Evidence for a Solid-Electrolyte Inductive Effect in the Superionic Conductor Li₁₀Ge_1–xSn_xP₂S₁₂

Strategies to enhance ionic conductivities in solid electrolytes typically focus on the effects of modifying their crystal structures or of tuning mobile-ion stoichiometries. A less-explored approach is to modulate the chemical bonding interactions within a material to promote fast lithium-ion diffusion. Recently, the idea of a solid-electrolyte inductive effect has been proposed, whereby changes in bonding within the solid-electrolyte host framework modify the potential energy landscape for the mobile ions, resulting in an enhanced ionic conductivity. Direct evidence for a solid-electrolyte inductive effect, however, is lacking—in part because of the challenge of quantifying changes in local bonding interactions within a solid-electrolyte host framework. Here, we consider the evidence for a solid-electrolyte inductive effect in the archetypal superionic lithium-ion conductor Li₁₀Ge_1–xSn_xP₂S₁₂. Substituting Ge for Sn weakens the {Ge,Sn}–S bonding interactions and increases the charge density associated with the S^2– ions. This charge redistribution modifies the Li⁺ substructure causing Li⁺ ions to bind more strongly to the host framework S^2– anions, which in turn modulates the Li⁺ ion potential energy surface, increasing local barriers for Li⁺ ion diffusion. Each of these effects is consistent with the predictions of the solid-electrolyte inductive effect model. Density functional theory calculations predict that this inductive effect occurs even in the absence of changes to the host framework geometry due to Ge → Sn substitution. These results provide direct evidence in support of a measurable solid–electrolyte inductive effect and demonstrate its application as a practical strategy for tuning ionic conductivities in superionic lithium-ion conductors.