Sodium-ion battery electrolyte as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of batteries, and particularly relates to a sodium-ion battery electrolyte and application thereof, the sodium-ion battery electrolyte comprises a sodium salt, an organic solvent and an additive; the sodium salt comprises sodium hexafluorophosphate and cyanogroup-containing sodium borate. The electrolyte has the beneficial effects that the highest occupied molecular orbital (HOMO) of the nitrile sodium salt contained in the electrolyte is lower than that of a carbon-oxygen bond, so that the oxidation stability of the sodium salt is improved, and meanwhile, the lowest unoccupied molecular orbital (LUMO) is also lower than that of the carbon-oxygen bond, so that the nitrile sodium salt is preferentially oxidized on the surface of a positive electrode to generate a CEI film and is reduced on the surface of a negative electrode to generate an SEI film, and the service life of the electrolyte is prolonged. Fluorine-containing groups in the sodium salt can construct a NaF-rich CEI membrane / SEI membrane, so that the first efficiency and the cycling stability of the battery are optimized; meanwhile, the cyano group (-CN) can promote the reductive decomposition of sodium salt, so that the problem that the impedance of an SEI film formed in a sodium ion battery is larger than that of an SEI film of a lithium battery is solved, and the cycling stability of the battery is effectively enhanced.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium ion batteries, and in particular relates to a sodium ion battery electrolyte and a preparation method and application thereof. Background Art

[0002] In recent years, with the increasing demand for renewable energy storage and concerns about the cost of lithium supply, sodium-ion batteries have attracted more and more attention. Sodium-ion batteries have become a powerful supplement to lithium-ion batteries due to their low cost, high safety and excellent high and low temperature performance, and their market demand continues to grow. However, compared with lithium-ion batteries, sodium-ion batteries still face problems such as low energy density and short cycle life. The development of new sodium-ion battery electrolytes is a key research direction to improve the performance and stability of sodium-ion batteries.

[0003] One way to develop sodium-ion battery electrolytes is to use salts and solvents that can improve battery performance and stability. Another way is to use additives or coatings to improve the performance of existing batteries. Among them, sodium salts, as an important component of battery electrolytes, not only provide abundant sodium ions, but also promote the formation of stable electrode-electrolyte interface films, which directly affects the cycle life and safety performance of sodium-ion batteries.

[0004] The commonly used inorganic sodium salt is sodium hexafluorophosphate (NaPF6). This sodium salt has excellent overall performance, is easy to dissociate, and has good compatibility with carbon-based negative electrodes and various positive electrode materials. However, this sodium salt has poor chemical stability and decomposes in an organic solvent to generate NaF and PF5, which affect the composition of the interface film. PF5 easily reacts with trace water in the solvent to produce HF gas. In addition, this sodium salt has poor thermal stability and high temperature will accelerate its decomposition. Therefore, research and search for suitable sodium salts is the key direction of sodium ion battery research and development. Summary of the invention

[0005] The present application provides a sodium ion battery electrolyte and a preparation method and application thereof, 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] A first aspect of the present application provides a sodium ion battery electrolyte, which includes a sodium salt, an organic solvent and an additive; the sodium salt includes sodium hexafluorophosphate and sodium cyanoborate.

[0007] The highest occupied molecular orbital (HOMO) of the nitrile sodium salt contained in the electrolyte described in the embodiment of the present application is lower than the carbon-oxygen bond, which is beneficial to improving the oxidation stability of the sodium salt. At the same time, the lowest unoccupied molecular orbital (LUMO) is also lower than the carbon-oxygen bond, so that it is preferentially oxidized on the positive electrode surface to form a CEI film and reduced on the negative electrode surface to form a SEI film. The fluorine-containing groups in the sodium salt can construct a CEI film / SEI film rich in NaF, thereby optimizing the initial efficiency and cycle stability of the battery.

