A gel electrolyte and a lithium-ion battery using the same.

Abstract

A gel electrolyte and a lithium-ion battery using the same are provided. The gel electrolyte comprises a polymer, which is polymerized from at least a first monomer, a second monomer, and a crosslinking monomer. The first monomer comprises isocyanate ethyl methacrylate, and the second monomer comprises a fluorinated derivative of urethane methacrylate. The gel electrolyte provided by this invention can actively interrupt reaction pathways in the early stages of thermal runaway through the isocyanate and urethane groups in the polymer's network structure, thereby suppressing thermal runaway. Furthermore, the fluorinated groups in the polymer's network structure can promote lithium salt dissociation, thereby increasing the concentration of free ions and further improving the fast-charging performance of the lithium-ion battery.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion batteries, and particularly relates to a gel electrolyte and a lithium-ion battery using the same. Background Technology

[0002] With the increasing popularity of new energy vehicles and portable electronic devices, the market has placed higher demands on the energy density and charging speed of lithium-ion batteries, making "fast charging" a key indicator for improving user experience. However, fast charging (high-rate charging) may cause a series of exothermic chain reactions inside the battery, leading to an uncontrollable rise in temperature and ultimately causing severe thermal runaway problems such as fire and explosion. Existing battery systems mainly rely on liquid electrolytes or traditional polymer gel electrolytes, and there is still room for optimization in balancing fast charging performance and thermal safety.

[0003] Liquid electrolytes offer the advantage of rapid ion transport; however, they are prone to side reactions under high temperature and pressure, leading to solvent decomposition and gas production. This increases the internal pressure of the battery, and when the pressure exceeds the design limits of the encapsulation or pressure relief valve, the liquid electrolyte may leak through weak points. While traditional gel electrolytes (GPEs) have addressed the leakage problem to some extent, they generally suffer from lower ionic conductivity than liquid electrolytes, hindering lithium-ion migration, causing severe polarization during fast charging, and increasing the risk of lithium plating. Furthermore, the limited thermal stability of traditional polymer matrices (such as polyethylene oxide) makes it difficult to provide an effective and active blocking mechanism in the early stages of battery thermal runaway. Summary of the Invention

[0004] To improve the thermal stability of lithium-ion batteries, this invention provides a gel electrolyte and a lithium-ion battery using the same.

[0005] According to one aspect of the present invention, a gel electrolyte is provided, the gel electrolyte comprising a polymer, the polymer being polymerized from at least a first monomer, a second monomer, and a crosslinking monomer; wherein the first monomer comprises ethyl isocyanate methacrylate; and the second monomer comprises a fluorinated derivative of urethane methacrylate.

[0006] Existing technologies employ polyolefin separators to improve the thermal stability of lithium-ion batteries using liquid electrolytes or traditional polymer matrices. Polyolefin separators typically shrink upon heating around 130°C, thus blocking current and preventing further temperature increases within the battery. However, if the temperature continues to rise above the separator's melting point, the separator melts through, leading to direct contact between the positive and negative electrodes and causing complete thermal runaway. The gel electrolyte provided in this invention, however, uses a polymer composed of at least a first monomer, a second monomer, and a crosslinking monomer. In a lithium-ion battery using this gel electrolyte, under normal operating conditions (e.g., below 70°C), the polymer network structure contains unreacted isocyanates (provided by the first monomer, -NCO) and urethane groups (provided by the second monomer, -NHCOO-). When the lithium-ion battery experiences the initial signs of thermal runaway, as the internal temperature rises from 170°C to 190°C, the urethane and isocyanates in the polymer network structure undergo polymerization, drastically increasing the crosslinking density of the network structure. This significantly increases the internal resistance of the lithium-ion battery, blocking ion migration. Therefore, the gel electrolyte provided by this invention can trigger protection in the early stages of thermal runaway, actively cutting off the reaction pathway to achieve battery melting and thus suppressing thermal runaway. On the other hand, the second monomer introduces strongly electron-withdrawing fluorine-containing groups into the polymer network structure. These groups promote lithium salt dissociation, thereby increasing the concentration of free ions and further improving the fast-charging performance of lithium-ion batteries. Furthermore, the introduction of fluorine-containing groups significantly reduces the HOMO Fermi level of the polymer chains in the polymer network structure (generally, the lower the HOMO level of a molecule, the stronger its antioxidant capacity and the better its high-voltage resistance), making it less susceptible to oxidation at high voltages. Simultaneously, the fluorine-containing groups help form a LiF-rich CEI film in situ on the surface of the cathode material, suppressing side reactions between the gel electrolyte and the cathode. In summary, the gel electrolyte provided by this invention can not only actively block battery thermal runaway but also operate stably at high voltages and has good fast-charging performance. Therefore, the gel electrolyte provided by this invention, through a "secondary curing" mechanism, enables lithium-ion batteries to "intelligently" and significantly increase their internal resistance at abnormally high temperatures. Compared to traditional closed-cell membranes, this molecular-level cross-linking blockage is more thorough and can effectively delay or prevent the occurrence of thermal runaway.

