Lagp solid-state electrolyte with gqd polymer buffer layer and preparation method and application thereof

Abstract

The application belongs to the field of lithium ion batteries, and particularly relates to a LAGP solid-state electrolyte with a GQD polymer buffer layer and a preparation method and application thereof. 1.5 A1 0.5 Ge 1.5 (PO4)3 powder is mixed and heated to obtain a GQD polymer precursor solution; the Li 1.5 A1 0.5 Ge 1.5 (PO4)3 powder is pressed into a sheet and sintered, and the LAGP solid-state electrolyte is obtained after polishing; the GQD polymer precursor solution is scraped onto the LAGP solid-state electrolyte to form a film, and the LAGP solid-state electrolyte with the GQD polymer buffer layer is obtained after drying. The GQD polymer buffer layer has high flexibility, which helps to alleviate the problem of poor interface contact between the LAGP and the lithium metal, thereby greatly reducing the interface resistance. Although only a few microns thick, the GQD polymer buffer layer effectively plays a role as a physical barrier, avoiding any direct electronic contact between the LAGP and the lithium metal, preventing the reduction of Ge 4+ and inhibiting the growth of lithium dendrites.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion batteries, specifically relating to LAGP solid electrolyte with GQD polymer buffer layer, its preparation method and application. Background Technology

[0002] The information disclosed in this background section is intended only to enhance understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] Among the solid electrolytes currently under research, NASICON-type solid electrolytes are favored due to their stability in air and good ion conductivity (10). -4 ~10 -3 S cm -1 The wide electrochemical window (up to 6V) of Li has attracted widespread attention. Currently, Li is the most extensively studied. 1.3 Al 0.3 Ti 1.7 (PO4)3(LATP) and Li 1.5 Al 0.5 Ge 1.5 Both LAGP and lithium metal anodes (PO4)3 exhibit high ionic conductivity. However, LAGP electrolytes are unstable when matched with lithium metal anodes, and their electrochemical window is very small (2.2-4V). LAGP solid electrolytes also possess high ionic conductivity and, due to their high oxidation limit (2.7V-4.27V vs. Li), good air stability, allowing for large-scale processing in air. Therefore, LAGP solid electrolytes have greater application feasibility. Despite these advantages, the interface problem between LAGP and lithium metal anodes limits LAGP applications. First, due to the rigidity of solid LAGP and electrode materials, the interfacial contact performance is typically poor; second, when in contact with lithium metal, GeO2 is observed on the LAGP surface. 4+ Restored to Ge 2+ This results in the formation of a GeO hybrid conductive interface. This unstable interface thickens continuously during cycling, increasing interface resistance and potentially causing battery failure. Simultaneously, the reduction reaction generates deep cracks within the ceramic mass, hindering Li... + The transmission.

[0004] Existing technologies employ a strategy of introducing buffer layers to address interface defects between the LAGP and lithium metal sides. For example, Han et al. introduced ultrathin alumina through atomic layer deposition and then converted it into an ion-conductive Li-Al-O layer, thereby improving the interface on the negative electrode side. The introduced buffer layer maintains interface stability and excellent electrochemical performance. However, the high cost, requiring complex thin-film fabrication equipment, limits its large-scale application. Furthermore, undesirable side reactions may occur during high-temperature processing, potentially damaging the Li metal at the interface. + Transport characteristics. Although existing technologies address the interface defects between LAGP and lithium metal by introducing a buffer layer, excessive thickness and insufficient conductivity of the buffer layer actually hinder lithium-ion transport. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a LAGP solid electrolyte with a GQD polymer buffer layer, its preparation method, and its applications. This invention constructs a GQD polymer buffer layer by combining graphene quantum dots (GQDs) with polyethylene oxide (PEO) and coating it onto the surface of LAGP. Due to the high flexibility of the GQD polymer buffer layer, it helps alleviate the problem of poor interfacial contact between LAGP and lithium metal, thereby significantly reducing the interfacial resistance. Although only a few micrometers thick, the GQD polymer buffer layer effectively acts as a physical barrier, preventing any direct electronic contact between LAGP and lithium metal, thus preventing the degradation of the LAGP by GQD polymer. 4+ The reduction inhibited the growth of lithium dendrites.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0007] In a first aspect, the present invention provides a method for preparing a LAGP solid electrolyte with a GQD polymer buffer layer, comprising the following steps:

[0008] S1, Graphene quantum dot solution, polyethylene oxide, lithium salt and Li 1.5 A1 0.5 Ge 1.5 (PO4)3 powder was mixed and heated to obtain a GQD polymer precursor solution;

[0009] S2, Li 1.5 A1 0.5 Ge 1.5 (PO4)3 powder is pressed into sheets and then sintered. After sintering, it is polished to obtain LAGP solid electrolyte.

[0010] S3. The GQD polymer precursor solution is scraped onto the LAGP solid electrolyte to form a film, and after drying, the LAGP solid electrolyte with the GQD polymer buffer layer is obtained.

[0011] Preferably, in the GQD polymer precursor solution, the mass ratio of polyethylene oxide to lithium salt is (7-9):1, and Li... 1.5 A1 0.5 Ge 1.5 The mass of (PO4)3 powder is 9% to 11% of the total mass of polyethylene oxide and lithium salt, and the mass of graphene quantum dots is 5% to 7% of polyethylene oxide. The lithium salt includes LiTFSI.

