Preparation method of modified electrolyte and sodium ion battery

Abstract

The invention provides a preparation method of a modified electrolyte and a sodium ion battery. The preparation method comprises the following steps: S1, mixing a fluorinated carboxylic ester solvent, a fluorine source and a film-forming additive to obtain a mixed solution; and S2, adding a sodium source or a lithium source into the mixed solution to obtain the modified electrolyte. According to the scheme, the problem that the performance of the battery is reduced due to insufficient ion conduction capability of the sodium ion battery electrolyte in a low-temperature environment in the prior art is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of battery manufacturing, and in particular to a method for preparing a modified electrolyte and a sodium ion battery. Background Art

[0002] Energy storage has received extensive attention as an important force in promoting the realization of the "dual carbon" strategic goals. Among electrochemical energy storage methods, lithium-ion batteries have been widely industrialized, but due to the low abundance of lithium in the earth's crust, their future development space is limited, and a new battery is urgently needed to solve this problem.

[0003] Sodium-ion batteries are expected to complement and effectively replace lithium-ion batteries due to their abundant sodium resources, low price and environmental friendliness. However, sodium-ion batteries also face the problem of performance degradation in low-temperature environments. The electrolyte has insufficient ion conductivity at low temperatures, resulting in low charging and discharging efficiency and easily causing safety issues, which seriously limits the application of sodium-ion batteries in cold regions.

[0004] To address the above problems, no effective solution has been proposed yet. Summary of the invention

[0005] The main purpose of the present invention is to provide a method for preparing a modified electrolyte and a sodium ion battery, so as to solve the problem in the prior art that the ion conductivity of the sodium ion battery electrolyte is insufficient under low temperature conditions, resulting in decreased battery performance.

[0006] In order to achieve the above object, according to one aspect of the present invention, a method for preparing a modified electrolyte is provided, the preparation method comprising:

[0007] Step S1: mixing a fluorinated carboxylic acid ester solvent, a fluorine source, and a film-forming additive to obtain a mixed solution;

[0008] Step S2: adding a sodium source or a lithium source to the mixed solution to obtain a modified electrolyte.

[0009] Furthermore, in step S1, the proportion of the fluorinated carboxylic acid ester solvent in the mixed solution is 40% to 100% by mass, and / or the proportion of the fluorine source in the mixed solution is 0% to 30%, and / or the proportion of the film-forming additive in the mixed solution is 0-30%.

[0010] Furthermore, the fluorinated carboxylic acid ester solvent is at least one of ethyl fluoroacetate, ethyl difluoroacetate and ethyl trifluoroacetate.

[0011] Furthermore, in step S2, the sodium source is selected from at least one of sodium bis(fluorosulfonyl)imide, sodium hexafluorophosphate, sodium bis(fluorooxyborate), and sodium tetrafluoroborate.

[0012] Further, the concentration of the sodium source is set to 1M-10M.

[0013] Furthermore, in step S1, the fluorine source is perfluorodiethylene glycol dimethyl ether solvent.

[0014] Furthermore, in step S1, the film-forming additive is a fluoroethylene carbonate film-forming additive.

[0015] Furthermore, in step S2, the lithium source is selected from at least one of lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, lithium trifluoromethanesulfonate, lithium perchlorate, and lithium borate.

[0016] Further, step S1 includes:

[0017] S11, mixing the fluorinated carboxylic acid ester solvent, the fluorine source, and the film-forming additive in a glove box containing an inert gas to isolate moisture and oxygen in the air;

[0018] S12, using a magnetic stirrer or other stirring equipment to fully mix the solvent components to obtain a mixed solution.

[0019] According to another aspect of the present invention, a sodium ion battery is provided.

[0020] Furthermore, the sodium ion battery comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, and the electrolyte is a modified electrolyte prepared by the above-mentioned method for preparing a modified electrolyte.

[0021] Step S1: mixing a fluorinated carboxylic acid ester solvent, a fluorine source, and a film-forming additive to obtain a mixed solution;

[0022] Step S2: adding a sodium source or a lithium source to the mixed solution to obtain a modified electrolyte.

