Effect of Si substitution on the structural and transport properties of superionic Li-argyrodites

Inspired by the recent interest in lithium ion conducting argyrodites as solid electrolytes for solid-state batteries, we have investigated the influence of aliovalent substitution in Li6PS5Br. Using Rietveld refinements against X-ray and neutron diffraction, coupled with impedance spectroscopy, we monitor the influence of Si4+ substitution for P5+ in Li6+xP1−xSixS5Br on the structure and ionic transport properties. A step-wise incorporation of Si4+ leads to an expansion of the unit cell, as well as the inclusion of additional Li+ within the structure. The increasing Li content occupies the structural transition state and, in combination with the structural changes, leads to a three-fold improvement of the ionic conductivity. This work demonstrates that the argyrodite material class can be optimized through aliovalent substitution, thereby making argyrodites an ideal system for studying solid electrolytes within the field of solid-state batteries.