[0012] Preferably, in step S1, the heating temperature is 40–60°C and the heating time is 11–13 hours.

[0013] Preferably, Li 1.5 A1 0.5 Ge 1.5 The preparation method of (PO4)3 powder includes the following steps:

[0014] Lithium carbonate, aluminum oxide, germanium dioxide, and ammonium dihydrogen phosphate were added according to Li 1.5 A1 0.5 Ge 1.5 The mixture of (PO4)3 in stoichiometric proportions was then wet-milled. The resulting mixture was sintered, followed by a second ball milling process, and then dried to obtain Li. 1.5 A1 0.5 Ge 1.5 (PO4)3 powder.

[0015] Further preferably, the ball milling medium for wet ball milling is isopropanol, the ball milling speed is 300-500 r / min, and the ball milling time is 9-11 h; the sintering specifically involves first sintering at 400-500℃ for 18-24 h, and then sintering at 800-1000℃ for 5-7 h; the second ball milling time is 9-11 h; and the drying is vacuum drying at a temperature of 110-130℃ for 2-4 h.

[0016] Preferably, in step S2, the pressing pressure is 5.9–6.1 MPa, the sintering temperature is 850–950 °C, the time is 7–9 h, and the surface is polished with 320-mesh, 500-mesh, 800-mesh, and 1500-mesh sandpaper in sequence.

[0017] Preferably, in step S3, a 50μm doctor blade is used to form a film, the drying temperature is 45-55℃, and the drying time is 48-60h.

[0018] In a second aspect, the present invention provides a LAGP solid electrolyte having a GQD polymer buffer layer, obtained by the preparation method described in the first aspect.

[0019] Thirdly, the present invention provides the application of LAGP solid electrolyte with GQD polymer buffer layer as described in the second aspect in the preparation of lithium-ion batteries.

[0020] Fourthly, the present invention provides a lithium-ion battery comprising a LAGP solid electrolyte having a GQD polymer buffer layer as described in the second aspect.

[0021] The beneficial effects achieved by one or more technical solutions of the present invention are as follows:

[0022] This invention combines GQD with PEO polymers, enhancing the interaction between GQD and PEO. GQD, to a certain extent, prevents the aggregation of PEO segments and increases the amorphous region. The increase in the amorphous phase in PEO allows the segments to move more freely and is unaffected by the crystalline phase. Therefore, compared to a buffer layer without GQD, the activation energy of the GQD polymer buffer layer is lower and the thermal decomposition temperature is higher, resulting in improved activity and thermal stability. By adding LAGP to the GQD polymer buffer layer, the electrochemical window of the buffer layer is expanded and its stability is improved.

[0023] The LAGP solid electrolyte with a GQD polymer buffer layer benefits from the high stability of the GQD polymer buffer layer, solving the problem of poor interfacial contact between LAGP and lithium metal. This significantly reduces the total impedance of the fabricated symmetric cell, and achieves a impedance of 0.1 mA / cm². -2 It can cycle stably for up to 800 hours at a current density without forming lithium dendrites, and the highest critical current density can reach 1.4 mA cm⁻¹. -2 .

[0024] Full cells constructed with LAGP solid-state electrolyte featuring a GQD polymer buffer layer exhibited excellent cycling and rate performance, achieving a maximum discharge specific capacity of 157.7 mAh g⁻¹ at 50°C and 0.1C. -1 The capacity retention rate after 100 cycles was 82.8%. After high-rate cycling at 0.3C-1C, the discharge capacity at 0.1C recovered to 154.9 mAh g. -1 It exhibits high reversible capacity. Attached Figure Description

[0025] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0026] Figure 1 SEM images of the surface of (a) LAGP solid electrolyte and (b) LAGP solid electrolytes with GQD polymer buffer layers in Examples 1, (c) 2, and (d) 3;

[0027] Figure 2 In Figure a, XRD patterns of LAGP solid electrolytes with GQD polymer buffer layers in Examples 1, 2, and 3, and LAGP solid electrolytes with buffer layers in Comparative Example 1 are shown. In Figure b, a side SEM image of LAGP solid electrolyte with GQD polymer buffer layer in Example 2 is shown.

[0028] Figure 3 The buffer layer TGA curves are for the LAGP solid electrolyte with GQD polymer buffer layer in Examples 1, 2, and 3, and for the LAGP solid electrolyte with buffer layer in Comparative Example 1.

[0029] Figure 4 LSVs of the GQD polymer buffer layer in (a) Example 2 and (b) Comparative Example 3;

[0030] Figure 5 Electrochemical impedance spectroscopy (EIS) of symmetrical cells of (a) Comparative Examples 7-11 and (b) Comparative Examples 6, 11, 4, 5, and 6.

[0031] Figure 6 The constant current long-cycle test diagrams are for (a) Comparative Example 10, (b) Comparative Example 7, (c) Comparative Example 8 and (d) Comparative Example 9.

[0032] Figure 7 The constant current long-cycle test diagrams are for (a) Comparative Example 6, (b) Example 4, (c) Example 5 and (d) Example 6.

