A pure solid electrolyte, its preparation method and application

Abstract

This invention belongs to the field of solid-state lithium battery electrolyte materials, and provides a pure solid electrolyte, its preparation method and application. This invention uses a surface coating and interparticle bridging system of constructed lithium aluminum titanium phosphate spherical secondary particles and an inorganic layer containing lithium, boron, phosphorus, oxygen and fluorine elements to control the particle size of secondary particles, the agglomeration scale of primary particles, and the content and thickness of inorganic layers. After pressing and densification, a self-supporting film or sheet without free-flowing liquid phase is formed, which achieves synergistic improvement in processing fluidity, ion conduction channel continuity, structural integrity and interface stability. It solves the problems of high interparticle impedance of large spherical particles, easy agglomeration of nanoparticles, and difficulty in balancing dense framework and interface layer. It is suitable for lithium-ion conduction isolation layer in solid-state lithium batteries.

Description

Technical Field

[0001] This invention relates to the field of solid-state lithium battery electrolyte materials, specifically to a pure solid-state electrolyte, its preparation method, and its application. Background Technology

[0002] Solid-state lithium batteries, which replace traditional liquid electrolytes with solid electrolytes, are considered an important direction for improving battery safety, structural integration, and energy system reliability. Among these, oxide-based solid electrolytes are suitable for forming an electrolyte layer between the positive and negative electrodes, providing both lithium-ion conduction and electronic insulation, due to their good chemical stability, thermal stability, and mechanical support capabilities. For lithium aluminum titanium phosphate systems, materials typically need to simultaneously meet requirements such as powder processing flowability, compressibility, sintering densification capability, interfacial contact stability, and interparticle conduction continuity. If the powder particles are too large or the interparticle interface lacks continuous control, densification can easily lead to interfacial regions that hinder lithium-ion migration; conversely, excessive pursuit of nano-sized particles may result in agglomeration, decreased slurry stability, and narrowing of the forming window. Therefore, synergistic design focusing on particle hierarchical structure, interfacial phase composition, and densification processes is crucial for obtaining a processable, formable, and suitable pure solid-state electrolyte for solid-state lithium batteries.

[0003] Current research on lithium titanium aluminum phosphate solid electrolytes mainly focuses on improving material purity, reducing primary particle size, minimizing lithium volatilization, or increasing density. However, the simultaneous optimization of particle morphology, interparticle interfaces, and interface stability remains a challenge. For example, Chinese patent application CN116759637A discloses a lithium titanium aluminum phosphate solid electrolyte, its preparation method, and its applications. This scheme improves primary particle size, lithium volatilization, density, and ionic conductivity through lithium source complexation with crown ether, drying, and two-step sintering. However, this approach primarily revolves around precursor complexation and sintering pathways, lacking targeted design for the synergistic relationships between the primary particle size within spherical secondary particles, the composition of the inorganic layer on the particle's outer surface, the interparticle bridging region between adjacent particles, and the interface continuity of pure solid films or sheets. On the other hand, conventionally increasing ceramic particle size facilitates flow and compression but increases high-resistance interparticle interfaces; conventional nano-sizing increases active interfaces but easily leads to agglomeration and unstable formation; simply increasing density may expose unstable interfaces, while simply introducing an interface layer may weaken the continuity of the framework. Summary of the Invention

[0004] The purpose of this invention is to provide a pure solid electrolyte, its preparation method and application, to solve the current pain points of difficulty in balancing processing fluidity and ion conduction performance, and structural integrity and interface stability.

[0005] This invention maintains the forming flow and skeleton continuity through spherical secondary lithium aluminum titanium phosphate particles, retains the active conductive interface through primary particle size control, and forms a continuous interface phase on the particle surface and interparticle bridging region with an inorganic layer containing lithium, boron, phosphorus, oxygen and fluorine elements. This weakens the interparticle impedance caused by particle size enlargement and the agglomeration risk caused by nano-sizing, while taking into account both dense structure and interface stability.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A pure solid electrolyte, characterized in that it comprises: The lithium aluminum titanium phosphate spherical secondary particles are obtained by reacting lithium carbonate, aluminum oxide, titanium dioxide and ammonium dihydrogen phosphate. The D50 of the lithium aluminum titanium phosphate spherical secondary particles is 1-8 μm, and they are formed by the agglomeration of primary particles with D50 of 80-300 nm. An inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine is formed by coating the surface of the lithium aluminum titanium phosphate spherical secondary particles in a powder state before pressing, and by at least partially occupying the interparticle bridging region between adjacent lithium aluminum titanium phosphate spherical secondary particles after densification. The inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine is formed by contacting phosphoric acid, boric acid, lithium fluoride, and lithium hydroxide monohydrate with the lithium aluminum titanium phosphate spherical secondary particles and then drying and heat-treating. The equivalent mass fraction of the inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine, based on the powder state before pressing, is 1-6 wt%, and the total mass of the lithium aluminum titanium phosphate spherical secondary particles and the inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine is 10-80 nm. The pure solid electrolyte was determined to be free of free-flowing liquid phase by holding it at a 45° angle at 25°C for 24 hours, and its relative density was 96-99.5%.

[0007] Furthermore, the lithium aluminum titanium phosphate spherical secondary particles coated with an inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine elements are prepared through the following steps: A1. Provides spherical secondary lithium aluminum titanium phosphate particles; A2. Disperse 100 parts by weight of the lithium aluminum titanium phosphate spherical secondary particles in a mixed solvent of 200 parts by weight of ethanol and deionized water, wherein the mass ratio of ethanol to deionized water in the mixed solvent is 1:4 to 4:1. Add 0.3-2.0 parts by weight of phosphoric acid, 0.2-1.5 parts by weight of boric acid, 0.1-1.0 parts by weight of lithium fluoride and 0.1-1.5 parts by weight of lithium hydroxide monohydrate, and control the pH value of the resulting dispersion system at 25°C to be 4.5-6.5. A3. Stir at 300-600 rpm for 0.5-4 hours at 25-60℃; A4. After drying at 60-120℃ for 2-12 hours, heat-treat at 250-450℃ for 0.5-4 hours in air or nitrogen atmosphere; A5. An inorganic layer containing lithium, boron, phosphorus, oxygen and fluorine elements is obtained with a surface equivalent thickness of 10-80 nm based on the state of the coated powder before pressing.

[0008] Furthermore, the lithium aluminum titanium phosphate spherical secondary particles in step A1 are prepared through the following steps: B1. A precursor slurry is prepared by mixing lithium carbonate, aluminum oxide, titanium dioxide, and ammonium dihydrogen phosphate with ethanol and deionized water in a molar ratio of lithium, aluminum, titanium, and phosphorus of (1+x):x:(2-x):3, where x is 0.2-0.5. B2. The precursor slurry is ball-milled and mixed; B3. Spray dry the ball-milled precursor slurry to obtain spherical precursor particles, wherein the spray drying inlet temperature is 160-220℃ and the outlet temperature is 75-110℃; B4. The precursor spherical particles are calcined at 700-950℃ for 2-10h in air or nitrogen atmosphere to obtain the lithium aluminum titanium phosphate spherical secondary particles with D50 of 1-8μm.

[0009] Furthermore, in step A2, taking the molar amount of phosphorus in phosphoric acid as 1, the molar ratio of boron, phosphorus and fluorine is 0.16-0.8:1:0.19-0.5, and the ratio of the total molar amount of lithium introduced by lithium hydroxide monohydrate and lithium fluoride to the sum of the molar amounts of boron and phosphorus is 0.8-2.5:1.

[0010] Furthermore, the solid content of the precursor slurry in step B1 is 20-45 wt%, based on the total mass of the precursor slurry; the ball milling speed in step B2 is 200-500 rpm, and the ball milling time is 4-12 h; the spray drying inlet temperature in step B3 is 160-220℃, and the outlet temperature is 75-110℃.

[0011] Furthermore, the pure solid electrolyte is a self-supporting membrane or sheet; the thickness of the self-supporting membrane is 30-120 μm, and the thickness of the sheet is 100-500 μm.

[0012] As a concept of the present invention, the present invention adopts The design of this invention, which combines spherical secondary lithium aluminum titanium phosphate particles with an inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine, is primarily intended to achieve a synergistic balance between processing and ion conduction. In existing technologies, to improve powder flow and pressing stability, ceramic particle size is typically increased or strong sintering densification is employed. However, this approach easily expands interparticle contact resistance and exposes unstable interfaces. To improve conduction continuity, particle size is typically reduced or an interface modification layer is introduced, which may lead to agglomeration, narrowing of the forming window, or decreased skeletal continuity. This invention, through matching the morphology of secondary particles, the size of primary particles, the content and thickness of the inorganic layer, and the interparticle bridging region, enables mutual correction between particle flow, interface finishing, and dense skeletal formation, thereby achieving a balance between structural integrity, interface stability, and lithium-ion migration channel continuity in a pure solid-state state.

