Impact of hydration on ion transport in Li2Sn2S5·xH2O

Abstract

This work investigates the structure and transport properties of the layered material Li₂Sn₂S₅·xH₂O. The anhydrous phase shows a room-temperature Li⁺ diffusivity below 10⁻⁹ cm² s⁻¹ and conductivity below 10⁻⁵ S cm⁻¹. Upon exposure to humidity, water intercalates between the layers and increases the interlayer distance, inducing first-order transitions to a hydrated phase (x ≈ 2–4) and then to a second hydrated phase (x ≈ 8–10). The latter is soft and sticky but remains solid. Diffusion of both Li⁺ ions and H₂O remains predominantly two-dimensional under all conditions. The Li⁺ diffusivity and conductivity both increase by three orders of magnitude upon hydration, reaching values of 5 × 10⁻⁷ cm² s⁻¹ and 10⁻² S cm⁻¹ in the second hydrate. These transport rates are extraordinary for a solid electrolyte and approach what is typically seen in aqueous solutions. The material Li₂Sn₂S₅·xH₂O thus bridges the gap between a hydrated solid electrolyte and a confined liquid electrolyte, which is scientifically interesting and potentially useful in battery applications. In the light of these findings, a previous work on Li₂Sn₂S₅ from our groups is revisited.