[0033] Figure 8 In the table, (a) represents the interface activation energy of the symmetrical cells of Comparative Example 6, Example 4, Example 5 and Example 6 in the temperature range of 35°C to 95°C, and (b) represents the interface activation energy of the symmetrical cells of Comparative Example 11 and Example 5 in the temperature range of 55°C to 95°C.

[0034] Figure 9 Electrochemical impedance spectroscopy was performed on (a) Comparative Example 11 and (b) Example 5 after cycling at constant current density for different times.

[0035] Figure 10 The critical current density for (a) Comparative Example 11 and (b) Example 5;

[0036] Figure 11 Comparative Example 11 and Example 5 were tested at 50°C and 0.1 mA cm. -2 Long-cycle performance under constant current density;

[0037] Figure 12Example 5: 0.05 mA cm at 50°C -2 up to 0.4mA cm -2 Voltage-time curves at current density;

[0038] Figure 13 SEM images of the solid electrolyte in (a) Comparative Example 11 and (b) Example 5 and (c) Comparative Example 11 and (d) Example 5 after 50h cycling.

[0039] Figure 14 Ge 3d spectra of (a) the initial LAGP solid electrolyte, (b) the solid electrolyte of Comparative Example 11 after 50 h of cycling, and (c) the solid electrolyte of Example 5 after 50 h of cycling; and Li 1s spectra of (d) the initial LAGP solid electrolyte, (e) the solid electrolyte of Comparative Example 11 after 50 h of cycling, and (f) the solid electrolyte of Example 5 after 50 h of cycling.

[0040] Figure 15 The XPS full spectrum and C1s spectrum of the transition layer in Example 5 are (a) and (b) respectively, and the C1s spectrum of the transition layer in Comparative Example 6 is (c) respectively.

[0041] Figure 16 The images show (a) Li 1s spectrum of the lithium anode surface in Comparative Example 11 after 50 hours of cycling, and (b) Li 1s spectrum and (c) F 1s spectrum of the lithium anode surface etched at 25 nm in Example 5.

[0042] Figure 17 The following images show the structural schematic diagram, cross-sectional SEM image, LFP surface SEM image, Fe element distribution on the LFP surface, and P element distribution on the LFP surface of the full cell in Example 7.

[0043] Figure 18 The long-cycle performance and coulombic efficiency of the full cells of Example 7 and Comparative Example 12 at 50°C and 0.1C;

[0044] Figure 19 The charge-discharge curves of the full cells of (a) Comparative Example 12 and (b) Example 7 at 50°C and 0.1C for different numbers of cycles are shown.

[0045] Figure 20 Rate performance of the full cells of Example 7 and Comparative Example 12;

[0046] Figure 21 The cycle performance of the full cell in Example 7 at 50°C and 0.5C is shown. Detailed Implementation

[0047] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments and comparative examples.

[0048] Example 1

[0049] Lithium carbonate, aluminum oxide, germanium dioxide, and ammonium dihydrogen phosphate were added according to Li 1.5 A1 0.5 Ge 1.5 The stoichiometric ratio of (PO4)3 was measured. The uniformly mixed raw materials were placed in a zirconia ball mill jar and wet-milled using isopropanol as the milling medium at a speed of 400 rpm for 10 hours. The milled mixture was then placed in an alumina crucible for high-temperature reaction, sintered at 450℃ for 20 hours, and then sintered at 900℃ for 6 hours. After sintering, the resulting powder was ball-milled again for 10 hours, and the second-milled powder was dried under vacuum at 120℃ for 3 hours to obtain LAGP powder. The LAGP powder was ground in an agate mortar and sieved through a 200-mesh sieve. 0.55g of powder was placed in a mold with a diameter of 16mm and pressed into a sheet shape under a pressure of 6MPa. The pressed green body was placed in an alumina crucible and then sintered in a muffle furnace at 900℃ for 8 hours. After sintering, LAGP ceramic sheets with a thickness of approximately 1mm and a diameter of 14mm were obtained. Next, the LAGP solid electrolyte was polished sequentially using 320-grit, 500-grit, 800-grit, and 1500-grit sandpaper on a grinding and polishing machine, finally obtaining a light green LAGP solid electrolyte with a mirror-like smooth surface. The polished LAGP solid electrolyte was then ultrasonically cleaned for 10 minutes in an ultrasonic instrument and dried in an oven to ensure the cleanliness of the electrolyte surface. A precursor solution was prepared by mixing 6.58 g / L graphene quantum dot (GQD) solution, polyethylene oxide (PEO), LiTFSI, and LAGP powder at a PEO to LiTFSI mass ratio of 8:1, with LAGP powder comprising 10% of the total mass of (PEO + LiTFSI). The mass of GQD in the precursor solution was 5% of the mass of PEO. The prepared precursor solution was stirred for 12 hours on a magnetic stirrer at a constant temperature of 50°C to promote PEO polymerization and give the precursor solution a certain viscosity.