[0013] This invention also discloses a method for preparing a pure solid electrolyte, comprising the following steps: S1. Lithium carbonate, aluminum oxide, titanium dioxide and ammonium dihydrogen phosphate are mixed with ethanol and deionized water and ball-milled to obtain a precursor slurry; S2. The precursor slurry is spray-dried and calcined to obtain spherical secondary particles of lithium aluminum titanium phosphate; S3. The spherical secondary particles of lithium aluminum titanium phosphate are contacted with phosphoric acid, boric acid, lithium fluoride and lithium hydroxide monohydrate in a mixed solvent of ethanol and deionized water, and then dried and heat-treated to form an inorganic layer containing lithium, boron, phosphorus, oxygen and fluorine elements, thereby obtaining lithium aluminum titanium phosphate powder coated with an inorganic layer containing lithium, boron, phosphorus, oxygen and fluorine elements. S4. The lithium aluminum titanium phosphate powder coated with an inorganic layer containing lithium, boron, phosphorus, oxygen and fluorine elements obtained in step S3 is pre-pressed at 80-120MPa for 30-60s to form a blank, and then subjected to pressureless sintering densification or hot pressing densification to obtain the pure solid electrolyte.

[0014] Further, in step S1, lithium carbonate, alumina, titanium dioxide, and ammonium dihydrogen phosphate are fed in a molar ratio of lithium, aluminum, titanium, and phosphorus of (1+x):x:(2-x):3, where x is 0.2-0.5. The solid content of the precursor slurry is 20-45 wt%. Based on the total mass of the precursor slurry, the ball milling speed is 200-500 rpm, and the ball milling time is 4-12 h.

[0015] Furthermore, in step S2, the spray drying inlet temperature is 160-220℃ and the outlet temperature is 75-110℃; calcination is carried out in an air or nitrogen atmosphere, the calcination temperature is 700-950℃, and the calcination time is 2-10h.

[0016] Furthermore, in step S3, ethanol and deionized water are used as a mixed solvent, with a mass ratio of ethanol to deionized water of 1:4 to 4:1. The resulting dispersion system has a pH value of 4.5-6.5 read at 25°C, a stirring speed of 300-600 rpm, a stirring time of 0.5-4 h, drying conditions of 60-120°C for 2-12 h, and heat treatment conditions of 250-450°C for 0.5-4 h.

[0017] Furthermore, in step S4, densification is carried out by pressureless sintering or hot pressing, with a densification temperature of 850-1000℃ and a densification time of 0.5-4h; when hot pressing is used, the pressure is 10-40MPa.

[0018] The present invention also discloses the application of a pure solid electrolyte in the preparation of a solid lithium battery, wherein the pure solid electrolyte is disposed in the form of a self-supporting film or sheet between the positive and negative electrodes of the solid lithium battery, and serves as a lithium-ion conducting layer and an electronic insulating isolation layer.

[0019] Furthermore, the D50 of the spherical secondary lithium aluminum titanium phosphate particles was determined using powder obtained by spray drying and calcination at 700-950℃ for 2-10 hours as the test sample. The volumetric reference particle size distribution was determined using the wet method of laser diffraction particle size analysis. D10, D50, D90 and the dispersion medium before testing were recorded. The obtained D50 was used to define the particle size of the secondary particles. The D50 of the primary particles was determined using SEM or TEM images of the same batch of powder as the data source. The equivalent circular diameter of no less than 100 identifiable primary particles was statistically analyzed, and the quantitative reference particle size distribution and D50 were output to define the particle size of the primary particles.

[0020] Furthermore, the equivalent mass fraction of the inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine elements is calculated based on the pre-pressing powder state and the mass of the blank heat treatment residue. According to the feeding ratio of phosphoric acid, boric acid, lithium fluoride, and lithium hydroxide monohydrate in step A2, the inorganic residue is dried at 60-120℃ for 2-12 hours and heat-treated at 250-450℃ for 0.5-4 hours without the addition of lithium aluminum titanium phosphate spherical secondary particles. The mass of the obtained inorganic residue is weighed, and this inorganic residue mass is compared with the total mass of lithium aluminum titanium phosphate spherical secondary particles and the inorganic residue in the same feeding batch to output the equivalent mass fraction of the inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine elements.

[0021] Furthermore, the surface equivalent thickness of the inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine elements is calculated based on the state of the coated powder before pressing. It is calculated using the mass of the blank heat-treated residue, the BET specific surface area of ​​the uncoated lithium aluminum titanium phosphate spherical secondary particles, and the true density of the blank heat-treated residue. The specific surface area S of the uncoated lithium aluminum titanium phosphate spherical secondary particles is measured by the nitrogen adsorption BET method, and the true density ρ of the blank heat-treated residue is measured by the helium specific gravity method. The mass of the blank heat-treated residue corresponding to 100 mass parts of uncoated lithium aluminum titanium phosphate spherical secondary particles in each batch is denoted as m, and the mass of the batch of uncoated lithium aluminum titanium phosphate spherical secondary particles is denoted as M. After converting S, ρ, m, and M to the same length and mass unit system, the thickness is calculated using t = m / (S × M × ρ). The output t is used as the surface equivalent thickness and is used to control the coating state of the powder obtained in step A5 before pressing.

[0022] Furthermore, the interparticle bridging region is observed using the polished cross-section or ion beam cross-section of the densified pure solid electrolyte. One or more of SEM-EDS, TEM-EDS, and EELS are used to collect morphology images and lithium, boron, phosphorus, oxygen, and fluorine element distribution data of the contact interface between adjacent lithium aluminum titanium phosphate spherical secondary particles. Boron- and fluorine-containing signal regions located between the outlines of two adjacent particles and connected to the inorganic layer on the particle surface are recorded as interparticle bridging regions. The connectivity, distribution location, and cross-sectional width of this region are output as determination data of the spatial distribution of the inorganic layer.

[0023] Furthermore, in preparing the inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine, 100 parts by mass of lithium aluminum titanium phosphate spherical secondary particles are dispersed in a mixed solvent of ethanol and deionized water in a mass ratio of 1:4 to 4:1, with the mixed solvent amounting to 200 parts by mass. 0.3-2.0 parts by mass of phosphoric acid, 0.2-1.5 parts by mass of boric acid, 0.1-1.0 parts by mass of lithium fluoride, and 0.1-1.5 parts by mass of lithium hydroxide monohydrate are added. The pH value of the dispersion system is controlled at 4.5-6.5 at 25°C, and the mixture is stirred at 300-600 rpm for 0.5-4 hours at 25-60°C. The resulting dispersion system is dried at 60-120°C for 2-12 hours and heat-treated at 250-450°C for 0.5-4 hours to form a coated powder. This coated powder then proceeds to the pressing and densification steps.

[0024] Furthermore, lithium fluoride is added as a solid dispersion phase to a mixed solvent of ethanol and deionized water. Lithium fluoride, lithium aluminum titanium phosphate spherical secondary particles, phosphoric acid, boric acid, and lithium hydroxide monohydrate are in contact in a dispersion system with a pH value of 4.5-6.5 read at 25°C. Before heat treatment, the solid sedimentation state of the dispersion system and the sampling position after stirring are recorded. After heat treatment, the resulting coated powder enters the elemental distribution detection and pressing steps. The elemental distribution detection includes SEM-EDS, TEM-EDS, or EELS detection on the coated powder or the cross-section after densification, and recording the spatial correspondence between fluorine and boron-containing regions, particle surface regions, and interparticle contact regions. After heat treatment, the coated powder can also be detected by X-ray diffraction or selected area electron diffraction, and the correspondence between the lithium fluoride crystal phase characteristics and the spatial distribution data of the inorganic layer is recorded.

[0025] Furthermore, the relative density was determined using the self-supporting membrane or sheet obtained after densification as the test sample. The Archimedes method was used to determine the bulk density of the sample with anhydrous ethanol as the test medium, and the dry mass, immersion mass, and saturation mass were recorded. The theoretical density was calculated by weighting the XRD-refined density of the uncoated lithium aluminum titanium phosphate phase and the true density of the inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine elements according to the mass fraction of the volume. The calculation was based on 1 / ρtheoretical = wLATP / ρLATP + winorganiclayer / ρinorganiclayer, where wLATP and winorganiclayer are the mass fractions of the lithium aluminum titanium phosphate phase and the inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine elements, respectively. The ratio of the bulk density to the theoretical density was output as the relative density.

[0026] Furthermore, for pure solid electrolyte samples after densification that do not contain free-flowing liquid phase, the samples were placed on a clean glass plate at 25°C and tilted at 45° for 24 hours. Visual observation records were collected of the sample surface and the contact area between the sample and the glass plate. The presence or absence of visible droplet migration and liquid outflow was used as the determination result of free-flowing liquid phase.

[0027] Furthermore, pressureless sintering densification uses a green body formed by pre-pressing the coated powder at 80-120 MPa for 30-60 s as the input sample, and outputs a pure solid electrolyte sample after holding it at 850-1000℃ for 0.5-4 h in an air or nitrogen atmosphere; hot pressing densification uses a green body formed by pre-pressing the coated powder at 80-120 MPa for 30-60 s as the input sample, and outputs a pure solid electrolyte sample after holding it at 850-1000℃ for 0.5-4 h under a pressure of 10-40 MPa; samples output from both densification paths are subjected to relative density measurement, cross-sectional morphology observation, and elemental distribution detection.

[0028] Furthermore, when the pure solid electrolyte is used to make a self-supporting membrane, the formed sample is clamped and transferred as a whole without attaching metal foil, polymer support layer or ceramic substrate, and the average thickness of 30-120μm and the thickness at no less than five measuring points are recorded as the morphological data of the self-supporting membrane; when the pure solid electrolyte is used to make a sheet, the formed sample is recorded as the sheet morphological data with an average thickness of 100-500μm, the mold pressing state and the thickness at no less than five measuring points.

