Minimizing the Grain Boundary Resistance of Li-Ion-Conducting Oxide Electrolyte by Controlling Liquid-Phase Formation During Sintering

The commercialization of sintered solid-state devices using oxide electrolyte is hindered by their low total conductivity, which largely originates from high ionic resistance at grain boundaries (GBs). Herein, we investigate a representative gamma-Li3PO4-type Li3.5Ge0.75S0.25O4 sintered sample, showing that its GB resistance mostly results from the presence of a residual Li-S-O rich phase produced after liquid-phase sintering and is also influenced by the sintering atmosphere. Finally, the GB resistance of the Li3.5Ge0.75S0.25O4 sintered sample is shown to be effectively reduced by the use of spark plasma sintering, which allows GB formation to occur quickly enough to prevent the formation of excess residual Li-S-O rich phase.