Atomic-scale mechanisms of interphase formation at lithium–glassy sulfide electrolyte interfaces

Abstract

Glassy sulfide solid electrolytes are promising candidates for all-solid-state lithium batteries owing to their high ionic conductivity and favorable mechanical properties. However, their thermodynamic instability against lithium metal leads to the formation of a complex solid electrolyte interphase (SEI), whose formation mechanisms remain poorly understood. Here, we employ machine-learning force-field molecular dynamics simulations to investigate SEI formation at Li metal interfaces with three representative glassy sulfide electrolytes: 50Li₂S–50SiS₂ (LiSiS), 60Li₂S–32SiS₂–8P₂S₅ (LiSiPS), and 75Li₂S–25P₂S₅ (LiPS). Our simulations reveal that SEI growth follows a power-law dependence across all compositions, with faster growth in P-rich systems. Interfacial reactions proceed through preferential decomposition of P–S and Si–S structural units, with phosphorus exhibiting more rapid reduction kinetics than silicon. The resulting SEI is dominated by an amorphous Li₂S-rich phase, whose composition and transport properties depend strongly on electrolyte chemistry. Notably, a stochastic crystallization event is observed in LiPS, forming a defect-rich, P-containing Li₂S phase that strongly slowed SEI thickening. These findings provide atomic-scale insights into the interplay between glass composition, reaction kinetics, and interphase stability, offering guidance for the rational design of stable lithium–electrolyte interfaces in solid-state batteries.