METHOD FOR PRODUCING A CUBIC PHASE LLZ

Abstract

A process for producing a cubic phase Li7La3Zr2O12(LLZ), comprising: dry mixing of Li2CO3, La2O3, ZrO2 and Al2O3 to form a mixture; firing the mixture for 5 to 7 hours at 800°C to 1000°C; natural cooling of the mixture and subsequent dry mixing of the mixture; producing a pellet comprising the mixture, with a size of 8 mm to 12 mm at 120 MPa to 150 MPa; and burning of the pellet for 20 to 36 hours at 1000°C to 1250°C, wherein Li in the cubic phase LLZ is substituted by Al, and wherein the substituting Al is present in an amount of 0.52 mol to 0.8 mol and the LLZ is doped with Al2O3 in an amount of 2.5 wt% to 3.76 wt%.

Description

TECHNICAL AREA

[0001] The present disclosure relates to a method for producing a cubic structure wherein a lithium position is substituted by aluminum (Al) when Al is converted to Li 7 La 3 Zr 2 O 12 (hereinafter referred to as LLZ), which has excellent ionic conductivity among garnet-based materials.

[0002] In particular, the present disclosure relates to a method for improving the physical properties of Li 7 La 3 Zr 2 O 12 (LLZ) by adding aluminum (Al) to LLZ, which exists in a cubic phase at normal temperature, to stabilize the cubic structure while substituting a lithium position with Al and achieving a liquid sintering effect, thereby increasing the density. BACKGROUND

[0003] Inorganic-based solid electrolytes are chemically divided into oxides and sulfides, and examples of suitable oxide-based solid electrolytes with excellent conductivity include perovskite and garnet. The present disclosure is limited to LLZ among the garnet-based materials.

[0004] Materials studies can be broadly divided into three steps: synthesis, analysis, and evaluation. Of these, the synthesis step represents a crucial part, determining the physical properties of a material and having a major impact on the future independent development of the material. Fig. 1 represents the synthesis process of LLZ.

[0005] European Patent Application Publication No. EP 2159867 A1 discloses a method for analyzing a relationship between Li conductivity and Al content in Al 2 O 3 , which is in Li 7 La 3 Zr 2 O12 is included among garnet-based materials.

[0006] The publication ”Synthesis of Garnet Structured Li 7 + × La 3Y × Zr 2-x O 12 (x = 0–0.4) by Modified Sol-Gel Method” discloses a method for synthesizing an electrolyte according to the temperature and the amount of oxygen when a cubic phase of Li 7 La 3 Zr 2 O 12 made from garnet-based materials.

[0007] The publication ”Synthesis of Cubic Li 7 La 3 Zr 2 O 12 by Modified Sol-gel Process” reveals the analysis of the relationship between Li conductivity and Al content in Al 2 O 3 , which is in Li 7 La 3 Zr 2 O 12 is included among garnet-based materials.

[0008] Korean Patent Application Publication No. KR-2010-0053543 A discloses the use of a solid ion conductor having a garnet-like structure and being chemically stable in batteries, storage batteries, electrochromic devices, and other electrochemical batteries, and a novel compound suitable for use therein. SUMMARY

[0009] The present disclosure provides a method for adding aluminum (Al) having a cubic structure of Li 7 La 3 Zr 2 O 12 stabilized while being substituted by lithium, and further provides an analysis result of changes in the density and sintering of the cubic structure that occur according to the amount of Al.

[0010] According to an exemplary embodiment of the present disclosure, a method for producing a cubic phase Li 7 La 3Zr 2 O 12 (LLZ) the dry mixing of Li 2 CO 3 , La 2 O 3 , ZrO 2 and Al 2 O 3 The mixture is fired for 5 to 7 hours at 800°C to 1,000°C, cooled naturally, and dry-mixed.

[0011] A pellet measuring 8 mm to 12 mm is produced using the mixture at 120 MPa to 150 MPa. The pellet is then fired at 1,000°C to 1,250°C for 20 to 36 hours.

[0012] According to the present disclosure, Li is substituted by Al in the cubic phase LLZ.

[0013] The substituted Al can be present in an amount of 0.52 mol to 0.80 mol, and the LLZ is reacted with Al 2 O 3 doped in an amount of 2.5 wt% to 3.76 wt%.

