Flame-retardant electrolyte, preparation method and application thereof
By introducing perfluorohexane derivative flame retardants and co-solvents into the electrolyte of lithium-ion batteries, a stable SEI film is formed, which solves the safety problem of lithium-ion batteries after the energy density is improved, and achieves high efficiency flame retardancy and improved safety.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- HUAZHONG UNIV OF SCI & TECH
- Filing Date
- 2022-11-08
- Publication Date
- 2026-06-02
AI Technical Summary
While existing lithium-ion batteries have improved energy density, safety issues have become prominent, especially the flammability and explosiveness of the electrolyte, and traditional flame retardants can damage battery performance.
Perfluorohexane derivatives are used as flame retardant additives, and a co-solvent that does not interact with lithium ions is introduced into the electrolyte. Through the bridging effect of the co-solvent, a stable SEI film is formed, preventing side reactions between the electrode material and the electrolyte at high temperatures.
It improves the flame retardant performance and safety of lithium-ion batteries, avoids thermal runaway, does not damage the electrochemical performance of the battery, and reduces the flammability and explosion risk of the electrolyte.
Smart Images
Figure CN115911552B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of lithium-ion battery flame retardant technology, and more specifically, relates to a flame-retardant electrolyte, its preparation method, and its application. Background Technology
[0002] Lithium-ion batteries are widely used in all aspects of life. In recent years, under the guidance and support of the Ministry of Science and Technology and the Ministry of Industry and Information Technology, my country's lithium-ion battery industry has been developing continuously, and its energy density has been constantly improving. However, while the energy density of lithium-ion batteries is constantly increasing, the energy that can be released in a concentrated manner is also increasing. Once a safety accident occurs, the risk factor is also constantly increasing. This is closely related to people's lives and property safety. Therefore, the safety issue of lithium-ion batteries cannot be ignored.
[0003] Thermal runaway in lithium-ion batteries is largely due to the increased temperature of electrode materials and electrolytes under high-temperature or high-rate charge-discharge conditions. This leads to the decomposition of the solid electrolyte interface and lithium salts on the surfaces of the positive and negative electrodes, releasing reactive oxygen species from the positive electrode. This promotes internal heat release within the lithium-ion battery, causing side reactions and resulting in a large accumulation of heat, ultimately leading to safety hazards such as battery fires and explosions. Furthermore, most lithium-ion battery electrolyte solvents have low boiling points, low flash points, and are flammable, making the electrolytes themselves flammable and explosive.
[0004] Adding flame retardants to traditional carbonate electrolytes is an effective way to reduce their flammability. Common flame retardants include various phosphorus-containing compounds such as phosphates, phosphites, and phosphate esters. However, adding flame retardants such as trimethyl phosphate (TMPa) to the electrolyte significantly degrades the battery's cycle performance. This is mainly because the degradation products of phosphate esters severely affect the formation of the solid electrolyte interphase (SEI), causing the electrolyte to continuously decompose on the graphite surface and co-intercalate with lithium ions between graphite layers. This leads to the peeling off of graphite sheets and a continuous decline in battery performance. Therefore, developing novel flame retardants that do not impair the performance of lithium-ion batteries is particularly important. Summary of the Invention
[0005] In view of the shortcomings of existing technologies and the need for improvement, this invention provides a flame-retardant electrolyte, its preparation method and application. The purpose is to provide a novel electrolyte that improves the flame-retardant capability of lithium-ion batteries while ensuring their electrochemical performance.
[0006] To achieve the above objectives, according to one aspect of the present invention, a flame-retardant electrolyte is provided, comprising: a lithium salt, a carbonate organic solvent, a co-solvent, and a flame-retardant additive;
[0007] The concentration of lithium salt in the carbonate electrolyte, which is composed of the lithium salt and the carbonate organic solvent, is 0.5–5 mol / L; the co-solvent is a solvent that can simultaneously dissolve the flame retardant additive, the lithium salt, and the carbonate organic solvent, and does not interact with lithium ions; the flame retardant additive is a perfluorohexane derivative; the volume ratio of the carbonate electrolyte to the flame retardant additive is 100:(1–30), and the volume ratio of the co-solvent to the flame retardant additive is (4–120):(1–5).