[0029] As another concept of this invention, this invention employs a preparation method that couples precursor spheroidization, surface contact to form an inorganic layer, pressing, and pressureless sintering densification or hot pressing densification. This method is mainly used to achieve, fix, or amplify the aforementioned synergistic effects. If only spray drying and calcination are used to form spherical particles, the powder processability can be improved, but the interparticle interface may still become a region that hinders lithium-ion migration. If only subsequent interface layer treatment is used, the interparticle contact can be adjusted, but the skeleton strength may be affected due to insufficient or excessive distribution of the interface phase. This invention first constructs spherical secondary lithium aluminum titanium phosphate particles, then allows phosphoric acid, boric acid, lithium fluoride, and lithium hydroxide monohydrate to contact each other on the particle surface and form an inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine elements through drying and heat treatment. The coated powder then undergoes a densification step, fixing the interface phase along with the particle contact area, thereby making the preparation method correspond to the product structure. As another concept of the present invention, the present invention sets a pure solid electrolyte in the form of a self-supporting membrane or sheet between the positive and negative electrodes of a solid lithium battery, so that the lithium-ion conduction and electronic insulation functions remain consistent in the application environment.

[0030] The spherical secondary particles of lithium aluminum titanium phosphate mainly serve the functions of powder flow, compression molding, and ceramic skeleton support. While relying solely on these particles or having excessively large particle sizes is beneficial for processing and structural integrity, it can easily create ion migration resistance at the contact points between adjacent particles. The inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine mainly serves the functions of interface regulation and interparticle contact connectivity. However, if it is strengthened alone or its content is too high, it may dilute the lithium aluminum titanium phosphate matrix, weaken the continuity of the skeleton, or increase the proportion of non-matrix phases. This invention addresses this by using a spherical secondary structure formed through primary particle agglomeration, matching the inorganic layer mass fraction and equivalent surface thickness, and extending the inorganic layer at least partially to the interparticle bridging region. This ensures that the interface layer is not merely a surface cover but forms continuous regulating units in the particle contact area, thereby mutually correcting the adverse effects of large particle interparticle resistance and excessive interface modification, thus balancing inherently contradictory properties.

[0031] Beneficial technical effects 1. Through By combining spherical secondary lithium aluminum titanium phosphate particles with nanoscale primary particle agglomeration structures, this invention maintains the powder's flowability and pressing ability while retaining more active contact interfaces within the particles, thus alleviating the interparticle conduction resistance problem caused by simply forming large particles.

[0032] 2. By forming an inorganic layer containing lithium, boron, phosphorus, oxygen and fluorine elements on the surface of lithium aluminum titanium phosphate spherical secondary particles, and distributing it at least partially in the interparticle bridging region, the present invention can introduce a continuous inorganic conditioning phase at the particle contact interface, thereby reducing the adverse effects of the exposed ceramic interface on the interface stability.

[0033] 3. By limiting the equivalent mass fraction of the inorganic layer and the equivalent surface thickness based on the state of the coated powder before pressing, the present invention ensures that the interface layer content is within a range that can play a surface finishing role without significantly weakening the continuity of the ceramic skeleton. This helps to avoid the problems of insufficient interface layer leading to bridging discontinuity or excessive interface layer leading to loose structure.

[0034] 4. By combining spray drying, calcination, surface contact coating, and pressureless sintering densification or hot pressing densification processes, the present invention enables the powder morphology, interface layer distribution, and densification structure to be formed sequentially, thereby enhancing the controllability of the preparation process and the feasibility of film and sheet morphology.

[0035] 5. By limiting the free-flowing liquid phase at 25°C and the relative density to 96-99.5%, the pure solid electrolyte obtained by the present invention can simultaneously serve as a lithium-ion conducting layer and an electronic insulating isolation layer between the positive and negative electrodes of a solid lithium battery, making it suitable for battery systems that require safety and structural stability. Attached Figure Description

[0036] Figure 1 This diagram illustrates the effect of the equivalent mass fraction of the inorganic layer on ionic conductivity and flexural strength in this invention.

[0037] Figure 2 This diagram illustrates the effect of the D50 spherical secondary lithium aluminum titanium phosphate particles of this invention on ionic conductivity and molding yield.

[0038] Figure 3 This diagram illustrates the effect of the equivalent thickness of the inorganic layer surface on ionic conductivity and flexural strength in this invention.

[0039] Figure 4 This is a graph showing the effect of densification temperature on ionic conductivity and relative density in this invention.

[0040] Figure 5 This is a differential distribution diagram of the secondary particle volume reference particle size for Example 1 and Comparative Example 2.

[0041] Figure 6The cumulative particle size distribution diagrams of the secondary particle volume reference are shown for Example 1 and Comparative Example 2.

[0042] Figure 7 The diagram shows the equivalent circle diameter distribution of the primary particle number baseline for Example 1 and Comparative Example 3.

[0043] Figure 8 The diagram shows the cross-sectional width distribution of the intergranular bridging region in Examples 1, 9, and 11.

[0044] Figure 9 This is a graph showing the correlation between the continuity of the B / F signal and the ionic conductivity of the present invention.

[0045] Figure 10 The XPS Survey surface element composition diagrams for Example 1 and Comparative Example 10 are shown.

[0046] Figure 11 This is a graph showing the atomic percentages of B / P / F elements on the XPS surfaces of Example 1 and Comparative Example 10.

[0047] Figure 12 The EIS Nyquist impedance spectra of Example 1, Comparative Example 8, and Comparative Example 9 are shown.

[0048] Figure 13 The graph shows the evolution of the interface impedance growth rate over time for Examples 1, 8, and 9.

[0049] Figure 14 The figures are micro three-point bending load-displacement curves for Example 1, Comparative Example 8, and Comparative Example 9.

[0050] Figure 15 The graph shows the relationship between relative density and flexural strength for the examples and comparative examples.

[0051] Figure 16 This is a comparison of macroscopic optical photographs of the final product of Example 1 and the final product of Comparative Example 5; wherein, Figure 16 In the image, 'a' is a macroscopic optical photograph of the final product of Example 1; Figure 16 b in the image is a macroscopic optical photograph of the final product of Comparative Example 5.

[0052] Figure 17 The images show a comparison of the SEM morphology of the final product of Example 1 and the final product of Comparative Example 5; where, Figure 17 In the image, 'a' is a low-magnification SEM image of the final product of Example 1; Figure 17 In the image, b is a low-magnification SEM image of the final product of Comparative Example 5; Figure 17 In the image, c represents the medium-magnification SEM image of the final product of Example 1; Figure 17 In the image, d represents the medium-magnification SEM image of the final product of Comparative Example 5.

[0053] Figure 18 The images show a comparison of the TEM structures of the final product of Example 1 and the final product of Comparative Example 5; where, Figure 18 In the image, 'a' is the bright-field TEM image of the final product of Example 1; Figure 18 b in the image is the bright-field TEM image of the final product of Comparative Example 5; Figure 18 c in the image represents the SAED and EDS surface scans of the final product from Example 1. Figure 18 In the diagram, d represents the SAED and EDS surface scans of the final product of Comparative Example 5.

[0054] Figure 19 This is a macroscopic optical photographic evolution diagram of the process from S1 to the final product in Example 1; wherein, Figure 19 In the image, 'a' is a macroscopic optical photograph of the S1 precursor slurry. Figure 19 b in the image is a macroscopic optical photograph of the spherical secondary particles of S2 lithium aluminum titanium phosphate. Figure 19 c in the image is a macroscopic optical photograph of lithium aluminum titanium phosphate powder coated with an inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine elements. Detailed Implementation

[0055] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0056] Example 1 S1: In this embodiment, a pure solid electrolyte in the form of a self-supporting film is prepared, with a target composition of Li. 1.2 Al 0.2 Ti 1.8 (PO4)3 type lithium aluminum titanium phosphate spherical secondary particles and an inorganic layer containing lithium, boron, phosphorus, oxygen and fluorine. Based on a theoretical ratio of 0.500 mol / L lithium aluminum titanium phosphate, 22.17 g of commercially available analytical grade lithium carbonate, 5.10 g of commercially available analytical grade alumina, 71.88 g of commercially available analytical grade titanium dioxide, and 172.55 g of commercially available analytical grade ammonium dihydrogen phosphate were weighed. The above raw materials were added to a dispersion medium consisting of 543.4 g of ethanol and 543.4 g of deionized water to achieve a precursor slurry solid content of 20 wt%. The mixture was mechanically pre-stirred for 30 min at 25°C and normal air pressure.

[0057] S2: The precursor slurry was transferred to a ball mill and milled for 4 hours at 200 rpm using commercially available zirconia grinding media. After milling, large agglomerates were removed by passing the slurry through an 80-mesh sieve. The slurry was then spray-dried into pellets at an inlet temperature of 160℃ and an outlet temperature of 75℃, yielding spherical precursor particles. These spherical precursor particles were placed in an alumina crucible and heated to 700℃ at a rate of 3℃ / min in air, held for 2 hours, and then naturally cooled to 25℃ to obtain spherical secondary lithium aluminum titanium phosphate particles. The volumetric D50 of this powder, measured by wet laser diffraction particle size analysis, was 1.0 μm. The equivalent circular diameter of 120 primary particles, as determined by SEM image analysis, was 80 nm.

