Electrolyte functional additive, preparation method thereof, electrolyte containing electrolyte functional additive and sodium ion battery

Abstract

The invention relates to an electrolyte functional additive, a preparation method thereof, an electrolyte containing the electrolyte functional additive and a sodium ion battery. The electrolyte functional additive is prepared from dopamine hydrochloride, imidazolium salt and fluorine-containing anion salt according to a specific mass ratio through the processes of auto-polymerization, heating and stirring, grafting and solvent removal.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage battery technology, specifically relating to an electrolyte functional additive and its preparation method, an electrolyte containing the additive, and a sodium-ion battery. Background Technology

[0002] Currently, lithium-ion batteries have been successfully applied in portable electronic devices and electric vehicles. However, lithium constitutes only about 0.0065% of the Earth's crust, making raw materials scarce and expensive. In 2020, my country's lithium raw material production accounted for only 24% of the global total, indicating a high dependence on imports. The scarcity of lithium-ion battery materials and rising costs have hindered the development of lithium batteries. In contrast, sodium is extremely abundant on Earth, and sodium-ion batteries (SIBs) offer similar performance to lithium-ion batteries, lower costs, excellent low-temperature performance, and a low redox potential (-2.71V relative to the standard hydrogen electrode). They show great potential for large-scale energy storage applications and have gradually become one of the best candidate batteries for next-generation commercial large-scale energy storage systems.

[0003] Hard carbon is considered one of the most promising anode materials for sodium-ion batteries due to its high sodium storage capacity and low sodium storage potential. The complex molecular structure of hard carbon creates various types of sodium storage active sites, and after optimized modification, it can exceed the theoretical specific capacity of lithium-ion battery graphite, demonstrating strong commercial potential. However, despite substantial progress in the screening of hard carbon sources, structural regulation, and research on sodium ion storage processes in hard carbon, poor gas generation and cycle stability under high-temperature conditions remain significant drawbacks of hard carbon anodes. The electrolyte readily reduces on the surface of the hard carbon anode to form an SEI film, generating gaseous products. This side reaction can lead to short circuits and subsequent safety hazards. The SEI film plays a crucial role in the electrochemical performance of sodium-ion batteries, including cycle stability and rate capability. The SEI film should possess a uniform morphology, good ionic conductivity to facilitate rapid sodium ion migration, and high mechanical strength, high insolubility, and cycle stability. However, high temperatures accelerate the side reactions in sodium-ion batteries and the decomposition of the SEI film, thereby exacerbating gas production and significantly reducing the cycle stability of the hard carbon anode.

[0004] Therefore, a novel electrolyte is needed to improve the electrochemical performance of hard carbon materials at high temperatures. A stable interfacial phase can delay unwanted reactions and prolong the duration of mild, reversible electrochemical reactions. However, there are very few commercially available electrolytes for improving gas generation during high-temperature cycling of hard carbon materials, and there are no reports on research into improving the high-temperature cycling stability of hard carbon materials in sodium-ion batteries by introducing special functional additives. Summary of the Invention

[0005] To address the technical problems of gas generation and instability in high-temperature cycling of hard carbon materials, the present invention aims to provide an electrolyte functional additive, its preparation method, an electrolyte containing the additive, and a sodium-ion battery. The electrolyte provided by the present invention exhibits excellent room-temperature cycling performance, high-temperature cycling performance, and high-temperature storage life, and can significantly reduce gas generation during high-temperature cycling. Furthermore, the preparation method of the electrolyte provided by the present invention is simple, convenient, and efficient, and produces high-quality products.

[0006] In a first aspect, the present invention provides an electrolyte functional additive, which is made by self-polymerization, heating and stirring, grafting and solvent removal processes of dopamine hydrochloride, imidazole salt and fluorine-containing anionic salt in a specific mass ratio.

[0007] Preferably, the mass ratio of dopamine hydrochloride to imidazole salt is 0.1-10:0.1-5, and more preferably 1.2:1; The mass of the fluorinated anionic salt is 0.1-10% of the total mass of dopamine hydrochloride and imidazole salt, preferably 2%.