[0014] The present disclosure implements a method of adding Al, which forms a cubic structure of Li 7 La3 Zr 2 O 12 stabilized while substituting lithium, and an analysis of the density changes and sintering of the cubic structure that occur according to the amount of Al. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Fig. 1 shows a synthesis process of Li 7 La 3 Zr 2 O 12 (LLZ).

[0016] Fig. Figure 2 is a graphical representation of the XRD phase change according to a synthesis method of LLZ.

[0017] Fig. 3 shows a final firing process and a photo example after firing.

[0018] Fig. Figure 4 is an XRD plot of an LLZ according to one phase.

[0019] Fig. Figure 5 shows an ICP-MS analysis method for analyzing the LLZ composition.

[0020] Fig.Figure 6 illustrates measuring conductivity by forming an electrode on the LLZ using Au sputtering and then inserting the LLZ into a jig to measure impedance.

[0021] Fig. Figure 7 is a graph showing the results of the impedance measurement of the LLZ.

[0022] Fig. 8 presents the XRD measurement results of Al-doped LLZ (amount of added Al 2 O 3 5 wt% to 20 wt%).

[0023] Fig. Figure 9 shows the result of a Raman measurement of Al-doped LLZ.

[0024] Fig. Figure 10 shows the result of an ICP-MS measurement of Al-doped LLZ.

[0025] Fig. Figure 11 shows the result of an XRD measurement of the addition of 0 wt% to 4 wt% of Al 2 O 3 represents.

[0026] Fig.Figure 12 illustrates the use of a BN plate and the use of a MgO crucible during the firing of LLZ.

[0027] Fig. 13 shows the result of an LLZ impedance evaluation of up to 4 wt% of Al 2 O 3 represents. DETAILED DESCRIPTION

[0028] The present disclosure provides a method for producing a cubic phase Li 7 La 3 Zr 2 O 12 (LLZ). The process involves the dry mixing of Li 2 CO 3 , La 2 O 3 , ZrO 2 and Al 2 O 3 .

[0029] The mixture is fired at 800°C to 1,000°C for 5 to 7 hours, naturally cooled, and then dry-blended. A pellet with a size of 8 mm to 12 mm is produced at 120 MPa to 150 MPa using the mixture, and then the pellet is fired at 1,000°C to 1,250°C for 20 to 36 hours. In the present disclosure, Li is substituted by Al in a cubic phase of LLZ. The substituted Al is present in an amount of 0.52 mol to 0.80 mol, and the LLZ is mixed with Al. 2 O 3 in an amount of 2.5 wt% to 3.76 wt%. The dry mix ratio of Li 2 CO 3 :La 2 O 3 :ZrO 2 :Al 2 O 3 can be 7 mol:3 mol:4 mol:0.813 mol.

[0030] The method for producing a cubic phase LLZ may further comprise a method of preparing a pellet using 10% to 80% of the dry mix before the pellet firing step and coating the pellet with the powder of the remaining dry mix.

[0031] The method for producing a cubic phase LLZ according to the present disclosure further comprises analyzing the produced LLZ by X-ray diffraction (XRD), Raman spectroscopy, or inductively coupled plasma mass spectrometry (ICP-MS). The method further comprises determining the phase of the LLZ and impurities by XRD.

[0032] The method of the present disclosure further includes determining the phase and impurities of a region several hundred microns in size or less, which cannot be determined by XRD or Raman. The composition ratio of each element in the LLZ is compared to a target composition ratio using ICP-MS.

