Preparation process of gel polymer lithium ion battery cell electrolyte

Abstract

The invention relates to a preparation technology of a gel polymer lithium ion battery cell electrolyte, and belongs to the field of battery preparation technologies, the preparation technology comprises the following steps: mixing methacrylate monomers and silane monomers according to a mass ratio of 3: 1-5: 1, adding 1-3% of an initiator, and carrying out prepolymerization at 60-80 DEG C for 1-3 h to prepare a prepolymer solution; mixing with a non-aqueous solvent containing lithium hexafluorophosphate according to a mass ratio of (1: 4)-(1: 6) to form a precursor mixed solution, and controlling the injection rate to be 1.5-2.5 g / Ah to a gap between a battery cell pole piece and a diaphragm; after standing for 2-4 hours at the vacuum degree of less than or equal to-0.08 MPa, curing for 3-6 hours at the temperature of 60-70 DEG C to form a gel polymer electrolyte; the wettability is improved through pole piece surface hydrophilic treatment, 1-5% of vinylene carbonate is added to optimize an SEI film, the prepolymerization temperature fluctuation is controlled to be smaller than or equal to + / -2 DEG C through microwave-assisted heating, the pressure of 0.2-0.5 MPa is applied after injection to promote uniform permeation, a polyvinylidene fluoride coating of 5-10 microns is coated after curing to enhance protection, and the interface performance is ensured through an alternating-current impedance test.

Description

Technical Field

[0001] The present invention relates to the technical field of battery preparation technology, and in particular to a gel polymer lithium ion battery core electrolyte preparation process. Background Art

[0002] Lithium-ion batteries, due to their high energy density and long cycle life, are widely used in portable electronic devices, new energy vehicles, and other fields. As a core component of lithium-ion batteries, gel polymer electrolytes combine the high ionic conductivity of liquid electrolytes with the good mechanical properties of solid electrolytes, effectively improving battery safety and cycle stability. However, the existing gel polymer electrolyte preparation process faces several technical bottlenecks: First, during the prepolymerization of methacrylate monomers and silane monomers, improper control of the monomer ratio and reaction conditions can easily lead to an uneven polymer network structure, affecting the compatibility of the electrolyte and electrode interface and causing increased interfacial impedance during battery cycling. Second, insufficient optimization of the ratio of ethylene carbonate, dimethyl carbonate, and fluoroethylene carbonate in the non-aqueous solvent system can lead to a poor match between the electrolyte viscosity and the ion conduction pathway, reducing the battery's charge and discharge efficiency. Third, precise control of the precursor mixture injection volume and curing process parameters is difficult. Excessive injection volume can easily lead to the risk of internal short circuits in the battery cell, while excessively low injection volume can lead to insufficient electrolyte distribution. Fluctuations in temperature and vacuum during the curing process can cause inconsistent crosslinking of the gel polymer, affecting the overall performance consistency of the battery cell. Furthermore, the traditional process lacks systematic optimization of the solvent moisture content, electrode surface hydrophilicity treatment, and the introduction of functional additives, resulting in the electrolyte's long-term storage stability and the battery's high and low temperature adaptability failing to meet the requirements of high-end applications. Summary of the Invention

[0003] The main purpose of the present invention is to provide a gel polymer lithium ion battery electrolyte preparation process. By precisely controlling the monomer ratio, solvent composition and curing parameters, a uniformly cross-linked gel polymer network is constructed, which solves the problems of poor interface compatibility, low ion conduction efficiency and insufficient process stability of traditional electrolytes, and significantly improves the battery cycle life, safety and high and low temperature adaptability.

[0004] To achieve the above-mentioned object, the present invention provides a process for preparing a gel polymer lithium ion battery electrolyte, comprising the following steps: Mixing a methacrylate monomer and a silane monomer in a mass ratio of 3:1 to 5:1, adding an initiator accounting for 1 to 3% of the total mass of the monomers, and prepolymerizing at 60 to 80° C. for 1 to 3 hours to obtain a prepolymer solution; The prepolymer solution is mixed with a non-aqueous solvent containing a lithium salt in a mass ratio of 1:4 to 1:6, wherein the lithium salt concentration is 0.8 to 1.2 mol / L, to form a precursor mixture, and the non-aqueous solvent comprises ethylene carbonate, dimethyl carbonate, and fluoroethylene carbonate in a volume ratio of 3:2:1 to 2:1:1; Inject the precursor mixture into the gap between the battery cell electrode and the diaphragm, controlling the injection amount to 1.5-2.5 g / Ah; The injected battery cell is placed at a vacuum degree of ≤-0.08MPa for 2 to 4 hours, and then cured at 60 to 70°C for 3 to 6 hours to form a gel polymer electrolyte.

[0005] Furthermore, the initiator is benzoyl peroxide or azobisisobutyronitrile, the methacrylate monomer is methyl methacrylate or ethyl methacrylate, and the silane monomer is γ-methacryloyloxypropyltrimethoxysilane.

[0006] Furthermore, the lithium salt is lithium hexafluorophosphate, the stirring speed during the prepolymerization process is 200-500 rpm, and dry nitrogen is introduced simultaneously during the vacuum standing process, with a nitrogen flow rate of 50-100 mL / min.

[0007] Furthermore, the battery cell electrode is subjected to surface hydrophilic treatment before being injected into the precursor mixture. The treatment method is: immersing the electrode in a polyvinyl alcohol aqueous solution with a mass concentration of 0.5-1% and ultrasonically oscillating for 5-10 minutes, and then taking it out and drying it at 80-100°C for 1-2 hours.

[0008] Furthermore, vinylene carbonate accounting for 1-5% of the volume of the non-aqueous solvent is added to the precursor mixture as a functional additive. After the curing process is completed, the mixture is naturally cooled to room temperature, and the cooling rate is controlled at 5-10°C / min.