[0023] By applying the technical solution of the present invention, the use of fluorinated carboxylic acid ester solvents enhances the oxidation stability and low temperature performance of the electrolyte, while the fluorine source and film-forming additives help to form a stable solid electrolyte interface film (SEI) on the electrode surface, reduce the side reactions between the electrode and the electrolyte, and extend the battery life. By forming a mixed solution with fluorinated carboxylic acid ester solvents, fluorine sources and film-forming additives, the performance of the electrolyte is significantly improved, especially in sodium ion batteries, which can effectively improve the cycle stability and energy density of the battery, and at the same time, the performance is more stable in high and low temperature environments, solving the problem of insufficient ion conductivity of sodium ion battery electrolytes in low temperature environments in the prior art, resulting in reduced battery performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The drawings constituting a part of the present application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0025] Figure 1 A flow chart of an embodiment of a method for preparing a modified electrolyte according to the present invention is shown. DETAILED DESCRIPTION

[0026] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0027] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0028] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the terms used in this way can be interchangeable where appropriate, so that the embodiments of the present application described herein can be implemented in an order other than those illustrated or described herein, for example. In addition, the terms "including" and "having" and any of their variations are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0029] Now, exemplary embodiments according to the present application will be described in more detail with reference to the accompanying drawings. However, these exemplary embodiments may be implemented in a variety of different forms and should not be construed as being limited to the embodiments described herein. It should be understood that these embodiments are provided to make the disclosure of the present application thorough and complete, and to fully convey the concepts of these exemplary embodiments to those of ordinary skill in the art. In the accompanying drawings, for the sake of clarity, the thickness of the layers and regions may be enlarged, and the same reference numerals are used to represent the same devices, and thus their descriptions will be omitted.

[0030] According to a specific embodiment of the present application, a method for preparing a modified electrolyte and a sodium ion battery are provided.

[0031] Specifically, Figure 1 As shown, the preparation method comprises:

[0032] Step S1: mixing a fluorinated carboxylic acid ester solvent, a fluorine source, and a film-forming additive to obtain a mixed solution;

[0033] Step S2: adding a sodium source or a lithium source to the mixed solution to obtain a modified electrolyte.

[0034] Optionally, fluorinated carboxylic acid ester solvents have a low freezing point, in which the highly electronegative F atoms can effectively regulate the affinity of Na+ and reduce its binding energy, thereby promoting rapid desolvation of Na+ under low temperature conditions and improving the ionic conductivity of the electrolyte.

[0035] The fluorine source has low viscosity, high flash point and high thermochemical stability, which can improve the oxidation resistance of the electrolyte, enable it to remain stable even under high voltage, and enhance the safety performance of the battery.

[0036] Film-forming additives can help the hard carbon negative electrode build a strong electrode / electrolyte interface (SEI film) during the battery charging and discharging process, especially under low temperature conditions. The formation of the SEI film can reduce battery polarization and improve the reversibility of the battery, thereby improving the low temperature performance and cycle life of the battery.

[0037] By applying the technical solution of the present invention, the use of fluorinated carboxylic acid ester solvents enhances the oxidation stability and low temperature performance of the electrolyte, while the fluorine source and film-forming additives help to form a stable solid electrolyte interface film (SEI) on the electrode surface, reduce the side reactions between the electrode and the electrolyte, and extend the battery life. By forming a mixed solution with fluorinated carboxylic acid ester solvents, fluorine sources and film-forming additives, the performance of the electrolyte is significantly improved, especially in sodium ion batteries, which can effectively improve the cycle stability and energy density of the battery, and at the same time, the performance is more stable in high and low temperature environments, solving the problem of insufficient ion conductivity of sodium ion battery electrolytes in low temperature environments in the prior art, resulting in reduced battery performance.

[0038] Specifically, in step S1, the proportion of the fluorinated carboxylic acid ester solvent in the mixed solution is 40% to 100% by mass, and / or the proportion of the fluorine source in the mixed solution is 0%-30%, and / or the proportion of the film-forming additive in the mixed solution is 0-30%. The inventors have finally determined the proportions of the fluorinated carboxylic acid ester solvent, the fluorine source, and the film-forming additive in the mixed solution through a large number of experiments and optimizations. The mixed solution is prepared in this proportion, which can make the electrolyte have better performance.