[0033] The LLZ has cubic and tetragonal phases. The cubic phase has a conductivity of 10 –4 / Ωcm, and the tetragonal phase has a conductivity of 10 –6 / Ωcm. The cubic phase is reported to be 100 times or more superior to the tetragonal phase in terms of conductivity. Accordingly, it is advantageous to synthesize only the cubic phase in order to improve physical properties, so that impurities and the secondary phase, or tetragonal phase, are not generated. Among the raw materials for the LLZ, La 2 O3 Hygroscopicity and was therefore used after a drying process of 24 hours at 900°C. Furthermore, to improve the physical properties, a small amount of Al 2 O 3used. Examples of the mixing process include a dry process and a wet process. Here, dry mixing was performed using a planetary mill (hereinafter referred to as PM) because of the risk of an increase in process time (an increase of one day or more until drying) and a side reaction with a solvent with the wet mixture. As a dry mixing condition, a condition under which an optimal powder size (a size of several microns) could be ensured in the shortest possible time was selected by analyzing, via SEM imaging, a powder for each step and a sample for each PM time point. During the synthesis of the LLZ, the LLZ is generally subjected to a firing process twice. With reference to Fig. 2, the LLZ is formed by primary burning, and part of the unstable phase (La 2 Zr 2 O 7, pyrochlore) and part of the raw materials exist together, and through secondary firing, all impurities participate in the reaction or disappear, and as a result, only an LLZ with a desired cubic structure exists. Especially during secondary firing, a change in firing temperature and time seriously affects the phase determination. Since a temperature of 1,250°C or more promotes the generation of an unstable phase, and a temperature below 1,150°C promotes the formation of a tetragonal phase, the temperature and time of the present synthesis method are determined accordingly.

[0034] The lithium composition, which affects conductivity, can also vary depending on the firing process. In particular, in the secondary firing process, the LLZ is exposed to a high temperature (1200°C) for a long time (20 hours), and the lithium in the LLZ evaporates. Fig. 3, a method for producing a desired lithium composition while preventing evaporation additionally comprises a method for producing a pellet using 10% to 80% of the dry mixture before Li 2 CO 3 in excess (10% excess) and a final firing (20 hours at approximately 1200°C) is carried out to allow for evaporation in the initial state. The pellet is coated with the remaining powder of the dry mix.

[0035] Analysis is used to determine whether LLZ is synthesized with a desired hexahedral (cubic) phase. Three analytical methods, such as XRD, Raman, and ICP-MS, can be performed. The LLZ phase and impurities can be confirmed by XRD, and Raman spectroscopy confirms the phase and impurities in a region several hundred microns or smaller, which cannot be determined by XRD. Furthermore, a difference between a target composition and a synthesis composition is compared by confirming the composition ratio of each element of the LLZ using ICP-MS.

[0036] Due to the lack of XRD data of the LLZ during the initial synthesis, the comparison and determination were performed by collecting the XRD data of the LLZ, which are presented in the documents.

[0037] A sintered pellet is ground to a powder using a mortar and pestle, and measurements are taken. The measurements can be performed using a Bruker D8 ADVANCE as the measuring instrument at a measuring rate of 3 degrees / minute in a range of 10 degrees (°) to 60 degrees (°). With reference to Fig. 4, the peak of the tetragonal LLZ (hereinafter referred to as T-LLZ) is broadly distributed compared to the peak of the cubic LLZ (hereinafter referred to as C-LLZ), and a splitting is observed. This phenomenon is observed due to the low crystallinity of the T-LLZ. Furthermore, when a small amount of Al is added, even if the phase is a cubic phase, a stronger peak is observed. Generally, when the cubic crystallinity in the LLZ is improved, the transfer of lithium is facilitated, and high ionic conductivity is measured.

[0038] During synthesis, it is difficult to synthesize a desired composition due to weighing errors in the raw materials, lithium evaporation caused by high-temperature sintering, Al doping in the pellet caused by an aluminum crucible, and the like. For accurate analysis of the composition of the synthesized LLZ, an ICP-MS evaluation method can be used. Unlike other materials, LLZ is a ceramic material, and it is difficult to completely dissolve the powder using a conventional pretreatment process for ICP analysis.

[0039] Fig.Figure 5 illustrates a method for subjecting the LLZ composition to ICP analysis. For complete dissolution, aqua regia (hydrochloric acid: nitric acid = 3:1 vol%) is prepared and boiled at 170°C to completely dissolve the powder, and then diluted to determine the composition. As a result of the reproducibility evaluation with the same sample, La, Zr, and Al ensured reproducibility with an error within 3%, while Li had an error rate of 12%.

