Effect of synthesis process on the Li-ion conductivity of LiTa2PO8 solid electrolyte materials for all-solid-state batteries†

Inorganic solid electrolytes are essential for developing safe and non-flammable all-solid-state batteries, with oxide-based ones having attracted attention owing to their excellent chemical stability. Recently, a new solid electrolyte material LiTa₂PO₈ (LTPO) was reported to have a bulk lithium-ion conductivity of 1.6 mS cm⁻¹ at room temperature, which is one of the highest among oxide solid electrolytes. However, oxide solid electrolytes tend to have a high grain boundary resistivity and must be formed into dense sintered pellets. In this study, different dense LTPO materials were synthesised by adjusting the size of the starting powder particles, and their ionic conductivities were systematically investigated. Counterintuitively, larger raw particles resulted in a lower grain boundary resistivity. This was attributed to the micromorphology of the sintered pellets. The grain boundary resistance varied by up to one order of magnitude under the investigated synthesis conditions, and the optimised total ionic conductivity (including the bulk and grain boundary contributions) of LTPO was 0.95 mS cm⁻¹ at 30 °C.