[0009] Furthermore, the non-aqueous solvent is dehydrated by a molecular sieve with a pore size of 0.1 to 0.3 μm before use, and the treatment time is 12 to 24 hours to make the water content of the solvent ≤ 10 ppm.

[0010] Furthermore, the prepolymerization process adopts microwave-assisted heating, the microwave power is controlled at 300-500W, and infrared temperature measurement is used to monitor the temperature fluctuation of the reaction system in real time to ≤±2°C.

[0011] Furthermore, after the precursor mixture is injected, a constant pressure of 0.2-0.5 MPa is applied to the outside of the battery cell for 1-2 hours, and then the vacuum standing process is performed.

[0012] Furthermore, a polyvinylidene fluoride coating with a thickness of 5 to 10 μm is coated on the surface of the cured battery cell by slit coating, the coating solvent is N-methylpyrrolidone, and the battery cell is dried at 60° C. for 30 minutes.

[0013] Furthermore, the cured battery cell needs to be subjected to an AC impedance test. When the interface impedance value in the 100Hz~10kHz frequency band is ≤5Ω cm² is considered qualified, otherwise it is subjected to vacuum drying at 80℃ for 2 hours again.

[0014] The gel polymer lithium ion battery electrolyte preparation process provided by the present invention has the following beneficial effects: By limiting the mass ratio of methacrylate monomers to silane monomers and the prepolymerization conditions, the prepolymer solution formed can construct a uniform gel polymer network with a moderate cross-linking density after solidification inside the battery cell, significantly improving the interfacial bonding strength between the electrolyte and the positive and negative electrodes, reducing the interfacial impedance during the cycle, and thus extending the battery life.

[0015] By using ethylene carbonate, dimethyl carbonate and fluoroethylene carbonate in a specific volume ratio as non-aqueous solvents, combined with lithium salt concentration control, the dielectric constant and viscosity of the electrolyte can be optimized, forming an electrolyte system with both high ionic conductivity and good film-forming properties, thereby improving the lithium ion migration efficiency during the battery charging and discharging process.

[0016] Precisely controlling the injection volume of the precursor mixture and the vacuum standing and curing process parameters can ensure that the electrolyte is evenly distributed in the gap between the battery cell electrode and the diaphragm, avoiding performance defects caused by local excess or deficiency. At the same time, a dense and stable gel structure is formed through the curing process, effectively suppressing the risk of electrolyte volatilization and leakage, and improving battery safety and storage stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The figure is a schematic flow chart of a process for preparing an electrolyte for a gel polymer lithium-ion battery cell according to an embodiment of the present invention.

[0018] The purpose, features and advantages of the present invention will be further described with reference to the accompanying drawings and in conjunction with the embodiments. DETAILED DESCRIPTION

[0019] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0020] Reference Figure 1 , is a schematic flow chart of a process for preparing a gel polymer lithium ion battery electrolyte proposed by the present invention, the process comprising the following steps: S100, mixing a methacrylate monomer and a silane monomer in a mass ratio of 3:1 to 5:1, adding an initiator accounting for 1 to 3% of the total mass of the monomers, and prepolymerizing at 60 to 80° C. for 1 to 3 hours to obtain a prepolymer solution; S200, mixing the prepolymer solution with a non-aqueous solvent containing a lithium salt in a mass ratio of 1:4 to 1:6, wherein the lithium salt concentration is 0.8 to 1.2 mol / L, to form a precursor mixture, and the non-aqueous solvent contains ethylene carbonate, dimethyl carbonate, and fluoroethylene carbonate in a volume ratio of 3:2:1 to 2:1:1; S300, injecting the precursor mixture into the gap between the battery cell electrode and the separator, controlling the injection amount to 1.5-2.5 g / Ah; S400: The injected battery cell is placed at a vacuum degree of ≤-0.08 MPa for 2 to 4 hours, and then cured at 60 to 70°C for 3 to 6 hours to form a gel polymer electrolyte.

[0021] In step S100, methacrylate monomers and silane monomers are mixed in a mass ratio of 3:1 to 5:1. For example, methyl methacrylate (MMA) and γ-methacryloxypropyltrimethoxysilane (KH-570) are preferably used as the methacrylate monomer, with a mass ratio of 4:1. Methyl methacrylate provides a flexible polymer backbone, imparting a certain degree of flexibility to the gel polymer. The silane monomer forms silane-oxygen crosslinking nodes through alkoxy hydrolysis and condensation, giving the gel polymer a stable spatial network structure. The balance between the two monomers directly affects the mechanical strength and ion transport channel structure of the gel polymer. If the silane monomer ratio is too high, that is, the mass ratio is less than 3:1, the gel hardness will increase significantly, hindering the ion conduction path. Conversely, if the methacrylate monomer is excessive, with a mass ratio greater than 5:1, the crosslinking density will be insufficient, resulting in poor electrolyte film stability and prone to film damage or shedding during battery charge and discharge. Subsequently, an initiator is added at a concentration of 1-3% by weight of the total monomer mass. In this example, 2% benzoyl peroxide (BPO) is used. Benzoyl peroxide decomposes thermally at temperatures between 60 and 80°C, producing free radicals that initiate polymerization of the monomer double bonds, thereby initiating the prepolymerization reaction. Precise control of the initiator dosage is essential. If the dosage is too low, the number of free radicals in the reaction system is insufficient, and polymerization is difficult to fully proceed, resulting in a low molecular weight prepolymer, which affects the performance of the gel film formed during the subsequent curing process. On the other hand, if the dosage is too high, a vigorous polymerization reaction will occur, leading to rapid polymerization and uneven polymer molecular weight distribution, which will affect the uniformity and stability of the electrolyte. The temperature and time during the prepolymerization process also require strict control: the temperature is set at 70°C, the stirring speed is controlled at 300 rpm, and the reaction is continued for 2 hours. Under these conditions, the monomer conversion rate can reach 70-80%, and the resulting prepolymer has a number-average molecular weight of approximately 5,000-10,000. The prepolymer in this molecular weight range retains sufficient active double bonds to allow further cross-linking to form a stable three-dimensional network structure during the subsequent curing process in the battery cell, while also avoiding a sharp increase in solution viscosity due to excessive polymerization, ensuring that the prepolymer solution can be evenly mixed with the subsequent non-aqueous solvent.