[0039] Optionally, the proportion of the fluorinated carboxylic acid ester solvent in the mixed solution is 40% to 100%, where % represents the mass fraction of the fluorinated carboxylic acid ester solvent in the mixed solution.

[0040] Furthermore, the fluorinated carboxylic acid ester solvent is at least one of ethyl fluoroacetate, ethyl difluoroacetate and ethyl trifluoroacetate.

[0041] Ethyl fluoroacetate (EFA) contains a fluorine atom, which gives it a low freezing point and high electronegativity. The low freezing point helps the electrolyte remain liquid at low temperatures, thereby maintaining good ionic conductivity. The high electronegativity helps to regulate the affinity of Na+ with solvent molecules, reduce the binding energy, and promote the rapid desolvation process of Na+ at low temperatures.

[0042] Ethyl difluoroacetate (EDFA) contains two fluorine atoms and has a higher degree of fluorination and a lower freezing point than EFA. In addition to the characteristics of EFA, EDFA has better performance at low temperatures and higher ion conductivity at low temperatures. This is because the difluoro structure further enhances the electronegativity and stability of the solvent, which is beneficial to improving the ion transmission efficiency of the electrolyte in a low temperature environment.

[0043] Ethyl trifluoroacetate (ETFA) contains three fluorine atoms and is the most fluorinated of the three solvents. ETFA has a very low freezing point and excellent chemical stability. In low temperature environments, ETFA can provide the best ionic conductivity. At the same time, its highly fluorinated structure enables it to remain stable under high voltage conditions and is not easy to decompose, which helps to improve the voltage window and cycle performance of the battery.

[0044] The three solvents of ethyl fluoroacetate, ethyl difluoroacetate and ethyl trifluoroacetate all have low freezing points and good chemical stability, and can maintain high ionic conductivity at low temperatures, which is the key to the good performance of sodium ion battery electrolytes under cold conditions. Therefore, at least one of ethyl fluoroacetate, ethyl difluoroacetate and ethyl trifluoroacetate is preferably used as a fluorinated carboxylic acid ester solvent for preparing the electrolyte.

[0045] Furthermore, in step S2, the sodium source is selected from at least one of sodium bis(fluorosulfonyl)imide, sodium hexafluorophosphate, sodium bis(fluorooxyborate), and sodium tetrafluoroborate. These sodium salts can not only provide stable sodium ions, but also optimize the conductivity of the electrolyte, thereby increasing the charge and discharge rate of the battery.

[0046] Sodium bis(fluorosulfonyl)imide (NaTFSI) has a perfluoroalkyl structure and a high degree of electron delocalization. It can maintain a high solubility even in solvents with low dielectric constants and is suitable for use in low-temperature environments. It can maintain good ionic conductivity at low temperatures and improve battery charge and discharge efficiency.

[0047] Sodium hexafluorophosphate (NaPF6) is an inorganic sodium salt with high conductivity and relatively stable chemical properties, and is a commonly used sodium salt in sodium-ion battery electrolytes within the conventional temperature range. However, its performance under low temperature and high voltage conditions may be relatively weak.

[0048] Sodium difluorooxyborate (NaDFOB) combines the properties of fluorinated and oxidized borates, has good chemical stability and electrical conductivity, can effectively broaden the electrochemical window of the battery, and improve the high voltage performance and energy density of the battery.

[0049] Sodium tetrafluoroborate (NaBF4) is another sodium salt with higher conductivity and better safety under certain conditions. It can provide a stable source of Na+ ions and may be more cost-effective.

[0050] Due to their different characteristics in ionic conductivity, electrolyte stability, solubility and cost safety, at least one of these four sodium salts can be selected as the sodium source for preparing the electrolyte according to actual needs to meet the performance requirements of sodium-ion batteries under different conditions, especially to meet their optimization in low-temperature environments.