[0040] For the development of a solid electrolyte, it is necessary to evaluate the physical properties of a solid phase as distinct from a liquid phase. The design of an apparatus, the establishment of an evaluation method, and the interpretation of evaluation results are important prerequisites for the development of a solid electrolyte. Optimization of the evaluation conditions was carried out based on experimental results according to the range of the LLZ, the material for forming an electrode, the thickness and area, the electrode pairing, the design of the measuring jig, and the conditions of an impedance analyzer. In the process, an in-depth study was conducted to overcome problems mainly arising in the synthesis of the material itself, which is different from commercially available materials.The LLZ is manufactured in the form of a pellet, and the result of an impedance evaluation is in the range of a thickness of 1 mm to 2 mm, a Au sputtering of 100 nm and an electrode area of ​​63 mm. 2 reliable.

[0041] As in Fig. As shown in Figure 6, the conductivity was measured by forming an electrode on the LLZ using Au sputtering and then inserting the LLZ into a jig for impedance measurement. When measuring conductivity, the frequency range and voltage intensity measured vary depending on the material. The LLZ was measured under conditions of a frequency range of 20 MHz to 1 Hz and a voltage of 30 mV using a Solartron 1260 device.

[0042] With reference to Fig.7, the resistance value was determined by inputting the impedance result into an equivalent circuit (single-RC circuit, using Z-VIEW software), and then a conductivity value was derived from it. Additionally, it is also possible to evaluate an asymmetric (Au / LLZ / Li) DC cell separately to measure ionic conductivity and electronic conductivity, or a symmetric (Li / LLZ / Li) DC cell to confirm the compatibility of lithium with the LLZ.

[0043] To improve the physical properties of LLZ, it is advantageous to increase the sintered density and allow the LLZ to exist as a cubic phase at normal temperature. As a method to simultaneously satisfy both conditions, Al is added to the LLZ. The addition of Al can stabilize the cubic structure while substituting the lithium position with Al and exhibit a liquid phase sintering effect, thereby expected to increase the density. In this case, 10% of the excess Li 2 CO 3 considering the evaporation of lithium. The results of the examples using an aluminum crucible and the ratio of Al 2 O 3, to which 0, 0.5, 1, 2, 3, 4, 5, 10, 15 and 20 wt% were added, are listed as follows in Table 1. The synthesis procedure was carried out as described, the analysis was carried out by XRD, Raman and ICP and an impedance analysis was performed. Nr. synthesis analysis Evaluation Amount of Al 2 O 3 (Wt.%) Doping Sintered density (%) XRD Raman ICP-MS (amount of Al 2 O 3 (wt%) Conductivity ( / Ωcm) 9-1 0 83 Cubic phase Cubic phase 2,50 8,48·10 –5 9-2 0.5 80 Cubic phase Cubic phase 2,96 1,49·10 –4 9-3 1 84 Cubic phase Cubic phase 3,14 8,92·10 –5 9-4 2 83 Cubic phase Cubic phase 3,63 1,30·10 –4 9-5 3 79 Cubic phase Cubic phase 3,76 1,60·10 –4 9-6 4 78 Cubic phase Cubic phase 3,68 2,35·10 –4 9-7 5 77 Cubic phase Al 3 Zr Impurity peak 4,58 5,12·10 –5 9-8 10 73 LaAlO 3 Lee 2 ZrO 3 Impurity peak 10,00 5,00·10 –7 9-9 15 72 LaAlO 3 Lee 2 ZrO 3 Impurity peak 16,25 4,63·10 –7 9-10 20 79 LaAlO 3 Lee 2 ZrO 3 Impurity peak 21,63 4,26·10 –7 [Table 1] Evaluation result of Al-doped LLZ

[0044] Referring to Table 1, the relative density tends to decrease as the amount of added Al 2 O 3 was increased during synthesis. In particular, when an amount of 3 wt% or more is added, a density of 80% or less is observed, providing a condition that impairs conductivity.

[0045] As a result of XRD analysis of Fig. 8 the production of impurities begins when Al 2 O 3in an amount of 5 wt% or more, and the LLZ is no longer observed during an addition in an amount of 10 wt% or more. Although research institutes report that Al in the LLZ is substituted by Li or Zr, studies on the amount limit of substitution have not yet been conducted. Based on the result of the present disclosure, it is determined that the substitution at an amount of 4 wt% of Al 2 O 3 can be carried out and it is possible to adjust the amount of added Al 2 O 3 to determine what is generally considered to have the best physical properties.

