Preparation method and application of semi-solid gel polymer electrolyte

Abstract

The application discloses a kind of semi-solid gel polymer electrolyte and preparation method and application thereof, to solve the technical problems of high-temperature easy decomposition of small molecule sulfate additive in existing semi-solid electrolyte, monomer and additive exist interaction.The application uses linear carbonate with unsaturated C=C double bond and cyclic sulfate as comonomer, and forms three-dimensional gel network in battery by in-situ thermal polymerization.The electrolyte has high ionic conductivity, excellent interface stability and high safety, which can significantly improve the cycle life and anti-piercing, high-temperature performance of battery, and is suitable for lithium-ion batteries in electric vehicles and energy storage field.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a method for preparing a semi-solid gel polymer electrolyte and its application. Background Technology

[0002] With the rapid development of new energy vehicles and energy storage power stations, higher requirements have been placed on the energy density and safety of lithium-ion batteries. Traditional liquid lithium-ion batteries use flammable organic carbonate electrolytes, posing serious safety hazards such as fire and explosion under thermal runaway, overcharging, or mechanical abuse. Semi-solid gel polymer electrolytes combine the advantages of high ionic conductivity of liquid electrolytes and high safety of solid electrolytes. By introducing polymerizable monomers into the liquid electrolyte, they form a three-dimensional network structure through in-situ polymerization inside the battery, "locking" the liquid components within the polymer skeleton, thereby significantly reducing the risk of electrolyte leakage and combustion.

[0003] However, existing in-situ polymerization techniques for preparing semi-solid gel polymer electrolytes have always faced the technical contradiction of balancing "polymer framework construction" and "interface stability," and the current mainstream technical routes cannot fundamentally solve this problem.

[0004] For example, patent CN116995294A discloses an in-situ polymerization system using pure carbonate monomers such as vinylene carbonate (VC) and ethylene ethylene carbonate. The limitation of this technical route is that the monomers themselves only have the single function of constructing the polymer backbone; to improve interfacial stability, small-molecule sulfate film-forming additives such as vinyl sulfate (DTD) must be added. However, the polycarbonate network formed after polymerization has a strong adsorption effect on sulfate additives, easily leading to non-uniform consumption of the additives. More importantly, the small-molecule sulfate additives themselves have poor thermal stability and are prone to decomposition at the high temperature environment of 45 to 75°C required for in-situ thermosetting, producing corrosive byproducts such as hydrofluoric acid (HF), which further deteriorates the interfacial properties.

[0005] To address the thermal decomposition problem of sulfate ester additives, patent CN119029290B discloses a modification technique that introduces nitrogen-containing Lewis basic groups into sulfate ester molecules to adsorb HF. However, this technique essentially still follows the traditional approach of using sulfate esters as additives. Sulfate esters always exist in the electrolyte as free small molecules, which can only alleviate, but not fundamentally eliminate, the risk of high-temperature decomposition and migration. At the same time, the polymerization function (acrylate monomers) and the film-forming function (sulfate ester additives) remain separate, and the two not only fail to produce a synergistic effect but may also interfere with each other.

[0006] It is evident that existing technologies have consistently failed to break free from the functional separation design framework of "monomers responsible for polymerization and additives responsible for film formation." This framework itself determines that the two major technical challenges of "monomer-additive interaction" and "high-temperature decomposition of small molecule sulfates" cannot be fundamentally solved. Therefore, developing a novel semi-solid gel polymer electrolyte system that can break through the above framework at the molecular design level and fundamentally eliminate the conflict between monomers and additives and the risk of sulfate decomposition is of significant practical importance. Summary of the Invention

[0007] This invention aims to overcome the shortcomings of existing technologies and provide a semi-solid gel polymer electrolyte with high safety and high interfacial stability, and its lithium-ion battery. This system achieves in-situ polymerization to form a uniform, stable, and smoothly ion-conducting semi-solid gel network through carefully designed molecular structures of polymer monomers, matching specific co-salt systems and film-forming additives.

