Preparation method for solid electrolyte

Abstract

A preparation method for a solid electrolyte. The preparation method comprises: adding an excess of a lithium precursor to a pre-prepared solid electrolyte blank, mixing same with a solvent, then ball-milling the resulting mixture, and then drying same; molding the mixture obtained after drying; and then sintering a molded body in a sintering atmosphere containing an inert gas, so as to obtain a solid electrolyte, wherein the prepared solid electrolyte is a tantalum-doped garnet-type solid electrolyte. The prepared solid electrolyte has a relatively high ionic conductivity, can improve the performance of a total battery, and is beneficial to commercial application of a lithium metal solid-state battery.

Description

A method for preparing a solid electrolyte Technical Field

[0001] The present invention relates to a method for preparing a solid-state electrolyte. More specifically, it relates to a method for preparing a tantalum-doped garnet-type solid-state electrolyte with high lithium ion conductivity, as well as a solid-state electrolyte prepared by the method and a solid-state lithium metal battery containing the solid-state electrolyte. Background Art

[0002] Lithium-ion batteries, with their long cycle life and high energy density, have become one of the most important energy storage devices in our daily lives. In recent years, the rapid development of electric vehicles has placed higher demands on battery energy density and safety. Solid-state lithium metal batteries use lithium metal as the negative electrode and a solid electrolyte instead of traditional separators and electrolytes. This not only significantly increases the battery's energy density but also significantly reduces the risk of spontaneous combustion and explosion.

[0003] At present, the common solid electrolytes include oxide solid electrolytes, sulfide solid electrolytes, composite solid electrolytes and emerging halide solid electrolytes. Among them, oxide solid electrolytes have the longest research time and the highest probability of commercialization. For example, studies have shown that garnet-type LLZO solid electrolytes (Li7La3Zr2O 12 ) After Ta element doping, the lithium ion conductivity can be greatly improved, so the research on tantalum-doped LLZTO solid electrolyte is more extensive.

[0004] The existing technology still faces many challenges in the preparation of LLZTO, such as low ceramic sheet density and low ionic conductivity. Currently, the lithium ion conductivity of LLZTO obtained by atmospheric pressure sintering is around 0.5mS / cm, which is still relatively low for commercial lithium battery use and far from the theoretical value. To meet commercial standards, the lithium ion conductivity of the electrolyte needs to be further improved, ideally to above 0.8mS / cm.

[0005] Therefore, designing a method to sinter LLZTO solid electrolyte with higher ionic conductivity is particularly critical for the commercial application of lithium metal solid-state batteries.

[0006] Summary of the Invention

[0007] The technical problem to be solved by the present invention is the low lithium ion conductivity of the ceramic sheet when the LLZTO solid electrolyte is currently sintered in air.

[0008] To address the above technical issues, the present invention provides a method for preparing high-conductivity solid-state electrolyte ceramic sheets. This method utilizes simple cold-pressing sintering to produce solid-state electrolyte ceramic sheets by controlling the lithium excess and sintering atmosphere. This simple process facilitates industrial production and application. During sintering in a protective atmosphere containing an inert gas, oxygen vacancies are generated in the solid electrolyte, thereby improving the ionic conductivity of the ceramic sheet.

[0009] A first aspect of the present invention provides a method for preparing a solid electrolyte, comprising the following steps:

[0010] (1) adding an excess amount of lithium precursor to a pre-prepared solid electrolyte blank, mixing with a solvent, performing ball milling, and then drying;

[0011] (2) shaping the mixture obtained in step (1);

[0012] (3) sintering the molded body obtained in step (2) in a sintering atmosphere containing an inert gas to obtain a solid electrolyte, wherein

[0013] The solid electrolyte is a tantalum-doped garnet-type solid electrolyte.

[0014] In some specific embodiments, the chemical formula of the tantalum-doped garnet-type solid electrolyte prepared according to the method of the present invention is Li 7-x La3Zr 2-x Ta x O 12 , wherein 0<x≤1, preferably 0.5≤x≤0.6.

[0015] In some specific embodiments, the sintering atmosphere in step (3) contains an inert gas of 50% by volume or greater, preferably 80% by volume or greater, and more preferably 95% by volume or greater. In the present invention, the inert gas is selected from helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xn), and the like.

[0016] In some embodiments, the inert gas in the sintering atmosphere comprises argon, and preferably the inert gas consists of argon.

[0017] In some specific embodiments, the sintering atmosphere in step (3) further comprises one or more gases selected from hydrogen, nitrogen, oxygen, and carbon dioxide.

