Composite electrolyte and preparation method and application thereof

Abstract

The invention discloses a composite electrolyte and a preparation method and application thereof, the composite electrolyte is a polyvinylidene fluoride garnet composite electrolyte, the composite electrolyte is milk white, the viscosity is 800cps to 50000cps, the ionic conductivity is 5 * 10 <-5 > Scm <-1 > to 5 * 10 <-4 > Scm <-1 >, and the oxidation potential is 4.5 V to 5V; the preparation method comprises the following steps: carrying out surface pretreatment on the garnet filler through anhydrous organic weak acid to obtain the treated garnet filler; adding the treated garnet filler into a polyvinylidene fluoride electrolyte solution to obtain composite electrolyte slurry; and preparing the composite electrolyte slurry into the composite electrolyte through a wet process. The composite electrolyte disclosed by the invention is greatly improved in the aspects of ionic conductivity, antioxidant window and ion transference number, and the preparation method is simple in process and suitable for industrial application.

Description

Technical Field

[0001] The present invention relates to the technical field of solid electrolytes, and more specifically, to a preparation method and application of a composite electrolyte. Background Art

[0002] With the popularization of portable electronic devices and the rise of new energy vehicles, the energy density of existing commercial liquid lithium-ion batteries is difficult to meet the growing demands of people. At the same time, traditional lithium-ion batteries based on organic electrolytes have safety hazards such as leakage, flammability, and explosiveness, which severely limit their development and application. Therefore, people are striving to find a battery system with high energy density and high safety that can replace liquid batteries. Solid electrolytes have a relatively wide electrochemical window and can theoretically match a metal lithium negative electrode with the lowest electrochemical potential (-3.04 V) and a high-voltage positive electrode to obtain a higher energy density. At the same time, the safety of solid electrolytes is much higher than that of electrolytes, which can fundamentally solve the safety problems of batteries.

[0003] Organic-inorganic composite electrolytes combine the flexibility of polymer electrolytes and the safety of ceramic electrolytes to become the most promising solid electrolyte materials. Among them, ceramic electrolyte fillers can not only improve the thermal stability of composite electrolytes, but also significantly improve the ionic conductivity and electrochemical window. Garnet-type ceramic electrolytes are regarded as the most promising composite electrolyte ceramic fillers due to their theoretical ionic conductivity as high as 10 -3 Scm -1 , an electrochemical window higher than 5 V, and excellent chemical stability to metallic lithium. In addition, polyvinylidene fluoride (PVDF) is regarded as the most promising polymer electrolyte due to its good mechanical properties and thermal stability. The PVDF-Garnet composite electrolyte prepared by combining the two can theoretically exhibit good performance. However, due to the addition of Garnet fillers, the PVDF molecules undergo a dehydrofluorination reaction, resulting in a decrease in the electrochemical window and lithium stability while increasing the ionic conductivity of the composite electrolyte. In addition, the dehydrofluorination reaction occurs during the slurry preparation process, leading to a significant increase in the slurry viscosity, making it difficult to disperse the fillers, and thus it is difficult to obtain a composite electrolyte membrane with a high filler content.

[0004] To address the above problems of PVDF-Garnet composite electrolytes, those skilled in the art usually adopt a method of coating the fillers to block the direct contact between PVDF molecules and Garnet fillers, thereby suppressing the occurrence of the dehydrofluorination reaction. The Y. Xu team (Advanced Energy Materials, 2023, 2204377) coated Li 6.4 La 3 Zr 1.4 Ta0.6 O 12 (LLZTO) achieved a PVDF / LLZTO composite electrolyte without hydrogen fluoride elimination. However, the ionic conductivity of the coating material is much lower than that of the LLZTO electrolyte, which weakens the lithium ion transport to a certain extent and makes the ionic conductivity of the composite electrolyte unable to reach the maximum value.

[0005] Therefore, there is an urgent need to provide a composite electrolyte that can ensure the stability of lithium ions and ionic conductivity, as well as its preparation method and application. Summary of the Invention

[0006] In view of this, the present invention provides a method for preparing a composite electrolyte and its application to ensure the stability of lithium ions and ionic conductivity.

