Garnet-type lithium ion solid electrolyte and method for preparing the same

Abstract

This invention discloses a garnet-type lithium-ion solid electrolyte and its preparation method. The preparation method involves ball milling LLZMO precursor powder with zirconium source A to obtain a mixed powder. The mixed powder is then pre-formed to obtain a rough blank, which is sintered in an air atmosphere to obtain a garnet-type lithium-ion solid electrolyte with lithium carbonate completely eliminated from the surface and grain boundaries and coated with lithium zirconate. This invention avoids the problems of lithium carbonate hindering sintering density, reducing lithium-ion conductivity, and increasing interfacial impedance. At the same time, the surface of the core material is coated with a lithium zirconate layer with good stability, which can further improve the air stability of the garnet electrolyte.

Description

Technical Field

[0001] This invention belongs to the fields of electrochemistry and new materials, specifically relating to a garnet-type lithium-ion solid electrolyte and its preparation method. Background Technology

[0002] All-solid-state batteries are highly promising rechargeable batteries. Compared to traditional liquid batteries, all-solid-state batteries offer advantages such as a wider operating temperature range, better safety, and higher energy density. The key to the normal operation of all-solid-state batteries lies in the high ionic conductivity, thermal stability, good mechanical properties, and stability to electrode materials of the solid electrolyte at room temperature. Among the reported solid electrolytes, garnet-type solid electrolytes have attracted much attention due to their high lithium-ion conductivity, good thermal stability, wide electrochemical operating window, and stability to lithium metal. However, current garnet-type lithium-ion solid electrolytes exhibit poor wettability to lithium metal, resulting in significant interfacial impedance and uneven current distribution at the lithium interface. Furthermore, the ionic conductivity of garnet-type lithium-ion solid electrolytes still does not meet the requirements of practical applications.

[0003] Currently, lithium carbonate forms on the surface and at grain boundaries of garnet-type lithium-ion solid electrolytes prepared and stored in air. Due to its lithium-phobic nature and extremely low ionic conductivity, the surface lithium carbonate hinders the densification process during electrolyte sintering, resulting in poor wettability of the electrolyte to lithium metal and a large interfacial impedance between the electrolyte and electrode. Lithium carbonate at grain boundaries increases grain boundary resistance, thereby reducing the ionic conductivity of the electrolyte. There are three main formation mechanisms of lithium carbonate: First, during preparation or storage in air, lithium in the garnet-type lithium-ion solid electrolyte undergoes proton exchange with hydrogen in water, initially forming lithium hydroxide, which then reacts with carbon dioxide to form lithium carbonate; second, the garnet electrolyte directly reacts with moisture in the air to form a garnet electrolyte with crystal water, which then reacts with carbon dioxide in the air to form lithium carbonate; third, the garnet electrolyte directly reacts with carbon dioxide in the air to form lithium carbonate. Research reports on solutions to the lithium carbonate problem in garnet-type lithium-ion solid electrolytes include: directly polishing the garnet electrolyte to remove surface lithium carbonate (Phys. Chem. Chem. Phys., 2014, 16(34): 18294-18300, Chem Mater, 2017, 29(18): 7961-7968). However, lithium carbonate at grain boundaries cannot be eliminated. This method is suitable for electrolytes with relatively smooth surfaces and cannot solve the problem of regeneration of contaminants such as lithium carbonate on the electrolyte surface. For example, lithium carbonate can be decomposed by acid treatment (J. Mater. Chem. A, 2019, 7(24): 14565-14574, Nano Energy, 2019, 61: 119-125, Mater. Lett, 2020, 280: 128543), but it is not suitable for electrolyte powder treatment and has certain limitations. In addition, lithium carbonate is still generated during the drying process. High-temperature heat treatment to decompose lithium carbonate (Nano Energy, 2020, 73:104836) also fails to inhibit the regeneration of lithium carbonate on the electrolyte surface. Surface coating can inhibit the formation of contaminants on the electrolyte surface to a certain extent. Patent (CN 110790573 A) discloses a method for coating garnet-type lithium-ion solid electrolytes with alumina, which effectively removes lithium carbonate from the electrolyte surface. However, the coating method is relatively complex and costly, and the lithium aluminate layer formed by the reaction of the coated alumina and lithium carbonate has low ionic conductivity (~10). -6 S cm -1 This hinders lithium-ion conduction. Patent (201911167109.3) discloses a method for coating garnet-type lithium-ion solid electrolytes with lithium silicate, but the surface-coated lithium silicate layer also has a low ionic conductivity (~10). -6 S cm -1 This is not conducive to the conduction of lithium ions at grain boundaries and between the electrolyte and the electrode, resulting in a low lithium conduction rate at the interface layer. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the first objective of this invention is to provide a garnet-type lithium-ion solid electrolyte with high density, ionic conductivity, and air stability.

[0005] The second objective of this invention is to provide a method for preparing a garnet-type lithium-ion solid electrolyte.