[0018] In some particularly preferred embodiments, the sintering atmosphere in step (3) is composed of an inert gas, preferably argon.

[0019] In some specific embodiments, in step (1), the excess ratio of the molar number of the added lithium element precursor relative to the molar content of the lithium element in the solid electrolyte is greater than or equal to 30%, preferably greater than or equal to 40%.

[0020] In some specific embodiments, in step (2), the forming is performed by cold pressing at a pressure of 15 to 20 MPa.

[0021] In some specific embodiments, in step (3), the sintering temperature is 1000-1500°C, preferably 1050-1200°C, and more preferably 1100-1150°C.

[0022] In some specific embodiments, the method for preparing the solid electrolyte blank used in step (1) comprises the following steps:

[0023] (a1) mixing precursors of various elements and optionally an excess of lithium with a solvent according to the chemical composition of the solid electrolyte, ball milling the mixture, and then drying the mixture;

[0024] (a2) pre-calcining the mixture obtained in step (a1) to obtain the solid electrolyte blank.

[0025] In some specific embodiments, the precursor of each element can be an oxide, hydroxide, or salt of the corresponding element. In some preferred embodiments, the precursor of each element includes a lithium source, a lanthanum source, a zirconium source, and a tantalum source; preferably, the precursor of each element includes LiOH·H2O, La2O3, ZrO2, and Ta2O5.

[0026] In some specific embodiments, the excess molar ratio of the lithium precursor in step (a1) is 1% to 20%, preferably 5% to 15%.

[0027] In some embodiments, the pre-firing temperature in step (a2) is lower than the sintering temperature in step (3). Specifically, the pre-firing temperature is 700-1000°C, preferably 800-900°C.

[0028] A second aspect of the present invention provides a solid electrolyte, which is prepared by the preparation method of the first aspect of the present invention.

[0029] In some specific embodiments, the solid electrolyte of the present invention is a tantalum-doped garnet-type solid electrolyte, preferably, its chemical formula is Li 6.4 La3Zr 1.4 Ta 0.6 O 12 .

[0030] In some specific embodiments, the ionic conductivity of the solid electrolyte of the present invention is greater than or equal to 0.8 mS / cm, preferably greater than or equal to 1.0 mS / cm.

[0031] The third aspect of the present invention provides the use of the solid electrolyte according to the second aspect of the present invention in a solid-state lithium metal battery.

[0032] A fourth aspect of the present invention provides a solid-state lithium metal battery, which includes the solid-state electrolyte according to the second aspect of the present invention.

[0033] Advantageous Effects of the Invention

[0034] The present invention adjusts the excess molar ratio of lithium during the sintering process and performs sintering in a protective atmosphere containing an inert gas (such as an argon atmosphere). The prepared solid electrolyte ceramic sheet has a high ionic conductivity (for example, greater than 1×10 -3 S / cm), 2-5 times higher than conventional air sintering (about 1-5×10 -4 S / cm). Higher lithium ion conductivity can improve the performance of the entire battery, such as better cycle performance, higher capacity retention, higher charge and discharge efficiency, and greater charge and discharge rates. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 shows exemplary optical images of a solid-state electrolyte LLZTO ceramic sheet prepared under different sintering atmospheres in Example 5 of the present invention. The sintering atmospheres, from left to right, are: (A) air; (B) nitrogen; (C) oxygen; (D) argon + hydrogen; (E) argon + oxygen; and (F) argon.

[0036] Figure 2 shows exemplary impedance test graphs of solid electrolytes prepared according to embodiments and comparative examples of the present invention. The circular marks on the left are LLZTO ceramic sheets sintered in air, and the triangle marks on the right are LLZTO ceramic sheets sintered in argon.

[0037] Figure 3 shows optical images of LLZTO ceramic sheets sintered under different lithium excess conditions in Example 4 of the present invention. (A) Sintered in argon, (B) Sintered in air. From left to right, the lithium excess ratios are 10%, 20%, 30%, and 40%, respectively.

[0038] 4 shows an XRD pattern of a solid electrolyte LLZTO ceramic sheet prepared according to an embodiment of the present invention. The preparation conditions are sintering in argon and a lithium excess of 40%.