[0007] Preferably, the crosslinking monomer comprises a multifunctional unsaturated monomer containing two or more carbon-carbon double bonds. The crosslinking monomer is used to crosslink the first monomer and the second monomer, thereby forming a stable polymer chain and enhancing the mechanical strength of the gel electrolyte.

[0008] Preferably, the multifunctional unsaturated monomer includes at least one of polyethylene glycol diacrylate (PEGDA), diurea dimethacrylate, trimethylolpropane triacrylate (TMPTA), N,N'-methylenebisacrylamide, and tris(2-acryloyloxyethyl) isocyanurate.

[0009] Preferably, the multifunctional unsaturated monomer includes polyethylene glycol diacrylate. Further, polyethylene glycol diacrylate can provide flexible polyethylene glycol (PEG) segments to the polymer network structure, which, in conjunction with the fluorinated groups in the polymer network structure, can promote the dissociation of lithium salts and assist lithium-ion transport, achieving a synergistic improvement in high ionic conductivity and high lithium-ion transference number, reducing concentration polarization, and thus improving the fast-charging performance of lithium ions.

[0010] Preferably, the sum of the molar numbers of the first monomer and the second monomer accounts for 70% to 90% of the total molar number of the first monomer, the second monomer and the crosslinking monomer.

[0011] Preferably, the molar ratio is calculated as follows: first monomer: second monomer = 1:0.5~2.

[0012] Preferably, the molar ratio of the first monomer to the second monomer is 1:0.95~1.05.

[0013] Preferably, the fluorinated urethane derivative of methacrylate is prepared by an addition reaction of isocyanate ethyl methacrylate (IEM) and a fluorinated alcohol. In this reaction, the -NCO group in the IEM reacts with the -OH group in the trifluoroethanol to form an urethane bond (-NHCOO-), thus introducing a fluorinated group.

[0014] Preferably, the fluorinated alcohol includes at least one of trifluoroethanol, hexafluoroisopropanol, and hexafluorobutanol.

[0015] Preferably, the raw materials for preparing the gel electrolyte also include an initiator, and the amount of initiator added is 0.5% to 2% of the total mass of the first monomer, the second monomer and the crosslinking monomer.

[0016] Preferably, the initiator is azobisisobutyronitrile (AIBN).

[0017] Preferably, the gel electrolyte comprises fluoroethylene carbonate (FEC).

[0018] Preferably, the gel electrolyte comprises a fluorinated solvent; the fluorinated solvent comprises at least one of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, bis(2,2,2-trifluoroethyl) ether, trifluorotoluene, methyl nonafluorobutyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, and hexafluorobenzene.

[0019] The gel electrolyte provided by this invention cleverly utilizes the fluorinated groups introduced into the polymer network structure by the second monomer, enabling the polymer network structure to achieve a "like-like compatibility" effect with FEC and fluorinated solvents. This significantly improves the polymer's compatibility in the system and allows for complete curing of the polymer network structure with a lower monomer concentration. Furthermore, the low polymer network content reduces the physical barrier to lithium-ion migration, thereby improving the ion transport efficiency of the gel electrolyte during fast charging.

[0020] Preferably, the gel electrolyte comprises 14% to 18% lithium salt by mass percentage.

[0021] Localized high-concentration electrolyte (LHCE) refers to a high-concentration solvated structure achieved while maintaining low viscosity by introducing a non-solventizable diluent. The unique solvation structure of LHCE lowers the energy barrier for lithium-ion desolvation, leading to attempts to prepare gel electrolytes using LHCE in existing technologies. However, due to the generally poor compatibility between traditional acrylic or methacrylic acid monomers and LHCE, the preparation of gel electrolytes using LHCE in existing technologies often requires high monomer concentrations (above 5% or even 10%) to form an effective polymer network structure, resulting in a significant increase in the battery's DC resistance (DCR). Additives and diluents are introduced into LHCE, and FEC, preferred in this invention, can be used as an additive, while fluorinated solvents can be used as diluents. Therefore, this invention introduces fluorinated groups into the polymer network structure of the gel electrolyte, creating a "like-like compatibility" effect with LHCE containing FEC and / or fluorinated solvents, thus improving the problem of requiring high monomer concentrations when using LHCE in gel electrolytes. Therefore, by reducing the monomer concentration and utilizing the solvation structure of LHCE, the present invention can significantly reduce the DC resistance (DCR) of the battery. On the other hand, the gel electrolyte provided by the present invention, through the combination of the antioxidant properties of LHCE and the strong binding of the polymer network structure, can suppress surface side reactions of the cathode material, thereby further improving the thermal stability of the lithium-ion battery.

