Non-combustible electrolyte suitable for sodium ion battery and application of non-combustible electrolyte

Abstract

The invention belongs to the technical field of sodium-ion batteries, and particularly relates to a non-combustible electrolyte suitable for a sodium-ion battery and an application of the non-combustible electrolyte. The competitive coordination mechanism of the high-polarity low-Lewis-alkalinity solvent and the non-fluorophosphate is utilized to regulate and control the proportion of the coordinated non-fluorophosphate solvent in the solvation structure, and the dissociation of the sodium salt is enhanced by utilizing the high polarity and low Lewis-alkalinity of the solvent; the ionic conductivity of the electrolyte is improved; meanwhile, the compatibility of the electrolyte and a hard carbon negative electrode is improved. On the basis, the molar ratio of the non-fluorophosphate to the fluorophosphate is cooperatively regulated and controlled, so that the compatibility of the electrolyte and the hard carbon negative electrode is remarkably improved. Besides, the viscosity of the electrolyte is improved by utilizing the carbonate solvent, so that the wettability of the incombustible electrolyte to the hard carbon negative electrode is improved, and the incombustible electrolyte which can be compatible with the hard carbon negative electrode of the sodium ion battery and has high ionic conductivity is obtained.

Description

Technical Field

[0001] The present application belongs to the technical field of sodium ion batteries, and more specifically, relates to a non-flammable electrolyte suitable for sodium ion batteries and its application. Background Art

[0002] Sodium-ion batteries have attracted widespread attention due to their abundant resources, wide distribution, and low cost. The development of sodium-ion batteries as a supplement to lithium-ion batteries can solve the problem of lithium resource shortage. However, sodium-ion batteries use the carbonate electrolyte used in the lithium-ion battery electrolyte system, which has the risk of flammability, increasing the risk of fire and explosion of sodium-ion batteries under extreme conditions. In addition, the electrolyte itself has a low boiling point and is volatile. After catching fire, the battery fluid can easily detonate surrounding batteries, causing the accident to spread. Therefore, the development of non-flammable electrolytes and the improvement of battery safety are crucial to the large-scale application of sodium-ion batteries.

[0003] The development of non-flammable electrolytes for sodium-ion batteries is mainly achieved with the help of flame-retardant solvents. Phosphate solvents have the advantages of low cost, environmental protection and high flame retardancy, and are considered to be a type of flame-retardant solvent with great application potential. However, phosphate solvents have strong coordination ability and are easy to decompose. The solid electrolyte interface film formed has poor ability to shield electrons and is difficult to be compatible with low-cost hard carbon negative electrodes with low sodium insertion potential. Patent document CN110518287A uses phosphate and fluoroether with a volume ratio of 1:1 to 2:1 as flame-retardant solvents. A large amount of fluoroether is used as a solvent, which increases the cost of the electrolyte. In addition, fluoroether has high vapor pressure and low boiling point, making it difficult to apply on a large scale. Patent document CN108736010A uses NaTi2(PO4)3, Na2TiCr(PO4)3, NaZr2(PO4)3, Na 1.5 Fe 0.5 Ti 1.5(PO4)3, TiP2O7 and other negative electrode materials reduce the degradation of the interface caused by the decomposition of phosphate esters. Patent document CN114792843A uses film-forming additives such as fluoroethylene carbonate, vinylene carbonate, 1,3-propane sultone, and vinyl sulfate to improve the interface, thereby improving the compatibility of the electrolyte with the hard carbon negative electrode. Patent document CN108028429A uses phosphate ester as the main solvent of the electrolyte, and inhibits the decomposition of the phosphate ester solvent by increasing the concentration of sodium salt, but the dielectric constant of phosphate ester is low, and the ionic conductivity of the electrolyte designed when it is used as the main solvent is low, usually less than 2mS / cm. In addition, the cost of electrolytes with high salt concentrations is high, and at the same time, the viscosity of the electrolyte increases, resulting in poor wettability of the electrolyte to the electrode and decreased conductivity. Patent document CN113555606A improves the viscosity and wettability of the electrolyte by adding a large amount of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, methyl nonafluorobutyl ether, ethyl nonafluorobutyl ether and other fluoroether solvents, but the dielectric constant of fluoroether solvents is low and the sodium salt is almost not dissociated, resulting in a decrease in the conductivity of the electrolyte and poor rate performance of the battery. Patent document CN117293390A increases the dissociation of sodium salts by adding carboxylates such as methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, butyl propionate, ethyl butyrate, methyl fluoropropionate, methyl fluoropropionate or ethyl pentafluoropropionate to the phosphate system to improve the ionic conductivity, but does not improve the poor compatibility of the electrolyte with the hard carbon negative electrode. Patent document CN115706263A improves the compatibility of electrolyte and hard carbon by adding ether solvents such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, dimethoxymethane, 1,2-dimethoxyethane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether, but ether solvents themselves are easy to decompose and produce peroxides, which leads to increased risk of electrolyte deterioration and explosion, and is not practical. Patent document CN114039095A combines the flame retardant additive fluorophosphate with the carbonate solvent, and needs to add the film-forming additive fluoroethylene carbonate to inhibit the increase of the battery internal resistance and improve the compatibility of the electrolyte with the hard carbon negative electrode. The film-forming additive is a consumable component. Once it is consumed, the electrolyte will become ineffective, the service life is short, and it cannot be used on a large scale.

