Composite solid electrolyte and preparation method and application thereof

Abstract

The application relates to a composite solid electrolyte and a preparation method and application thereof. The preparation method of the composite solid electrolyte comprises the following steps: S1, mixing inorganic fillers and fluorine-containing compounds, reacting under acidic conditions at 70-90 DEG C to obtain inorganic fillers wrapped by fluorination layers; then adding a fluorine ion complexing agent and a silane coupling agent to react to obtain modified fillers; adding a thermal decomposition type crosslinking agent in a polymer, and performing melt blending to obtain a crosslinked polymer; S2, uniformly mixing the modified fillers, the crosslinked polymer, lithium salt and a solvent to obtain multiple slurries containing modified fillers with different volume concentrations; forming the slurry to prepare the composite solid electrolyte. The fluorination layer and the siloxane network are constructed in steps, the interface impedance is reduced, and the efficient dissociation of lithium salt is promoted; the three-dimensional network structure is combined to improve the tensile strength; the modified fillers are further gradient designed to improve the lithium ion migration number.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of solid electrolyte materials, and particularly relates to a composite solid electrolyte and a preparation method and application thereof. BACKGROUND

[0002] Traditional polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) based solid electrolytes are difficult to meet the demand of high energy density batteries for electrolyte materials due to insufficient mechanical strength, high filler-matrix interface impedance, and limited dielectric properties. In the prior art, single surface modification or crosslinking treatment can partially improve the performance, but often leads to a decrease in ionic conductivity or an increase in processing difficulty. For example, although the conventional silane coupling agent modified filler can enhance the interface bonding, it cannot simultaneously increase the dielectric constant; while peroxide crosslinking can strengthen the mechanical properties, excessive crosslinking will limit the movement of polymer chain segments, hindering the migration of lithium ions. In addition, the homogeneous composite structure causes the filler to be uniformly dispersed in the overall electrolyte, resulting in the inability to form a high-concentration filler barrier in the surface layer to inhibit dendrite growth, and the internal region is hindered by excessive fillers, which hinders the continuity of the ion channel.

[0003] For example, the prior art CN114566699B discloses a preparation method of a fluorine-containing composite lithium ion solid-state electrolyte, comprising the following steps: step 1, adding a silane coupling agent, a fluorine-containing monomer, and an initiator into an organic solvent, heating to obtain a reaction product, and removing the solvent in the reaction product to obtain a fluorine-containing polymer; step 2, dissolving the fluorine-containing polymer in an organic solvent, adding a lithium salt to obtain a polymer system; dispersing an inorganic filler in an organic solvent and adding it into the polymer system to mix, obtaining a mixture; pouring the polymer system on a polytetrafluoroethylene plate to form a film, and drying to obtain a composite solid-state electrolyte film; based on the total weight of the synthesized fluorine-containing composite lithium ion solid-state electrolyte, the content of the fluorine-containing monomer is 24-86 wt%, the content of the silane coupling agent is 3-56 wt%, the content of the lithium salt is 4-28 wt%, and the content of the inorganic filler is 3-16 wt%; the molar ratio of the silane coupling agent to the fluorine-containing monomer is 1:1-19; the silane coupling agent is 3-(methacryloyloxy)propyl trimethoxysilane; and the fluorine-containing monomer is trifluoroethyl methacrylate, trifluoroethyl acrylate, hexafluorobutyl methacrylate, or hexafluorobutyl acrylate. However, the prior art is to obtain a fluorine-containing polymer by copolymerization of a fluorine-containing monomer and a silane, and then mix it with a lithium salt to improve ionic conductivity, but it lacks a crosslinking network design, and the mechanical properties have not been effectively improved.

[0004] The prior art CN115000499A discloses a fluoride-composite solid-state electrolyte film and a preparation method and application thereof. The solid-state electrolyte is prepared by pouring and hot pressing after uniformly mixing an organic polymer, a sodium salt and a fluoride in an organic solvent. The introduction of fluoride is to form a stable sodium fluoride interface directly or through a chemical reaction at the interface between the electrode and the electrolyte during the charging and discharging of the battery, which not only can improve the ion transmission rate in the solid-state electrolyte, but also can avoid the occurrence of adverse side reactions at the interface between the electrolyte and the sodium metal, enhance the interface stability of the sodium negative electrode, and thus improve the electrochemical performance of the solid-state electrolyte. When applied to a solid-state sodium ion battery, the sodium ion battery has excellent cycle and rate performance, the service life of the sodium battery is prolonged, and the electrochemical window of the all-solid-state sodium battery is widened. However, the prior art introduces a fluoride additive to form an interface layer and modify the organic polymer, which is difficult to balance the synergistic optimization of dendrite protection and ion transmission efficiency. SUMMARY

[0005] The purpose of the present application is to provide a composite solid-state electrolyte and a preparation method and application thereof, which significantly reduces the interface impedance and promotes the efficient dissociation of lithium salt by constructing a fluorinated layer and a siloxane network in steps; and improves the tensile strength and lithium ion migration number by combining the three-dimensional network structure of the polymer and the gradient filler design.

[0006] In order to achieve the above purpose, the technical scheme adopted by the present application is:

[0007] A preparation method of a composite solid-state electrolyte, comprising the following steps:

[0008] S1, mixing inorganic fillers and fluorine-containing compounds, reacting under acidic conditions at 70-90 DEG C to obtain inorganic fillers wrapped with fluorinated layer; then adding fluorine ion complexing agent and silane coupling agent to obtain modified fillers;

[0009] Adding a thermally decomposable crosslinking agent to the polymer and melt blending to obtain a crosslinked polymer;

[0010] S2, uniformly mixing the modified fillers, the crosslinked polymer, the lithium salt and the solvent to obtain a slurry containing different volume concentrations of modified fillers; forming the slurry to prepare a composite solid-state electrolyte;

[0011] The composite solid-state electrolyte comprises a surface layer, a middle layer and an inner layer stacked in sequence; the volume fraction of the modified fillers in the solid components in the surface layer is 30-40 vol%, the volume fraction of the modified fillers in the solid components in the middle layer is 15-25 vol%, and the volume fraction of the modified fillers in the solid components in the inner layer is 5-10 vol%;

[0012] The fluorine-containing compound is one or more of ammonium fluoride, ammonium bifluoride and sodium fluoride.

