Composite solid electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile, preparation method and application of composite solid electrolyte and all-solid-state battery

Abstract

The invention belongs to the technical field of solid-state electrolytes, and particularly relates to a lithium lanthanum zirconium oxide and polyacrylonitrile composite solid-state electrolyte, a preparation method and application thereof and an all-solid-state battery. The preparation method comprises the following steps: 1) preparing cubic phase LLZTO powder; (2) carrying out surface silanization treatment on the cubic phase LLZTO powder under an acidic condition; 3) preparing polyacrylonitrile, a polyion liquid, a lithium salt and the cubic phase LLZTO of which the surface is silanized into composite electrolyte slurry at room temperature; and 4) casting the composite electrolyte slurry to form a film, and then performing vacuum drying to obtain the lithium lanthanum zirconium oxide and polyacrylonitrile composite solid electrolyte. The LLZTO and polyacrylonitrile composite solid electrolyte provided by the invention can integrate the advantages of organic and inorganic materials, solves the problem of contact between traditional polyacrylonitrile and LLZTO and a positive electrode side interface, and realizes a stable composite structure with high voltage resistance, high temperature resistance and high ionic conductivity.

Description

Technical Field

[0001] This invention belongs to the field of solid electrolyte technology, specifically relating to a composite solid electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile, its preparation method, its application and all-solid-state battery. Background Technology

[0002] Li 6.5 La3Zr 1.5 Ta 0.5 O 12 LLZTO solid electrolytes are widely used as ceramic fillers or 3D frameworks for composite solid electrolytes (CPEs) due to their high ionic conductivity, wide electrochemical window, and good stability to lithium anodes. However, due to their large specific surface area, nano- or submicron-sized garnet particles are prone to agglomeration. Furthermore, incompatibility at the organic / inorganic interface exacerbates the agglomeration of garnet particles in the polymer, hindering the enhancement of ionic conductivity in CPES. Although LLZTO has a wide electrochemical window, its brittleness makes the contact between the solid interface and the cathode extremely difficult to address. Polyacrylonitrile (PAN) is a relatively unique polymer solid electrolyte with stable electrochemical performance, high mechanical strength, and oxidation resistance. It also boasts a wide electrochemical window; pure PAN can reach 4.6V, making it compatible with various high-voltage cathodes. However, PAN has a low ionic conductivity; typically, pure PAN membranes at room temperature only achieve 10V. −5 Scm -1 Meanwhile, the relatively hard material does not allow for sufficient contact with the electrode, and the polar nitrile groups in PAN can undergo a strong passivation reaction with lithium metal, leading to increased interfacial impedance and poor interfacial compatibility. Furthermore, PAN is difficult to process and is more brittle than other polymer electrolytes, deterring most researchers. From the perspective of solid electrolyte preparation, inorganic-organic composite solid electrolytes have simpler synthesis conditions, which is conducive to large-scale industrial production.

[0003] Currently, the main methods for preparing composite electrolytes include solution casting, in-situ polymerization, solvent-free hot pressing, and coating. The most common and simplest process is solution casting. Solution casting involves pouring a mixed solution containing polymer, lithium salt, inorganic filler, and organic solvent into a mold, and then evaporating the solvent under vacuum and high temperature conditions to obtain the composite solid electrolyte. This process is relatively simple and is the most commercially viable preparation method. Summary of the Invention

[0004] To address the issues of poor interfacial contact between the pure inorganic LLZTO system and the positive electrode, and its poor cycle performance when matched with various positive electrodes; and the instability of the pure polyacrylonitrile (PAN) polymer system with lithium metal anodes, as well as its high brittleness and low ionic conductivity within polymer systems, this invention provides a composite solid-state electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile, its preparation method, its applications, and an all-solid-state battery. The LLZTO / polyacrylonitrile composite solid-state electrolyte provided by this invention combines the advantages of organic and inorganic materials, solves the problem of interfacial contact between traditional polyacrylonitrile / LLZTO and the positive electrode, and achieves a stable composite structure with high voltage resistance, high temperature resistance, and high ionic conductivity.

[0005] Meanwhile, this invention also solves the problem that the cyano group in polyacrylonitrile will undergo a cyclization reaction at room temperature under the strong alkaline environment of LLZTO and the action of the metal active sites therein. Although the stability and ionic conductivity of polyacrylonitrile after cyclization are improved, excessive cyclization makes it more difficult to cast polyacrylonitrile into films, and it is very easy to gel during stirring. The process is irreversible and the processability is greatly reduced.