[0008] Secondly, the present invention provides a method for preparing the above-mentioned electrolyte functional additive, the preparation method comprising the following steps: (1) Add imidazole salt to dopamine hydrochloride solution to carry out self-polymerization reaction. After the reaction is completed, the product is separated and purified for the first time. (2) Disperse the product after the first purification into the grafting reaction solvent, add fluorine-containing anion salt and iodomethane, and carry out the grafting reaction by heating and stirring. (3) The product after the grafting reaction is completed is subjected to a second separation and a second purification to obtain the electrolyte functional additive.

[0009] Preferably, the dopamine hydrochloride is at least one of 3,4-dihydroxyphenylethylamine hydrochloride and 6-hydroxydopamine hydrochloride, and more preferably 3,4-dihydroxyphenylethylamine hydrochloride; The imidazole salt includes at least one of 1-(3-aminopropyl)imidazolium and 2-ethylaminomethylimidazolium dihydrochloride, preferably 1-(3-aminopropyl)imidazolium; The fluorinated anionic salt is at least one of sodium hexafluorophosphate, sodium difluorosulfonyl imide, and sodium bis(trifluoromethylsulfonyl)imide, preferably sodium difluorosulfonyl imide.

[0010] Preferably, the temperature of the self-polymerization reaction is 0-50°C, and the reaction time is 6-12 hours.

[0011] Preferably, the grafting reaction solvent is at least one of methanol, isopropanol, and dimethyl sulfoxide, with methanol being the most preferred. The concentration of the product after the first purification in the grafting reaction solvent is 0.1-10 mg / mL, preferably 2 mg / mL; The ratio of the grafting reaction solvent to iodomethane is 10-20 mL: 10-15 mL.

[0012] Preferably, the grafting reaction is carried out at a temperature of 60–80°C for 6–12 hours.

[0013] Thirdly, the present invention provides an electrolyte for improving the high-temperature circulating gas production of hard carbon materials, comprising an organic solvent, a sodium salt, and the above-mentioned electrolyte functional additives. The concentration of the sodium salt is 0.1-10M, preferably 0.5-3.0M, and more preferably 1.0M; the electrolyte functional additive accounts for 1-50% of the total mass of the electrolyte.

[0014] Preferably, the organic solvent includes at least one of propylene carbonate, ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, fluoroethylene carbonate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, or tetraethylene glycol dimethyl ether. The sodium salt includes at least one of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate, sodium bis(oxalate di)borate, sodium trifluoromethanesulfonate, sodium bis(trifluoromethanesulfonyl)imide, and sodium bis(fluorosulfonyl)imide.

[0015] Fourthly, the present invention provides a sodium-ion battery comprising a hard carbon negative electrode and an electrolyte that improves the high-temperature cycling gas production of the hard carbon material.

[0016] Beneficial effects The electrolyte provided by this invention is simple to prepare and the raw materials are easy to obtain. By adding imidazole salt-dopamine hydrochloride grafted functional additives to the traditional organic sodium-ion battery electrolyte, it is possible to ensure that a dense and uniform SEI with high inorganic components is formed on the surface of the carbon anode, which greatly improves the high-temperature stability of the SEI. This is beneficial to effectively improve the cycle stability of hard carbon anode and improve the problem of high-temperature cycle gas generation of hard carbon materials. Attached Figure Description

[0017] Figure 1 This is the first-cycle charge-discharge curve of the hard carbon anode in Example 1; Figure 2 This is a charge-discharge curve of the hard carbon anode in Example 2 during the first cycle. Figure 3 The first-cycle charge-discharge curve of the hard carbon anode in Comparative Example 1 is shown. Figure 4 The first-cycle charge-discharge curve of the hard carbon anode in Comparative Example 2 is shown. Figure 5The first-cycle charge-discharge curve of the hard carbon anode in Comparative Example 3 is shown. Figure 6 The first-cycle charge-discharge curve of the hard carbon anode in Comparative Example 4 is shown. Detailed Implementation

[0018] The present invention will be further illustrated by the following embodiments. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the present invention.