[0022] Preferably, the lithium salt includes at least one of lithium hexafluorophosphate (LiPF6), lithium bisfluorosulfonylimide (LiFSI), and lithium bistrifluorosulfonylimide (LiTFSI).

[0023] Preferably, the locally high-concentration electrolyte also includes a solvent, which includes chain carbonates.

[0024] Preferably, the chain carbonate includes at least one of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate (EMC).

[0025] Preferably, the method for preparing the gel electrolyte includes the following steps: S1. Preparing a precursor solution, wherein, by mass percentage, the precursor solution includes 10%~20% lithium salt, 3%~12% fluoroethylene carbonate, 15%~55% fluorinated solvent, and the total mass percentage of the first monomer, the second monomer, the crosslinking monomer, and the initiator is 2%~7%; S2. The precursor solution is cured to obtain the gel electrolyte.

[0026] Preferably, in S2, the curing reaction conditions are: curing at 55℃~70℃ and 0.1MPa~0.7MPa for 1 hour to 20 hours.

[0027] In another aspect, the present invention provides a lithium-ion battery comprising a gel electrolyte as described above. Detailed Implementation

[0028] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0029] In this document, unless otherwise stated, “%” indicates weight percentage.

[0030] A second aspect of this disclosure provides a lithium-ion battery, including the gel electrolyte described above. Here, the lithium-ion battery can be a primary lithium-ion battery or a secondary lithium-ion battery. Furthermore, secondary lithium-ion batteries can be broadly classified into pouch batteries, hard-case batteries, and cylindrical batteries according to their casing type. Although the external casing forms of these three types of batteries differ, the combination system of positive and negative electrodes and electrolytes assembled inside the battery is common. This disclosure does not limit the scope of the above.

[0031] A lithium-ion battery comprises a positive electrode, a negative electrode, a separator between the positive and negative electrodes, and a gel electrolyte. During charging and discharging, active ions repeatedly insert and extract between the positive and negative electrodes. The gel electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0032] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material.

[0033] The positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on one or both of the two opposite surfaces of the positive current collector.

[0034] The positive electrode active material described in this invention can be a lithium-containing composite oxide. Specific examples include LiMnO2, LiFeO2, LiMn2O4, Li2FeSiO4, and LiNi. 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi5CO2Mn3O2, Li z Ni （1-x-y） Co x M y O2 (where 0.01≤x≤0.20, 0≤y≤0.20, 0.97≤z≤1.20, and M represents at least one element selected from Mn, V, Mg, Mo, Nb, and Al), LiFePO4, and Li z CO （1-x） M x O2 (where 0≤x≤0.1, 0.97≤z≤1.20, and M represents at least one element selected from the group consisting of Mn, Ni, V, Mg, Mo, Nb, and Al).

[0035] The negative electrode includes a negative current collector and a film layer optionally disposed on at least one surface of the negative current collector.

[0036] The negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on one or both of the two opposite surfaces of the negative electrode current collector.

[0037] The negative electrode active material described in this invention is a material capable of lithium insertion and extraction. It includes, but is not limited to, carbon materials such as crystalline carbon (natural graphite and artificial graphite), amorphous carbon, carbon-coated graphite, and resin-coated graphite, or silicon-based materials such as silicon oxide (SiO₂). xThe active materials can be silicon-carbon materials (0≤x≤2), or metal oxide materials such as indium oxide, tin oxide, lithium titanate, and zinc oxide. Lithium metal or metals that can form alloys with lithium can also be used, such as Cu, Sn, Si, Co, Mn, Fe, Sb, and Ag. Binary or ternary alloys containing these metals and lithium can also be used as negative electrode active materials. These negative electrode active materials can be used alone or in combination of two or more. From the perspective of achieving high energy density, carbon materials such as graphite can also be combined with Si-based materials such as Si, Si alloys, and Si oxides.

[0038] Active materials usually also contain binders, conductive agents and other substances. The amount added can be adjusted from 1% to 50% of the total amount of positive electrode active material, depending on different needs.

[0039] Conductive agents are reagents used to ensure that electrodes have good charge and discharge performance. Examples include carbon black materials such as Super P, carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking black; conductive fibers such as carbon fiber and metal fiber; metal powders such as fluorinated carbon powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; and conductive metal oxides or polyphenylene derivatives such as titanium dioxide.