[0008] At the same time, cyanide (—CN) can promote the reduction and decomposition of sodium salt, thereby reducing the deposition of BO on the surface of SEI film and ions containing oxygen bonds and by-products on the surface of the negative electrode, forming a uniform, dense and thin SEI film. Because the binding force between anions and Na ions in sodium cyanoborate is relatively strong, the binding of Na ions with other organic functional groups is reduced to form an organic-based and inorganic-rich SEI, which increases the proportion of inorganic matter in the SEI film, thereby reducing the impedance of the SEI film and improving its flexibility, solving the problem that the impedance of the SEI film formed in the sodium ion battery is greater than that of the SEI film of the lithium battery, and effectively enhancing the cycle stability of the battery.

[0009] According to some embodiments of the sodium ion battery electrolyte described in the present application, the structural formula of the sodium cyanoborate is as follows:

[0010]

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

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

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

[0014] According to some embodiments of the sodium ion battery electrolyte described in the present application, the structural formula of the sodium cyanoborate is as follows:

[0015]

[0016] According to some embodiments of the sodium ion battery electrolyte described in the present application, the organic solvent includes a carbonate organic solvent.

[0017] According to some embodiments of the sodium ion battery electrolyte described in the present application, the carbonate 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 the present application, the additive includes sodium tetrafluoroborate, sodium nitrate, vinylene carbonate, fluoroethylene carbonate, trans-difluoroethylene carbonate, vinyl sulfite, 1,3-propane sultone, dicyandiamide, propenyl-1,3-sultone, vinyl sulfate, ethoxy (pentafluoro) cyclotriphosphazene, methyl nonafluorobutyl ether, tris (2,2,2-trifluoroethyl) phosphite and biphenyl. One or more.

[0019] According to some embodiments of the sodium ion battery electrolyte described in the present application, the additive includes one or more of sodium tetrafluoroborate, vinylene carbonate and fluoroethylene carbonate.

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

[0021] According to some embodiments of the sodium ion battery electrolyte described in the present application, the electrolyte includes the following raw materials in mass percentage: 15%-20% sodium salt, 69-87% organic solvent, and 2%-10% additive.

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

[0023] In a second aspect of the present application, a sodium ion battery is provided, wherein the sodium ion battery comprises the sodium ion battery electrolyte described in the first aspect of the present application.

[0024] Because the electrolyte described in the present application can promote the formation of stable and uniform SEI film and CEI film, the cycle stability and impedance of the sodium ion battery can be improved.

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

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

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

[0028] According to some embodiments of the sodium ion battery described in the present 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 the present application, the negative electrode active material includes one or more of graphene, graphene oxide, soft carbon, hard carbon, expanded graphite, silicon, and mesophase carbon microbeads.

[0030] According to some embodiments of the sodium ion battery described in the present 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 the present application, the negative electrode active material includes hard carbon.

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

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

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

[0035] In the present invention, the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" etc. mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the described specific features, structures, materials or characteristics may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0036] An embodiment of the present application provides a sodium ion battery electrolyte, which includes a sodium salt, an organic solvent and an additive; the sodium salt includes sodium hexafluorophosphate and sodium cyanoborate.

[0037] The highest occupied molecular orbital (HOMO) of the nitrile sodium salt contained in the electrolyte described in the embodiment of the present application is lower than the carbon-oxygen bond, which is beneficial to improving the oxidation stability of the sodium salt. At the same time, the lowest unoccupied molecular orbital (LUMO) is also lower than the carbon-oxygen bond, so that it is preferentially oxidized on the positive electrode surface to form a CEI film and reduced on the negative electrode surface to form a SEI film. The fluorine-containing groups in the sodium salt can construct a CEI film / SEI film rich in NaF, thereby optimizing the initial efficiency and cycle stability of the battery.

