Difunctional polyion liquid binder as well as preparation method and application thereof

Abstract

The invention discloses a difunctional polyion liquid binder as well as a preparation method and application thereof, and belongs to the technical field of solid-state battery materials. The simple formula of the structure of the binder is poly ([VEIm] [FSI]-co-[UPhos]) modified by trifluoroacetoxy, wherein [VEIm] [FSI] represents an ionic liquid monomer unit consisting of 1-vinyl-3-ethylimidazolium cations and bis (fluorosulfonyl) imide anions; [UPhos] represents an ethylenically unsaturated phosphate ester monomer unit. The method is based on the ionic liquid, and the ionic liquid is polymerized by utilizing an initiator at a relatively high temperature. Diffusion and transmission of Li ions in the electrode material are effectively improved, and the internal resistance of the battery is reduced, so that the specific capacity and the cycling stability of the battery are improved.

Description

Technical Field

[0001] This invention belongs to the field of solid-state battery materials technology, specifically relating to a bifunctional polyionic liquid binder that can simultaneously stabilize the positive and negative electrode interfaces of sulfide all-solid-state batteries, its preparation method, and its application in sulfide all-solid-state batteries. Background Technology

[0002] In recent years, with the development of new energy vehicles, electronic communication equipment, and large-scale energy storage grids, portable electronic products and electric vehicles urgently require lithium-ion batteries with high capacity, long lifespan, high stability, and fast charge / discharge performance. To further expand the market, such as for stationary energy storage of renewable energy and large vehicles beyond automobiles, batteries must have higher energy density and safety performance than currently available. Given that current lithium-ion batteries using liquid electrolytes cannot meet the demands for significantly increased energy density and safety, the development of next-generation batteries is of great significance. Sulfide-based all-solid-state lithium batteries, based on non-flammable sulfide solid electrolytes, are expected to improve energy density and alleviate safety issues through a unique bipolar stacking method. As a key supporting material for electrode microstructures, the performance defects of binders are increasingly becoming a bottleneck restricting the industrialization of sulfide systems. Traditional polyvinylidene fluoride (PVDF) and styrene-butadiene rubber (SBR) binders expose three fatal problems in solid-state batteries: firstly, ionic insulation (conductivity <10). -8 The high nickel cathode NCM811 has a high impedance (S / cm), which severely hinders lithium-ion transport at the interface with the sulfide electrolyte (such as Li6PS5Cl), resulting in an interface impedance as high as 150 Ω·cm. 2 The above factors contribute to several issues. First, severe polarization occurs, reducing the initial efficiency to below 83% (Zhang et al., J. Power Sources 2023). Second, insufficient mechanical compatibility leads to cracks in the silicon anode due to volume expansion exceeding 300% during cycling, resulting in physical contact failure between the active material and the electrolyte, causing a sharp drop in capacity retention to 60% after 100 cycles (Chen et al., Adv. Energy Mater. 2022). Third, a lack of chemical stability results in reactive oxygen species released by NCM811 at a high voltage of 4.3V reacting with sulfides to form a high-resistivity Li2S / Li2O layer, which PVDF cannot suppress, accelerating battery performance degradation. Even more serious is the fact that physical blending modification strategies (such as simple mixing of PVDF and ionic liquids) lead to phase separation of components, causing modifier leaching after long-term cycling, resulting in a capacity retention of less than 65% after 200 cycles (Liu et al., ACS Appl. Mater. Interfaces 2024).

[0003] To overcome these limitations, researchers have attempted various solutions, but all have significant drawbacks. For example, while a two-component binder system (using conductive polymer PEDOT for the positive electrode and elastomer PEO for the negative electrode) can optimize the positive and negative electrode interfaces separately, poor interphase interface compatibility results in a critical current density of only 0.75 mA / cm² for the full cell. 2 (Wang et al., NanoEnergy 2023); Inorganic coating technologies (such as LiNbO3 coating of NCM811) can delay positive electrode side reactions, but rigid coatings exacerbate interfacial stress concentration, further worsening the silicon anode expansion rate to 55% (Kato et al., J. Electrochem. Soc2022); Ionic liquid blending modification causes electrode component degradation due to small molecule migration, with interfacial impedance increasing by 300% after 300 cycles. The essential contradiction of these solutions lies in the fact that they attempt to solve chemical-mechanical coupling failure through "physical superposition" but ignore the functional synergy of materials at the molecular scale, leading to process complexity and limited effectiveness. Existing solutions such as CN114512628A, which uses PVDF-HFP binders, only improve mechanical strength, while US2022003776A1, which uses elastomer binders, cannot suppress chemical side reactions. Therefore, developing a single-component multifunctional binder that combines ion conduction, interfacial stability, and mechanical buffering has become an inevitable path to solve the interfacial dilemma of sulfide all-solid-state batteries.

