Bottleneck of Diffusion and Inductive Effects in Li10Ge1–xSnxP2S12

The lithium-ion conductor Li10GeP2S12 (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 Li10Ge1–xSnxP2S12, 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 Sn4+ fraction in Li10Ge1–xSnxP2S12 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 S2– become stronger, further increasing the activation barrier. This work provides a likely explanation for the lower conductivity exhibited by Li10SnP2S12 and demonstrates that there is more to the underlying lithium diffusion mechanism in the Li10MP2S12 structure.