[0006] The technical solution provided by this invention is as follows: A method for preparing a composite solid electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile includes the following steps: 1) Preparation of cubic LLZTO powder; 2) The cubic LLZTO powder obtained in step 1) is subjected to surface silanization treatment under acidic conditions; 3) A composite electrolyte slurry was prepared at room temperature by mixing polyacrylonitrile, polyionic liquid, lithium salt, and surface-silanized cubic phase LLZTO, wherein: The mass ratio of polyacrylonitrile to polyionic liquid is 1:0.5~1.2; The mass ratio of polyacrylonitrile to lithium salt is 1:0.5-1.2; The mass of LLZTO is 5-50% of polyacrylonitrile; 4) Cast the composite electrolyte slurry obtained in step 3) into a film, and then vacuum dry it to obtain the composite solid electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile.

[0007] The purpose of adding LLZTO is to improve the stability, ionic conductivity, and high voltage resistance of solid electrolytes, but the addition of LLZTO will exacerbate the brittleness of polyacrylonitrile membranes.

[0008] The above technical solution, by surface silanizing cubic LLZTO, can suppress the problem of excessive cyclization of polyacrylonitrile caused by the strong alkaline environment and metal active sites of LLZTO during the mixing process of polyacrylonitrile and LLZTO, which leads to excessively brittle films that cannot be processed during the film formation process.

[0009] Specifically, step 1) includes: 1a) The lithium source, lanthanum source, and zirconium source are mixed, ground, and then dried; 1b) The mixed powder was heated to 950-1050°C in air at a heating rate of 2-5°C / min, held for 6-12 hours, and sintered to obtain the primary cubic phase LLZTO. 1c) The primary cubic phase LLZTO is ground twice and then dried; 1d) The material obtained in step 1c) is calcined at 1050-1150℃ for 4-8 hours, with the heating rate controlled within 2℃ / min-5℃ / min during the calcination process. Then, it is ground, dried and passed through a 200-300 mesh sieve to obtain the cubic phase LLZTO powder.

[0010] Specifically, step 2) includes: 2a) The cubic LLZTO powder is vacuum dried; 2b) The cubic LLZTO powder obtained in step 2a) is added to an acidic silane hydrolysate with a pH of 3-5 and heated to carry out the reaction. The solvent in the silane hydrolysate is ethanol and water in a volume ratio of 95:3-95:6, and the specific ratio depends on the amount of LLZTO added later. 2c) The reaction product is centrifuged and cleaned, the cleaned solid is vacuum dried, and then heated to react, so that the silane layer is fully cross-linked and cured to obtain surface silanized S-LLZTO.

[0011] Specifically: In step 2b), the reaction temperature is 55-65°C and the reaction time is 4-8 hours; In step 2c), the vacuum drying temperature is 55-65°C, and the vacuum drying time is 4-6 hours; In step 2 c), the reaction temperature is 100-120°C and the reaction time is 1-2 hours.

[0012] Specifically, step 3) includes the following steps: 3a) Prepare a polymer solution by mixing polyacrylonitrile, polyionic liquid and lithium salt; 3b) Prepare a suspension of surface-silanized cubic LLZTO (denoted as S-LLZTO); 3c) The polymer solution is continuously stirred at room temperature while the suspension is slowly added dropwise. After the addition is complete, magnetic stirring is continued at room temperature to obtain the composite electrolyte slurry.

[0013] In the above technical solution, by modifying LLZTO with silanization under acidic conditions, the silanization process can be controlled, and a monolayer silanized coating can be formed as much as possible. This greatly improves the problem of difficult dispersion of LLZTO in organic solutions (such as DMF or NMP) (as can be seen from SEM images, LLZTO dispersion is uniform) and the problem of polyacrylonitrile cyclization caused by the addition of LLZTO. The acidic conditions are adjusted by adding acetic acid when preparing the silane hydrolysate. Specifically, after preparing the ethanol / water solution, acetic acid is added while stirring to adjust the pH.

[0014] Preferred: The polyionic liquid is 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt; The lithium salt is lithium bis(trifluoromethanesulfonylimide); The solvent for the polymer solution is N,N-dimethylformamide; The total mass concentration of the solute in the polymer solution is 8-12 wt%; The dispersion of the suspension is N,N-dimethylformamide.

[0015] In the above technical solution: Silanized LLZTO, as an inorganic additive, can improve the stability, ionic conductivity, and high-voltage resistance of solid electrolytes. [BMIM][TFSI], as a plasticizer, can improve the flexibility of composite films and enhance ionic conductivity; When both are added to the polymer solid electrolyte membrane, the problem of excessive cyclization of polyacrylonitrile is successfully solved, and the problem of high brittleness and difficulty in processing that easily occurs when polyacrylonitrile is used as a solid electrolyte membrane is further solved.