[0004] The root cause of interface failure in sulfide all-solid-state batteries (taking the typical NCM811 / LPSCl / Si system as an example) can be attributed to the combined effect of positive electrode chemical degradation and negative electrode mechanical instability. On the positive electrode side, NCM811 in the delithiation state (Ni...) 4+ The high oxidizing power of 2Ni drives the oxygen release reaction. 3+ → 2Ni 2+ The release of oxygen atoms from the lithium-ion intercalation of silicon particles (Li₆PS₅Cl + O₂ → Li₂S + Li₃PO₄ + Cl₂, ΔG = -287 kJ / mol) attacks the PS bonds in LPSCl, triggering irreversible decomposition (Li₆PS₅Cl + O₂ → Li₂S + Li₃PO₄ + Cl₂, ΔG = -287 kJ / mol), generating a nanoscale insulating layer that blocks ion transport (Chem. Mater. 2021, 33, 7745). On the negative electrode side, the volume expansion of silicon particles during lithium intercalation generates interfacial stresses >1 GPa, which not only damages the integrity of the electrode structure but also promotes the formation of new Li₂. x Si surface continuous reduction sulfide electrolyte (4Li) x Si + Li6PS5Cl → Li 15 Si4 + Li3P + Li2S) consumes active lithium and thickens the SEI layer.

[0005] Polyionic liquids (PILs), as a novel functional polymer, are valued for their unique molecular designability and intrinsic ionic conductivity (10⁻⁶). -5 -10 -3 S / cm) makes it an ideal carrier for solving the above problems. Its advantages are: First, the cationic / anionic groups can be customized, such as introducing imidazolium cations to provide lithium-ion migration channels, or bonding functional anions to achieve interface passivation; Second, flexible segments (such as polyethylene glycol) can be grafted onto the polymer backbone, giving the material the ability to dynamically adapt to volume changes; Third, it has excellent thermal stability (decomposition temperature > 300℃), far higher than PVDF (about 200℃), and is suitable for high-temperature battery conditions (Prog. Polym. Sci. 2020, 105, 101250). However, existing PIL binder research mostly focuses on single-function optimization (such as improving ionic conductivity or mechanical strength), and has not yet achieved a dual-function synergistic design of positive electrode anchoring and negative electrode film formation, and lacks molecular engineering strategies for the interface characteristics of sulfide batteries. Summary of the Invention

[0006] To overcome the shortcomings of existing technologies, this invention provides a bifunctional polyionic liquid binder, its preparation method, and its applications. This binder can reduce electrode damage, mitigate interfacial side reactions, stabilize the positive and negative electrode interfaces, and improve the cycle stability of sulfide all-solid-state batteries.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A bifunctional polyionic liquid binder, the binder having the structural formula of trifluoroacetoxy-modified poly([VEIm][FSI]-co-[UPhos]), wherein: [VEIm][FSI] represents a 1-vinyl-3-ethylimidazolium cation ([VEIm]...). + ) and difluorosulfonamide anion ([FSI]) - The binder consists of ionic liquid monomer units composed of 1-vinyl-3-ethylimidazolium cations and olefinic unsaturated phosphate monomer units through free radical copolymerization; the positive electrode functional group is a trifluoroacetoxy group with the structure -OC(O)-CF3; the negative electrode functional group is a phosphate group introduced as a comonomer and suspended in the main chain with the structure -PO(OR)2; R is a C1-C4 alkyl group.