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

[0039] At the same time, cyanide (—CN) can promote the reduction and decomposition of sodium salt, thereby reducing the deposition of BO on the surface of SEI film and ions containing oxygen bonds and by-products on the surface of the negative electrode, forming a uniform, dense and thin SEI film. Because the binding force between anions and Na ions in sodium cyanoborate is relatively strong, the binding of Na ions with other organic functional groups is reduced to form an organic-based and inorganic-rich SEI, which increases the proportion of inorganic matter in the SEI film, thereby reducing the impedance of the SEI film and improving its flexibility, solving the problem that the impedance of the SEI film formed in the sodium ion battery is greater than that of the SEI film of the lithium battery, and effectively enhancing the cycle stability of the battery.

[0040] In some embodiments of the present application, the structural formula of the sodium cyanoborate is as shown below:

[0041]

[0042] Wherein: R1 and R2 are each independently selected from substituted or unsubstituted C1-C20 alkyl, alkoxy, alkyleneoxy, acyl, ether, isocyanate, phenylamino and amino, halogen; the substituent used in the substitution includes halogen.

[0043] The introduction of F element into sodium cyanoborate can effectively reduce the viscosity of the electrolyte, improve the conductivity of the electrolyte, and have good film-forming properties, thereby improving the performance of sodium-ion batteries.

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

[0045] In some embodiments of the present application, the structure of the sodium cyanoborate is shown below:

[0046]

[0047] In some embodiments of the present application, the synthesis reaction formula of the compound represented by structural formula (1) is as follows:

[0048]

[0049] In some embodiments of the present application, the synthesis reaction formula of the compound represented by structural formula (2) is as follows:

[0050]

[0051] In some embodiments of the present application, the organic solvent includes a carbonate organic solvent.

[0052] In some embodiments of the present 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 the present application, the additive includes one or more of sodium tetrafluoroborate, sodium nitrate, vinylene carbonate, fluoroethylene carbonate, trans-difluoroethylene carbonate, vinyl sulfite, 1,3-propane sultone, dicyandiamide, propenyl-1,3-sultone, vinyl sulfate, ethoxy (pentafluoro) cyclotriphosphazene, methyl nonafluorobutyl ether, tris (2,2,2-trifluoroethyl) phosphite and biphenyl; the addition of the additive can preferentially react with the organic solvent and the sodium salt for reduction (or oxidation) to form a dense, uniform and thin SEI film (or CEI film) on the surface of the negative electrode (or positive electrode), so that the internal electrode material is protected.

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

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

[0056] In some embodiments of the present application, the electrolyte includes the following raw materials in the following mass percentages: 15%-20% sodium salt, 69%-87% organic solvent, and 2%-10% additive.

[0057] In some embodiments of the present application, the mass ratio of sodium hexafluorophosphate to sodium cyanoborate in the sodium salt is (2.8-4):2, such as 2.8:2, 3:2, 3.6:2, 4:2, etc. The sodium salt is mainly sodium hexafluorophosphate, partially replaced by sodium cyanoborate, and the specific mass ratio is the best mass ratio verified by subsequent experiments to be 3:2.

[0058] An embodiment of the present application also provides a sodium ion battery, comprising the sodium ion battery electrolyte described in the first aspect of the present application.

[0059] Because the electrolyte described in the present application can promote the formation of stable and uniform SEI film and CEI film, the cycle stability and impedance of the sodium ion battery can be improved.

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

[0061] In some embodiments of the present application, the positive electrode plate includes a layered oxide active material.

[0062] In some embodiments of the present 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 NaCu1 / 9 Ni 2 / 9 Fe 1 / 3 Zn 1 / 3 O2 active substances.

[0063] In some embodiments of the present application, the negative electrode plate includes a carbon-based negative electrode active material, and the negative electrode active material 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 the present application, the sodium ion battery is a sodium ion layered oxide battery.

[0065] In some embodiments of the present application, the application temperature of the sodium ion battery is -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, 60°C, etc. The sodium ion battery described in the present application has good electrochemical properties at both high and low temperatures.

[0066] The technical solution of the present application is described in detail below in conjunction with specific embodiments.

