Enhancing ionic conductivity in $Li_{6+ x} Ge_x P_{1−x}S_5Br$: 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 $Li_{6+ x} Ge_x P_{1−x}S_5Br$ 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 $Li_{6+ x} Ge_x P_{1−x}S_5Br$. Solid-state nuclear magnetic resonance spectroscopy measurements reveal the gradual changes in cation environments ($^6Li$ and $^{31}P$) 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 $Li_{6.31}Ge_{0.31}P_{0.69}S_5Br$ and decreasing activation energies. Employing $Li_{6.31}Ge_{0.31}P_{0.69}S_5Br$ as a catholyte in $LiNi_xMn_yCo_zO_2$ based solid-state batteries results in reproducibly higher active material utilization and rate stability in comparison to $Li_6PS_5Br$. 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.