A sodium-ion battery electrolyte, its preparation method and application

Abstract

This application belongs to the field of battery technology, specifically relating to a sodium-ion battery electrolyte and its application. The sodium-ion battery electrolyte includes sodium salt, organic solvent, and additives; the sodium salt includes sodium hexafluorophosphate and sodium cyanoborate. The beneficial effects of this application include: the highest occupied molecular orbital (HOMO) of the nitrile sodium salt in the electrolyte of this application is lower than that of the carbon-oxygen bond, which is beneficial to improving the oxidation stability of the sodium salt. Simultaneously, the lowest unoccupied molecular orbital (LUMO) is also lower than that of the carbon-oxygen bond, allowing it to preferentially oxidize on the positive electrode surface to form a CEI film and reduce on the negative electrode surface to form an SEI film. The fluorine-containing groups in the sodium salt can construct a NaF-rich CEI / SEI film, optimizing the initial efficiency and cycle stability of the battery. At the same time, the cyano group (-CN) can promote the reduction and decomposition of the sodium salt, solving the problem that the SEI film formed in sodium-ion batteries has a higher impedance than that in lithium-ion batteries, effectively enhancing the cycle stability of the battery.

Description

Technical Field

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

[0002] In recent years, sodium-ion batteries have received increasing attention due to rising demand for renewable energy storage and concerns about lithium supply costs. Sodium-ion batteries, with their low cost, high safety, and excellent high and low temperature performance, have become a powerful complement to lithium-ion batteries, leading to continuous market demand growth. However, compared to lithium-ion batteries, sodium-ion batteries still face challenges such as low energy density and short cycle life. Therefore, developing novel sodium-ion battery electrolytes is a key research direction for improving the performance and stability of sodium-ion batteries.

[0003] One approach to developing electrolytes for sodium-ion batteries is to use salts and solvents that can improve battery performance and stability. Another approach is to use additives or coatings to improve existing battery performance. Sodium salts, as a crucial component of battery electrolytes, must not only provide abundant sodium ions but also promote the formation of a stable electrode-electrolyte interface film, which directly affects the cycle life and safety performance of sodium-ion batteries.

[0004] The most commonly used inorganic sodium salt is sodium hexafluorophosphate (NaPF6). This sodium salt has excellent overall performance, is easily dissociated, and has good compatibility with carbon-based anodes and various cathode materials. However, it has poor chemical stability, decomposing to equilibrium in organic solvents to generate NaF and PF5, which affect the interfacial film composition. PF5 readily reacts with trace amounts of water in the solvent to produce HF gas. Furthermore, this sodium salt has poor thermal stability, and high temperatures accelerate its decomposition. Therefore, researching and finding suitable sodium salts is a key focus in sodium-ion battery development. Summary of the Invention

[0005] This application provides a sodium-ion battery electrolyte, its preparation method, and its application, aiming to solve the problems of poor cycle stability, high gas production during high-temperature storage, and low energy density of existing sodium-ion batteries.

[0006] The first aspect of this application provides a sodium-ion battery electrolyte, which includes a sodium salt, an organic solvent, and additives; the sodium salt includes sodium hexafluorophosphate and sodium cyanoborate.

[0007] The sodium nitrile salts in the electrolyte of this application contain a highest occupied molecular orbital (HOMO) lower than that of carbon-oxygen bonds, which is beneficial to improving the oxidation stability of sodium salts. At the same time, the lowest unoccupied molecular orbital (LUMO) is also lower than that of carbon-oxygen bonds, so that they preferentially oxidize to form a CEI film on the positive electrode surface and reduce to form an SEI film on the negative electrode surface. The fluorine-containing groups in the sodium salts can construct a NaF-rich CEI / SEI film, optimizing the initial efficiency and cycle stability of the battery.

[0008] Meanwhile, the cyano group (-CN) can promote the reduction and decomposition of sodium salt, thereby reducing the deposition of BO and oxygen-containing ions and byproducts on the surface of the SEI film, forming a uniform, dense and thin SEI film. Because the binding force between the anion and Na ion in sodium cyanoborate is relatively strong, it reduces the combination of Na ions with other organic functional groups, forming an SEI that is mainly organic and rich in inorganic matter, increasing the proportion of inorganic matter in the SEI film, thereby reducing the impedance of the SEI film and improving its flexibility. This solves the problem that the impedance of the SEI film formed in sodium-ion batteries is larger than that of the SEI film in lithium batteries, and effectively enhances the cycle stability of the battery.