[0013] The polymer is polyvinylidene fluoride-hexafluoropropylene copolymer or polyvinylidene fluoride.

[0014] When polyvinylidene fluoride is used, the melt blending temperature is ≥175℃.

[0015] The solid component in the surface layer, middle layer or inner layer of the composite solid electrolyte is a crosslinked polymer and a modified filler.

[0016] The present application innovatively integrates a triple synergistic mechanism: through step-by-step construction of a fluorinated layer and a siloxane network, the interface impedance is significantly reduced by more than 30% and the efficient dissociation of lithium salt is promoted; combined with a three-dimensional network structure formed by a thermal decomposition type crosslinking agent crosslinking the polymer, the tensile strength is increased to ≥8MPa, while the ionic conductivity is maintained at ≥1×10 -3 S / cm (25℃); further through gradient design of the modified filler, a dense dendritic crystal barrier of 30-40vol% high dielectric filler is formed in the surface layer, 15-25% is reserved in the middle layer, and 5-10vol% of the filler is reserved in the inside to maintain continuous ion channels, so that the lithium ion migration number is increased to ≥0.6.

[0017] According to the embodiments of the present application, the present application can be further optimized, and the following is the technical scheme formed after optimization:

[0018] In one preferred embodiment, the thickness of the fluorinated layer is 10-50nm, and the dielectric constant is not less than 45.

[0019] When the thickness of the fluorinated layer is too low, the fluorinated layer (LiF / Li2O composite phase) forms a dense structure due to too high grain boundary density, which blocks the Li + transport, resulting in a decrease in ionic conductivity. In addition, when the thickness of the fluorinated layer is too low, it may not completely cover the surface defects of the filler, increasing the risk of dendritic puncture.

[0020] When the thickness of the fluorinated layer is too thick, the high dielectric constant induces Li + enrichment at the fluorinated layer / polymer interface, accelerating dendritic nucleation. At the same time, when the thickness of the fluorinated layer is too thick, during the hot pressing and cooling process, the fluorinated layer and the polymer may have a thermal expansion mismatch, causing interface stress cracking and causing microcracks.

[0021] In one preferred embodiment, the amount of the fluorine-containing compound added is 0.5-2wt% of the inorganic filler.

[0022] In one preferred embodiment, the inorganic filler is a perovskite type ceramic powder or titanium dioxide; the perovskite type ceramic powder is barium titanate or strontium titanate.

[0023] In one preferred embodiment, when the inorganic filler is titanium dioxide, it is in the rutile crystal form.

[0024] In one preferred embodiment, the average particle size of the inorganic filler is 1-10 μm.

[0025] In one preferred embodiment, the silane coupling agent is one or more of γ-glycidoxypropyltrimethoxysilane (KH560), γ-aminopropyltriethoxysilane (KH550), or γ-methacryloyloxypropyltrimethoxysilane (KH570).

[0026] In one preferred embodiment, the silane coupling agent is added in an amount of 1-3 wt% of the inorganic filler.

[0027] Too little silane coupling agent will result in insufficient surface coverage, causing ion transport blockage; too much silane coupling agent will cause side reactions to consume crosslinking agents, and too much silane coupling agent will also cause the interface to become brittle due to excessive multilayer self-condensation of the silane coupling agent. In addition, too much silane coupling agent will also increase the viscosity of the system, making slurry processing more difficult.

[0028] In one preferred embodiment, the acidic condition is an environment with a pH of 3-6.

[0029] In one preferred embodiment, the fluoride ion complexing agent is an aliphatic hydroxy acid containing 2-4 carboxyl groups, preferably citric acid, tartaric acid, or malic acid.

[0030] In one preferred embodiment, the thermal decomposition type crosslinking agent is one or more of dicumyl peroxide (DCP), benzoyl peroxide (BPO), or azobisisobutyronitrile (AIBN).

[0031] In one preferred embodiment, the thermal decomposition type crosslinking agent is added in an amount of 0.1-1.5 wt% of the polymer.

[0032] By adjusting the amount of crosslinking agent added (0.1-1.5 wt%), the mechanical properties are strengthened while maintaining the ability of chain segment movement, ensuring ion transport efficiency.

[0033] In one preferred embodiment, the lithium salt is one or more of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), or lithium hexafluorophosphate (LiPF6).

[0034] In one preferred embodiment, the lithium salt is added in an amount of 10-15 wt% of the crosslinked polymer.

[0035] In one preferred embodiment, the solvent is added in an amount of 60-80 wt% of the total mass of the slurry.

[0036] In one preferred embodiment, the solvent is one or more of N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), tetrahydrofuran (THF), dimethylacetamide (DMAc), and acetone (ACE).

[0037] In one preferred embodiment, the slurry is one-step formed or two-step compounded in step S2.

[0038] In one preferred embodiment, the two-step compounding of the slurry includes: first, preparing a single-layer homogeneous film for each slurry by hot-pressing, solution casting, or 3D printing, then stacking the single-layer homogeneous films according to the concentration of the modified filler from low to high, and finally hot-pressing at 180-200℃ and 8-12MPa for 10-15 minutes to obtain the composite solid-state electrolyte.

[0039] In one preferred embodiment, the hot-pressing process is at a temperature of 160-180℃, a pressure of 5-10MPa, and a time of 5-10 minutes.

