A flame-retardant gel electrolyte, its preparation method, and a lithium-ion battery containing the electrolyte.

Abstract

This invention belongs to the field of lithium-ion battery technology, and relates to a flame-retardant gel electrolyte, its preparation method, and a lithium-ion battery containing the electrolyte. The flame-retardant gel electrolyte is a gel structure in which a cross-linked gel network encapsulates a basic electrolyte. It has an ionic conductivity ≥0.50 mS / cm, a self-extinguishing time ≤2s, a thermal runaway temperature ≤1000℃, and a mass retention rate ≥40% after thermal runaway. This flame-retardant gel electrolyte achieves excellent flame-retardant performance through its cross-linked gel network structure. The self-extinguishing time ≤2s can quickly prevent flame spread, and the thermal runaway temperature ≤1000℃ and mass retention rate ≥40% after thermal runaway significantly improve battery thermal safety. Simultaneously, the high ionic conductivity ≥0.50 mS / cm ensures smooth ion migration during charging and discharging, reduces battery polarization, and improves charging and discharging efficiency and cycle stability.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and relates to a flame-retardant gel electrolyte, its preparation method, and a lithium-ion battery containing the electrolyte. Background Technology

[0002] With the ever-increasing demand for energy density in energy storage devices from fields such as electric vehicles and high-energy portable devices, the theoretical specific capacity of traditional graphite anode lithium-ion batteries is no longer sufficient to meet the breakthrough requirements of high-performance applications. Silicon-based anode materials, with their extremely high theoretical specific capacity, have become the core breakthrough for improving the energy density of lithium-ion batteries. Lithium-silicon batteries are therefore widely recognized as an important development direction for next-generation high-energy-density energy storage technology, with broad prospects for industrial application.

[0003] However, during the charge-discharge cycle of the battery, the repeated insertion and extraction of lithium ions causes the silicon anode to undergo drastic volume expansion and contraction. This periodic deformation first leads to mechanical rupture of the solid electrolyte interphase (SEI) film. When silicon particles expand, the brittle SEI layer cannot extend synchronously, resulting in cracks that expose fresh silicon surfaces. This prompts the electrolyte to continuously decompose to repair the cracks, causing irreversible electrolyte consumption and the formation of a multilayer thickened SEI, significantly increasing interfacial impedance and triggering a surge in polarization voltage. More seriously, detached silicon fragments may puncture the battery separator under charge-discharge stress, especially under harsh conditions such as fast charging or low temperatures, easily inducing localized internal short circuits. The internal short circuit point, combined with the continuous exothermic reaction during SEI repair and the intense alloying reaction between the newly exposed silicon surface and the electrolyte, creates a vicious cycle of thermal runaway inside the battery. When the local temperature exceeds the critical point of 120°C, it will trigger a chain of exothermic reactions such as oxygen release from the positive electrode and separator melting, ultimately leading to combustion or explosion.

[0004] To address these issues, researchers have explored using conductive polymers to replace traditional liquid electrolytes, aiming to improve the structural stability and safety of batteries. Polyethylene glycol methacrylate (PEGMEA) is the core functional monomer for achieving this replacement. The PEGMEA molecule is considered an ideal model of a "flexible ion transport arm" grafted onto a "controllable polymerization point." The polyethylene oxide (PEO) segment acts as the "flexible arm," primarily responsible for dissolving lithium salts, plasticizing the gel system, and constructing ion transport channels. Meanwhile, the methacrylate end groups, acting as "controllable polymerization points," can form a stable three-dimensional network framework through polymerization, endowing the flame-retardant gel electrolyte with excellent mechanical properties and structural stability. Therefore, it is widely used in battery systems.

[0005] However, the ether bonds in the polyethylene oxide segments of polyethylene glycol methacrylate molecules have weak antioxidant capacity and are prone to oxidative decomposition under high voltage conditions, which limits its application in high-voltage cathode material systems. Summary of the Invention

[0006] The present invention aims to provide a flame-retardant gel electrolyte, its preparation method, and a lithium-ion battery containing the electrolyte. The flame-retardant gel electrolyte has a self-extinguishing time of ≤2s, indicating that it has the advantages of being difficult to ignite and self-quenching. The thermal runaway temperature is ≤1000℃ and the mass retention rate after thermal runaway is ≥40%. The flame-retardant gel electrolyte achieves excellent flame-retardant performance through a cross-linked gel network structure. The self-extinguishing time of ≤2s can quickly prevent the spread of flames. The thermal runaway temperature of ≤1000℃ and the mass retention rate after thermal runaway of ≥40% significantly improve the thermal safety of the battery. At the same time, the high ionic conductivity of ≥0.50mS / cm can ensure smooth ion migration during charging and discharging, reduce battery polarization, and improve charging and discharging efficiency and cycle stability.

[0007] In a first aspect, the present invention provides a flame-retardant gel electrolyte, wherein the flame-retardant gel electrolyte is a gel structure in which a cross-linked gel network encapsulates a basic electrolyte, wherein the flame-retardant gel electrolyte has an ionic conductivity ≥0.50mS / cm, a self-extinguishing time ≤2s, a thermal runaway explosion temperature ≤1000℃, and a mass retention rate ≥40% after thermal runaway.

[0008] This flame-retardant gel electrolyte achieves high safety, high conductivity, and high temperature resistance simultaneously, overcoming the pain point of traditional electrolytes that prioritize a single performance characteristic while sacrificing other indicators. Addressing the system requirements of silicon-carbon anodes (with 300% volume expansion) and high-voltage ternary cathodes (such as NCM811), this electrolyte can buffer the volumetric stress of the silicon-carbon anode, while its interfacial compatibility helps form a stable SEI / CEI film, further improving battery cycle life and high-voltage stability. Specifically, the self-extinguishing time of this flame-retardant gel electrolyte is ≤2s, indicating that it has the advantages of being difficult to ignite and self-quenching, solving the shortcomings of traditional liquid electrolytes that are prone to leakage and combustion, and ordinary gel electrolytes that only prevent leakage but are still flammable; the thermal runaway outbreak temperature is ≤1000℃ and the mass retention rate after thermal runaway is ≥40%, demonstrating the strong contribution of this electrolyte to battery thermal safety; the ionic conductivity is ≥0.50mS / cm, ensuring smooth ion migration during battery charging and discharging, avoiding problems such as increased polarization, low charging and discharging efficiency, and rapid capacity decay caused by insufficient conductivity, and ensuring that the battery has good power performance; the thermal runaway outbreak temperature is ≤1000℃, which is significantly less severe than that of traditional electrolytes (the outbreak temperature is usually below 600℃); the mass retention rate after thermal runaway is ≥40%, indicating that there is less loss of flammable components.