[0008] Furthermore, the number of solvent donors in the carbonate organic solvent is greater than 10; and the number of solvent donors and the dielectric constant of the cosolvent are both less than 10.
[0009] Furthermore, the general structural formula of the flame retardant additive is as follows:
[0010]
[0011] In the formula, R is one of halogen atoms, fluorosulfonyl groups, haloalkyl groups, halobenzene groups, and trimethylsilyl groups.
[0012] Furthermore, one or more hydrogen atoms in the trimethylsilyl group are each independently substituted by one or more of the following substituents: fluorine, trifluoromethyl, fluorosulfonyl, fluorophenyl, chlorophenyl, bromophenyl, iodophenyl, trifluoromethylphenyl, and trifluoromethyl(sulfonylimide lithium).
[0013] Furthermore, the co-solvent is at least one of aromatic compounds, fluorinated ether compounds, and fluorinated ester compounds.
[0014] Furthermore, the lithium salt is at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluorosulfonyl)imide, lithium perchlorate, lithium bis(oxalate-borate), and lithium difluorooxalate-borate.
[0015] Furthermore, the carbonate organic solvent is at least one selected from ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, vinylene carbonate, and fluoroethylene carbonate.
[0016] The present invention also provides a method for preparing the flame-retardant electrolyte as described above, comprising:
[0017] S1. Dissolve the lithium salt in a carbonate organic solvent to obtain a carbonate electrolyte;
[0018] S2. Add flame retardant and co-solvent to the carbonate electrolyte and dissolve to obtain the flame retardant electrolyte.
[0019] Further, the specific steps of step S2 are as follows: first, add a co-solvent to the carbonate electrolyte, then add a flame retardant additive to obtain a flame retardant electrolyte, thus completing the electrolyte preparation; or, first, add a flame retardant additive to the carbonate electrolyte, then add a co-solvent to obtain a flame retardant electrolyte, thus completing the electrolyte preparation; or, first, add a flame retardant additive to the co-solvent to obtain a flame retardant solution, then add the flame retardant solution to the carbonate electrolyte to obtain a flame retardant electrolyte, thus completing the electrolyte preparation.
[0020] Wherein, the co-solvent is a solvent that can simultaneously dissolve the flame retardant additive, the lithium salt, and the carbonate organic solvent, and does not interact with lithium ions; the concentration of lithium salt in the electrolyte composed of lithium salt and carbonate organic solvent is 0.5-5 mol / L; the volume ratio of the carbonate electrolyte to the flame retardant additive is 100:(1-30); the flame retardant additive is a perfluorohexane derivative; and the volume ratio of the co-solvent to the flame retardant additive is (4-120):(1-5).
[0021] The present invention also provides a lithium-ion battery, wherein the electrolyte is the flame-retardant electrolyte as described above.
[0022] In summary, the above-described technical solutions conceived in this invention can achieve the following beneficial effects:
[0023] (1) This invention proposes introducing a co-solvent that does not interact with lithium ions into the electrolyte, thereby introducing an insoluble flame retardant into the conventional electrolyte through the bridging effect of the co-solvent. The solvation result of the flame-retardant electrolyte prepared by this method will not deteriorate the electrochemical performance due to the addition of flame retardant, thus solving the problem of incompatibility between the flame-retardant performance and electrochemical performance of lithium-ion battery electrolytes.