[0007] On the one hand, the present invention provides a composite electrolyte, which is a polyvinylidene fluoride garnet composite electrolyte. The composite electrolyte is milky white, with a viscosity of 800 cps - 50000 cps and an ionic conductivity of 5×10 -5 Scm -1 -5×10 -4 Scm -1 , and the oxidation potential is 4.5V - 5V.

[0008] On the other hand, the present invention provides a method for preparing a composite electrolyte, including:

[0009] Performing surface pretreatment on garnet fillers with an anhydrous organic weak acid to obtain pretreated garnet fillers;

[0010] Adding the pretreated garnet fillers to a polyvinylidene fluoride electrolyte solution to obtain a composite electrolyte slurry;

[0011] Manufacturing the composite electrolyte slurry into a composite electrolyte through a wet process.

[0012] Optionally, the performing surface pretreatment on garnet fillers with an anhydrous organic weak acid to obtain pretreated garnet fillers includes:

[0013] Adding garnet fillers to an anhydrous organic weak acid for reaction, performing solid-liquid separation to obtain reacted garnet fillers;

[0014] Washing the reacted garnet fillers with a volatile solvent and performing solid-liquid separation;

[0015] Removing the residual volatile solvent to obtain pretreated garnet fillers.

[0016] Optionally, the organic weak acid includes formic acid (HCOOH), acetic acid (CHCOOH), or oxalic acid (HOOCCOOH);

[0017] and / or, the volatile solvent includes ethanol (CH 3 CH 2 OH).

[0018] Optionally, the method for preparing the polyvinylidene fluoride electrolyte solution includes:

[0019] Adding polyvinylidene fluoride into a solvent and stirring to prepare a polyvinylidene fluoride solution with a viscosity of 800 cps - 50000 cps, and then dissolving a lithium salt in the polyvinylidene fluoride solution to obtain the polyvinylidene fluoride electrolyte solution.

[0020] Optionally, the lithium salt includes at least one of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium fluorosulfonylimide (LiFSI), and lithium difluoro(oxalato)borate (LiDFOB).

[0021] Optionally, the garnet filler includes at least one of lithium lanthanum zirconium oxide (LLZO), tantalum-doped lithium lanthanum zirconium oxide (LLZTO15), and aluminum-doped lithium lanthanum zirconium oxide (LLZAO), and the particle size of the garnet filler is 0.1 μm - 2 μm.

[0022] Optionally, the wet process includes one of coating, casting, spraying, and electrospinning.

[0023] On the other hand, the present invention also provides an application of the composite electrolyte in at least one of devices such as a solid-state lithium battery, a lithium-sulfur battery, a lithium battery with a hybrid electrolyte system, a new fuel cell, and an electrochemical sensor, where the composite electrolyte is the above composite electrolyte prepared by the above preparation method.

[0024] Optionally, the composite electrolyte is applied to an electrode composite ion conductive network, an electrode coating, a porous electrolyte membrane, and a dense electrolyte membrane.

[0025] Compared with the prior art, the composite electrolyte, its preparation method, and application provided by the present invention achieve at least the following beneficial effects:

[0026] The present invention first performs surface treatment on the garnet electrolyte material with an organic weak acid to remove the impurity layer, and then prepares it into a composite electrolyte slurry. Since no dehydrofluorination reaction occurs, this slurry is milky white and has high fluidity. Finally, the composite electrolyte material is prepared by a wet process. Compared with the traditional composite electrolyte with dehydrofluorination, the composite electrolyte has good improvements in ionic conductivity, antioxidant window, and ion transference number. And this preparation method has a simple process and is suitable for industrial application.

[0027] The polyvinylidene fluoride garnet composite electrolyte slurry prepared by the present invention has stable properties, has no limitation on the filler content, and is suitable for a variety of application scenarios.

[0028] The polyvinylidene fluoride garnet composite electrolyte without hydrogen fluoride elimination prepared by the present invention has a better ion transport path and exhibits higher ionic conductivity.

[0029] In the present invention, the garnet filler without an impurity layer participates in ion transport, improving the ion transference number of the composite electrolyte.