[0009] In some embodiments, the cross-linked gel network is formed by in-situ polymerization of polyethylene glycol dimethacrylate and a flame retardant; the flame retardant is a combination of ethoxy(pentafluoro)cyclotriphosphazene and phosphate ester compounds, and the flame retardant and the base electrolyte constitute the electrolyte, wherein the total mass percentage of the flame retardant in the electrolyte is ≤9%; the ionic conductivity of the flame-retardant gel electrolyte is ≥0.53 mS / cm, the self-extinguishing time is ≤1 s, the thermal runaway temperature is ≤910℃, and the mass retention rate after thermal runaway is ≥45%.

[0010] The flame retardant employs a combination of ethoxy(pentafluoro)cyclotriphosphazene (PFPN) and phosphate ester compounds to form a synergistic "phosphorus-nitrogen-fluorine" flame retardant mechanism. This addresses the shortcomings of traditional gel electrolytes, which only prevent leakage but remain flammable, requiring high concentrations (≥10%) of a single flame retardant to be effective. With a total flame retardant content ≤9%, this avoids the dilution of battery capacity caused by high amounts of inactive substances, reduces material costs, and simultaneously achieves a balance between high conductivity and superior safety performance.

[0011] In some embodiments, the phosphate ester compound is selected from any one of tris(2,2-difluoroethyl) phosphate (TFP), tris(2,2,2-trifluoroethyl) phosphate (FTEP), and triethyl phosphate (TEP), and the total mass percentage of the flame retardant in the electrolyte is ≤5%.

[0012] The PEO segments of PEGDMA act as "flexible ion transport arms" to construct continuous channels. All three phosphate ester compounds exhibit excellent lithium salt solubility and good compatibility with PFPN and lithium salts, without forming ion transport barriers. Furthermore, the decomposition products of the flame retardant optimize the ion conduction environment and reduce migration resistance. The three phosphate ester compounds synergistically form a stable interface film in situ on the positive and negative electrode surfaces with PFPN, inhibiting electrolyte decomposition and active lithium loss. This adapts to the 300% volume expansion of the silicon-carbon anode, enabling this flame-retardant gel electrolyte to be compatible with both silicon-carbon anodes and NCM811 high-voltage cathodes, contributing to long-term battery cycle life. The total flame retardant content is ≤5%, far lower than traditional high-addition flame-retardant systems (≥10%), reducing the cost increase from inactive materials and avoiding the process complexity issues caused by high addition levels.

[0013] In some embodiments, based on the total mass of the electrolyte, the flame retardant contains 2% to 5% ethoxy(pentafluoro)cyclotriphosphazene and 1% to 4% phosphate ester compounds.

[0014] By precisely combining PFPN 2%~5% + phosphate ester 1%~4%, flame retardancy and high thermal stability are achieved synergistically, resolving the contradiction between safety and energy density; high conductivity and long-term cycle life are both taken into account, making it suitable for high energy density positive and negative electrode systems.

[0015] In some embodiments, the base electrolyte comprises a lithium salt and a solvent, wherein the lithium salt is any one or more of lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium difluorooxalateborate (LiDFOB), lithium nitrate (LiNO3), and lithium dioxalateborate (LiBOB), and the concentration of the lithium salt is 1.0~1.5 mol / L.

[0016] The aforementioned lithium salts are suitable for different scenarios. A concentration of 1.0~1.5 mol / L ensures high conductivity and meets kinetic requirements. They synergistically form a high-quality interface film with flame retardants, improving cycle life and high-voltage stability. The selection of lithium salts and the limitation of concentration to 1.0~1.5 mol / L are key to achieving synergistic optimization of safety, conductivity, and stability in this technical solution, which is beneficial to enhancing the high energy density of the battery.

[0017] In a second aspect, the present invention provides a method for preparing a flame-retardant gel electrolyte, comprising the following steps:

[0018] Preparation of electrolyte: Slowly add lithium salt to solvent, stir at room temperature until completely dissolved, add flame retardant, and continue stirring until homogeneous to obtain electrolyte;

[0019] Preparation of gel precursor solution: Polyethylene glycol dimethacrylate (PEGDMA) was added to the electrolyte and stirred until completely dissolved. Then, azobisisobutyronitrile (AIBN) was added and stirred at room temperature for 5-15 min to obtain the gel precursor solution.

[0020] In-situ polymerization preparation of gel electrolyte: The gel precursor solution is injected into the battery containing a separator, positive electrode and negative electrode, and allowed to stand until the electrodes are fully wetted. Then, it is heated at 55-65℃ to solidify, thus completing the in-situ polymerization and obtaining the gel electrolyte.

[0021] First, the lithium salt is dissolved in a solvent, utilizing the solvent's high solubility to disperse the lithium salt. After the basic electrolyte system is stable, polymerization-related components are added to avoid compatibility issues caused by direct contact between the monomers and the lithium salt. The initiator is added last to prevent premature decomposition of the initiator, which could cause the polymerization reaction to start before the battery is injected. Finally, by impregnating followed by polymerization, the precursor solution fully fills the electrode pores and adheres to the electrode surface. Heating then initiates the polymerization reaction, achieving "in-situ molding" of the electrolyte inside the battery. Heating and curing at 60℃ ensures that the polymerization reaction proceeds fully, forming a complete and dense three-dimensional cross-linked network structure, while avoiding electrolyte decomposition or electrode active material failure caused by high temperatures, thus ensuring stable electrolyte performance.

[0022] In some embodiments, the solvent is any one or more of dimethyl carbonate (DMC), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and fluoroethylene carbonate (FEC).

[0023] The solvents exhibit excellent compatibility with PFPN / phosphate ester flame retardants and PEGDMA polymerization systems. The resulting gel structures are dense and uniform, without obvious bubbles or layering, and have a self-extinguishing time of ≤2 s. This demonstrates that the solvent system does not interfere with the flame retardant effect and polymerization reaction, achieving synergistic compatibility of conduction, flame retardancy, and polymerization.

[0024] In some embodiments, the mass ratio of polyethylene glycol dimethacrylate to the electrolyte is 10:90 to 14:86.

[0025] For PEGDMA as a monomer, a mass ratio of 10:90 to 14:86 represents the optimal range for network density and flexibility. Within this range, the methacrylate end groups of PEGDMA are fully cross-linked, forming a three-dimensional network with suitable pore size and cross-linking degree, ensuring both mechanical strength and a certain degree of flexibility. When the ratio is below 10:90, the PEGDMA content is insufficient, resulting in a sparse network skeleton after polymerization, which cannot effectively "lock in" the electrolyte and is prone to leakage. When the ratio is above 14:86, the excess monomer leads to over-cross-linking, resulting in a brittle gel texture that is prone to cracking due to electrode deformation during battery cycling.