[0019] First, this invention provides an electrolyte functional additive. The electrolyte functional additive is prepared by a specific mass ratio of dopamine hydrochloride, imidazole salt, and fluorinated anionic salt through a process involving self-polymerization, heating and stirring, grafting, and solvent removal.

[0020] The following is an exemplary description of a method for preparing the electrolyte functional additive provided by the present invention. The preparation method may include the following steps: First, imidazole salt was added to the dopamine hydrochloride solution to carry out a self-polymerization reaction. After the reaction was completed, the product was separated and purified for the first time. Then, the product after the first purification is dispersed in the grafting reaction solvent, a fluorine-containing anion salt and iodomethane are added, and the grafting reaction is carried out by heating and stirring. Finally, the product after the grafting reaction is completed undergoes a second separation and a second purification to obtain the electrolyte functional additive.

[0021] In some embodiments, the dopamine hydrochloride may be at least one of 3,4-dihydroxyphenylethylamine hydrochloride and 6-hydroxydopamine hydrochloride, preferably 3,4-dihydroxyphenylethylamine hydrochloride; more preferably, the solvent of the dopamine hydrochloride solution may be a tris(hydroxymethyl)aminomethane-hydrochloric acid buffer (Tris-HCl) with a molar concentration of 10 mM and a pH of 8.5.

[0022] In some embodiments, the imidazole salt may include at least one of 1-(3-aminopropyl)imidazolium and 2-ethylaminomethylimidazolium dihydrochloride, preferably 1-(3-aminopropyl)imidazolium.

[0023] In some embodiments, the mass ratio of dopamine hydrochloride to imidazole salt can be 0.1-10:0.1-5, preferably 1.2:1. The reason for controlling the mass ratio within this specific range is to control the amount of self-polymerization and grafting reactions, thereby regulating the component morphology of the prepared functional additive. If the mass ratio is too high, the amount of grafting reaction will be too small, resulting in an excessively high content of polydopamine in the obtained additive component, making it difficult to promote the formation of the SEI film on the hard carbon surface during the electrochemical reaction. If the mass ratio is too low, the amount of self-polymerization reaction of dopamine hydrochloride will be too small, making it difficult to induce a grafting reaction with imidazole salt, resulting in too little grafted product in the obtained electrolyte additive, thus failing to achieve the technical effects of the present invention.

[0024] In some embodiments, the temperature of the self-polymerization reaction can be 0–50°C, and the reaction time can be 6–12 hours. The self-polymerization reaction refers to the polymerization of dopamine hydrochloride to form polydopamine. Excessively high temperatures in the self-polymerization reaction can lead to excessively rapid polymerization and difficulty in controlling the structure, while excessively low temperatures can hinder the reaction from occurring. Excessively long reaction times can cause the product to decompose due to prolonged exposure at that temperature, while excessively short reaction times can result in incomplete reaction.

[0025] In some embodiments, the grafting reaction solvent can be at least one of methanol, isopropanol, and dimethyl sulfoxide, preferably methanol.

[0026] In some embodiments, the concentration of the product after the first purification in the grafting reaction solvent can be controlled to be 0.1-10 mg / mL, preferably 2 mg / mL.

[0027] The fluorinated anion salt can be at least one of sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, and sodium bis(trifluoromethylsulfonyl)imide, preferably sodium bis(fluorosulfonyl)imide. In some embodiments, the mass of the fluorinated anion salt can be 0.1-10% of the total mass of dopamine hydrochloride and imidazole salt, preferably 2%. If the mass ratio of the fluorinated anion salt is too high, it will hinder the grafting reaction, resulting in too little grafted product in the obtained electrolyte additive, and failing to achieve the technical effect of the present invention; conversely, if its mass ratio is too low, it will result in less inorganic components in the SEI film on the hard carbon surface, hindering the transport of sodium ions and deteriorating the electrochemical performance of the hard carbon anode.

[0028] The addition of iodomethane can promote the grafting reaction. In some embodiments, the ratio of the grafting reaction solvent to iodomethane can be controlled at 10-20 mL: 10-15 mL.