[0009] Furthermore, the molar ratio of [VEIm][FSI] to [UPhos] is (6-8):(2-4), preferably, the molar ratio of [VEIm][FSI] to [UPhos] is 8:1.

[0010] Furthermore, the negative electrode functional group accounts for 2%-30% of the total mass of the bifunctional polyionic liquid copolymer binder, preferably, the negative electrode functional group accounts for 5.6% of the total mass of the bifunctional polyionic liquid copolymer binder.

[0011] Furthermore, the positive electrode functional group is covalently modified at the C2 position of the imidazole ring; the negative electrode functional group is covalently linked to the alkenyl group and the 1-vinyl-3-ethylimidazolium cation on the main chain to form a random carbon chain.

[0012] The olefinic unsaturated phosphate monomers include phosphate monomers containing vinyl, acryloyl, methacryloyl, allyl, styrene, or maleimide groups.

[0013] A method for preparing the aforementioned bifunctional polyionic liquid binder includes the following steps:

[0014] S1. Mix 1-vinyl-3-ethylimidazolium bromide with an equimolar LiFSI acetone solution and perform Br... - With FSI - [VEIm][FSI] monomers were obtained by anion exchange, and high-purity [VEIm][FSI] monomers were obtained by filtration and rotary evaporation.

[0015] S2. The olefinic unsaturated phosphate monomer is dissolved in an organic solvent and copolymerized with [VEIm][FSI] monomer under an initiator and an inert gas atmosphere. After the reaction is completed, the precipitate is washed and vacuum dried to obtain a pure precursor polymer.

[0016] S3. Dissolve the precursor polymer obtained in S2 in anhydrous DMF, add a strong base and stir under an ice bath and inert gas atmosphere, while simultaneously adding trifluoroacetic anhydride dropwise to raise the temperature to room temperature and react. After the reaction is complete, precipitate, wash, and vacuum dry to obtain a bifunctional polyionic liquid binder.

[0017] S4. In the preparation of wet-coated Si anodes, the bifunctional polyionic liquid binder obtained in S3 is dissolved in a suitable solvent to prepare a homogeneous solution. Nano-Al2O3 particles are added, and the solution is dispersed by high-intensity ultrasonication to form a homogeneous composite binder slurry for electrode preparation. The Al2O3 particles are used in the Si anode to assist in constructing the anode interface and to work with the binder to build a conductive network, addressing the problem of poor conductivity in Si.

[0018] Further, in step S2, the organic solvent includes at least one of anhydrous acetonitrile, anhydrous γ-butyrolactone, anhydrous N,N-dimethylformamide, anhydrous tetrahydrofuran, anhydrous chloroform, 1,2-ethylenediamine, and 1,2-ethylenedithiol; the initiator is one of azobisisobutyronitrile, azobisisoheptanenitrile, dimethyl azobisisobutyrate, azoisobutylcyanoformamide, ammonium persulfate, potassium persulfate, and sodium persulfate; and the inert gas atmosphere is at least one of argon, nitrogen, and helium, preferably nitrogen.

[0019] Furthermore, in step S2, the content of the initiator accounts for 0.1-0.5 wt% of the total mass of all monomers; preferably, the amount of initiator is 0.2-0.3%.

[0020] Furthermore, in step S2, the copolymerization reaction temperature is 65-75℃, and the reaction time is 12-24h. Preferably, the reaction temperature is 70℃, and the reaction time is 12h.

[0021] Furthermore, in step S3, the amount of strong base is not limited, ensuring that the pH of the reaction system is between 10 and 12; the molar ratio of the amount of trifluoroacetic anhydride to the imidazolium group in step S1 is 1.05-1.2:1, ensuring that the trifluoroacetic anhydride is not in excess.

[0022] Furthermore, in step S4, the content of nano-Al2O3 particle filler accounts for 1-5 wt% of the binder mass, with a particle size of 30-100 nm and a specific surface area >100 m². 2 / g.

[0023] An application of the aforementioned bifunctional polyionic liquid binder, wherein the bifunctional ionic liquid binder is used in sulfide solid-state batteries.

[0024] Furthermore, the bifunctional ionic liquid binder is used in the positive and / or negative electrodes of sulfide solid-state batteries.