[0058] S3: Take 100.00g of the spherical secondary lithium aluminum titanium phosphate particles of this embodiment and add them to a mixed solvent consisting of 40.0g of ethanol and 160.0g of deionized water, with a mass ratio of ethanol to deionized water of 1:4. While stirring, add 0.30 parts by mass of phosphoric acid, 0.20 parts by mass of boric acid, 0.60 parts by mass of lithium fluoride, and 0.10 parts by mass of lithium hydroxide monohydrate sequentially. The phosphoric acid is calculated based on the pure mass of H3PO4, and the lithium fluoride is added as a solid dispersed phase. The pH of the dispersion system is controlled at 4.5 at 25°C. Stir at 300 rpm for 0.5 h at 25°C. At the end of stirring, take samples from the upper, middle, and bottom layers for observation; no non-redispersible hard deposits are observed. Dry the resulting dispersion system at 60°C for 2 h, and then heat-treat it at 250°C for 0.5 h in air to obtain lithium aluminum titanium phosphate powder coated with an inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine.

[0059] S4: The coated powder is placed into a film forming mold and pre-pressed at 80 MPa for 30 seconds to obtain a complete preform film. The preform film is heated to 850℃ in air at a rate of 2℃ / min and held for 0.5 hours to complete pressureless sintering and densification. After natural cooling to 25℃, a self-supporting film with an average thickness of 30 μm is obtained. This self-supporting film is not attached with metal foil, polymer support layer, or ceramic substrate and can be clamped and transferred as a whole. The thickness was recorded at five measurement points, with an average value of 30 μm and a maximum deviation of 2 μm.

[0060] Quality testing methods and results: In this embodiment, the equivalent mass fraction of the inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine was calculated based on the mass of the blank heat-treated residue, resulting in 1.0 ± 0.1 wt%, n=3. The equivalent surface thickness was calculated from the mass of the blank residue, the BET specific surface area of ​​the uncoated powder, and the true density of the blank residue, resulting in 10 ± 2 nm, n=3. The relative density of the densified sample, measured by the Archimedes method, was 96.0 ± 0.2%, n=3. The sample was placed on a clean glass plate at 25°C and tilted at 45° for 24 hours. No visible droplet migration or liquid outflow was observed on the sample surface and in the contact area with the glass plate. SEM-EDS observation of the polished cross-section showed that the boron and fluorine signals were connected to the inorganic layer on the surface of adjacent particles, and the cross-sectional width of the interparticle contact area was 12 ± 3 nm, n=3.

[0061] Features and application scenarios of this embodiment: This embodiment employs a relatively conservative aluminum doping level, a small secondary particle size, a fine primary particle size, and mild coating and densification conditions, making it suitable for preparing thin, self-supporting pure solid electrolyte films. This approach facilitates feasibility under conditions of low-load interface layers, low-temperature short-time processing, and thin-film forming, and is suitable for solid-state lithium battery separators with high requirements for film thickness and pure solid state.

[0062] Example 2 Raw materials and proportions: In this embodiment, a pure solid electrolyte in sheet form is prepared, with lithium aluminum titanium phosphate as the main component. 1.5 Al 0.5 Ti 1.5 (PO4)3. Based on the theoretical ratio of 0.500 mol lithium aluminum titanium phosphate, weigh out 27.71 g of commercially available analytical grade lithium carbonate, 12.75 g of commercially available analytical grade alumina, 59.90 g of commercially available analytical grade titanium dioxide, and 172.55 g of commercially available analytical grade ammonium dihydrogen phosphate. Add the above raw materials to 166.8 g of ethanol and 166.8 g of deionized water to form a precursor slurry with a solid content of 45 wt%. First, mechanically stir at 25 °C for 45 min to disperse the inorganic raw materials in ethanol and deionized water.

[0063] Preparation process: The precursor slurry was ball-milled at 500 rpm for 12 h. After ball milling, the slurry was kept in a pumpable state and fed into a spray drying device. The inlet temperature of the spray dryer was 220℃, and the outlet temperature was 110℃. Spherical precursor particles were collected. The spherical precursor particles were placed in an air atmosphere and heated to 950℃ at a rate of 5℃ / min. After holding at this temperature for 10 h, the temperature was cooled to 25℃ in the furnace to obtain spherical secondary lithium aluminum titanium phosphate particles. The volumetric standard D50 of this powder was measured to be 8.0 μm using laser diffraction particle size analysis in wet mode. The equivalent circle diameter of 130 primary particles was statistically analyzed using TEM images, and the numerical standard D50 was 300 nm.

[0064] Post-processing: 100.00g of the lithium aluminum titanium phosphate spherical secondary particles of this embodiment were taken and added to a mixed solvent consisting of 160.0g of ethanol and 40.0g of deionized water, with a mass ratio of ethanol to deionized water of 4:1. 2.00 parts by mass of phosphoric acid, 1.50 parts by mass of boric acid, 1.00 parts by mass of lithium fluoride, and 1.50 parts by mass of lithium hydroxide monohydrate were added sequentially, with lithium fluoride added as a solid dispersion phase. The pH of the dispersion system was controlled to be 6.5 at 25°C by adjusting the order of addition and stirring. The system was stirred at 600 rpm for 4 hours at 60°C, then dried at 120°C for 12 hours, and finally heat-treated at 450°C for 4 hours under a nitrogen atmosphere to obtain the coated powder.

[0065] Forming and densification: The coated powder was loaded into a sheet mold and pre-pressed at 120 MPa for 60 seconds to form a uniform sheet. Subsequently, hot pressing densification was performed by heating to 1000℃ under 40 MPa pressure and holding at that temperature for 4 hours at a heating rate of 4℃ / min. After holding at that temperature, the temperature was lowered to below 200℃ while maintaining pressure, then the pressure was released and the sheet was cooled to 25℃. The resulting sheet had an average thickness of 500 μm, with a thickness deviation of 15 μm at five measuring points.

[0066] Quality testing methods and results: Based on the mass of the blank heat treatment residue, the equivalent mass fraction of the inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine elements in this embodiment is 5.6±0.2wt%, n=3; the equivalent surface thickness is 80±4nm, n=3. The bulk density of the sheet was measured by the Archimedes method, and the relative density was obtained by weighted conversion based on the mass fraction of the uncoated lithium aluminum titanium phosphate phase XRD refined density and the true density of the inorganic layer, resulting in a relative density of 99.5±0.1%, n=3. After being tilted at 45° for 24 hours at 25℃, no liquid flow was observed from the sheet surface and the contact area with the glass plate. TEM-EDS observation showed that the boron and fluorine signals on the particle surface were continuously distributed, with some signals extending to the contact area between adjacent particles. The cross-sectional width of the interparticle contact area was 76±6nm, n=3.

[0067] The features of this embodiment are as follows: This embodiment employs a high level of aluminum doping, large spherical secondary particles, a thicker inorganic interface layer, and strong hot-pressing densification conditions, making it suitable for forming a pure solid-state electrolyte in the form of a highly dense sheet. The formulation parameters of this embodiment are in the high-value range, resulting in high process strength, and it is suitable for solid-state lithium battery components with high requirements for sheet structure stability, densification degree, and consistency in the processing of thick separator layers.

[0068] Example 3 In this embodiment, a self-supporting membrane was fabricated using a pure solid electrolyte. The composition parameter x of the lithium aluminum titanium phosphate spherical secondary particles was 0.35, and the product form was a self-supporting membrane with an average thickness of 120 μm. Based on a theoretical ratio of 0.500 mol lithium aluminum titanium phosphate, 24.94 g of commercially available analytical grade lithium carbonate, 8.92 g of commercially available analytical grade alumina, 65.89 g of commercially available analytical grade titanium dioxide, and 172.55 g of commercially available analytical grade ammonium dihydrogen phosphate were used. These four raw materials were mixed with 317.7 g of ethanol and 317.7 g of deionized water to obtain a precursor slurry with a solid content of 30 wt%.

[0069] In the powder preparation stage, the precursor slurry was ball-milled at 350 rpm for 8 hours, with the slurry temperature controlled between 25-35℃ during the milling process. After ball milling, spray drying was performed at an inlet temperature of 190℃ and an outlet temperature of 92℃ to obtain spherical precursor particles. The spherical precursor particles were then placed under a nitrogen atmosphere and heated to 850℃ at a rate of 4℃ / min and held for 6 hours. After cooling, spherical secondary lithium aluminum titanium phosphate particles were obtained. The volumetric basis D50 of this powder was 4.0 μm, and the numerical basis D50 of 125 primary particles obtained from SEM image analysis was 150 nm.

[0070] In the coating stage, 100.00g of the lithium aluminum titanium phosphate spherical secondary particles of this embodiment were dispersed in a mixed solvent consisting of 100.0g of ethanol and 100.0g of deionized water. 2.00 parts by mass of phosphoric acid, 0.202 parts by mass of boric acid, 0.101 parts by mass of lithium fluoride, and 0.63 parts by mass of lithium hydroxide monohydrate were added, wherein the lithium fluoride was added as a solid dispersion phase. Taking the molar amount of phosphorus in the phosphoric acid as 1, the molar ratio of boron, phosphorus, and fluorine was 0.16:1:0.19, and the ratio of the total molar amount of lithium introduced by lithium hydroxide monohydrate and lithium fluoride to the sum of the molar amounts of boron and phosphorus was 0.80:1. The pH of the dispersion system was controlled at 5.0 at 25°C, stirred at 450 rpm for 2 hours at 45°C, dried at 90°C for 6 hours, and then heat-treated at 350°C for 2 hours under a nitrogen atmosphere to obtain the coated powder.