[0016] Specifically, step 4) includes: The composite electrolyte slurry is poured into a flat mold, the mold opening is covered, and then left to stand at room temperature to form a preliminary film. The initial film was vacuum dried at 55-65°C for 24-48 hours to obtain the composite solid electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile.

[0017] Preferably, the drying process employs a gradually increasing vacuum level (e.g., from -0.2 MPa to -0.6 MPa), which is suitable for different DMF solution contents.

[0018] The present invention also provides a composite solid electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile, which is prepared according to the preparation method described above.

[0019] The composite film described above has no obvious pores inside without the need for additional hot pressing or surface polymerization, and can withstand a voltage of not less than 4.9V (preferably 5.0V).

[0020] The present invention also provides an all-solid-state battery, comprising a positive electrode, a solid electrolyte membrane and a negative electrode, wherein the solid electrolyte membrane is made of the aforementioned lithium lanthanum zirconium oxide and polyacrylonitrile composite solid electrolyte.

[0021] Specifically, the positive electrode can be a ternary positive electrode, a lithium-rich manganese positive electrode, or a lithium iron phosphate positive electrode, and the negative electrode can be a lithium metal negative electrode to form a high-voltage or long-term stable lithium battery.

[0022] This invention also provides the application of a composite solid electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile as a solid electrolyte material or a quasi-solid electrolyte material. A quasi-solid electrolyte is a composite material with a solid polymer / inorganic material as a continuous framework or main body, wherein a considerable amount of liquid electrolyte is filled or impregnated, with the liquid electrolyte content ranging from 0% to 5%.

[0023] The beneficial effects of this invention are as follows: 1) Significantly improved interfacial compatibility and reduced interfacial impedance: After chemical grafting modification of the LLZTO surface with epoxy silane coupling agent (KH-560), the silane layer and the polymer matrix (PAN) form a stable chemical interaction, which greatly improves the compatibility between inorganic fillers and organic phases and avoids interfacial defects caused by the agglomeration of inorganic particles in traditional composite electrolytes. At the same time, the cross-linked structure of the silane layer reduces the interfacial impedance in the lithium ion transport process and improves the ion conduction efficiency, solving the key technical pain points of poor interfacial compatibility and excessive impedance in inorganic-organic composite systems.

[0024] 2) Inhibiting PAN cyclization and improving membrane flexibility: The entire process of composite slurry preparation and film formation adopts room temperature / low temperature / acidic process (maximum curing temperature 120°C), which fundamentally inhibits the cyclization reaction of PAN at high temperature, avoiding the membrane material from becoming brittle and reducing its toughness; at the same time, the synergistic effect of polyionic liquid ([BMIM][TFSI]) and LiTFSI not only improves the lithium ion migration rate, but also enhances the membrane's flexibility, high temperature resistance, and high pressure resistance (compared to traditional PAN-based electrolytes, the high pressure resistance is improved to above 4.9V, and it is also difficult to burn at high temperatures), solving the problems of easy embrittlement and poor processing performance of existing PAN-based composite membranes.

[0025] 3) The process is mild and controllable, and easy to scale up: Except for the unavoidable need for high-temperature equipment in the preparation of LLZTO powder in step 1, the preparation process in steps 2 and 3 does not require high-temperature and high-pressure equipment. The solution casting method for film formation is simple and easy to operate, and the parameters of each step (such as ball milling speed, reaction temperature, drying time, etc.) are highly controllable and have good repeatability. Compared with other methods for preparing PAN composite membranes, the reaction conditions in this scheme are at room temperature, which reduces production energy consumption and equipment costs, while avoiding complex post-processing steps, making it more suitable for large-scale industrial preparation of composite solid electrolyte membranes.

[0026] 4) The membrane material has a balanced and excellent overall performance: The final product has high ionic conductivity, good flexibility (no cracks when bent at 90°), excellent mechanical strength and stable electrochemical performance, which solves the problems of "high conductivity and flexibility cannot be obtained at the same time" and "imbalance between mechanical properties and electrochemical stability" in existing composite solid electrolyte membranes, and is more suitable for the actual application needs of solid lithium-ion batteries.

[0027] 5) Existing PAN composite membranes prepared using the same process technology have a porous structure, making them unsuitable for high-pressure environments, or requiring the addition of other polymers to the membrane surface for further polymerization before they can withstand high pressure. This invention strictly controls the evaporation of DMF solvent in the membrane, resulting in a composite membrane without obvious pores, eliminating the need for surface polymerization of other polymers or hot pressing of the membrane. Attached Figure Description

[0028] Figure 1 This is a flowchart of the preparation method of the present invention.

[0029] Figure 2 This is a comparison chart of discharge specific capacity.

