The effect of aliovalent dopants on the structural and transport properties of Li6La2BaTa2O12 garnet Li-ion solid electrolytes

Li-rich garnet solid electrolytes are promising candidates for all-solid-state batteries, allowing for increased energy densities, compatibility with Li-metal anodes and improved safety by replacing flammable organic-based liquid electrolytes. Li-stuffed garnets typically require aliovalent doping to stabilise the highly ionic conductive Ia (3) over bard cubic phase. The role of dopants and their location within the garnet framework can greatly affect the conduction properties of these garnets, yet their impact on the structure and resulting ion transport is not fully understood. Here, we evaluate the effect of aliovalent doping with Al3+, Ga3+ and Zn2+ in the Li6BaLa2Ta2O12 (LBLTO) garnet material. A combination of PXRD and XAS reveals a linear cell parameter contraction with an increase in doping and the preference of the 24d Li+ sites for Al3+ and Zn2+ dopants, with Ga3+ occupying both the 24d and 48g Li+ sites. Macroscopic ionic conductivity analyses by EIS demonstrate an enhancement of the transport properties where addition of small amounts of Al3+ decreases the activation energy to Li+ diffusion to 0.35(4) eV. A detrimental effect on ionic conductivities is observed when dopants were introduced in Li+ pathways and upon decreasing the Li+ concentration. Insights into this behaviour are gleaned from microscopic diffusion studies by muon spin relaxation (mu+SR) spectroscopy, which reveals a low activation energy barrier for Li+ diffusion of 0.16(1) eV and a diffusion coefficient comparable to those of Li7La3Zr2O12 (LLZO) benchmark garnet materials.