[0040] Adhesives are components that facilitate the bonding between active materials and conductive agents, and also facilitate the bonding between active materials and current collectors. They are typically selected from polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers.

[0041] The current collector, serving as the substrate supporting the electrode active material, is typically a metal foil with a thickness of 3μm to 500μm. There are no particular restrictions on the material, as long as it has high conductivity and will not produce a chemical reaction in the secondary battery system. For example, it can be a foil formed by surface treatment of copper, nickel, titanium, aluminum, silver, stainless steel, carbon, etc. The current collector usually has a smooth surface, but fine textures can also be formed on its surface to improve the adhesion between the positive electrode active material and the current collector. Besides foil, the current collector can also be used in any combination of one or more forms, such as film, mesh, porous, foam, or non-woven fabric.

[0042] The separator placed between the positive and negative electrode plates uses an insulating film with high ion permeability and high mechanical strength. The separator typically has a thickness of 9 μm to 18 μm; a pore size of 5 μm to 300 μm; an air permeability of 180 s / 100 mL to 380 s / 100 mL; and a porosity of 30% to 50%. The separator is made of sheets or nonwoven fabrics made of olefin polymers such as polypropylene; glass fiber; or polyethylene, which possess chemical resistance and hydrophobicity.

[0043] Example 1 This embodiment describes the preparation of a gel electrolyte and the application of it in a lithium-ion battery according to the following formulation and method.

[0044] Preparation of fluorinated urethane methacrylate derivatives: Under nitrogen protection, 0.1 mol of fluorinated alcohol (trifluoroethanol) and 300 mL of dichloromethane were mixed and stirred until dissolved. Then, 0.1 mol of isocyanate methacrylate (IEMA) was slowly added dropwise under ice bath conditions. After the addition was complete, the mixture was stirred for 12 hours. The reaction was quenched with deionized water, and the organic phase was separated and washed three times with saturated sodium chloride solution. The organic phase was dried with anhydrous magnesium sulfate, and the solvent was removed by rotary evaporation to obtain the fluorinated urethane methacrylate derivatives.

[0045] Preparation of precursor solution: In a glove box containing 99.999% argon, less than 0.1 ppm oxygen and less than 0.1 ppm moisture, weigh the first monomer (isocyanate methacrylate), the second monomer (urethane methacrylate fluorinated derivative), and the crosslinking monomer (polyethylene glycol diacrylate, PEGDA, Mn = 400) and dissolve them in a locally high-concentration electrolyte (LHCE). Add 1% of the total mass of the first monomer, the second monomer, and the crosslinking monomer as an initiator (azobisisobutyronitrile, AIBN) and stir for 1 hour to obtain a uniform and transparent precursor solution. In the precursor solution, the molar ratio of the first monomer, the second monomer, and the crosslinking monomer is 40:40:20. LHCE is composed of lithium salt (lithium hexafluorophosphate, LiPF6), fluoroethylene carbonate (FEC), fluorinated solvent (1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether), and solvent (ethyl methyl carbonate, EMC). Calculated by mass percentage, the precursor solution contains 15% LiPF6, 7% FEC, 40% 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and the total mass percentage of the first monomer, the second monomer, the crosslinking monomer, and the initiator is 3%. The remaining component in the precursor solution is EMC.

[0046] Preparation of positive electrode: LiNi 0.9 Mn 0.05 Co 0.05O2, acetylene black, and PVDF are mixed in a mass ratio of 95:3:2 and dissolved in the organic solvent NMP to achieve a solid content of 50 wt%. The mixture is stirred under vacuum until it becomes homogeneous, thus obtaining a positive electrode slurry. The positive electrode slurry is then uniformly coated onto aluminum foil, dried at room temperature, and then transferred to an oven for drying. After cold pressing and slitting, the positive electrode sheet is obtained.

[0047] Preparation of the negative electrode sheet: Artificial graphite and silicon oxide compound are mixed evenly at a mass ratio of 8:2 to obtain the negative electrode active material. The negative electrode active material, acetylene black, CMC-Na and SBR are mixed at a mass ratio of 96:2:1:1, and deionized water solvent is added to make the solid content 40 wt%. Then, the mixture is thoroughly stirred and mixed evenly under the action of a vacuum stirrer to obtain a negative electrode slurry. The negative electrode slurry is coated on copper foil, and after being dried at room temperature, it is transferred to an oven for drying. After cold pressing and slitting, the negative electrode sheet is obtained.

[0048] Preparation of dry cell: The above-prepared positive electrode, the separator loaded with electroactive organic material interlayer and the above-prepared negative electrode are placed in sequence, wrapped with aluminum-plastic film, and transferred to 80°C for baking to remove moisture.