[0030] Figure 3 This is an LSV curve graph.

[0031] Figure 4 This is a cross-sectional SEM image of the composite membrane.

[0032] Figure 5 This is a cross-sectional SEM image of another composite membrane.

[0033] Figure 6 This is a surface SEM image of the composite membrane. Detailed Implementation

[0034] The principles and features of the present invention are described below. The embodiments given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0035] Unless otherwise specified, the test methods used in the embodiments are conventional methods; unless otherwise specified, the materials and reagents used are commercially available.

[0036] In one specific embodiment, the preparation method of the composite solid electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile includes the following steps: Step 1: Preparation of cubic phase LLZTO powder Raw material weighing: according to Li 6.5 La3Zr 1.5 Ta 0.5 O 12The stoichiometric ratio is used to accurately weigh the lithium source (preferably lithium hydroxide monohydrate LiOH·H2O), lanthanum source (preferably lanthanum oxide La2O3), zirconium source (preferably zirconium oxide ZrO2), and tantalum source (preferably tantalum oxide Ta2O5); wherein the lithium source is in excess by 5-15% to compensate for the loss of lithium volatilization during calcination.

[0037] Mixing and grinding: Place all raw materials in a ball mill jar, add isopropanol as solvent, and use zirconia balls as grinding media (ball-to-material ratio 10:1~20:1). Grind in a planetary ball mill at a speed of 200-400 r / min for 6-12 hours to achieve thorough mixing and refinement of the raw materials.

[0038] Drying and sieving: The ball-milled slurry is placed in a vacuum drying oven at 60°C for 8-12 hours to remove the solvent. After drying, the blocky material is ground and sieved to obtain a uniformly mixed powder.

[0039] High-temperature sintering: The mixed powder is placed in an alumina crucible and heated to 950-1050°C at a heating rate of 2-5°C / min in an air atmosphere. The temperature is held for 6-12 hours to sinter and initially form the cubic phase LLZTO.

[0040] Secondary grinding and drying: The sintered raw materials are ground and dried under the following conditions: ball-to-material ratio 5:2, solvent-to-raw material ratio 2:1. The drying conditions are: vacuum degree -0.6 MPa, time 24 h.

[0041] Secondary sintering: The material after secondary grinding and drying is placed in a small (10ml) magnesium oxide crucible, covered with a magnesium oxide crucible lid, and calcined at 1100℃ for 6 hours. The heating rate during calcination is controlled within 2℃ / min-5℃ / min. After sintering, the material is ground under the same grinding and drying conditions as after secondary grinding, and passed through a 200-300 mesh sieve to obtain cubic phase LLZTO powder, which is then stored in a glove box for later use.

[0042] Step 2: Surface silanization treatment of LLZTO Pretreatment: Place the LLZTO powder prepared in step 1 under vacuum drying at 100-120°C for 6-12 hours to completely remove the physically adsorbed water on the surface.

[0043] Preparation of silane hydrolysate: Add acetic acid to a mixed solvent of ethanol and deionized water (volume ratio of 95:3-95:6) to adjust the pH to 4.0, then add 1-5% of the mass of LLZTO as an epoxy silane coupling agent (preferably 3-glycidyloxypropyltrimethoxysilane KH-560), and stir magnetically at room temperature for 20-40 minutes to allow the coupling agent to be fully hydrolyzed to generate silanol, thus obtaining the silane hydrolysate.

[0044] Surface coating: The pretreated LLZTO powder was added to the silane hydrolysate and reacted at a constant temperature of 60°C with magnetic stirring for 6 hours to achieve chemical grafting of silane molecules on the LLZTO surface.

[0045] Centrifugal washing: After the reaction is complete, centrifuge to separate the mixture and discard the supernatant; add anhydrous ethanol to the precipitate, redisperse by vortexing or brief sonication, and centrifuge again. Repeat this washing operation 2-4 times until the supernatant is clear and transparent.

[0046] Vacuum curing: The cleaned solid is placed in a vacuum dryer at 60°C for 4-6 hours, and then subjected to vacuum heat treatment at 100-120°C for 1-2 hours to fully crosslink and cure the silane layer, thus obtaining surface silanized LLZTO (abbreviated as S-LLZTO).

[0047] LLZTO solution preparation: Disperse S-LLZTO in N,N-dimethylformamide (DMF) to prepare a solution at a concentration of 0.1 g / mL, and sonicate for 15 minutes before use.