[0050] The LAGP electrolyte sheet was embedded in a specially made polytetrafluoroethylene (PTFE) template with a thickness of 1 mm. The precursor solution was scraped onto the LAGP electrolyte sheet to form a film using a 50 μm scraper. The film was then placed in a vacuum oven at 50 °C for 50 hours to evaporate the moisture and obtain a solid LAGP electrolyte with a GQD polymer buffer layer.

[0051] Example 2

[0052] Unlike Example 1, the mass of GQD in the precursor solution was 6% of that of PEO.

[0053] Example 3

[0054] Unlike Example 1, the mass of GQD in the precursor solution was 7% of that of PEO.

[0055] like Figure 1 As shown, the original LAGP surface still has slight scratches from the polishing process. Figure 1 As shown in a), after multiple coats of GQD polymer, the grooves caused by the scratches are covered by a buffer layer, resulting in a smoother and more even surface, which is beneficial for improving the interfacial wettability with the lithium anode. In Example 1, the low concentration of graphene quantum dots causes LAGP powder to precipitate, leading to uneven dispersion of the LAGP powder (e.g., ...). Figure 1 As shown in b), in Example 3, the high concentration of graphene quantum dots causes LAGP powder to agglomerate, which also leads to uneven dispersion of LAGP powder. Figure 1 In Example 2, the concentration was moderate and the LAGP powder was evenly dispersed (e.g., d). Figure 1 As shown in c), its expected performance is quite good.

[0056] Comparative Example 1

[0057] Unlike Example 1, GQD was not added to the precursor solution, resulting in a LAGP solid electrolyte with a buffer layer.

[0058] like Figure 2 As shown in a, the presence of the steamed bun peak in the XRD of Examples 1-3 and Comparative Example 1 indicates that the PEO in the GQD polymer buffer layer has been successfully polymerized, while the LAGP diffraction peaks shown above indicate that the LAGP powder has been completely added to it. Figure 2 Figure b shows a cross-sectional SEM image of Example 2, where the buffer layer and LAGP are observed to be tightly bonded together, with uniform LAGP grain distribution. At the interface between LAGP and the buffer layer, the LAGP maintains its original morphology without grain collapse or breakage, indicating a highly stable interface between LAGP and the buffer layer. The flexible GQD polymer buffer layer covers the rigid LAGP solid electrolyte to enhance interfacial wettability. The GQD polymer buffer layer has a thickness of 17 μm, which is moderate. A buffer layer that is too thin would be insufficient to protect the LAGP / Li interface, but due to the low ionic conductivity of the polymer film itself, an excessively thick buffer layer would allow Li to... + Increased transmission distance limits the improvement of interface performance.

[0059] The thermal stability of the transition layer in Examples 1-3 and Comparative Example 1 was analyzed using thermogravimetric analysis (TGA). The sample testing temperature range was 30–700 °C, and the heating rate was 10 °C / min under a nitrogen atmosphere. Figure 3As shown, the TGA curves clearly reveal two weight loss processes. The first weight loss is due to the thermal decomposition of the PEO matrix, while the second weight loss is related to the degradation of LiTFSI. In the first weight loss segment, the temperatures at which the transition layer thermal weight loss occurred in Comparative Example 1, Example 1, Example 2, and Example 3 were 222℃, 295℃, 305℃, and 290℃, respectively. This indicates that after combining with GQD, the thermal decomposition temperature of PEO increases, and the weight loss decreases. GQD, to a certain extent, prevents the aggregation of PEO segments and increases the amorphous region. The increase in the amorphous phase in PEO allows the segments to move more freely and is unaffected by the crystalline phase. The better mechanical properties and thermal stability of the solid-state electrolyte will provide a strong guarantee for the high safety of solid-state lithium metal batteries.

[0060] Comparative Example 2

[0061] Unlike Example 1, no LAGP powder was added to the precursor solution, resulting in a LAGP solid electrolyte with a buffer layer.

[0062] Comparative Example 3

[0063] Unlike Example 2, no LAGP powder was added to the precursor solution, resulting in a LAGP solid electrolyte with a buffer layer.

[0064] like Figure 4 As shown, the buffer layer of Example 2 exhibits an electrochemical stability window as high as 5.1 V vs. Li / Li+ at 50°C. Figure 4 The value in (a) meets the requirements for matching most cathode materials. However, the buffer layer in Comparative Example 3 without LAGP powder has an electrochemical stability window of only 4.6 V vs. Li / Li+ at 50°C. Figure 4 In example b), the reduction is significant compared to example 2 with added LAGP.

[0065] Comparative Example 4

[0066] Unlike Example 3, no LAGP powder was added to the precursor solution, resulting in a LAGP solid electrolyte with a buffer layer.

[0067] Comparative Example 5

[0068] Unlike Example 1, GQD and LAGP powder were not added to the precursor solution, resulting in a LAGP solid electrolyte with a buffer layer.

[0069] Example 4

[0070] In an argon-filled glove box with water and oxygen content below 0.01 ppm, a symmetrical battery was fabricated by attaching lithium foils with a diameter of 6 mm and a thickness of approximately 100 μm to both sides of the LAGP solid electrolyte with a GQD polymer buffer layer from Example 1. The lithium foils on both sides were positioned symmetrically to ensure efficient lithium-ion transport. The structure of the symmetrical battery is similar to a sandwich, consisting of lithium foil, solid electrolyte, and lithium foil in that order. The sandwich structure was then placed inside a CR2032 coin cell and pressed using a hydraulic press under a pressure of 5.0 MPa.