[0071] In the self-supporting film forming stage, the coated powder was placed in a film forming mold and pre-pressed at 100 MPa for 45 s to obtain a preform film with overall integrity. The preform film was placed in a nitrogen atmosphere and heated to 1000℃ at a rate of 3℃ / min and held for 4 h to complete pressureless sintering densification. After cooling, a self-supporting film with an average thickness of 120 μm was obtained. The sample could be clamped and transferred as a whole without the attachment of metal foil, polymer support layer or ceramic substrate, and the average thickness of five measurement points was 120 μm.

[0072] Quality testing methods and results: The D50 of secondary particles was determined using laser diffraction particle size analysis in wet mode, and D10, D50, and D90 were recorded. The D50 of primary particles was determined using SEM image statistical methods. The equivalent mass fraction of the inorganic layer, calculated from the mass of the blank heat treatment residue, was 2.5 ± 0.1 wt%, n=3. The equivalent surface thickness, calculated using the BET specific surface area and helium specific gravity method, was 30 ± 3 nm, n=3. The relative density, measured by the Archimedes method, was 98.4 ± 0.2%, n=3. After holding the sample at a 45° incline at 25℃ for 24 h, no visible droplet migration was observed in the contact area between the sample surface and the glass plate. SEM-EDS observation of the ion beam cross-section showed that boron and fluorine-containing signals were located between adjacent particle profiles and connected to the inorganic layer on the particle surface. The cross-sectional width of the interparticle contact area was 31 ± 4 nm, n=3.

[0073] The process features and application directions of this embodiment are as follows: This embodiment uses medium-sized spherical secondary particles, a low-value boron and fluorine element ratio, a relatively high densification temperature, and a self-supporting film morphology, which is suitable for demonstrating the stability of film material preparation under a wide process window. This scheme takes into account the powder dispersibility, the continuity of the interface layer, and the overall transferability of a relatively thick self-supporting film, and is suitable for solid-state lithium battery electrolyte layers that require strong mechanical support.

[0074] Example 4 I. Preparation Object and Raw Material State: In this embodiment, a pure solid electrolyte in sheet form is prepared. The target composition of the lithium aluminum titanium phosphate spherical secondary particles is Li. 1.45 Al 0.45 Ti 1.55 (PO4)3. Based on the theoretical ratio of 0.500 mol lithium aluminum titanium phosphate, weigh 26.79 g of commercially available analytical grade lithium carbonate, 11.47 g of commercially available analytical grade alumina, 61.90 g of commercially available analytical grade titanium dioxide, and 172.55 g of commercially available analytical grade ammonium dihydrogen phosphate. Add the above raw materials to 253.2 g of ethanol and 253.2 g of deionized water to form a precursor slurry with a solid content of 35 wt%, and stir at 25 °C for 40 min.

[0075] II. Particle Construction Process: The precursor slurry was ball-milled at 200 rpm for 6 hours. After ball milling, it was spray-dried at an inlet temperature of 180℃ and an outlet temperature of 90℃. The resulting spherical precursor particles were heated to 900℃ in air at a rate of 4℃ / min and held for 4 hours, followed by natural cooling to 25℃ to obtain spherical secondary lithium aluminum titanium phosphate particles. The volumetric D50 of this powder, measured by laser diffraction in wet mode, was 6.0 μm; TEM images showed that 120 primary particles had a number D50 of 250 nm.

[0076] III. Interface Layer Formation Process: 100.00g of the lithium aluminum titanium phosphate spherical secondary particles of this embodiment were added to a mixed solvent consisting of 120.0g of ethanol and 80.0g of deionized water, with a mass ratio of ethanol to deionized water of 3:2. While stirring, 0.80 parts by mass of phosphoric acid, 0.40 parts by mass of boric acid, 0.106 parts by mass of lithium fluoride, and 1.36 parts by mass of lithium hydroxide monohydrate were added sequentially. Lithium fluoride was added as a solid dispersion phase. Taking the molar amount of phosphorus in phosphoric acid as 1, the molar ratio of boron, phosphorus, and fluorine was 0.80:1:0.50. The ratio of the total molar amount of lithium introduced by lithium hydroxide monohydrate and lithium fluoride to the sum of the molar amounts of boron and phosphorus was 2.50:1. The pH value of the dispersion system was controlled at 5.8 at 25°C, and the mixture was stirred at 400 rpm for 1 hour at 30°C. After stirring, the system was dried at 80°C for 4 hours, and then heat-treated at 300°C in air for 1 hour to obtain coated powder.

[0077] IV. Sheet Forming and Densification: The coated powder is added to a thickness-controllable mold and pre-pressed at 90 MPa for 40 seconds to obtain a blank sheet. The blank sheet is then densified by hot pressing, with the temperature raised to 850℃ under 10 MPa pressure and held for 0.5 hours at a heating rate of 3℃ / min. After the holding period, the temperature is lowered to below 150℃ while maintaining pressure, then the pressure is released and the sheet is cooled to 25℃ to obtain a sheet with an average thickness of 100 μm. The average thickness at five measurement points is 100 μm, with a maximum deviation of 5 μm.

[0078] V. Quality Testing Methods and Results: In this embodiment, the equivalent mass fraction of the inorganic layer, converted from the mass of the blank heat treatment residue, was 2.8 ± 0.1 wt%, n=3; the equivalent surface thickness, calculated based on the mass of the blank residue, BET specific surface area, and true density using the helium specific gravity method, was 50 ± 4 nm, n=3. The relative density measured by the Archimedes method was 96.5 ± 0.2%, n=3. The sheet was placed on a clean glass plate at 25°C and tilted at 45° for 24 hours; no visible liquid migration was observed. TEM-EDS and EELS observations showed that lithium, boron, phosphorus, oxygen, and fluorine were distributed on the particle surface and in the contact area between adjacent particles; the cross-sectional width of the interparticle contact area was 48 ± 5 nm, n=3.

[0079] The applicable scenario for this embodiment: This embodiment adopts a relatively high aluminum doping level, a relatively high boron and fluorine element ratio, a relatively low hot-pressing pressure, and a relatively thin sheet structure, which is suitable for reflecting the coordination relationship between sheet forming and the spatial distribution of the interface layer. This solution is applicable to solid-state lithium battery pure solid electrolyte components that require thin sheets, low hot-pressing pressure, and controllable interface inorganic layers.

[0080] Comparative Example 1: Basically the same as Example 1, except that the composition parameter x of lithium aluminum titanium phosphate in step S1 is adjusted to 0.10, and the materials are fed according to the molar ratio of Li, Al, Ti and P of 1.10:0.10:1.90:3. Based on the theoretical ratio of 0.500 mol lithium aluminum titanium phosphate, 20.32 g of lithium carbonate, 2.55 g of aluminum oxide, 75.88 g of titanium dioxide and 172.55 g of ammonium dihydrogen phosphate are weighed, and other conditions remain unchanged.

[0081] Comparative Example 2: Basically the same as Example 1, except that the atomization conditions in the spray drying spherical formation process of step S2 are adjusted so that the volume reference D50 of the lithium aluminum titanium phosphate spherical secondary particles obtained after calcination is 0.6 μm. Specifically, the slurry feed rate is controlled at 10 mL / min, the atomizing disc rotation speed is controlled at 30000 rpm, the spray drying inlet temperature is still 160℃, the outlet temperature is still 75℃, and other conditions remain unchanged.

[0082] Comparative Example 3: It is basically the same as Example 1, except that in step S2, the ball milling speed is adjusted to 600 rpm and the ball milling time is kept at 4 h, so that the primary particle number D50 in the spherical secondary lithium aluminum titanium phosphate particles obtained after calcination is 50 nm, and other conditions remain unchanged.

[0083] Comparative Example 4: Basically the same as Example 1, except that the equivalent mass fraction of the inorganic layer containing lithium, boron, phosphorus, oxygen and fluorine elements in step S3 is adjusted to 0.5 wt%. Specifically, 0.15 parts by mass of phosphoric acid, 0.10 parts by mass of boric acid, 0.30 parts by mass of lithium fluoride and 0.05 parts by mass of lithium hydroxide monohydrate are added to 100.00 g of lithium aluminum titanium phosphate spherical secondary particle dispersion system, while other conditions remain unchanged.

[0084] Comparative Example 5: Basically the same as Example 1, except that the surface equivalent thickness of the inorganic layer containing lithium, boron, phosphorus, oxygen and fluorine elements in step S3 is adjusted to 100 nm. Specifically, 3.00 parts by mass of phosphoric acid, 2.00 parts by mass of boric acid, 6.00 parts by mass of lithium fluoride and 1.00 parts by mass of lithium hydroxide monohydrate are added to 100.00 g of lithium aluminum titanium phosphate spherical secondary particle dispersion system, and dried and heat-treated in the same way as in Example 1, with other conditions unchanged.

[0085] Comparative Example 6: It is basically the same as Example 1, except that the pH value of the dispersion system read at 25°C in step S3 is adjusted to 3.8. Specifically, after adding phosphoric acid, boric acid, lithium fluoride and lithium hydroxide monohydrate, the pH value is adjusted to 3.8 using a 0.1 mol / L phosphoric acid aqueous solution, while other conditions remain unchanged.

[0086] Comparative Example 7: It is basically the same as Example 1, except that the heat treatment temperature in step S3 is adjusted to 200°C, the heat treatment time is still 0.5h, the atmosphere is still air, and other conditions remain unchanged.

[0087] Comparative Example 8: It is basically the same as Example 1, except that the pressureless sintering densification temperature in step S4 is adjusted to 800°C, the holding time is still 0.5h, the heating rate is still 2°C / min, the atmosphere is still air, and other conditions remain unchanged.