[0009] According to some embodiments of the sodium-ion battery electrolyte described in this application, the structural formula of the sodium cyanoborate-containing sodium is shown below:

[0010]

[0011] Wherein: R1 and R2 are each independently selected from substituted or unsubstituted C1-C20 alkyl, alkoxy, alkeneoxy, acyl, ether, isocyanate, phenylamino or amino, halogen; the substituents used in the substitution include halogens.

[0012] The substituted or unsubstituted C1-C20 alkyl, alkoxy, acyl, ether, isocyanate, phenylamino, and amino groups refer to C1-C20 alkyl, C1-C20 alkoxy, C1-C20 acyl, C1-C20 ether, C1-C20 isocyanate, C1-C20 phenylamino, C1-C20 amino, halogen-substituted C1-C20 alkyl, halogen-substituted C1-C20 alkoxy, halogen-substituted C1-C20 acyl, halogen-substituted C1-C20 ether, halogen-substituted C1-C20 isocyanate, halogen-substituted C1-C20 phenylamino, and halogen-substituted C1-C20 amino groups.

[0013] The electrolyte described in this application contains sodium hexafluorophosphate and sodium cyanoborate sodium salt. The sodium cyanoborate sodium salt contains a cyano group (-CN) with a triple bond structure. This structure can polymerize on the positive electrode surface to form a CEI film with good oxidation resistance and permeability. The CEI film in the positive electrode electrolyte intermediate phase can isolate the positive electrode from the electrolyte, inhibit electrolyte decomposition and prevent the dissolution of positive electrode metal, thus improving the battery cycle performance.

[0014] According to some embodiments of the sodium-ion battery electrolyte described in this application, the structural formula of the sodium cyanoborate-containing sodium is shown below:

[0015]

[0016] According to some embodiments of the sodium-ion battery electrolyte described in this application, the organic solvent includes carbonate-based organic solvents.

[0017] According to some embodiments of the sodium-ion battery electrolyte described in this application, the carbonate-based organic solvent includes one or more of ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, and propylene carbonate.

[0018] According to some embodiments of the sodium-ion battery electrolyte described in this application, the additives include one or more of sodium tetrafluoroborate, sodium nitrate, vinylene carbonate, fluoroethylene carbonate, trans-difluoroethylene carbonate, vinyl sulfite, 1,3-propanesulfonate lactone, succinate, propenyl-1,3-sulfonate lactone, vinyl sulfate, ethoxy(pentafluoro)cyclotriphosphazene, methyl nonafluorobutyl ether, tris(2,2,2-trifluoroethyl) phosphite, and biphenyl.

[0019] According to some embodiments of the sodium-ion battery electrolyte described in this application, the additives include one or more of sodium tetrafluoroborate, vinylene carbonate, and fluoroethylene carbonate.

[0020] According to some embodiments of the sodium-ion battery electrolyte described in this application, the electrolyte comprises the following raw materials by mass percentage: sodium salt 11%-21%, organic solvent 69-87%, and additives 2%-10%.

[0021] According to some embodiments of the sodium-ion battery electrolyte described in this application, the electrolyte comprises the following raw materials in weight percentages: 15%-20% sodium salt, 69%-87% organic solvent, and 2%-10% additives.

[0022] According to some embodiments of the sodium-ion battery electrolyte described in this application, the mass ratio of sodium hexafluorophosphate to sodium cyanoborate in the sodium salt is (2.8-4):2.

[0023] A second aspect of this application provides a sodium-ion battery, the sodium-ion battery comprising the sodium-ion battery electrolyte described in the first aspect of this application.

[0024] Because the electrolyte described in this application can promote the formation of stable and uniform SEI and CEI films, it can improve the cycle stability and impedance of sodium-ion batteries.

[0025] According to some embodiments of the sodium-ion battery described in this application, the sodium-ion battery further includes a positive electrode, a negative electrode, and a separator.