[0025] The cathode material in the cathode includes the following components: ternary material LiNi x Co y Mn z O2 (NCM, x+y+z=1), conductive carbon black, the polyionic liquid binder, and N-methylpyrrolidone;

[0026] The negative electrode material in the negative electrode includes silicon active material, conductive carbon black, and N-methylpyrrolidone.

[0027] Positive electrode slurry: NCM: binder: conductive carbon black = 90: (3-8): (2-7) by mass, and dispersed in NMP solvent.

[0028] Negative electrode slurry: Si active material: binder: conductive carbon black = 90: (5-10): (5-10) by mass ratio, and dispersed in NMP solvent.

[0029] The slurry is stirred for 0.1-1 hours, coated onto the current collector, and dried under vacuum at 80°C.

[0030] Compared with the prior art, the beneficial effects of the present invention are:

[0031] This invention uses a random copolymer carbon chain formed by free radical copolymerization of 1-vinyl-3-ethylimidazolium cation and olefinic unsaturated phosphate monomer units as the main chain. The positive electrode functional group is a trifluoroacetoxy group, and the negative electrode functional group is a phosphate ester group (-PO(OR)2), with 3 wt% nano-Al2O3 added. On the positive electrode side, -OC(O)CF3 anchors oxygen vacancies on the NCM surface to suppress side reactions, while on the negative electrode side, -PO(OR)2 reacts with Si to form a Li3PO4 / Li2SiO3 mixed SEI layer. When applied to a LiPSCl electrolyte / NCM positive electrode / Si negative electrode all-solid-state battery, the capacity retention rate reaches 86.7% after 500 cycles, and the positive electrode interface impedance decreases to 16.3 Ω·cm. 2 The negative electrode expansion rate is <20%. This invention simultaneously achieves positive electrode oxygen release suppression and negative electrode SEI regulation through a single-component binder; the interface impedance is reduced by 70%, and -OC(O)CF3 blocks the NCM / LiPSCl side reaction; the expansion rate is reduced by 60%, and -PO(OR)2 and Al2O3 synergistically buffer silicon volume stress. In other words, this invention simultaneously solves the positive and negative electrode interface failure problem through a single-component binder, significantly improving battery cycle stability.

[0032] This invention is based on ionic liquids, which are polymerized at higher temperatures using an initiator. This effectively enhances the diffusion and transport of Li ions in the electrode material and reduces the internal resistance of the battery, thereby improving the battery's specific capacity and cycle stability. Attached Figure Description

[0033] Figure 1 This is a synthetic route diagram of adhesive molecules;

[0034] Figure 2 These are cycle performance test graphs of the sulfide solid-state full cells prepared in Example 1 and Comparative Example 1.

[0035] Figure 3 The first charge-discharge curves of the sulfide solid-state full cells prepared in Example 1 and Comparative Example 1 are shown.

[0036] Figure 4 These are cycle performance test graphs of the sulfide solid-state full cells prepared in Example 2 and Comparative Example 1.

[0037] Figure 5 These are rate performance test graphs of the sulfide solid-state full cells prepared in Example 2 and Comparative Example 1. Detailed Implementation

[0038] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are only some embodiments of the invention, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0039] Unless otherwise specified, the experimental methods and conditions used in the embodiments of this invention are conventional methods and conditions. Unless otherwise specified, the materials, reagents, or instruments used in the embodiments can be obtained commercially or prepared by conventional methods. Some embodiments are listed below to further illustrate the advantages of the technical solution of this invention.

[0040] Example 1

[0041] A method for preparing and applying a bifunctional polyionic liquid binder, the preparation method comprising the following steps:

[0042] S1. At room temperature, 1-vinyl-3-ethylimidazolium bromide ([VEIm]Br, 50.0 g, 0.24 mol) was dissolved in 200 mL of anhydrous acetone. While stirring, an acetone solution containing an equimolar amount of lithium difluorosulfonylimide (LiFSI, 44.8 g, 0.24 mol) was slowly added. A white precipitate (LiBr) immediately formed. The reaction was continued with stirring for 12 hours. After the reaction was complete, the mixture was filtered three times to remove the LiBr precipitate. The filtrate was rotary evaporated at 40 °C to remove the acetone solvent, yielding a viscous liquid. This liquid was dissolved in a small amount of dichloromethane and purified again by precipitation in diethyl ether. Finally, the product was dried under high vacuum at 60 °C for 48 hours to obtain a colorless to pale yellow, high-purity [VEIm][FSI] ionic liquid monomer as a polymerization precursor. The yield was approximately 80 g, with a yield of approximately 90%.