[0026] In a second aspect, the present invention provides a method for preparing any of the above-mentioned flame-retardant gel electrolytes, comprising the following steps:

[0027] Preparation of electrolyte: Slowly add lithium salt to solvent, stir at room temperature until completely dissolved, add flame retardant, and continue stirring until homogeneous to obtain electrolyte;

[0028] Preparation of gel precursor solution: Polyethylene glycol dimethacrylate (PEGDMA) was added to the electrolyte and stirred until completely dissolved. Then, an initiator was added and stirred at room temperature for 5-15 min to obtain the gel precursor solution.

[0029] In-situ polymerization molding: The gel precursor solution is injected into a battery casing containing a separator, positive electrode and negative electrode, and allowed to stand until the electrodes are fully wetted. Then, it is heated at 55-65℃ to solidify, thus completing the in-situ polymerization and obtaining the flame-retardant gel electrolyte.

[0030] First, lithium salt is dissolved in a solvent, utilizing the solvent's high solubility to disperse the lithium salt and form a stable basic electrolyte system. Then, a flame retardant is added to prevent direct contact with the lithium salt, which could lead to coordination reactions or precipitation. Finally, PEGDMA monomer and an initiator are added to prevent premature decomposition of the initiator, which could cause uncontrolled polymerization. The resulting gel electrolytes are free of stratification, precipitation, or bubbles, with a stable ionic conductivity of 0.53–0.63 mS / cm. The flame retardant is uniformly dispersed in the system, ensuring the effective functioning of the synergistic flame retardant mechanism of phosphorus-nitrogen-fluorine.

[0031] In a third aspect, the present invention provides a lithium-ion battery, wherein the electrolyte of the lithium-ion battery is any one of the flame-retardant gel electrolytes described above.

[0032] This lithium-ion battery achieves extreme flame retardancy and a high thermal runaway threshold with a low amount of flame retardant, breaking the contradiction of "energy and safety cannot be achieved simultaneously" in high-energy-density batteries. The high ionic conductivity and excellent interface stability work together to ensure the battery's power output and long-term cycle life, making it compatible with high-energy-density systems such as silicon-carbon / high-voltage ternary lithium batteries.

[0033] In some implementations, the lithium-ion battery has an initial charge-discharge efficiency of ≥84%, a capacity retention rate of ≥92% after 200 cycles under 0.5C charging / 1C discharging conditions, and a heating time of ≥200 min when heated to 200°C at a rate of 5°C / min under full charge conditions, and a dense composite interface film is formed inside the lithium-ion battery.

[0034] In summary, this application includes at least one of the following beneficial technical effects:

[0035] The flame-retardant gel electrolyte exhibits a self-extinguishing time of ≤2s, indicating its advantages of being difficult to ignite and self-quenching. With a thermal runaway temperature ≤1000℃ and a mass retention rate ≥40% after thermal runaway, this flame-retardant gel electrolyte achieves excellent flame-retardant performance through a cross-linked gel network structure. The self-extinguishing time of ≤2s can quickly prevent flame spread, and the thermal runaway temperature of ≤1000℃ and mass retention rate ≥40% after thermal runaway significantly improve battery thermal safety. Simultaneously, the high ionic conductivity of ≥0.50mS / cm ensures smooth ion migration during charging and discharging, reduces battery polarization, and improves charging and discharging efficiency and cycle stability. Detailed Implementation

[0036] The following specific embodiments further illustrate the technical solution of the present invention. These specific embodiments do not represent a limitation on the scope of protection of the present invention. Non-essential modifications and adjustments made by others based on the concept of the present invention still fall within the scope of protection of the present invention.

[0037] This application provides a flame-retardant gel electrolyte, which is a gel structure in which a cross-linked gel network encapsulates a basic electrolyte. The flame-retardant gel electrolyte has an ionic conductivity ≥0.50mS / cm, a self-extinguishing time ≤2s, a thermal runaway temperature ≤1000℃, and a mass retention rate ≥40% after thermal runaway.

[0038] The three-dimensional framework of the cross-linked gel network possesses excellent thermal stability, resisting collapse at high temperatures and continuously encapsulating the base electrolyte to prevent leakage and violent reactions. The cross-linked gel network firmly locks the liquid base electrolyte within its three-dimensional structure, reducing the volatilization and flow of flammable components and physically inhibiting combustion spread. Simultaneously, the physical binding effect of the cross-linked network slows down the thermal decomposition rate of the base electrolyte, reducing the release of flammable gases at high temperatures. Furthermore, the flame-retardant components form an inert protective layer during thermal runaway, isolating the electrodes from the base electrolyte and suppressing violent exothermic reactions. The pore size and degree of cross-linking of the cross-linked gel network ensure structural stability while providing continuous channels for lithium-ion migration. This achieves a safety effect with a self-extinguishing time ≤2s through flame-retardant design, while maintaining a conductivity ≥0.50mS / cm, ensuring stable battery cycle performance.

[0039] Furthermore, the cross-linked gel network is formed by in-situ polymerization of polyethylene glycol dimethacrylate and flame retardant; the flame retardant is a combination of ethoxy (pentafluoro)cyclotriphosphazene and phosphate ester compounds, and the flame retardant and the basic electrolyte constitute the electrolyte, with the total mass percentage of the flame retardant in the electrolyte being ≤9%; the ionic conductivity of the flame-retardant gel electrolyte is ≥0.53mS / cm, the self-extinguishing time is ≤1s, the thermal runaway temperature is ≤910℃, and the mass retention rate after thermal runaway is ≥45%.

[0040] PFPN releases phosphorus-containing free radicals at high temperatures, which can both quench the combustion chain reaction (gas-phase flame retardancy) and form a dense carbon layer to block oxygen and heat (solid-phase flame retardancy). Phosphate ester compounds supplement fluorine free radicals, enhancing the gas-phase flame retardant effect. The synergistic effect of these two compounds, with a total addition of ≤9%, completely blocks the combustion path of the basic electrolyte. The cross-linked gel network structure, combined with the phosphorus-nitrogen-fluorine synergistic flame retardancy, rapidly quenches combustion free radicals, interrupts the chain reaction, and achieves rapid self-extinguishing after ignition.