[0067] Example 1

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

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

[0070] The organic solvent is a mixed solution of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate, and the mass ratio of ethylene carbonate: propylene carbonate: dimethyl carbonate: ethyl methyl carbonate: diethyl carbonate in the organic solvent is 45:30:11:8:6;

[0071] The additive is a mixture of vinylene carbonate, fluoroethylene carbonate, propylene-1,3-sultone and vinyl sulfate, wherein the mass ratio of vinylene carbonate, fluoroethylene carbonate, propylene-1,3-sultone and vinyl sulfate in the additive is 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. The mass ratio of sodium hexafluorophosphate to sodium difluoro(1,2-dihydroxyethane-1,1,2,2-tetracyano)borate in the sodium salt is 3:2.

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

[0074]

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

[0076] Example 2

[0077] The sodium ion battery electrolyte described in Example 2 is different from the sodium ion battery electrolyte described in Example 1 only in that the contents of sodium salt, organic solvent and additives are different.

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

[0079] Example 3

[0080] The sodium ion battery electrolyte described in Example 3 is different from the sodium ion battery electrolyte described in Example 1 only in that the contents of sodium salt, organic solvent and additives are different.

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

[0082] Example 4

[0083] The sodium ion battery electrolyte described in Example 4 is different from the sodium ion battery electrolyte described in Example 1 only in that 1,2-dihydroxyethane-1,1,2,2-tetracyanobis sodium borate is used instead of sodium difluoro(1,2-dihydroxyethane-1,1,2,2-tetracyano)borate.

[0084] The structural formula of 1,2-dihydroxyethane-1,1,2,2-tetracyanooxalate sodium borate is as follows:

[0085]

[0086] Example 5

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

[0090] Comparative Example 1

[0091] The sodium ion battery electrolyte described in Comparative Example 1 is different from the sodium ion battery electrolyte described in Example 1 only in that:

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

[0093] Comparative Example 2

[0094] The sodium ion battery electrolyte described in Comparative Example 2 is different from the sodium ion battery electrolyte described in Example 1 only in that:

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

[0096] Comparative Example 3

[0097] The sodium ion battery electrolyte described in Comparative Example 3 is different from the sodium ion battery electrolyte described in Example 1 only in that:

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

[0099] Comparative Example 4

[0100] The sodium ion battery electrolyte described in Comparative Example 4 is different from the sodium ion battery electrolyte described in Example 1 only in that:

[0101] The sodium salt used in the sodium ion battery electrolyte of 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 the present application:

[0103] The sodium ion battery electrolyte described in Examples 1-6 and Comparative Examples 1-4 was used to prepare a soft-pack 505060 sodium ion battery using a soft-pack lamination process. 1 / 9 Ni 2 / 9 Fe 1 / 3 Zn 1 / 3The O2 positive electrode sheet, the hard carbon negative electrode sheet and the polyethylene diaphragm are stacked and assembled, and the positive electrode sheet is baked to a moisture content of less than 100ppm, and the negative electrode sheet is baked to a moisture content of less than 200ppm, and the sodium ion battery electrolyte described in Examples 1-6 and Comparative Examples 1-4 is injected respectively, and hot pressing is carried out at a temperature of 45°C and a pressure of 425KGF, and then the battery is placed at a temperature of 45°C for 24 hours before packaging. After volume division, the battery is placed at room temperature to obtain a soft-pack 505060 finished battery, and the HF content of the above electrolyte is tested, and the electrochemical performance of the above battery is tested.

[0104] HF content test in electrolyte

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

[0106] High temperature storage gas production performance test of the battery cell prepared by the electrolyte described in this application

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

[0108] The DC internal resistance DCR test of the battery prepared by 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 separately at 25°C on an electrochemical workstation battery test system, the battery capacity was adjusted to 50% SOC and left for 2 hours, the end-of-stay voltage U1 was recorded, the battery was discharged at a current of I (2C) for 10 seconds, and the end-of-discharge voltage U2 was recorded, DCR = (U1-U2) / I, and the test results are shown in Table 1.