[0039] Figure 5 shows SEM images of solid electrolyte LLZTO ceramic sheets prepared according to examples and comparative examples of the present invention. (A) and (B) are surface SEM images, and (C) and (D) are cross-sectional SEM images. The preparation conditions were sintering in argon. (A) and (C) had a 40% excess lithium content, while (B) and (D) had a 10% excess lithium content. DETAILED DESCRIPTION

[0040] The present invention will be further described below by way of specific embodiments. Unless otherwise specified, the terms used herein have the same meanings as those generally understood by those of ordinary skill in the art. The numerical limits or ranges set forth herein include endpoints, specifically all values ​​and subranges within the numerical limits or ranges.

[0041] A first aspect of the present invention provides a method for preparing a solid electrolyte, comprising the following steps:

[0042] (1) adding an excess amount of lithium precursor to a pre-prepared solid electrolyte blank, mixing with a solvent, performing ball milling, and then drying;

[0043] (2) shaping the mixture obtained in step (1);

[0044] (3) sintering the molded body obtained in step (2) in a sintering atmosphere containing an inert gas to obtain a solid electrolyte, wherein

[0045] The solid electrolyte is a tantalum-doped garnet-type solid electrolyte.

[0046] In some specific embodiments of the present invention, the chemical formula of the tantalum-doped garnet-type solid electrolyte prepared according to the method of the present invention is Li 7-x La3Zr 2-x Ta x O 12 , wherein 0<x≤1, preferably 0.5≤x≤0.6.

[0047] In some particularly preferred embodiments, the chemical formula of the tantalum-doped garnet-type solid electrolyte prepared according to the method of the present invention is Li 6.4 La3Zr 1.4 Ta 0.6 O 12 .

[0048] In some specific embodiments of the present invention, the sintering atmosphere in step (3) contains an inert gas of greater than or equal to 50% by volume, preferably greater than or equal to 80% by volume, and more preferably greater than or equal to 95% by volume. In the present invention, inert gas refers to a gaseous substance corresponding to a Group 0 element on the periodic table, including helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xn), etc. The chemical reactivity of inert gas is very low and can be used as a protective gas under high temperature conditions. Under the preparation conditions of the present invention, it does not react substantially with the solid electrolyte of the present invention or any of its precursors or intermediates, thereby avoiding the occurrence of potential side reactions in the preparation process and ensuring that the prepared solid electrolyte has an ideal lattice structure.

[0049] In some specific embodiments of the present invention, the inert gas in the sintering atmosphere comprises argon, and preferably, the inert gas consists of argon. The expression "consisting of argon" means that the argon used is commercially available industrial argon or laboratory argon with a purity greater than 99%, greater than 99.9%, or greater than 99.99%, but may still contain unavoidable impurities.

[0050] In some specific embodiments of the present invention, the sintering atmosphere in step (3) further comprises one or more gases selected from hydrogen, nitrogen, oxygen, and carbon dioxide. The content of the gases other than the inert gases in the sintering atmosphere is less than or equal to 50% by volume, preferably less than or equal to 20% by volume, and more preferably less than or equal to 5% by volume.

[0051] In the preparation method of the solid electrolyte of the present invention, the lithium source needs to be provided in excess. In the context of the present invention, "excess" refers to the molar number of the lithium element precursor added during the preparation process being greater than the molar content of the lithium element in the solid electrolyte calculated according to the chemical composition of the solid electrolyte based on the molar number of other element precursors (such as lanthanum source, zirconium source, tantalum source, etc.). For example, the chemical formula of the tantalum-doped garnet-type solid electrolyte prepared by the present invention is Li 7-x La3Zr 2-x Ta x O 12 If the molar ratio of lithium, lanthanum, zirconium and tantalum elements in the lithium source, lanthanum source, zirconium source and tantalum source is Li:La:Zr:Ta=M:3:2-x:x, then the molar ratio M of lithium element in the lithium source is greater than 7-x; the excess percentage is calculated as (M-(7-x)) / (7-x).

[0052] In some particularly preferred embodiments, the sintering atmosphere in step (3) is composed of an inert gas, preferably argon, that is, the sintering atmosphere does not contain any gas other than the inert gas.

[0053] In some specific embodiments of the present invention, in step (1), the molar excess ratio of the added lithium precursor relative to the molar content of the lithium element in the solid electrolyte is greater than or equal to 30%, preferably greater than or equal to 40%. According to research of the present invention, it has been found that the excess provision of lithium source can not only compensate for the volatilization of lithium components during high-temperature sintering, but also, when the excess lithium source is greater than or equal to 30%, preferably greater than or equal to 40%, can further improve the ionic conductivity of the prepared solid electrolyte.

