Reduction in Formation Temperature of Ta-Doped Lithium Lanthanum Zirconate by Application of Lux–Flood Basic Molten Salt Synthesis

Abstract

Garnets such as Li7La3Zr2O12 (LLZO) are important Li+ conducting ceramics for potential use as solid electrolytes in solid-state batteries. However, LLZO is predominately prepared by using solid-state reaction methods, despite the high energy cost, multiple steps involved, and large particle sizes of the resultant material. Herein, molten salt synthesis (MSS) is applied to prepare Ta-doped LLZO (Li6.4La3Zr1.4Ta0.6O12, LLZTO), demonstrating that control over the Lux–Flood basicity of the molten salt medium enables drastic reduction in the formation temperature relative to other synthetic methods. Each of the reaction media investigated, including eutectic LiCl–KCl, a mixture of LiCl–LiOH, and highly basic ternary mixtures of LiNO3–LiOH–Li2O2, can be used to synthesize LLZTO under the appropriate experimental conditions. In the last case, garnet powders with predominately submicrometer particle sizes are obtained at temperatures as low as 550 °C. Sintered LLZTO pellets with high room temperature ionic conductivity can be obtained by using powders from each MSS method. LLZTO powders synthesized from the highly basic melts show good densification due to the small particle sizes (0.2–1 μm) and exhibit total ionic conductivity as high as 0.61 mS cm–1. The results show that molten salt synthesis in media with high Lux–Flood basicity is an attractive low-temperature synthetic approach to achieving highly conducting garnet electrolytes.