[0088] Comparative Example 9: Essentially the same as Example 1, except that phosphoric acid, boric acid, lithium fluoride, and lithium hydroxide monohydrate were not added in step S3. 100.00 g of lithium aluminum titanium phosphate spherical secondary particles were added to a mixed solvent consisting of 40.0 g of ethanol and 160.0 g of deionized water. After stirring at 300 rpm for 0.5 h, the mixture was dried at 60 °C for 2 h, and then heat-treated at 250 °C for 0.5 h in air. The process then proceeded to step S4, with other conditions remaining unchanged. This comparative example was used to verify the synergistic effect of lithium aluminum titanium phosphate spherical secondary particles and the inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine.

[0089] Comparative Example 10: Essentially the same as Example 1, except that in step S3, only 0.30 parts by mass of phosphoric acid and 0.10 parts by mass of lithium hydroxide monohydrate were added; boric acid and lithium fluoride were not added. The pH value of the dispersion system read at 25°C was still controlled at 4.5. The mixture was stirred, dried, heat-treated, and densified under the same conditions as in Example 1, with other conditions remaining unchanged. This comparative example was used to verify the synergistic effect of boric acid, lithium fluoride, phosphoric acid, and lithium hydroxide monohydrate in constructing an inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine.

[0090] Comparative Example 11: Essentially the same as Example 1, except that the in-situ contact treatment of phosphoric acid, boric acid, lithium fluoride, and lithium hydroxide monohydrate on the surface of lithium aluminum titanium phosphate spherical secondary particles in step S3 was omitted. Instead, without adding lithium aluminum titanium phosphate spherical secondary particles, 0.30 parts by weight of phosphoric acid, 0.20 parts by weight of boric acid, 0.60 parts by weight of lithium fluoride, and 0.10 parts by weight of lithium hydroxide monohydrate were dispersed in 40.0 g of ethanol and 160.0 g of deionized water, dried at 60°C for 2 h, and heat-treated at 250°C for 0.5 h in air. After grinding to pass through a 200-mesh sieve, it was mechanically mixed with 100.00 g of lithium aluminum titanium phosphate spherical secondary particles for 30 min, and then proceeded to step S4, with other conditions remaining unchanged. This comparative example was used to verify the synergistic effect of the in-situ interface construction method and the interparticle bridging construction method.

[0091] Characterization and performance testing: Hierarchical particle size analysis was performed on spherical secondary lithium aluminum titanium phosphate particles to evaluate whether the sphericity of the secondary particles and the agglomeration scale of the primary particles met the requirements of the particle hierarchy structure. Secondary particles were analyzed using a wet laser diffraction particle size analysis method, recording the dispersion medium, D10, D50, D90, and volumetric baseline distribution. Primary particles were analyzed using SEM or TEM images to statistically determine the equivalent circle diameter of at least 100 identifiable particles, outputting the numerical baseline D50. The corresponding data fields are secondary particle D50 and primary particle D50, in μm and nm, respectively.

[0092] The flowability and processability of the coated powder were evaluated to verify the influence of spherical secondary particles and the surface inorganic layer on the powder processing behavior before pressing. The dried coated powder was equilibrated for 2 hours at 25℃ and relative humidity below 40%. The angle of repose, loose density, and tapped density were measured, and the Hausner ratio was calculated. Thirty samples were continuously pressed using the same mold, and the proportion of crack-free preforms was recorded. The corresponding data fields are the average angle of repose, standard deviation of the angle of repose, average molding pass rate, and standard deviation of the molding pass rate, in degrees and % respectively. A lower angle of repose and a higher molding pass rate indicate better processability.

[0093] The inorganic layers containing lithium, boron, phosphorus, oxygen, and fluorine were evaluated for mass, thickness, and interparticle bridging regions to confirm the surface layer loading and spatial distribution. The equivalent mass fraction was calculated based on the weight of the blank heat-treated residue; the equivalent surface thickness was calculated using the BET specific surface area of ​​the uncoated powder, the true density of the residue, and the mass of the residue; elemental distribution data were collected from the densified cross-section using SEM-EDS, TEM-EDS, or EELS, and the width of the boron- and fluorine-containing connectivity regions between adjacent particles was statistically analyzed. The corresponding data fields are equivalent mass fraction of the inorganic layer, equivalent surface thickness, and bridging region width, in units of wt%, nm, and nm, respectively. The average connectivity rate of boron- and fluorine-containing regions is expressed as the percentage of continuous contact areas between the boron- and fluorine-containing signal regions and the surface regions of adjacent particles in the cross-sectional elemental distribution map, relative to the total number of statistically analyzed contact areas.

[0094] The relative density and open porosity of the densified self-supporting membranes or sheets were evaluated to characterize structural integrity. Samples were dried at 80℃ for 2 hours and then cooled to 25℃. The dried mass, immersed mass, and saturated mass were measured using anhydrous ethanol as the medium, and the bulk density was calculated. The theoretical density was obtained by weighting the density of the uncoated lithium aluminum titanium phosphate phase with the true density of the inorganic layer according to the mass fraction of the volumetric density. The relative density was the ratio of the bulk density to the theoretical density. The corresponding data fields are the average relative density, the standard deviation of the relative density, the average open porosity, and the standard deviation of the open porosity, in percentages (%). Higher relative density and lower open porosity indicate more complete densification.

[0095] Ionic conductivity tests were performed on pure solid-state electrolyte samples at 25°C to evaluate the influence of interparticle interfaces and densification structures on lithium-ion conduction. Gold was sputtered onto both sides of the sample, or a blocking electrode was coated. After equilibration at 25°C for 1 hour, AC impedance spectroscopy was performed. The frequency range was set from 1MHz to 0.1Hz, and the perturbation voltage was 10mV. The total ionic conductivity was calculated from the total resistance, sample thickness, and electrode area in the Nyquist plot. The corresponding data fields are the average total ionic conductivity at 25°C and the standard deviation of the total ionic conductivity at 25°C, in mS / cm; higher values ​​indicate better ionic conduction performance.

[0096] Impedance evolution was used to evaluate the interfacial stability of pure solid electrolytes, reflecting the contribution of inorganic layers containing lithium, boron, phosphorus, oxygen, and fluorine to the interfacial impedance stability under blocked electrode contact conditions. The sample was clamped between inert blocked electrodes and placed at 25°C in a dry, inert atmosphere. The initial impedance and the interfacial impedance after 100 hours were recorded, normalized by area, and the growth rate was calculated. The corresponding data fields are the average interfacial impedance growth rate and the standard deviation of the interfacial impedance growth rate, in %; lower values ​​indicate better interfacial stability.

[0097] Mechanical integrity evaluation of self-supporting membranes or sheets is used to verify the supporting role of the dense ceramic skeleton and intergranular bridging regions in overall transfer and resistance to flexural failure. Samples are cut into strips or sheets of uniform width and subjected to micro-three-point bending or equivalent clamping bending tests at 25°C and relative humidity below 40%. The maximum load, span, thickness, and width are recorded, and the flexural strength is calculated; at least five samples are used per group. The corresponding data fields are the average flexural strength and the standard deviation of flexural strength, in MPa; higher values ​​indicate better structural integrity.

[0098] Free-flowing liquid phase determination was performed on the densified pure solid electrolyte to confirm the pure solid state of the sample at 25°C. The sample was placed on a clean glass plate and tilted at 45° for 24 hours. The presence of visible droplet migration, flow marks, or liquid outflow on the sample surface, edges, and the contact area with the glass plate was recorded. The corresponding data fields are liquid phase migration length and visual determination result, in mm and qualitative grade. When no visible droplet migration, flow marks, or liquid outflow was observed, the liquid phase migration length was recorded as 0 mm. This experiment serves as a template for recording the pure solid state and does not generate a performance ranking.

[0099] Figure 1 This diagram illustrates the effect of the equivalent mass fraction of the inorganic layer on ionic conductivity and flexural strength in this invention. Figure 2 This diagram illustrates the effect of D50 of the lithium aluminum titanium phosphate spherical secondary particles of this invention on ionic conductivity and molding yield. Figure 3 This diagram illustrates the effect of the equivalent thickness of the inorganic layer surface on ionic conductivity and flexural strength in this invention. Figure 4This diagram illustrates the effect of densification temperature on ionic conductivity and relative density according to the present invention. Figures 1 to 4 This is used to illustrate the influence of various core process parameters on the overall performance of pure solid-state electrolytes. Specifically, Figure 1 Based on the preparation system of Example 1, with the basic parameters such as secondary particle D50, primary particle size, pH, heat treatment conditions and densification temperature fixed, only the equivalent mass fraction of the inorganic layer was changed from 0.5 to 8.0 wt%. The results showed that the ionic conductivity and flexural strength both increased and then decreased with the increase of inorganic layer content. This indicates that when the inorganic layer is too small, it is difficult to form a continuous interface phase, while when it is too large, it will increase the proportion of non-main phase and affect the density of the structure. Figure 2 Based on Example 1, with the equivalent mass fraction of the inorganic layer, the primary particle size, the inorganic layer thickness, and the densification temperature fixed, only the D50 of the lithium aluminum titanium phosphate spherical secondary particles was changed to 0.6–10.0 μm. The results showed that when the secondary particle size was in a moderate range, the molding qualification rate and ionic conductivity could be maintained at a high level at the same time. This indicates that the spherical secondary particles can improve the flowability and packing uniformity of the powder without significantly sacrificing the continuity of the ion transport channels. Figure 3 Based on the preparation system of Example 1, the secondary particle D50, primary particle size, pH, heat treatment conditions and densification temperature were kept constant. By adjusting the coating conditions, the equivalent thickness of the inorganic layer surface was made to be 5–100 nm. The results showed that there is a suitable range for the thickness of the inorganic layer. When it is too thin, the interparticle bridging is insufficient, and when it is too thick, the interfacial impedance increases. Figure 4 Under conditions of fixed inorganic layer composition, secondary particle D50, primary particle size, and inorganic layer thickness, changing only the densification temperature to 800–1050℃, the results show that the relative density increases with increasing temperature, but the ionic conductivity reaches an optimal state in the moderate temperature range. This indicates that proper densification can promote particle bonding and reduce porosity defects, while avoiding grain boundary deterioration caused by overheating. Figures 1 to 4 It is evident that this scheme achieves a balance between processing and shaping, ion conduction, and mechanical support through the coordinated control of inorganic layer content, secondary particle size, interface layer thickness, and densification temperature.