[0054] In some embodiments of the present invention, in step (2), the forming is performed by cold pressing at a pressure of 15 to 20 MPa. For example, the powder after ball milling and drying can be placed in a circular mold with a diameter of 15 mm and tabletted at a pressure of 15 to 20 MPa to obtain a formed round tablet.

[0055] In some specific embodiments of the present invention, in step (3), the sintering temperature is 1000-1500° C., preferably 1050-1200° C., and more preferably 1100-1150° C. In some specific embodiments of the present invention, in step (3), the heating rate is 5-20° C. / min, preferably 10° C. / min; and the sintering time is 6-24 hours, preferably 12 hours.

[0056] In some specific embodiments of the present invention, the method for preparing the solid electrolyte blank used in step (1) comprises the following steps:

[0057] (a1) mixing precursors of various elements and optionally an excess of lithium with a solvent according to the chemical composition of the solid electrolyte, ball milling the mixture, and then drying the mixture;

[0058] (a2) pre-calcining the mixture obtained in step (a1) to obtain the solid electrolyte blank.

[0059] In some specific embodiments of the present invention, the precursor of each element may be an oxide, hydroxide or salt (such as nitrate) of the corresponding element. For example, as a lithium source, lithium hydroxide monohydrate (LiOH·H2O) can be used, as a lanthanum source, La2O3 can be used, and as a zirconium source, ZrO2 can be used. As a tantalum source, Ta2O5 can be used. The precursor can be pretreated according to actual conditions. For example, when La2O3 is used, it needs to be calcined at above 900°C for more than 12 hours in advance to remove moisture. In some preferred embodiments, the precursors of each element include LiOH·H2O, La2O3, ZrO2 and Ta2O5.

[0060] In some specific embodiments of the present invention, the molar excess ratio of the lithium element precursor in step (a1) is 1% to 20%, preferably 5% to 15%. In the preparation process of the solid electrolyte blank, the definition of excess lithium element is the same as above, that is, the molar number of the lithium element precursor added in step (a1) is greater than the molar content of lithium element in the solid electrolyte blank calculated according to the chemical composition of the solid electrolyte based on the molar number of other element precursors (such as lanthanum source, zirconium source, tantalum source, etc.).

[0061] In some specific embodiments of the present invention, the pre-firing temperature in step (a2) is lower than the sintering temperature in step (3). Specifically, the pre-firing temperature in step (a2) is 700-1000°C, preferably 800-900°C. In some specific embodiments of the present invention, the heating rate in step (2a) is 5-20°C / min, preferably 10°C / min; the pre-firing time is 6-24 hours, preferably 12 hours. There is no particular limitation on the atmosphere used during the pre-firing process. For the purpose of facilitating operation and reducing costs, air can be used.

[0062] In some specific embodiments of the present invention, in the two ball milling steps (step (a1) and step (1)), the solid-liquid ratio of the solid (i.e., the precursor or blank) to the solvent can be 1:1-5, the ball-to-solid ratio can be 1:1-10, the ball milling time can be 1-24 hours, the drying temperature can be 50-200° C., and the drying time can be 2-24 hours. In addition, the solvent can be isopropyl alcohol.

[0063] In an exemplary embodiment of the present invention, the chemical formula is Li 6.4 La3Zr 1.4 Ta 0.6 O 12 The preparation method of the solid electrolyte comprises the following steps:

[0064] (a1) Based on the chemical composition of the solid electrolyte and a 10% excess of lithium source, the precursors of each element were mixed with a solvent in a molar ratio of LiOH·H2O:La2O3:ZrO2:Ta2O5=7.04:1.5:1.4:0.3, and then ball-milled and dried;

[0065] (a2) pre-calcining the mixture obtained in step (a1) at 900° C. to obtain a solid electrolyte blank;

[0066] (1) adding a lithium precursor LiOH·H2O in a molar excess of 40% to the solid electrolyte blank prepared in step (a2), mixing with a solvent, ball milling, and then drying;

[0067] (2) cold pressing the mixture obtained in step (1);

[0068] (3) The molded body obtained in step (2) is sintered in an argon sintering atmosphere to obtain a molten body having a chemical formula of Li 6.4 La3Zr 1.4 Ta 0.6 O 12 of solid electrolytes.