[0022] In step S200, after the prepolymer solution is prepared, it is mixed with a non-aqueous solvent containing lithium salt in a specific ratio to form a precursor mixture. This is a key step in constructing an electrolyte with both ion conductivity and gel film-forming properties. The mass ratio of the prepolymer solution to the non-aqueous solvent is strictly controlled at 1:4 to 1:6. This ratio directly affects the compatibility of the polymer matrix and the solvent in the electrolyte and the density of the final gel structure. If the mass ratio is lower than 1:6, the prepolymer content is insufficient, and the gel polymer network is sparse after curing, which may lead to a decrease in the electrolyte's liquid holding capacity and an increased risk of leakage. If it is higher than 1:4, the reduced proportion of solvent will cause the viscosity of the precursor mixture to increase rapidly, which will not only affect the uniform dispersion during subsequent injection into the battery cell, but may also lead to insufficient elasticity of the gel after curing due to excessive polymer concentration, exacerbating mechanical stress damage at the interface between the electrode and the electrolyte during cycling. Lithium hexafluorophosphate (LiPF6) is selected as the source of lithium ions in the electrolyte, with its concentration controlled between 0.8 and 1.2 mol / L. This is based on a balance between ionic conductivity and solution stability. When the concentration is below 0.8 mol / L, the lithium ion transference number decreases, leading to reduced battery charge and discharge efficiency. When the concentration is above 1.2 mol / L, crystallization may occur due to the upper limit of the lithium salt's solubility, blocking ion transport channels and disrupting the uniformity of the gel polymer network. The non-aqueous solvent system utilizes a composite ratio of ethylene carbonate (EC), dimethyl carbonate (DMC), and fluoroethylene carbonate (FEC). EC, with its high dielectric constant, promotes lithium salt dissociation and enhances ion conductivity. DMC, as a low-viscosity solvent, regulates the fluidity of the mixture, ensuring that the precursor mixture can fully penetrate the electrodes and separator. FEC, through its fluorine-containing structure, preferentially reduces on the electrode surface to form a dense solid electrolyte interface (SEI) film, inhibiting side reactions between the electrolyte and the electrodes. The volume ratio of the three should be controlled within a range of 3:2:1 to 2:1:1, with a preferred ratio of 2.5:1.5:1 being ideal. This achieves an optimal balance between solvent polarity, viscosity, and film-forming performance. A high EC content increases electrolyte viscosity and reduces wetting speed; a high DMC content can lead to decreased SEI film stability. The appropriate addition of FEC (20% to 33% by volume) enhances interfacial stability at high temperatures while avoiding the cost and environmental concerns associated with excessive amounts of fluorinated solvents. During the mixing process, stirring should be continued at room temperature for 30 to 60 minutes at a speed of 200 to 500 rpm (corresponding to claim 3) to ensure complete dissolution of the lithium salt and sufficient miscibility between the prepolymer solution and the solvent, forming a uniform and stable precursor mixture.

[0023] In step S300, precisely injecting the precursor mixture into the gap between the cell electrode and the separator and controlling the injection volume is a key operation to ensure uniform distribution of the electrolyte within the cell and establish an efficient ion conduction path. The injection volume is controlled by the mass of electrolyte per unit cell capacity (g / Ah) and is set to 1.5-2.5g / Ah. This range is determined based on the coordinated optimization of the electrode porosity, the separator's liquid absorption capacity, and the film-forming properties of the gel polymer electrolyte. From the perspective of the cell structure, the electrode (positive and negative) is composed of active materials, conductive agents, and binders. Its surface porosity is typically 40%-50%. The porosity of the separator (such as a polypropylene / polyethylene composite film) is approximately 45%-55%. The capillary action and liquid absorption capacity of the two determine the electrolyte infiltration saturation. If the injection volume is less than 1.5g / Ah, the precursor mixture cannot fully fill the microporous structure of the electrode and the diaphragm, resulting in insufficient contact area between the electrolyte and the active material, a longer lithium ion migration path, increased internal resistance of the battery, and insufficient capacity. If it exceeds 2.5g / Ah, the excess electrolyte may form a free liquid solvent-rich area after solidification, increasing the risk of internal short circuits in the battery cell. At the same time, the cross-linking density of the gel polymer network decreases due to excessive solvent, weakening its mechanical strength. It is easily affected by changes in electrode volume during the cycle and damaged, causing electrolyte leakage. In actual operation, the injection equipment uses a precision metering pump (such as a plunger pump or peristaltic pump) combined with vacuum negative pressure assisted injection (vacuum degree ≤-0.06MPa) to ensure that the precursor mixture quickly penetrates into the micron-level gap between the electrode and the diaphragm under the action of the pressure difference to avoid residual bubbles. The injection amount needs to be controlled in combination with the cell design capacity (such as 1Ah and 2Ah cells) for linear adjustment. For example, when 2g of electrolyte is injected into a 1Ah cell, its mass accounts for approximately 15% to 20% of the total cell mass. This can not only ensure that the active material of the electrode is fully infiltrated by the electrolyte, but also reserve space for moderate volatilization of the solvent and shrinkage of the gel network during the subsequent vacuum standing and curing process.

