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-50SiS 2 (LiSiS), 60Li2S-32SiS2-8P2S5 (LiSiPS), and 75Li2S-25P2S5 (LiPS). Our simulations reveal that SEI growth follows diffusion-limited kinetics with 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-doped Li 2 S phase that effectively passivates further interfacial growth. 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.