[0006] This invention provides a garnet-type lithium-ion solid electrolyte, composed of a core material and a coating layer, wherein the molecular formula of the core material is Li. 7-x La3Zr 2-x M x O 12 (LLZMO), where x = 0-0.6, the coating layer is lithium zirconate, and M is selected from at least one of Ta, Nb, Hf, Ge, Ga, Sc, Ti, Al, Si, Sn, Y, and V.

[0007] The garnet-type lithium-ion solid electrolyte provided by this invention does not contain lithium carbonate on the surface and at the grain boundaries of the core material. Therefore, it avoids the problems of lithium carbonate hindering sintering density, reducing lithium-ion conductivity, and increasing interfacial impedance. At the same time, the surface of the core material is coated with a lithium zirconate layer with good air stability, which can further improve the air stability of the garnet electrolyte.

[0008] In a preferred embodiment, the zirconium content in the coating layer is 1-4 mol% of the lithium content in the core material.

[0009] The inventors discovered that when the content of the coating layer is within the above range, the performance of the garnet-type lithium-ion solid electrolyte is optimal. However, the content of the coating layer needs to be effectively controlled. If the zirconium content in the coating layer is too high, the excess zirconium will react with the LLZMO electrolyte, generating La3Zr2O7 impurities at the grain boundaries or surface, affecting the sintering performance of the electrolyte, resulting in a loose and porous garnet electrolyte, and further reducing the ionic conductivity of the electrolyte.

[0010] In a preferred embodiment, the molecular formula of the core material is Li. 7-x La3Zr 2-x M x O 12 Where x = 0.25-0.6, and M is selected from at least one of Ta, Nb, and Ga.

[0011] This invention provides a method for preparing a garnet-type lithium-ion solid electrolyte. The method involves ball milling LLZMO precursor powder with zirconium source A to obtain a mixed powder, pre-pressing the mixed powder to obtain a rough blank, and sintering the rough blank in air to obtain the garnet-type lithium-ion solid electrolyte. The sintering process is as follows: first, the temperature is raised to 300-500℃ at a heating rate of 3-5℃ / min and held for 1-5 hours; then, the temperature is raised to 800-1100℃ at a heating rate of 3-5℃ / min; then, the temperature is raised to 1150-1300℃ at a heating rate of 15-25℃ / min and held for 0.5-2 hours; finally, the temperature is naturally cooled to room temperature.

[0012] When garnet-type lithium-ion electrolyte LLZMO precursor powder is prepared, stored, or used in an air atmosphere, lithium carbonate will form on its surface or at grain boundaries, as Li7La3Zr2O. 12 For example, the formation mechanism of lithium carbonate is as follows:

[0013] Li7La3Zr2O 12 +xH2O→Li 7-x H x La3Zr2O 12 +xLiOH

[0014] 2LiOH + CO2 → Li2CO3 + H2O

[0015] Li7La3Zr2O 12 +xH₂O→Li₇La₃Zr₂O 12 (xH2O)

[0016] Li7La3Zr2O 12 (xH2O) + CO2 → Li 7-x H x La3Zr2O 12 +(x-2)LiOH+Li2CO3

[0017] Li7La3Zr2O 12 +CO2→Li 7-2x La3Zr2O 12-x +xLi2CO3

[0018] The lithium carbonate generated above exists on the surface of the solid electrolyte and at grain boundaries. Because lithium carbonate is lithium-repellent and has extremely low ionic conductivity, the surface lithium carbonate hinders the densification process of the electrolyte during sintering, resulting in poor wettability of the electrolyte to lithium metal and a large interfacial impedance between the electrolyte and the electrode. Lithium carbonate at the grain boundaries increases the grain boundary resistance, thereby reducing the ionic conductivity of the electrolyte.

[0019] This invention involves ball milling garnet-type lithium-ion electrolyte LLZMO precursor powder with a zirconium source, adhering zirconium sources of different masses or thicknesses to the surface or gaps of the electrolyte powder. When predetermined conditions are met, zirconium oxide reacts with lithium carbonate, consuming the lithium carbonate already generated in the synthesized garnet-type lithium-ion solid electrolyte powder. The reaction equation between zirconium oxide and lithium carbonate is: Li2CO3 + ZrO2 = Li2ZrO3 + CO2(g). Through the reaction, lithium carbonate is eliminated, and a lithium zirconate coating layer is formed simultaneously.

[0020] The inventors discovered that the sintering process is crucial. Only by adopting the above-mentioned sintering procedure can lithium carbonate be eliminated, while simultaneously forming a lithium zirconate coating layer and obtaining a dense garnet-type lithium-ion solid electrolyte. If the temperature is not maintained at low temperature or the temperature is raised too quickly, the reaction will be incomplete. If the temperature is raised too slowly at high temperature or the holding time is too long, more lithium will be lost and the electrolyte will decompose to generate La3Zr2O7 impurities, affecting the density of the electrolyte sintering and leading to a decrease in performance.