[0043] S2. In an argon-filled glove box (H2O, O2 < 0.1ppm), the above-mentioned monomers ([VEIm][FSI]) (90.0g, 0.309mol) and vinyl diethyl phosphate monomer (V-POEP) (10.0g, 0.054mol) were dissolved in 200mL of anhydrous acetonitrile. 0.5g of the free radical initiator azobisisobutyronitrile (AIBN), approximately 0.5wt% of the total monomer mass, was added. The reaction system was removed from the glove box, and the reaction was carried out under nitrogen protection and a 75°C oil bath with stirring for 18 hours. The viscous solution after the reaction was slowly added dropwise to 1.5L of anhydrous diethyl ether under vigorous stirring, resulting in the precipitation of fibrous or flocculent solids. After standing, the supernatant was discarded, and the precipitate was washed three times with diethyl ether. The resulting white solid was then heated under high vacuum (< 10) at 60°C. -3 After drying for 48 hours, the precursor copolymer Poly([VEIm][FSI]-co-VPOEP) was obtained, with a yield of approximately 85g.

[0044] S3. Weigh 20.0 g of the above precursor copolymer and dissolve it in 150 mL of anhydrous DMF. While stirring continuously in an ice-water bath for 1 h, slowly add 0.7 g of sodium hydride (NaH) dropwise to completely deprotonate the C2 site of the imidazole ring. Slowly add 3.4 mL of trifluoroacetic anhydride (TFAA), remove from the ice bath, and allow the reaction mixture to slowly rise to room temperature while stirring continuously for 24 h. After the reaction is complete, repeat the precipitation purification and solvent removal by vacuum distillation to obtain a pale yellow viscoelastic solid with an ionic conductivity of 1.2 × 10⁻⁶. -4 S / cm.

[0045] The binder material obtained in this embodiment is applied to both the positive and negative electrodes of the sulfide solid-state battery, and the full cell is assembled using the following method:

[0046] Positive electrode preparation: NCM811 active material (LiNi) 0.8 Co 0.1 Mn 0.1 O2), Super P conductive carbon black, and the binder prepared by the above method are mixed in a mass ratio of 90:5:5. N-methylpyrrolidone (NMP) is added to adjust the viscosity, and the mixture is stirred for 6 hours. The mixture is then coated onto a 20 μm thick aluminum foil, resulting in a wet film thickness of 150 μm. After vacuum drying at 80 °C for 12 hours, in-situ crosslinking of the binder is initiated by UV curing (wavelength 365 nm, exposure 60 s), yielding a positive electrode sheet (area capacity 3.2 mAh / cm²). 2 ).

[0047] Negative electrode preparation: Nano-silicon powder, Super P conductive carbon black, and the binder prepared by the above method were mixed at a mass ratio of 90:5:5. Nano-Al2O3 particles (3 wt% of the binder mass) were added, and 0.5 mL of acetonitrile was added dropwise to wet the mixture. The mixture was stirred for 8 hours to obtain a homogeneous slurry. This slurry was coated onto a copper foil (15 μm thick), resulting in a wet film thickness of 100 μm. After drying at 70℃ for 10 hours, it was cured under UV light to obtain a negative electrode sheet (area capacity 3.5 mAh / cm²). 2 ).

[0048] Full cell assembly: In an argon atmosphere, the Li6PS5Cl sulfide electrolyte layer is cold-pressed at 360MPa, with the positive electrode on the positive side and the negative electrode on the negative side. The cell is cold-pressed at 360MPa for 1 minute, then inserted into a steel shell and kept at 360MPa to obtain a full cell for testing.

