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 Li_6+xGe_xP_1−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 Li_6+xGe_xP_1−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 Li_6.31Ge_0.31P_0.69S₅Br and decreasing activation energies. Employing Li_6.31Ge_0.31P_0.69S₅Br as a catholyte in LiNi_xMn_yCo_zO₂ 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.