[0021] In a preferred embodiment, the zirconium source A is one or more of zirconium oxide, zirconium carbonate, zirconium nitrate, zirconium oxynitrate, and zirconium ethoxide.

[0022] In a preferred embodiment, the amount of zirconium source added is 1-4 mol% of the lithium content in the LLZMO precursor powder.

[0023] In a preferred embodiment, the ball milling is a wet ball milling process, the ball milling medium is ethanol or isopropanol, the ball milling speed is 200-300 rpm, and the ball milling time is 8-16 h.

[0024] By controlling the ball milling process under the above conditions, it can be ensured that the garnet precursor powder and the zirconium source are mixed evenly.

[0025] In the preferred embodiment, the mixture is dried at 50-90℃ for 6-12 hours after ball milling to obtain the mixed powder.

[0026] In a preferred embodiment, the predetermined molding pressure is 100-400 MPa.

[0027] In a further preferred embodiment, the predetermined molding method is non-isostatic pressing.

[0028] In a preferred embodiment, the sintering process is as follows: first, the temperature is increased to 300-400℃ at a heating rate of 3-5℃ / min, and held for 1-5 hours; then, the temperature is increased to 800-950℃ at a heating rate of 3-5℃ / min; then, the temperature is increased to 1150-1250℃ at a heating rate of 15-25℃ / min, and held for 0.5-2 hours; finally, the temperature is allowed to cool naturally to room temperature.

[0029] In this invention, the sintering conditions are one of ordinary solid-state sintering, hot pressing sintering, and field-assisted sintering.

[0030] In a preferred embodiment, the preparation process of the LLZMO precursor powder is as follows: lithium source, lanthanum source, zirconium source B, and M source are mixed in a designed ratio, wet ball milled to obtain a mixed raw material, dried, and sintered in an air atmosphere to obtain the LLZMO precursor powder.

[0031] In a further preferred embodiment, the lithium source is one or more of lithium hydroxide, lithium carbonate, lithium oxide, and lithium nitrate; the lanthanum source is one or more of lanthanum hydroxide, lanthanum oxide, lanthanum nitrate, and lanthanum carbonate; the zirconium source B is one or more of zirconium oxide, zirconium nitrate, and zirconium carbonate; and the M source is one or more of oxides, nitrates, carbonates, and hydroxides of elements Ta, Nb, Hf, Ge, Ga, Sc, Ti, Al, Si, Sn, Y, and V.

[0032] In a further preferred embodiment, the medium used in the wet ball milling is ethanol or isopropanol, the rotation speed of the wet ball milling is 150-450 rpm, and the wet ball milling time is 4-8 hours. This promotes uniform mixing of the raw materials.

[0033] In a further preferred embodiment, the drying temperature is 50-90℃, and the drying time is 6-12 hours.

[0034] In a further preferred embodiment, the sintering temperature is 900-1100℃ and the sintering time is 6-12h.

[0035] In actual operation, the raw materials are placed in an alumina or magnesium oxide crucible for sintering.

[0036] The particle size of the LLZMO precursor powder prepared by the above method is in the micrometer range.

[0037] Beneficial effects

[0038] This invention provides a garnet-type lithium-ion solid electrolyte, consisting of a core material and a coating layer. The core material is LLZMO, and the coating layer is lithium zirconate. The garnet-type lithium-ion solid electrolyte provided by this invention does not contain lithium carbonate on the surface and at the grain boundaries of the core material. Therefore, it avoids the problems caused by the presence of lithium carbonate, such as hindering sintering density, reducing lithium-ion conductivity, and increasing interfacial impedance. At the same time, the lithium zirconate layer, which has good stability, can further improve the air stability of the garnet electrolyte.

[0039] The preparation method of this invention, taking zirconium oxide as the zirconium source in step 3 as an example, requires no other raw materials besides the garnet solid electrolyte precursor material during the entire coating process. No other impurity elements are introduced during the process. The garnet electrolyte precursor powder is uniformly mixed with a certain amount of zirconium oxide through a simple ball milling process. Then, the mixed powder is shaped and sintered to obtain a garnet-type lithium-ion solid electrolyte with lithium carbonate completely eliminated from the surface and grain boundaries and coated with lithium zirconate. Since lithium carbonate is lithium-phobic, it leads to increased interfacial impedance. The presence of lithium carbonate at the electrolyte grain boundaries affects sintering performance. The added zirconium oxide reacts with contaminants such as lithium carbonate and lithium hydroxide to generate lithium zirconate, which can remove contaminants such as lithium carbonate from the surface and grain boundaries, and also coat the garnet electrolyte with a layer of lithium zirconate with good stability, improving the air stability of the garnet electrolyte. Furthermore, compared with contaminants such as lithium carbonate, the lithium zirconate coated on the electrolyte surface has three-dimensional lithium-ion channels and high ionic conductivity (~5-8×10⁻⁸). -5 S·cm -1 This method promotes lithium-ion conduction at grain boundaries and interfaces. In summary, this method is low-cost, simple in process, and effectively improves the density, ionic conductivity, and air stability of solid electrolytes. Attached Figure Description

[0040] Figure 1 This is a flowchart of a method for completely removing lithium carbonate using a garnet-type lithium-ion solid electrolyte according to an embodiment of the present invention.