[0046] The measurement result of Raman spectroscopy in Fig. 9 is also equally observed in the analysis result of the XRD phase. When Al 2 O 3in an amount of 0 to 4 wt%, C-LLZ is observed in all results, but during the addition of 5 wt% or more of Al 2 O 3 different peaks and different intensities are observed.

[0047] As in Fig. 10 by ICP-MS, 2.5 wt% doped Al is observed due to the aluminum crucible, even if Al 2 O 3 is not added. During the addition of a small amount of Al 2 O 3 (0 to 3 wt%), the amount of Al detected due to the crucible is significantly increased, but during the substitution of 4 wt% or more of Al 2 O 3 a content similar to the added amount of Al 2 O 3detected. Thus, it is difficult to control the amount of aluminum added. The present disclosure further provides a method for preventing the addition of Al by preventing direct contact of an aluminum crucible with a sample. In particular, the use of a boron nitride (BN) plate or an MgO crucible during firing prevents the addition of Al.

[0048] Fig.Figure 12 shows an evaluation result of using a BN plate over an aluminum crucible and an evaluation result of using an MgO crucible instead of an aluminum crucible. It is not possible to secure a sample because a phenomenon in which the sample is fused with a binder component due to the elution of the binder component of the BN plate is caused by using the BN plate during firing at 1200°C. Although the final firing is performed when the MgO crucible is used, a pellet cannot be formed, and a sintering phenomenon between powders cannot occur at all.

[0049] Meanwhile, the result of the impedance evaluation is a similar conductivity (a measure of σ = 10 –4 / Ωcm) up to 4 wt.% Al 2 O 3 observed (see Fig.13), but during the addition of 5 wt% or more, the conductivity decreases sharply while impurities are produced (a measure of σ = 10 –7 / Ωcm).

[0050] Therefore, the physical properties can be improved while maintaining the cubic phase of LLZ due to the substitution of Al in the LLZ, but the physical properties may deteriorate due to the production of impurities during the addition of 4.6 wt% or more of Al 2 O 3 worsen. QUOTES CONTAINED IN THE DESCRIPTION

[0051] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0052] EP 2159867 A1

[0005] KR 2010-0053543 A

[0008]

Claims

[1] Method for producing a cubic phase Li 7 La 3 Zr 2 O 12 (LLZ), where the procedure includes: Dry mixing of Li 2 CO 3 , La 2 O 3 , ZrO 2 and Al 2 O 3 to form a mixture; Firing the mixture for 5 to 7 hours at 800°C to 1,000°C; natural cooling of the mixture and subsequent dry mixing of the mixture; Producing a pellet comprising the mixture having a size of 8 mm to 12 mm at 120 MPa to 150 MPa; and Firing the pellets for 20 to 36 hours at 1,000°C to 1,250°C. [2] The process according to claim 1, wherein Li in the cubic phase LLZ is substituted by Al. [3] The process according to claim 2, wherein the substituted Al is present in an amount of 0.52 mol to 0.80 mol and the LLZ is reacted with Al 2 O 3is doped in an amount of 2.5 wt% to 3.76 wt%. [4] The method according to claim 1, wherein a dry mix ratio of Li 2 CO 3 :La 2 O 3 :ZrO 2 :Al 2 O 3 7 mol:3 mol:4 mol:0.7 to 0.9 mol. [5] The method of claim 1, further comprising: Making a pellet using 10% to 80% of the dry mix before firing the pellet, and coating the pellet with a powder of the remaining dry mix. [6] The method of claim 1, further comprising: Analyzing the produced LLZ, where analysis is carried out by X-ray diffraction (XRD), Raman spectroscopy or inductively coupled plasma mass spectrometry (ICP-MS). [7] The method of claim 6, wherein the analyzing determines the phase of the LLZ and the impurities by XRD. [8] The method of claim 6, wherein the analyzing determines the phase and impurities of a region several hundred microns in size or less, which cannot be determined by XRD or Raman. [9] The method of claim 6, wherein analyzing compares a composition ratio of each element in the LLZ with a target composition ratio by ICP-MS.

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