[0030] The polyvinylidene fluoride matrix without hydrogen fluoride elimination reaction in the present invention has a lower highest occupied molecular orbital (HOMO) energy level, exhibits a higher antioxidant potential, is beneficial to matching with a high-voltage positive electrode, and improves the battery energy density.

[0031] Of course, any product implementing the present invention does not necessarily need to achieve all the above-described technical effects simultaneously.

[0032] Other features and advantages of the present invention will become clear from the following detailed description of the exemplary embodiments of the present invention with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The drawings incorporated in and constituting a part of this specification illustrate embodiments of the present invention and, together with the description, serve to explain the principles of the present invention.

[0034] Figure 1 is a flowchart of a method for preparing a composite electrolyte provided by the present invention;

[0035] Figure 2 is an optical photograph of the PVDF-LLZTO composite electrolyte slurry prepared without surface treatment in the comparative example;

[0036] Figure 3 is an optical photograph of the PVDF-LLZTO composite electrolyte slurry prepared by the method of the present invention in Example 1;

[0037] Figure 4 is a cross-sectional SEM image of the hydrogen fluoride elimination-free PVDF-LLZTO composite electrolyte membrane prepared in Example 1;

[0038] Figure 5 is a cross-sectional SEM image of the PVDF-LLZTO composite electrolyte membrane prepared without surface treatment in the comparative example;

[0039] Figure 6 is the electrochemical impedance spectra of the composite electrolyte with hydrogen fluoride elimination reaction in the comparative example and the hydrogen fluoride elimination-free PVDF:LLZTO composite electrolyte prepared in Example 1 at 30 °C;

[0040] Figure 7 LSV curves of the traditional composite electrolyte with dehydrofluorination reaction and the PVDF:LLZTO composite electrolyte without dehydrofluorination reaction prepared in Example 1 in the comparative example;

[0041] Figure 8 It is the DC polarization curve of the lithium symmetric battery with the PVDF-LLZTO composite electrolyte prepared based on the method of the present invention in Example 1. Detailed implementation manners

[0042] Various exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It should be noted that: unless otherwise specifically stated, the relative arrangements, numerical expressions, and numerical values of the components and steps set forth in these embodiments do not limit the scope of the present invention.

[0043] The following description of at least one exemplary embodiment is merely illustrative in nature and in no way serves as a limitation on the present invention, its application, or use.

[0044] Technologies, methods, and devices known to those of ordinary skill in the relevant art may not be discussed in detail, but where appropriate, such technologies, methods, and devices should be regarded as part of the specification.

[0045] In all the examples shown and discussed here, any specific value should be construed as merely exemplary and not as a limitation. Therefore, other examples of the exemplary embodiments may have different values.

[0046] It should be noted that: similar reference numerals and letters denote similar items in the following drawings, and thus, once an item is defined in one drawing, it does not need to be further discussed in subsequent drawings.

[0047] The present invention provides a composite electrolyte, which is a polyvinylidene fluoride garnet composite electrolyte without dehydrofluorination. The composite electrolyte is milky white, has a viscosity of 800 cps - 50000 cps, an ionic conductivity of 5×10 -5 Scm -1 -5×10 -4 Scm -1 , and an oxidation potential of 4.5 V - 5 V.

[0048] In the prior art, there is a polyvinylidene fluoride garnet composite electrolyte with dehydrofluorination. The composite electrolyte is reddish-brown, has poor fluidity, and is not conducive to later use. And the ionic conductivity of the polyvinylidene fluoride garnet composite electrolyte with dehydrofluorination is in the range of 5×10 -6 Scm -1 -5×10 -5 Scm -1between 4V and 4.5V.

[0049] The polyvinylidene fluoride garnet composite electrolyte without hydrogen fluoride elimination of the present invention has a good improvement in ionic conductivity and antioxidant window.

[0050] Combined Figure 1 , the embodiment of the present invention provides a method for preparing a composite electrolyte, including:

[0051] S101, performing surface pretreatment on the garnet filler with an anhydrous organic weak acid to obtain a pretreated garnet filler;

[0052] S102, adding the pretreated garnet filler into the polyvinylidene fluoride electrolyte solution to obtain a composite electrolyte slurry;

[0053] S103, fabricating the composite electrolyte slurry into a composite electrolyte through a wet process.