[0041] Figure 2 This is a schematic diagram illustrating the principle of lithium carbonate removal according to an embodiment of the present invention;

[0042] Figure 3 This is a scanning electron microscope (SEM) image of the sintered garnet-type lithium-ion solid electrolyte of Example 1 of the present invention.

[0043] Figure 4 This is a cross-sectional scanning electron microscope image of the garnet lithium-ion solid electrolyte after sintering in Comparative Example 2 of this invention;

[0044] Figure 5 This is a cross-sectional scanning electron microscope image of the garnet lithium-ion solid electrolyte after sintering in Comparative Example 2 of this invention;

[0045] Figure 6 These are the room temperature impedance diagrams of Embodiment 1 and Comparative Example 1 of the present invention;

[0046] Figure 7 This is a comparison chart of Raman curves of Embodiment 1 and Comparative Example 1 of the present invention;

[0047] Figure 8This is a comparison of the Raman curves of garnet-type lithium-ion solid electrolytes from Example 1 and Comparative Example 1 after sintering, which were then exposed to air for a period of time. Detailed Implementation

[0048] The technical problems, technical solutions, and advantages of the present invention will be described in detail below with reference to exemplary embodiments. These embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Unless otherwise specified, specific conditions in the embodiments are performed under conventional conditions or conditions recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0049] This invention provides a preparation method using zirconium oxide as the zirconium source. The method employs a simple ball milling process to uniformly mix garnet electrolyte precursor powder with zirconium oxide. The mixed powder is then shaped and sintered at high temperature. The zirconium oxide adhering to the surface or interstices of the garnet electrolyte precursor powder particles can decompose lithium carbonate on the surface and at grain boundaries of the garnet-type lithium-ion solid electrolyte. Through the above ball milling and sintering processes, a garnet-type lithium-ion solid electrolyte with completely eliminated lithium carbonate on the surface and at grain boundaries can be obtained, effectively improving the density and ionic conductivity of the solid electrolyte. Solid-state batteries assembled using this solid electrolyte can effectively suppress lithium dendrite growth and improve electrochemical cycle life.

[0050] This invention employs a simple ball milling process to uniformly mix garnet electrolyte precursor powder with zirconium oxide. After the mixed powder is formed, during the high-temperature sintering process, the zirconium oxide adhering to the surface or gaps of the garnet electrolyte precursor powder particles can decompose lithium carbonate on the surface and at the grain boundaries of the garnet-type lithium-ion solid electrolyte.

[0051] Figure 1 This is a schematic diagram of the process for completely removing lithium carbonate using a garnet-type lithium-ion solid electrolyte.

[0052] Step (1): Weigh the lithium source, lanthanum source, zirconium source, and M source according to a stoichiometric ratio of 5-8:2-3:1-2:0.1-0.6. Preferably, in this step, the lithium source is one or more of lithium hydroxide, lithium carbonate, lithium oxide, and lithium nitrate; the lanthanum source is one or more of lanthanum hydroxide, lanthanum oxide, lanthanum nitrate, and lanthanum carbonate; the zirconium source is one or more of zirconium oxide, zirconium nitrate, and zirconium carbonate; and the M source is one or more of oxides, nitrates, carbonates, or hydroxides of elements Ta, Nb, Hf, Ge, Sc, Ti, Al, Si, Sn, Y, and V. The ball milling medium is ethanol or isopropanol, the ball milling speed is 150-450 rpm, and the ball milling time is 4-8 hours to promote uniform mixing of the raw materials.

[0053] In step (2), preferably, the raw material drying temperature is 50-90℃, the drying time is 6-12 hours, the crucible material is alumina or magnesium oxide crucible, the sintering temperature is 900-1100℃, the sintering time is 6-12 hours, and the synthesized garnet precursor powder is in the micron range.

[0054] The garnet-type electrolyte precursor powder synthesized in this step via calcination is produced using a solid-state method. The chemical formula of the synthesized garnet-type electrolyte precursor powder is Li. 7-x La3Zr 2-x M x O 12 , where x = 0 - 0.6.

[0055] Furthermore, the surface of the garnet solid electrolyte precursor powder has a lithium carbonate layer.