[0008] The core of this invention lies in: A semi-solid gel polymer electrolyte, wherein the semi-solid gel polymer electrolyte is formed by in-situ thermally initiated free radical polymerization of a precursor solution inside a battery; The precursor solution contains an ester-based organic mixed solvent, a lithium salt, a film-forming additive, a polymerization monomer, and a thermal initiator; The polymerizable monomer is a combination of a linear carbonate (Formula I) having an unsaturated C=C double bond and a cyclic sulfate (Formula II) compound having a C=C double bond, with the following general structural formulas: Formula I: ; Formula II: ; In Formula I, R1 and R2 are each independently selected from C2-C6 alkenyl groups and C2-C6 haloalkenyl groups; and at least one of R1 and R2 contains an unsaturated C=C double bond. In Formula II, R2 is selected from C2-C6 alkenyl groups and C2-C6 haloalkenyl groups. The mass ratio of linear carbonate to cyclic sulfate is (1-3):1; and the total content of polymer monomers accounts for 0.5%-7.5% of the mass fraction of the precursor solution. The linear carbonate with unsaturated C=C double bonds is selected from one or more of diallyl carbonate and bis(2-methylallyl) carbonate; the cyclic sulfate with C=C double bonds includes compounds 1-3, with the following structures:

[0009] In the precursor solution, the volume ratio of each component of the ester organic mixed solvent is ethylene carbonate (EC): propylene carbonate (PC): dimethyl carbonate (DMC): ethyl methyl carbonate (EMC): ethyl acetate (EA): ethyl propionate (EP) = (5-15): (0.1-8): (20-30): (25-35): (15-25): (5-15).

[0010] The precursor solution contains a combination of lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI). The total concentration of the lithium salt is 0.9-1.5 mol / L, and LiFSI accounts for 10%-50% of the total lithium salt moles.

[0011] The film-forming additives in the precursor solution include one or more of VC, fluoroethylene carbonate (FEC), and DTD, and the total mass fraction of the additives in the precursor solution is 1% to 8%.

[0012] In the precursor solution, the thermal initiator is azobisisobutyronitrile (AIBN), and its addition amount is 0.5%-2% of the mass of the polymerizable monomer.

[0013] A method for preparing a semi-solid gel polymer electrolyte lithium-ion battery includes the following steps: S1: Under low dew point (dew point ≤ -20℃) argon protection at room temperature, inject the precursor solution used to prepare the above semi-solid gel polymer electrolyte into the battery casing that has been assembled with positive and negative electrodes and separator, and seal it. S2: In-situ thermally initiated free radical polymerization of the sealed battery: S21: Maintain a constant temperature of 25±5℃ for 8-16 hours to allow the precursor to fully penetrate into the pores of the electrode and diaphragm, thus completing the full impregnation; S22: Maintain a constant temperature of 45-75℃ for 8-24 hours until the monomers are completely polymerized to form a three-dimensional gel polymer network, thereby obtaining the semi-solid gel polymer electrolyte lithium-ion battery.

[0014] A semi-solid gel polymer electrolyte lithium-ion battery includes a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the electrolyte is the aforementioned semi-solid gel polymer electrolyte; the positive electrode active material is selected from one or more of lithium nickel cobalt manganese oxide, lithium iron phosphate, and lithium cobalt oxide; the negative electrode active material is selected from one or more of graphite, silicon-carbon composite material, and silicon-based material; and the separator is a polyolefin porous membrane, a ceramic-coated separator, or an aramid separator.

[0015] The beneficial effects of this invention are as follows: 1. This invention eliminates the decomposition risk of small-molecule sulfate ester additives and overcomes the biases of existing technologies. It breaks through the inherent limitation of existing technology that "sulfate esters can only be used as additives," by introducing polymerizable C=C double bonds into cyclic sulfate ester molecules, transforming them from "free small-molecule additives" into "covalently bonded polymer backbone components." Unlike existing technologies that introduce nitrogen-containing groups to adsorb HF, this invention solves the problems of small-molecule migration and high-temperature decomposition of sulfate esters at the molecular design level. 2. This invention achieves functional integration of polymerization and film formation, eliminating the inherent conflict between monomers and additives. The binary monomer system of this invention realizes a "one molecule, dual function" design. The linear carbonate monomer provides good compatibility with the electrolyte, constructing a smooth ion transport channel; the cyclic sulfate monomer simultaneously undertakes the dual functions of constructing the polymer backbone and participating in interfacial film formation. Compared with the existing technology's separate design of "pure carbonate monomer and exogenous sulfate additive," this invention eliminates the problem of polycarbonate network adsorption and consumption of sulfate additives, avoiding the technical defect of increased interfacial impedance. 3. Formation of an organic-inorganic nanocomposite interface film, improving interface stability. The unsaturated cyclic sulfate monomers of this invention preferentially undergo reductive ring-opening polymerization on the electrode surface, first forming a flexible organic polymer network framework, simultaneously decomposing to generate inorganic particles. Together, these form a dense organic-inorganic nanocomposite solid electrolyte interphase (SEI) film and a positive electrode electrolyte interphase (CEI) film. Compared to inorganic salt films formed solely through physical stacking of traditional small-molecule sulfate additives, this composite film exhibits stronger adhesion, lower impedance, and better stability, effectively buffering the volume expansion of the negative electrode material and improving battery cycle life and high-temperature storage performance. 4. It combines high ionic conductivity with high safety, exhibiting excellent overall performance. The polymer network of this invention is rich in polar carbonate and sulfate groups, making it highly compatible with the electrolyte system. After polymerization, it maintains high ionic conductivity, approaching the level of liquid electrolytes. The resulting semi-solid gel polymer electrolyte is essentially non-flowing, improving the battery's heat resistance, puncture resistance, impact resistance, and leak-proof performance. 5. Strong process adaptability and easy industrialization. The in-situ polymerization process of this invention can be seamlessly integrated with existing lithium-ion battery production processes without major equipment modifications. It requires low monomer addition and has limited cost increase, making it promising for large-scale industrial application. Attached Figure Description

