Production of Ta-Doped Li₇La₃Zr₂O₁₂ Solid Electrolyte with High Critical Current Density

Solid-state electrolytes are key materials for developing high-safety solid-state Li-ion batteries. The garnet-type Li7La3Zr2O12 (LLZO) solid electrolyte is one of the most promising solid electrolytes due to its high conductivity and feasible preparation in ambient air. Among several dopants, Ta-doped LLZO (Ta-LLZO) delivers high stability against lithium metal and high conductivity, which attracts lots of researchers. However, production of Ta-LLZO ceramics is less problematic due to the complicated gas–liquid–solid sintering mechanism. This work aims to develop an efficient method to produce Ta-LLZO solid electrolyte ceramic pellets, including providing a stacking sintering method and optimizing the amount of excessive Li source, the wet milling duration, and the sintering temperature. First, a low-cost, efficient “gravel-separator” strategy is provided for multiple sintering of LLZO. Second, the Li6.5La3Zr1.5Ta0.5O12 (Ta5) samples with different excessive Li levels (2, 4, 6, and 8%) are sintered at various sintering conditions of 1240 °C for 60 min, 1280 °C for 20 min, and 1320 °C for 2 min to search for the optimum excessive amount of Li. Third, Ta5 samples with optimized excessive Li (4, 6%) after milling for longer duration are sintered at 1280 °C for 30 min, 1300 °C for 10 min, 1320 °C for 10 min, 1320 °C for 30 min to finally optimize the preparation craft. After optimization, the Ta5 ceramics with excessive 4 and 6% Li sintered at 1320 °C for 10 min deliver homogeneous and dense cross-sectional morphology, high conductivity of over 1 × 10–3 S cm–1 at room temperature, and relative densities of above 96%. Furthermore, symmetric Li cells assembled with the Ta-LLZO solid electrolyte prepared with the optimized method deliver a critical current density of 1.28 mA cm–2 at 30 °C and over 1000 h of stable cycling at 0.2 mA cm–2 (0.2 mA h cm–2) at 30 °C.