[0040] In one preferred embodiment, the solution casting process is: casting the slurry onto a substrate, leveling, and vacuum drying at 80-100℃ for 24-48 hours.

[0041] In one preferred embodiment, the 3D printing process is: a 3D printing nozzle temperature of 170-190℃, a printing speed of 5-15mm / s, and a layer thickness of 50-100μm.

[0042] In one preferred embodiment, the one-step forming of the slurry includes: depositing each slurry by multi-channel 3D printing according to the concentration of the modified filler from low to high, and one-step forming to obtain the composite solid-state electrolyte.

[0043] In one preferred embodiment, the multi-channel 3D printing process is: using a coaxial multi-channel 3D printer, loading the slurry into the hoppers, and extruding the three layers of slurry in order according to the concentration gradient of the filler; a nozzle temperature of 180-220℃, a printing speed of 8-12mm / s, a layer thickness of 50-200μm, and an interlayer cooling time of ≤10 seconds.

[0044] Based on the same inventive concept, the present application also claims the composite solid-state electrolyte prepared by the preparation method.

[0045] In one preferred embodiment, the ionic conductivity of the composite solid-state electrolyte is ≥8×10 -4 S / cm (25℃, EIS test), a tensile strength of ≥8MPa (ASTM D638), and an electrochemical window of ≥5.0V vs. Li +Linear sweep voltammetry (Li, scan rate 0.5 mV / s), lithium ion transference number ≥ 0.6 (Bruce-Vincent method).

[0046] In one preferred embodiment, the composite solid-state electrolyte has a thickness of 50-200 μm.

[0047] Based on the same inventive concept, the application also claims a battery comprising the composite solid-state electrolyte.

[0048] In one preferred embodiment, the battery is a lithium-ion battery, a sodium-ion battery or a lithium-sulfur battery.

[0049] The lithium-ion battery has a stable cycle life of ≥ 500 hours at 0.2 mA / cm 2

[0050] The application is further explained as follows:

[0051] The application innovatively combines a triple synergistic mechanism: through step-by-step construction of a fluorinated layer and a siloxane network; combined with a three-dimensional network structure formed by crosslinking the polymer with a thermal decomposition type crosslinking agent and gradient filler design, so that the three of “interface modification to reduce transmission resistance - crosslinking to strengthen the stability of the structure - gradient design to optimize the force / electric field distribution” form a closed loop optimization, forming the following core effects: (1) breaking the contradiction between “high ionic conductivity - low mechanical strength” of traditional electrolytes; (2) solving the coupled problem of dendrite growth and interface failure in solid-state batteries; (3) achieving long-term lithium symmetric battery cycling (Example 1, lithium symmetric battery stable cycle life ≥ 500 hours at 0.2 mA / cm 2 Current density, 600 hours of stable cycle life of lithium symmetric battery at 0.2 mA / cm

[0052] The synergistic system enables the electrolyte to achieve a stable cycle life of lithium symmetric battery ≥ 500 hours at 0.2 mA / cm 2 Current density, 40% longer cycle life, 3 times higher ionic conductivity, and compatible with hot-pressing, 3D printing and other industrialization processes, providing a reliable solution for high-energy-density, high-safety energy storage devices.

[0053] The mechanism of the application is explained as follows:

[0054] Mechanism and mechanism of synchronous construction of fluorinated layer and siloxane network

[0055] The fluorinated layer (rigid ion conductor) and the siloxane network (flexible interface bridge) are atomically constructed on the surface of the filler in a three-step coupling reaction.

[0056] I. Stepwise reaction path and spatial positioning ​

[0057] (1) Fluorinated layer in-situ construction

[0058] Under acidic conditions, fluorine-containing compounds decompose to release F - , which is embedded into the perovskite lattice through ion exchange reaction, reducing the Li

[0059] .

[0060] (2) Siloxane network epitaxial growth

[0061] After the construction of the fluorinated layer, the reaction system is not changed, and silane coupling agent is directly added to trigger the following reactions (taking KH560 as an example) in the residual acidic environment:

[0062] Hydrolysis reaction:

[0063] .

[0064] Condensation reaction:

[0065] (bonding with the fluorinated layer).

[0066] (network self-assembly).

[0067] II. Interface chemical bonding mechanism

[0068] (1) Covalent bonding

[0069] The Si-O bonds of the siloxane network form Si-O-Ti bonds with the Ti atoms on the surface of the fluorinated layer; the residual F- forms Si-F bonds with Si-OH.

[0070] (2) Ion-dipole interaction

[0071] The high electronegativity of F- in the fluorinated layer produces electrostatic attraction with the epoxy groups of silane.

[0072] The fluorinated layer is an inner rigid skeleton, and the siloxane network is an outer flexible coating layer.

[0073] III. Synergistic reinforcement reaction of synchronous construction

[0074] The fluorinated layer reduces the Li + migration energy barrier, and the siloxane network eliminates the interface gap, synergistically reducing the total interface impedance; the fluorinated layer increases the modulus of the filler, and the siloxane network forms an elastic buffer layer, synergistically improving the mechanical strength; the fluorinated layer promotes the dissociation of lithium salt, and the siloxane network provides continuous transition sites, synergistically improving the ion transport efficiency.

[0075] 2. Three-dimensional crosslinking network formation mechanism of thermal decomposition type crosslinking agent in polymer

[0076] (1) Radical initiation and chain reaction

[0077] Thermal decomposition of crosslinking agent occurs homolytic decomposition at a certain temperature, generating high activity free radicals (take DCP as an example):

[0078] .

[0079] Cumene oxide free radical (·C(CH3)2C(O)CH3) further decomposes into acetone and methyl free radical (·CH3)

[0080] (2) Polymer chain activation

[0081] The generated free radicals attack the molecular chain of polymers (such as PVDF-HFP):

[0082] Free radicals (R·) take the hydrogen atom of the methylene (-CH2-) or tertiary carbon site in the molecular chain of polymers such as PVDF-HFP (the strong electronegativity of fluorine atom makes the adjacent C-H bond easier to be attacked), forming macromolecular free radicals (P·);

[0083] .