[0049] Preparation of lithium-ion batteries: The precursor solution prepared above was injected into a dry cell at an injection volume of 2.2 g / Ah, and then sealed. After soaking at room temperature for 24 hours, the cell was obtained. Subsequently, the cell was placed in a 60℃ constant temperature oven for 5 hours to complete in-situ polymerization. After formation, aging and capacity testing, a lithium-ion battery with a capacity of 1.6 Ah containing gel electrolyte was obtained.

[0050] Example 2 This embodiment prepares a gel electrolyte and a lithium-ion battery using the formulation and method provided in Example 1. The difference from Example 1 is that, in preparing the fluorinated methacrylate carbamate derivative, this embodiment uses an equimolar amount of hexafluoroisopropanol instead of trifluoroethanol used in Example 1. Apart from the above differences, the operational steps for preparing the gel electrolyte and lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0051] Example 3 This embodiment prepares a gel electrolyte and a lithium-ion battery using the formulation and method provided in Example 1. The difference from Example 1 is that, in preparing the fluorinated methacrylate carbamate derivative, this embodiment uses an equimolar amount of hexafluorobutanol instead of trifluoroethanol used in Example 1. Apart from the above differences, the operational steps for preparing the gel electrolyte and lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0052] Comparative Example 1 This comparative example prepares a gel electrolyte and a lithium-ion battery using the formulation and method provided in Example 1. The difference from Example 1 is that no second monomer is added when preparing the precursor solution, and the molar ratio of the first monomer to the crosslinking monomer is 80:20 (the total mass percentage of the first monomer, crosslinking monomer, and initiator remains 3%). Apart from the above differences, the operational steps for preparing the gel electrolyte and lithium-ion battery in this comparative example are strictly consistent with those in Example 1.

[0053] Comparative Example 2 This comparative example prepares a gel electrolyte and a lithium-ion battery using the formulation and method provided in Example 1. The difference from Example 1 is that, in preparing the precursor solution, the first monomer is not added, and the molar ratio of the second monomer to the crosslinking monomer is 80:20 (the total mass percentage of the second monomer, crosslinking monomer, and initiator remains 3%). Apart from the above differences, the operational steps for preparing the gel electrolyte and lithium-ion battery in this comparative example are strictly consistent with those in Example 1.

[0054] Comparative Example 3 This comparative example prepares a gel electrolyte and a lithium-ion battery using the formulation and method provided in Example 1. The difference from Example 1 is that, in preparing the fluorinated methacrylate carbamate derivative, this example uses an equimolar amount of ethanol instead of the trifluoroethanol used in Example 1 (i.e., the second monomer used in this comparative example does not contain a fluorinated group). Apart from the above differences, the operational steps for preparing the gel electrolyte and lithium-ion battery in this comparative example are strictly consistent with those in Example 1.

[0055] Comparative Example 4 This comparative example uses the formulation and method provided in Comparative Example 3 to prepare a gel electrolyte and a lithium-ion battery. The difference from Comparative Example 3 is that, in this example, the total mass percentage of the first monomer, second monomer, crosslinking monomer, and initiator in preparing the precursor solution is 5% (corresponding to a reduction in the mass percentage of EMC). Apart from the above differences, the operational steps for preparing the gel electrolyte and lithium-ion battery in this comparative example are strictly consistent with those in Comparative Example 3.

[0056] Comparative Example 5 This comparative example prepares a gel electrolyte and a lithium-ion battery using the formulation and method provided in Example 1. The difference from Example 1 is that no crosslinking monomer is added when preparing the precursor solution, and the molar ratio of the first monomer to the second monomer is 50:50 (the total mass percentage of the first monomer, the second monomer, and the initiator remains 3%). Apart from the above differences, the operational steps for preparing the gel electrolyte and the lithium-ion battery in this comparative example are strictly consistent with those in Example 1.

[0057] Test Example 1 1. Test Object The gel electrolytes prepared in Examples 1 to 3 and Comparative Examples 1 to 5 and the lithium-ion batteries using them.

[0058] 2. Testing Methods (1) Ionic conductivity test: Before curing, the ionic conductivity probe is inserted into the precursor solution. After curing, the ionic conductivity of the gel is tested using an ionic conductivity meter.

[0059] (2) Lithium-ion transport number test: Lithium-ion symmetric batteries (Li|GPE|Li) assembled from gel electrolytes prepared in each example and comparative example were used. The Bruce-Vincent steady-state current method was employed to apply a 10 mV DC polarization voltage to each of the prepared lithium-ion symmetric batteries, and the impedance spectra before and after DC polarization were measured. Lithium-ion transport number (LiI) t + The calculation is as follows:

[0060] in, Iss This represents the steady-state current (A). I 0 represents the initial current (A), Δ V This indicates the applied DC polarization voltage (V). R 0: Electrode interface resistance before polarization (Ω). R ss : Steady-state electrode interface resistance after polarization (Ω).