[0024] (2) The flame retardant additive used in this invention, on the one hand, can be heated and vaporized before the electrolyte burns, releasing a large number of fluorine-containing free radicals, which can combine with the hydrogen free radicals generated by the decomposition of the electrolyte, effectively preventing the combustion or explosion of organic electrolytes and greatly improving the safety of lithium-ion batteries; on the other hand, the large amount of fluorine in the flame retardant additive can increase the flash point of the electrolyte, thereby improving the flame retardant effect of the electrolyte. In addition, the flame retardant additive contains a large amount of F element, which can help form a stable and dense protective film on the electrode surface when used to prepare ion batteries, preventing side reactions of electrode materials and electrolyte solvents at high temperatures and avoiding the occurrence of battery thermal runaway.
[0025] (3) The flame retardant additive provided by the present invention can introduce a large number of silicon-containing groups, and can generate a large number of inorganic heat insulation layers containing Si-O bonds when the electrolyte temperature rises. It can also achieve the effect of blocking oxygen, thereby making the electrolyte self-extinguishing.
[0026] (4) The flame-retardant electrolyte based on perfluorohexane derivative flame-retardant additives provided by the present invention has a simple preparation method and low cost. It not only broadens the application field of perfluorohexane derivative organic compounds, but also provides new ideas for the research and development of safe electrolytes. Attached Figure Description
[0027] Figure 1 A schematic diagram of a flame retardant test for comparative example 1 provided in an embodiment of the present invention;
[0028] Figure 2 A schematic diagram of a flame retardant test for Example 8 provided in this embodiment of the invention;
[0029] Figure 3 A schematic diagram of a flame retardant test for Example 14 provided in this embodiment of the invention;
[0030] Figure 4 The self-extinguishing time of electrolytes in Comparative Example 1, Comparative Example 2, and Examples 1-14 are shown in the bar chart for the embodiments of the present invention.
[0031] Figure 5 The first charge-discharge curve of the lithium-graphite half-cell assembled with the electrolyte prepared in Example 14 of the present invention at a rate of 0.1C.
[0032] Figure 6 A comparison chart of the cycling performance of graphite |NCM9811 full cells assembled with the electrolyte of Example 14 and the electrolyte of Comparative Example 1 at 1C rate, provided as an embodiment of the present invention.
[0033] Figure 7 Rate performance diagrams of lithium-graphite half-cells in Example 14 and Comparative Example 1 provided for embodiments of the present invention. Detailed Implementation
[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.
[0035] Example 1
[0036] A flame-retardant electrolyte comprises: a lithium salt, a carbonate organic solvent, a co-solvent, and a flame-retardant additive A.
[0037] The concentration of lithium salt in the carbonate electrolyte, which is composed of lithium salt and carbonate organic solvent, is 0.5–5 mol / L; the volume ratio of carbonate electrolyte to flame retardant additive is 100:(1–30); the volume ratio of co-solvent to flame retardant additive is (4–120):(1–5); the co-solvent is a solvent that can simultaneously dissolve flame retardant additive, lithium salt and carbonate organic solvent, and does not interact with lithium ions; flame retardant additive A is a perfluorohexane derivative.
[0038] To address the incompatibility between flame-retardant electrolytes and electrochemical performance, this embodiment proposes introducing a co-solvent that does not interact with lithium ions into the electrolyte. The co-solvent acts as a bridge to introduce the insoluble flame retardant into the conventional electrolyte. The solvation result of the flame-retardant electrolyte prepared by this method does not deteriorate the electrochemical performance due to the addition of the flame retardant.
[0039] In addition, flame retardant additive A contains a large amount of fluorine, so it can achieve the following effects: (1) It can be heated and vaporized before the electrolyte burns to release a large amount of fluorine free radicals, which can combine with the hydrogen free radicals generated by the decomposition of the electrolyte, effectively preventing the combustion or explosion of organic electrolytes and greatly improving the safety of lithium-ion batteries; (2) The large amount of fluorine contained therein can increase the flash point of the electrolyte, thereby improving the flame retardant effect of the electrolyte; (3) When the electrolyte composed of it and other components is used to prepare lithium-ion batteries, it can generate a stable and dense protective film on the surface of the positive and negative electrodes, preventing the electrode materials and electrolyte solvents or lithium salts from undergoing side reactions at high temperatures, delaying or avoiding the occurrence of battery thermal runaway.