[0069] The inventors have discovered that in the solid electrolyte preparation method of the present invention, a secondary ball milling step, performed after the solid electrolyte blank is prepared, can result in a finer powder particle size, a denser LLZTO pellet after sintering, and higher ionic conductivity. The primary purpose of the initial ball milling of the blank followed by low-temperature sintering is to align the powder phases and produce the LLZTO phase. The 10% excess lithium source added during the initial ball milling compensates for lithium loss during the powder calcination at 900°C. Insufficient excess lithium source in this step results in poor phase formation of the sintered LLZTO powder, insufficient LLZTO phase formation, and compromised solid electrolyte product quality. Excessive excess lithium source introduces a significant amount of impurity phases. The secondary ball milling step refines impurity phases, coarse particles, and less active phases within the solid electrolyte blank, enhancing the subsequent sintering activity of the ceramic pellet. Furthermore, the addition of excess lithium source during the secondary ball milling allows for thorough mixing of the supplemented lithium hydroxide with the phase powder to be reacted, facilitating a full reaction during the secondary sintering step. SEM and XRD characterizations have shown that adding 40% molar excess lithium source during the secondary spheroidal graphite sintering can produce a solid electrolyte with large LLZTO grains, few grain boundaries, no pores, and no other impurity phases.

[0070] The solid electrolyte preparation method of the present invention can improve the ionic conductivity of the solid electrolyte. Specifically, the solid electrolyte prepared according to the method of the present invention has an ionic conductivity of greater than or equal to 0.8 mS / cm, preferably greater than or equal to 1.0 mS / cm. This is because the LLZTO solid electrolyte sintered in argon has a higher density, fewer grain boundaries, and a more complete crystal structure.

[0071] The second aspect of the present invention provides a solid electrolyte obtained by the preparation method according to the first aspect of the present invention.

[0072] In some specific embodiments of the present invention, the solid electrolyte of the present invention is a tantalum-doped garnet-type solid electrolyte, preferably, its chemical formula is Li 7-x La3Zr 2-x Ta x O 12 , wherein 0<x≤1, preferably 0.5≤x≤0.6.

[0073] In some particularly preferred embodiments, the chemical formula of the tantalum-doped garnet-type solid electrolyte according to the present invention is Li 6.4 La3Zr 1.4 Ta 0.6 O 12 .

[0074] The solid electrolyte of the second aspect of the present invention has an ionic conductivity superior to that of prior art solid electrolytes. In some embodiments of the present invention, the ionic conductivity of the solid electrolyte of the present invention is greater than or equal to 0.8 mS / cm, preferably greater than or equal to 1.0 mS / cm.

[0075] The third aspect of the present invention provides the use of the solid electrolyte according to the second aspect of the present invention in a solid-state lithium metal battery.

[0076] A fourth aspect of the present invention provides a solid-state lithium metal battery. In some specific embodiments, the solid-state lithium metal battery of the present invention includes the solid-state electrolyte of the second aspect of the present invention. The materials and preparation methods of the other components of the lithium battery, other than the solid-state electrolyte, are not particularly limited; conventional materials and preparation methods in the art may be used.

[0077] Example

[0078] The present invention is described in detail below by way of examples, which are not intended to limit the present invention. The experimental methods in the following examples are all conventional methods unless otherwise specified.

[0079] Example 1 Preparation of LLZTO solid electrolyte blank

[0080] To prepare the chemical formula Li 6.4 La3Zr 1.4 Ta 0.6 O 12 The LLZTO solid electrolyte is prepared by weighing the following raw materials according to the stoichiometric ratio and the molar ratio of 10% excess lithium source: 3.3198g of lithium hydroxide monohydrate (LiOH·H2O), 5.4948g of lanthanum oxide (La2O3), 1.9396g of zirconium oxide (ZrO2) and 1.4945g of tantalum oxide (Ta2O5).

[0081] Place the weighed precursor powders of each element into a zirconia ball mill. Add 9-10 mL of isopropyl alcohol and 50 g of zirconia ball milling beads. Wet mill at 500 rpm for 3 hours to obtain a mixed powder. Then, dry the mixed powder in an oven at 80°C to completely remove the isopropyl alcohol solvent.

[0082] The above powder was placed in a muffle furnace for pre-sintering, and the temperature was increased from 50°C to 900°C at a heating rate of 10°C / min, and kept warm for 12 hours. Then, the temperature was reduced to 500°C at a cooling rate of 10°C / min, and naturally cooled to room temperature and taken out to obtain a pre-sintered LLZTO solid electrolyte blank.

[0083] Unless otherwise specified, the LLZTO solid electrolyte blank prepared above was used as the starting material in the subsequent examples for the preparation of LLZTO solid electrolyte.

