LLZO solid electrolyte and preparation method thereof

Abstract

The invention relates to the technical field of solid electrolytes, and discloses an LLZO solid electrolyte and a preparation method thereof. A lithium iron phosphate positive electrode material and an LLZO pre-pressed sheet are placed in the same container at an interval, and are sintered to obtain an LLZO ceramic sheet. According to the invention, the waste lithium iron phosphate positive electrode material is used as mother powder, and Li2O gas is generated at high temperature to make up the lithium volatilization problem during LLZO sintering. By recycling and reusing the lithium element, effective utilization of resources can be realized.

Description

Technical Field

[0001] The present invention relates to the technical field of solid electrolytes, and in particular to an LLZO solid electrolyte and a preparation method thereof. Background Art

[0002] All-Solid-State Lithium Batteries (ASSLBs) have become one of the promising options for lithium-ion batteries (LIBs) due to their advantages such as safety and high energy density. Among the many types of solid electrolytes, Li7La3Zr2O 12 LLZO (Low Lithium Zinc Oxide) has attracted considerable attention due to its high ionic conductivity, wide electrochemical stability window, high shear modulus, and chemical stability to lithium metal. However, during the sintering process, the problem of "lithium volatilization" (such as Li2O) becomes serious, leading to a series of problems such as insufficient bonding between LLZO solid electrolyte grains, low relative density, and insufficient ionic conductivity. Therefore, Li2O atmosphere compensation is one of the keys to producing high-performance LLZO. The current lithium-supplementing material is a powder obtained by calcining LLZO once and cannot be recycled. Due to its high cost, it is not conducive to large-scale production.

[0003] In the existing technology, La2Zr2O7 (LZO) is added as a sintering aid during the ceramic sintering process. The Li2O generated by decomposition during the sintering process can compensate for the lithium loss in situ, and sintering without buried powder can be achieved. The problems with this method are as follows: First, although LZO as a sintering aid can reduce the sintering temperature and improve the conductivity, when the amount of LZO added is too high, not all LZO can react with Li2O to generate LLZO with lithium ion conductivity. Excess LZO may remain in the sintered body, and these unreacted LZO will hinder the Li + Second, the introduction of LZO as a sintering aid increases material cost. Third, the amount of LZO added and the sintering conditions must be precisely controlled to ensure optimal sintering and conductivity, which increases process complexity. Summary of the Invention

[0004] Technical problems solved by the present invention: Used to solve the problem of LLZO lithium volatilization during the preparation of ceramic sheets.

[0005] The technical solution adopted in the present invention is: In response to the above technical problems, the present invention aims to provide an LLZO solid electrolyte and a preparation method thereof. The specific contents are as follows: The lithium iron phosphate positive electrode material and the LLZO pre-pressed sheet are placed in the same container with a spacing between them, and sintered to obtain the LLZO ceramic sheet.

[0006] According to some preferred embodiments, the lithium iron phosphate positive electrode material is placed below the LLZO pre-pressed sheet.

[0007] According to some preferred embodiments, the insulation may be performed using heat-resistant materials, such as magnesium oxide supports.

[0008] According to some preferred embodiments, the lithium iron phosphate positive electrode material is currently produced or retired lithium iron phosphate.

[0009] According to some preferred embodiments, sintering: heating the temperature to 1200-1400° C. at a heating rate of 3-8° C. / min and maintaining the temperature for 3-6 h.