[0040] Overall, the flame-retardant electrolyte proposed in this embodiment can effectively solve the problem of incompatibility between flame-retardant performance and electrochemical performance.
[0041] Preferably, the number of solvent donors in the carbonate organic solvent is greater than 10; and the number of solvent donors and the dielectric constant of the cosolvent are both less than 10.
[0042] The higher the number of donors in the solvent, the stronger its binding ability with lithium ions. Carbonate organic solvents have a donor number greater than 10, thus exhibiting a strong binding ability with lithium ions. Conversely, cosolvents have a donor number and dielectric constant both less than 10, resulting in weak or no binding ability with lithium ions. Therefore, the addition of a cosolvent does not affect the solvation structure of the electrolyte, and thus does not affect the electrochemical performance.
[0043] Preferably, the general structural formula of flame retardant additive A can be:
[0044]
[0045] In the formula, R is one of halogen atoms, fluorosulfonyl groups, haloalkyl groups, halobenzene groups, and trimethylsilyl groups.
[0046] Halogen atoms, when heated before combustion in the electrolyte, can generate halogen free radicals, which synergistically retard flame with fluorine-containing free radicals. Fluorosulfonyl groups, when heated before combustion in the electrolyte, also generate halogen free radicals, which synergistically retard flame with fluorine-containing free radicals and can participate in the construction of an SEI film rich in inorganic sulfides, improving the thermal stability of the SEI film. Halogenated alkyl groups, when heated before combustion in the electrolyte, generate halogen free radicals, which synergistically retard flame with fluorine-containing free radicals. Halogenated benzene groups, when heated before combustion in the electrolyte, generate halogen free radicals, which synergistically retard flame with fluorine-containing free radicals. Trimethylsilyl groups, when heated before combustion in the electrolyte, generate silicon oxide particles that isolate oxygen, synergistically retard flame, and also participate in the construction of an SEI film rich in inorganic silicon compounds, improving the thermal stability of the SEI film.
[0047] Therefore, this embodiment preferably designs a novel additive structure containing a large number of fluorine atoms and chlorine- or silicon-containing groups. The chlorine or silicon atoms and fluorine atoms achieve a synergistic flame-retardant effect, further enhancing the flame-retardant performance. Furthermore, a silicon-containing group replaces one terminal fluorine atom in the perfluorohexane backbone, introducing a new silicon-containing group. Besides the aforementioned synergistic flame-retardant effect with fluorine atoms, this introduced silicon-containing group also participates in constructing a dense, thermally stable SEI film rich in silicon. When the electrolyte temperature rises, it generates a large number of inorganic insulating layers containing Si-O bonds. This highly thermally stable SEI film does not decompose or break down at high temperatures, effectively blocking oxygen and thus suppressing or blocking exothermic side reactions between the lithium-ion graphite anode and the electrolyte, reducing the accumulation of heat inside the battery, thereby causing the electrolyte to self-extinguish and preventing the battery from catching fire or exploding due to thermal runaway.
[0048] Furthermore, one or more hydrogen atoms in the above-mentioned trimethylsilyl group can be replaced by the following substituents: fluorine, trifluoromethyl, fluorosulfonyl, fluorophenyl, chlorophenyl, bromophenyl, iodophenyl, trifluoromethylphenyl, trifluoromethyl(sulfonylimide lithium).
[0049] Replacing one or more hydrogen atoms in a trimethylsilyl group with a substituent containing halogen atoms can increase the halogen content in the structure. The added halogen atoms can be released at high temperatures to additionally capture highly reactive and flammable hydrogen free radicals, thereby enhancing the flame retardant effect of the electrolyte.