[0016] Figure 1Electrochemical impedance spectroscopy (EIS) spectra of lithium-ion batteries prepared in Examples 1 and 2 of this invention at room temperature; wherein, the solid line represents the impedance spectrum curve of the semi-solid gel polymer electrolyte lithium-ion battery described in Example 1, and the dashed line represents the impedance spectrum curve of the liquid electrolyte lithium-ion battery described in Example 2; two curves of the same line type represent parallel experimental samples of the corresponding examples. Detailed Implementation

[0017] The present invention will be further described below with reference to specific embodiments and accompanying drawings, but is not limited thereto. Furthermore, the experimental methods described in the following embodiments are conventional methods unless otherwise specified; the reagents and materials described are commercially available unless otherwise specified.

[0018] It should be noted that this embodiment uses NCM811 positive electrode, silicon-carbon negative electrode, and ceramic-coated polyolefin separator as an example. The semi-solid gel polymer electrolyte of this invention is designed based on a carbonate-based system, which has natural compatibility with all mainstream electrode and separator materials in the lithium-ion battery field and does not require special modification. 1. Cathode material: Lithium iron phosphate has low operating voltage and good chemical stability, and its high-temperature cycling performance is even better when combined with the electrolyte of this invention; Lithium cobalt oxide has high compaction density and is suitable for high energy density applications; 2. Anode material: Graphite is the most widely used commercial anode, and the film formation system of this invention can form a stable SEI film on its surface; silicon-based materials have the same lithium intercalation mechanism as silicon-carbon anodes, and the flexible composite SEI film of this invention can effectively buffer its volume expansion; 3. Separator material: Ordinary polyolefin porous membranes are a low-cost and conventional choice, while aramid membranes have higher thermal stability and can improve battery safety.

[0019] The film-forming additives that can be added to the precursor solution of this invention are one or more of VC, FEC, and DTD. These additives preferentially decompose on the electrode surface to form stable SEI and CEI films. Among them, VC preferentially undergoes free radical polymerization to form a dense polycarbonate SEI film, effectively inhibiting the continuous decomposition of the electrolyte; FEC can improve low-temperature performance and inhibit high-temperature gas generation, and the resulting SEI film is rich in LiF and has good mechanical stability; DTD can reduce the internal resistance of the battery and improve high-temperature storage performance, and its decomposition products can optimize the ion conductivity of the SEI film.

[0020] VC, FEC, and DTD can be used individually or in any combination, producing a synergistic effect when used together: VC provides a dense base membrane, FEC enhances the membrane's thermal stability and low-temperature adaptability, and DTD further reduces interfacial impedance and improves high-temperature performance. Improving interfacial stability is achieved as long as the total mass fraction is within the range of 1% to 8%. However, a total addition of less than 1% cannot form a continuous and complete SEI / CEI membrane, while an addition of more than 8% will produce excessive side reactions leading to increased interfacial impedance. In Example 1, the total addition of 4.5% (0.5% VC, 3% FEC, 1% DTD) represents the optimal value for overall performance.

[0021] Example 1: Preparation of a semi-solid gel polymer electrolyte lithium-ion battery 1. Preparation of precursor solution: In a glove box filled with argon (H2O, O2 < 0.1 ppm, dew point ≤ -20℃) at room temperature (25℃), EC, PC, DMC, EMC, EA, and EP are added sequentially in a volume ratio of 10:4:25:30:20:10 and stirred for 10 minutes to mix evenly; then LiPF6 and LiFSI are added and stirred for 30 minutes until completely dissolved, so that the total lithium salt concentration is 1.2 mol / L, of which LiFSI accounts for 30% of the total lithium salt moles. Next, film-forming additives VC, FEC, and DTD were added, with mass fractions of 0.5%, 3%, and 1% of the total mass of the precursor solution, respectively. The mixture was stirred for 15 minutes to mix thoroughly. Finally, the polymeric monomers bis(2-methylallyl) carbonate and compound 2 (mass ratio 2:1) were added, with the total amount added being 4% of the total mass of the precursor solution, as well as the thermal initiator AIBN, with the amount added being 1% of the total mass of the polymeric monomers. The mixture was stirred for another 20 minutes until completely dissolved, resulting in a clear and transparent precursor solution.