[0054] For step S101, acid treatment of the Garnet filler can effectively remove the basic impurities on the surface of the Garnet filler, thereby eliminating the hydrogen fluoride elimination reaction brought by adding the Garnet filler to the electrolyte slurry, making the performance of the composite electrolyte slurry stable and not gelating. Even if the filler content is increased, the slurry still remains stable.

[0055] For step S102, the pretreated garnet filler can be added into the polyvinylidene fluoride electrolyte solution by stirring or ultrasonic means to obtain a composite electrolyte slurry. The composite electrolyte prepared from this composite electrolyte slurry has a wide electrochemical window and stability to lithium. At the same time, it can ensure sufficient contact between the Garnet phase and the PVDF chain segments, reduce the interfacial resistance, optimize the ion transport path, and improve the ionic conductivity and lithium ion transference number of the composite electrolyte.

[0056] Optionally, the mass ratio of the garnet filler to PVDF can be (1:4) to (4:1). If higher mechanical strength and wear resistance are required, the proportion of the garnet filler can be increased; if better flexibility and processability are required, the proportion of PVDF can be increased.

[0057] For step S103, optionally, the prepared composite electrolyte slurry is uniformly coated on a substrate (such as a PET film, a ceramic sheet, etc.) by means of equipment such as a doctor blade and a coater. Then, the coated sample is placed in an oven and dried at an appropriate temperature to remove the solvent. The dried sample is immersed in a coagulation bath (such as deionized water, alcohols, etc.). Through the diffusion between the solvent and the coagulation bath, the polymer solution gradually solidifies into a film. During this process, cross-linking occurs between the PVDF molecular chains, forming a stable network structure. At the same time, Garnet solid electrolyte is uniformly embedded in the polymer matrix to form a composite electrolyte membrane. The solidified composite electrolyte membrane is taken out of the coagulation bath and subjected to subsequent treatments such as cleaning and drying. Optionally, heat pressing, stretching and other treatments can also be carried out to further improve its mechanical properties and ionic conductivity.

[0058] Optionally, the surface of the garnet filler is pretreated with an anhydrous organic weak acid to obtain a treated garnet filler, including:

[0059] Add the garnet filler to the anhydrous organic weak acid for reaction, and perform solid-liquid separation to obtain the reacted garnet filler;

[0060] Wash the reacted garnet filler with a volatile solvent and perform solid-liquid separation;

[0061] Remove the residual volatile solvent to obtain the treated garnet filler.

[0062] Add the garnet filler to the anhydrous organic weak acid. Utilize the chemical properties of the weak acid to react with the basic impurities on the surface of the garnet filler. While removing the impurities on the surface of the garnet filler, it can also change its surface chemical composition and structure to enhance its compatibility with other materials or improve its performance. After the reaction is completed, separate the garnet filler from the reaction solution through solid-liquid separation techniques (such as filtration, centrifugation, etc.) to ensure that the treatment effect on the surface of the garnet filler is retained, and at the same time remove the unreacted anhydrous organic weak acid and other possible impurities.

[0063] Wash the reacted garnet filler with a volatile solvent to remove the residues attached to the surface. After the washing is completed, perform solid-liquid separation again to ensure the cleanliness of the surface of the garnet filler.

[0064] Remove the residual volatile solvent on the surface of the garnet filler by heating, vacuum drying or other methods to ensure the quality and stability of the treated garnet filler.

[0065] Optionally, the organic weak acid includes formic acid (HCOOH), acetic acid (CHCOOH) or oxalic acid (HOOCCOOH);

[0066] and / or, the volatile solvent includes ethanol (CH3 CH 2 OH).

[0067] Organic weak acids are organic acids that are partially ionized in aqueous solution and have a relatively low degree of ionization. Selecting HCOOH, CHCOOH, or HOOCCOOH as organic weak acids can avoid the corrosion of Garnet grains by inorganic strong acids. At the same time, anhydrous organic weak acids are used, which can avoid the secondary pollution of the powder by the water in the diluted acid solution.