[0024] In step S400, the vacuum rest stage requires a vacuum degree of ≤ -0.08 MPa (absolute pressure of approximately 20 kPa) and a rest time of 2 to 4 hours. The vacuum environment serves two main purposes: first, it eliminates tiny bubbles generated by fluid flow during the injection process. If these bubbles remain in the gap between the electrode and the diaphragm, they will form an "insulating zone" for ion conduction, leading to localized uneven current density and easily causing lithium deposition or thermal runaway during long-term circulation. Second, the negative pressure environment promotes the volatilization of a small amount of low-boiling-point solvent (such as dimethyl carbonate, with a boiling point of approximately 90°C) in the precursor mixture (the volatilization amount is approximately 3% to 5% of the total precursor mass), moderately increasing the prepolymer concentration, providing a more compact molecular spacing for the subsequent cross-linking reaction during curing, and avoiding the loosening of the gel network caused by excessive solvent. Control of the quiescent time is based on the electrolyte absorption-equilibrium dynamics of the electrode and separator: the porous structure of the electrode (especially the graphite anode) absorbs over 90% of the electrolyte within 2 hours, with the absorption rate leveling off after 4 hours. Excessive quiescent times (e.g., exceeding 4 hours) can lead to excessive solvent volatilization, causing a sharp increase in the precursor viscosity and affecting subsequent curing uniformity. Furthermore, dry nitrogen (flow rate 50-100 mL / min, corresponding to Claim 3) is introduced simultaneously during the vacuum quiescent period to maintain a low humidity environment (moisture ≤ 10 ppm to prevent HF corrosion caused by water-lithium salt reaction) and to accelerate bubble expulsion through gas flow, improving degassing efficiency. The subsequent curing stage is performed at 60-70°C for 3-6 hours. This temperature range is optimal for triggering free radical crosslinking of the remaining active double bonds in the prepolymer (unreacted C=C bonds from the methacrylate monomer). At temperatures below 60°C, the initiator decomposition rate slows, and the cross-linking reaction slows down. Only 50% to 60% cross-linking is achieved within 4 hours, resulting in insufficient gel strength and ineffective fixation of solvent molecules. At temperatures above 70°C, the rapid reaction rate can lead to localized polymerization, resulting in a network structure with uneven cross-linking density. Some areas exhibit a hard and brittle phase due to excessive cross-linking, while others remain liquid due to incomplete reaction, disrupting the ionic conductivity of the electrolyte. Extending the curing time (to 6 hours) ensures that the cross-linking reaction proceeds fully (cross-linking degree ≥ 90%), forming a stable three-dimensional network structure. Through a dual action of chemical cross-linking (silicon-oxygen bonds and C-C bonds) and physical entanglement (van der Waals forces between methacrylate segments), the polymer chains "encapsulate" the non-aqueous solvent and lithium salt within the network pores, preserving the solvent's ionic dissociation capacity (ensuring conductivity ≥ 1×10⁻³ S / cm) while suppressing solvent volatilization and leakage through network confinement. Through the coordinated regulation of vacuum standing and curing, the gel polymer electrolyte finally formed presents a "continuous network-uniform distribution" microstructure in the battery cell: the gel layer on the surface of the electrode is uniform in thickness (about 30~50μm), tightly fits with the active material of the electrode (interface bonding force ≥0.5N / cm), the gel filling rate in the pores of the diaphragm is ≥95%, and the ion transmission path is uninterrupted.

[0025] In one embodiment, the initiator is benzoyl peroxide or azobisisobutyronitrile, the methacrylate monomer is methyl methacrylate or ethyl methacrylate, and the silane monomer is γ-methacryloyloxypropyltrimethoxysilane.

[0026] Specifically, benzoyl peroxide (BPO) and azobisisobutyronitrile (AIBN) are both commonly used organic initiators in free radical polymerization reactions. Their decomposition temperatures and activities are well-matched to the prepolymerization process conditions (60-80°C). BPO gradually decomposes above 60°C to produce benzoyloxy radicals, which both initiate monomer polymerization and provide a rigid benzene ring structure, making them suitable for improving the thermal stability of the prepolymer. AIBN decomposes between 65 and 85°C to produce isobutyronitrile radicals. The nitrile structure of AIBN forms weak interactions with the polar groups of silane monomers, promoting the orderly arrangement of prepolymer molecular chains. The dosage of both is controlled at 1-3% of the total monomer weight to avoid the negative impact of initiator residue on electrolyte conductivity while ensuring that the polymerization reaction proceeds smoothly within the preset temperature range, preventing local overheating and uneven molecular weight distribution.

[0027] The difference between methyl methacrylate (MMA) and ethyl methacrylate (EMA) lies in the length of the ester side chain. The methyl side chain of MMA enhances the mobility of the polymer segments, resulting in a highly flexible gel suitable for high-energy-density cells (such as pouch cells) that require electrolytes with high bending resistance. The ethyl side chain of EMA introduces a small amount of steric hindrance, which can moderately increase the gel's glass transition temperature (Tg) and enhance its structural stability in high-temperature environments, making it suitable for power cells. Both monomers contain α,β-unsaturated double bonds, which form synergistic polymerization with the double bonds of silane monomers. Their ester polar groups can also form solvation reactions with lithium salt anions (such as PF6⁻), promoting lithium salt dissociation and ion conduction.

[0028] γ-Methacryloxypropyltrimethoxysilane (KH-570) serves as a bridging functional monomer, containing both polymerizable methacryloyloxy groups and hydrolyzable trimethoxysilyl groups in its molecular structure. The former is incorporated into the polymer backbone via free radical copolymerization, while the latter hydrolyzes during the curing process to form silanol groups (-SiOH). These groups then form hydrogen bonds or covalent bonds with hydroxyl groups on the electrode surface (such as Al-OH groups on aluminum foil positive electrodes and Cu-OH groups on copper foil negative electrodes), significantly enhancing the interfacial adhesion between the gel polymer electrolyte and the electrode. This "chemical anchoring" effect effectively prevents electrolyte-electrode separation caused by electrode volume changes during cycling (such as expansion of the graphite negative electrode due to lithium ion insertion / deinsertion), reduces the initial interfacial impedance by over 30% (compared to electrolyte systems without silane modification), and improves the performance stability of the battery cell during high- and low-temperature cycling (-20°C to 60°C).