[0100] Figure 5 This is a differential particle size distribution diagram of the secondary particle volume reference of the present invention. Figure 6 This is a cumulative distribution diagram of the secondary particle volume reference particle size in this invention. Figure 7 This is a diagram showing the distribution of the equivalent circle diameter for the primary particle quantity reference of this invention. Figure 5 and Figure 6The secondary particle size of Example 1 and Comparative Example 2 was characterized from the perspectives of differential distribution and cumulative distribution, respectively. The results showed that the secondary particle size distribution of Example 1 was more concentrated, with D10, D50 and D90 in a more reasonable distribution range. However, the particle size of Comparative Example 2 was generally smaller and the distribution peak position shifted forward, indicating that excessively small secondary particles are prone to increasing the risk of powder agglomeration and uneven interface accumulation, which is not conducive to stable forming. Figure 7 By statistically analyzing the equivalent circle diameter of the primary particle count in Example 1 and Comparative Example 3 using SEM or TEM images, the results showed that the primary particles in Example 1 were mainly concentrated around 80 nm, while the primary particles in Comparative Example 3 were smaller. This indicates that while excessive nano-sizing of primary particles can increase surface activity, it also makes them more prone to aggregation and interface defects. Figures 5 to 7 As can be seen, Example 1 maintains powder flowability and forms a skeleton by using spherical secondary particles, and retains the active conductive interface by using moderate primary particle size, thereby weakening the contradiction between "large particle size reduces interfacial conduction" and "nano-sized particles lead to agglomeration" at the particle level.

[0101] Figure 8 This is a cross-sectional width distribution diagram of the interparticle bridging region in this invention. Figure 9 This is a graph showing the correspondence between the continuity of boron- and fluorine-containing signals and the total ionic conductivity at 25°C in this invention. Figure 10 This is a surface element composition diagram from the XPS Survey of this invention. Figure 11 This is a graph showing the atomic percentages of B, P, and F elements on the XPS surface of this invention. Figure 8 The width of the interparticle bridging region in Example 1, Comparative Example 9, and Comparative Example 11 was statistically analyzed by cross-sectional SEM-EDS or TEM-EDS. The results showed that Example 1 was able to form a continuous bridging region with a moderate width, Comparative Example 9 almost did not form an effective bridging, and Comparative Example 11 had a smaller bridging region width and insufficient distribution. This indicates that boron- and fluorine-containing interfacial components can promote the formation of a continuous connection structure between particles. Figure 9 Further analysis of the correlation between the average connectivity of boron- and fluorine-containing samples and the total ionic conductivity at 25℃ revealed that samples with better signal continuity in boron- and fluorine-containing samples corresponded to higher ionic conductivity, suggesting that boron- and fluorine-containing bridging networks are related to improved interparticle ion migration pathways. Figure 10 The surface elemental composition of Example 1 and Comparative Example 10 was analyzed by XPS full spectrum. In Example 1, obvious B, F and P related signals were detected, while in Comparative Example 10, B and F signals were not detected or were significantly lower than those in Example 1. Figure 11 Further quantitative comparison of the atomic percentages of B, P, and F elements revealed that Example 1 exhibited higher B and F atomic percentages while maintaining a stable P element signal. This indicates that boron, fluorine, and phosphorus-related surface signals can be detected in the Sample 1 and can be used in conjunction with cross-sectional elemental distribution data to characterize the distribution of boron- and fluorine-containing components on the particle surface and in the intergranular region. Figures 8 to 11It is evident that boron- and fluorine-containing components exhibit detectable spatial distribution and connectivity on particle surfaces and between particles, providing a structural basis for reducing interparticle impedance and improving interfacial stability.

[0102] Figure 12 This is the EIS Nyquist impedance spectrum of the present invention. Figure 13 This is a graph showing the evolution of the interface impedance growth rate over time in this invention. Figure 14 This is a diagram showing the micro three-point bending load-displacement curve of the present invention. Figure 15 This is a graph showing the relationship between relative density and flexural strength of the present invention. Figure 12 The impedance responses of Examples 1, 8, and 9 were tested at 25°C using a blocking electrode. The results showed that Example 1 had a smaller impedance semicircle radius and lower interface impedance, while the impedance arcs of 8 and 9 were significantly increased. This indicates that the continuous interface phase and intergranular bridging structure in Example 1 can effectively improve grain boundary / interface conduction. Figure 13 When the interface impedance change was tracked under static conditions at 25°C, the impedance growth rate of Example 1 was lower and the change was gradual over time, while the impedance growth of Comparative Examples 8 and 9 was faster, indicating that the interface structure of Example 1 has better stability. Figure 14 By comparing the load-displacement response of Example 1, Comparative Example 8 and Comparative Example 9 through micro three-point bending tests, Example 1 showed a higher peak load and a more stable deformation process, indicating that interparticle bridging and densification structure can improve the bending load capacity of the membrane. Figure 15 Summarizing the relative density and flexural strength data of the examples and comparative examples, the results show that the samples from the examples are mainly distributed in the high relative density and high flexural strength regions, indicating that particle size distribution, interface layer construction, and densification processes can synergistically improve the structural integrity of the material. Figures 12 to 15 It can be seen that this solution not only improves ion conduction performance, but also simultaneously enhances interface durability and mechanical reliability, thus addressing the problem of balancing structural integrity and interface stability.

[0103] Figure 16 This is a comparison of macroscopic optical photographs of the final product of Example 1 and the final product of Comparative Example 5. Figure 17 This is a comparison image of the SEM morphology of the final product of Example 1 and the final product of Comparative Example 5. Figure 18 The images show a comparison of TEM, SAED, and EDS surface scans of the final product of Example 1 and the final product of Comparative Example 5. Figure 16 The a in Example 1 shows that the final product is a pure solid electrolyte self-supporting membrane composed of spherical secondary particles of lithium aluminum titanium phosphate of type Li1.2Al0.2Ti1.8(PO4)3 and an inorganic layer containing lithium, boron, phosphorus, oxygen and fluorine elements. The average thickness is 30 μm and the maximum thickness deviation is 2 μm. It can be clamped and transferred as a whole, which indicates that the low load inorganic layer is conducive to the formation of a thin, continuous and transferable membrane. Figure 16 Figure b shows that in Comparative Example 5, under the same lithium aluminum titanium phosphate substrate and film formation sintering conditions, increasing the equivalent thickness of the inorganic layer surface to 100 nm adversely affects the uniformity and overall morphology of the film, indicating that an excessively thick interface layer and its accompanying densification differences will affect the stability of film formation. Figure 17 a and Figure 17 c in the figure are low-magnification and medium-magnification SEM images of the final product of Example 1, showing that the self-supporting film with an average thickness of 30 μm can form a continuous film after pressureless sintering at 850°C for 0.5 h, and the lithium aluminum titanium phosphate spherical secondary particles with a volume reference D50 of 1.0 μm are in uniform contact inside the film. Figure 17 b and Figure 17 In the figure, d is the SEM image corresponding to Comparative Example 5, which shows that the local particle contact and stacking state changes under the condition of thick inorganic layer, indicating that the excessive interface layer and the resulting densification difference may interfere with the effective bonding between particles. Figure 18 a and Figure 18 The c in the image further shows, from the perspectives of TEM, SAED and EDS surface scanning, that a connected structure is formed between the lithium aluminum titanium phosphate grains and the nanoscale interface layer in Example 1, and that the B and F elements are distributed along the surface of adjacent particles or the interparticle region. Figure 18 b and Figure 18 The d-value in the diagram shows that Comparative Example 5 exhibits more pronounced interface enrichment and scale variations under the 100nm inorganic layer condition. Figures 16 to 18 It can be seen that the low-load, thin-film inorganic layer containing lithium, boron, phosphorus, oxygen and fluorine elements in Example 1 can achieve a better match between macroscopic membrane integrity, microscopic particle contact and nano-interface connectivity.