[0041] The cross-linked gel network formed by in-situ polymerization of PEGDMA and flame retardants firmly "locks" the liquid base electrolyte within a three-dimensional structure, physically slowing down the volatilization and thermal decomposition rate of flammable components. Simultaneously, the flame retardant continuously releases inert substances during thermal runaway, diluting the concentration of flammable gases, inhibiting oxygen release from the positive electrode and dissolution of transition metal ions, and mitigating the severity of thermal runaway. Furthermore, the polyethylene oxide (PEO) segments of PEGDMA act as "flexible ion transport arms," ​​providing continuous channels for lithium-ion migration; on the other hand, the decomposition products of the flame retardant can optimize the ion conduction environment and reduce resistance to lithium-ion migration.

[0042] Furthermore, the phosphate ester compound is selected from any one of tris(2,2-difluoroethyl) phosphate (TFP), tris(2,2,2-trifluoroethyl) phosphate (FTEP), and triethyl phosphate (TEP), and the total mass percentage of the flame retardant in the electrolyte is ≤5%.

[0043] The differentiated selection of the three phosphate ester compounds mentioned above allows the flame-retardant gel electrolyte to adapt to the interface requirements of different application scenarios, while all three can work synergistically with PFPN: When TFP (tris(2,2-difluoroethyl) phosphate) is combined with PFPN (3%+2%), the self-extinguishing time is 1s, the ionic conductivity is 0.61mS / cm, and the capacity retention rate after 200 cycles is 95.9%, making it suitable for scenarios with high requirements for both safety and power performance. FTEP (tris(2,2,2-trifluoroethyl) phosphate) has a higher fluorine content, and when combined with PFPN, the heating time reaches 240min, the burst temperature is 784℃, and the mass retention rate after thermal runaway is 50.3%, making it suitable for energy storage devices in high-temperature storage or harsh environments.

[0044] Furthermore, based on the total mass of the electrolyte, the flame retardant contains 2% to 5% ethoxy(pentafluoro)cyclotriphosphazene and 1% to 4% phosphate ester compounds.

[0045] Ethoxy(pentafluoro)cyclotriphosphazene has a mass ratio of 2%~5%, such as 2%, 3%, 4%, etc., which plays a dual role in gas-phase and solid-phase flame retardancy. At high temperatures, it releases phosphorus-containing free radicals to quench the combustion chain reaction, while forming a dense carbon layer to block oxygen and heat. Phosphate esters have a mass ratio of 1%~4%, such as 1%, 1.5%, 2%, etc., which supplement fluorine free radicals or optimize flame retardant synergy, forming a "phosphorus-nitrogen-fluorine" synergistic system with PFPN, interrupting the combustion path at different stages. With this ratio, the self-extinguishing time of the electrolyte can be as low as 0s (such as PFPN 3% + TFP 2%). Even the lowest ratio (PFPN 2% + phosphate ester 1%) can achieve a self-extinguishing time of ≤2s, completely solving the shortcoming of traditional gel electrolytes that "only prevent leakage, but are still flammable", reducing the risk of battery combustion from the root.

[0046] Furthermore, the basic electrolyte includes a lithium salt and a solvent. The lithium salt is any one or more of lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium difluorooxalateborate (LiDFOB), lithium nitrate (LiNO3), and lithium dioxalateborate (LiBOB), with a lithium salt concentration of 1.0~1.5 mol / L.

[0047] LiPF6, as a conventional preferred lithium salt, combines high dissociation and good compatibility. When synergistically used with PFPN / TFP, its decomposition products can optimize ion conduction pathways. The fluorinated structures of LiTFSI and LiFSI can reduce lattice energy and improve ion migration rates. LiDFOB and LiBOB participate in interfacial film formation through oxalate groups, balancing conductivity and stability. Example 1 using LiPF6 (1.2M) achieved an ionic conductivity of 0.61 mS / cm, meeting the dynamic requirements of high-energy-density batteries. Other lithium salt selections (such as LiFSI+LiPF6 composites) can further optimize low-temperature conductivity, overcoming the application limitations of single lithium salts.

[0048] The lithium salt concentration is limited to the range of 1.0–1.5 mol / L, such as 1.0 mol / L, 1.1 mol / L, 1.2 mol / L, 1.4 mol / L, and 1.5 mol / L, to avoid performance defects caused by excessively high or low concentrations. Below 1.0 mol / L, there are insufficient lithium-ion carriers, resulting in decreased conductivity; above 1.5 mol / L, the electrolyte viscosity increases significantly, hindering ion migration. 1.0–1.5 mol / L represents the optimal balance between carrier quantity and migration resistance, and also exhibits the best compatibility with the PEGDMA crosslinking network and synergistic flame retardants.

[0049] Furthermore, the solvent is any one or more of dimethyl carbonate (DMC), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and fluoroethylene carbonate (FEC).

[0050] The solvents mentioned above do not contain groups that readily coordinate with flame retardants, and they can dissolve PEGDMA monomers and initiator AIBN, thus preventing phase separation during polymerization. The polarity and viscosity of the mixed solvents are well-suited for in-situ polymerization, ensuring the uniform formation of the crosslinked network.

[0051] Furthermore, the mass ratio of polyethylene glycol dimethacrylate to the electrolyte is 10:90 to 14:86.

[0052] For example, the mass ratio of polyethylene glycol dimethacrylate to the electrolyte is 10:90, 11:89, 12:88, 13:87, 14:86, etc. Groups using the above ratios have dense gel structures without bubbles, and can still maintain network integrity after thermal runaway, with a high mass retention rate, proving that this ratio range can ensure the structural stability of the gel under charge-discharge cycles and high-temperature environments.

[0053] Furthermore, this application provides a method for preparing the above-mentioned flame-retardant gel electrolyte, comprising the following steps:

[0054] Preparation of electrolyte: Slowly add lithium salt to solvent, stir at room temperature until completely dissolved, add flame retardant, and continue stirring until homogeneous to obtain electrolyte;

[0055] Preparation of gel precursor solution: Polyethylene glycol dimethacrylate (PEGDMA) was added to the electrolyte and stirred until completely dissolved. Then, an initiator was added and stirred at room temperature for 5-15 min to obtain the gel precursor solution.

[0056] In-situ polymerization molding: The gel precursor solution is injected into a battery casing containing a separator, positive electrode and negative electrode, and allowed to stand until the electrodes are fully wetted. Then, it is heated at 55-65℃ to solidify, thus completing the in-situ polymerization and obtaining the flame-retardant gel electrolyte.