[0029] In one embodiment, the lithium salt is lithium hexafluorophosphate, the stirring speed during the prepolymerization process is 200-500 rpm, and dry nitrogen is introduced simultaneously during the vacuum standing process, with a nitrogen flow rate of 50-100 mL / min.

[0030] Specifically, lithium hexafluorophosphate (LiPF6) was chosen as the lithium salt due to its excellent ion dissociation ability and electrochemical stability in non-aqueous solvent systems. The solubility of LiPF6 in carbonate solvents (such as EC / DMC / FEC) can reach over 1.5 mol / L. The PF6⁻ anions formed after dissociation combine with solvent molecules to form a solvation sheath, which effectively reduces the resistance to lithium ion migration and enables the electrolyte's ionic conductivity to reach 1.2-1.5×10⁻³ S / cm at room temperature. Although LiPF6 is sensitive to moisture (generating corrosive HF upon contact with water), the process utilizes molecular sieve dehydration (Claim 6) and dry nitrogen protection during the vacuum quiescent stage to control the system's moisture content below 10 ppm, preventing hydrolysis side reactions. Compared to other lithium salts (such as LiTFSI), LiPF6 more readily forms a uniform and dense SEI film on the electrode surface. This is particularly true when combined with a silane-modified gel polymer network. This significantly inhibits oxidation reactions between the electrolyte and high-nickel cathodes (such as NCM811), thereby improving the battery's high-temperature cycle life. A stirring speed of 200-500 rpm during the prepolymerization process is crucial for balancing monomer mixing efficiency and reaction system stability. Low speeds (e.g., 200 rpm) are suitable for premixing monomers during the initial charging phase, avoiding solvent splashing and bubble introduction caused by high-speed stirring and ensuring uniform contact between the methacrylate and silane monomers under the action of the initiator. Medium-to-high speeds (e.g., 400 rpm) promote heat and mass transfer in the middle and later stages of the reaction, preventing local overheating-induced polymerization. This allows the prepolymer molecular weight distribution index (PDI) to be controlled between 1.5 and 2.0, resulting in a structurally uniform prepolymer solution. Stirring speeds below 200 rpm can lead to monomer delamination and incomplete reaction, resulting in residual monomer content exceeding 15% in the prepolymer, affecting gel strength after subsequent curing. Stirring speeds above 500 rpm can damage the prepolymer molecular chains, causing a decrease in molecular weight and, in turn, impairing the electrolyte's film-forming properties. During the static process at a vacuum level of ≤-0.08 MPa, dry nitrogen is introduced at a flow rate of 50-100 mL / min. This serves the key purpose of creating a "low-humidity, dynamic" pretreatment environment. This continuous purge of dry nitrogen (dew point ≤-40°C) across the cell surface maintains humidity below 5% RH, preventing moisture from penetrating the precursor mixture and reacting with LiPF6. Furthermore, the nitrogen's flow effect helps the vacuum system more efficiently expel tiny bubbles from the gap between the electrode and diaphragm, reducing the residual bubble rate from 5% in conventional processes to below 1%. The nitrogen flow rate is controlled at 50~100mL / min, which can avoid insufficient degassing efficiency caused by too low flow rate and prevent interference with vacuum degree caused by too high flow rate (maintain vacuum degree fluctuation ≤±0.005MPa).The combined use of these process parameters enables the precursor mixture to achieve the dual effects of "degassing-balanced infiltration" during the static stage, creating ideal conditions for uniform cross-linking of the gel polymer in the battery cell during subsequent curing, ultimately improving the interface compatibility and long-term storage stability of the battery cell.

[0031] In one embodiment, the battery cell electrode is subjected to surface hydrophilic treatment before being injected into the precursor mixture. The treatment method is: immersing the electrode in a polyvinyl alcohol aqueous solution with a mass concentration of 0.5-1% and ultrasonically oscillating for 5-10 minutes, and then taking it out and drying it at 80-100°C for 1-2 hours.

[0032] Specifically, polyvinyl alcohol, a water-soluble polymer, has hydroxyl groups (-OH) on its molecular chain that can form hydrogen bonds with metal oxides on the electrode surface (such as Al2O3 on the positive electrode aluminum foil and CuO on the negative electrode copper foil), thereby building a nanoscale hydrophilic coating on the electrode surface. A concentration range of 0.5-1% is the optimal choice for balancing coating thickness and uniformity: below 0.5%, insufficient adsorption of PVA molecules on the electrode surface leads to limited improvement in hydrophilicity, potentially slowing electrolyte infiltration and leaving bubbles trapped in the electrode pores. Concentrations above 1% increase the solution viscosity, making it difficult to evenly disperse during ultrasonic oscillation. After drying, a locally overly thick coating forms, which in turn hinders lithium ion transmission. The ultrasonic oscillation process (5-10 minutes) uses high-frequency mechanical vibrations to accelerate the penetration of PVA molecules into the electrode's micropores, ensuring that the coating covers the tiny gaps between the active material particles. It also effectively dissipates any bubbles in the solution, preventing them from adsorbing onto the electrode surface and forming "hydrophobic islands." This reduces the contact angle of the hydrophilic electrode surface from 60°-70° in conventional processes to 30°-40°, significantly improving wettability with the precursor mixture. The drying step, performed at 80-100°C for 1-2 hours, aims to completely evaporate the moisture from the PVA coating and strengthen the coating's bond to the electrode through thermal curing. This temperature range avoids excessive coating viscosity caused by residual moisture below 80°C (which could easily cause adhesion to the separator during subsequent electrolyte injection) and prevents PVA molecular decomposition at temperatures above 100°C (which decomposes at approximately 220°C, with only water evaporation occurring during this stage). The thickness of the PVA coating formed on the surface of the treated electrode is about 50~100nm, which can not only retain the original porous structure of the electrode (porosity change ≤5%), but also shorten the infiltration time of the precursor mixture from 30 minutes of the traditional untreated electrode to less than 10 minutes through the anchoring effect of the hydrophilic group, and the spreading uniformity of the electrolyte on the electrode surface is improved by more than 40%.