[0061] (3) Direct Current Resistance (DCR) Test: At 25 °C, after the battery was left to stand for 10 minutes, it was charged at a constant current rate of 0.33 C to 4.2 V, then charged at a constant voltage rate to 0.05 C. After standing for 10 minutes, it was discharged at a rate of 0.33 C to 2.5 V to obtain the theoretical capacity of the battery. Then, it was charged at a constant current rate of 0.33 C to 4.2 V, then charged at a constant voltage rate to 0.05 C. After standing for 10 minutes, it was discharged at a rate of 0.33 C to 50% SOC. After standing for 1 hour, the initial voltage of the battery was recorded. V 1. Then, the battery is charged with a 4C current. I 1. Discharge for 30 seconds and record the battery voltage after discharge. V 2. Calculate the DCR at 50% SOC before the battery thermally melts using the following formula.

[0062] DCR=( V 1- V 2) / I 1 (4) 4C Fast Charging Capacity Retention Test: At 25℃, the lithium-ion battery was sequentially charged at different rates (0.33C, 1C, 2C, 3C, and 4C) to 4.3V, then charged at a constant voltage of 4.2V until the current was less than 0.05C. After resting for 10 minutes, it was discharged at a constant current of 0.33C to 2.5V. Each charging rate cycle was repeated twice, then the cycle was adjusted to the next rate. The charging capacity of the second cycle at 0.33C was recorded. C 1. The second charging cycle capacity at a 4C rate is C 2. Calculate the capacity retention rate of high-rate 4C fast charging according to the following formula.

[0063] Fast charging capacity retention rate (%) = C 2 / C 1×100% (5) Thermal runaway temperature test: After the battery was left to stand at 25 °C for 10 min, it was charged at a constant current rate of 0.33 C to 4.2 V, and then charged at a constant voltage rate to 0.05 C. After standing for 10 min, the battery was placed in an insulated chamber and heated to 130 °C at a rate of 5 °C / min, and held at that temperature for 30 min. Then the temperature was increased to 300 °C at a rate of 5 °C / min. The battery temperature was recorded at an interval of 0.1 s. When the cell temperature rise rate was first ≥1 °C / s, it was recorded as the thermal runaway temperature.

[0064] (6) 15-day high-temperature storage capacity retention test: At 25 ℃, after the battery was left to stand for 10 min, it was charged at a constant current rate of 0.33C to 4.2 V, then charged at a constant voltage rate to 0.05C. After being left to stand for 30 min, it was discharged at a constant current rate of 0.33C to 2.5 V. The discharge capacity at this time was recorded as follows. C 3. After standing for 10 minutes, charge the battery at a constant current rate of 0.33C to 4.2V, then charge it at a constant voltage rate to 0.05C. Place the battery in a 60℃ constant temperature chamber and let it stand for 15 days. Remove the battery and let it stand at 25℃ for 1 hour. Discharge it at a constant current rate of 0.33C to 2.5V. After standing for 10 minutes, charge the battery at a constant current rate of 0.33C to 4.2V, then charge it at a constant voltage rate to 0.05C. After standing for 30 minutes, discharge it at a constant current rate of 0.33C to 2.5V. Record the discharge capacity at this point. C 4.

[0065] High-temperature storage capacity retention rate (%) = C 4 / C 3×100% 3. Test Results and Analysis The experimental data for this test example are shown in Table 1. Based on the data from Examples 1 to 3, it can be seen that different fluorinated groups in the polymer network structure have different strong electron-withdrawing inductive effects, thereby affecting the local electron cloud density of the polymer backbone, and thus promoting lithium salt dissociation and reducing the HOMO Fermi level. Therefore, those skilled in the art can select specific fluorinated groups in the polymer network structure according to actual needs.

[0066] Comparing the data from Example 1 and Comparative Example 1, it is evident that the lithium-ion battery using a gel electrolyte without the addition of a second monomer exhibits significantly lower ionic conductivity (1.7 mS / cm) and lithium-ion transference number (0.49) compared to Example 1. This is likely because the formation of the fluorine-rich CEI film in Example 1 effectively suppresses lattice oxygen release and gel electrolyte decomposition on the surface of the high-nickel material, significantly reducing gas generation and capacity loss during high-temperature storage. Furthermore, comparative data confirms that the fluorine-containing groups introduced into the polymer network structure in Example 1 promote lithium salt dissociation through a strong electron-withdrawing effect. Additionally, the lack of urethane groups in the polymer network structure as sites for secondary crosslinking results in a significantly lower thermal runaway temperature (208°C).