[0049] Example 2

[0050] The difference between this embodiment and Example 1 is that in step S2, 16.3 g (0.09 mol) of bis(2-acryloyloxy)ethyl vinyl phosphate (BVPO) and 82.5 g (0.2 mol) of the high-purity [VEIm][FSI] monomer obtained in S1 are dissolved in 200 mL of anhydrous THF, and 0.49 g of AIBN (approximately 0.5 wt% of the total monomer mass) is added. The operation is the same as in Example 1.

[0051] The difference between this embodiment and Embodiment 1 is that, during electrode preparation, the slurry is coated onto the current collector and heat-treated in a vacuum drying oven at 100°C for 2 hours to initiate free radical copolymerization of the acryloyloxy groups at the ends of the BVPO monomer units under the action of the residual initiator, forming a robust three-dimensional network. Then, it is dried in a vacuum drying oven at 70°C for 12 hours.

[0052] This embodiment assembles the battery using the same method as in Embodiment 1.

[0053] Example 3

[0054] The difference between this embodiment and Example 1 is that in step S2, the monomer feed ratio is adjusted to increase the proportion of olefinically unsaturated phosphate monomers. Pure ([VEIm][FSI]) monomer (70.0 g, 0.24 mol) and vinyl diethyl phosphate monomer (V-POEP) (30.0 g, 0.19 mol) are dissolved in 200 mL of anhydrous acetonitrile. The remaining steps are the same as in Example 1.

[0055] This embodiment differs from Embodiment 1 in that, in the preparation of the negative electrode, nano-silicon powder, Super P conductive carbon black, and the binder prepared by the above method are mixed at a mass ratio of 95:3:2. Nano-Al2O3 particles, accounting for 3 wt% of the binder mass, are added, and after wetting with 0.5 mL of acetonitrile, the mixture is stirred for 8 hours to ensure thorough mixing and obtain a homogeneous slurry. This slurry is coated onto a copper foil (15 μm thick), resulting in a wet film thickness of 100 μm. After drying at 70°C for 10 hours, it is also cured under ultraviolet light to obtain the negative electrode sheet. In full-cell cycle tests with higher capacity silicon negative electrodes, a lower capacity decay rate and superior cycle life are observed due to the formation of a more effective protective interface.

[0056] Comparative Example 1

[0057] Polyvinylidene fluoride (PVDF) was dissolved in NMP to prepare a 5% (w / w) binder solution with a viscosity of 1000 mPa·s. For the negative electrode preparation, Si, PVDF, and VGCF were weighed out in a 90:5:5 mass ratio and mixed into a slurry, which was then coated onto copper foil and vacuum dried in an oven at 80°C for 6 hours before being cut into electrode sheets. For the positive electrode preparation, NCM811, PVDF, and VGCF were weighed out in a 80:10:10 mass ratio and mixed into a slurry, which was then coated onto aluminum foil.

[0058] Figure 2 The long-cycle performance of the batteries prepared in Example 1 and Comparative Example 1 was measured at a rate of 1C (1C = 200 mAh / g) and a voltage range of 2.0-4.2V. Figure 3 The first charge-discharge curves of the batteries prepared in Example 1 and Comparative Example 1 at a rate of 0.1C show that the polyionic liquid binder has excellent cycle performance and good stability when applied in sulfide solid-state batteries.

[0059] Figure 4 The charge-discharge cycle curves for Example 2 and Comparative Example 1 are obtained by constant current charging to 4.2V and discharging to 2V at 55°C and a 0.1C (1C = 200 mAh / g) rate. The initial efficiency was measured to be 85.7%, and the capacity retention rate after 200 cycles was 73.5%. Figure 5 The rate curves of the batteries prepared in Example 2 and Comparative Example 1 at room temperature are shown.

[0060] Long-cycle performance tests of Example 1 and Comparative Example 1 show that the electrode of Example 1 has superior cycle stability. Comparative Example 1 exhibits rapid capacity decay after less than 200 cycles, while Example 1 can stably cycle for 300 cycles. Furthermore, the electrode of Example 1 has a higher coulombic efficiency than the electrode of Comparative Example 1, indicating that lithium ions are fully inserted and extracted, which is attributed to the rapid ion transport characteristics of the polyionic liquid binder.