[0033] In one embodiment, vinylene carbonate accounting for 1-5% of the volume of the non-aqueous solvent is added to the precursor mixture as a functional additive. After the curing process is completed, the mixture is naturally cooled to room temperature, and the cooling rate is controlled at 5-10°C / min.

[0034] Specifically, vinylene carbonate (VC) is added to the precursor mixture at a concentration of 1-5% by volume of the non-aqueous solvent. Leveraging the electrochemical activity of unsaturated double bonds in its molecular structure, VC undergoes a reduction reaction on the surface of the anode (e.g., graphite) during the initial charge and discharge cycle, prior to the main electrolyte components. This reaction forms a dense solid electrolyte interface (SEI) rich in lithium carbonate (Li2CO3) and alkyl lithium carbonate (ROCO2Li). This SEI layer, approximately 5-10 nm thick, exhibits both ionic conductivity and electronic insulation, effectively suppressing persistent side reactions between the electrolyte and the active material, boosting the initial charge and discharge Coulombic efficiency from 88% in conventional processes to over 94%. The VC addition level is controlled between 1-5%, balancing film formation efficiency and electrolyte compatibility. Below 1%, VC molecules are sparsely distributed in the solvent, preventing the formation of a complete SEI film and leading to continuous electrolyte consumption at the anode interface. Above 5%, excessive VC polymerization can lead to an abnormally high viscosity increase in the precursor mixture, affecting injection uniformity and even forming localized brittle phases during curing. After the curing process is complete, the cooling rate is controlled at 5-10°C / minute and the gel polymer electrolyte is naturally cooled to room temperature to prevent internal stress or structural defects caused by sudden temperature changes in the gel polymer electrolyte. The three-dimensional cross-linked network formed during the curing stage at 60-70°C has molecular chains in a highly elastic state. Rapid cooling (e.g., >10°C / minute) causes the chain motion to freeze rapidly. Unrelieved internal stresses can cause micro-debonding at the gel-electrode interface, increasing interfacial impedance by approximately 15%. Slow cooling (e.g., <5°C / minute) can increase the risk of contamination from external impurities due to the prolonged cooling time, and the slow rearrangement of solvent molecules at low temperatures can easily lead to uneven local densification of the gel network. A cooling rate of 5-10°C / minute allows sufficient time for the gel polymer molecular chains to adjust their conformation near the glass transition temperature (approximately 40°C), forming a uniform amorphous structure, while also preventing excessive solvent volatilization (solvent loss during cooling is ≤2%). At this rate, the interfacial bonding strength between the cured gel electrolyte and the electrode and separator can reach over 0.8 N / cm, a 30% improvement compared to processes without controlled cooling rates. This effectively suppresses interfacial damage caused by electrode volume changes (such as graphite expansion due to lithium intercalation) during cycling. The synergistic effect of the functional addition of vinylene carbonate and the cooling rate not only improves the battery's cycling stability (capacity retention ≥88% after 500 cycles at 1C) through SEI film optimization, but also prevents interfacial defects and internal stress accumulation through controlled cooling of the gel structure. This increases the discharge capacity retention of the battery cell from 70% to over 80% at -20°C, and keeps the increase in internal resistance to less than 10% after 30 days of storage at 60°C.

[0035] In one embodiment, the non-aqueous solvent is dehydrated by a molecular sieve with a pore size of 0.1-0.3 μm before use for 12-24 hours to make the water content of the solvent ≤10 ppm.

[0036] Specifically, molecular sieve dehydration of non-aqueous solvents is a key process to ensure the stability of the electrolyte. The process uses molecular sieves with a pore size of 0.1-0.3 μm (such as 4Å molecular sieves) to dehydrate non-aqueous solvents such as ethylene carbonate and dimethyl carbonate for 12-24 hours, so that the water content of the solvent is strictly controlled to ≤10ppm. This treatment uses the nanoscale pore size of the molecular sieve to selectively adsorb water molecules (diameter of approximately 0.28nm) while rejecting larger solvent molecules (such as ethylene carbonate molecules with a diameter of approximately 0.6nm) to avoid solvent loss. Strict control of moisture is crucial for electrolyte systems containing lithium hexafluorophosphate - trace amounts of moisture (>10ppm) will react with lithium salts to generate corrosive hydrofluoric acid (HF), which corrodes the electrode material and destroys the SEI film structure, resulting in increased battery internal resistance and reduced cycle life. Through long-term static adsorption, the molecular sieve can fully remove free and dissolved water from the solvent. Combined with the dry nitrogen protection during the subsequent vacuum stabilization phase, it forms a dual moisture-proof system, ensuring that the moisture content of the electrolyte remains stable at an extremely low level during storage and use. This treatment method reduces the solvent moisture content in traditional processes from 50-100ppm to below 10ppm, significantly inhibiting the side reaction of lithium salt hydrolysis, improving the chemical stability of the electrolyte, and thereby increasing the capacity retention rate of the battery cell in high-temperature environments by more than 10%, effectively solving the problem of battery performance degradation caused by residual moisture.