[0067] Based on the data comparison between Example 1 and Comparative Example 2, it can be seen that although it achieved better conductivity (2.8 mS / cm) by relying on the second monomer, the thermal runaway temperature (227℃) was significantly lower than that of Example 1 (249℃). This indicates that without the isocyanate (-NCO) group in the first monomer, the polymer network structure cannot undergo the secondary cross-linking reaction of "isocyanate-carbamate" with monomer B at high temperatures, resulting in the lithium-ion battery using it losing its high-temperature self-sealing blocking mechanism.

[0068] The data from Example 1, Comparative Examples 3 and 4 demonstrate that the gel electrolyte provided by this invention has the advantage of curing at low concentrations in LHCE. Compared to Example 1, Comparative Example 3 failed to form a gel at a low concentration of 3%. This may be because the network structure of the polymer without fluorinated groups has poor extensibility in LHCE, making it difficult to crosslink into a network. In Comparative Example 4, the total mass percentage of the first monomer, second monomer, crosslinking monomer, and initiator had to be increased to 5% to achieve gelation, resulting in an overly dense polymer network structure that severely hindered ion transport, significantly increased DCR, and poor thermal stability of the lithium-ion battery using the gel electrolyte formed in Comparative Example 4, with a high-temperature capacity retention rate of only 87.4%.

[0069] In Comparative Example 5, no cross-linking monomer was added, and the first monomer and the second monomer were polymerized together. As a result, the system could not form a gel with a three-dimensional network structure, which made it impossible to lock in LHCE.

[0070] Table 1. Test results of Test Case 1

[0071] Example 4 This embodiment prepares a gel electrolyte and a lithium-ion battery using the formulation and method provided in Example 1. The difference from Example 1 is that, in preparing the precursor solution, the molar ratio of the first monomer, the second monomer, and the crosslinking monomer is 45:45:10 (the total mass percentage of the first monomer, the second monomer, the crosslinking monomer, and the initiator is maintained at 3%). Apart from the above differences, the operational steps for preparing the gel electrolyte and the lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0072] Example 5 This embodiment prepares a gel electrolyte and a lithium-ion battery using the formulation and method provided in Example 1. The difference from Example 1 is that, in preparing the precursor solution, the molar ratio of the first monomer, the second monomer, and the crosslinking monomer is 35:35:30 (the total mass percentage of the first monomer, the second monomer, the crosslinking monomer, and the initiator remains at 3%). Apart from the above differences, the operational steps for preparing the gel electrolyte and the lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0073] Example 6 This embodiment prepares a gel electrolyte and a lithium-ion battery using the formulation and method provided in Example 1. The difference from Example 1 is that, in preparing the precursor solution, the molar ratio of the first monomer, the second monomer, and the crosslinking monomer is 25:25:50 (the total mass percentage of the first monomer, the second monomer, the crosslinking monomer, and the initiator remains 3%). Apart from the above differences, the operational steps for preparing the gel electrolyte and the lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0074] Example 7 This embodiment prepares a gel electrolyte and a lithium-ion battery using the formulation and method provided in Example 1. The difference from Example 1 is that, in preparing the precursor solution, an equimolar amount of N,N'-methylenebisacrylamide is used instead of PEGDA. Apart from the above differences, the operational steps for preparing the gel electrolyte and lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0075] Example 8 This embodiment prepares a gel electrolyte and a lithium-ion battery using the formulation and method provided in Example 1. The difference from Example 1 is that, in preparing the precursor solution, an equimolar amount of TMPTA is used instead of PEGDA. Apart from the above differences, the operational steps for preparing the gel electrolyte and lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0076] Test Example 2 1. Test Object The gel electrolytes prepared in Examples 1 and 4 to 8, and the lithium-ion batteries using them.

[0077] 2. Testing Methods In this test case, the test was conducted strictly in accordance with the experimental method provided in Test Case 1.

[0078] 3. Test Results and Analysis The test results of this test example are shown in Table 2. For ease of comparison, Table 2 also includes the test results of the gel electrolyte provided in Example 1 and the lithium-ion battery using it in Test Example 1. The experimental results of Examples 1 and 4-6 of this test example show that by optimizing the ratio of the first monomer, the second monomer, and the crosslinking monomer, introducing appropriate amounts of the first and second monomers to construct a secondary crosslinking mechanism, and using an appropriate amount of crosslinking monomer as the polymer backbone, the excellent fast-charging performance (high conductivity, high mobility number) and superior thermal safety performance (high thermal runaway temperature) of the gel electrolyte provided by this invention are further optimized. Furthermore, based on the experimental results of Examples 1 and 7-8 of this test example, the optimal selection of crosslinking monomer materials can balance the fast-charging performance and thermal safety performance of the lithium-ion battery. In particular, the network structure of the polymer in the gel electrolyte provided in Example 1 achieves rapid ion transport through the synergistic effect of fluorine-containing groups and PEG segments in PEGDA.