[0068] Optionally, the preparation method of the polyvinylidene fluoride electrolyte solution includes:

[0069] Adding polyvinylidene fluoride to a solvent and stirring to prepare a polyvinylidene fluoride solution with a viscosity of 800 cps - 50000 cps, and then dissolving a lithium salt in the polyvinylidene fluoride solution to obtain a polyvinylidene fluoride electrolyte solution.

[0070] Optionally, the mass ratio of PVDF to the lithium salt can be 4:3 to 1:1.

[0071] Optionally, the lithium salt includes at least one of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium fluorosulfonylimide (LiFSI), and lithium difluoro(oxalato)borate (LiDFOB).

[0072] LiTFSI has high thermal stability and good solubility, which can improve the ionic conductivity and battery performance. The anion structure of LiTFSI is stable, which helps to reduce the crystallinity of the electrolyte, thereby improving the ionic conduction performance. LiFSI has properties similar to LiTFSI, but has a lower melting point and higher ionic conductivity. LiDFOB is a boron-containing lithium salt used to improve the thermal stability and cycling performance of lithium-ion batteries. It can form a protective film on the surface of the battery cathode material, reducing the direct contact between the active material and the electrolyte, thereby extending the service life of the battery.

[0073] Optionally, the Garnet filler includes at least one of lithium lanthanum zirconium oxide (LLZO), tantalum-doped lithium lanthanum zirconium oxide (LLZTO15), and aluminum-doped lithium lanthanum zirconium oxide (LLZAO).

[0074] LLZO has the advantages of high ionic conductivity at room temperature, wide electrochemical window, and chemical stability to lithium metal anode. LLZTO is obtained by doping the element tantalum (Ta) on the basis of LLZO, with further improved density and significantly increased lithium ion conductivity, making the performance of LLZO reach the optimal. LLZAO is another garnet-type solid electrolyte obtained by doping and modifying LLZO with aluminum element. Aluminum doping can stabilize the cubic phase structure, optimize the preparation route, reduce the interface resistance and grain boundary resistance, and improve the room temperature ionic conductivity.

[0075] Garnet filler, whose morphology includes but is not limited to: particles, linear and three-dimensional continuous structures.

[0076] Garnet fillers usually exist in the form of particles. Linear-structured garnets can enhance the mechanical strength of composite materials or improve the ionic conductivity of electrolytes. In the field of solid electrolytes, three-dimensional continuous structure garnet electrolytes have higher ionic conductivity and better mechanical properties, thus improving the performance and safety of batteries.

[0077] Optionally, the particle size of the garnet filler is 0.1 μm - 2 μm.

[0078] Specifically, when the particle size is less than 0.1 μm, the garnet filler has a large specific surface area and serious agglomeration, which is not conducive to the removal of the surface impurity layer; when the particle size is greater than 2 μm, it is not conducive to the preparation of a light, thin and uniform solid electrolyte layer.

[0079] Optionally, the wet process includes one of coating, casting, spraying and electrospinning.

[0080] The coating method includes the pretreatment of the current collector, the modulation of the positive electrode slurry, the coating of the positive electrode, drying and the cutting of the electrode sheet. The coating method can ensure that the electrolyte slurry is evenly coated on the substrate to form a uniform electrolyte layer.

[0081] The casting method is to mix the electrolyte powder evenly with the solvent and the binder, and then cast a certain thickness of electrolyte film on the film belt through a casting machine. The casting method is suitable for the preparation of large-area and uniform-thickness electrolyte films.

[0082] The spraying method is a method of evenly spraying the composite electrolyte slurry on the substrate through a spraying device. The spraying method is suitable for the preparation of electrolyte layers with complex shapes and structures, such as three-dimensional structured electrolytes. The spraying method can ensure that the electrolyte slurry is evenly distributed on the substrate and at the same time form a dense electrolyte layer.

[0083] The electrospinning method is a method of using the electrostatic field force to draw the electrolyte slurry into nanofibers. The electrospinning method can prepare nanofiber materials with high specific surface area and porosity, providing a fast channel for ion transport.

[0084] On the other hand, the present invention also provides an application of the composite electrolyte in at least one of devices such as solid-state lithium batteries, lithium-sulfur batteries, lithium-ion batteries in a mixed electrolyte system, new fuel cells and electrochemical sensors. The composite electrolyte is the above composite electrolyte prepared by the above preparation method.