[0084] Example 2 Preparation of LLZTO solid electrolyte

[0085] Weigh 10g of the pre-calcined LLZTO solid electrolyte blank, weigh lithium hydroxide monohydrate (LiOH·H2O) according to the required Li excess molar ratio, place it in a zirconia ball mill, add 8-10mL of isopropanol and 50g of zirconia ball milling beads, and wet-mill at 500rpm for 8h to mix thoroughly to obtain a mixed powder. The mixed powder is then placed in an oven and dried at 80°C to completely remove the solvent isopropanol.

[0086] 1.1g of dried powder was weighed and placed in a 15mm diameter circular mold. The pellets were pressed under a pressure of 15-20MPa to obtain a shaped pellet. The pellets were then placed in a crucible and placed in a muffle furnace. Under the appropriate sintering atmosphere, the temperature was increased from 50°C to 1150°C at a rate of 10°C / min. The temperature was maintained for 12 hours, then decreased to 500°C at a rate of 5°C / min. The pellets were then naturally cooled to room temperature and removed to obtain the LLZTO solid electrolyte.

[0087] Example 3 Characterization of LLZTO solid electrolyte

[0088] The surface of the sintered LLZTO ceramic sheet was polished to be smooth, then immersed in dilute hydrochloric acid for 30 seconds to remove impurities such as Li2CO3 and LiOH on the surface, and then placed in a glove box for standby use.

[0089] The diameter and thickness of the LLZTO ceramic sheet were measured using a micrometer.

[0090] To test the ionic conductivity of the LLZTO solid electrolyte, a layer of Ag was deposited on each side of the LLZTO electrolyte sheet using a thermal evaporation coating apparatus to form an Ag / LLZTO / Ag blocking system. This was then encapsulated in a coin cell battery. AC impedance measurements were performed using a Metrohm Autolab with an AC voltage of 10 mV and a frequency range of 1-10 MHz. After measuring the AC impedance of the solid electrolyte ceramic sheet, the ionic conductivity σ was calculated using the formula σ = L / RS, where L is the thickness of the ceramic sheet, R is the AC impedance of the ceramic sheet, and S is the area per side of the ceramic sheet.

[0091] In addition, XRD and SEM tests were performed on some LLZTO ceramic sheets.

[0092] The XRD test instrument was a Rigaku MiniFlex, Japan, with a scanning angle of 10-60° and a scanning rate of 8° / min.

[0093] The SEM test instrument is a Zeiss Sigma 300, which directly tests the sintered solid electrolyte. The test content includes surface SEM and cross-sectional SEM.

[0094] Example 4 Effect of Lithium Excess Molar Ratio on the Conductivity of LLZTO Solid Electrolyte

[0095] In order to determine the influence of different sintering conditions on the conductivity and other parameters of LLZTO ceramic sheets, the conductivity law of lithium ions in LLZTO ceramic sheets sintered under different lithium excess molar ratios was first investigated.

[0096] LLZTO solid electrolytes were prepared according to the method of Example 2. The excess molar ratios of the newly added lithium source (LiOH·H2O) relative to the molar content of lithium in the LLZTO solid electrolyte blank were 10%, 20%, 30%, and 40%, respectively. Air and argon were used as sintering atmospheres, respectively. After the LLZTO solid electrolyte was prepared, various parameters of the resulting LLZTO ceramic sheet were measured according to the method of Example 3. The results are listed in Table 1.

[0097] Table 1 Parameters and conductivity of LLZTO ceramic sheets prepared under different lithium excess conditions

[0098] In addition, optical images of LLZTO ceramic sheets sintered under the aforementioned different lithium excess conditions are shown in Figure 3. Figure 3(A) shows, from left to right, LLZTO solid electrolytes sintered in argon with lithium excess molar ratios of 10%, 20%, 30%, and 40%. Figure 3(B) shows, from left to right, LLZTO solid electrolytes sintered in air with lithium excess molar ratios of 10%, 20%, 30%, and 40%. As shown in Figure 3, the LLZTO solid electrolytes obtained after sintering all appear to be round and have a regular appearance. The LLZTO solid electrolyte sintered in argon is white in color and does not change significantly with increasing lithium excess molar ratio. In contrast, the color of the LLZTO solid electrolyte sintered in argon gradually turns yellow and darker with increasing lithium excess molar ratio, which indirectly indicates an increase in impurities in the electrolyte.