[0004] Therefore, it is crucial to provide a low-cost, high ionic conductivity and non-flammable electrolyte that can be used in Na-ion batteries. Summary of the invention

[0005] In view of the defects of the prior art, the purpose of the present application is to provide a non-flammable electrolyte and application suitable for sodium ion batteries, so as to solve the technical problems of the existing sodium ion battery electrolyte such as high cost, low ionic conductivity, and poor compatibility with hard carbon negative electrodes.

[0006] To achieve the above-mentioned object, the present application provides a non-flammable electrolyte suitable for sodium ion batteries, comprising a sodium salt, a solvent with high polarity and low Lewis alkalinity, a phosphate solvent and a carbonate solvent;

[0007] The dielectric constant of the high polarity and low Lewis basicity solvent is greater than 30, and the electron donation capacity is less than 20; the phosphate solvent includes non-fluorinated phosphate and fluorinated phosphate, and the molar ratio of the non-fluorinated phosphate to the fluorinated phosphate is less than or equal to 10:1;

[0008] The molar ratio of the sodium salt, the solvent with high polarity and low Lewis basicity, the phosphate solvent and the carbonate is 1:(1-3):(2.2-3.5):(1-4).

[0009] Preferably, the molar ratio of the non-fluorinated phosphate ester to the fluorinated phosphate ester is (2-10):1.

[0010] Preferably, the general structural formula of the non-fluorinated phosphate is

[0011]

[0012] Here, R1 and R2 are each independently an alkyl group having 1 to 10 carbon atoms, and R3 is an alkyl group having 1 to 10 carbon atoms or an alkoxy group having 1 to 10 carbon atoms.

[0013] Preferably, the fluorophosphate ester is at least one of tris(2,2,2-trifluoroethyl) phosphate, bis(2,2,2-trifluoroethyl) phosphate methyl ester and bis(2,2,2-trifluoroethyl) phosphate ethyl ester.

[0014] Preferably, the concentration of the sodium salt in the non-flammable electrolyte is 0.9 mol / L to 1.5 mol / L.

[0015] Preferably, the sodium salt is one or more of sodium hexafluorophosphate, sodium bis(trifluoromethylsulfonyl)imide, sodium bis(fluorosulfonyl)imide, sodium tetrafluoroborate and sodium bis(oxalatoborate).

[0016] Preferably, the solvent with high polarity and low Lewis basicity is at least one of ethylene carbonate and propylene carbonate.

[0017] Preferably, the viscosity of the carbonate solvent is less than 1 mPa·s -1 , selected from one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, methyl trifluoroethyl carbonate and di(2,2,2-trifluoroethyl) carbonate.

[0018] On the other hand, the present application also provides a sodium ion battery, which includes a positive electrode, a negative electrode and the above-mentioned non-flammable electrolyte.

[0019] Preferably, the negative electrode is hard carbon.

