A Cluster-Based Model of Anionic Frameworks for Evaluating Lithium Conductivity in Sulfide Solid-State Electrolytes

Sulfides constitute an important group of ionic conductive solids for all-solid-state lithium-ion batteries, whereas their poor stability against air and humidity inhibits the accurate experimental evaluation of their intrinsic conductivity. In this paper, a new structural tool, the cluster-plus-glue-atom model, is used to correlate the lithium conduction and crystal structure in sulfide solid-state electrolytes (SSEs). This model identifies the anion-based composition unit in any sulfide as being composed of an anion unit and stoichiometrically matched cations. The anion unit covers a nearest-neighbor anion cluster plus next-neighbor “glue” anions, generally containing 16 or 24 anions. Cations occupy interstitials within the anion unit, with transmission-active Li ions inside anionic triangular dipyramids and octahedra. It is assumed that the Li transmission is realized through adjacent active Li sites of inter-distances falling close to the anion nearest-neighbor distances. The number of such Li–Li pairs per anion (n) is proposed to correlate with room-temperature ionic conductivities (σ) of typical sulfide SSEs. It is revealed for SSEs with 3D Li diffusion channels that the upper limits of the measured σ‘s follow approximately log(σ) = −3 + n/3, enabling a fast evaluation of these SSEs. Accordingly, Li₇SiPS₈, Li₁₀SnP₂S₁₂, and Li₁₀GeP₂S₁₂, with their n's falling in 3–5, should be promising SSEs.