[0099] By comparing the characterization data of solid electrolytes sintered in air and argon atmospheres, it can be found that when sintered in an argon atmosphere, the ionic conductivity of the LLZTO ceramic sheet shows a continuous upward trend as the lithium excess ratio increases; the ionic conductivity is highest when the lithium excess ratio is 40%. When sintered in an air atmosphere, even if the lithium excess ratio increases, the ionic conductivity of the LLZTO ceramic sheet remains at a low level and does not increase with the increase in lithium excess ratio. This is because the excess lithium element may produce side reactions with the sintering atmosphere, generating impurities such as LiCO3, LiOH, and Li3N, which are incompatible with the LLZTO crystal phase structure, thereby affecting the structure and ionic conductivity of the solid electrolyte.

[0100] As shown in the XRD pattern of Figure 4, the LLZTO solid electrolyte sintered in argon when the lithium excess ratio is 40% shows the characteristic peaks of the LLZTO crystal phase, indicating that the chemical formula is Li 6.4 La3Zr 1.4 Ta 0.6 O 12 The XRD pattern shows no characteristic peaks other than the LLZTO phase, indicating that there are basically no other impurity phases in the solid electrolyte and that the excess lithium precursor has no effect on the crystal structure of the solid electrolyte.

[0101] As shown in the SEM image in Figure 5, compared to a 10% Li excess, at a 40% Li excess, the synthesized LLZTO has large grains, few grain boundaries, and no pores, indicating that it is essentially free of impurities. However, when the Li excess is low, the loss of lithium source through volatilization leads to insufficient Li in the sintered solid electrolyte, which may affect the crystal structure.

[0102] Further experiments by the inventors have shown that increasing the lithium excess ratio further increases the amount of lithium-containing impurities (e.g., LiOH·H₂O, LiOH, Li₂O, etc.) in the sintered LLZTO ceramic sheet, decreasing ionic conductivity. Therefore, in the present invention, the most preferred lithium excess ratio is 40%.

[0103] Example 5 Effect of sintering atmosphere on the conductivity of LLZTO solid electrolyte

[0104] In order to further determine the effect of different sintering atmospheres on the conductivity and other parameters of LLZTO ceramic sheets, the lithium ion conductivity of LLZTO ceramic sheets sintered in different sintering atmospheres was investigated under the condition of using a 40% lithium excess ratio.

[0105] The LLZTO solid electrolyte was prepared according to the method of Example 2. The excess molar ratio of the newly added lithium source (LiOH·H2O) relative to the molar content of the lithium element in the LLZTO solid electrolyte blank was 40%. The sintering atmospheres used were air, argon, nitrogen, oxygen, a mixture of argon (95 volume %) and hydrogen (5 volume %), and a mixture of argon (95 volume %) and oxygen (5 volume %). After the LLZTO solid electrolyte was prepared, the various parameters of the obtained LLZTO ceramic sheet were measured according to the method of Example 3. The results are listed in Table 2.

[0106] Table 2 Parameters and electrical conductivity of LLZTO ceramic sheets prepared under different sintering atmospheres

[0107] In addition, optical images of LLZTO ceramic sheets sintered in the above different atmospheres are shown in Figure 1. The sintering atmospheres are: (A) air; (B) nitrogen; (C) oxygen; (D) argon + hydrogen; (E) argon + oxygen; (F) argon. As can be seen from Figure 1, the LLZTO solid electrolytes obtained after sintering are all discs with regular appearance and shape, but there are certain differences in color. The LLZTO solid electrolyte sintered in argon (F) is white; the LLZTO solid electrolyte sintered in argon and hydrogen (D) is basically white; the LLZTO solid electrolyte sintered in other atmospheres is darker and yellower, which can indirectly indicate the increase of impurities in the electrolyte.

[0108] FIG2 shows exemplary impedance test graphs of LLZTO ceramic sheets sintered in air and argon. The circular marks on the left are LLZTO ceramic sheets sintered in air; the triangle marks on the right are LLZTO ceramic sheets sintered in argon.

[0109] Characterization data for solid electrolytes sintered in the above different atmospheres show that the ionic conductivity of LLZTO ceramic sheets is significantly better when sintered in an inert gas atmosphere such as argon than when sintered in air. In addition, the ionic conductivity of LLZTO ceramic sheets sintered in pure nitrogen and pure oxygen atmospheres is similar to that of sintered in air. This is because a larger amount of Li3N impurities may be generated in a nitrogen atmosphere, while a large amount of impurities such as LiCO3 and LiOH will be generated in an oxygen atmosphere, affecting the density and ionic conductivity of LLZTO. When argon is mixed with a reducing gas such as hydrogen, the ionic conductivity of the resulting solid electrolyte is similar to that of a pure argon atmosphere. When argon is mixed with an oxidizing gas (oxygen), even if the oxygen content is only 5%, the ionic conductivity of the resulting solid electrolyte is significantly reduced. This indicates that argon and a trace amount of reducing atmosphere promote the formation of LLZTO crystals and can inhibit the formation of impurities such as LiCO3 and LiOH on the LLZTO surface. However, an oxidizing atmosphere (oxygen) promotes the formation of impurities and has no inhibitory effect.