[0029] In some embodiments, the grafting reaction temperature can be 60–80°C, and the reaction time can be 6–12 hours. The grafting reaction refers to imidazole grafting, a chemical reaction process mainly involving the bonding of imidazole compounds to the surface or molecular chains of other substances. Excessively high grafting temperatures reduce the number of active sites, thus affecting the efficiency and grafting rate; excessively low temperatures result in a slow reaction rate, hindering the grafting process and reducing the grafting rate, thus affecting product performance. Excessively long reaction times lead to decreased grafted product performance and increased energy consumption, while excessively short reaction times result in incomplete reactions, reduced grafting rates, and poor product performance.

[0030] A functional additive with surface-modified groups is formed through the self-polymerization and grafting reactions of dopamine hydrochloride and imidazole salt. This invention differs from conventional methods that utilize the nitrogen-containing properties of dopamine hydrochloride itself to dope graphene oxide. During hydrothermal processing, dopamine hydrochloride undergoes a cross-linking reaction in the graphene oxide solution, achieving the purpose of modifying the graphene oxide. Simultaneously, the imidazole salt used in this invention also differs from conventional methods where it is used as an electrolyte solvent, i.e., an ionic liquid. In conventional methods, imidazole salts generally only act as dispersants and do not chemically react with components such as silica.

[0031] The technical solution described in this invention utilizes a chemical reaction between dopamine hydrochloride and imidazole salts and fluorinated anionic salts, i.e., a grafting reaction occurs under certain conditions, generating new additive substances. The special additive prepared is an electrolyte additive that mitigates the adverse effects of organic solvent decomposition on the carbon anode surface in the battery system, promoting the formation of a dense and uniform SEI with high inorganic components on the carbon anode surface.

[0032] The present invention also provides an electrolyte for improving the circulating gas generation of hard carbon materials at high temperatures (50-70°C).

[0033] The electrolyte for improving the high-temperature circulating gas production of hard carbon materials may include an organic solvent, a sodium salt, and the above-mentioned electrolyte functional additives; wherein the concentration of the sodium salt can be controlled to be 0.1-10M, preferably 0.5-3.0M, more preferably 1.0M; the electrolyte functional additives may account for 1-50% of the total mass of the electrolyte.

[0034] Too low a sodium salt concentration results in low electrolyte ionic conductivity, making it difficult to meet the sodium ion intercalation / extraction requirements and affecting battery performance. Too high a sodium salt concentration increases electrolyte viscosity, reducing ion conduction efficiency and impacting charge / discharge performance. An excessively high additive percentage can lead to incomplete dissolution in the electrolyte, forming precipitates or deposits that may block ion transport channels, affecting battery performance. Furthermore, excessive additives can reduce sodium ion mobility (the rate at which sodium ions conduct in the electrolyte), further decreasing charge / discharge performance. Conversely, an excessively low additive percentage will result in ineffective additive activity.

[0035] In some embodiments, the organic solvent may include at least one of propylene carbonate, ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, fluoroethylene carbonate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, or tetraethylene glycol dimethyl ether.

[0036] In some embodiments, the sodium salt may include at least one of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate, sodium bis(oxalate di)borate, sodium trifluoromethanesulfonate, sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), and sodium bis(fluorosulfonyl)imide (NaFSI).

[0037] This invention relates to a functional additive formed by grafting dopamine hydrochloride and imidazole salt into the electrolyte of a sodium-ion battery. This functional additive can form inorganic components such as sodium nitride and sodium fluoride on the surface of the hard carbon anode during the electrochemical reaction, ensuring the formation of a thin, dense, and uniform SEI with high inorganic content on the carbon anode surface. This significantly improves the stability of the SEI under high-temperature conditions, thereby reducing direct contact between the electrolyte and the hard carbon anode during high-temperature cycling, mitigating the adverse effects of organic solvent decomposition and gas generation on the carbon anode surface, effectively improving the cycle stability of the carbon anode, suppressing gas generation in the hard carbon material during high-temperature cycling, and enhancing the high-temperature cycle performance and safety of hard carbon-based sodium-ion batteries.