[0056] When garnet-type lithium-ion solid electrolyte powder or electrolyte sheets are prepared, stored, or used in air, lithium carbonate will form on their surface or at grain boundaries, as Li7La3Zr2O. 12 For example, the formation mechanism of lithium carbonate is as follows:

[0057] Li7La3Zr2O 12 +xH2O→Li 7-x H x La3Zr2O 12 +xLiOH

[0058] 2LiOH + CO2 → Li2CO3 + H2O

[0059] Li7La3Zr2O 12 +xH₂O→Li₇La₃Zr₂O 12 (xH2O)

[0060] Li7La3Zr2O 12 (xH2O) + CO2 → Li 7-x H x La3Zr2O 12 +(x-2)LiOH+Li2CO3

[0061] Li7La3Zr2O 12 +CO2→Li 7-2x La3Zr2O 12-x +xLi2CO3

[0062] The lithium carbonate generated above exists on the surface of the solid electrolyte and at grain boundaries. Because lithium carbonate is lithium-repellent and has extremely low ionic conductivity, the surface lithium carbonate hinders the densification process of the electrolyte during sintering, resulting in poor wettability of the electrolyte to lithium metal and a large interfacial impedance between the electrolyte and the electrode. Lithium carbonate at the grain boundaries increases the grain boundary resistance, thereby reducing the ionic conductivity of the electrolyte.

[0063] In step (3), preferably, the amount of zirconium oxide added is 1-4 mol% of the total lithium content in the garnet-type lithium-ion electrolyte, without introducing any impurity elements.

[0064] After two-stage ball milling, drying, and powder forming, zirconium oxide of varying mass or thickness adheres to the surface or gaps of the electrolyte powder. Under predetermined conditions, the zirconium oxide reacts with lithium carbonate, consuming the lithium carbonate already generated in the synthesized garnet-type lithium-ion solid electrolyte powder. The reaction equation for zirconium oxide and lithium carbonate is: Li₂CO₃ + ZrO₂ = Li₂ZrO₃ + CO₂(g). Thermodynamic calculations show that ΔG = 0 in the above reaction, and the reaction temperature is 780℃. Therefore, when the sintering temperature reaches or exceeds 780℃, the above reaction can occur, and the lithium carbonate in the synthesized garnet-type lithium-ion solid electrolyte can be eliminated, improving the electrolyte density and ionic conductivity.

[0065] Further, in step (4), the drying temperature of the mixed powder is 50-90℃, the drying time is 6-12 hours, the predetermined molding pressure is 100-400 MPa, and the sintering process is as follows: first, the temperature is raised to 300-500℃ at a heating rate of 3-5℃ / min, held for 1-5 hours, then raised to 800-1100℃ at a heating rate of 3-5℃ / min, then raised to 1150-1300℃ at a heating rate of 15-25℃ / min, held for 0.5-2 hours, and finally naturally cooled to room temperature. The predetermined molding conditions are non-isostatic pressing molding, the predetermined sintering atmosphere is air, and the sintering conditions are one of ordinary solid-state sintering, hot pressing sintering, and field-assisted sintering.

[0066] Figure 2 The diagram shown illustrates the principle of lithium carbonate elimination in an embodiment of the present invention. Figure 2As shown, after two-stage ball milling in step (3), drying and powder forming in step (4), zirconium oxide layers of different masses or thicknesses adhere to the surface or gaps of the electrolyte precursor powder particles. Since lithium carbonate is generated during the preparation, storage and use of the electrolyte precursor powder in an air atmosphere, when predetermined conditions are met, such as the calcination temperature reaching or exceeding 780°C, zirconium oxide reacts with lithium carbonate on the surface and at the grain boundaries of the solid electrolyte. Then, sintering is carried out according to the sintering procedure provided by this invention, which can achieve the purpose of removing lithium carbonate from the surface and at the grain boundaries. At the same time, lithium zirconate, a lithium-ion conductor, is generated on the surface and at the grain boundaries, promoting the sintering of the material to be dense, reducing the grain boundary impedance and improving the ionic conductivity of the solid electrolyte. In addition, since lithium zirconate has good stability, it can inhibit the regeneration of contaminants such as lithium carbonate on the surface of garnet-type lithium-ion solid electrolytes.

[0067] To facilitate understanding of the embodiments of the present invention, the present invention will be further described below with reference to the accompanying drawings and examples, but is not limited thereto.