[0026] According to some embodiments of the sodium-ion battery described in this application, the positive electrode comprises a layered oxide active material.

[0027] According to some embodiments of the sodium-ion battery described in this application, the negative electrode sheet comprises a carbon-based negative electrode active material.

[0028] According to some embodiments of the sodium-ion battery described in this application, the layered oxide active material includes NaMO2, wherein M is selected from one or more of Fe, Ni, Co, Mn, Cu, Cr, Al, Mg, Ti, Li and Zn.

[0029] According to some embodiments of the sodium-ion battery described in this application, the negative electrode active material includes one or more of graphene, graphene oxide, soft carbon, hard carbon, expanded graphite, silicon, and mesophase carbon microspheres.

[0030] According to some embodiments of the sodium-ion battery described in this application, the layered oxide active material includes NaCu. 1 / 9 Ni 2 / 9 Fe 1 / 3 Zn 1 / 3 O2 active substances.

[0031] According to some embodiments of the sodium-ion battery described in this application, the negative electrode active material includes hard carbon.

[0032] According to some embodiments of the sodium-ion battery described in this application, the sodium-ion battery is a sodium-ion layered oxide battery.

[0033] According to some embodiments of the sodium-ion battery described in this application, the application temperature of the sodium-ion battery is -40°C to 60°C. Detailed Implementation

[0034] The embodiments of the present invention are described in detail below. These embodiments are exemplary and intended to explain the present invention, and should not be construed as limiting the invention.

[0035] In this invention, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0036] This application provides a sodium-ion battery electrolyte, which includes sodium salt, organic solvent, and additives; the sodium salt includes sodium hexafluorophosphate and sodium cyanoborate.

[0037] The sodium nitrile salts in the electrolyte of this application contain a highest occupied molecular orbital (HOMO) lower than that of carbon-oxygen bonds, which is beneficial to improving the oxidation stability of sodium salts. At the same time, the lowest unoccupied molecular orbital (LUMO) is also lower than that of carbon-oxygen bonds, so that they preferentially oxidize to form a CEI film on the positive electrode surface and reduce to form an SEI film on the negative electrode surface. The fluorine-containing groups in the sodium salts can construct a NaF-rich CEI / SEI film, optimizing the initial efficiency and cycle stability of the battery.

[0038] The electrolyte described in this application contains sodium hexafluorophosphate and sodium cyanoborate sodium salt. The sodium cyanoborate sodium salt contains a cyano group (-CN) with a triple bond structure. This structure can polymerize on the positive electrode surface to form a CEI film with good oxidation resistance and permeability. The CEI film in the positive electrode electrolyte intermediate phase can isolate the positive electrode from the electrolyte, inhibit electrolyte decomposition and prevent the dissolution of positive electrode metal, thus improving the battery cycle performance.

[0039] Meanwhile, the cyano group (-CN) can promote the reduction and decomposition of sodium salt, thereby reducing the deposition of BO and oxygen-containing ions and byproducts on the surface of the SEI film, forming a uniform, dense and thin SEI film. Because the binding force between the anion and Na ion in sodium cyanoborate is relatively strong, it reduces the combination of Na ions with other organic functional groups, forming an SEI that is mainly organic and rich in inorganic matter, increasing the proportion of inorganic matter in the SEI film, thereby reducing the impedance of the SEI film and improving its flexibility. This solves the problem that the impedance of the SEI film formed in sodium-ion batteries is larger than that of the SEI film in lithium batteries, and effectively enhances the cycle stability of the battery.

[0040] In some embodiments of this application, the structural formula of the sodium cyanoboronate is shown below:

[0041]

[0042] Wherein: R1 and R2 are each independently selected from substituted or unsubstituted C1-C20 alkyl, alkoxy, alkeneoxy, acyl, ether, isocyanate, phenylamino and amino groups, halogens; the substituents used in the substitution include halogens.

[0043] Introducing fluorine (F) into sodium cyanoborate can effectively reduce the viscosity of the electrolyte and improve its conductivity by reducing the defluorination capacity of the sodium salt due to the cyano group. It also has good film-forming properties, thus improving the performance of sodium-ion batteries.