[0085] Solid-state lithium batteries have higher energy density, better safety, and longer cycle life compared to liquid lithium batteries. The hydrogen fluoride-free PVDF garnet composite electrolyte can effectively improve the ionic conductivity of solid-state lithium batteries, reduce the interfacial impedance between the electrolyte and the electrode, and thus enhance the overall performance of the battery.

[0086] Lithium-sulfur batteries have the advantages of high energy density and low cost, but suffer from problems such as low utilization rate of the sulfur cathode active material and poor cycle stability. The hydrogen fluoride-free PVDF garnet composite electrolyte can stabilize the electrochemical environment of the sulfur cathode, reduce the loss of active materials, and thus improve the cycle stability and capacity retention rate of lithium-sulfur batteries.

[0087] Lithium batteries with a hybrid electrolyte system combine the advantages of solid electrolytes and liquid electrolytes, having higher energy density, better safety, and a wider electrochemical window.

[0088] New types of fuel cells such as solid oxide fuel cells (SOFC) and proton exchange membrane fuel cells (PEMFC) have extremely high requirements for the performance of electrolytes. The hydrogen fluoride-free PVDF garnet composite electrolyte has excellent ionic conduction performance and chemical stability, and can meet the high requirements of new fuel cells for electrolytes.

[0089] Electrochemical sensors require a stable electrolyte to provide accurate measurement signals. This composite electrolyte has stable electrochemical performance and good ionic conduction performance, and can provide a reliable working environment for electrochemical sensors.

[0090] Optionally, the composite electrolyte is applied to electrode composite ion conductive networks, electrode coatings, porous electrolyte membranes, and dense electrolyte membranes.

[0091] Example 1

[0092] This example provides a method for preparing a hydrogen fluoride-free PVDF-LLZTO composite electrolyte membrane, which includes the following steps:

[0093] (1) Preparation of PVDF electrolyte solution: Add PVDF powder to DMF solvent and stir. After PVDF is completely dissolved, add LiTFSI and stir to dissolve to form an electrolyte solution. Among them, the mass ratio of PVDF to LiTFSI is 4:3.

[0094] (2) Surface treatment of LLZTO filler: Remove the surface impurity layer of LLZTO particles with an average particle size of 0.1 micron using glacial acetic acid.

[0095] (3) Preparation of slurry: Add the treated LLZTO filler to the PVDF electrolyte solution and disperse to obtain a uniform composite electrolyte slurry. Among them, the mass ratio of LLZTO to PVDF is 1:4.

[0096] (4) Preparation of composite electrolyte: The composite electrolyte slurry is formed by the casting method to obtain a PVDF-LLZTO composite electrolyte membrane without hydrogen fluoride elimination.

[0097] Example 2

[0098] This example provides a method for preparing a PVDF-LLZTO composite electrolyte membrane without hydrogen fluoride elimination, including the following steps:

[0099] (1) Preparation of PVDF electrolyte solution: PVDF powder is added to DMF solvent and stirred. After PVDF is completely dissolved, LiTFSI is added and stirred to dissolve to form an electrolyte solution. Among them, the mass ratio of PVDF to LiTFSI is 4:3.

[0100] (2) Surface treatment of LLZTO filler: LLZTO particles with an average particle size of 0.1 μm are treated with glacial acetic acid to remove the surface impurity layer.

[0101] (3) Preparation of slurry: The treated LLZTO filler is added to the PVDF electrolyte solution and dispersed to obtain a uniform composite electrolyte slurry, where the mass ratio of LLZTO to PVDF is 1:1.

[0102] (4) Preparation of composite electrolyte: The composite electrolyte slurry is formed by the casting method to obtain a PVDF-LLZTO composite electrolyte membrane without hydrogen fluoride elimination.

[0103] Example 3

[0104] This example provides a method for preparing a PVDF-LLZTO composite electrolyte membrane without hydrogen fluoride elimination, including the following steps:

[0105] (1) Preparation of PVDF electrolyte solution: PVDF powder is added to DMF solvent and stirred. After PVDF is completely dissolved, LiTFSI is added and stirred to dissolve to form an electrolyte solution. Among them, the mass ratio of PVDF to LiTFSI is 4:3.