[0050] Specifically, when one or more hydrogen atoms of the trimethylsilyl group are replaced by the above-mentioned substituents, halogen free radicals can be generated at high temperatures because the substituents contain halogens. In addition, when the substituents include fluorine-containing groups (such as fluorine, trifluoromethyl, fluorophenyl, chlorophenyl, etc.) and sulfur-containing groups (such as fluorosulfonyl, trifluoromethyl (sulfonylimide lithium)), fluorine-containing free radicals and inorganic sulfides will be generated at high temperatures. Halogen free radicals can synergistically retard flame with fluorine-containing free radicals and can participate in the construction of SEI films rich in inorganic sulfides, thereby improving the thermal stability of SEI films.
[0051] Preferably, the co-solvent is one of a fluorinated aromatic compound, a fluorinated ether compound, and a fluorinated ester compound.
[0052] The cosolvent selected in this embodiment has low polarity, thus achieving miscibility with the electrolyte solvent without dissolving the lithium salt. Secondly, the selected cosolvent contains a large number of fluorine atoms, enabling it to achieve similar miscibility with the perfluorohexane derivative flame retardant of this embodiment. Ultimately, the perfluorohexane derivative flame retardant was successfully introduced into a traditional carbonate electrolyte without interacting with the lithium salt and altering the solvation structure of the electrolyte.
[0053] Preferably, the lithium salt is one or more of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluorosulfonyl)imide, lithium perchlorate, lithium bis(oxalate-borate), and lithium difluorooxalate-borate.
[0054] Preferably, the carbonate organic solvent is at least one or more selected from ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, vinylene carbonate, and fluoroethylene carbonate.
[0055] Example 2
[0056] A method for preparing a flame-retardant electrolyte containing a flame retardant, comprising:
[0057] S1. Add lithium salt to carbonate organic solvent and dissolve to obtain carbonate electrolyte;
[0058] S2. Add flame retardant and co-solvent to carbonate electrolyte, dissolve, and obtain the flame retardant lithium-ion battery electrolyte.
[0059] In some preferred embodiments, step S2 specifically involves adding a co-solvent to a carbonate electrolyte composed of lithium salt and carbonate organic solvent, and then adding a flame retardant additive to obtain a flame retardant electrolyte, thus completing the electrolyte preparation.
[0060] In some preferred embodiments, step S2 specifically involves adding a flame retardant additive to a carbonate electrolyte composed of lithium salt and carbonate organic solvent, and then adding a co-solvent to obtain a flame retardant electrolyte, thus completing the electrolyte preparation.
[0061] In some preferred embodiments, step S2 specifically involves: first, adding a flame retardant additive to a co-solvent to obtain a flame retardant solution; then, adding the flame retardant solution to a carbonate electrolyte composed of a lithium salt and a carbonate organic solvent to obtain a flame-retardant electrolyte, thus completing the electrolyte preparation; wherein, the co-solvent is a solvent capable of simultaneously dissolving the flame retardant additive, the lithium salt, and the carbonate organic solvent, and does not interact with lithium ions; the concentration of lithium salt in the electrolyte composed of the lithium salt and the carbonate organic solvent is 0.5–5 mol / L; the volume ratio of the carbonate electrolyte to the flame retardant additive is 100:(1–30); the flame retardant additive is a perfluorohexane derivative; and the volume ratio of the co-solvent to the flame retardant additive is (4–120):(1–5).
[0062] To better illustrate the present invention and its effects, the following Examples 1-14 and Comparative Examples 1-2 are provided. The electrolytes involved in these examples are all prepared according to the following methods:
[0063] Ethylene carbonate (EC, solvent) and dimethyl carbonate (DMC, solvent) are mixed evenly at a volume ratio of EC:DMC = 1:1. Lithium hexafluorophosphate (lithium salt) is added until the lithium salt concentration is 1 mol / L. Flame retardant additive A and co-solvent are added (the amounts of both are shown in Table 1 in mL). The mixture is dissolved and stirred evenly to prepare the desired flame retardant electrolyte.