[0061] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A bifunctional polyionic liquid binder, characterized by: The structural formula of the binder is poly([VEIm][FSI]-co-[UPhos]) modified by trifluoroacetoxy, wherein: [VEIm][FSI] represents an ionic liquid monomer unit composed of 1-vinyl-3-ethyl imidazolium cation and bis-fluorosulfonylimide anion; [UPhos] represents an olefinically unsaturated phosphate monomer unit; the binder main chain is a random copolymer carbon chain formed by radical copolymerization of 1-vinyl-3-ethyl imidazolium cation and the olefinically unsaturated phosphate monomer unit; the positive electrode functional group is a trifluoroacetoxy group, and the structure is -O-C(O)-CF3; the negative electrode functional group is a phosphate group introduced as a comonomer and hanging on the main chain, and the structure is -PO(OR)2; R is C1-C4 alkyl.

2. The bifunctional polyionic liquid binder of claim 1, wherein: The molar ratio of [VEIm][FSI] to [UPhos] is (6-8):(2-4); the negative electrode functional group accounts for 2%-30% of the total mass of the bifunctional polyionic liquid copolymer binder.

3. The bifunctional polyionic liquid binder of claim 1, wherein: The positive electrode functional group is modified on the C2 position of the imidazole ring through a covalent bond; and the negative electrode functional group is covalently connected to the 1-vinyl-3-ethyl imidazolium cation on the main chain through an alkenyl group to form a random carbon chain.

4. The bifunctional polyionic liquid binder of claim 1, wherein: The olefinically unsaturated phosphate monomer includes a phosphate monomer containing a vinyl group, an acryloyl group, a methacryloyl group, an allyl group, a styryl group, or a maleimide group.

5. A process for the preparation of the bifunctional polyionic liquid binder as claimed in any one of claims 1 to 4, characterized in that, The method comprises the following steps: S1, mix 1-vinyl-3-ethylimidazolium bromide with equimolar LiFSI acetone solution, carry out Br - exchange with the anion of FSI - to obtain [VEIm][FSI] monomer, filter, rotary evaporate to obtain high-purity [VEIm][FSI] monomer; S2, dissolving the olefinically unsaturated phosphate monomer in an organic solvent, and copolymerizing with the [VEIm][FSI] monomer under the action of an initiator and an inert gas atmosphere, and then precipitating, washing, and vacuum drying to obtain a pure precursor polymer; S3, dissolving the precursor polymer obtained in S2 in anhydrous DMF, adding a strong base under the action of an inert gas atmosphere and ice bath, and stirring, and then adding trifluoroacetic anhydride dropwise to room temperature, and then precipitating, washing, and vacuum drying to obtain the bifunctional polyionic liquid binder.

6. The method of claim 5, wherein the method further comprises: In the step S2, the organic solvent includes at least one of anhydrous acetonitrile, anhydrous γ-butyrolactone, anhydrous N,N-dimethylformamide, anhydrous tetrahydrofuran, anhydrous chloroform, 1,2-ethanediamine, and 1,2-ethanedithiol; the initiator is one of azobisisobutyronitrile, azobisisoheptyl nitrile, dimethyl azobis isobutyrate, azoisobutyryl cyanamide, ammonium persulfate, potassium persulfate, and sodium persulfate, and the inert gas atmosphere is at least one of argon, nitrogen, and helium.

7. The method of claim 5, wherein the method further comprises: In the step S3, the amount of the strong base added is to ensure that the pH of the reaction system is in the range of 10-12; and the molar ratio of the amount of trifluoroacetic anhydride to the imidazolium group in step S1 is 1.05-1.2:

1.

8. The method of claim 5, wherein the method further comprises: In the step S2, the content of the initiator accounts for 0.1-0.5wt% of the total mass of all monomers.

9. The method of claim 5, wherein the method further comprises: In the step S2, the reaction temperature of the copolymerization reaction is 65-75°C, and the reaction time is 12-24h.

10. Use of the bifunctional polyionic liquid binder according to any one of claims 1 to 4, characterized in that: The bifunctional polyionic liquid binder is used in the positive electrode and / or the negative electrode of a sulfide solid-state battery.

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