[0071] Example 5

[0072] Unlike Example 4, the LAGP solid electrolyte with a GQD polymer buffer layer from Example 2 was used.

[0073] Example 6

[0074] Unlike Example 4, the LAGP solid electrolyte with a GQD polymer buffer layer from Example 3 was used.

[0075] Comparative Example 6

[0076] Unlike Example 4, the LAGP solid electrolyte with a buffer layer of Comparative Example 1 was used.

[0077] Comparative Example 7

[0078] Unlike Example 4, the LAGP solid electrolyte with a buffer layer of Comparative Example 2 was used.

[0079] Comparative Example 8

[0080] Unlike Example 4, the LAGP solid electrolyte with a buffer layer of Comparative Example 3 was used.

[0081] Comparative Example 9

[0082] Unlike Example 4, the LAGP solid electrolyte with a buffer layer of Comparative Example 4 was used.

[0083] Comparative Example 10

[0084] Unlike Example 4, the LAGP solid electrolyte with a buffer layer of Comparative Example 5 was used.

[0085] Comparative Example 11

[0086] Unlike Example 4, a buffer-free modified LAGP solid electrolyte (obtained according to the preparation method of Example 1) was used.

[0087] The symmetrical cells obtained in Examples 4-6 and Comparative Examples 6-11 were characterized by electrochemical impedance spectroscopy (EIS). All electrochemical performance tests were performed at 50°C. Figure 5 As shown in Figure a, the total resistance of the symmetric cell without buffer layer modification in Comparative Example 11 is very high, with a measured value as high as 4463 Ω, which is due to point contacts at the interface. In Comparative Examples 7-10 without LAGP powder, the impedance gradually decreases with increasing graphene quantum dot concentration, and the symmetric cell in Comparative Example 9 has the lowest total resistance (1835 Ω), which is most conducive to improving interface performance. Figure 5 As shown in b, the resistance of the symmetric cell using a solid electrolyte with added LAGP powder in the buffer layer is further reduced, with the symmetric cell of Example 5 having the lowest interface resistance (1460Ω), which is most conducive to the improvement of interface performance.

[0088] Constant current long-cycle testing can be used to evaluate Li in the buffer layer + Long-term stability of transmission, such as Figure 6 As shown, compared to Comparative Example 11, which only cycled for 136 hours without the buffer layer modification, the cycle times of the modified symmetric cells in Comparative Examples 7-9 were all improved. This is because the GQD polymer buffer layer significantly improved the interfacial contact between LAGP and Li. However, the polarization voltage of the modified symmetric cells was unstable and showed a slow increasing trend, which is due to the low ionic conductivity of the GQD polymer buffer layer itself. Therefore, it is necessary to add solid electrolyte powder LAGP to the GQD polymer buffer layer to increase ion transport efficiency. Figure 7 As shown, the polarization voltage is relatively stable after the addition of LAGP powder, with the polarization voltage of Example 5 being ( Figure 6 The lowest value of c is 0.1V.

[0089] Electrochemical impedance spectroscopy was performed on the interface of a symmetrical cell within a temperature range of 35℃ to 95℃, and the activation energy of the interface within this temperature range can be obtained according to Arrhenius's law. Figure 8 As shown in Figure a, the interface activation energy of Comparative Example 6 is 0.420 eV, that of Example 4 is 0.421 eV, and that of Example 6 is 0.423 eV. Among these, Example 5 has the lowest interface activation energy at 0.403 eV. A lower activation energy indicates that Li... + Transport is easier at the interface. The significant reduction in interface resistance and activation energy clearly indicates that the transition layer modification in Example 2 is the most effective. Figure 8As shown in b, the activation energy of the LAGP / Li interface in Comparative Example 11 (0.42 eV) and the interface activation energy of Example 5 (0.39 eV) at 55℃-95℃ indicate that the interfacial performance between the solid electrolyte and the lithium metal anode was significantly improved after modification with a buffer layer containing 6% GQD.

[0090] At 50℃, 0.1mA cm -2 Electrochemical impedance spectroscopy (EIS) tests were performed after constant current density cycling for different times, and the results are shown in Figure 9. The initial total resistance of Comparative Example 11 was 4463 Ω, due to the rigid nature of the LAGP electrolyte, which resulted in only point-to-point contact with metallic lithium. After 64 hours, the interfacial resistance rapidly increased to 8600 Ω, and climbed to 46000 Ω after 112 hours. Figure 9 (a) In Comparative Example 11, the impedance reached 5000Ω after 40 hours of cycling, indicating that the reaction between LAGP and lithium metal began shortly after contact. As the contact time increased, the reaction continued, and the area and depth of the reaction interface layer increased, leading to a continuous increase in battery impedance. During long-term contact, the rapid interface reaction hindered the charge transfer process and disrupted the bonding between LAGP grains. Conversely, for Example 5, the initial interface impedance decreased from 4463Ω to 1460Ω compared to Comparative Example 11. Due to the presence of the GQD polymer buffer layer, the stability of LAGP to lithium metal was improved, and after 400 hours of cycling, the interface impedance slowly increased to 4130Ω. Figure 9 (b)

