Influence of the Lithium Substructure on the Diffusion Pathways and Transport Properties of the Thio-LISICON Li₄Ge_{1- x}Sn_xS₄

Inorganic lithium-ion conductors have garnered considerable attention as separators for all-solid-state lithium-ion battery applications, given their potential to solve the safety issues and improve the energy and power densities of conventional devices possessing liquid electrolytes. However, achieving this transition requires the optimization of solid electrolyte materials and thus, developing a better understanding of the structure-property relationships, that govern ionic transport, is of crucial importance. Herein, inspired by the growing technological interest, a systematic study on the correlations between structural modifications and transport properties in the thio-LISICON family resulting from the substitution of Ge⁴⁺ by Sn⁴⁺ within Li₄Ge_1-xSn_xS₄ has been conducted. Using Rietveld refinements against neutron diffraction data coupled with maximum-entropy method analyses of nuclear densities, a rigorous investigation into the Li⁺ diffusion pathways was performed. The substitution of Ge⁴⁺ by Sn⁴⁺ is shown to broaden the diffusion bottleneck, modify the lithium distribution, and enhance the connectivity between the conduction channels, thereby leading to an increase of the ionic conductivity for Li₄SnS₄. The correlations between composition, structure, and transport behavior found in this work provide insights into design strategies for new electrolytes belonging to the thio-LISICON family.