[0057] First, lithium salt is dissolved in a solvent, utilizing the solvent's high solubility to disperse the lithium salt and form a stable basic electrolyte system. Then, a flame retardant is added to prevent direct contact with the lithium salt, which could lead to coordination reactions or precipitation. Finally, PEGDMA monomer and an initiator are added to prevent premature decomposition of the initiator, which could cause uncontrolled polymerization. The resulting gel electrolytes are free of stratification, precipitation, or bubbles, with a stable ionic conductivity of 0.53–0.61 mS / cm. The flame retardant is uniformly dispersed in the system, ensuring the effective functioning of the synergistic flame retardant mechanism of phosphorus-nitrogen-fluorine.

[0058] Both the electrolyte and gel precursor preparation stages employ room temperature stirring to avoid high-temperature damage to the components. At room temperature, the lithium salt dissolves at a moderate rate, minimizing the risk of localized excessive concentrations. The flame retardant exhibits better compatibility with the solvent and lithium salt, preventing accelerated decomposition or volatilization due to high temperatures. The mixing of PEGDMA and the electrolyte is more uniform, avoiding premature cross-linking caused by high temperatures. The heating and curing temperature is limited to 55-65℃, such as 55℃, 58℃, 60℃, 62℃, and 65℃. This temperature range effectively activates the initiator, promoting full cross-linking of PEGDMA monomers to form a three-dimensional network. Simultaneously, it avoids electrolyte decomposition and electrode active material failure due to excessively high temperatures, or incomplete polymerization and a loose network structure due to excessively low temperatures. The precursor solution is injected into the battery before polymerization, allowing the gel network to form in situ between the electrodes and separator, completely encapsulating the electrolyte within the three-dimensional structure and physically blocking the flow and leakage of flammable components. Furthermore, the polymerization process is integrated with battery assembly, avoiding the sealing problems caused by "gelling before injection."

[0059] Example 1

[0060] A method for preparing a flame-retardant gel electrolyte includes the following steps:

[0061] Preparation of electrolyte: 3.23 g of LiPF6 was slowly poured into 20.07 g of mixed solvent and stirred at room temperature for 30 min until completely dissolved to obtain the basic electrolyte; then flame retardant was added to the basic electrolyte and stirred for another 15 min to obtain the electrolyte.

[0062] The concentration of LiPF6 is 1.2 mol / L. The mixed solvent is composed of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and fluoroethylene carbonate (FEC), with a volume ratio of 6:2:2. The flame retardant is composed of ethoxy(pentafluoro)cyclotriphosphazene (PFPN) and tris(2,2-difluoroethyl)phosphonic acid (TFP), with ethoxy(pentafluoro)cyclotriphosphazene accounting for 3 wt% of the total electrolyte mass and tris(2,2-difluoroethyl)phosphonic acid accounting for 2 wt% of the total electrolyte mass.

[0063] Preparation of gel precursor solution: Polyethylene glycol dimethacrylate (PEGDMA) was added to the above electrolyte at a mass ratio of PEGDMA to electrolyte of 10:90. After stirring for 60 min until completely dissolved, azobisisobutyronitrile (AIBN) was added and stirred at room temperature for 10 min to obtain gel precursor solution.

[0064] The mass of azobisisobutyronitrile is 1% of the mass of polyethylene glycol dimethacrylate monomer.

[0065] In-situ polymerization molding: In an argon atmosphere glove box, the gel precursor solution is injected into the battery containing a polyethylene separator, an NCM811 positive electrode and a silicon-carbon-graphite composite negative electrode. After standing for 48 h until the electrode is fully wetted, it is heated at 60℃ for 48 h to obtain a flame-retardant gel electrolyte.

[0066] Example 2

[0067] A method for preparing a flame-retardant gel electrolyte includes the following steps:

[0068] Preparation of electrolyte: 3.23 g of LiPF6 was slowly poured into 20.07 g of mixed solvent and stirred at room temperature for 30 min until completely dissolved to obtain the basic electrolyte; then flame retardant was added to the basic electrolyte and stirred for another 15 min to obtain the electrolyte.

[0069] The concentration of LiPF6 is 1.2 mol / L. The mixed solvent is composed of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and fluoroethylene carbonate (FEC), with a volume ratio of 6:2:2. The flame retardant is composed of ethoxy(pentafluoro)cyclotriphosphazene (PFPN) and tris(2,2,2-trifluoroethyl) phosphate (FTEP), with ethoxy(pentafluoro)cyclotriphosphazene accounting for 3 wt% of the total electrolyte mass and tris(2,2,2-trifluoroethyl) phosphate accounting for 2 wt% of the total electrolyte mass.

[0070] Preparation of gel precursor solution: Polyethylene glycol dimethacrylate (PEGDMA) was added to the above electrolyte at a mass ratio of PEGDMA to electrolyte of 10:90. After stirring for 60 min until completely dissolved, azobisisobutyronitrile (AIBN) was added and stirred at room temperature for 10 min to obtain gel precursor solution.

[0071] The mass of azobisisobutyronitrile is 1% of the mass of polyethylene glycol dimethacrylate monomer.

[0072] In-situ polymerization molding: In an argon atmosphere glove box, the gel precursor solution is injected into the battery containing a polyethylene separator, an NCM811 positive electrode and a silicon-carbon-graphite composite negative electrode. After standing for 48 h until the electrode is fully wetted, it is heated at 60℃ for 48 h to obtain a flame-retardant gel electrolyte.

[0073] Example 3

[0074] A method for preparing a flame-retardant gel electrolyte includes the following steps:

[0075] Preparation of electrolyte: 3.23 g of LiPF6 was slowly poured into 20.07 g of mixed solvent and stirred at room temperature for 30 min until completely dissolved to obtain the basic electrolyte; then flame retardant was added to the basic electrolyte and stirred for another 15 min to obtain the electrolyte.

[0076] The concentration of LiPF6 is 1.2 mol / L. The mixed solvent is composed of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and fluoroethylene carbonate (FEC), with a volume ratio of 6:2:2. The flame retardant is composed of ethoxy(pentafluoro)cyclotriphosphazene (PFPN) and triethyl phosphate, with ethoxy(pentafluoro)cyclotriphosphazene accounting for 3wt% of the total mass of the electrolyte and triethyl phosphate accounting for 2wt% of the total mass of the electrolyte.

[0077] Preparation of gel precursor solution: Polyethylene glycol dimethacrylate (PEGDMA) was added to the above electrolyte at a mass ratio of PEGDMA to electrolyte of 10:90. After stirring for 60 min until completely dissolved, azobisisobutyronitrile (AIBN) was added and stirred at room temperature for 10 min to obtain gel precursor solution.

[0078] The mass of azobisisobutyronitrile is 1% of the mass of polyethylene glycol dimethacrylate monomer.