[0020] In general, the above technical solutions conceived by this application have the following technical advantages compared with the prior art:

[0021] (1) This application utilizes the competitive coordination mechanism between high-polarity and low-Lewis-base solvents and non-fluorinated phosphates to regulate the proportion of coordinated non-fluorinated phosphate solvents in the solvation structure, and utilizes the high polarity and low Lewis-base of the solvent to enhance the dissociation effect on the sodium salt, thereby improving the ionic conductivity of the electrolyte and improving its compatibility with the hard carbon negative electrode. On this basis, the molar ratio of non-fluorinated phosphates and fluorinated phosphates is synergistically regulated to significantly improve the compatibility of the electrolyte with the hard carbon negative electrode. In addition, carbonate solvents are used to improve the viscosity of the electrolyte, thereby improving the wettability of the non-flammable electrolyte to the hard carbon negative electrode, and a non-flammable electrolyte with high ionic conductivity suitable for sodium ion batteries is designed. The electrolyte is compatible with low-cost hard carbon negative electrodes at conventional sodium salt concentrations, breaking through the problem of low ionic conductivity and poor compatibility of existing non-flammable electrolytes with hard carbon negative electrodes.

[0022] (2) Compared with the existing electrolytes, the present application can obtain a non-flammable electrolyte with high ionic conductivity, good wettability to electrodes, low cost, high safety and strong practicality by compounding sodium salts, high polarity and low Lewis alkalinity solvents, phosphate esters and carbonate ester solvents without adding ether solvents, fluoroether solvents and film-forming additives. The electrolyte is compatible with the hard carbon negative electrode commonly used in sodium ion batteries, and its application in sodium ion batteries exhibits relatively excellent electrochemical performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is the first cycle charge and discharge curve of the hard carbon negative electrode of the sodium ion battery assembled with the electrolyte provided in Example 1 of the present application;

[0024] Figure 2 The first cycle charge and discharge curve of the hard carbon negative electrode of the sodium ion battery assembled with the electrolyte provided in Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical solution and advantages of the present application more clear, the present application is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described here are only used to explain the present application and are not used to limit the present application.

[0026] The present application provides a non-flammable electrolyte suitable for a sodium ion battery, comprising a sodium salt, a solvent with high polarity and low Lewis alkalinity, a phosphate solvent and a carbonate solvent;

[0027] The dielectric constant of the high polarity and low Lewis basicity solvent is greater than 30, and the electron donation capacity value (DN) is less than 20; the phosphate solvent includes non-fluorinated phosphate and fluorinated phosphate, and the molar ratio of the non-fluorinated phosphate to the fluorinated phosphate is less than or equal to 10:1;

[0028] The molar ratio of the sodium salt, the solvent with high polarity and low Lewis basicity, the phosphate solvent and the carbonate is 1:(1-3):(2.2-3.5):(1-4).

[0029] In some embodiments, the concentration of sodium salt in the non-flammable electrolyte is 0.9 mol / L to 1.5 mol / L.

[0030] In some embodiments, the sodium salt is one or more of sodium hexafluorophosphate, sodium bis(trifluoromethylsulfonyl)imide, sodium bis(fluorosulfonyl)imide, sodium tetrafluoroborate and sodium bis(oxalatoborate).

[0031] In the present application, the high polarity of the high-polarity, low-Lewis-base solvent can increase the dissociation of the sodium salt, so that the electrolyte has a high ionic conductivity. The low Lewis base of the solvent makes the ion-solvation cluster formed by it have a moderate action strength, which can improve the compatibility of the electrolyte with the hard carbon negative electrode. When the Lewis base of the solvent is too high, the new ion-solvent component formed after coordination with the sodium ions is difficult to remove the solvent on the surface of the negative electrode, thereby causing the irreversible decomposition of a large amount of solvent, making the electrolyte incompatible with the negative electrode. In some embodiments, the high-polarity, low-Lewis-base solvent is at least one of ethylene carbonate and propylene carbonate.

[0032] In some embodiments, the molar ratio of the non-fluorinated phosphate ester to the fluorinated phosphate ester is (2-10): 1. The inventors have experimentally found that compounding the non-fluorinated phosphate ester and the fluorinated phosphate ester and adjusting the molar ratio can significantly improve the compatibility of the electrolyte with the hard carbon negative electrode.

[0033] In some embodiments, the above-mentioned non-fluorinated phosphate has the general structural formula:

[0034]

[0035] Here, R1 and R2 are each independently an alkyl group having 1 to 10 carbon atoms, and R3 is an alkyl group having 1 to 10 carbon atoms or an alkoxy group having 1 to 10 carbon atoms.

[0036] In some embodiments, the non-fluorinated phosphate ester may be, but is not limited to, trimethyl phosphate.