[0106] (2) Surface treatment of LLZTO filler: LLZTO particles with an average particle size of 0.1 μm are treated with glacial acetic acid to remove the surface impurity layer.

[0107] (3) Preparation of slurry: The treated LLZTO filler is added to the PVDF electrolyte solution and dispersed to obtain a uniform composite electrolyte slurry, where the mass ratio of LLZTO to PVDF is 4:1.

[0108] (4) Preparation of composite electrolyte: The composite electrolyte slurry is formed by the casting method to obtain a PVDF-LLZTO composite electrolyte membrane without hydrogen fluoride elimination.

[0109] Comparative Example

[0110] (1) Preparation of PVDF electrolyte solution: PVDF powder was added to DMF solvent and stirred. After PVDF was completely dissolved, LiTFSI was added and stirred to dissolve to prepare an electrolyte solution. Among them, the mass ratio of PVDF to LiTFSI was 4:3.

[0111] (3) Preparation of slurry: LLZTO particles with an average particle size of 0.1 μm were added to the PVDF electrolyte solution and dispersed to obtain a uniform composite electrolyte slurry. Among them, the mass ratio of LLZTO to PVDF was 4:1.

[0112] (4) Preparation of composite electrolyte: The composite electrolyte slurry was formed by the casting method to obtain a PVDF-LLZTO composite electrolyte membrane.

[0113] Comparison of experimental results:

[0114] Figure 2 is an optical photograph of the PVDF-LLZTO composite electrolyte slurry prepared in the comparative example without surface treatment. It can be found that due to the occurrence of dehydrofluorination reaction, the slurry presents a reddish-brown gel state and has poor fluidity, which is not conducive to later use.

[0115] Figure 3 is an optical photograph of the PVDF-LLZTO composite electrolyte slurry prepared by the method of the present invention in Example 1. It can be found that the electrolyte slurry without dehydrofluorination reaction presents milky white and has good fluidity.

[0116] Figure 4 is a cross-sectional SEM image of the PVDF-LLZTO composite electrolyte membrane without dehydrofluorination prepared in Example 1. It can be found that the thickness of the electrolyte membrane is 33 μm, and the upper and lower surfaces are flat without obvious undulations, which is beneficial to the interface contact of the solid-state battery. At the same time, the LLZTO powder is evenly dispersed in the PVDF matrix without obvious agglomerated particles.

[0117] Figure 5 is a cross-sectional SEM image of the PVDF-LLZTO composite electrolyte membrane without surface treatment prepared in the comparative example. It can be found that the thickness of the electrolyte membrane is 32 μm, and the upper and lower surfaces are flat without obvious undulations. At the same time, there are a large number of high-brightness regions representing agglomerated particles, which is due to the increase in viscosity caused by the dehydrofluorination of the slurry, resulting in difficulty in powder dispersion.

[0118] Figure 6 is the electrochemical (EIS) impedance spectrum of the composite electrolyte with dehydrofluorination reaction in the comparative example and the PVDF:LLZTO composite electrolyte without dehydrofluorination reaction prepared in Example 1 at 30°C. Figure 6Orange represents the composite electrolyte with dehydrofluorination reaction in the comparative example, and blue represents the PVDF:LLZTO composite electrolyte without dehydrofluorination reaction prepared in Example 1. It can be calculated that the ionic conductivity of the composite electrolyte with dehydrofluorination reaction is 5.06×10 -5 Scm -1 , and the ionic conductivity of the composite electrolyte without dehydrofluorination reaction is 1.14×10 -4 Scm -1 . This is because after removing the impurity layer on the surface of LLZTO, lithium ions conduct through the bulk phase of LLZTO, optimizing the ion transport path and increasing the ionic conductivity.