[0038] The electrolyte provided by this invention has excellent room temperature cycling performance, high temperature cycling performance and high temperature storage life, and can significantly reduce the amount of gas generated during high temperature cycling.

[0039] The following is an exemplary description of a method for preparing an electrolyte that improves high-temperature circulating gas generation in hard carbon materials, provided by the present invention. The preparation method may include the following steps: First, the sodium salt is added to an organic solvent to form a mixed solution; Then, an electrolyte functional additive is added to the mixed solution, stirred evenly, and dehydrated using a 3A molecular sieve to obtain the electrolyte that improves the high-temperature circulating gas production of hard carbon materials.

[0040] In some embodiments, the temperature at which the sodium salt is added to the organic solvent can be controlled between -5 and 15°C. Too low a temperature will affect the dissolution of the sodium salt, making it difficult to dissolve; too high a temperature will lead to side reactions.

[0041] In some embodiments, the organic solvent is purified to ensure that the moisture content is less than 5 ppm, and then the organic solvent, sodium salt and additive are uniformly mixed under a pressure of 0.01-0.1 MPa, wherein the moisture content in the organic solvent is less than 6 ppm.

[0042] The electrolyte preparation method provided by this invention is simple, requiring only the organic solvent, sodium salt and additives to be mixed evenly in proportion. It is convenient to prepare, has high production efficiency and low production cost, and can effectively improve the phenomenon of gas generation during high-temperature cycling of hard carbon materials, thereby improving the cycling stability of sodium-ion batteries assembled with hard carbon anodes at high temperatures.

[0043] The electrolyte obtained by the preparation method provided by this invention, which improves the high-temperature cycling gas production of hard carbon materials, can be applied to sodium-ion batteries with hard carbon anodes.

[0044] The following examples further illustrate the present invention in detail. It should also be understood that the following examples are only for further explanation of the present invention and should not be construed as limiting the scope of protection of the present invention. Non-essential improvements and adjustments made by those skilled in the art based on the above description of the present invention are all within the scope of protection of the present invention. The specific process parameters, etc., in the following examples are merely examples within a suitable range; that is, those skilled in the art can make appropriate selections within the range based on the description herein, and are not intended to be limited to the specific values ​​in the examples below. Unless otherwise specified, the experimental materials involved in the following examples can be purchased commercially or obtained through conventional preparation methods in the art.

[0045] Example 1

[0046] The method for preparing an electrolyte that improves high-temperature circulating gas production in hard carbon materials, as provided in this embodiment, includes the following steps: (1) Preparation of electrolyte functional additives: First, 1-(3-aminopropyl)imidazole was added to a 3,4-dihydroxyphenylethylamine hydrochloride solution at 30°C, and the reaction was carried out by continuous stirring for 10-12 hours to carry out the polymerization reaction. The mass ratio of 3,4-dihydroxyphenylethylamine hydrochloride to 1-(3-aminopropyl)imidazole was controlled at 1.2:1. The solvent of the 3,4-dihydroxyphenylethylamine hydrochloride solution was a 10 mM tris(hydroxymethyl)aminomethane-hydrochloric acid buffer (Tris-HCl) with a pH of 8.5. After the reaction was completed, the product was separated and purified. Then, the purified product obtained above was dispersed in methanol (concentration of 2 mg / mL), and sodium difluorosulfonamide and iodomethane were added. The mass of sodium difluorosulfonamide accounted for 2% of the total mass of dopamine hydrochloride and 1-(3-aminopropyl)imidazole. The amount of iodomethane was controlled to be 10-15 mL for every 10-20 mL of methanol dispersion. The grafting reaction was carried out by stirring continuously at 60-80°C for 6-12 h. Finally, the product after the grafting reaction is completed is separated and purified to obtain the electrolyte functional additive. (2) Preparation of electrolyte for improving high-temperature circulating gas generation of hard carbon materials: First, sodium perchlorate is added to an organic solvent composed of ethylene carbonate (EC) and propylene carbonate (PC) (the volume ratio of EC to PC is 1:1) to form a mixed solution. Then, the electrolyte functional additive prepared in step (1) is added to the mixed solution, stirred evenly and dehydrated using 3A molecular sieve to obtain an electrolyte that improves the high-temperature circulating gas production of hard carbon materials. In the electrolyte used to improve the high-temperature circulating gas production of hard carbon materials, the molar concentration of sodium perchlorate is 1M, and the mass fraction of the electrolyte functional additive is 4% (the mass fraction of the total mass of the electrolyte).