[0048] Step 3: Room temperature preparation of composite electrolyte slurry Polymer solution preparation: PAN and polyionic liquid (preferably 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt [BMIM][TFSI]) are dissolved in DMF at a mass ratio of 1:0.5~1.2, and lithium salt (preferably lithium(trifluoromethanesulfonyl)imide LiTFSI) is added at the same time. The mass ratio of PAN to lithium salt is 1:0.5-1:1. The solution is magnetically stirred at room temperature (15-30°C) for 4-8 hours to form a clear and homogeneous polymer solution with a total polymer concentration of 8-12wt%.

[0049] Preparation of S-LLZTO suspension: Weigh S-LLZTO powder and disperse it in DMF to prepare a 0.1 g / mL S-LLZTO / DMF solution. Before each use, sonicate for 10-30 minutes to form a homogeneous suspension. Add LLZTO at 5%-50% of the PAN mass.

[0050] Slurry compounding: The polymer solution is continuously stirred at room temperature while S-LLZTO suspension is slowly added dropwise. After the addition is complete, magnetic stirring is continued at room temperature for 2-6 hours to obtain a uniform, stable compound electrolyte slurry without significant yellowing or reddening.

[0051] Step 4: Solution casting to form a film Pouring: Pour the composite electrolyte slurry into a flat mold (preferably a polytetrafluoroethylene mold or a glass plate).

[0052] Solvent evaporation: Place the mold in a semi-enclosed environment (such as a fume hood with all windows closed) and let it stand for 12-24 hours at room temperature without strong convection to allow the solvent to slowly evaporate and form a preliminary film.

[0053] Vacuum drying: The pre-cured membrane is transferred to a vacuum drying oven and dried at 60°C for 24-48 hours to completely remove residual solvent, thus obtaining a cubic phase LLZTO / PAN composite solid electrolyte membrane.

[0054] like Figure 1 The diagram shown is a flowchart of the preparation method of the present invention.

[0055] Example 1 15% LLZTO-doped PAN composite film Experimental materials Lithium hydroxide monohydrate (LiOH・H2O, purity ≥99.0%); Lanthanum oxide (La2O3, purity ≥99.9%); Zirconium oxide (ZrO2, purity ≥99.5%); 3-glycidyloxypropyltrimethoxysilane (KH-560, industrial grade); Polyacrylonitrile (PAN, number average molecular weight 160,000); 1-Butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][TFSI], purity ≥98.0%); Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, purity ≥99.0%); Isopropanol (IPA, anhydrous grade); Anhydrous ethanol (analytical grade); N,N-dimethylformamide (DMF, analytical grade); Acetic acid (analytical grade); Deionized water.

[0056] Preparation steps 1. Preparation of cubic phase LLZTO powder Using 20g of LLZTO product with an excess of 15% lithium salt as designed product, LiOH·H2O, La2O3, ZrO2, and Ta2O5 were weighed according to stoichiometric coefficients and subjected to planetary ball milling at 320 rpm in an 80mL nylon container. During milling, 30g of zirconia grinding beads (10g Φ10mm + 20g Φ5mm) were used as the grinding media, and 30g of isopropanol (IPA) was used as the solvent. The resulting viscous slurry was dried in a vacuum drying oven at 55-65 °C and -0.4 MPa for 12 hours. The dried powder was sieved through a No. 20 sieve. The powder was then dried in an open Al2O3 crucible at 5 °C for 1 minute. -1The LLZTO precursor was calcined at 950℃ for 6 hours to facilitate the evaporation of solvent and H2O gas. After the first grinding step, a second grinding was performed with a ball-milling bead to raw material ratio of 5:2 and a grinding media isopropanol to raw material mass ratio of 2:1, while maintaining the same grinding time and speed. The LLZTO precursor from the second grinding was then vacuum dried for 24 hours at a vacuum of -0.6 MPa and a temperature of 60℃. The dried product was then placed in a small magnesium oxide crucible (with a magnesium oxide crucible lid) for a second calcination, heated to 1100℃ and held for 6 hours. The subsequent grinding step after sintering was repeated to activate the prepared LLZTO powder, which was then filtered through a 200-300 mesh sieve and stored in a brown glass bottle for later use.

[0057] 2. LLZTO powder silanization treatment First, a precursor solution of 3-glycidyloxypropyltrimethoxysilane (KH-560) needs to be prepared. Its composition is 95 mL ethanol and 5 mL water, with acetic acid added to adjust the pH to approximately 4. Then, 2 mL of KH-560 is added, and the solution is magnetically stirred for 30 min at room temperature. The Si-OC group can be easily converted to a silanol group (Si-OH) through hydrolysis. Next, 2-3 g of the LLZTO powder prepared in step 1 is added to the precursor solution, and the mixture is stirred at 60 °C for 6 h at room temperature at 8000 rpm. −1 The slurry was separated by centrifugation, washed with ethanol, and dried at 60°C for 12 h to obtain KH-560 modified S-LLZTO particles. The modified powder was sieved and then mixed with DMF solvent to prepare a 0.1 g / mL S-LLZTO / DMF solution. The prepared solution was ultrasonicated for 20 min before subsequent addition processes.