[0051] Optionally, the concentration of the sodium source is set to 1M-10M (M represents the unit mol / l). The concentration of sodium ions (Na+) in the electrolyte has a direct impact on ionic conductivity. A higher Na+ concentration means more free-moving ions, thereby improving the ionic conductivity of the electrolyte, which is a key factor in the efficiency and rate of battery charge and discharge. Within the concentration range of 1M-10M, it can be ensured that the electrolyte has sufficient ionic conductivity at different temperatures, especially in low temperature environments, to support the normal operation of the battery. The cost of sodium salt increases with increasing concentration. Selecting the appropriate concentration as needed can improve battery performance while considering cost-effectiveness and avoid unnecessary costs due to the use of excessively high concentrations of sodium salt.

[0052] Specifically, in step S1, the fluorine source is a perfluorodiethylene glycol dimethyl ether solvent. Perfluorodiethylene glycol dimethyl ether solvent (PFDF) is selected as a fluorine source because it can provide high ionic conductivity in a low temperature environment while maintaining the thermal stability, chemical stability and safety of the electrolyte, which helps to form a stable SEI film and broaden the voltage window of the battery, thereby significantly improving the performance and safety of sodium ion batteries under low temperature conditions. When preparing sodium ion battery electrolytes, using PFDE as a fluorine source can effectively solve the problem of insufficient ion conductivity at low temperatures, while improving the cycle life of the battery.

[0053] Specifically, in step S1, the film-forming additive is a fluoroethylene carbonate film-forming additive. As a film-forming additive, fluoroethylene carbonate (FEC) can maintain the ionic conductivity of the electrolyte while preferentially performing a reduction reaction on the surface of the negative electrode during the first charge and discharge process of the battery, forming a dense and stable solid electrolyte interface (SEI) film, which can maintain good electrochemical contact even at low temperatures, preventing the battery from significantly degrading due to freezing of the electrolyte or reduced ion transfer efficiency. FEC can also improve cycle stability and high-voltage resistance, enhance chemical stability and safety, and improve the cycle stability and energy density of the battery in one step.

[0054] Specifically, in step S2, the lithium source is selected from at least one of lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, lithium trifluoromethanesulfonate, lithium perchlorate, and lithium borate.

[0055] When preparing the electrolyte of lithium-ion batteries, the main function of the lithium source is to provide lithium ions (Li+) in the electrolyte, thereby ensuring that the electrolyte has sufficient ionic conductivity to support the battery's charge and discharge process. These lithium salts can provide stable lithium ions, promote the formation of SEI film in the electrolyte, optimize the conductivity of the electrolyte and the electrochemical performance of the battery. When selecting lithium salts, factors such as cost and environmental impact are comprehensively considered to achieve the best battery performance.

[0056] Further, step S1 includes:

[0057] S11, mixing the fluorinated carboxylic acid ester solvent, the fluorine source, and the film-forming additive in a glove box containing an inert gas to isolate moisture and oxygen in the air;

[0058] S12, using a magnetic stirrer or other stirring equipment to fully mix the solvent components to obtain a mixed solution.

[0059] The environmental conditions in the glove box are: H2O≤0.1ppm, O2 less than 0.1ppm.

[0060] By mixing the fluorinated carboxylic acid ester solvent, fluorine source and film-forming additive in an inert gas environment, the influence of moisture and oxygen on the electrolyte composition is effectively avoided, thereby ensuring the purity and stability of the electrolyte.

[0061] Optionally, the uniformly mixed electrolyte can be filtered using a microporous filter membrane, and the filtered electrolyte can be degassed and dehumidified to obtain a pure electrolyte.

[0062] According to another aspect of the present invention, a sodium ion battery is provided. The sodium ion battery comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte. The electrolyte is prepared by the above-mentioned method for preparing the modified electrolyte.

[0063] The modified electrolyte prepared by the above method is compared with the conventional battery electrolyte:

[0064] [Comparative Example 1]:

[0065] 1. Preparation of conventional sodium ion battery electrolyte:

[0066] The electrolyte consists of electrolyte sodium salt and an organic solvent.

[0067] The organic solvent is composed of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a weight ratio of EC:PC:EMC:DEC=10:20:30:40.