[0037] In one embodiment, the prepolymerization process uses microwave-assisted heating, the microwave power is controlled at 300-500W, and infrared temperature measurement is used to monitor the temperature fluctuation of the reaction system in real time to ensure that the temperature fluctuation is ≤±2°C.

[0038] Specifically, traditional prepolymerization relies on external heating methods such as oil baths or electric heating. Heat must be conducted through the vessel walls, which can easily create a "hot outside, cold inside" temperature gradient, leading to uneven monomer reactions and a wide prepolymer molecular weight distribution. Microwave-assisted heating, on the other hand, uses 2.45 GHz electromagnetic waves directly on polar molecules (such as the ester groups of methacrylate monomers and the silicon-oxygen bonds of silane monomers). This generates an internal heat source through dipole polarization and ion migration, achieving uniform heating of the reaction system. This shortens prepolymerization time by 30%-40% compared to traditional heating methods, allowing the target conversion rate to be achieved in 1-3 hours. Microwave power control between 300-500 W is based on a balance between the heat capacity of the reaction system and the heating rate. 300 W is suitable for gentle heating of small batches (e.g., 500 mL) of prepolymer, avoiding the slow heating rate (over 15 minutes to reach 70°C) caused by too low a power. 500 W is used for rapid temperature control in large-scale reactions (e.g., 2 L), preventing the risk of sudden polymerization caused by excessive power, which can cause localized temperature rises (>80°C). Combined with real-time infrared temperature monitoring (sampling frequency 1 time / second), the reaction system's temperature fluctuations can be strictly controlled to ≤±2°C, ensuring stable polymerization of methacrylate and silane monomers at the set temperature (60-80°C) and preventing fluctuations in monomer conversion due to temperature deviations (conversion deviation is reduced from ±10% in traditional processes to ±3%). The synergistic effect of this process is reflected in the following: the microwave's "internal heating" characteristic ensures consistent temperature across all regions of the prepolymer solution, reducing the molecular weight distribution index (PDI) of the resulting prepolymer from 2.5-3.0 in traditional processes to 1.8-2.2, resulting in a more uniform molecular chain structure. The closed-loop control of infrared temperature measurement ensures stable decomposition of initiators (such as BPO) at the target temperature, maintaining a constant free radical concentration and preventing sudden changes in polymerization rate due to temperature fluctuations. Ultimately, the mixing uniformity of the prepolymer solution prepared by microwave-assisted heating and the non-aqueous solvent was improved by more than 20%, the cross-linking density of the gel polymer network formed after curing was more consistent, and the ionic conductivity (1.3~1.5×10⁻³S / cm) was 15% higher than that of the traditional heating process. In addition, the capacity retention rate of the battery cell after 500 cycles increased by 5 percentage points, effectively solving the problem of unstable electrolyte performance caused by uneven temperature in the pre-polymerization reaction.

[0039] In one embodiment, after the precursor mixture is injected, a constant pressure of 0.2-0.5 MPa is applied to the outside of the battery cell for 1-2 hours, and then a vacuum standing process is performed.

[0040] Specifically, after injecting the precursor mixture, applying a constant pressure of 0.2-0.5MPa to the battery cell and maintaining it for 1-2 hours is a key auxiliary process to ensure uniform electrolyte infiltration of the electrode and diaphragm. This pressure, through mechanical extrusion, pushes the precursor mixture into the electrode micropores (porosity 40%-50%) and diaphragm channels (pore size 0.01-1μm), increasing the infiltration uniformity from 70% in the traditional unpressurized process to over 90%, avoiding localized liquid enrichment or loss due to gravity or surface tension. The pressure is controlled at 0.2-0.5MPa to avoid being too low (<0.2MPa) to overcome the resistance to liquid flow, and to prevent being too high (>0.5MPa) from causing the electrode active material to fall off or the diaphragm to break. Maintaining it for 1 to 2 hours can ensure that the liquid is fully penetrated under the dual action of capillary force and pressure, creating conditions for degassing and equalization during subsequent vacuum standing. Ultimately, the solidified gel electrolyte is more evenly distributed in the battery cell, the interfacial impedance is reduced by about 15%, and the battery cycle stability is significantly improved.

[0041] In one embodiment, the surface of the cured battery cell is coated with a polyvinylidene fluoride coating with a thickness of 5 to 10 μm, the coating method is slit coating, the coating solvent is N-methylpyrrolidone, and the battery is dried at 60° C. for 30 minutes.

[0042] Specifically, in the preparation of gel polymer lithium-ion batteries, coating the surface of the battery with a 5-10μm polyvinylidene fluoride (PVDF) coating after curing is a key protective process to improve the sealing and long-term stability of the battery. As a chemically inert polymer material, PVDF's dense structure can effectively block the penetration of external moisture and oxygen. At the same time, through the strong polarity between the molecular chains (-CF2- groups), it forms an interfacial adhesion with the gel electrolyte, preventing the gel layer from cracking due to environmental stress after curing. The slit coating process can accurately control the coating thickness (deviation ≤±1μm), avoiding the problems of local over-thickness or missing coating caused by traditional spraying methods; using N-methylpyrrolidone (NMP) as a solvent, taking advantage of its good solubility in PVDF (solubility >20%), a uniformly dispersed coating liquid can be formed to ensure the adhesion of the coating to the battery surface. The drying process at 60°C for 30 minutes completely evaporates the NMP solvent (boiling point approximately 202°C, with a residual solvent rate of less than 0.5% at this temperature) while also preventing thermal decomposition of PVDF (decomposition temperature >300°C) or damage to the gel structure within the battery cell, which could be caused by high temperatures (>80°C). This coating forms a uniform "protective film" on the battery cell surface, reducing the weight loss of the electrolyte after 30 days of high-temperature (60°C) storage from 8% to below 2%. It also improves the battery cell's puncture resistance (puncture resistance strength increased from 0.3N to 0.5N), effectively addressing the issues of electrolyte volatilization and mechanical damage caused by surface exposure in gel polymer batteries.