[0022] 2. Battery Assembly and Electrolyte Filling: A positive electrode sheet was prepared using NCM811 as the positive active material, conductive carbon black as the conductive agent, and polyvinylidene fluoride as the binder, at a mass ratio of 96:2:2. A negative electrode sheet was prepared using silicon-carbon composite material (5% CVD) as the negative active material, conductive carbon black as the conductive agent, and sodium carboxymethyl cellulose and styrene-butadiene rubber as binders, at a mass ratio of 95:2:1.5:1.5. A 9μm thick ceramic-coated polyolefin separator was used. The positive electrode sheet, separator, and negative electrode sheet were stacked sequentially to assemble a pouch battery casing. In the same glove box, the precursor solution prepared in step 1 was injected into the pouch battery casing at a volume of 2.5 g / Ah. After standing for 30 minutes, vacuum sealing was completed.

[0023] 3. In-situ thermally initiated free radical polymerization: The sealed pouch cell was placed in a constant temperature chamber and kept at 25°C for 12 hours to allow the precursor to fully penetrate into the pores of the electrode and separator, achieving complete wetting. Then, the temperature of the chamber was raised to 65°C and kept there for 24 hours to allow AIBN to decompose and generate free radicals, initiating the gradual cross-linking polymerization of monomers until a uniform and stable three-dimensional gel polymer network was formed. After polymerization, the battery was cooled to room temperature to obtain the semi-solid gel polymer electrolyte lithium-ion battery.

[0024] Example 2: Comparative Experiment (Liquid Electrolyte Battery) Except for the absence of polymer monomers and AIBN initiators, the electrolyte composition and battery assembly process were exactly the same as in Example 1, and a conventional liquid electrolyte lithium-ion battery was prepared as a comparative example.

[0025] Example 3: Monomer Substitution Experiment In Example 1, the polymer monomers were replaced with an equal mass of ethylene glycol dimethacrylate, while the other steps remained unchanged, to prepare battery C, which was used to compare the effect of changes in polymer network structure on performance.

[0026] Tests and Results The batteries prepared in Examples 1-3 were tested: 1. Safety Test (Needle Penetration): Example 1 (semi-solid gel polymer electrolyte lithium-ion battery) only emitted smoke after being punctured, without open flame, and the voltage dropped slowly. Example 2 (liquid battery) instantly and violently burned.

[0027] 2. Ionic conductivity (25°C): Example 1 electrolyte (before polymerization) was 12.5 mS / cm; the gel electrolyte after polymerization was 8.7 mS / cm. Example 2 liquid electrolyte was 12.6 mS / cm. Example 1 exhibited a higher retention rate of ionic conductivity.

[0028] 3. Cyclic performance (1C, 2.75-4.2V): After 500 cycles, the capacity retention rate of the battery in Example 1 was 91.5%; that of the battery in Example 2 was 85.2%; and that of the battery in Example 3 was 89.1%.

[0029] 4. High-temperature storage performance (60°C, 7 days of full charge storage): The thickness expansion rate of the battery in Example 1 was 8.5%; that of the battery in Example 2 was 22.3%.

[0030] 5. Electrochemical Impedance Spectroscopy (Room Temperature): The batteries prepared in Examples 1 and 2 were subjected to room temperature electrochemical impedance spectroscopy using an electrochemical workstation. Two parallel samples were tested for each example to verify data repeatability. The test frequency range was 10⁻²Hz to 10⁻²Hz. 5 Hz, AC amplitude is 5mV, Figure 1The two parallel sample curves from the same embodiment show a high degree of overlap, indicating that the experimental process is stable and the data is reliable. Furthermore, the ohmic impedance and interfacial impedance of the semi-solid gel polymer electrolyte battery in Example 1 are essentially equivalent to those of the liquid electrolyte battery in Example 2. This indicates that the polymerization process of the present invention does not damage the electrode interface, and the organic-inorganic composite SEI / CEI film formed with the participation of unsaturated cyclic sulfate monomers exhibits low impedance characteristics, further verifying the interfacial stability advantage of the electrolyte system of the present invention.