[0010] According to some preferred embodiments, the LLZO pre-compressed tablets are prepared by weighing LiOH·H2O (99.99%, Aladdin), La2O3 (99.99%, Aladdin), Ta2O5 (99.99%, Aladdin), and ZrO2 (99.99%, Aladdin) in a stoichiometric ratio, with a 5-10 wt% excess of Li. The mixture is then ball-milled (solid content: 20%) using isopropyl alcohol as a dispersant. The powder is then placed in a milling jar containing zirconia balls (ball-to-material ratio: 10:1) at a speed of 300 rpm for 10 hours. The resulting slurry is dried in a forced-air oven, sieved, and placed in an Al2O3 crucible. The temperature is then increased to 900°C at a rate of 2-5°C / min and maintained at this temperature for 6 hours. After calcination, the powder was collected and placed in a ball mill at 300 rpm for 8 hours. Isopropyl alcohol was added as a dispersant and ball milled a second time to further reduce the powder particle size and increase its activity. The slurry was dried at 60°C and sieved. An appropriate amount of powder was weighed and pressed at 200-300 MPa for 3 minutes to obtain LLZO pre-compressed tablets.

[0011] The technical mechanism and beneficial effects adopted by the present invention are: This invention uses discarded lithium iron phosphate cathode material as a master powder, generating Li2O gas at high temperatures to compensate for lithium volatilization during the LLZO sintering process. By recovering and reusing lithium, this allows for efficient resource utilization. Test results show that using lithium iron phosphate cathode material instead of LLZO powder from a single calcination as the master powder yields ceramic sheets with superior density. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 These are the XRD patterns of the LLZO ceramic sheets prepared in Example 1 and Comparative Examples 1-7. DETAILED DESCRIPTION

[0013] To make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are described clearly and completely below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer are used. Where the manufacturer of the reagents or instruments is not specified, all are conventional products that can be purchased commercially.

[0014] Example 1: LiOH·H₂O (99.99%, Aladdin), La₂O₃ (99.99%, Aladdin), Ta₂O₅ (99.99%, Aladdin), and ZrO₂ (99.99%, Aladdin) were weighed in a stoichiometric ratio, with a 10 wt% excess of Li. The mixture was ball-milled (solid content: 20%) using isopropyl alcohol as a dispersant. The powder was placed in a milling jar filled with zirconia balls (ball-to-material ratio: 10:1) at a speed of 300 rpm for 10 h. The resulting slurry was dried in a forced-air oven, sieved, and placed in an Al₂O₃ crucible. The temperature was then increased to 900°C at a rate of 3°C / min and held for 6 h. After calcination, the powder was collected and subjected to a secondary ball milling process. The powder was placed in a ball mill at 300 rpm for 8 hours. Isopropyl alcohol was added as a dispersant and the secondary milling process further reduced the powder particle size and increased its activity. The slurry was dried at 60°C and sieved. An appropriate amount of powder was weighed and pressed at 300 MPa for 3 minutes to obtain compacted ceramic discs for later use.

[0015] The discarded lithium iron phosphate cathode material was pretreated by leaching it with 0.3 mol / L sulfuric acid and hydrogen peroxide (the volume ratio of the two was 1:1) at 60 °C for 2 h, removing the filter residue, and drying the remaining solution to obtain the pretreated lithium iron phosphate cathode material.

[0016] The pretreated lithium iron phosphate cathode material was used as a master powder. 4 g of discarded lithium iron phosphate (corresponding to 0.45 g of LLZO electrolyte sheet, also 4 g of the original LLZO master powder) was placed approximately 5 cm below the ceramic sheet. A magnesium oxide support was used to separate the ceramic sheet and the master powder. The temperature was then increased to 1250°C at a rate of 5°C / min and sintered for 5 hours to obtain a densely sintered ceramic sheet.

[0017] Comparative Example 1 The difference between this comparative example and Example 1 is that the mother powder (lithium iron phosphate positive electrode material) is not placed under the ceramic sheet (LLZO), but the ceramic sheet is directly sintered.

[0018] Comparative Example 2 The difference between this comparative example and Example 1 is that the mother powder is a powder obtained by calcining LLZO once.

[0019] Comparative Example 3 The difference between this comparative example and Example 1 is that the discarded lithium iron phosphate positive electrode material is directly used as the mother powder.

