried out in a glove box environment: S1: Configure the first non-aqueous organic solvent, mix uniformly and freeze; S2: Add sodium salt to the frozen first non-aqueous organic solvent and stir until completely dissolved; ​ S3: adding a first functional additive into the first non-aqueous organic solvent in which the sodium salt is dissolved, continuously stirring to obtain a first electrolyte; The preparation of the second electrolyte is carried out in a glove box environment: T1: configuring a second non-aqueous organic solvent, and freezing after uniform mixing; T2: adding a sodium salt into the frozen second non-aqueous organic solvent and stirring until completely dissolved; T3: adding a second functional additive into the second non-aqueous organic solvent in which the sodium salt is dissolved, continuously stirring to obtain a second electrolyte.

8. The method according to claim 7, wherein the method is characterized by, The glove box environment is an argon atmosphere with a H2O content of less than 10 ppm; the freezing is carried out at-20℃ for 12 hours; and the continuously stirring time is 2 hours.

9. The liquid injection process of the sodium-ion battery secondary liquid injection electrolyte according to any one of claims 1-6, characterized in that, The method comprises the following steps: injecting a first electrolyte before formation of the sodium ion battery; injecting a second electrolyte after aging of the sodium ion battery. 10.The liquid injection process of a secondary injection electrolyte of a sodium-ion battery according to claim 9, characterized in that, The mass ratio of the first electrolyte to the second electrolyte is 8:2-7:3.

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