[0047] Figure 1 This is the first-cycle charge-discharge curve of the hard carbon anode in Example 1.

[0048] Example 2

[0049] The preparation method of the electrolyte for improving high-temperature circulating gas production of hard carbon materials provided in this embodiment is the same as in Example 1, with the main difference being: The mass fraction of electrolyte functional additives in the final prepared electrolyte for improving high-temperature circulating gas production of hard carbon materials is 2%.

[0050] Figure 2 This is the first-cycle charge-discharge curve of the hard carbon anode in Example 2.

[0051] Comparative Example 1

[0052] The preparation method of the electrolyte provided in this comparative example is the same as that in Example 1, with the main difference being: The additive in step (1) is prepared by the following process: the 3,4-dihydroxyphenylethylamine hydrochloride solution is continuously stirred and reacted for 10-12 hours. The solvent of the 3,4-dihydroxyphenylethylamine hydrochloride solution is controlled to be a tris(hydroxymethyl)aminomethane-hydrochloric acid buffer (Tris-HCl) with a molar concentration of 10 mM and a pH of 8.5. After stirring, the product is separated and purified to obtain the additive.

[0053] Figure 3The first-cycle charge-discharge curve of the hard carbon anode in Comparative Example 1 is shown.

[0054] Comparative Example 2

[0055] The preparation method of the electrolyte provided in this comparative example is the same as that in Example 1, with the main difference being: The additives in step (1) are prepared using the following process: First, 1-(3-aminopropyl)imidazole was added to a 10 mM tris(hydroxymethyl)aminomethane-hydrochloric acid buffer (Tris-HCl) with a pH of 8.5, and the mixture was stirred continuously for 10–12 h. After stirring, the product was separated and purified. Then, the purified product was dispersed in methanol (2 mg / mL), and iodomethane was added, with 10–15 mL of iodomethane corresponding to every 10–20 mL of methanol dispersion. The mixture was stirred continuously at 60–80 °C for 6–12 h. Finally, the product after stirring is separated and purified to obtain the additive.

[0056] Figure 4 The first-cycle charge-discharge curve of the hard carbon anode in Comparative Example 2 is shown.

[0057] Comparative Example 3

[0058] The preparation method of the electrolyte provided in this comparative example is the same as that in Example 1, with the main difference being: The additive in step (1) is prepared by the following process: sodium difluorosulfonamide is dispersed in methanol (concentration of 0.04 mg / mL), iodomethane is added, and 10-15 mL of iodomethane is added for every 10-20 mL of methanol dispersion. The mixture is stirred continuously at 60-80°C for 6-12 h. Finally, the product after stirring is separated and purified to obtain the additive.

[0059] Figure 5 The first-cycle charge-discharge curve of the hard carbon anode in Comparative Example 3 is shown.

[0060] Comparative Example 4

[0061] The preparation method of the electrolyte provided in this comparative example includes the following steps: adding sodium perchlorate to an organic solvent composed of ethylene carbonate (EC) and propylene carbonate (PC) (the volume ratio of EC to PC is 1:1) to form a mixed solution, stirring evenly and then treating with 3A molecular sieve to remove water, to obtain the electrolyte; wherein, the molar concentration of sodium perchlorate in the electrolyte is 1M.

[0062] Figure 6 The first-cycle charge-discharge curve of the hard carbon anode in Comparative Example 4 is shown.

