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, Li 7 SiPS 8 , Li 10 SnP 2 S 12 , and Li 10 GeP 2 S 12 , with their n 's falling in 3–5, should be promising SSEs.