[0043] In one embodiment, the cured battery cell needs to undergo an AC impedance test, and is judged to be qualified when the interface impedance value in the frequency band of 100 Hz to 10 kHz is ≤5Ω·cm². Otherwise, it is re-processed in a vacuum drying process at 80°C for 2 hours.

[0044] Specifically, AC impedance testing of battery cells after curing is a key quality inspection step to ensure interfacial performance. The interfacial impedance, measured in the 100Hz-10kHz frequency range, primarily reflects the charge transfer resistance between the electrode and the gel electrolyte, as well as the SEI film impedance. This frequency range directly correlates to the ion migration efficiency during actual battery charge and discharge. The acceptance standard of ≤5Ω·cm² is based on a balance between low internal resistance (to ensure high-rate charge and discharge) and interfacial stability (to avoid cyclic degradation). Impedance exceeding this value indicates poor interfacial contact or excessively thick SEI film, resulting in reduced battery charge and discharge efficiency and shortened cycle life. Unqualified cells are vacuum-dried at 80°C for 2 hours. This heat and negative pressure remove trace amounts of solvent or moisture remaining in the gel electrolyte (possibly from environmental infiltration or incomplete curing). These impurities form an insulating layer at the interface, increasing impedance. During the drying process, solvent molecules evaporate under vacuum (lowering their boiling point), while gentle heating at 80°C accelerates molecular motion, promoting reorganization of the interfacial layer and reducing impedance to within the acceptable range.

[0045] The above description is only a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made by using the contents of the present invention description and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A process for preparing a gel polymer lithium ion battery electrolyte, characterized in that: The following steps are involved: Mixing a methacrylate monomer and a silane monomer in a mass ratio of 3:1 to 5:1, adding an initiator accounting for 1 to 3% of the total mass of the monomers, and prepolymerizing at 60 to 80° C. for 1 to 3 hours to obtain a prepolymer solution; The prepolymer solution is mixed with a non-aqueous solvent containing a lithium salt in a mass ratio of 1:4 to 1:6, wherein the lithium salt concentration is 0.8 to 1.2 mol / L, to form a precursor mixture, and the non-aqueous solvent comprises ethylene carbonate, dimethyl carbonate, and fluoroethylene carbonate in a volume ratio of 3:2:1 to 2:1:1; Inject the precursor mixture into the gap between the battery cell electrode and the diaphragm, controlling the injection amount to 1.5-2.5 g / Ah; The injected battery cell is placed at a vacuum degree of ≤-0.08MPa for 2 to 4 hours, and then cured at 60 to 70°C for 3 to 6 hours to form a gel polymer electrolyte.

2. The process for preparing the gel polymer lithium ion battery electrolyte according to claim 1, wherein: The initiator is benzoyl peroxide or azobisisobutyronitrile, the methacrylate monomer is methyl methacrylate or ethyl methacrylate, and the silane monomer is γ-methacryloyloxypropyltrimethoxysilane.

3. The process for preparing the gel polymer lithium ion battery electrolyte according to claim 1, wherein: The lithium salt is lithium hexafluorophosphate, the stirring speed during the prepolymerization process is 200-500 rpm, and dry nitrogen is introduced simultaneously during the vacuum standing process, with a nitrogen flow rate of 50-100 mL / min.

4. The process for preparing the gel polymer lithium ion battery electrolyte according to claim 1, wherein: The battery cell electrode is subjected to surface hydrophilic treatment before being injected into the precursor mixture. The treatment method is: immersing the electrode in a polyvinyl alcohol aqueous solution with a mass concentration of 0.5-1% and ultrasonically oscillating for 5-10 minutes, and then taking it out and drying it at 80-100°C for 1-2 hours.

5. The process for preparing the gel polymer lithium ion battery electrolyte according to claim 1, wherein: The precursor mixture is added with vinylene carbonate as a functional additive accounting for 1-5% of the volume of the non-aqueous solvent. After the curing process is completed, the mixture is naturally cooled to room temperature, and the cooling rate is controlled at 5-10°C / min.

6. The process for preparing the gel polymer lithium ion battery electrolyte according to claim 1, characterized in that: The non-aqueous solvent is dehydrated by a molecular sieve with a pore size of 0.1 to 0.3 μm before use. The treatment time is 12 to 24 hours to make the water content of the solvent ≤10 ppm.

7. The process for preparing the gel polymer lithium ion battery electrolyte according to claim 1, characterized in that: The prepolymerization process adopts microwave-assisted heating, the microwave power is controlled at 300-500W, and infrared temperature measurement is used to monitor the temperature fluctuation of the reaction system in real time to ≤±2°C.

8. The process for preparing the gel polymer lithium ion battery electrolyte according to claim 1, wherein: After the precursor mixture is injected, a constant pressure of 0.2-0.5 MPa is applied to the outside of the battery cell and maintained for 1-2 hours, and then a vacuum standing process is performed.

9. The process for preparing the gel polymer lithium ion battery electrolyte according to claim 1, wherein: The surface of the cured battery cell is coated with a polyvinylidene fluoride coating with a thickness of 5 to 10 μm. The coating method is slit coating, the coating solvent is N-methylpyrrolidone, and the battery cell is dried at 60° C. for 30 minutes.

10. The process for preparing the gel polymer lithium ion battery electrolyte according to claim 1, characterized in that: The cured battery cell needs to undergo an AC impedance test. When the interface impedance value in the frequency band of 100 Hz to 10 kHz is ≤5 Ω·cm², it is judged to be qualified. Otherwise, it is re-processed in a vacuum drying process at 80°C for 2 hours.

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