[0079] Table 2. Test results of Test Case 2

[0080] Example 9 This embodiment prepares a gel electrolyte and a lithium-ion battery using the formulation and method provided in Example 1. The difference from Example 1 is that in this embodiment, the total mass percentage of the first monomer, second monomer, crosslinking monomer, and initiator is 2% when preparing the precursor solution. Apart from the above differences, the operational steps for preparing the gel electrolyte and lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0081] Example 10 This embodiment prepares a gel electrolyte and a lithium-ion battery using the formulation and method provided in Example 1. The difference from Example 1 is that in this embodiment, the total mass percentage of the first monomer, second monomer, crosslinking monomer, and initiator is 7% when preparing the precursor solution. Apart from the above differences, the operational steps for preparing the gel electrolyte and lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0082] Test Example 3 1. Test Object The gel electrolytes prepared in Examples 1 and 9-10, and the lithium-ion batteries using them.

[0083] 2. Testing Methods In this test case, the test was conducted strictly in accordance with the experimental method provided in Test Case 1.

[0084] 3. Test Results and Analysis The test results for this test example are shown in Table 3. For ease of comparison, Table 3 also includes the test results of the gel electrolyte provided in Example 1 and the lithium-ion battery using it in Test Example 1. The experimental results of Examples 1, 9-10 of this test example demonstrate that the gel electrolyte provided by this invention has a low gelation threshold. Furthermore, based on the test results, it can be seen that as the total mass percentage of the first monomer, the second monomer, the crosslinking monomer, and the initiator increases, the thermal runaway temperature increases, but the fast-charging capability decreases.

[0085] Table 3. Test results of Test Case 3

[0086] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A gel electrolyte, characterized in that, The gel electrolyte comprises a polymer, which is polymerized from at least a first monomer, a second monomer, and a crosslinking monomer; wherein the first monomer comprises ethyl isocyanate methacrylate; and the second monomer comprises a fluorinated derivative of urethane methacrylate.

2. The gel electrolyte as described in claim 1, characterized in that, The crosslinking monomer includes a multifunctional unsaturated monomer containing two or more carbon-carbon double bonds.

3. The gel electrolyte as described in claim 2, characterized in that, The multifunctional unsaturated monomers include at least one of polyethylene glycol diacrylate, diurea dimethacrylate, trimethylolpropane triacrylate, N,N'-methylenebisacrylamide, and tris(2-acryloyloxyethyl) isocyanurate.

4. The gel electrolyte as described in claim 1, characterized in that, The sum of the molar numbers of the first monomer and the second monomer accounts for 70% to 90% of the total molar number of the first monomer, the second monomer and the crosslinking monomer; according to the molar ratio, the first monomer: the second monomer = 1:0.5 to 2.

5. The gel electrolyte as described in claim 1, characterized in that, The fluorinated methacrylate carbamate derivative is prepared by an addition reaction of isocyanate methacrylate and fluorinated alcohol.

6. The gel electrolyte as described in claim 1, characterized in that, The gel electrolyte includes fluoroethylene carbonate.

7. The gel electrolyte as described in claim 1, characterized in that: The gel electrolyte comprises a fluorinated solvent; the fluorinated solvent comprises at least one of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, bis(2,2,2-trifluoroethyl) ether, trifluorotoluene, methyl nonafluorobutyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, and hexafluorobenzene.

8. The gel electrolyte according to any one of claims 1 to 7, characterized in that, The method for preparing the gel electrolyte includes the following steps: S1. Prepare a precursor solution, wherein, by mass percentage, the precursor solution comprises 10%~20% lithium salt, 3%~12% fluoroethylene carbonate, and 15%~55% fluorinated solvent, and the total mass percentage of the first monomer, the second monomer, the crosslinking monomer, and the initiator is 2%~7%; S2. The precursor solution is cured to obtain the gel electrolyte.

9. The gel electrolyte as described in claim 8, characterized in that, In S2, the curing reaction conditions are: curing at 55℃~70℃ and 0.1MPa~0.7MPa for 1 hour to 20 hours.

10. A lithium-ion battery, characterized in that, The lithium-ion battery includes the gel electrolyte as described in any one of claims 1 to 9.

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