[0037] In some embodiments, the fluorophosphate ester is at least one of tris(2,2,2-trifluoroethyl) phosphate, bis(2,2,2-trifluoroethyl) phosphate methyl ester and bis(2,2,2-trifluoroethyl) phosphate ethyl ester.

[0038] In some embodiments, the viscosity of the carbonate solvent is less than 1 mPa·s -1 , such as but not limited to one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, methyl trifluoroethyl carbonate and bis(2,2,2-trifluoroethyl) carbonate.

[0039] The present application provides a sodium ion battery, comprising a positive electrode, a negative electrode and the above-mentioned non-flammable electrolyte.

[0040] In some embodiments, the negative electrode is hard carbon.

[0041] The above technical solutions are described in detail below in conjunction with specific embodiments. It should be understood that these are merely exemplary and are not intended to limit the present application. Materials identical or similar to the types, models, qualities, properties or functions of the following reagents and instruments can be used to implement the present application. The experimental methods used in the following embodiments are conventional methods unless otherwise specified. The materials, reagents, etc. used in the following embodiments, unless otherwise specified, can all be obtained from commercial sources.

[0042] The following are examples and comparative examples:

[0043] Example 1

[0044] This embodiment provides a sodium ion battery, and the specific steps are as follows:

[0045] Preparation of electrolyte: In a glove box, sodium hexafluorophosphate, propylene carbonate, and phosphate (including triethyl phosphate and tri(2,2,2-trifluoroethyl) phosphate) were stirred and mixed at room temperature, and then dimethyl carbonate was added and mixed to obtain electrolyte A1, wherein the sodium salt concentration was 1.05 mol / L, the molar ratio of sodium hexafluorophosphate, propylene carbonate, phosphate and dimethyl carbonate was 1:2:2.5:3, and the molar ratio of triethyl phosphate and tri(2,2,2-trifluoroethyl) phosphate was 4:1.

[0046] Assembling a sodium ion battery: using a hard carbon negative electrode as a working electrode, metallic sodium as a counter electrode and a reference electrode, and using a 2032-type battery shell to assemble the above-mentioned electrolyte, hard carbon negative electrode, and metallic sodium into a button battery.

[0047] Electrolyte A1 performance test: The non-flammable properties of the electrolyte were determined by the diaphragm ignition method; the wettability of the electrolyte was determined by a contact angle tester; and the ionic conductivity of the electrolyte was determined by a Lei-Magnetic DDS-307A conductivity meter. The test results are shown in Table 1.

[0048] Sodium ion battery performance test: The assembled battery was charged and discharged on a charge and discharge instrument. The test voltage range was 0.005V~2V, the first cycle test current density was 0.1C, and the current density during the cycle was 0.5C. The test results are shown in Table 1. The first cycle charge and discharge curve is as follows: Figure 1 shown.

[0049] Example 2

[0050] This embodiment provides a sodium ion battery, and the preparation of the electrolyte is as follows: in a glove box, sodium hexafluorophosphate, propylene carbonate, and phosphate (including triethyl phosphate and tris(2,2,2-trifluoroethyl) phosphate) are stirred and mixed at room temperature, and then diethyl carbonate is added and mixed to obtain electrolyte A2, wherein the sodium salt concentration is 1.5 mol / L, the molar ratio of sodium hexafluorophosphate, propylene carbonate, phosphate and diethyl carbonate is 1:2:2.2:3, the molar ratio of triethyl phosphate and tris(2,2,2-trifluoroethyl) phosphate is 10:1, and the other parameters of the battery are the same as those in Example 1. The performance of electrolyte A2 and the assembled sodium ion battery is shown in Table 1.

[0051] Example 3

[0052] This embodiment provides a sodium ion battery, and the preparation of the electrolyte is as follows: in a glove box, sodium hexafluorophosphate, propylene carbonate, and phosphate (including triethyl phosphate and tris(2,2,2-trifluoroethyl) phosphate) are stirred and mixed at room temperature, and then ethyl methyl carbonate is added and mixed to obtain electrolyte A3, wherein the sodium salt concentration is 1.1 mol / L, the molar ratio of sodium hexafluorophosphate, propylene carbonate, phosphate and ethyl methyl carbonate is 1:2:3:3, the molar ratio of triethyl phosphate and tris(2,2,2-trifluoroethyl) phosphate is 2:1, and the other parameters of the battery are the same as those in Example 1. The performance of electrolyte A3 and the assembled sodium ion battery is shown in Table 1.