[0084] Adjacent macromolecular free radicals form C-C covalent crosslinking bonds through coupling reaction, which continues in the polymer melt, eventually building a three-dimensional network structure throughout the matrix (such as Figure 2 ).

[0085] .

[0086] 3. Synergistic mechanism of interface modification-crosslinking reinforcement-gradient structure

[0087] (1) Functional complementarity synergy

[0088] Filler interface modification: reduce interface impedance, provide low resistance ion channels for gradient structure;

[0089] Polymer crosslinking reinforcement: improve mechanical strength, support gradient structure against dendrite puncture;

[0090] Gradient structure design: optimize ion / stress distribution, amplify the effectiveness of interface modification and crosslinking.

[0091] (2) Cross-scale structure coupling mechanism

[0092] Nanoscale: interface-matrix molecular interlocking

[0093] ​​The epoxy group of silane coupling agent forms "dipole-dipole" interaction with C-F bond of cross-linked PVDF-HFP, reducing the filler / matrix interface energy; free radicals in the cross-linked network capture residual chloride ions, preventing them from corroding lithium salts (such as LiPF6), and improving interface stability.

[0094] Microscale: Gradient-crosslinking stress synergy

[0095] The surface layer (high filler) bears most of the external stress and disperses it to the interior through the cross-linked network; the interior (low filler) absorbs stress from the flexible matrix, inhibiting the propagation of microcracks.

[0096] Macro scale: Electrochemical-mechanical performance balance

[0097] The gradient surface layer high dielectric barrier (fluoride + high filler) increases the critical penetration pressure of dendrites; the cross-linked network maintains the toughness of the matrix, enabling high ionic conductivity and high mechanical strength to be achieved simultaneously.

[0098] (3) Kinetic synergy

[0099] The strong polarization field of the fluorinated layer reduces the energy barrier of lithium salts (TFSI + anions (TFSI - , promoting the accelerated dissociation of lithium salts;

[0100] The siloxane network provides low-energy barrier transition sites, improving interface transitions;

[0101] The gradient internal low filler region forms a continuous percolation channel, improving ion migration number and enhancing lithium ion bulk transport.

[0102] Compared with the prior art, the present application has the following beneficial effects:

[0103] The present application realizes the comprehensive improvement of the performance of composite solid electrolyte through the synergistic effect of interface modification, cross-linking reinforcement, and gradient structure design. This is specifically reflected in the following aspects:

[0104] 1. Interface and electrochemical performance synergy optimization

[0105] Through the synchronous construction of the fluorinated layer and the siloxane network, the filler-matrix interface impedance is significantly reduced by more than 30%, while promoting the efficient dissociation of lithium salts, making the ionic conductivity of the electrolyte increase to ≥8×10 -4 S / cm (25℃).

[0106] 2. Mechanical strength and thermal stability enhancement

[0107] The thermal decomposition type crosslinking agent such as dicumyl peroxide (DCP), benzoyl peroxide (BPO) and the like is used to form a three-dimensional crosslinked network in the fluorine-containing polymer matrix, so that the tensile strength of the material reaches ≥8 MPa (ASTM D638 standard test), and the thermal decomposition temperature is increased to above 380℃. While strengthening the mechanical properties, the chain segment movement ability is maintained to ensure the ion transmission efficiency.

[0108] 3. Gradient structure inhibits dendrite growth

[0109] Through the design of gradient composite structure, 20-30vol% high dielectric filler is introduced in the surface layer of the electrolyte to form a dense barrier, which effectively inhibits the penetration of lithium dendrites; the internal region maintains a filler content of 5-15vol% to retain continuous ion channels. Combined with the optimization of lithium salt concentration, the polymer crystallinity is reduced to below 30%, and the lithium ion transference number is increased to ≥0.6, realizing the synergistic effect of ion transmission and dendrite protection.

[0110] 4. Process adaptability and application expandability

[0111] The electrolyte is compatible with various preparation processes such as hot pressing, 3D printing, etc., and is suitable for solid-state lithium ion batteries, solid-state sodium ion batteries and lithium-sulfur batteries. By adjusting the fluorinated layer thickness (10-50nm), crosslinking density (gel fraction 60-90%) and lithium salt concentration (5-20wt%), the electrochemical performance (electrochemical window ≥5.0V vs. Li+ / Li) and mechanical performance (elongation at break ≥150%) can be customized to balance. Lithium symmetric battery at 0.2mA / cm 2 Current density, stable cycle ≥500 hours, providing a reliable technical solution for high-safety energy storage devices. BRIEF DESCRIPTION OF DRAWINGS

[0112] Figure 1 is a schematic diagram of the principle and structure of inorganic filler modification.

[0113] Figure 2 is a schematic diagram of polymer crosslinking.

[0114] Figure 3 is a cross-section high magnification microscope of the gradient electrolyte membrane.

[0115] Figure 4 is the ion conductivity curve of Example 1.

[0116] Figure 5 is the ion conductivity curve of Example 2.

[0117] Figure 6 is the ion conductivity curve of Example 3.

[0118] Figure 7 is the ion conductivity curve of Example 4.

[0119] Figure 8 Ion conductivity curve of Comparative Example 1.

[0120] Figure 9 Ion conductivity curve of Comparative Example 2.

[0121] Figure 10 Ion conductivity curve of Comparative Example 3.

[0122] Figure 11 Ion conductivity curve of Comparative Example 4.

[0123] Figure 12 Lithium symmetric cell voltage-time curve of Example 1.

