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.