[0053] Example 4

[0054] This embodiment provides a sodium ion battery, and the preparation of the electrolyte is as follows: in a glove box, sodium bis(oxalatoborate), propylene carbonate, and phosphate (including triethyl phosphate and bis(2,2,2-trifluoroethyl) methyl phosphate) are stirred and mixed at room temperature, and then dimethyl carbonate is added and mixed to obtain electrolyte A4, wherein the sodium salt concentration is 1.04 mol / L, the molar ratio of sodium hexafluorophosphate, propylene carbonate, phosphate and dimethyl carbonate is 1:1:3.5:1, the molar ratio of triethyl phosphate and bis(2,2,2-trifluoroethyl) methyl phosphate is 6:1, and the other parameters of the battery are the same as those in Example 1. The performance of electrolyte A4 and the assembled sodium ion battery is shown in Table 1.

[0055] Example 5

[0056] This embodiment provides a sodium ion battery, and the preparation of the electrolyte is as follows: in a glove box, sodium hexafluorophosphate, propylene carbonate, and phosphate (including triethyl phosphate and bis(2,2,2-trifluoroethyl) ethyl phosphate) are stirred and mixed at room temperature, and then dipropyl carbonate is added and mixed to obtain electrolyte A5, wherein the sodium salt concentration is 1.2 mol / L, the molar ratio of sodium hexafluorophosphate, propylene carbonate, phosphate and dipropyl carbonate is 1:3:3.5:2, the molar ratio of triethyl phosphate and bis(2,2,2-trifluoroethyl) ethyl phosphate is 4:1, and the other parameters of the battery are the same as those in Example 1. The performance of electrolyte A5 and the assembled sodium ion battery is shown in Table 1.

[0057] Example 6

[0058] This embodiment provides a sodium ion battery, and the preparation of the electrolyte is as follows: in a glove box, sodium bis(fluorosulfonyl)imide, ethylene carbonate, and phosphate (including triethyl phosphate and tris(2,2,2-trifluoroethyl) phosphate) are stirred and mixed at room temperature, and then dimethyl carbonate is added and mixed to obtain electrolyte A6, wherein the sodium salt concentration is 0.9 mol / L, the molar ratio of sodium bis(fluorosulfonyl)imide, ethylene carbonate, phosphate and dimethyl carbonate is 1:3:3:4, the molar ratio of triethyl phosphate and tris(2,2,2-trifluoroethyl) phosphate is 2:1, and the other parameters of the battery are the same as those in Example 1. The performance of electrolyte A6 and the assembled sodium ion battery is shown in Table 1.

[0059] Comparative Example 1

[0060] This comparative example provides a sodium ion battery, and the preparation of the electrolyte is as follows: in a glove box, sodium hexafluorophosphate and phosphate (including triethyl phosphate and tris(2,2,2-trifluoroethyl) phosphate) are stirred and mixed at room temperature, and then dimethyl carbonate is added and mixed to obtain electrolyte B1, wherein the molar ratio of sodium hexafluorophosphate, phosphate and dimethyl carbonate is 1:2.5:3, and the molar ratio of triethyl phosphate, dimethyl carbonate and tris(2,2,2-trifluoroethyl) phosphate is 4:1, and the other parameters of the battery are the same as those in Example 1. The performance test results of electrolyte B1 and the assembled sodium ion battery are shown in Table 1, and the first cycle charge and discharge curve of the hard carbon negative electrode of the sodium ion battery is shown in Table 1. Figure 2 .

[0061] Comparative Example 2

[0062] This comparative example provides a sodium ion battery, and the preparation of the electrolyte is as follows: in a glove box, sodium hexafluorophosphate, dimethyl sulfoxide, and phosphate (including triethyl phosphate and tris(2,2,2-trifluoroethyl) phosphate) are stirred and mixed at room temperature, and then dimethyl carbonate is added and mixed to obtain electrolyte B2, wherein the molar ratio of sodium hexafluorophosphate, dimethyl sulfoxide, phosphate, and triethyl phosphate is 1:2:2.5:3, the molar ratio of triethyl phosphate, dimethyl carbonate, and tris(2,2,2-trifluoroethyl) phosphate is 4:1, and the other parameters of the battery are the same as those in Example 1. The performance test results of electrolyte B2 and the assembled sodium ion battery are shown in Table 1.