[0124] Figure 13 SEM image of porous electrolyte membrane of Example 3. DETAILED DESCRIPTION

[0125] The present application is not limited to the following detailed description, and those skilled in the art can implement the present application in other various embodiments according to the disclosure of the present application, or fall within the scope of the present application by using the design structure and ideas of the present application with simple changes or modifications. It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict.

[0126] In the present application, the test temperature of the ion conductivity of the electrolyte membrane is 25±5℃, which is obtained by EIS test. The tensile strength is obtained by ASTM D638 test. The test method of the electrochemical window is linear sweep voltammetry, the scan rate is 0.5mV / s, and the test method of the lithium ion transference number is Bruce-Vincent method.

[0127] Example 1

[0128] A preparation method of a composite solid-state electrolyte based on a modified barium titanate filler, the specific implementation steps are as follows:

[0129] 1. Surface modification of filler

[0130] 10 g of barium titanate (BaTiO3, average particle size 1 μm) was dispersed in 200 mL of ethanol / water mixture (volume ratio 8:2) to form a uniform suspension by ultrasonic treatment for 30 minutes. First, 0.15 g of ammonium fluoride (1.5% of the mass of the filler) was added, the pH of the system was adjusted to 4.5 with dilute hydrochloric acid, and the system was stirred at 80°C for 4 hours to allow the fluoride ions to replace the surface oxygen ions of the barium titanate to form a fluorinated layer; then, while maintaining the same pH and temperature, 0.2 g of silane coupling agent KH560 (2% of the mass of the filler) and 0.03 g of citric acid (0.3% of the mass of the filler) were added, and the stirring was continued for 2 hours to complete the hydrolysis and condensation reaction of KH560, while the citric acid complexed the residual fluoride ions to inhibit side reactions. After the reaction was completed, the product was centrifuged (8000 rpm, 10 minutes), washed with ethanol three times, and dried at 80°C under vacuum for 12 hours to obtain modified barium titanate (BT@F-KH560) coated with a fluorinated layer and a siloxane network. The schematic diagram of the principle of modification of barium titanate and the schematic diagram of the structure are shown in Figure 1

[0131] 2. Crosslinking of the polymer matrix

[0132] 10 g of PVDF-HFP (HFP content 12 wt%) and 0.05 g of dicumyl peroxide (DCP, 0.5% of the mass of PVDF-HFP) were dry mixed uniformly, and then melt blended by a twin-screw extruder at a temperature of 180-200°C and a rotation speed of 50 rpm, with a residence time of 5 minutes. The free radicals generated by the decomposition of DCP at high temperature initiate the crosslinking of the C-F bonds between the molecular chains of PVDF-HFP, forming a three-dimensional network structure of the crosslinked polymer. The schematic diagram of the crosslinked polymer is shown in Figure 2

[0133] 3. Formation of the composite electrolyte membrane

[0134] The crosslinked polymer, surface-modified barium titanate (BT@F-KH560), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, 15% of the mass of the crosslinked polymer) were ball-milled in N-methylpyrrolidone (75 wt% of the total mass of the slurry) for 4 hours (300 rpm) to obtain a surface layer slurry. The volume fraction of BT@F-KH560 filler in the solid components (crosslinked polymer + surface-modified barium titanate) in the surface layer slurry was 30%, and the volume fraction of the crosslinked polymer was 70%.

[0135] The crosslinked polymer, surface-modified barium titanate (BT@F-KH560), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, 15% of the mass of the crosslinked polymer) were ball-milled in N-methylpyrrolidone solvent for 4 hours (300 rpm) to obtain a middle layer slurry. The volume fraction of BT@F-KH560 filler in the solid components in the middle layer slurry was 15%, and the volume fraction of the crosslinked polymer was 85%. ​​

[0136] The cross-linked polymer, surface-modified barium titanate (BT@F-KH560), and lithium bis (trifluoromethanesulfonyl) imide (LiTFSI, accounting for 15% of the mass of the cross-linked polymer) were ball-milled in N-methylpyrrolidone solvent for 4 hours (300 rpm) to obtain a bottom layer slurry. In the bottom layer slurry, the volume fraction of the BT@F-KH560 filler in the solid components was 5%, and the volume fraction of the cross-linked polymer was 95%.

[0137] The three-layer slurry was successively stacked and coated, and was formed by hot pressing at 185°C and 12 MPa for 12 minutes to obtain a three-layer gradient structure electrolyte film with a total thickness of 150μm. The cross-sectional gradient distribution of the three-layer gradient structure electrolyte film was characterized by a high-power microscope (Keyence VHX-X1) as shown in Figure 3 .

[0138] The three-layer gradient structure electrolyte film was subjected to scanning electron microscopy (SEM), as shown in Figure 4 .The results showed that the surface layer was dense and had no pores, and the ionic conductivity at 25°C was 9.43×10 -4 S / cm Figure 4 ; the lithium ion transference number (Bruce-Vincent method) was 0.65.

[0139] The three-layer gradient structure electrolyte film was assembled into a lithium symmetric battery and its performance was tested. The process was as follows: a three-layer gradient structure electrolyte film with a diameter of 16mm was sandwiched between two metal lithium sheets with a diameter of 15mm and a thickness of 200μm, and was packaged in a CR2032 type battery shell under a pressure of 10MPa to obtain a lithium symmetric battery. The cycle performance of the lithium symmetric battery was tested at 0.2mA / cm 2 . The results showed that the lithium symmetric battery was stable and cycled for 600 hours without dendrite formation at 0.2mA / cm 2 . Figure 12

[0140] The mechanical properties of the three-layer gradient structure electrolyte film were analyzed, and the results showed that the tensile strength was 9.5MPa and the elongation at break was 180%.

[0141] Example 2

[0142] On the basis of Example 1, barium titanate was replaced by strontium titanate (SrTiO3, average particle size 0.8μm), and the addition amounts of NH4F and KH560 were 1% and 3% of the mass of the filler, respectively. The modified SrTiO3@F-KH560 filler was prepared by reacting at 75°C for 7 hours at pH 5.0.