[0064] Table 1 shows the types and amounts of flame retardant additives used in Examples 1-14 and Comparative Examples 1-2.
[0065] Table 1 shows the types and amounts of flame retardant additives and co-solvents used in Example 1-14 and Comparative Example 1-2.
[0066]
[0067]
[0068] Table 2 shows the types and dosages of flame retardant additives and the dosage of cosolvents in Examples 15-28.
[0069]
[0070]
[0071] In Tables 1 and 2, the flame retardant additive A corresponding to structural formula 1 is perfluorohexyl chloride, and its structural formula is:
[0072]
[0073] In Tables 1 and 2, the flame retardant additive A corresponding to structural formula 2 is diisopropyl(3,3,4,4,5,5,6,6,7,7,8,8,8,8-tert-butylfluorooctyl)silane, whose structural formula is:
[0074]
[0075] In the lithium-ion battery preparation of Examples 1-28 and Comparative Examples 1-2, the positive electrode, negative electrode, separator, and electrolyte are stacked to form the electrode core. The electrode core is then placed in an aluminum-plastic film and undergoes top-side sealing, baking, electrolyte injection, and formation processes to produce a soft-pack battery. The positive electrode is NCM9811, the negative electrode is graphite, and the separator is a PP separator.
[0076] The electrolytes prepared in Examples 1-28 and Comparative Examples 1-2 were subjected to flame retardancy tests as follows:
[0077] A glass fiber diaphragm of approximately 1.0 × 0.5 cm is thoroughly immersed in the electrolyte to be tested. The diaphragm is then fixed with a clip and ignited using a gas ignition device. The time from when the ignition device is removed until the flame extinguishes automatically is recorded; this time is called the self-extinguishing time. Another method for testing the self-extinguishing time of an electrolyte is to dispense a certain mass of the electrolyte to be tested into a battery casing, ignite it using a gas ignition device, and record the time from when the ignition device is removed until the flame extinguishes automatically. The flammability of different electrolytes is compared using the self-extinguishing time per unit mass of electrolyte. Figure 1 , Figure 2 , Figure 3 As shown, the results are as follows Figure 4 And as shown in Table 3 below.
[0078] In addition, the batteries of Examples 1-28 and Comparative Example 1-2 were subjected to a nail penetration safety performance test (nail penetration test). The pouch batteries of Examples 1-28 and Comparative Example 1-2 were charged at 25°C with a constant current of 1C to a voltage of 4.3V, and then charged at a constant voltage of 4.3V to a current of 0.05C, which is considered a fully charged state. The fully charged pouch batteries were then placed in a nail penetration test chamber with a needle diameter of 3mm. After the needle penetrated the battery and remained in place for 3 minutes, the battery was observed to see if thermal runaway occurred.
[0079] The test results of the self-extinguishing time and high-temperature safety performance of the lithium-ion battery electrolytes prepared in Examples 1-28 and Comparative Examples 1-2 are shown in Table 3.
[0080] Table 3 Self-extinguishing time and high-temperature safety performance of the electrolyte
[0081]
[0082]
[0083]
[0084] Based on the battery test results of Examples 1-14 and Comparative Examples 1-2, the electrolyte provided in Example 1 has a shorter self-extinguishing time, enabling it to be flame-retardant or even non-flammable. The electrolyte provided in Example 1 reduces the risk of thermal runaway, preventing fires and explosions during nail penetration tests. This further confirms that the flame-retardant additive A used in Example 1 releases fluorine-containing free radicals upon heating and vaporization, which can combine with hydrogen free radicals in the electrolyte, effectively preventing the combustion or explosion of organic solvents and greatly improving the safety of lithium-ion batteries. Secondly, its structure contains a large amount of fluorine, thus increasing the flash point of the electrolyte and enhancing its flame-retardant effect. Furthermore, because the flame-retardant additive A provided in Example 1 contains a large amount of fluorine and silicon-containing groups, it can form a stable and dense protective film on the electrode surface when used in the preparation of lithium-ion batteries, preventing side reactions between the electrode material and the electrolyte solvent at high temperatures and avoiding thermal runaway.