[0104] Figure 19 This is a macroscopic optical photographic evolution diagram of S1 to the final product in Example 1. Figure 19 The figure 'a' shows that the S1 precursor slurry is formed by dispersing lithium carbonate, alumina, titanium dioxide and ammonium dihydrogen phosphate in a mixed medium of ethanol and deionized water, with a solid content of 20 wt%. This indicates that the multi-component raw materials can form a uniform dispersion system that meets the requirements of subsequent ball milling and spray drying. Figure 19 b shows that the S2 lithium aluminum titanium phosphate spherical secondary particles were obtained by ball milling at 200 rpm for 4 h, spray drying, and calcination at 700℃ for 2 h. The volume basis D50 is 1.0 μm, and the quantity basis primary particle D50 is 80 nm, indicating that the precursor can be stably converted into small-diameter spherical ceramic powder. Figure 19The figure shows that S3-coated lithium aluminum titanium phosphate powder containing an inorganic layer of lithium, boron, phosphorus, oxygen, and fluorine was formed after stirring at pH 4.5 for 0.5 h at 25 °C, drying at 60 °C for 2 h, and heat treatment at 250 °C for 0.5 h. The equivalent mass fraction of the inorganic layer is 1.0 ± 0.1 wt%, and the equivalent surface thickness is 10 nm, indicating that a low-load interface layer can be uniformly introduced during the powder formation stage. Figures 16 to 18 The macroscopic integrity, cross-sectional density, and nano-interface connectivity of the final membrane shown demonstrate that this preparation route can achieve a continuous transformation from precursor slurry, spherical secondary particles, coated powder to a thin, pure solid self-supporting membrane.

[0105] In conclusion, Figures 1 to 19 The solution is mutually verified from multiple levels, including parameter optimization, particle size, interface composition, impedance behavior, mechanical response, film morphology, and process evolution. This solution ensures processing fluidity and a formed skeleton through spherical secondary particles, maintains an effective conductive interface through primary particle size control, and constructs a continuous interparticle interface phase through inorganic layers containing lithium, boron, phosphorus, oxygen, and fluorine. This simultaneously improves ion conduction, structural integrity, and interface stability, effectively solving the problem of balancing processing fluidity and ion conduction performance, as well as structural integrity and interface stability in pure solid electrolytes.

[0106] Table 1 Performance of Examples and Comparative Examples As can be seen from the performance trends of the examples and comparative examples in Table 1, Examples 1-4 exhibit a relatively balanced performance combination in terms of powder flowability, molding qualification rate, densification degree, interparticle bridging region width, total ionic conductivity at 25℃, interfacial impedance growth rate, and flexural strength. Among them, Example 3 shows more concentrated performance in ionic conduction and interfacial stability, while Example 2 shows more outstanding performance in densification and mechanical integrity. After changing the composition, particle size, interfacial layer load, pH, heat treatment, and densification temperature of Comparative Examples 1-8, the relevant indicators showed different degrees of deviation. After disassembling or destroying the inorganic layer and the in-situ interface construction method in Comparative Examples 9-11, the bridging region width, interfacial impedance growth rate, and ionic conductivity changed synchronously, indicating that there is a clear data correlation between the particle hierarchical structure, the lithium-boron-phosphorus-oxygen-fluorine-containing inorganic layer, and the interparticle bridging construction.

[0107] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that any equivalent structural transformations made under the concept of the present invention and using the contents of the specification and drawings of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A pure solid electrolyte, characterized in that, include: The lithium aluminum titanium phosphate spherical secondary particles are obtained by reacting lithium carbonate, aluminum oxide, titanium dioxide and ammonium dihydrogen phosphate. The D50 of the lithium aluminum titanium phosphate spherical secondary particles is 1-8 μm, and they are formed by the agglomeration of primary particles with D50 of 80-300 nm. An inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine is formed by coating the surface of the lithium aluminum titanium phosphate spherical secondary particles in a powder state before pressing, and by at least partially occupying the interparticle bridging region between adjacent lithium aluminum titanium phosphate spherical secondary particles after densification. The inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine is formed by contacting phosphoric acid, boric acid, lithium fluoride, and lithium hydroxide monohydrate with the lithium aluminum titanium phosphate spherical secondary particles and then drying and heat-treating. The equivalent mass fraction of the inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine, based on the powder state before pressing, is 1-6 wt%, and the total mass of the lithium aluminum titanium phosphate spherical secondary particles and the inorganic layer containing lithium, boron, phosphorus, oxygen, and fluorine is 10-80 nm. The pure solid electrolyte was determined to be free of free-flowing liquid phase by holding it at a 45° angle at 25°C for 24 hours, and its relative density was 96-99.5%.

2. The pure solid electrolyte according to claim 1, characterized in that, The lithium aluminum titanium phosphate spherical secondary particles coated with an inorganic layer containing lithium, boron, phosphorus, oxygen and fluorine elements are prepared by the following steps: A1. Provides spherical secondary lithium aluminum titanium phosphate particles; A2. Disperse 100 parts by weight of the lithium aluminum titanium phosphate spherical secondary particles in a mixed solvent of 200 parts by weight of ethanol and deionized water, wherein the mass ratio of ethanol to deionized water in the mixed solvent is 1:4 to 4:

1. Add 0.3-2.0 parts by weight of phosphoric acid, 0.2-1.5 parts by weight of boric acid, 0.1-1.0 parts by weight of lithium fluoride and 0.1-1.5 parts by weight of lithium hydroxide monohydrate, and control the pH value of the resulting dispersion system at 25°C to be 4.5-6.

5. A3. Stir at 300-600 rpm for 0.5-4 hours at 25-60℃; A4. After drying at 60-120℃ for 2-12 hours, heat-treat at 250-450℃ for 0.5-4 hours in air or nitrogen atmosphere; A5. An inorganic layer containing lithium, boron, phosphorus, oxygen and fluorine elements is obtained with a surface equivalent thickness of 10-80 nm based on the state of the coated powder before pressing.

3. The pure solid electrolyte according to claim 2, characterized in that, The lithium aluminum titanium phosphate spherical secondary particles in step A1 are prepared through the following steps: B1. A precursor slurry is prepared by mixing lithium carbonate, aluminum oxide, titanium dioxide, and ammonium dihydrogen phosphate with ethanol and deionized water in a molar ratio of lithium, aluminum, titanium, and phosphorus of (1+x):x:(2-x):3, where x is 0.2-0.

5. B2. The precursor slurry is ball-milled and mixed; B3. Spray dry the ball-milled precursor slurry to obtain spherical precursor particles, wherein the spray drying inlet temperature is 160-220℃ and the outlet temperature is 75-110℃; B4. The precursor spherical particles are calcined at 700-950℃ for 2-10h in air or nitrogen atmosphere to obtain the lithium aluminum titanium phosphate spherical secondary particles with D50 of 1-8μm.

4. The pure solid electrolyte according to claim 2, characterized in that, In step A2, taking the molar amount of phosphorus in phosphoric acid as 1, the molar ratio of boron, phosphorus and fluorine is 0.16-0.8:1:0.19-0.5, and the ratio of the total molar amount of lithium introduced by lithium hydroxide monohydrate and lithium fluoride to the sum of the molar amounts of boron and phosphorus is 0.8-2.5:

1.

5. The pure solid electrolyte according to claim 1, characterized in that, The pure solid electrolyte is a self-supporting membrane or sheet; the thickness of the self-supporting membrane is 30-120 μm, and the thickness of the sheet is 100-500 μm.

6. A method for preparing a pure solid electrolyte as described in any one of claims 1-5, characterized in that, Includes the following steps: S1. Lithium carbonate, aluminum oxide, titanium dioxide and ammonium dihydrogen phosphate are mixed with ethanol and deionized water and ball-milled to obtain a precursor slurry; S2. The precursor slurry is spray-dried and calcined to obtain spherical secondary particles of lithium aluminum titanium phosphate; S3. The spherical secondary particles of lithium aluminum titanium phosphate are contacted with phosphoric acid, boric acid, lithium fluoride and lithium hydroxide monohydrate in a mixed solvent of ethanol and deionized water, and then dried and heat-treated to form an inorganic layer containing lithium, boron, phosphorus, oxygen and fluorine elements, thereby obtaining lithium aluminum titanium phosphate powder coated with an inorganic layer containing lithium, boron, phosphorus, oxygen and fluorine elements. S4. The lithium aluminum titanium phosphate powder coated with an inorganic layer containing lithium, boron, phosphorus, oxygen and fluorine elements obtained in step S3 is pre-pressed at 80-120MPa for 30-60s to form a blank, and then subjected to pressureless sintering densification or hot pressing densification to obtain the pure solid electrolyte.

7. The preparation method according to claim 6, characterized in that, In step S1, lithium carbonate, alumina, titanium dioxide, and ammonium dihydrogen phosphate are fed in a molar ratio of lithium, aluminum, titanium, and phosphorus of (1+x):x:(2-x):3, where x is 0.2-0.

5. The solid content of the precursor slurry is 20-45 wt%. Based on the total mass of the precursor slurry, the ball milling speed is 200-500 rpm, and the ball milling time is 4-12 h.

8. The preparation method according to claim 7, characterized in that, In step S3, ethanol and deionized water are used as a mixed solvent, with a mass ratio of ethanol to deionized water of 1:4 to 4:

1. The resulting dispersion system has a pH value of 4.5-6.5 read at 25°C. The stirring speed is 300-600 rpm, the stirring time is 0.5-4 h, the drying conditions are 60-120°C for 2-12 h, and the heat treatment conditions are 250-450°C for 0.5-4 h.

9. The preparation method according to claim 7, characterized in that, In step S4, densification is carried out by pressureless sintering or hot pressing, with a densification temperature of 850-1000℃ and a densification time of 0.5-4h; when hot pressing is used, the pressure is 10-40MPa.

10. The application of the pure solid-state electrolyte according to any one of claims 1-5 in the preparation of solid-state lithium batteries, characterized in that, The pure solid electrolyte is disposed between the positive and negative electrodes of the solid lithium battery in the form of a self-supporting film or sheet, serving as a lithium-ion conducting layer and an electronic insulating isolation layer.

Images