[0143] The gradient electrolyte film was prepared by the same three-layer structure hot pressing process as in Example 1, i.e. hot pressing at 185°C and 12 MPa for 12 minutes. The performance of the prepared gradient electrolyte film was as follows:

[0144] Regarding structural characteristics, scanning electron microscopy (SEM) images show that its surface exhibits a dense, non-porous structure. Electrochemical performance tests indicate that the ionic conductivity measured by electrochemical impedance spectroscopy (EIS) at 25℃ is 8.84 × 10⁻⁶. -4 S / cm ( Figure 5 The lithium-ion transport number, determined using the Bruce-Vincent method, was 0.65. Mechanical property tests showed a tensile strength of 12.8 MPa and an elongation at break of 180%, as measured according to ASTM D638. The lithium-ion symmetric battery operates at 0.2 mA / cm². 2 No dendrite formation was observed after 500 hours of cycling at the current density.

[0145] Example 3

[0146] Based on Example 1, DCP was replaced with benzoyl peroxide (BPO, added at 0.8 wt% of the polymer), and PVDF-HFP crosslinking was completed at a melting temperature of 170°C using a twin-screw extruder (temperature gradient 165-185°C, speed 45 rpm).

[0147] Everything else is the same as in Example 1.

[0148] Crosslinked PVDF-HFP, modified filler (BT@F-KH560), and LiFSI (based on 15% of the mass of crosslinked PVDF-HFP) were dissolved in DMF solvent (75wt% of the total slurry mass). After thorough mixing, the three slurries were deposited layer by layer using a 3D printing device (using a coaxial multi-channel 3D printer, where the slurries were loaded into the hoppers and extruded in sequence according to the filler concentration gradient; three nozzles were used to print the three slurries respectively, with a nozzle temperature of 190℃ and a printing speed of 10mm / s). After vacuum drying at 80℃ for 48 hours, a porous electrolyte membrane was obtained. The porous electrolyte membrane was analyzed by SEM, and the results are as follows: Figure 13 As shown in the figure. Tests indicate that the membrane porosity is 35% and the ionic conductivity reaches 8.60 × 10⁻⁶. -4 S / cm ( Figure 6 ).

[0149] Example 4

[0150] Based on Example 1, lithium salt LiTFSI was replaced with sodium salt NaTFSI (10 wt% of the crosslinked polymer mass), and La was used as the filler. 3+ Barium titanate doped (BaTiO3:La, chemical formula Ba) 0.95 La 0.05TiO3, preparation method and structure characterization refer to the prior art: GUO S, TAN S, MA J, etc. A dielectric electrolyte composite with high lithium-ion conductivity for high-voltage solid-state lithium metal batteries [J]. Energy & Environmental Science, 2024, 17: 3797-3806 and the prior art SHI P, MA J, LIU M, etc. Dissociation mechanism of lithium salt by BaTiO3 with spontaneous polarization [J]. Nature Nanotechnology, 2023, 18: 602-610), modified and composite film preparation according to the process of Example 1.

[0151] The results of the performance test of the composite film are:

[0152] The obtained electrolyte film (diameter 16 mm) was sandwiched between a sodium metal negative electrode (diameter 15 mm, thickness 200 μm) and a Na3V2(PO4)3 positive electrode, which was composed of 85wt% Na3V2(PO4)3, 10wt% Super P conductive agent, and 5wt% PVDF binder, and the positive electrode active material surface load was 8 mg / cm 2 ; packaged in a CR2032 type battery shell under a pressure of 10 MPa, and the whole process was operated in an argon glove box (water and oxygen value <0.1 ppm) to obtain a Na| electrolyte | Na3V2(PO4)3 battery (structure reference prior art: SHANG J, MA Y, FAN H J S. Preparation of Na3V2(PO4)3 sodium-ion battery cathode material [J]. Asia-Pacific Journal of Chemical Engineering, 2023, 18(3): e2882). After testing, the sodium ion conductivity at 25°C was 9.66×10 -4 S / cm ( Figure 7 ); the Na| electrolyte | Na3V2(PO4)3 battery was tested by a new Wei BTS-5V10mA battery test system, and the capacity retention rate was more than 90% after 200 cycles at 0.5C rate, and the coulombic efficiency was stable at more than 99.2%.

[0153] Comparative Example 1

[0154] Unmodified filler and un-crosslinked PVDF-HFP

[0155] On the basis of Example 1, unmodified BaTiO3 and un-crosslinked PVDF-HFP were used, and the rest of the process was the same as Example 1. The ionic conductivity of the prepared electrolyte was only 2.32 x 10 -4 S / cm (T=25℃) Figure 8 , the tensile strength dropped to 1.7 MPa, and the lithium symmetrical battery 0.2 mA / cm 2 After 200 hours of cycling, dendrites appeared.

[0156] Comparative Example 2

[0157] On the basis of Example 1, unmodified BaTiO3 was used, and the rest of the process was the same as Example 1. The prepared electrolyte was analyzed by scanning electron microscopy, and it was found that there was obvious phase separation at the filler-matrix interface. The ionic conductivity at 25℃ was 4.37 x 10 -4 S / cm (T=25℃) Figure 9 The lithium ion transference number measured by Bruce-Vincent method was 0.31. The tensile strength decreased to 5.2 MPa. The lithium symmetrical battery 0.2 mA / cm 2 After 73 hours of cycling, dendrite penetration occurred.