[0110] Cyclic performance test of the battery prepared by the electrolyte described in this application:

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

[0112] Table 1

[0113]

[0114] It can be seen from Table 1 that the cyano group in the difluoronitrile sodium borate and the nitrile-containing sodium borate in the electrolyte described in the present application improves the chemical stability of the sodium salt through the strong binding force between the anion and the metal cation, thereby reducing the occurrence of oxidative decomposition and side reactions, and further forming a dense, uniform and thin interface film on the electrode surface, reducing the generation of HF and other gases during the battery cycle, reducing the interface impedance and improving the cycle stability of the battery.

[0115] Combining the experimental data of Examples 1 to 4 of the present application with Comparative Examples 1 to 4 of the present application, it can be seen that when sodium difluoronitrile borate salt is added to the electrolyte of the present invention, the sodium battery is cycled to 80% SOH, and Example 1 has the largest number of cycles, the best cycle stability, and the lowest DCR. When the amount of sodium salt, organic solvent and additive added is within the preferred range, the performance is more excellent.

[0116] Combined with the test data of Examples 4 to 6 of the present 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 form a too thick film, resulting in excessive impedance of the battery, which is not conducive to the cycle of the battery.

[0117] Although the above embodiments have been shown and described, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations on the present invention. Changes, modifications, substitutions and variations of the above embodiments by those of ordinary skill in the art are all within the scope of protection of the present invention.

Claims

1. A sodium ion battery electrolyte, characterized in that: The sodium ion battery electrolyte comprises sodium salt, organic solvent and additives; The sodium salts include sodium hexafluorophosphate and sodium cyanoborate.

2. The sodium ion battery electrolyte according to claim 1, characterized in that: The structural formula of the sodium cyanoborate is shown below: Wherein: R1 and R2 are each independently selected from substituted or unsubstituted C1-C20 alkyl, alkoxy, alkyleneoxy, acyl, ether, isocyanate, phenylamino or amino, halogen; the substituent used in the substitution includes halogen.

3. The sodium ion battery electrolyte according to claim 1 or 2, characterized in that: The structure of the sodium cyanoborate is shown below:

4. The sodium ion battery electrolyte according to claim 1 or 2, characterized in that: The organic solvent includes a carbonate organic solvent; Preferably, the carbonate organic solvent includes one or more of ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate and propylene carbonate; And / or, the additive includes one or more of sodium tetrafluoroborate, sodium nitrate, vinylene carbonate, fluoroethylene carbonate, trans-difluoroethylene carbonate, vinyl sulfite, 1,3-propane sultone, dicyandiamide, propenyl-1,3-sultone, vinyl sulfate, ethoxy (pentafluoro) cyclotriphosphazene, methyl nonafluorobutyl ether, tris (2,2,2-trifluoroethyl) phosphite and biphenyl; Preferably, the additive includes one or more of sodium tetrafluoroborate, vinylene carbonate and fluoroethylene carbonate.

5. The sodium ion battery electrolyte according to claim 1 or 2, characterized in that: The electrolyte comprises the following raw materials in mass percentages: 11%-21% sodium salt, 69%-87% organic solvent, and 2%-10% additives; Preferably, the electrolyte comprises the following raw materials in mass percentage: 15%-20% sodium salt, 69%-87% organic solvent, and 2%-10% additive; Preferably, the mass ratio of sodium hexafluorophosphate to sodium cyanoborate in the sodium salt is (2.8-4):

2.

6. A sodium ion battery, characterized in that: The sodium ion battery comprises the sodium ion battery electrolyte according to any one of claims 1 to 5.

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

8. The sodium ion battery according to claim 7, characterized in that: The layered oxide active material comprises 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 microbeads.

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 6, characterized in that: The sodium ion battery is a sodium ion layered oxide battery; And / or, the application temperature of the sodium ion battery is -40°C to 60°C.

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