[0058] 3. Preparation of composite membranes First, weigh 0.5g and 0.25g of PAN and LiTFSI in a mass ratio of 1:0.5 in a glove box, and dissolve them in a DMF solution with a total polymer concentration of 10% (10.29g of PAN and [BMIM][TFSI]). After stirring at room temperature for 3 hours, add 0.6g of [BMIM][TFSI] as a plasticizer and continue stirring at room temperature for 2-3 hours to finally obtain the polymer solution.

[0059] Add 0.75 mL of a pre-prepared 0.1 g / mL S-LLZTO / DMF solution, then continue stirring at room temperature overnight to form a homogeneous solution. The solution is then cast onto a PTFE plate, with strict control over DMF evaporation. After standing in a fume hood for one day, transfer to a 60°C vacuum drying oven for 24 hours. During the first 3 hours, maintain a vacuum level of -0.2 MPa; from 3 to 6 hours, adjust the vacuum level to -0.4 MPa; and after 6 hours, adjust the vacuum level to -0.6 MPa. This yields a PAN / LLZTO composite membrane.

[0060] The aforementioned composite membrane was assembled into a battery using lithium metal and lithium iron phosphate cathode materials. Electrochemical performance tests were conducted at room temperature (25°C). During constant current charge-discharge cycling, the initial discharge specific capacity at a current density of 0.2C was 151.53 mAh / g, and the capacity retention after 80 cycles was approximately 94.8%. The LSV curve of the assembled SS / composite membrane / Li semi-symmetric battery was tested, and it could withstand a high voltage of 5.0V.

[0061] Example 2 4% LLZTO-doped PAN composite film The experimental materials and procedures in steps 1 and 2 are the same as in Example 1. In step 3, during the preparation of the composite membrane: First, weigh 0.5g and 0.25g of PAN and LiTFSI (mass ratio 1:0.5) in a glove box and dissolve them in a 10% DMF solution (10.81g). After stirring at room temperature for 3 hours, add 0.6g of [BMIM][TFSI] as a plasticizer and continue stirring at room temperature for 2-3 hours. Then add 0.2mL of a pre-prepared 0.1g / mL S-LLZTO / DMF solution, and continue stirring at room temperature overnight to form a homogeneous solution.

[0062] The solution was then cast onto a polytetrafluoroethylene (PTFE) plate. DMF solution evaporation was strictly controlled. After standing in a fume hood for one day, the solution was transferred to a 60°C vacuum drying oven for 24 hours. During the first 3 hours, the vacuum level of the drying oven was controlled at -0.2 MPa; from 3 to 6 hours, the vacuum level was adjusted to -0.4 MPa; and after 6 hours, the vacuum level was adjusted to -0.6 MPa. This yielded a PAN / LLZTO composite membrane.

[0063] The aforementioned composite membrane was assembled into a battery using lithium metal and lithium iron phosphate cathode materials. Electrochemical performance was tested at room temperature (25°C). In constant current charge-discharge cycle testing, its initial discharge specific capacity at a current density of 0.2C was 152.45 mAh / g, and its capacity retention after 80 cycles was approximately 78.7%. When assembled into an SS / composite membrane / Li semi-symmetric battery, its LSV curve was tested, and it could withstand a high voltage of 4.9V.

[0064] Example 3 30% LLZTO-doped PAN composite film The experimental materials and procedures in steps 1 and 2 are the same as in Example 1. In step 3, during the preparation of the composite membrane: First, weigh 0.5g and 0.25g of PAN and LiTFSI (mass ratio 1:0.5) in a glove box and dissolve them in a 10% DMF solution (9.579g). After stirring at room temperature for 3 hours, add 0.6g of [BMIM][TFSI] as a plasticizer and continue stirring at room temperature for 2-3 hours. Then add 1.5mL of a pre-prepared 0.1g / mL S-LLZTO / DMF solution, and continue stirring at room temperature overnight to form a homogeneous solution.

[0065] The solution was then cast onto a polytetrafluoroethylene (PTFE) plate. DMF solution evaporation was strictly controlled. After standing in a fume hood for one day, the solution was transferred to a 60°C vacuum drying oven for 24 hours. During the first 3 hours, the vacuum level of the drying oven was controlled at -0.2 MPa; from 3 to 6 hours, the vacuum level was adjusted to -0.4 MPa; and after 6 hours, the vacuum level was adjusted to -0.6 MPa. This yielded a PAN / LLZTO composite membrane.