[0158] Comparative Example 3

[0159] On the basis of Example 1, un-crosslinked PVDF-HFP was used, and the rest of the process was the same as Example 1. The prepared electrolyte was analyzed by differential scanning calorimetry (DSC), and it was found that the glass transition temperature was greatly reduced (Tg=-42℃), and the thermal shrinkage rate at 80℃ reached 15.8%. The ionic conductivity at 25℃ was 7.05 x 10 -4 S / cm (T=25℃) Figure 10 The lithium ion transference number measured by Bruce-Vincent method was 0.38. The tensile strength decreased to 5.2 MPa. The lithium symmetrical battery 0.2 mA / cm 2 After 48 hours of cycling, dendrite penetration occurred. After 24 hours of high-temperature storage at 60℃, the membrane body deformed significantly.

[0160] Comparative Example 4

[0161] On the basis of Example 1, no gradient filler design was used, and a single concentration of filler design was used, with the following specific operations:

[0162] 8g of crosslinked polymer, 2g of surface-modified barium titanate (BT@F-KH560), and 1.2g of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, accounting for 15% of the crosslinked polymer mass) were ball-milled in N-methylpyrrolidone solvent for 4 hours (300 rpm). After vacuum degassing, the mixture was hot-pressed at 190℃ and 10MPa for 10 minutes to obtain a homogeneous electrolyte membrane with a thickness of 100μm. Other steps were the same as in Example 1.

[0163] The properties of the prepared electrolyte are as follows: Electrochemical impedance spectroscopy (EIS) analysis showed an ionic conductivity of 1.30 × 10⁻⁶ at 25 °C. -3 S / cm ( Figure 11 The dielectric constant (1kHz) test showed ε'=48; the tensile strength test showed 6.5MPa; and the elongation at break was 135%. The test was conducted on a lithium-ion symmetric battery at 0.2mA / cm². 2 Cycling performance at current density showed that dendrite penetration occurred after 500 hours of stable cycling in the lithium symmetric battery.

[0164] Comparative Example 5

[0165] Based on Example 1, instead of using fluoride-modified fillers, only silane coupling agents were used for modification, resulting in inorganic fillers coated solely with coupling agents. Everything else remained the same as in Example 1.

[0166] The performance characterization of the obtained electrolyte membrane shows that:

[0167] In terms of microstructure, scanning electron microscopy (SEM) showed local voids at the interface between the filler and the polymer matrix, and energy dispersive spectroscopy (EDS) confirmed the absence of fluorine at the interface.

[0168] In the electrochemical performance test, the ionic conductivity at 25℃ was 7.85 × 10⁻⁶. -4 S / cm (significantly lower than in Example 1), and lithium-ion transference number reduced to 0.35.

[0169] The mechanical properties were as follows: tensile strength of 9.2 MPa and elongation at break of 125%, which were 29.2% and 30.6% lower than those of Example 1, respectively.

[0170] In battery cycle testing, the lithium symmetric battery achieved a speed of 0.2 mA / cm². 2 Voltage fluctuations occurred after 214 hours of cycling at the current density.

[0171] Comparative Example 6

[0172] Based on Example 1, instead of using silane coupling agents to modify the filler, only fluoride modification was used to obtain an inorganic filler with sole fluoride coating. Everything else is the same as in Example 1.

[0173] Performance test results show that:

[0174] Microstructure analysis showed that the filler particles agglomerated under high magnification microscopy, and X-ray photoelectron spectroscopy (XPS) confirmed that the characteristic peak of surface silicon element disappeared, and the distribution of fluorine element showed local enrichment.

[0175] In terms of electrochemical performance, the ionic conductivity at 25°C was 6.92×10 -4 S / cm (significantly lower than Example 1), the lithium ion transference number was 0.37, and the electrochemical window was narrowed to 4.5V (vs. Li + / Li).

[0176] Mechanical performance test showed that the tensile strength was 10.8MPa, and the elongation at break decreased to 142%.

[0177] Battery performance showed that the lithium symmetric battery cycled at 0.2mA / cm 2 Voltage oscillation occurred after 158 hours of cycling, and delamination of the electrode-electrolyte interface was observed after disassembly.

[0178] Comparative Example 7

[0179] Based on Example 1, the filler surface modification process was adjusted: silane coupling agent KH560 and ammonium fluoride (NH4F) were pre-mixed and reacted in ethanol solvent to prepare a fluorinated polymer solution, and then the barium titanate filler was coated once, and the rest of the process parameters were kept consistent with Example 1.

[0180] Performance characterization showed:

[0181] In microstructure analysis, scanning electron microscopy (SEM) showed that the thickness of the coating layer on the surface of the filler was uneven, and X-ray photoelectron spectroscopy (XPS) detected the characteristic peak of Si-F bond (687.2eV), while the intensity of the filler characteristic peak attenuated.

[0182] In terms of electrochemical performance, the ionic conductivity at 25°C decreased to 6.24×10 -4 S / cm (significantly lower than Example 1), the lithium ion transference number was 0.33.

[0183] In thermal mechanical performance test, the tensile strength was 7.5MPa (42.3% lower than Example 1), and the thermal shrinkage rate at 80°C reached 8.7%.

[0184] Battery cycle test showed that the lithium symmetric battery cycled at 0.2mA / cm 2 Internal short circuit occurred after 110 hours of cycling.

[0185] Comparative Example 8

[0186] On the basis of Example 1, methyl acrylate trifluoroethyl ester was used as a fluorine-containing compound to replace ammonium fluoride, and the rest was the same as Example 1.

[0187] The performance test results show that:

[0188] In the microstructure characterization, transmission electron microscopy (TEM) shows that an amorphous organic coating layer is formed on the surface of the filler, with a thickness of about 25 nm.

[0189] In terms of electrochemical performance, the ionic conductivity at 25°C is 6.21×10 -4 S / cm (significantly lower than Example 1), and the lithium ion transference number decreases to 0.31.

[0190] Mechanical performance testing shows that the tensile strength is 8.3 MPa, and the elongation at break decreases to 118%.

