Enhancing Ionic Conductivity in Li6+xGexP1−xS5Br: Impact of Li+ Substructure on Ionic Transport and Solid-State Battery Performance

Solid-state batteries have been investigated as efficient energy storage systems due to the increased power and energy densities that they can offer, compared to liquid-based batteries. The search for solid electrolytes with high ionic conductivities, sufficient electrochemical and mechanical stability is indispensable. In this work, the Li6+xGexP1−xS5Br substitution series is investigated via X-ray and neutron powder diffraction, as well as impedance and solid-state nuclear magnetic resonance spectroscopy. Structural analyses reveal the expansion of the cage-like Li+ substructure with increasing degree of substitution of Ge(IV) for P(V) in Li6+xGexP1−xS5Br. Solid-state nuclear magnetic resonance spectroscopy measurements reveal the gradual changes in cation environments (6Li and 31P) and the effect of Ge(IV) substitution on local Li+ transport. Impedance spectroscopy shows an improvement of ionic conductivity at room temperature up to fivefold for Li6.31Ge0.31P0.69S5Br and decreasing activation energies. Employing Li6.31Ge0.31P0.69S5Br as a catholyte in LiNixMnyCozO2 based solid-state batteries results in reproducibly higher active material utilization and rate stability in comparison to Li6PS5Br. This work emphasizes the importance of understanding the Li+ substructure of argyrodites in correlation with the Li+ transport properties to systematically develop highly conductive Li+ solid electrolytes for improved solid-state batteries.