[0031] Results Analysis: Example 1, employing the specific monomers and polymerization process of this invention, outperforms traditional liquid electrolyte lithium-ion batteries in terms of safety, cycle life, and high-temperature stability. Compared to Example 3, which uses other monomers, it also exhibits superior overall performance, demonstrating the unique advantages of the combined monomer of bis(2-methylallyl) carbonate and unsaturated cyclic sulfate in balancing ionic conductivity and interfacial stability. The combined monomer of bis(2-methylallyl) carbonate and unsaturated cyclic sulfate transcends the single function of ethylene glycol dimethacrylate as merely a "physical framework." By introducing electrolyte-compatible carbonate segments and highly polar, film-forming sulfate functional groups, it achieves the synergistic integration of three functions: "structural support," "ion transport," and "interfacial stability."

Claims

1. A semi-solid gel polymer electrolyte, characterized in that, The semi-solid gel polymer electrolyte is formed by in-situ thermally initiated free radical polymerization of the precursor solution inside the battery; The precursor solution contains an ester-based organic mixed solvent, a lithium salt, a film-forming additive, a polymerization monomer, and a thermal initiator; The polymerizable monomer is a combination of a linear carbonate (Formula I) having an unsaturated C=C double bond and a cyclic sulfate (Formula II) compound having a C=C double bond, with the following general structural formulas: Formula I: ; Formula II: ; In Formula I, R1 and R2 are each independently selected from C2-C6 alkenyl groups and C2-C6 haloalkenyl groups; and at least one of R1 and R2 contains an unsaturated C=C double bond. In Formula II, R2 is selected from C2-C6 alkenyl groups and C2-C6 haloalkenyl groups. The mass ratio of linear carbonate to cyclic sulfate is (1-3):1; and the total content of polymer monomers accounts for 0.5%-7.5% of the mass fraction of the precursor solution.

2. The semi-solid gel polymer electrolyte according to claim 1, characterized in that, The linear carbonate with unsaturated C=C double bonds is selected from one or more of diallyl carbonate and bis(2-methylallyl) carbonate; the cyclic sulfate with C=C double bonds includes compounds 1-3, with the following structures: 。 3. The semi-solid gel polymer electrolyte according to claim 1, characterized in that, In the precursor solution, the volume ratio of each component of the ester organic mixed solvent is ethylene carbonate (EC): propylene carbonate (PC): dimethyl carbonate (DMC): ethyl methyl carbonate (EMC): ethyl acetate (EA): ethyl propionate (EP) = (5-15): (0.1-8): (20-30): (25-35): (15-25): (5-15).

4. The semi-solid gel polymer electrolyte according to claim 1, characterized in that, In the precursor solution, the lithium salt is a combination of lithium hexafluorophosphate (LiPF6) and lithium bisfluorosulfonylimide (LiFSI). The total concentration of the lithium salt is 0.9-1.5 mol / L, and LiFSI accounts for 10%-50% of the total molar amount of the lithium salt.

5. The semi-solid gel polymer electrolyte according to claim 1, characterized in that, The film-forming additives in the precursor solution include one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), and ethylene sulfate (DTD), with a total mass fraction of 1% to 8% in the precursor solution.

6. The semi-solid gel polymer electrolyte according to claim 1, characterized in that, In the precursor solution, the thermal initiator is azobisisobutyronitrile (AIBN), and its addition amount is 0.5%-2% of the mass of the polymerizable monomer.

7. A method for preparing a semi-solid gel polymer electrolyte lithium-ion battery, characterized in that, Includes the following steps: S1: Under a low dew point (dew point ≤ -20 ℃) ​​argon protection environment at room temperature, the precursor solution used to prepare the semi-solid gel polymer electrolyte of claim 1 is injected into the battery casing that has been assembled with positive and negative electrodes and separator and then sealed. S2: In-situ thermally initiated free radical polymerization of the sealed battery: S21: Maintain a constant temperature of 25±5 ℃ for 8-16 hours to allow the precursor to fully penetrate into the pores of the electrode and diaphragm, thus completing the full impregnation; S22: Maintain a constant temperature of 45-75 ℃ for 8-24 hours until the monomers are completely polymerized to form a three-dimensional gel polymer network, thereby obtaining the semi-solid gel polymer electrolyte lithium-ion battery.

8. A semi-solid gel polymer electrolyte lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The electrolyte is the semi-solid gel polymer electrolyte of claim 1; the positive electrode active material is selected from one or more of lithium nickel cobalt manganese oxide, lithium iron phosphate, and lithium cobalt oxide; the negative electrode active material is selected from one or more of graphite, silicon-carbon composite material, and silicon-based material; the separator is a polyolefin porous membrane, a ceramic-coated separator, or an aramid separator.

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