[0020] Comparative Example 4 The difference between this comparative example and Example 1 is that the mother powder is not placed under the ceramic sheet. Instead, 2 wt% LZO is added as a sintering aid during the LLZO powder process, and then the ceramic sheet is prepared through ball milling, drying, tableting, and sintering.

[0021] Comparative Example 5 The difference between this comparative example and comparative example 4 is that 4 wt% LZO is added as a sintering aid during the LLZO powder process.

[0022] Comparative Example 6 The difference between this comparative example and comparative example 4 is that 6 wt% LZO is added as a sintering aid during the LLZO powder process.

[0023] Comparative Example 7 The difference between this comparative example and comparative example 4 is that 8 wt% LZO is added as a sintering aid during the LLZO powder process.

[0024] Test example The LLZO ceramic sheets prepared in Example 1 and Comparative Examples 1-7 were subjected to XRD tests. The results are as follows: Figure 1 shown.

[0025] Depend on Figure 1 The results show that the LLZO ceramic pieces obtained by sintering completely overlap with the PDF cards, which proves that the prepared ceramic pieces are all LLZO pure phase.

[0026] The LLZO ceramic sheets prepared in Example 1 and Comparative Examples 1-7 were subjected to density tests. The density test was performed by weighing the sheets with a solid density meter, and then calculating the actual density of the electrolyte sheet according to the Archimedean principle. The actual density was divided by the theoretical density of the ceramic sheet, 5.41 g cm -3 , the density can be obtained. The results are shown in Table 1.

[0027] The LLZO ceramic sheets prepared in Example 1 and Comparative Examples 1-7 were tested for ionic conductivity, with the results shown in Table 1. To test ionic conductivity, the sintered ceramic sheets were surface-polished. Conductive silver paste was applied to both sides of the polished sheets and dried. After drying, the surrounding silver paste was removed by grinding. Testing was performed using a Donghua electrochemical workstation. Ionic conductivity was calculated using the formula σ = L / S × R.

[0028] Table 1

[0029] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A method for preparing an LLZO solid electrolyte, characterized in that: The steps include: The lithium iron phosphate positive electrode material and the LLZO pre-pressed sheet are placed in the same container with a spacing between them, and sintered to obtain the LLZO ceramic sheet.

2. The method for preparing the LLZO solid electrolyte according to claim 1, wherein The lithium iron phosphate positive electrode material is placed under the LLZO pre-pressed sheet.

3. The method for preparing the LLZO solid electrolyte according to claim 1, wherein Insulate with heat-resistant material.

4. The method for preparing the LLZO solid electrolyte according to claim 3, wherein: The heat-resistant material is magnesium oxide bracket.

5. The method for preparing the LLZO solid electrolyte according to claim 1, wherein Sintering: Heat up to 1200-1400°C at a heating rate of 3-8°C / min and maintain for 3-6 hours.

6. The method for preparing the LLZO solid electrolyte according to claim 1, wherein: The lithium iron phosphate positive electrode material is current or retired lithium iron phosphate.

7. The method for preparing the LLZO solid electrolyte according to any one of claims 1 to 6, characterized in that: The preparation method of LLZO pre-compressed tablets is as follows: LiOH·H2O, La2O3, Ta2O5 and ZrO2 are weighed according to the stoichiometric ratio and dispersed by ball milling once to obtain a slurry; after the slurry is dried, heat treatment, secondary ball milling, drying and pressing are performed to obtain LLZO pre-compressed tablets.

8. The method for preparing the LLZO solid electrolyte according to claim 7, wherein: Heat treatment: Heat to 900℃ at a heating rate of 2-5℃ / min and keep warm for 6h.

9. The method for preparing the LLZO solid electrolyte according to claim 7, wherein: Pressing: Maintain pressure at 200-300 MPa for 3 minutes.

10. An LLZO solid electrolyte obtained by the preparation method according to any one of claims 1 to 9.

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