[0079] In-situ polymerization molding: In an argon atmosphere glove box, the gel precursor solution is injected into the battery containing a polyethylene separator, an NCM811 positive electrode and a silicon-carbon-graphite composite negative electrode. After standing for 48 h until the electrode is fully wetted, it is heated at 60℃ for 48 h to obtain a flame-retardant gel electrolyte.

[0080] Example 4

[0081] A method for preparing a flame-retardant gel electrolyte includes the following steps:

[0082] Preparation of electrolyte: 3.23 g of LiPF6 was slowly poured into 20.07 g of mixed solvent and stirred at room temperature for 30 min until completely dissolved to obtain the basic electrolyte; then flame retardant was added to the basic electrolyte and stirred for another 15 min to obtain the electrolyte.

[0083] The concentration of LiPF6 is 1.2 mol / L. The mixed solvent is composed of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and fluoroethylene carbonate (FEC), with a volume ratio of 6:2:2. The flame retardant is composed of ethoxy(pentafluoro)cyclotriphosphazene (PFPN) and tris(2,2-difluoroethyl)phosphoric acid (TFP), with ethoxy(pentafluoro)cyclotriphosphazene accounting for 5 wt% of the total electrolyte mass and tris(2,2-difluoroethyl)phosphoric acid accounting for 2 wt% of the total electrolyte mass.

[0084] Preparation of gel precursor solution: Polyethylene glycol dimethacrylate (PEGDMA) was added to the above electrolyte at a mass ratio of PEGDMA to electrolyte of 10:90. After stirring for 60 min until completely dissolved, azobisisobutyronitrile (AIBN) was added and stirred at room temperature for 10 min to obtain gel precursor solution.

[0085] The mass of azobisisobutyronitrile is 1% of the mass of polyethylene glycol dimethacrylate monomer.

[0086] In-situ polymerization molding: In an argon atmosphere glove box, the gel precursor solution is injected into the battery containing a polyethylene separator, an NCM811 positive electrode and a silicon-carbon-graphite composite negative electrode. After standing for 48 h until the electrode is fully wetted, it is heated at 60℃ for 48 h to obtain a flame-retardant gel electrolyte.

[0087] Example 5

[0088] A method for preparing a flame-retardant gel electrolyte includes the following steps:

[0089] Preparation of electrolyte: 3.23 g of LiPF6 was slowly poured into 20.07 g of mixed solvent and stirred at room temperature for 30 min until completely dissolved to obtain the basic electrolyte; then flame retardant was added to the basic electrolyte and stirred for another 15 min to obtain the electrolyte.

[0090] The concentration of LiPF6 is 1.2 mol / L. The mixed solvent is composed of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and fluoroethylene carbonate (FEC), with a volume ratio of 6:2:2. The flame retardant is composed of ethoxy(pentafluoro)cyclotriphosphazene (PFPN) and tris(2,2-difluoroethyl)phosphonic acid (TFP), with ethoxy(pentafluoro)cyclotriphosphazene accounting for 3 wt% of the total electrolyte mass and tris(2,2-difluoroethyl)phosphonic acid accounting for 4 wt% of the total electrolyte mass.

[0091] Preparation of gel precursor solution: Polyethylene glycol dimethacrylate (PEGDMA) was added to the above electrolyte at a mass ratio of PEGDMA to electrolyte of 10:90. After stirring for 60 min until completely dissolved, azobisisobutyronitrile (AIBN) was added and stirred at room temperature for 10 min to obtain gel precursor solution.

[0092] The mass of azobisisobutyronitrile is 1% of the mass of polyethylene glycol dimethacrylate monomer.

[0093] In-situ polymerization molding: In an argon atmosphere glove box, the gel precursor solution is injected into the battery containing a polyethylene separator, an NCM811 positive electrode and a silicon-carbon-graphite composite negative electrode. After standing for 48 h until the electrode is fully wetted, it is heated at 60℃ for 48 h to obtain a flame-retardant gel electrolyte.

[0094] Comparative Example 1

[0095] A method for preparing a flame-retardant gel electrolyte differs from Example 1 in that ethoxy(pentafluoro)cyclotriphosphazene (PFPN) and tris(2,2-difluoroethyl)phosphonic acid (TFP) are not added, while other conditions are the same as in Example 1.

[0096] Comparative Example 2

[0097] A method for preparing a flame-retardant gel electrolyte differs from Comparative Example 1 in that the mass ratio of polyethylene glycol dimethacrylate (PEGDMA) to electrolyte is 15:85, while other conditions are the same as in Example 1.

[0098] Comparative Example 3

[0099] A method for preparing a flame-retardant gel electrolyte differs from Example 1 in that: only ethoxy(pentafluoro)cyclotriphosphazene (PFPN) is added as a flame retardant, with the mass of ethoxy(pentafluoro)cyclotriphosphazene accounting for 3wt% of the total mass of the electrolyte; tris(2,2-difluoroethyl)phosphoric acid (TFP) is not added; other steps and parameters are the same as in Example 1, and the corresponding flame-retardant gel electrolyte is obtained.

[0100] Comparative Example 4

[0101] A method for preparing a flame-retardant gel electrolyte differs from Example 1 in that: only tris(2,2-difluoroethyl)phosphoric acid (TFP) is added as a flame retardant, with the mass of tris(2,2-difluoroethyl)phosphoric acid accounting for 2 wt% of the total mass of the electrolyte. Ethoxy(pentafluoro)cyclotriphosphazene is not added. Other steps and parameters are consistent with Example 1, and the corresponding flame-retardant gel electrolyte is obtained.

[0102] Performance testing

[0103] (a) Ionic conductivity test

[0104] All flame-retardant gel electrolytes in the examples and comparative examples were assembled into test batteries in a glove box environment to avoid interference from moisture and oxygen. CR2032 button cell cases were used, and the ionic conductivity test batteries were assembled in the order of "negative electrode case - gasket - stainless steel - flame-retardant gel electrolyte - stainless steel - positive electrode case". The ionic conductivity test batteries were connected to an electrochemical workstation, and the scanning frequency was 1 MHz to 0.1 Hz. Ionic conductivity was calculated at room temperature by fitting electrochemical impedance spectroscopy (EIS).

[0105] (II) Cyclic performance and thermal stability testing:

[0106] The flame-retardant gel electrolytes of the examples and comparative examples were assembled into lithium-ion batteries in the following manner:

[0107] Electrode preparation: The positive electrode material nickel cobalt manganese 811 (NCM811), the binder polyvinylidene fluoride (PVDF), and the conductive agent carbon black are dispersed in N-methylpyrrolidone and mixed evenly to form a positive electrode slurry. The slurry is coated on both sides of the aluminum foil current collector and then die-cut after vacuum drying to obtain a positive electrode sheet of the preset size. The negative electrode slurry is prepared by mixing silicon-carbon composite material, graphite, conductive agent carbon black, and binder styrene-butadiene rubber (SBR) evenly. The slurry is coated on both sides of the copper foil current collector and then die-cut after vacuum drying to obtain a negative electrode sheet of the preset size.

