Sn Substitution in the Lithium Superionic Argyrodite Li₆PCh₅I (Ch = S and Se)

The lithium argyrodites Li₆PS₅X (X = Cl, Br, and I) have attracted interest as fast solid ionic conductors for solid-state batteries. Within this class of materials, it has been previously suggested that more polarizable anions and larger substituents should influence the ionic conductivity (e.g., substituting S by Se). Building upon this work, we explore the influence of Sn substitution in lithium argyrodites Li_6+xSn_xP_1–xSe₅I in direct comparison to the previously reported Li_6+xSn_xP_1–xS₅I series. The (P⁵⁺/Sn⁴⁺)Se₄^3/4– polyhedral volume, unit cell volume, and lithium coordination tetrahedra Li(48h)-(S/Se)₃-I increase with Sn substitution in this new selenide series. Impedance spectroscopy reveals that increasing Sn⁴⁺ substitution results in a fivefold improvement in the ionic conductivity when compared to Li₆PSe₅I. This work provides further understanding of compositional influences for optimizing the ionic conductivity of solid electrolytes.