[0068] Example 1

[0069] Prepared by solid-state reaction method with chemical formula Li 6.4 La3Zr 1.4 Ta 0.6 O 12 The preparation steps for lithium lanthanum zirconium oxygarnet type lithium-ion solid electrolyte precursor powder include: selecting LiOH(H2O), La2O3, ZrO2, and Ta2O5 as raw materials according to the molar ratio of Li, La, Zr, and Ta of 6.4:3:1.4:0.6, with LiOH(H2O) in 15 mol% excess, ball milling with isopropanol as solvent for 6 hours, followed by drying; then calcining at 900℃ for 6 hours at a heating rate of 3℃ / min; and finally pulverizing and sieving after sintering to obtain Li. 6.4 La3Zr 1.4 Ta 0.6 O 12 Powder. Taking zirconium dioxide as the zirconium source in step 3 as an example, according to the reaction of zirconium dioxide with Li... 6.4 La3Zr 1.4 Ta 0.6 O 12The lithium content in the electrolyte precursor powder was in a molar ratio of 0.01:1. The synthesized electrolyte precursor powder and zirconium oxide were weighed separately and ball-milled for 12 hours at 300 rpm using isopropanol as solvent until homogeneous. The mixture was then dried in an oven at 50°C for 12 hours. The dried powder was pressed into electrolyte sheets under a pressure of 150 MPa and placed in a covered magnesium oxide crucible. The temperature was first increased to 300°C at a rate of 5°C / min and held for 2 hours. Then, the temperature was increased to 850°C at a rate of 5°C / min, followed by an increase to 1150°C at a rate of 15°C / min and held for 30 minutes to obtain a garnet-type lithium-ion solid electrolyte with lithium carbonate removed. The cross-section of the lithium carbonate-removed garnet solid electrolyte described in this embodiment was observed using a scanning electron microscope. The results are as follows: Figure 3 As shown, the solid electrolyte particles are in close contact with each other, with only a few voids, and the sintered density of the electrolyte is 94.7%.

[0070] Comparative Example 1

[0071] Comparative Example 1 prepared Li using essentially the same steps as in Example 1. 6.4 La3Zr 1.4 Ta 0.6 O 12 The lithium lanthanum zirconium oxygarnet type lithium-ion solid electrolyte precursor powder differs in that zirconium oxide is not added during the two-stage ball milling.

[0072] The above-mentioned comparative samples were observed using a scanning electron microscope, and the results were obtained. Figure 4 The results show that a lot of lithium carbonate exists in the grain boundaries and pores of the electrolyte after sintering in the comparative example. At the same time, the sintering is not dense, with a density of 77.8%, which is much lower than the density of the solid electrolyte without lithium carbonate in the examples. This shows that the method of the present invention can effectively improve the density of solid electrolytes.

[0073] The comparative solid electrolyte and the solid electrolyte with lithium carbonate eliminated in the examples were crushed and ground separately to obtain powder, and then Raman tests were performed. Figure 5 The figure shows the Raman test comparison curves of the solid electrolytes described in Comparative Example 1 and Example 1. No lithium carbonate peak was observed in the powder of the solid electrolyte described in Example 1 after it was broken up, indicating that the method described in the example can effectively remove lithium carbonate. The solid electrolytes of Example 1 and the comparative example were placed in air for a period of time before Raman testing was performed. Figure 6 Example 1, when exposed to air, still showed no lithium carbonate peak, demonstrating that the solid electrolyte of Example 1 has good air stability and can effectively inhibit the regeneration of lithium carbonate contaminants. To compare the impedance of the prepared comparative solid electrolyte and the lithium carbonate-free solid electrolyte of Example 1, ionic conductivity was calculated, and the corresponding impedances are shown below. Figure 7 As shown, the ionic conductivity of the comparison sample solid electrolyte is only 6.5 × 10⁻⁶.-5 S·cm -1 The solid electrolyte that eliminates lithium carbonate has an ionic conductivity as high as 3.87 × 10⁻⁶. -4 S·cm -1 This demonstrates that the method described in this invention can effectively improve the ionic conductivity of garnet-type lithium-ion solid electrolytes.

[0074] Comparative Example 2

[0075] Comparative Example 2 prepared Li using essentially the same steps as in Example 1. 6.4 La3Zr 1.4 Ta 0.6 O 12 The lithium lanthanum zirconium oxygarnet type lithium-ion solid electrolyte precursor powder differs in that 6 mol% excess zirconium oxide is added during the two-stage ball milling.

[0076] The above-mentioned comparative samples were observed using a scanning electron microscope, and the results were obtained. Figure 5 The results show that, in Comparative Example 2, the sintered electrolyte contains a large amount of La3Zr2O7 at grain boundaries and pores, resulting in a less dense electrolyte with a density of 77.8%, far lower than the density of the lithium carbonate-free solid electrolyte in the examples. This indicates that excess zirconium oxide reacts with the LLZMO electrolyte to form La3Zr2O7, affecting the electrolyte's sintering performance. Impedance testing was performed on the prepared solid electrolyte, and the calculated ionic conductivity was 4.0 × 10⁻⁶. -5 S·cm -1 This indicates that adding too much zirconium oxide will not further improve the sintering density and conductivity, but will instead worsen the sintering performance of the electrolyte.