[0063] Comparative Example 3

[0064] This comparative example provides a sodium ion battery, and the preparation of the electrolyte is as follows: in a glove box, sodium hexafluorophosphate, propylene carbonate, and tris(2,2,2-trifluoroethyl) phosphate are stirred and mixed at room temperature, and then dimethyl carbonate is added and mixed to obtain electrolyte B2, wherein the molar ratio of sodium hexafluorophosphate, propylene carbonate, tris(2,2,2-trifluoroethyl) phosphate and dimethyl carbonate is 1:2:2.5:3, and the other parameters of the battery are the same as those in Example 1. The performance test results of electrolyte B3 and the assembled sodium ion battery are shown in Table 1.

[0065] Comparative Example 4

[0066] This comparative example provides a sodium ion battery, and the preparation of the electrolyte is as follows: in a glove box, sodium hexafluorophosphate, propylene carbonate, and triethyl phosphate are stirred and mixed at room temperature, and then dimethyl carbonate is added and mixed to obtain electrolyte B4, wherein the molar ratio of sodium hexafluorophosphate, propylene carbonate, triethyl phosphate, and dimethyl carbonate is 1:2:2.5:3, and the other parameters of the battery are the same as those in Example 1. The performance of electrolyte B4 and the assembled sodium ion battery is shown in Table 1.

[0067] Comparative Example 5

[0068] This comparative example provides a sodium ion battery, and the preparation of the electrolyte is as follows: in a glove box, sodium hexafluorophosphate, propylene carbonate, and phosphate (including triethyl phosphate and tris(2,2,2-trifluoroethyl) phosphate) are stirred and mixed at room temperature, and then dimethyl carbonate is added and mixed to obtain electrolyte B2, wherein the molar ratio of sodium hexafluorophosphate, propylene carbonate, phosphate and dimethyl carbonate is 1:2:2.6:3, the molar ratio of triethyl phosphate and tris(2,2,2-trifluoroethyl) phosphate is 12:1, and the other parameters of the battery are the same as those in Example 1. The performance of electrolyte B5 and the assembled sodium ion battery is shown in Table 1.

[0069] Comparative Example 6

[0070] This comparative example provides a sodium ion battery, and the preparation of the electrolyte is as follows: in a glove box, sodium hexafluorophosphate, propylene carbonate, triethyl phosphate and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether are stirred and mixed at room temperature, and then dimethyl carbonate and bromobenzene are added and mixed to obtain electrolyte B5, wherein the molar ratio of sodium hexafluorophosphate, propylene carbonate, triethyl phosphate, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether and dimethyl carbonate is 1:2:2:0.5:3, and the other parameters of the battery are the same as those in Example 1. The performance of electrolyte B6 and the assembled sodium ion battery is shown in Table 1.

[0071] Comparative Example 7

[0072] This comparative example provides a sodium ion battery, and the preparation of the electrolyte is as follows: in a glove box, sodium hexafluorophosphate, propylene carbonate, and phosphate (including triethyl phosphate and tris(2,2,2-trifluoroethyl) phosphate) are stirred and mixed at room temperature to obtain electrolyte B7, wherein the molar ratio of sodium hexafluorophosphate, propylene carbonate, and phosphate is 1:2:2.5, and other parameters are the same as those in Example 1. The performance of electrolyte B7 and the assembled sodium ion battery is shown in Table 1.

[0073] Table 1 Performance of electrolytes prepared in Examples and Comparative Examples and assembled sodium ion batteries

[0074]

[0075] The test results show that the electrolytes provided in the embodiments of the present application are all non-flammable, have a contact angle with the diaphragm of less than 40°, have good wettability to the diaphragm, and have an ionic conductivity greater than or equal to 6mS / cm. The first-cycle charging capacity of the sodium ion battery assembled using the electrolyte provided in the embodiments of the present application is greater than 290mA / g, the first-cycle coulomb efficiency of the hard carbon negative electrode is greater than or equal to 75%, and the capacity retention rate after 300 cycles is maintained at 90% or above, indicating that the electrolyte provided in the present application is compatible with the hard carbon negative electrode.

