Bottleneck of Diffusion and Inductive Effects in Li₁₀Ge1–xSnxP₂S₁₂

The lithium-ion conductor Li 10 GeP 2 S 12 (LGPS) is known to exhibit ionic conductivity values as high as 12 mS·cm –1 . Unfortunately, counter to chemical intuition, many attempts to enhance the ionic transport in LGPS, e.g., by increasing the Sn fraction in Li 10 Ge 1– x Sn x P 2 S 12, have even led to a reduction in the conductivity. Employing a combination of Rietveld refinements against X-ray diffraction data, speed of sound measurements, and electrochemical impedance spectroscopy, we investigate the structure–property relationships governing this behavior. Herein, it is shown that with increasing Sn 4+ fraction in Li 10 Ge 1– x Sn x P 2 S 12 a structural bottleneck along the diffusion channels in the z -direction begins to tighten, and with the concomitant increase in the lattice softness, the local ionic bonding interactions between Li + and S 2– become stronger, further increasing the activation barrier. This work provides a likely explanation for the lower conductivity exhibited by Li 10 SnP 2 S 12 and demonstrates that there is more to the underlying lithium diffusion mechanism in the Li 10 MP 2 S 12 structure.