[0091] like Figure 10 As shown, Comparative Example 11 operates at a current density of only 0.2 mA cm⁻¹. -2 This will cause the voltage to increase rapidly and exceed the safe voltage. Figure 10 (a) This is related to the side reactions and poor contact between LAGP and lithium metal during cycling. Uniform Li deposition at the interface is difficult under high current densities. + This exacerbates the formation of voids and high internal resistance byproducts, ultimately leading to battery failure. In contrast, the critical current density of the symmetrical battery in Example 5 can reach 1.4 mA cm⁻¹. -2 ( Figure 10 In section b), the significant increase in critical current density stems from excellent interfacial stability. At all current densities, the polarization voltage of the symmetrical cell in Example 5 is lower than that in Comparative Example 11, indicating that Example 5 has lower internal resistance. The comparison of critical current densities shows that the transition layer in Example 5 not only reduces interfacial impedance during cycling but also improves the interfacial stability between LAGP and Li metal.

[0092] The symmetrical cells of Comparative Example 11 and Example 5 were compared at 50°C and 0.1 mA cm⁻¹.-2 Under constant current density, long-cycle testing was performed, such as... Figure 11 As shown in the figure, the test results show that the polarization voltage of Comparative Example 11 is around 0.12V. With increasing cycle time, the polarization voltage gradually increases, eventually leading to battery failure after 136 hours of cycling. This is due to severe side reactions between the lithium metal anode and the LAGP solid electrolyte, resulting in lithium dendrite formation. In contrast, Example 5, due to the GQD polymer transition layer suppressing interfacial side reactions and improving the contact between the lithium metal anode and the LAGP solid electrolyte interface, allows the symmetrical battery to cycle stably for 800 hours, demonstrating a significant performance improvement.

[0093] By comparing the polarization voltage at the same current density, the magnitude of the interfacial internal resistance of different modification methods can be obtained, such as... Figure 12 As shown, Example 5 was subjected to a rate cycling test at 50°C, at 0.05 mA cm⁻¹. -2 0.1mA cm -2 0.2mAcm -2 0.4mA cm -2 The polarization voltages at the current densities were 0.05V, 0.1V, 0.16V, and 0.38V, respectively. When the current density returned to 0.1mA / cm... -2 At that time, the polarization voltage of Example 5 also decreased back to a stable 0.11V. This demonstrates that the transition layer can accelerate the Li + It facilitates the transport of substances and suppresses the generation of side reactions, thus maintaining the long-term stability of the interface.

[0094] The symmetrical cells of Comparative Example 11 and Example 5, after 50 hours of cycling, were disassembled in an argon-filled glove box, and the lithium anode and solid electrolyte surface were characterized by SEM. Figure 13 As shown in Figure a, Comparative Example 11 exhibits significant cracks near the surface where LAGP contacts lithium metal, indicating that Li reduction of LAGP severely damages the electrolyte particles at the interface. The LAGP surface after the reaction is no longer flat, becoming highly uneven, which is caused by the interfacial reaction between Li metal and LAGP during cycling. The rough interface resulting from the reaction may be the main reason for the extremely high internal resistance and poor cycle stability of the battery. Figure 13 As shown in b, the electrolyte structure of Example 5 remained intact, and the electrolyte surface remained relatively smooth without cracks. The buffer layer played a crucial role in protecting the interface between LAGP and Li metal during cycling. Meanwhile, to determine the impact of the buffer layer on the lithium metal anode, the surface of the lithium anode after 50 hours of cycling was characterized by SEM. Figure 13As shown in c, the surface of lithium metal without a buffer layer exhibits an uneven morphology, with small areas of unreacted lithium metal remaining, but the surface is already covered with byproducts of severe LAGP reaction and lithium dendrites. In contrast, the surface of lithium metal with a buffer layer is very dense and smooth. Figure 13 (d) This clearly demonstrates that the buffer layer has excellent protective function and can form a robust solid electrolyte interface (SEI) layer.

[0095] Further X-ray photoelectron spectroscopy (XPS) analysis was performed on the cycled solid electrolyte, comparing it with the initial LAGP solid electrolyte. For the initial LAGP, Ge only exists at 32.5 eV. 4+ Characteristic peaks ( Figure 14 (a) In Comparative Example 11, a new peak at 29.4 eV appeared in the Ge 3d spectrum of LAGP after cycling, indicating that 0-valent Ge (a) appeared on the surface of the LAGP electrolyte. Figure 14 b) indicates that when LAGP contacts Li, Ge 4+ The Ge is reduced, resulting in the formation of a highly conductive mixed conductive interphase. In contrast, the Ge in the symmetric cell of Example 5... 4+ The signal did not significantly transfer to lower binding energies under the protection of the buffer layer. Figure 14 c). Li 1s indicates that in Comparative Example 11, LAGP was reduced by lithium, and the signal intensity of Li increased ( Figure 14 In Example 5, no significant changes were observed (e). Figure 14 f in the middle.