[0044] When R1 and R2 are independently haloalkyl, haloalkoxy, alkyl, or alkoxy groups, they can synergistically alter the Na+ content in the electrolyte with fluoroethylene carbonate (FEC). + The coordination environment promotes the formation of organic SEI rich in sodium fluoride, sodium alkyl, and sodium alkoxy on the negative electrode surface, generating a uniform, dense, and low-resistance SEI film, which can reduce sodium ion consumption, thereby improving the battery's initial efficiency and reducing the cell's internal resistance.

[0045] In some embodiments of this application, the structure of the sodium cyanoboronate is shown in the following formula:

[0046]

[0047] In some embodiments of this application, the synthetic reaction formulas of the compounds represented by structural formula (1) are shown below:

[0048]

[0049] In some embodiments of this application, the synthetic reaction formulas of the compounds represented by structural formula (2) are shown below:

[0050]

[0051] In some embodiments of this application, the organic solvent includes carbonate-based organic solvents.

[0052] In some embodiments of this application, the carbonate organic solvent includes one or more of ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, and propylene carbonate.

[0053] In some embodiments of this application, the additives include one or more of sodium tetrafluoroborate, sodium nitrate, vinylene carbonate, fluoroethylene carbonate, trans-difluoroethylene carbonate, vinyl sulfite, 1,3-propanesulfonate lactone, succinate, propylene-1,3-sulfonate lactone, vinyl sulfate, ethoxy(pentafluoro)cyclotriphosphazene, methyl nonafluorobutyl ether, tris(2,2,2-trifluoroethyl) phosphite, and biphenyl. The addition of the additives can preferentially undergo reduction (or oxidation) reactions with organic solvents and sodium salts to form a dense, uniform, and relatively thin SEI film (or CEI film) on the surface of the negative electrode (or positive electrode), thereby protecting the internal electrode material.

[0054] In some embodiments of this application, the additive includes one or more of sodium tetrafluoroborate, vinylene carbonate, and fluoroethylene carbonate.

[0055] In some embodiments of this application, the electrolyte comprises the following raw materials by mass percentage: 11%-21% sodium salt, 69%-87% organic solvent, and 2%-10% additives.

[0056] In some embodiments of this application, the electrolyte comprises the following raw materials in weight percentages: 15%-20% sodium salt, 69%-87% organic solvent, and 2%-10% additives.

[0057] In some embodiments of this application, the mass ratio of sodium hexafluorophosphate to sodium cyanoborate in the sodium salt is (2.8-4):2, for example, 2.8:2, 3:2, 3.6:2, 4:2, etc. The sodium salt is mainly composed of sodium hexafluorophosphate, with sodium cyanoborate partially substituted. The optimal mass ratio, verified by subsequent experiments, is 3:2.

[0058] This application also provides a sodium-ion battery, including the sodium-ion battery electrolyte described in the first aspect of this application.

[0059] Because the electrolyte described in this application can promote the formation of stable and uniform SEI and CEI films, it can improve the cycle stability and impedance of sodium-ion batteries.

[0060] In some embodiments of this application, the sodium-ion battery further includes a positive electrode, a negative electrode, and a separator.

[0061] In some embodiments of this application, the positive electrode sheet comprises a layered oxide active material.

[0062] In some embodiments of this application, the layered oxide active material includes NaMO2, where M is selected from one or more of Fe, Ni, Co, Mn, Cu, Cr, Al, Mg, Ti, Li, and Zn. In some preferred embodiments, the layered oxide active material is NaCu.1 / 9 Ni 2 / 9 Fe 1 / 3 Zn 1 / 3 O2 active substances.

[0063] In some embodiments of this application, the negative electrode sheet comprises a carbon-based negative electrode active material, which includes one or more of graphene, graphene oxide, soft carbon, hard carbon, expanded graphite, and silicon. In some preferred embodiments, the negative electrode active material is hard carbon.

[0064] In some embodiments of this application, the sodium-ion battery is a sodium-ion layered oxide battery.

[0065] In some embodiments of this application, the sodium-ion battery is used at temperatures ranging from -40°C to 60°C, such as -40°C, -35°C, -30°C, -28°C, -20°C, -10°C, 0°C, 15°C, 20°C, 28°C, 35°C, 40°C, 48°C, 55°C, and 60°C. The sodium-ion battery described in this application exhibits good electrochemical performance at both high and low temperatures.

