Properties of Li7La3Zr2O12 Solid Electrolytes Synthesized from Highly Basic Molten Salts Fluxes

Abstract

Molten salt synthesis (MSS) is an alternative synthetic route to solid state reaction to obtain dopant stabilized, garnet-type Li7La3Zr2O12 (LLZO) with both fine particle size and high ionic conductivity. Molten salt fluxes are high temperature solvents with properties that can be tuned based on the salt composition, use of additives, and Lux-Flood basicity of the media. Compared to solid state reaction, molten salt synthesis can produce highly crystalline powders with smaller particle sizes at lower temperature and with shorter reaction times. Herein, our work investigating the effect of basicity on the reaction kinetics, formation temperature, phase purity, dopant distribution, and particle size distribution for the formation of doped LLZO garnets will be presented. We find that a variety of halide, hydroxide, and nitrate salt fluxes are effective for preparation of LLZO garnets with a range of particle sizes and crystal morphologies. Furthermore, highly basic salt fluxes can enable Ta-doped LLZO with cubic crystal structure to be obtained at 400 oC, an unprecedented low temperature synthesis for LLZO. The distribution of Ta5+ dopant in individual LLZO particles is assessed using high resolution transmission/scanning electron microscopy and reveals that the composition is inhomogeneous. The origin of this inhomogeneity is attributed to differences in solubility of the Ta and Zr oxides in the molten salt melt, but is mitigated by using a novel quasi-single source precursor that enables better delivery of the reagents in the salt flux. Finally, the Li ionic conductivity of LLZO pellets prepared using molten salt synthesis using various sintering conditions is compared to LLZO made by conventional solid state reaction.