[0085] Furthermore, while ensuring that flame retardant additive A can be introduced into the electrolyte composed of lithium salt and carbonate organic solvent, the amount of co-solvent is fixed. For example, in Examples 15-28, the amount of co-solvent is 1.2 ml. The test results in Examples 15-28 show that the suppression of combustion or explosion of lithium-ion batteries is only related to the amount of flame retardant additive A. The addition of co-solvent does not slow down the combustion or explosion of lithium-ion batteries.
[0086] In addition, the present invention also tested and compared the electrochemical performance of the lithium-ion batteries prepared in the above examples. For example... Figure 5 The first-cycle charge / discharge efficiency of the lithium-graphite half-cell assembled using the electrolyte of Example 14, which contains a perfluorohexane derivative flame retardant, was measured. The first-cycle coulombic efficiency of the lithium-graphite half-cell using the electrolyte of Example 14 reached 93.4%, indicating that the introduced perfluorohexane derivative flame retardant can form a stable SEI film and does not cause co-intercalation of additives. Meanwhile, as... Figure 6 The cycling performance of graphite|NCM9811 full cells assembled using the electrolyte of Comparative Example 1 (without perfluorohexane derivative flame retardants) and the electrolyte of Example 14 (containing perfluorohexane derivative flame retardants) is shown. The full cell performance using the electrolyte of Example 14 is better than that using the electrolyte of Comparative Example 1, indicating that the introduction of perfluorohexane derivative flame retardants does not affect the performance of graphite|NCM9811 full cells. Finally, as Figure 7The table shows the rate performance of lithium-graphite half-cells assembled using the electrolyte of Comparative Example 1 (without perfluorohexane derivative flame retardant) and the electrolyte of Example 14 (containing perfluorohexane derivative flame retardant). The full-cell performance using the electrolyte of Example 14 is superior to that using the electrolyte of Comparative Example 1, indicating that the introduction of the perfluorohexane derivative flame retardant does not affect the solvation structure of the electrolyte. The superior rate performance indicates that the introduction of the perfluorohexane derivative flame retardant forms a more stable SEI film, accelerating lithium ion transport at the graphite-electrolyte interface.
[0087] In summary, the flame-retardant electrolyte based on perfluorohexane derivative flame-retardant additive A provided by this invention generates a large number of fluorine-containing free radicals upon heating and vaporization. These free radicals can combine with hydrogen free radicals generated from the decomposition of carbonate electrolytes, effectively reducing the self-extinguishing coefficient of the electrolyte. The large number of fluorine atoms contained in flame-retardant additive A and other components can form a dense and uniform solid electrolyte interphase film (SEI film) on the electrode surface, preventing the decomposition and breakage of the SEI film at high temperatures and preventing exothermic side reactions between the electrolyte solvent, lithium salt, and electrode materials at high temperatures. Simultaneously, the large amount of fluorine in its structure can increase the flash point of the electrolyte itself, reduce its flammability, and thus improve the safety of lithium-ion batteries. Furthermore, replacing a terminal fluorine atom in the perfluorohexane main chain with a chlorine- or silicon-containing group can synergistically retard the flame with the fluorine atom. Finally, when a large number of silicon-containing groups are introduced into the perfluorohexane derivative, a large number of inorganic insulating layers containing Si-O bonds can be generated when the electrolyte temperature rises, which can also achieve the effect of blocking oxygen, thereby enabling the electrolyte to self-extinguish. The electrolyte preparation method improved by this invention is simple and low in cost, which not only broadens the application field of perfluorohexane derivative organic compounds, but also provides new ideas for the research and development of safe electrolytes.