[0077] Comparative Example 3

[0078] Comparative Example 3 prepared Li using essentially the same steps as in Example 1. 6.4 La3Zr 1.4 Ta 0.6 O 12 The precursor powder for the lithium lanthanum zirconium oxygarnet type lithium-ion solid electrolyte was prepared using the same zirconium source in the two-stage ball milling process. The difference lay in the sintering process of the shaped electrolyte sheet: the temperature was first increased to 300℃ at a rate of 5℃ / min and held for 2 hours, then directly increased to 1150℃ at a rate of 5℃ / min and held for 30 minutes. Impedance testing was performed on the prepared solid electrolyte, and the calculated ionic conductivity was 2.5 × 10⁻⁶. -5 S·cm -1This is because at temperatures above 900℃, lithium in the garnet electrolyte volatilizes as lithium oxide, resulting in lithium loss and the formation of a lithium-deficient La3Zr2O7 phase. This affects the electrolyte's sintering performance and reduces ionic conductivity. This indicates that the sintering time at high temperatures should not be too long to avoid further lithium loss.

[0079] Comparative Example 4

[0080] Comparative Example 4 prepared Li using essentially the same steps as in Example 1. 6.4 La3Zr 1.4 Ta 0.6 O 12 The precursor powder for the lithium lanthanum zirconium oxygarnet type lithium-ion solid electrolyte was prepared using the same zirconium source added during the two-stage ball milling process. The difference lay in the sintering process of the shaped electrolyte sheet: the temperature was first increased to 850℃ at a rate of 5℃ / min, followed by an increase to 1150℃ at a rate of 15℃ / min, and held for 30 minutes. Impedance testing was performed on the prepared solid electrolyte, and the calculated ionic conductivity was 8.0 × 10⁻⁶. -6 S·cm -1 The ionic conductivity was much lower than that of the sintered electrolyte in Example 1. This was because the heating rate was too fast and the low-temperature sintering time was too short, resulting in poor electrolyte density. This indicates that the sintering time in the low-temperature section should not be too short, as this leads to poor sintering density.

[0081] Example 2

[0082] Prepared by solid-state reaction method with chemical formula Li 6.25 Ga 0.25 La3Zr2O 12 The preparation steps for lithium lanthanum zirconium oxygarnet type lithium-ion solid electrolyte precursor powder include: selecting LiOH(H2O), La2O3, ZrO2, and Ga2O3 as raw materials according to the molar ratio of Li, La, Zr, and Ga of 6.25:3:2:0.25, with LiOH(H2O) in 15 mol% excess, ball milling with isopropanol as solvent for 8 hours, and then drying; then calcining at 950℃ for 10 hours at a heating rate of 4℃ / min; and finally pulverizing and sieving after sintering to obtain Li 6.25 Ga 0.25 La3Zr2O 12 Powder. According to the relationship between zirconium dioxide and Li... 6.25 Ga 0.25 La3Zr2O 12The lithium content in the electrolyte precursor powder was in a molar ratio of 0.02:1. The synthesized electrolyte precursor powder and zirconium oxide were weighed separately and ball-milled for 12 hours at a speed of 250 rpm using isopropanol as solvent. The mixture was dried in an oven at 60°C for 12 hours. The dried powder was pressed into electrolyte sheets under a pressure of 250 MPa and placed in a covered magnesium oxide crucible. The temperature was first increased to 350°C at a rate of 4°C / min and held for 3 hours. Then the temperature was increased to 900°C at a rate of 4°C / min and then increased to 1200°C at a rate of 20°C / min and held for 1 hour to obtain a garnet-type lithium-ion solid electrolyte with lithium carbonate removed. Scanning electron microscopy (SEM), Raman spectroscopy, density, and impedance analysis were performed on the lithium carbonate-removed garnet-type lithium-ion solid electrolyte obtained above. The results showed that the lithium carbonate-removed garnet-type lithium-ion electrolyte had a relatively dense cross-section, with a density of approximately 95%. The Raman spectroscopy showed no peak corresponding to lithium carbonate, confirming that lithium carbonate had been eliminated. The ionic conductivity of the lithium carbonate-removed garnet-type lithium-ion electrolyte was approximately 3.2 × 10⁻⁶. -4 S·cm -1 .