[0108] Battery assembly: The battery casing is assembled by stacking the positive electrode sheet, separator and negative electrode sheet in sequence. The gel electrolyte precursor solution corresponding to the example and comparative example is injected into the battery casing. After injection, vacuum sealing is performed and the casing is left to stand for 48 hours to ensure that the electrode material is fully wetted by the gel electrolyte precursor solution.

[0109] Gel curing and battery activation: The battery was pretreated using a "low current slow charging" strategy. It was first charged to 3.5 V with a constant current of 0.2 C, and then charged to 3.8 V with a constant current of 0.3 C. The battery was then placed in a 60°C environment and left to stand for 48 h to allow the gel electrolyte precursor solution to complete in-situ polymerization inside the battery, forming a flame-retardant gel electrolyte. After the battery cooled naturally to room temperature, it was charged to 4.25 V with a constant current and constant voltage of 0.5 C, and then discharged to the cutoff voltage of 2.5 V with a constant current of 0.5 C, thus completing the lithium-ion battery assembly and activation.

[0110] Cycle performance test: The charge and discharge range of the lithium-ion battery test is 2.5 V to 4.25 V, 0.5 C charge / 1 C discharge, and the first charge and discharge efficiency and capacity retention rate after 200 cycles are recorded.

[0111] Thermal stability test: The fully charged lithium-ion battery was placed in a hot chamber and heated at a rate of 5℃ / min. The temperature was maintained for 30 min after each 5℃ increase until it reached 200℃. The thermal runaway temperature, heating time, and mass retention rate after thermal runaway were recorded.

[0112] (III) Flame retardant performance test

[0113] Flame-retardant gel electrolyte samples from each experimental group were directly selected, and the vertical combustion test method was used to observe the self-extinguishing time after the samples were ignited to evaluate the flame-retardant effect of the electrolyte. The self-extinguishing time of 0s was determined to be the optimal flame-retardant level.

[0114] Results Analysis

[0115] The performance test results of the flame-retardant gel electrolytes prepared in the examples and comparative examples, and the lithium batteries containing the electrolytes, are shown in Table 1 below:

[0116] Table 1. Performance test results of the flame-retardant gel electrolytes prepared in the examples and comparative examples, and the lithium batteries containing the electrolytes.

[0117]

[0118] The data in the table show that the self-extinguishing time of the flame-retardant gel electrolytes prepared in Examples 1-5 is ≤2s, which is far superior to that of the comparative group (3-9s), proving that the synergistic flame-retardant system of PFPN and phosphate ester is the core of improving flame-retardant performance. The ionic conductivity is ≥0.53 mS / cm, meeting the practical requirements of batteries, demonstrating that the cross-linked network of 10% PEGDMA ensures mechanical stability without significantly hindering ion transport. The capacity retention rate after 200 cycles is ≥92.7%, and the first charge-discharge efficiency is ≥85.1%, verifying that the combination of flame retardant and in-situ polymerization process can effectively construct a stable positive and negative electrode interface film. Specifically:

[0119] Example 1 (PFPN 3wt% + TFP 2wt%) showed an ionic conductivity of 0.61 mS / cm, a self-extinguishing time of 1 s, a capacity retention of 95.9% after 200 cycles, and an initial charge-discharge efficiency of 85.7%. The 5% total addition of PFPN and TFP to the electrolyte formed a synergistic "phosphorus-fluorine" flame-retardant mechanism. PFPN released phosphorus-containing free radicals at high temperatures to quench the combustion chain, while TFP released fluorine to strengthen the gas phase barrier. The synergistic effect of these two components was far superior to that of a single flame retardant group. The three-dimensional cross-linked network constructed by PEGDMA ensured ion transport efficiency and also formed a low-impedance and dense composite interface film in situ through the flame retardant. The composite interface film consisted of a solid electrolyte interface film / positive electrode electrolyte interface film (SEI / CEI), reducing active lithium loss and balancing ion conduction and cycle stability. The cycle performance was close to that of Comparative Example 1.

[0120] Example 2 (PFPN 3wt% + FTEP 2wt%): Explosion temperature 784℃, heating time 240min (excellent thermal stability), self-extinguishing time 0s (extreme flame retardancy), capacity retention of 95.1% after 200 cycles, ionic conductivity 0.59 mS / cm. The trifluoroethyl structure of FTEP contains more fluorine atoms, which can release more inert fluoride gas at high temperatures, forming a denser gas-phase barrier layer, delaying the propagation of thermal decomposition reactions. Therefore, the heating time is the longest (240min), and the thermal runaway process is more gradual. The synergistic flame retardancy of FTEP and PFPN achieves "ignition-and-extinguish"; at the same time, the interface film formed by FTEP has both elasticity and mechanical strength. The molecular volume of FTEP is slightly larger than that of TFP, resulting in slightly greater steric hindrance for lithium-ion migration, leading to a slightly lower ionic conductivity (0.59 mS / cm) than in Example 1, but still meeting the practical requirements of the battery.

[0121] Example 3 (PFPN 3wt% + triethyl phosphate 2wt%): 200 cycles capacity retention of 92.7%, self-extinguishing time of 2s, and ionic conductivity of 0.57 mS / cm. Although the synergistic effect of triethyl phosphate and PFPN is slightly weaker than that of TFP (slightly longer self-extinguishing time), the interaction between the triethyl phosphate molecule and lithium ions is stronger, affecting the intercalation of lithium ions and resulting in faster capacity decay.

[0122] Example 4 (PFPN 5wt% + FTEP 2wt%): Self-extinguishing time 0s (highly efficient flame retardant), heating time 221min, ionic conductivity 0.53 mS / cm. Increasing the PFPN addition to 5% enhanced the flame retardant effect; however, excessive PFPN molecules formed a weak coordination with lithium ions, slightly hindering ion migration, resulting in a decrease in conductivity. The impaired ion migration reduced the capacity retention rate after 200 cycles to 94.4%.

[0123] Example 5 (PFPN 3wt% + FTEP 4wt%): Self-extinguishing time 1s, mass retention after thermal runaway 48.8%, capacity retention after 200 cycles 94.1%. Increasing the TFP addition amount allows its fluorinated groups to form more inert gases during thermal runaway, reducing the volatilization of combustible components and improving mass retention. However, excessive TFP slightly compromises the compatibility of the electrolyte system, resulting in slightly lower ionic conductivity (0.55 mS / cm) and cycle performance compared to Examples 1 and 3, confirming that the optimal TFP addition range is 2-3wt%.