[0076] The first cycle coulomb efficiency of the hard carbon negative electrode of the sodium ion battery assembled with the electrolyte provided in Comparative Example 1 is low, and the hard carbon negative electrode cannot be charged and discharged normally ( Figure 2), the capacity retention rate of the sodium ion battery was not tested, and it is indicated by "-". The electrolyte provided by Comparative Example 2 showed poor compatibility with the hard carbon negative electrode, and the first-cycle coulomb efficiency of the hard carbon negative electrode was low at only 15%. The reason may be that the competitive coordination solvent dimethyl sulfoxide (electron donation capacity value of 29.8) used in the electrolyte has high polarity and Lewis alkalinity higher than propylene carbonate (electron donation capacity value of 15.1) and ethylene carbonate (electron donation capacity value of 16.4). The new ion-solvent component formed after coordination with sodium ions is difficult to remove the solvent on the negative electrode surface, resulting in irreversible decomposition of a large amount of solvent, making the electrolyte incompatible with the negative electrode. The capacity retention rate of the sodium ion battery was not tested, and it was indicated by "-". By comparing Example 1 and Comparative Examples 3 to 5, it can be seen that compounding an appropriate amount of fluorophosphate and non-fluorophosphate can effectively improve the compatibility of the electrolyte with the hard carbon negative electrode. The electrolyte provided in Comparative Example 3 also has non-flammable properties. The performance of the sodium ion battery assembled based on this electrolyte is slightly lower than that of the embodiment, but the cost of the electrolyte is high, which is not conducive to promotion and application. Compared with Comparative Example 4, Comparative Example 6 compounds fluoroether with other components to prepare an electrolyte, which can improve the ionic conductivity of the electrolyte, but cannot effectively improve the compatibility of the electrolyte with the hard carbon negative electrode. The electrolyte provided in Comparative Example 7 has poor wettability, and the sodium ion battery assembled based on this electrolyte has a low capacity retention rate of only 70% after 300 cycles, which does not meet the use requirements of sodium ion batteries.

[0077] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A non-flammable electrolyte suitable for sodium ion batteries, characterized in that: including sodium salts, highly polar solvents with low Lewis base, phosphate solvents, and carbonate solvents; Wherein, the dielectric constant of the high polarity and low Lewis basicity solvent is greater than 30, and the electron donation capacity value is less than 20; the phosphate solvent includes non-fluorinated phosphate ester and fluorinated phosphate ester, and the molar ratio of the non-fluorinated phosphate ester to the fluorinated phosphate ester is less than or equal to 10:1; The molar ratio of the sodium salt, the solvent with high polarity and low Lewis basicity, the phosphate solvent and the carbonate is 1:(1-3):(2.2-3.5):(1-4).

2. The non-flammable electrolyte according to claim 1, characterized in that ,, the molar ratio of the non-fluorinated phosphate ester to the fluorinated phosphate ester is (2-10):

1.

3. The non-flammable electrolyte according to claim 1 or 2, characterized in that: The general structural formula of the non-fluorinated phosphate is Here, R1 and R2 are each independently an alkyl group having 1 to 10 carbon atoms, and R3 is an alkyl group having 1 to 10 carbon atoms or an alkoxy group having 1 to 10 carbon atoms.

4. The non-flammable electrolyte according to claim 1 or 2, characterized in that: The fluorophosphate ester is at least one of tris(2,2,2-trifluoroethyl) phosphate, bis(2,2,2-trifluoroethyl) phosphate methyl ester and bis(2,2,2-trifluoroethyl) phosphate ethyl ester.

5. The non-flammable electrolyte according to claim 1, characterized in that ,, the concentration of sodium salt in the non-flammable electrolyte is 0.9 mol / L to 1.5 mol / L.

6. The non-flammable electrolyte according to claim 1, characterized in that ,, the sodium salt is one or more of sodium hexafluorophosphate, sodium bis(trifluoromethylsulfonyl)imide, sodium bis(fluorosulfonyl)imide, sodium tetrafluoroborate and sodium bis(oxalatoborate).

7. The non-flammable electrolyte according to claim 1, characterized in that: The solvent with high polarity and low Lewis basicity is at least one of ethylene carbonate and propylene carbonate.

8. The non-flammable electrolyte according to claim 1, characterized in that: The viscosity of the carbonate solvent is less than 1 mPa·s -1 , selected from one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, methyl trifluoroethyl carbonate and di(2,2,2-trifluoroethyl) carbonate.

9. A sodium ion battery, characterized in that: The sodium ion battery comprises a positive electrode, a negative electrode and the non-flammable electrolyte according to any one of claims 1 to 8.

10. The sodium ion battery according to claim 9, characterized in that: The negative electrode is hard carbon.

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