[0066] The technical solution of this application will be described in detail below with reference to specific embodiments.

[0067] Example 1

[0068] A sodium-ion battery electrolyte, comprising, by mass percentage, the following raw materials:

[0069] Sodium salt 18.5%, organic solvent 76.8%, and additives 4.7%;

[0070] The organic solvent is a mixture of ethylene carbonate, propylene carbonate, dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate, with a mass ratio of ethylene carbonate: propylene carbonate: dimethyl carbonate: methyl ethyl carbonate: diethyl carbonate of 45:30:11:8:6.

[0071] The additive is a mixture of vinylene carbonate, fluoroethylene carbonate, propylene-1,3-sulfonyl lactone, and vinyl sulfate, with a mass ratio of 1:2.5:1.5:1.

[0072] The sodium salt includes sodium hexafluorophosphate and sodium difluoro(1,2-dihydroxyethane-1,1,2,2-tetracyano)borate, with a mass ratio of sodium hexafluorophosphate to sodium difluoro(1,2-dihydroxyethane-1,1,2,2-tetracyano)borate of 3:2.

[0073] The structural formula of sodium difluoro(1,2-dihydroxyethane-1,1,2,2-tetracyano)borate is:

[0074]

[0075] The preparation method of the sodium-ion battery electrolyte described in Example 1 includes the following steps: mixing the raw materials according to the above mass ratio under the conditions of an ambient temperature of 20°C, an ambient humidity of ≤1%, an oxygen content of ≤1ppm, and a water content of ≤0.1ppm.

[0076] Example 2

[0077] The sodium-ion battery electrolyte described in Example 2 differs from that described in Example 1 only in the content of sodium salt, organic solvent, and additives.

[0078] Specifically, the sodium-ion battery electrolyte described in Example 2 comprises, by mass percentage, the following raw materials: 17.5% sodium salt, 77.8% organic solvent, and 4.7% additives.

[0079] Example 3

[0080] The sodium-ion battery electrolyte described in Example 3 differs from that described in Example 1 only in the content of sodium salt, organic solvent, and additives.

[0081] Specifically, the sodium-ion battery electrolyte described in Example 3 comprises, by mass percentage, the following raw materials: 19.5% sodium salt, 75.8% organic solvent, and 4.7% additives.

[0082] Example 4

[0083] The only difference between the sodium-ion battery electrolyte in Example 4 and the sodium-ion battery electrolyte in Example 1 is that sodium 1,2-dihydroxyethane-1,1,2,2-tetracyanoxazoborate is used instead of sodium difluoro(1,2-dihydroxyethane-1,1,2,2-tetracyano)borate.

[0084] The structural formula of sodium 1,2-dihydroxyethane-1,1,2,2-tetracyanoxarate-borate is shown below:

[0085]

[0086] Example 5

[0087] The sodium-ion battery electrolyte described in Example 5 differs from that described in Example 1 only in that the mass ratio of sodium hexafluorophosphate to sodium difluoro(1,2-dihydroxyethane-1,1,2,2-tetracyano)borate in the sodium salt is 3:3.

[0088] Example 6

[0089] The sodium-ion battery electrolyte described in Example 6 differs from that described in Example 1 only in that the mass ratio of sodium hexafluorophosphate to sodium difluoro(1,2-dihydroxyethane-1,1,2,2-tetracyano)borate in the sodium salt is 3:1.

[0090] Comparative Example 1

[0091] The only difference between the sodium-ion battery electrolyte described in Comparative Example 1 and the sodium-ion battery electrolyte described in Example 1 is that...

[0092] In Comparative Example 1, sodium difluorooxalate borate was used instead of sodium difluoro(1,2-dihydroxyethane-1,1,2,2-tetracyano)borate in the sodium-ion battery electrolyte.

[0093] Comparative Example 2

[0094] The only difference between the sodium-ion battery electrolyte described in Comparative Example 2 and the sodium-ion battery electrolyte described in Example 1 is that...

[0095] In Comparative Example 2, sodium tetrafluoroborate was used instead of sodium difluoro(1,2-dihydroxyethane-1,1,2,2-tetracyano)borate in the sodium-ion battery electrolyte.