[0119] Figure 7 The LSV curves of the traditional composite electrolyte with dehydrofluorination reaction in the comparative example and the PVDF:LLZTO composite electrolyte without dehydrofluorination reaction prepared in Example 1 are shown. Figure 7 Orange represents the composite electrolyte with dehydrofluorination reaction in the comparative example, and blue represents the PVDF:LLZTO composite electrolyte without dehydrofluorination reaction prepared in Example 1. It can be seen that the oxidation potential of the composite electrolyte with dehydrofluorination reaction is 4.45V, and the ionic conductivity of the composite electrolyte without dehydrofluorination reaction is 4.71V. This is because the PVDF matrix without dehydrofluorination has a lower HOMO energy level, making the composite electrolyte exhibit a higher oxidation potential.

[0120] Figure 8 The DC polarization curve and the EIS spectra before and after polarization of the lithium symmetric battery with the PVDF-LLZTO composite electrolyte prepared based on the method of the present invention in Example 1 are shown. Figure 8 The inset in it is the EIS spectrum of the lithium symmetric battery before and after polarization. It can be calculated that the lithium ion transference number of this composite electrolyte is 0.33.

[0121] Although some specific embodiments of the present invention have been described in detail by way of examples, those skilled in the art should understand that the above examples are only for illustration and not for limiting the scope of the present invention. Those skilled in the art should understand that the above embodiments can be modified without departing from the scope and spirit of the present invention. The scope of the present invention is defined by the appended claims.

Claims

1. A composite electrolyte, characterized in that The composite electrolyte is a polyvinylidene fluoride garnet composite electrolyte, which is milky white, has a viscosity of 800cps-50000cps, and an ionic conductivity of 5×10 -5 Scm -1 -5×10 -4 Scm -1 , the oxidation potential is 4.5V-5V.

2. A method for preparing a composite electrolyte, characterized in that: include: The treated garnet filler is obtained by surface pre-treating the garnet filler with anhydrous organic weak acid; adding the treated garnet filler into a polyvinylidene fluoride electrolyte solution to obtain a composite electrolyte slurry; The composite electrolyte slurry is prepared into a composite electrolyte through a wet process.

3. The method for preparing a composite electrolyte according to claim 2, characterized in that: The garnet filler is surface pretreated by anhydrous organic weak acid to obtain the treated garnet filler, comprising: Adding a garnet filler into an anhydrous organic weak acid to react, performing solid-liquid separation, and obtaining a reacted garnet filler; The reacted garnet filler is washed with a volatile solvent to separate the solid from the liquid; The residual volatile solvent is removed to obtain the treated garnet filler.

4. The method for preparing a composite electrolyte according to claim 3, characterized in that: The organic weak acid includes formic acid, acetic acid or oxalic acid; And / or, the volatile solvent comprises ethanol.

5. The method for preparing a composite electrolyte according to claim 2, characterized in that: The preparation method of the polyvinylidene fluoride electrolyte solution comprises: Polyvinylidene fluoride is added into a solvent and stirred to prepare a polyvinylidene fluoride solution with a viscosity of 800 cps-50000 cps, and then lithium salt is dissolved in the polyvinylidene fluoride solution to obtain the polyvinylidene fluoride electrolyte solution.

6. The method for preparing a composite electrolyte according to claim 5, characterized in that: The lithium salt includes at least one of lithium bis(trifluoromethylsulfonyl)imide, lithium fluorosulfonyl imide and lithium difluorooxalatoborate.

7. The method for preparing a composite electrolyte according to claim 2, characterized in that: The garnet filler includes at least one of lithium lanthanum zirconium oxide, tantalum-doped lithium lanthanum zirconium oxide, and aluminum-doped lithium lanthanum zirconium oxide, and the particle size of the garnet filler is 0.1 μm-2 μm.

8. The method for preparing a composite electrolyte according to claim 2, characterized in that: The wet process includes one of coating, casting, spraying and electrospinning.

9. Application of a composite electrolyte in at least one of a solid-state lithium battery, a lithium-sulfur battery, a mixed electrolyte system lithium battery, a new fuel cell and an electrochemical sensor, characterized in that: The composite electrolyte is the composite electrolyte of claim 1, which is prepared by the preparation method of any one of claims 2 to 8.

10. The use according to claim 9, characterized in that: The composite electrolyte is applied to electrode composite ion conductive networks, electrode coatings, porous electrolyte membranes and dense electrolyte membranes.

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