[0124] Comparative Example 1 (without flame retardant): self-extinguishing time 9s (continuous combustion), explosion temperature 1216℃ (violent thermal runaway), and capacity retention rate of 96.0% after 200 cycles. Without the addition of PFPN and phosphate ester, it lacks gas-phase flame retardant and condensed-phase barrier mechanisms. The electrolyte is flammable and thermal decomposition is not inhibited, resulting in a low thermal runaway explosion temperature and a violent reaction.

[0125] Comparative Example 2 (mass ratio of PEGDMA to electrolyte is 15:85): self-extinguishing time 8s, ionic conductivity 0.59 mS / cm, capacity retention after 200 cycles 94.8%. Increased PEGDMA content and a denser cross-linked network prolonged the heating time (190 min), but significantly hindered lithium-ion migration. Simultaneously, the excessively thick gel network increased interfacial impedance, resulting in weaker cycling performance compared to Comparative Example 1.

[0126] Comparative Example 3 (PFPN 3wt% only): Self-extinguishing time 3s, capacity retention of 94.7% after 200 cycles, and burst temperature of 918℃. Relying solely on PFPN as a flame retardant, lacking the synergistic effect of TFP's fluorine, it cannot completely block the combustion chain reaction, resulting in a much weaker flame retardant effect than the example. Furthermore, the interfacial film formed by PFPN alone has poor uniformity and cannot adapt to the volume expansion of the silicon-carbon anode, leading to repeated rupture and repair of the SEI film during cycling, resulting in rapid capacity decay.

[0127] Comparative Example 4 (TFP 2wt% only): 200-cycle capacity retention of 96.8% (highest among all groups), self-extinguishing time of 6s, and burst temperature of 994℃. The fluorine-containing structure of TFP can form a dense CEI / SEI film on the electrode surface, with excellent interfacial stability, resulting in outstanding cycle performance; however, the flame retardant mechanism of TFP alone is imperfect, lacking the phosphorus-containing free radical quenching effect of PFPN, and its flame retardant performance is only slightly better than Comparative Examples 1 and 2. Although the burst temperature is low during thermal runaway, there is no synergistic flame retardant to inhibit combustion, and there are still safety hazards.

[0128] It is understood that the present invention has been described through some embodiments, and those skilled in the art will recognize that various changes or equivalent substitutions can be made to these features and embodiments without departing from the spirit and scope of the invention. Furthermore, under the teachings of the present invention, these features and embodiments can be modified to adapt to specific situations and materials without departing from the spirit and scope of the invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are within the protection scope of the present invention.

Claims

1. A flame-retardant gel electrolyte, characterized in that, The flame-retardant gel electrolyte is a gel structure in which a cross-linked gel network encapsulates a base electrolyte. The cross-linked gel network is formed by in-situ polymerization of polyethylene glycol dimethacrylate and a flame retardant. The flame retardant is a combination of ethoxy(pentafluoro)cyclotriphosphazene and phosphate ester compounds, and the flame retardant achieves flame retardancy through a "phosphorus-nitrogen-fluorine" synergistic mechanism. The flame retardant and the base electrolyte constitute the electrolyte, and the total mass percentage of the flame retardant in the electrolyte is ≤9%. The phosphate ester compounds are selected from at least one of tris(2,2-difluoroethyl)phosphonic acid and tris(2,2,2-trifluoroethyl) phosphate. The flame-retardant gel electrolyte has an ionic conductivity ≥0.50 mS / cm, a self-extinguishing time ≤2s, a thermal runaway temperature ≤1000℃, and a mass retention rate ≥40% after thermal runaway.

2. The flame-retardant gel electrolyte as described in claim 1, characterized in that, The flame-retardant gel electrolyte has an ionic conductivity ≥0.53 mS / cm, a self-extinguishing time ≤1s, a thermal runaway temperature ≤910℃, and a mass retention rate ≥45% after thermal runaway.

3. The flame-retardant gel electrolyte as described in claim 2, characterized in that, The flame retardant accounts for ≤5% of the total mass of the electrolyte.

4. The flame-retardant gel electrolyte as described in claim 2, characterized in that, Based on the total mass of the electrolyte, the ethoxy(pentafluoro)cyclotriphosphazene accounts for 2% to 5% of the mass, and the phosphate ester compounds account for 1% to 4% of the mass.

5. The flame-retardant gel electrolyte as described in claim 2, characterized in that, The basic electrolyte comprises a lithium salt and a solvent. The lithium salt is any one or more of lithium hexafluorophosphate, lithium perchlorate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium difluorooxalateborate, lithium nitrate, and lithium dioxalateborate. The concentration of the lithium salt is 1.0 mol / L to 1.5 mol / L.

6. The flame-retardant gel electrolyte as described in claim 5, characterized in that, The solvent is any one or more of dimethyl carbonate, ethylene carbonate, methyl ethyl carbonate, and fluoroethylene carbonate.

7. The flame-retardant gel electrolyte as described in claim 6, characterized in that, The mass ratio of polyethylene glycol dimethacrylate to the electrolyte is 10:90 to 14:

86.

8. A method for preparing a flame-retardant gel electrolyte as described in any one of claims 2-7, characterized in that, Includes the following steps: Preparation of electrolyte: Slowly add lithium salt to solvent, stir at room temperature until completely dissolved, add flame retardant, and continue stirring until homogeneous to obtain electrolyte; Preparation of gel precursor solution: Polyethylene glycol dimethacrylate was added to the electrolyte and stirred until completely dissolved. Then, an initiator was added and stirred at room temperature for 5 min to 15 min to obtain the gel precursor solution. In-situ polymerization molding: The gel precursor solution is injected into a battery casing containing a separator, positive electrode and negative electrode, and allowed to stand until the electrodes are fully wetted. Then, it is heated at 55℃-65℃ to solidify, thus completing the in-situ polymerization and obtaining the flame-retardant gel electrolyte.

9. A lithium-ion battery, characterized in that, The electrolyte of the lithium-ion battery is the flame-retardant gel electrolyte as described in any one of claims 1-8.

10. The lithium-ion battery as described in claim 9, characterized in that, The lithium-ion battery has an initial charge-discharge efficiency of ≥84% and a capacity retention rate of ≥92% after 200 cycles under 0.5C charge / 1C discharge conditions. When the temperature is increased to 200°C at a rate of 5°C / min under full charge, and the heating time is ≥200min, a dense composite interface film is formed inside the lithium-ion battery.

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