[0096] Comparative Example 3

[0097] The only difference between the sodium-ion battery electrolyte described in Comparative Example 3 and the sodium-ion battery electrolyte described in Example 1 is that...

[0098] The sodium salt used in the sodium-ion battery electrolyte described in Comparative Example 3 is sodium hexafluorophosphate.

[0099] Comparative Example 4

[0100] The only difference between the sodium-ion battery electrolyte described in Comparative Example 4 and the sodium-ion battery electrolyte described in Example 1 is that...

[0101] The sodium salt used in the sodium-ion battery electrolyte described in Comparative Example 4 is sodium difluoro(1,2-dihydroxyethane-1,1,2,2-tetracyano)borate.

[0102] Performance study of the sodium-ion battery electrolytes described in Examples 1-6 and Comparative Examples 1-4 of this application:

[0103] Using the sodium-ion battery electrolytes described in Examples 1-6 and Comparative Examples 1-4, soft-pack 505060 sodium-ion batteries were fabricated using a soft-pack stacking process. Specifically, the process involved: NaCu... 1 / 9 Ni 2 / 9 Fe 1 / 3 Zn 1 / 3The O2 positive electrode, hard carbon negative electrode, and polyethylene separator are stacked and assembled. The positive electrode is baked until the moisture content is below 100 ppm, and the negative electrode is baked until the moisture content is below 200 ppm. They are then injected into the sodium-ion battery electrolytes described in Examples 1-6 and Comparative Examples 1-4, respectively. Hot pressing formation is carried out at a temperature of 45°C and a pressure of 425 kGF. After standing at 45°C for 24 hours, the electrolytes are packaged. After capacity testing, the batteries are left to stand at room temperature to obtain soft-pack 505060 finished batteries. The HF content of the electrolytes is tested, and the electrochemical performance of the batteries is tested.

[0104] Electrolyte HF content test

[0105] The method for testing HF content is as follows: After storing the sodium-ion battery electrolytes described in Examples 1-6 and Comparative Examples 1-4 at 60°C for 0D and 30D, respectively, the electrolytes were dissolved in ice water and titrated with a 0.01mol / L sodium hydroxide standard solution to obtain the HF content of the electrolytes after storage at 60°C for 0D and 30D (determined according to the method in 3.9.1 of T19282-2014). The results are shown in Table 1.

[0106] High-temperature storage gas generation performance test of the battery cell prepared with the electrolyte described in this application.

[0107] High-temperature storage gas production performance test method: The cells prepared by the sodium-ion battery electrolytes described in Examples 1-6 and Comparative Examples 1-4 were stored at 60°C for 0D and 30D. The thickness H1 of the cell before storage and the thickness H2 after storage were recorded. The cell gas production expansion rate was recorded as (H2-H1) / H1*100%. The results are shown in Table 1.

[0108] DC internal resistance (DCR) test of the battery prepared with the electrolyte described in this application:

[0109] DC internal resistance (DCR) test method: The batteries prepared by the sodium-ion battery electrolytes described in Examples 1-6 and Comparative Examples 1-4 were tested at 25°C on the battery testing system of the electrochemical workstation. The battery capacity was adjusted to 50% SOC and left to stand for 2 hours. The end voltage U1 of the stand was recorded. The batteries were discharged with a current I (2C) for 10 seconds and the end voltage U2 of the discharge was recorded. DCR = (U1-U2) / I. The test results are shown in Table 1.

[0110] Cycle performance testing of batteries prepared with the electrolyte described in this application:

[0111] Cyclic performance testing method: Batteries prepared with the sodium-ion battery electrolytes described in Examples 1-6 and Comparative Examples 1-4 were tested at 25°C on an electrochemical workstation battery testing system. The charge / discharge current density was 1C / 1C, and the charge / discharge voltage window was 1.5V-3.95V. The test results are shown in Table 1.

[0112] Table 1

[0113]

[0114] As can be seen from Table 1, the cyano groups in the difluoronitrile-based sodium borate and nitrile-containing sodium borate in the electrolyte of this application enhance the chemical stability of the sodium salt through the strong binding force between the anion and the metal cation, thereby reducing oxidative decomposition and side reactions. This leads to the formation of a dense, uniform, and thin interfacial film on the electrode surface, reducing the generation of HF and other gases during battery cycling, lowering interfacial impedance, and improving the cycle stability of the battery.