[0068] The sodium salt is sodium bis(fluorosulfonyl)imide (NaTFSI). The concentration of the sodium salt is 2M.

[0069] The preparation of the electrolyte is carried out in an argon-protected glove box (H2O≤0.1ppm, O2 less than 0.1ppm). During the preparation, the components constituting the above-mentioned organic solvent are first mixed uniformly according to their mass fractions, and then the sodium salt is slowly added and fully stirred to form a mixed solution, thereby obtaining a sodium ion battery electrolyte.

[0070] 2. Sodium ion soft pack battery:

[0071] Sodium-ion batteries include a positive electrode, a negative electrode, a separator and an electrolyte.

[0072] The process humidity is 22% and the process temperature is 25°C. Preparation of positive electrode sheets for sodium ion batteries: Sodium nickel iron manganese oxide, polyvinylidene fluoride, conductive agent Super-P and N-methylpyrrolidone are mixed in a mass ratio of 95:2.5:2.5, and a slurry is obtained by stirring. The slurry is coated on the current collector aluminum foil on a coating machine and then dried. The positive electrode sheets are obtained by cold pressing, cutting, sheeting, powdering and sheet selection.

[0073] Preparation of negative electrode sheets for sodium ion batteries: hard carbon is used as the negative electrode active material, mixed with thickener CMC, binder SBR (styrene-butadiene rubber latex), conductive agent Super-P and deionized water in a mass ratio of 94:2:2:2, and a slurry is obtained by stirring. The slurry is coated on the current collector aluminum foil on a coater and then dried. The negative electrode sheets are obtained after cold pressing, cutting, sheet making, powder sweeping and sheet selection.

[0074] The positive electrode and the negative electrode are stacked into a single cell by a Z-type stacking process in the manner of coating the positive electrode with the negative electrode, and the separator is placed between adjacent positive and negative electrode sheets. The soft-pack sodium battery is then prepared by aluminum-plastic film packaging, liquid injection, and chemical formation. The rated capacity of the battery is 2.0Ah.

[0075] [Comparative Example 2]:

[0076] Compared with Comparative Example 1, only the sodium salt is replaced by sodium hexafluorophosphate (NaPF6), and the remaining steps are the same as those of Comparative Example 1.

[0077] [Example 1]:

[0078] 1. Preparation of modified electrolyte:

[0079] A modified electrolyte consists of sodium salt and organic solvent. The organic solvent consists of the following raw materials: 50% fluorinated carboxylic acid ester compound solvent, 30% perfluorodiethylene glycol dimethyl ether solvent and 20% fluoroethylene carbonate film-forming additive.

[0080] The fluorinated carboxylic acid ester solvent is ethyl difluoroacetate (EDFA).

[0081] The sodium salt is sodium bis(fluorosulfonyl)imide (NaTFSI). The concentration of the sodium salt is 2M.

[0082] The preparation of the modified electrolyte is carried out in an argon-protected glove box (H2O≤0.1ppm, O2 less than 0.1ppm). During the preparation, the components in the above-mentioned organic solvent are first mixed uniformly according to their mass fractions, and then the sodium salt is slowly added and fully stirred to form a mixed solution to obtain the modified electrolyte.

[0083] 2. Sodium ion soft pack battery:

[0084] The sodium ion battery of the present invention comprises a positive electrode material, a negative electrode material, a separator and an electrolyte.

[0085] Preparation of positive electrode sheets for sodium ion batteries: The process humidity is 22% and the process temperature is 25°C. Sodium nickel iron manganese oxide, polyvinylidene fluoride, conductive agent Super-P and N-methylpyrrolidone are mixed in a mass ratio of 94:3:3, and a slurry is obtained by stirring. The slurry is coated on the current collector aluminum foil on a coating machine and then dried. The positive electrode sheets are obtained after cold pressing, sheet cutting, sheet making, powder sweeping and sheet selection.