[0096] XPS analysis was performed on the transition layers in Example 5 and Comparative Example 6, such as... Figure 15 As shown in a, the transition layer of Example 5 mainly contains C (66.29%), O (23.39%), and F (10.32%). Figure 15 As shown in b and c, the CO peak in the transition layer of Example 5 is enhanced, partly due to the introduction of GQD, and partly due to the weakening of Li. + The coordination with oxygen in diethyl ether affects the strength of CO.

[0097] The 1s XPS spectrum of the lithium metal anode Li separated from Comparative Example 11 after 50 hours of cycling is as follows: Figure 16 As shown in Figure a, the Li 1s peaks near 54 eV and 55.5 eV may correspond to Li₂O₂ and Li₂CO₃, respectively. The inorganic components on the lithium anode are generally considered to be byproducts of the reaction between lithium metal and LAGF, leading to a porous and uneven surface. For example... Figure 16As shown in b and c, the peak value of 55.7 eV in the Li 1s spectrum and the peak value of 685.7 eV in the F 1s spectrum both correspond to LiF. This indicates that during cycling, LiF is formed by the reaction of LiTFSI in the GQD polymer buffer layer with the lithium metal anode at the interface, and also shows that LiF is widely distributed inside the buffer layer. Due to its high surface energy, the LiF component can guide the horizontal growth of Li and avoid vertical deposition, thereby effectively suppressing the generation and growth of lithium dendrites and improving the cycle stability of the battery.

[0098] Example 7

[0099] The preparation process of LiFePO4 (LFP) cathode is as follows: LFP, SuperP conductive carbon black (SP), and polyvinylidene fluoride (PVDF) are added to N-methylpyrrolidone (NMP) in a mass ratio of 8:1:1. The mixture is stirred on a magnetic stirrer for 24 hours to obtain a cathode slurry. The cathode slurry is then coated onto aluminum foil using an automatic coating machine, followed by drying in a 90℃ oven for 3 hours to remove organic solvents, and then vacuum drying at 110℃ for 12 hours to remove moisture. The resulting cathode sample is black and has a dense surface. Finally, it is cut into cathode sheets with a diameter of 12 mm using a manual cutting machine, with an active material content of approximately 1.3 mg / cm³. -2 Store in the glove box for later use. The theoretical specific capacity of LFP is 170mAh g. -1 The solid-state full cell with LAGP solid electrolyte and GQD polymer buffer layer of Example 2 was assembled using LiFePO4 (LFP) as the positive electrode and lithium metal as the negative electrode.

[0100] The structure of a full battery is as follows Figure 17 As shown in a, from Figure 17 As shown in b, the LAGP, buffer layer, and LFP are tightly bonded together, providing good contact with the positive electrode side even without electrolyte added. The SEM image of the LFP surface is shown below. Figure 17 As shown in c, the surface is uniform and dense, and the energy dispersive spectroscopy image shows a uniform distribution of P and Fe elements, representing LFP. Figure 17 (d and e in the text).

[0101] Example 8

[0102] Unlike Example 7, the LAGP solid electrolyte with a GQD polymer buffer layer from Example 1 was used.

[0103] Example 9

[0104] Unlike Example 7, Example 3 has an LAGP solid electrolyte with a GQD polymer buffer layer.

[0105] Comparative Example 12

[0106] Unlike Example 7, a buffer-free modified LAGP solid electrolyte (obtained according to the preparation method of Example 1) was used, and 5 μL of liquid electrolyte (1M LiPF6 in EC / DEC, 1:1 v / v) was added between the LFP cathode and the buffer-free modified LAGP.

[0107] The long-cycle specific capacity and coulombic efficiency performance of the full cells of Example 7 and Comparative Example 12 at 50°C are as follows: Figure 18 As shown, in Comparative Example 12, the rigid point-to-point contact between LAGP and the electrode material results in excessively high interfacial resistance, hindering the Li... + It passes freely at the interface, thus its first-cycle discharge specific capacity is 147mAh g. -1 The coulomb efficiency is 91.29%. More importantly, LAGP and Li + The ongoing chemical reactions between the LAGP and Li interfaces lead to a continuous deterioration in the stability of the LAGP / Li interface, resulting in a sharp decline in the battery's cycle performance. After 20 cycles, the discharge specific capacity is only 40.5 mAh g⁻¹. -1 The severe degradation of the battery in Comparative Example 12 is mainly due to two factors: First, the increased interfacial impedance and interphase thickness of LAGP / Li lead to unstable interfacial ion transport channels and poor electrode / electrolyte contact; second, the accumulation of interfacial stress causes microcracks on the electrolyte surface to extend into the LAGP matrix, potentially leading to electrolyte fracture and ultimately self-discharge. The full cell in Example 7 maintained relatively stable cycling performance, exhibiting excellent cycling characteristics. After the initial few cycles of activation and stabilization, its discharge specific capacity increased from 147 mAh g⁻¹ of the bare full cell. -1 Increased to 157.7mAh g -1 The capacity retention rate after 100 cycles is 82.8%. Due to the activation of lithium iron phosphate, the capacity will increase slightly in the first 10 cycles.