[0191] The battery performance shows that the lithium symmetric battery at 0.2mA / cm 2 After 85 hours of cycling at 0.2mA / cm

[0192] Comparative Example 9

[0193] On the basis of Example 1, uncrosslinked PVDF-HFP was used, and ammonium fluoride was used to modify PVDF-HFP, with the following specific operations:

[0194] 10g of PVDF-HFP (HFP content 12wt%) was weighed, and 10wt% of ammonium fluoride was dissolved in 10mL of acetonitrile, stirred on a stirring table at 25°C for 12h, to form a uniform solution. The rest was the same as Example 1.

[0195] The performance test results show that:

[0196] In the microstructure characterization, scanning electron microscopy (SEM) observed that sodium fluoride formed 1-5μm agglomerates in the polymer matrix, and energy spectrum area scanning confirmed that the fluorine element was locally enriched.

[0197] In terms of electrochemical performance, the ionic conductivity at 25°C is 7.02×10 -4 S / cm (significantly lower than Example 1), and the lithium ion transference number decreases to 0.28.

[0198] Mechanical performance testing shows that the tensile strength decreases to 7.8 MPa, and the thermal shrinkage rate at 80°C reaches 17.3%.

[0199] The battery performance shows that the lithium symmetric battery at 0.2mA / cm 2 Short circuit occurs after 87 hours of cycling at 0.2mA / cm

[0200] Comparative Example 10

[0201] A composite solid electrolyte was prepared according to the method described in Example 1 of CN114566699B: 3-(methacryloyloxy)propyltrimethoxysilane and trifluoroethyl methacrylate were dissolved in acetonitrile at a molar ratio of 1:5, 0.5wt% azobis isobutyronitrile was added, and the mixture was reacted at 80°C for 12 hours to obtain a fluorine-containing polymer; the fluorine-containing polymer, 10wt% lithium bis-trifluoromethanesulfonimide, and 15vol% barium titanate were mixed and cast into a film with a thickness of 100μm.

[0202] After mechanical property testing, the tensile strength of the electrolyte was 2.8MPa (ASTM D638), the elongation at break was 45%, and the lithium symmetrical battery had a voltage fluctuation (>100mV) after 150 hours of cycling at 0.1mA / cm 2 The mechanical properties of the electrolyte were significantly worse than those of Example 1.

[0203] Compared with Comparative Example 10, the tensile strength of the composite electrolyte film prepared in Example 1 of the present application was increased by 239%, the cycle life was prolonged by 300%, and the dendrite critical penetration pressure was >8.0MPa (microprobe indentation method).

[0204] It should be noted that the above examples are merely examples for clearly illustrating the present application and are not intended to limit the embodiments of the present application. Based on the above description, other different forms of changes or modifications can be made by those of ordinary skill in the art. It is impossible to exhaust all embodiments here. Any obvious changes or modifications derived from the technical solutions of the present application are still within the scope of protection of the present application.

Claims

1. A method for preparing a composite solid electrolyte, characterized in that, Includes the following steps: S1. Inorganic filler and fluorine-containing compound are mixed and reacted under acidic conditions at 70-90℃ to obtain inorganic filler coated with fluorine layer; then fluoride ion complexing agent and silane coupling agent are added and reacted to obtain modified filler; A thermally decomposable crosslinking agent is added to the polymer, and the polymer is melt-blended to obtain a crosslinked polymer. S2. Mix the above modified filler, crosslinked polymer, lithium salt and solvent evenly to obtain a variety of slurries containing different volume concentrations of modified filler; mold the various slurries to prepare a composite solid electrolyte; The composite solid electrolyte comprises a top layer, a middle layer, and an inner layer stacked sequentially; the modified filler in the top layer accounts for 30-40 vol% of the solid component by volume, the modified filler in the middle layer accounts for 15-25 vol% of the solid component by volume, and the modified filler in the inner layer accounts for 5-10 vol% of the solid component by volume. The fluorine-containing compound is one or more of ammonium fluoride, ammonium bifluoride, and sodium fluoride; The polymer is polyvinylidene fluoride-hexafluoropropylene copolymer or polyvinylidene fluoride.

2. The preparation method according to claim 1, characterized in that, The amount of the fluorinated compound added is 0.5-2 wt% of the inorganic filler.

3. The preparation method according to claim 1, characterized in that, The inorganic filler is perovskite ceramic powder or titanium dioxide, and the perovskite ceramic powder is barium titanate or strontium titanate.

4. The preparation method according to claim 1, characterized in that, The silane coupling agent is one or more of γ-glycidoxypropyltrimethoxysilane, γ-aminopropyltriethoxysilane, or γ-methacryloyloxypropyltrimethoxysilane; the amount of the silane coupling agent added is 1-3 wt% of the inorganic filler.

5. The preparation method according to claim 1, characterized in that, The acidic conditions are an environment with a pH of 3-6; the fluoride ion complexing agent is citric acid.

6. The preparation method according to claim 1, characterized in that, The thermally decomposable crosslinking agent is one or more of dicumyl peroxide, benzoyl peroxide, or azobisisobutyronitrile; the amount of the thermally decomposable crosslinking agent added is 0.1-1.5 wt% of the polymer.

7. The preparation method according to claim 1, characterized in that, In the slurry, the amount of lithium salt added is 10-15 wt% of the crosslinked polymer; and / or, the lithium salt is one or more of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, and lithium hexafluorophosphate.

8. The composite solid electrolyte prepared by the preparation method according to any one of claims 1-7.

9. The composite solid electrolyte according to claim 8, characterized in that, The ionic conductivity of the composite solid electrolyte is ≥8×10⁻⁶. -4 S / cm, tensile strength ≥8MPa, electrochemical window ≥5.0V, lithium-ion transference number ≥0.

6.

10. A battery, characterized in that, The battery includes the composite solid electrolyte as described in claim 8 or 9.

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