[0086] Preparation of negative electrode sheets for sodium ion batteries: The process humidity is 22% and the process temperature is 25°C. Hard carbon is used as the negative electrode active material, mixed with thickener CMC, binder SBR (styrene butadiene rubber latex), conductive agent Super-P and deionized water in a mass ratio of 94:2:2:2, and a slurry is obtained by stirring. The slurry is applied to the current collector aluminum foil on a coating machine and then dried. The negative electrode sheet is obtained after cold pressing, cutting, sheet making, powder sweeping and sheet selection.

[0087] The positive electrode and the negative electrode are stacked into a single cell by a Z-type stacking process in the manner of coating the positive electrode with the negative electrode, and the separator is placed between adjacent positive and negative electrode sheets. The soft-pack sodium battery is then prepared by aluminum-plastic film packaging, liquid injection, and chemical formation. The rated capacity of the battery is 2.0Ah.

[0088] [Example 2]:

[0089] Compared with Example 1, only the fluorinated carboxylic acid ester solvent was replaced with ethyl fluoroacetate (EFA), and the remaining steps were the same as those in Example 1.

[0090] [Example 3]:

[0091] Compared with Example 1, only the fluorinated carboxylic acid ester solvent is replaced with ethyl trifluoroacetate (ETFA), and the remaining steps are the same as those in Example 1.

[0092] [Example 4]:

[0093] Compared with Example 1, only the raw material composition of the organic solvent is replaced with: 80% fluorinated carboxylic acid ester compound solvent and 20% fluoroethylene carbonate film-forming additive, and the remaining steps are the same as Example 1.

[0094] [Example 5]:

[0095] Compared with Example 1, only the raw material composition of the organic solvent is replaced with: 80 wt % perfluorodiethylene glycol dimethyl ether solvent and 20 wt % fluoroethylene carbonate film-forming additive, and the remaining steps are the same as those in Example 1.

[0096] The electrochemical performance tests of the above-mentioned Examples 1-5 and Comparative Examples 1-2 were conducted using a Titan 5V60A battery test cabinet and the tests were conducted within a voltage range of 1.5-4.0V.

[0097] The voltage range of 1.5-4.0V is a common electrochemical window for sodium-ion batteries. The positive and negative electrode materials of sodium-ion batteries show good electrochemical activity within this voltage range, and can carry out effective charge and discharge processes. Testing the performance within this range can accurately reflect the performance of the battery in actual use.

[0098] The voltage test range of 1.5-4.0V covers most of the electrochemical reactions that the electrolyte may undergo in a sodium-ion battery. By testing within this range, the ionic conductivity of the electrolyte at different voltages, the formation and stability of the SEI film, and the cycle performance and rate performance of the battery can be effectively evaluated, thereby judging the quality of the electrolyte formulation.

[0099] The test results are as follows:

[0100] As shown in Table 1, the capacities of Examples 1-5 and Comparative Examples 1-2 all meet the standards at room temperature. When the number of cycles of the battery reaches 400, 800, and 2000, the discharge capacity retention rates of Examples 1-5 are all higher than those of the Comparative Examples. This indicates that the modified electrolytes formulated in Examples 1-5 can improve the cycle life of sodium ion batteries.

[0101] It can be seen from Table 2 that when the battery is discharged at -20°C, -40°C, and -50°C, the discharge capacity of Examples 1-5 is higher than that of the comparative example. In particular, under a -50°C environment, the discharge capacity of Examples 1-5 is between 0.7 and 1.0 Ah, while the comparative example can no longer release capacity, which indicates that the modified electrolytes of Examples 1-5 can significantly improve the low-temperature discharge performance of sodium ion batteries.

[0102] Table 1: Cycle life of batteries of the embodiments and comparative examples at room temperature.

[0103]

[0104] Table 2: Low temperature charge and discharge test results of the embodiments and comparative examples.

[0105]

[0106] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0107] 1. By using a specific ratio of fluorinated carboxylic acid ester solvents, perfluorodiethylene glycol dimethyl ether solvents and fluoroethylene carbonate film-forming additives, the electrolyte of this scheme can maintain high ionic conductivity in a low temperature environment, significantly improving the charging and discharging efficiency and safety of sodium ion batteries under low temperature conditions.