[0063] Electrode coating: A slurry containing 80% hard carbon, 10% super P, and 10% carboxymethyl cellulose was coated onto aluminum foil to prepare a hard carbon anode. After drying at 100℃ for 10 h, the mass loading of the active material was approximately 15 mg / cm³. -2 A hard carbon anode was prepared by coating an aluminum foil with a slurry containing 80% sodium vanadium phosphate, 10% super P, and 10% polyvinylidene fluoride. After drying at 100°C for 10 hours, the mass loading of the active material was approximately 12 mg / cm³. -2 .

[0064] Assemble sodium-ion batteries: Assemble pouch cells using a PE separator with a thickness of 9 μm, the electrolyte prepared in the examples and comparative examples, and the negative and positive electrode sheets prepared above. After the batteries have been left to stand for 12 hours, the battery volume (V1) is tested using the water displacement method.

[0065] Battery activation: The assembled batteries are placed in a 45°C constant temperature chamber and activated using a Blue Electric charge / discharge device. The activation voltage range is 1.2–4.3V, the charge / discharge rate is 0.05C, and the charge / discharge process is performed for 3 weeks.

[0066] Battery cycle stability test: The activated batteries were subjected to 1C and 1000-cycle cycle stability tests at room temperature and 60°C, respectively, with a charge / discharge voltage range of 1.2–4.3V. During the test, the instrument automatically recorded the charge / discharge data and related curves.

[0067] Volume change ΔV: The volume (V2) of the battery after 1000 cycles at 60℃ and 1C using the water displacement method is calculated as ΔV = V2 - V1.

[0068] The experimental results comparing the cycling stability at room temperature and high temperature and the volume change after high temperature cycling in Examples 1 and 2 and Comparative Examples 1-4 are shown in Table 1 below:

[0069] The results in Table 1 show that the addition of 4% imidazole salt grafted functional additive significantly suppressed gas generation during the charge-discharge cycle of hard carbon materials under high temperature conditions. Compared with the carbonate electrolyte in Comparative Example 4, the high-temperature cycle performance of hard carbon-based sodium-ion batteries assembled with the electrolytes in Examples 1 and 2 and Comparative Examples 1 and 2 was significantly improved, which is attributed to the technical effect of the additive.

[0070] The results in Table 1 show that, compared to the electrolyte test results in Comparative Example 4, the cycle stability of the hard carbon-vanadium phosphate electrolytes in Examples 1 and 2, and Comparative Examples 1 and 2, was improved. Compared to the carbonate electrolyte in Comparative Example 3, the cycle stability of the hard carbon anode in Examples 1 and 2 was also significantly improved. The hard carbon anode showed the best cycle stability in the electrolyte test in Example 1. Compared to the electrolytes with single dopamine hydrochloride additive, single imidazole salt functional additive, and single fluorinated anion salt additive in Comparative Examples 1, 2, and 3, the sodium-ion battery exhibited better high-temperature cycle stability and less high-temperature gas production. This indicates that the synergistic effect of imidazole salt and dopamine hydrochloride, and fluorinated anion salt after grafting reaction is superior to the suppression effect of any single component on high-temperature cycle gas production of hard carbon materials. The hard carbon anode exhibited the best cycle stability in the electrolyte test of Example 1. Compared with the electrolyte with a low amount of functional additives in Example 2, it showed relatively better cycle stability and less gas production, further verifying the role of the imidazole salt-dopamine salt grafted functional additives in improving the high-temperature cycle stability of hard carbon materials and suppressing high-temperature cycle gas production.

[0071] In summary, this invention proposes a sodium-ion battery electrolyte and its preparation method to improve gas generation during high-temperature cycling of hard carbon materials. This involves adding imidazole salt-dopamine salt grafted functional additives to a traditional organic electrolyte. Hard carbon sodium-ion batteries assembled based on this electrolyte can maintain a capacity retention of over 90% after 1000 cycles at 60°C and 1C, with gas generation suppressed. The electrolyte preparation process of this invention is simple, and the raw materials are abundant. It can effectively improve the stability of the solid electrolyte interface (SEI) on the surface of hard carbon materials under high-temperature conditions, extend the high-temperature cycle life of the hard carbon anode, and significantly improve the safety and cycle stability of sodium-ion batteries based on hard carbon materials in practical applications, demonstrating significant practical value.