[0083] Example 3

[0084] Prepared by solid-state reaction method with chemical formula Li 6.4 La3Zr 1.4 Nb 0.6 O 12 The preparation steps for lithium lanthanum zirconium oxygarnet type lithium-ion solid electrolyte precursor powder include: selecting LiOH(H2O), La2O3, ZrO2, and Ga2O3 as raw materials according to the molar ratio of Li, La, Zr, and Nb of 6.4:3:1.4:0.6, wherein LiOH(H2O) is in 10 mol% excess, ball milling with isopropanol as solvent for 10 hours and then drying; then calcining at 1000℃ for 12 hours at a heating rate of 5℃ / min, and after sintering, pulverizing and sieving to obtain Li 6.4 4La3Zr 1.4 Nb 0.6 O 12 Powder. According to the relationship between zirconium dioxide and Li... 6.4 4La3Zr 1.4 Nb 0.6 O 12The lithium content in the electrolyte precursor powder was in a molar ratio of 0.03:1. The synthesized electrolyte precursor powder and zirconium oxide were weighed separately and ball-milled for 14 hours at a speed of 200 rpm using isopropanol as solvent. The mixture was dried in an oven at 80°C for 12 hours. The dried powder was pressed into electrolyte sheets under a pressure of 300 MPa and placed in a covered magnesium oxide crucible. The temperature was first increased to 400°C at a rate of 3°C / min and held for 4 hours. Then, the temperature was increased to 950°C at a rate of 3°C / min and then increased to 1250°C at a rate of 25°C / min and held for 2 hours to obtain a garnet-type lithium-ion solid electrolyte with lithium carbonate removed. Scanning electron microscopy (SEM), Raman spectroscopy, density, and impedance analysis were performed on the lithium carbonate-removed garnet-type lithium-ion solid electrolyte obtained above. The results showed that the lithium carbonate-removed garnet-type lithium-ion electrolyte had a relatively dense cross-section, with a density of approximately 95.8%. The Raman spectroscopy showed no corresponding peak for lithium carbonate, confirming that lithium carbonate had been eliminated. The ionic conductivity of the lithium carbonate-removed garnet-type lithium-ion electrolyte was approximately 4.5 × 10⁻⁶. -4 S·cm -1 .

[0085] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a garnet-type lithium ion solid electrolyte, characterized by: The LLZMO precursor powder is mixed with the zirconium source A to obtain a mixed powder, the mixed powder is preformed to obtain a rough blank, and the rough blank is sintered in an air atmosphere to obtain the garnet lithium ion solid electrolyte, wherein the sintering process is as follows: first, the temperature is raised to 300-500 ℃ at a temperature raising rate of 3-5 ℃ / min, and then the temperature is raised to 800-1100 ℃ at a temperature raising rate of 3-5 ℃ / min, and then the temperature is raised to 1150-1300 ℃ at a temperature raising rate of 15-25 ℃ / min, and then the temperature is kept for 0.5-2 h, and finally the temperature is naturally cooled to room temperature. The amount of the zirconium source A is 1-4 mol% of the lithium content in the LLZMO precursor powder. The garnet lithium ion solid electrolyte is composed of a core material and a coating layer, the molecular formula of the core material is Li 7-x La3Zr 2-x M x O 12 , wherein x=0-0.6, the coating layer is lithium zirconate, and the M is at least one selected from Ta, Nb, Hf, Ge, Ga, Sc, Ti, Al, Si, Sn, Y, V.

2. The method of claim 1, wherein the method is characterized by: The zirconium source A is one or more of zirconium oxide, zirconium carbonate, zirconium nitrate, zirconyl nitrate and zirconium ethoxide.

3. The method of claim 1, wherein the method comprises: The ball milling is wet ball milling, the ball milling medium is ethanol or isopropyl alcohol, the ball milling rotation speed is 200-300 r / min, and the ball milling time is 8-16 h. ​ 4. The method of claim 1, wherein the method is characterized by: The preforming pressure is 100-400 Mpa.

5. The method of claim 1, wherein the method is characterized by: The sintering process is as follows: first, the temperature is raised to 300-400 ℃ at a temperature raising rate of 3-5 ℃ / min, and then the temperature is raised to 800-950 ℃ at a temperature raising rate of 3-5 ℃ / min, and then the temperature is raised to 1150-1250 ℃ at a temperature raising rate of 15-25 ℃ / min, and then the temperature is kept for 0.5-2 h, and finally the temperature is naturally cooled to room temperature.

6. The method of claim 1, wherein the method is characterized by: The preparation process of the LLZMO precursor powder is as follows: the lithium source, the lanthanum source, the zirconium source B and the M source are mixed by wet ball milling according to a designed proportion, and then the mixture is dried and sintered in an air atmosphere to obtain the LLZMO precursor powder.

7. The method of claim 6, wherein the method is characterized by: The lithium source is one or more of lithium hydroxide, lithium carbonate, lithium oxide and lithium nitrate; the lanthanum source is one or more of lanthanum hydroxide, lanthanum oxide, lanthanum nitrate and lanthanum carbonate; the zirconium source B is one or more of zirconium oxide, zirconium nitrate and zirconium carbonate; and the M source is one or more of oxides, nitrate salts, carbonate salts and hydroxides of elements Ta, Nb, Hf, Ge, Ga, Sc, Ti, Al, Si, Sn, Y and V.

8. The method of claim 6, wherein the method is characterized by: The wet ball milling medium is ethanol or isopropyl alcohol, the wet ball milling rotation speed is 150-450 r / min, and the wet ball milling time is 4-8 h; the sintering temperature is 900-1100 ℃, and the sintering time is 6-12 h.

9. The method of claim 1, wherein the method is characterized by: The content of zirconium in the coating layer is 1-4 mol% of the lithium content in the core material.

Images