[0066] The aforementioned composite membrane was assembled into a battery using lithium metal and lithium iron phosphate cathode materials. Electrochemical performance was tested at room temperature (25°C). In constant current charge-discharge cycle testing, its initial discharge specific capacity at a current density of 0.2C was 154.73 mAh / g, and its capacity retention after 80 cycles was approximately 83.6%. When assembled into an SS / composite membrane / Li semi-symmetric battery, its LSV curve was tested, and it could withstand a high voltage of 5.2V.

[0067] Comparative Example 1: PAN composite film without LLZTO doping The experimental materials and procedures in steps 1 and 2 are the same as in Example 1. In step 3, during the preparation of the composite membrane: First, weigh 0.5g and 0.25g of PAN and LiTFSI in a mass ratio of 1:0.5 in a glove box, and dissolve them in a 10% DMF (11g) solution. After stirring at room temperature for 3 hours, add 0.6g of [BMIM][TFSI] as a plasticizer, and continue stirring at room temperature overnight to form a homogeneous solution.

[0068] The solution was then cast onto a polytetrafluoroethylene (PTFE) plate, and the evaporation of the DMF solution was strictly controlled. After standing in a fume hood for one day, it was transferred to a 60°C vacuum drying oven for 24 hours. During the first 3 hours, the vacuum level of the drying oven was controlled at -0.2 MPa; from 3 to 6 hours, the vacuum level was adjusted to -0.4 MPa; and after 6 hours, the vacuum level was adjusted to -0.6 MPa. A PAN composite membrane was obtained. This composite membrane was then assembled with lithium metal and lithium iron phosphate cathode materials to form a battery.

[0069] Electrochemical performance tests were conducted at room temperature (25°C). During constant current charge-discharge cycling, the initial discharge specific capacity at a current density of 0.2C was 153.01 mAh / g, and the capacity retention after 80 cycles was only 42.9%. When assembled into an SS / composite film / Li semi-symmetric battery, the LSV curve was tested, but it could not withstand voltages above 4.8V. The poor cycle stability is attributed to side reactions at the interface between the pure PAN system and the lithium metal anode.

[0070] Comparative Example 2: Unmodified 15% LLZTO-doped composite film Without performing LLZTO surface silanization treatment, the LLZTO powder prepared in step 1 was used directly, and the remaining steps were the same as in Example 1. The resulting slurry turned reddish-brown and gradually gelled after stirring for 2 hours, and could not be cast into a film after overnight.

[0071] Comparative Example 3: Unmodified 4% LLZTO-doped composite film Similar to Comparative Example 2, but with LLZTO added at 4% of the PAN content. The resulting slurry turned reddish-brown after stirring at room temperature, but could still be cast into a film. However, during the drying process after casting, the prepared composite film, due to excessive PAN cyclization and excessive rigidity, developed numerous cracks regardless of whether the drying was carried out at room temperature or in a vacuum drying oven. After repeated experiments, the film also cracked during the cutting process, making experimental reproduction difficult.

[0072] Comparative Example 4: Modified 4% LLZTO doped composite film without [BMIM][TFSI] Step 3: In the preparation of the composite membrane: Similar to Example 1, except that 0.5g and 0.25g of PAN and LiTFSI (mass ratio 1:0.5) were weighed in a glove box and dissolved in a 10% (11g) DMF solution. After stirring at room temperature for 3 hours, 0.2ml of S-LLZTO / DMF solution was added, and subsequent operations were the same as in Example 1. The dried film was brittle, and under an electron microscope, large surface defects and numerous pores were clearly visible. After assembling into a full cell, most cells short-circuited immediately. For cells that did not short-circuit, the average capacity, tested using an electrochemical workstation, was approximately 70mAh / g.

[0073] like Figure 2 The figure shows a comparison of the discharge specific capacity of Example 1, Example 2, Experimental Example 3, and Comparative Example 1. It can be seen that using an appropriate amount of LLZTO can significantly improve the battery cycle stability, i.e., reduce the side reactions between PAN and the lithium metal anode.

[0074] like Figure 3 The figure shows the LSV curves for Examples 1, 2, and 3. It can be seen that the composite electrolyte membrane of the present invention can withstand high voltages above 4.8V, thus meeting the requirements for high-voltage positive electrodes.

[0075] like Figure 4 The image shown is a cross-sectional SEM image of the PAN / LLZTO composite membrane obtained in Example 1. It can be seen from the image that the modified S-LLZTO is uniformly distributed in the membrane, resulting in a homogeneous overall membrane structure.

[0076] like Figure 5 The image shows a cross-sectional SEM image of the incomplete PAN / LLZTO composite membrane obtained in Comparative Example 4. The image reveals that the unmodified LLZTO is unevenly distributed within the membrane, exhibiting severe gravity deposition.