[0072] Although the present invention has been described in detail through the preferred embodiments above, it should be understood that the above description should not be considered as a limitation of the present invention. Various modifications and substitutions to the present invention will be apparent to those skilled in the art after reading the above description. Therefore, the scope of protection of the present invention should be defined by the appended claims.

Claims

1. An electrolyte functional additive, characterized in that, The electrolyte functional additive is prepared by self-polymerization, heating and stirring, grafting and solvent removal processes of dopamine hydrochloride, imidazole salt and fluorine-containing anion salt in a specific mass ratio.

2. The electrolyte functional additive according to claim 1, characterized in that, The mass ratio of dopamine hydrochloride to imidazole salt is 0.1-10:0.1-5, preferably 1.2:1; The mass of the fluorinated anionic salt is 0.1-10% of the total mass of dopamine hydrochloride and imidazole salt, preferably 2%.

3. A method for preparing the electrolyte functional additive according to claim 1 or 2, characterized in that, The preparation method includes the following steps: (1) Add imidazole salt to dopamine hydrochloride solution to carry out self-polymerization reaction. After the reaction is completed, the product is separated and purified for the first time. (2) Disperse the product after the first purification into the grafting reaction solvent, add fluorine-containing anion salt and iodomethane, and carry out the grafting reaction by heating and stirring. (3) The product after the grafting reaction is completed is subjected to a second separation and a second purification to obtain the electrolyte functional additive.

4. The preparation method according to claim 3, characterized in that, The dopamine hydrochloride is at least one of 3,4-dihydroxyphenylethylamine hydrochloride and 6-hydroxydopamine hydrochloride, preferably 3,4-dihydroxyphenylethylamine hydrochloride; The imidazole salt includes at least one of 1-(3-aminopropyl)imidazolium and 2-ethylaminomethylimidazolium dihydrochloride, preferably 1-(3-aminopropyl)imidazolium; The fluorinated anionic salt is at least one of sodium hexafluorophosphate, sodium difluorosulfonyl imide, and sodium bis(trifluoromethylsulfonyl)imide, preferably sodium difluorosulfonyl imide.

5. The preparation method according to claim 3 or 4, characterized in that, The self-polymerization reaction is carried out at a temperature of 0-50℃ for 6-12 hours.

6. The preparation method according to any one of claims 3-5, characterized in that, The grafting reaction solvent is at least one of methanol, isopropanol, and dimethyl sulfoxide, preferably methanol; The concentration of the product after the first purification in the grafting reaction solvent is 0.1-10 mg / mL, preferably 2 mg / mL; The ratio of the grafting reaction solvent to iodomethane is 10-20 mL: 10-15 mL.

7. The preparation method according to any one of claims 3-6, characterized in that, The grafting reaction is carried out at a temperature of 60–80°C for 6–12 hours.

8. An electrolyte for improving high-temperature circulating gas generation in hard carbon materials, characterized in that, Includes organic solvents, sodium salts, and electrolyte functional additives as described in claim 1 or 2; The concentration of the sodium salt is 0.1-10M, preferably 0.5-3.0M, and more preferably 1.0M; the electrolyte functional additive accounts for 1-50% of the total mass of the electrolyte.

9. The electrolyte for improving high-temperature circulating gas generation in hard carbon materials according to claim 8, characterized in that, The organic solvent includes at least one of propylene carbonate, ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, fluoroethylene carbonate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, or tetraethylene glycol dimethyl ether. The sodium salt includes at least one of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate, sodium bis(oxalate di)borate, sodium trifluoromethanesulfonate, sodium bis(trifluoromethanesulfonyl)imide, and sodium bis(fluorosulfonyl)imide.

10. A sodium-ion battery comprising a hard carbon negative electrode and an electrolyte as described in claim 8 or 9 that improves the high-temperature cycling gas production of hard carbon materials.

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