[0108] The charge-discharge curves of the full cells of Example 7 and Comparative Example 12 at 50°C and 0.1C for different numbers of cycles are shown below. Figure 19 As shown. The full cell of Example 7 exhibits a typical charge-discharge voltage plateau on its charge-discharge curves, while the cell's polarization voltage hardly increases with cycling. The substantially overlapping curves indicate that the buffer layer promotes the formation of a tight interface, thereby reducing the interface resistance and achieving Li + The efficient and stable transfer of the Li / LAGP interface was observed. In contrast, the cycle performance of the Comparative Example 12 battery was significantly worse, with rapid capacity decay consistent with the electrochemical behavior of lithium-symmetric batteries. The charge-discharge curves at the LAGP / Li interface also showed increased voltage polarization and voltage fluctuations, indicating rough contact and undesirable reduction reactions at the Li / LAGP interface.

[0109] like Figure 20 As shown, the full battery was first charged and discharged 6 times at a current of 0.1C. The battery was fully activated at low current. The discharge capacity of the full battery in Example 7 reached 150.40 mAh g in the first discharge cycle at a current of 0.1C. -1 The full battery was then cycled sequentially at currents of 0.3C, 0.5C, 0.8C, and 1C. The modified full battery also exhibited excellent discharge capacity, reaching 147.2 mAh g⁻¹. -1 130.7mAh g -1 103.9mAh g -1 and 75.2mAh g -1 After high-rate cycling, the discharge capacity of the full cell in Example 7 at 0.1C can be restored to 154.9 mAh g. -1 The battery exhibited high reversible capacity. The discharge capacity of all-solid-state batteries with and without a buffer layer differed significantly, indicating that the buffer layer plays a role in improving the intercalation and deintercalation capabilities of lithium ions between the electrolyte and the electrodes.

[0110] like Figure 21 As shown, the full battery of Example 7 was first charged and discharged three times at a current of 0.1C to fully activate the battery at a low rate. Then, a charge-discharge test was conducted at 0.5C, and its initial discharge capacity was 106.1 mAh g. -1 With a coulombic efficiency of 97.41% and a capacity retention of 74.6% after 50 cycles, its excellent electrochemical performance indicates its potential for industrial application.

[0111] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a LAGP solid electrolyte with a GQD polymer buffer layer, characterized in that, Includes the following steps: S1, Graphene quantum dot solution, polyethylene oxide, lithium salt and Li 1.5 A1 0.5 Ge 1.5 (PO4)3 powder was mixed and heated to obtain a GQD polymer precursor solution; S2, Li 1.5 A1 0.5 Ge 1.5 (PO4)3 powder is pressed into sheets and then sintered. After sintering, it is polished to obtain LAGP solid electrolyte. S3. The GQD polymer precursor solution is scraped onto the LAGP solid electrolyte to form a film, and after drying, the LAGP solid electrolyte with the GQD polymer buffer layer is obtained. In the GQD polymer precursor solution, the mass ratio of polyethylene oxide to lithium salt is (7~9):1, Li 1.5 A1 0.5 Ge 1.5 The mass of (PO4)3 powder is 9% to 11% of the total mass of polyethylene oxide and lithium salt, and the mass of graphene quantum dots is 5% to 7% of polyethylene oxide.

2. The preparation method according to claim 1, characterized in that, The lithium salt includes LiTFSI.

3. The preparation method according to claim 1, characterized in that, In step S1, the heating temperature is 40~60 ℃ and the time is 11~13 h.

4. The preparation method according to claim 1, characterized in that, Li 1.5 A1 0.5 Ge 1.5 The preparation method of (PO4)3 powder includes the following steps: Lithium carbonate, aluminum oxide, germanium dioxide, and ammonium dihydrogen phosphate were added according to Li 1.5 A1 0.5 Ge 1.5 The mixture of (PO4)3 in stoichiometric proportions was then wet-milled. The resulting mixture was sintered, followed by a second ball milling process, and then dried to obtain Li. 1.5 A1 0.5 Ge 1.5 (PO4)3 powder.

5. The preparation method according to claim 4, characterized in that, The ball milling medium for wet ball milling is isopropanol, the ball milling speed is 300~500 r / min, and the ball milling time is 9~11 h; the sintering process is as follows: first sintering at 400~500 ℃ for 18~24 h, and then sintering at 800~1000 ℃ for 5~7 h; the second ball milling time is 9~11 h; the drying process is vacuum drying at 110~130 ℃ for 2~4 h.

6. The preparation method according to claim 1, characterized in that, In step S2, the pressing pressure is 5.9~6.1 MPa, the sintering temperature is 850~950 ℃, the time is 7~9 h, and the surface is polished with 320 mesh, 500 mesh, 800 mesh and 1500 mesh sandpaper in sequence.

7. The preparation method according to claim 1, characterized in that, In step S3, a 50 μm doctor blade is used to coat the film, and the drying temperature is 45~55 ℃, and the drying time is 48~60 h.

8. A LAGP solid electrolyte with a GQD polymer buffer layer, characterized in that, Obtained by the preparation method described in any one of claims 1 to 7.

9. The application of the LAGP solid electrolyte with a GQD polymer buffer layer as described in claim 8 in the preparation of lithium-ion batteries.

10. A lithium-ion battery, characterized in that, Includes the LAGP solid electrolyte with a GQD polymer buffer layer as described in claim 8.

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