[0077] like Figure 6 The image shows a surface SEM image of the PAN / LLZTO composite membrane obtained in Example 1. The image shows that the composite membrane surface has no obvious pores and is relatively dense. High density helps to improve the electrochemical performance and stability of the membrane.

[0078] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a composite solid electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile, characterized in that, Includes the following steps: 1) Preparation of cubic LLZTO powder; 2) The cubic LLZTO powder obtained in step 1) is subjected to surface silanization treatment under acidic conditions to obtain surface silanized cubic LLZTO. 3) A composite electrolyte slurry is prepared at room temperature by mixing polyacrylonitrile, polyionic liquid, lithium salt, and the surface-silanized cubic phase LLZTO, wherein: The mass ratio of polyacrylonitrile to polyionic liquid is 1:0.5~1.2; The mass ratio of polyacrylonitrile to lithium salt is 1:0.5~1.2; The mass of the surface-silanized cubic phase LLZTO is 5-50% of that of polyacrylonitrile; 4) Cast the composite electrolyte slurry obtained in step 3) into a film, and then vacuum dry it to obtain the composite solid electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile.

2. The method for preparing the composite solid electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile according to claim 1, characterized in that, Step 1) includes: 1a) The lithium source, lanthanum source and zirconium source are mixed, ground and then dried to obtain a mixed powder; 1b) The mixed powder was heated to 950-1050°C in air at a heating rate of 2-5°C / min, held for 6-12 hours, and sintered to obtain the primary cubic phase LLZTO. 1c) The primary cubic phase LLZTO is ground twice and then dried; 1d) The material obtained in step 1c) is calcined at 1050-1150℃ for 4-8 hours, with the heating rate controlled within 2℃ / min-5℃ / min during the calcination process. Then, it is ground, dried and passed through a 200-300 mesh sieve to obtain the cubic phase LLZTO powder.

3. The method for preparing the composite solid electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile according to claim 1, characterized in that, Step 2) includes: 2a) The cubic LLZTO powder is vacuum dried; 2b) The cubic phase LLZTO powder obtained in step 2a) is added to an acidic silane hydrolysate with a pH of 3-5 and heated to carry out the reaction. The solvent in the silane hydrolysate is an aqueous ethanol solution, and the volume ratio of ethanol to water in the aqueous ethanol solution is 95:3-95:

6. 2c) The reaction product is centrifuged and cleaned, the cleaned solid is vacuum dried, and then heated to react, so that the silane layer is fully cross-linked and cured to obtain the cubic phase LLZTO with surface silanization.

4. The method for preparing the composite solid electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile according to claim 3, characterized in that: In step 2b), the reaction temperature is 55-65°C and the reaction time is 4-8 hours; In step 2c), the vacuum drying temperature is 55-65°C, and the vacuum drying time is 4-6 hours; In step 2c), the heating reaction temperature is 100-120°C and the reaction time is 1-2 hours.

5. The method for preparing the composite solid electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile according to claim 1, characterized in that, Step 3) includes: 3a) Prepare a polymer solution by mixing polyacrylonitrile, polyionic liquid and lithium salt; 3b) Prepare a suspension of the surface-silanized cubic phase LLZTO; 3c) The polymer solution is continuously stirred at room temperature while the suspension is slowly added dropwise. After the addition is complete, magnetic stirring is continued at room temperature to obtain the composite electrolyte slurry.

6. The method for preparing the composite solid electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile according to claim 5, characterized in that: The polyionic liquid is 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt; The lithium salt is lithium bis(trifluoromethanesulfonylimide); The solvent for the polymer solution is N,N-dimethylformamide; The total mass concentration of the solute in the polymer solution is 8-12 wt%; The dispersion of the suspension is N,N-dimethylformamide.

7. The method for preparing the composite solid electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile according to any one of claims 1 to 6, characterized in that, Step 4) includes: The composite electrolyte slurry is poured into a flat mold and placed in a fume hood, then left to stand at room temperature to form a preliminary film. The initial film was vacuum dried at 55-65°C for 24-48 hours to obtain the composite solid electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile.

8. A composite solid electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile, characterized in that: It is prepared according to any one of claims 1 to 7.

9. An all-solid-state battery, comprising a positive electrode, a solid electrolyte membrane, and a negative electrode, characterized in that: The solid electrolyte membrane is made of the composite solid electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile as described in claim 8.

10. An application of the composite solid electrolyte of lithium lanthanum zirconium oxide and polyacrylonitrile according to claim 8, characterized in that: As a solid electrolyte material or a quasi-solid electrolyte material.

Images