[0088] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.
Claims
1. A flame-retardant electrolyte, characterized in that, include: Lithium salts, carbonate organic solvents, cosolvents, and flame retardant additives; The concentration of lithium salt in the carbonate electrolyte, composed of the lithium salt and the carbonate organic solvent, is 0.5–5 mol / L; the co-solvent is a solvent capable of simultaneously dissolving the flame retardant additive, the lithium salt, and the carbonate organic solvent without interacting with lithium ions; the flame retardant additive is a perfluorohexane derivative; the volume ratio of the carbonate electrolyte to the flame retardant additive is 100:(1–30); the volume ratio of the co-solvent to the flame retardant additive is (4–120):(1–5); the solvent donor number of the carbonate organic solvent is greater than 10; and the solvent donor number and dielectric constant of the co-solvent are both less than 10. The general structural formula of the flame retardant additive is as follows: ； In the formula, R is one of the trimethylsilyl groups; wherein, the trimethylsilyl group is heated before the electrolyte is burned to generate silicon oxide particles that isolate oxygen, cooperate in flame retardancy, and also participate in the construction of an SEI film rich in inorganic silicon compounds. The co-solvent is at least one of aromatic compounds, fluorinated ether compounds, and fluorinated ester compounds.
2. The flame-retardant electrolyte according to claim 1, characterized in that, One or more hydrogen atoms in the trimethylsilyl group are each independently substituted by one or more of the following substituents: fluorine, trifluoromethyl, fluorosulfonyl, fluorophenyl, chlorophenyl, bromophenyl, iodophenyl, trifluoromethylphenyl, trifluoromethyl(sulfonylimide lithium).
3. The flame-retardant electrolyte according to claim 1, characterized in that, The lithium salt is at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluorosulfonyl)imide, lithium perchlorate, lithium bis(oxalatoborate), and lithium difluorooxalatoborate.
4. The flame-retardant electrolyte according to claim 1, characterized in that, The carbonate organic solvent is at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, vinylene carbonate, and fluoroethylene carbonate.
5. A method for preparing a flame-retardant electrolyte as described in any one of claims 1 to 4, characterized in that, Includes the following steps: S1. Dissolve the lithium salt in a carbonate organic solvent to obtain a carbonate electrolyte; S2. Add flame retardant and co-solvent to the carbonate electrolyte to dissolve and obtain the flame retardant electrolyte.
6. The preparation method according to claim 5, characterized in that, The specific steps of step S2 are as follows: first, add a co-solvent to the carbonate electrolyte, then add a flame retardant additive to obtain a flame retardant electrolyte, thus completing the electrolyte preparation; Alternatively, flame retardant additives can be added to the carbonate electrolyte first, followed by the addition of a co-solvent to obtain a flame retardant electrolyte, thus completing the electrolyte preparation. Alternatively, flame retardant additives can be added to the co-solvent to obtain a flame retardant solution, and then the flame retardant solution can be added to the carbonate electrolyte to obtain a flame retardant electrolyte, thus completing the electrolyte preparation. Wherein, the co-solvent is a solvent that can simultaneously dissolve the flame retardant additive, the lithium salt, and the carbonate organic solvent, and does not interact with lithium ions; the concentration of lithium salt in the electrolyte composed of lithium salt and carbonate organic solvent is 0.5–5 mol / L; the volume ratio of the carbonate electrolyte to the flame retardant additive is 100:(1–30); the flame retardant additive is a perfluorohexane derivative; the volume ratio of the co-solvent to the flame retardant additive is (4–120):(1–5).
7. A lithium-ion battery, characterized in that, Its electrolyte is the flame-retardant electrolyte as described in any one of claims 1 to 4.