[0110] In summary, the present invention can prepare a tantalum-doped garnet-type solid electrolyte with high ionic conductivity by sintering in a protective atmosphere containing an inert gas and regulating the size of the excess molar ratio of lithium during the sintering process, which is beneficial to the commercial application of lithium metal solid-state batteries.

[0111] The above examples are used to describe exemplary embodiments of the present invention, but the present invention is not limited thereto. It should be understood by those skilled in the art that the above examples are for illustrative purposes only, and the specific embodiments and examples of the present invention should not be construed as limiting the scope of the present invention. The embodiments can be changed and modified within the scope of the present invention, and such changes and modifications should fall within the scope of protection of the present invention.

Claims

1. A method for preparing a solid electrolyte, It is characterized in that The preparation method comprises the following steps: (1) adding an excess amount of lithium precursor to a pre-prepared solid electrolyte blank, mixing with a solvent, performing ball milling, and then drying; (2) forming the mixture obtained in step (1); (3) sintering the molded body obtained in step (2) in a sintering atmosphere containing an inert gas to obtain a solid electrolyte, wherein The solid electrolyte is a tantalum-doped garnet-type solid electrolyte.

2. The preparation method according to claim 1, It is characterized in that The chemical formula of the tantalum-doped garnet-type solid electrolyte is Li 7-x L a3 Zr 2-x T ax O 12 , wherein 0<x≤1, preferably 0.5≤x≤0.

6.

3. The preparation method according to claim 1 or 2, It is characterized in that The sintering atmosphere in step (3) contains 50 volume % or more of inert gas, preferably 80 volume % or more, and more preferably 95 volume % or more.

4. The preparation method according to any one of claims 1 to 3, It is characterized in that The inert gas comprises argon, and preferably the inert gas consists of argon.

5. The preparation method according to any one of claims 1 to 4, It is characterized in that The sintering atmosphere in step (3) further comprises one or more gases selected from hydrogen, nitrogen, oxygen and carbon dioxide.

6. The preparation method according to any one of claims 1 to 5, It is characterized in that In the step (1), the excess ratio of the molar number of the lithium element precursor added is greater than or equal to 30%, preferably greater than or equal to 40%, relative to the molar content of the lithium element in the solid electrolyte.

7. The preparation method according to any one of claims 1 to 6, It is characterized in that In the step (2), the molding is performed by cold pressing at a pressure of 15 to 20 MPa.

8. The preparation method according to any one of claims 1 to 7, It is characterized in that In the step (3), the sintering temperature is 1000-1500°C, preferably 1050-1200°C, and more preferably 1100-1150°C.

9. The preparation method according to any one of claims 1 to 8, It is characterized in that The method for preparing the solid electrolyte blank in step (1) comprises the following steps: (a1) according to the chemical composition of the solid electrolyte, the precursors of each element and optionally the precursor of an excess lithium element are mixed with a solvent, followed by ball milling, and then drying; (a2) pre-calcining the mixture obtained in step (a1) to obtain the solid electrolyte blank.

10. The preparation method according to claim 9, It is characterized in that The precursors of the elements include LiOH·H 2 O.La 2 O 3 、ZrO 2 and Ta 2 O 5 .

11. The preparation method according to claim 9 or 10, It is characterized in that The excess ratio of the lithium precursor in step (a1) is 1% to 20% in mole, preferably 5% to 15%.

12. The preparation method according to any one of claims 9 to 11, It is characterized in that The pre-firing temperature in the step (a2) is lower than the sintering temperature in the step (3), and the pre-firing temperature is preferably 700 to 1000°C, more preferably 800 to 900°C.

13. A solid electrolyte, It is characterized in that The solid electrolyte is prepared by the preparation method described in any one of claims 1 to 12; Preferably, the ionic conductivity of the solid electrolyte is greater than or equal to 0.8 mS / cm, preferably greater than or equal to 1.0 mS / cm.

14. Use of the solid electrolyte according to claim 13 in a solid-state lithium metal battery.

15. A solid-state lithium metal battery, It is characterized in that The solid-state lithium metal battery comprises the solid-state electrolyte of claim 13.

Images