[0108] 2. Use a mixed new sodium salt, among which sulfonates (NaTFSI, NaFSI) have strong perfluoroalkyl electron-withdrawing groups and stable conjugated structures, resulting in a high degree of electron delocalization, which can achieve high solubility even in solvents with low dielectric constants, further improving the rate performance of sodium-ion batteries at low temperatures.

[0109] 3. Compared with conventional electrolyte formulas, the electrolyte of this scheme can promote the formation of a more stable and stronger SEI film, effectively reduce the capacity decay of the battery during the cycle process, and significantly extend the cycle life of the sodium-ion battery.

[0110] For ease of description, spatially relative terms such as "above", "above", "on the upper surface of", "above", etc. may be used here to describe the spatial positional relationship between a device or feature and other devices or features as shown in the figure. It should be understood that spatially relative terms are intended to include different orientations of the device in use or operation in addition to the orientation described in the figure. For example, if the device in the accompanying drawings is inverted, the device described as "above other devices or structures" or "above other devices or structures" will be positioned as "below other devices or structures" or "below other devices or structures". Thus, the exemplary term "above" can include both "above" and "below". The device can also be positioned in other different ways (rotated 90 degrees or in other orientations), and the spatially relative descriptions used here are interpreted accordingly.

[0111] In addition to the above, it should be noted that "one embodiment", "another embodiment", "embodiment", etc. mentioned in this specification refer to the specific features, structures or characteristics described in conjunction with the embodiment included in at least one embodiment generally described in this application. The same expression appearing in multiple places in the specification does not necessarily refer to the same embodiment. Further, when describing a specific feature, structure or characteristic in conjunction with any embodiment, it is claimed that the realization of such feature, structure or characteristic in conjunction with other embodiments also falls within the scope of the present invention.

[0112] In the above embodiments, the description of each embodiment has its own emphasis. For parts that are not described in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0113] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a modified electrolyte, characterized in that: The preparation method comprises: Step S1: mixing a fluorinated carboxylic acid ester solvent, a fluorine source, and a film-forming additive to obtain a mixed solution; Step S2: adding a sodium source or a lithium source to the mixed solution to obtain a modified electrolyte.

2. The method for preparing the modified electrolyte according to claim 1, characterized in that: In the step S1, the proportion of the fluorinated carboxylic acid ester solvent in the mixed solution is 40% to 100% by mass, and / or the proportion of the fluorine source in the mixed solution is 0% to 30%, and / or the proportion of the film-forming additive in the mixed solution is 0-30%.

3. The method for preparing the modified electrolyte according to claim 1 or 2, characterized in that: The fluorinated carboxylic acid ester solvent is at least one of ethyl fluoroacetate, ethyl difluoroacetate and ethyl trifluoroacetate.

4. The method for preparing a modified electrolyte according to claim 1 or 2, characterized in that: In step S2, the sodium source is selected from at least one of sodium bis(fluorosulfonyl)imide, sodium hexafluorophosphate, sodium bis(fluorooxyborate), and sodium tetrafluoroborate.

5. The method for preparing the modified electrolyte according to claim 4, characterized in that: The concentration of the sodium source is set to 1M-10M.

6. The method for preparing the modified electrolyte according to claim 1, characterized in that: In the step S1, the fluorine source is a perfluorodiethylene glycol dimethyl ether solvent.

7. The method for preparing the modified electrolyte according to claim 1, characterized in that: In the step S1, the film-forming additive is a fluoroethylene carbonate film-forming additive.

8. The method for preparing the modified electrolyte according to claim 1, characterized in that: In step S2, the lithium source is selected from at least one of lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, lithium trifluoromethanesulfonate, lithium perchlorate, and lithium borate.

9. The method for preparing the modified electrolyte according to claim 1, characterized in that: The step S1 comprises: S11, mixing the fluorinated carboxylic acid ester solvent, the fluorine source and the film-forming additive in a glove box containing an inert gas to isolate moisture and oxygen in the air; S12, using a magnetic stirrer or other stirring equipment to fully mix the solvent components to obtain the mixed solution.

10. A sodium ion battery, characterized in that: The sodium ion battery comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, and the electrolyte is prepared by the preparation method of the modified electrolyte described in any one of claims 1 to 9.

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