[0115] Based on the experimental data from Examples 1-4 and Comparative Examples 1-4 of this application, it can be seen that when sodium difluoronitrile borate salt is added to the electrolyte of this invention, the sodium battery achieves the highest number of cycles and the best cycle stability when cycling to 80% SOH, with Example 1 exhibiting the lowest DCR. Even better performance is achieved when the amounts of sodium salt, organic solvent, and additives are within the preferred range.

[0116] Based on the experimental data from Examples 4 to 6 of this application, it can be seen that the optimization of the mass ratio of sodium hexafluorophosphate and nitrile-containing sodium borate in the electrolyte of the present invention promotes the improvement of battery performance. The addition of nitrile-containing sodium borate can improve the cycle performance of the battery. However, excessive nitrile-containing sodium borate will result in an excessively thick film, leading to excessive battery impedance, which is not conducive to battery cycle performance.

[0117] Although the above embodiments have been shown and described, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Any changes, modifications, substitutions and variations made to the above embodiments by those skilled in the art are within the protection scope of the present invention.

Claims

1. A sodium-ion battery electrolyte, characterized in that, The sodium-ion battery electrolyte comprises the following raw materials by mass percentage: sodium salt 11%-21%, organic solvent 69%-87%, and additives 2%-10%. The sodium salt comprises sodium hexafluorophosphate and sodium cyanoborate; the mass ratio of sodium hexafluorophosphate to sodium cyanoborate in the sodium salt is (2.8-4):2; The structure of the sodium cyanoboronate is shown in the following formula:

2. The sodium-ion battery electrolyte according to claim 1, characterized in that, The organic solvent includes carbonate-based organic solvents; And / or, the additives include one or more of sodium tetrafluoroborate, sodium nitrate, vinylene carbonate, fluoroethylene carbonate, trans-difluoroethylene carbonate, vinyl sulfite, 1,3-propanesulfonate lactone, succinate, propenyl-1,3-sulfonate lactone, vinyl sulfate, ethoxy(pentafluoro)cyclotriphosphazene, methyl nonafluorobutyl ether, tris(2,2,2-trifluoroethyl) phosphite, and biphenyl.

3. The sodium-ion battery electrolyte according to claim 2, characterized in that, The carbonate-based organic solvents include one or more of ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, and propylene carbonate. And / or, the additives include one or more of sodium tetrafluoroborate, vinylene carbonate, and fluoroethylene carbonate.

4. The sodium-ion battery electrolyte according to claim 1, characterized in that, The electrolyte comprises, by mass percentage, the following raw materials: 15%-20% sodium salt, 69%-87% organic solvent, and 2%-10% additives.

5. A sodium-ion battery, characterized in that, The sodium-ion battery includes the sodium-ion battery electrolyte according to any one of claims 1-4.

6. The sodium-ion battery according to claim 5, characterized in that, The sodium-ion battery also includes a positive electrode, a negative electrode, and a separator.

7. The sodium-ion battery according to claim 6, characterized in that, The positive electrode sheet includes a layered oxide active material; the negative electrode sheet includes a carbon-based negative electrode active material.

8. The sodium-ion battery according to claim 7, characterized in that, The layered oxide active material includes NaMO2, wherein M is selected from one or more of Fe, Ni, Co, Mn, Cu, Cr, Al, Mg, Ti, Li and Zn; And / or, the negative electrode active material includes one or more of graphene, graphene oxide, soft carbon, hard carbon, expanded graphite, silicon, and mesophase carbon microspheres.

9. The sodium-ion battery according to claim 7, characterized in that, The layered oxide active material includes NaCu 1 / 9Ni 2 / 9 Fe 1 / 3 Zn 1 / 3 O2 active substances; And / or, the negative electrode active material includes hard carbon.

10. The sodium-ion battery according to claim 5, characterized in that, The sodium-ion battery is a sodium-ion layered oxide battery; And / or, the sodium-ion battery is used at a temperature of -40°C to 60°C.

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