Reaction Pathways Driving the Solid Electrolyte Interphase Formation in Lithium-Metal Batteries in the Presence of Solid Polymer Electrolytes

The formation of the solid electrolyte interphase (SEI) governs the stability and performance of lithium metal batteries, yet the reaction pathways driving such SEI formation in the presence of solid polymer electrolytes (SPEs) remains poorly understood. Here, we develop a reactive force field (ReaxFF) to capture the reaction pathways dictating the SEI formation at a Lithium anode in the presence of LiTFSI-doped poly-ethylene oxide (PEO) SPE. By extending existing forcefield and retraining C–O, Li–O, and ACKS2 charge-transfer terms, we reproduce key bond-breaking and implicit charge-transfer processes that drive the SEI formation. Large-scale reactive molecular dynamics (RMD) simulations reveal that upon contact with Li⁰, TFSI and PEO initiate charge transfer that weakens N–S and C–O bonds, triggering reduction pathways consistent with ab initio predictions. These pathways identify a large number of inorganic and organic products with these products eventually coalescing into a heterogeneous passivation layer (principal component of the SEI) whose composition depends on local concentration of LiTFSI and PEO and the Li⁰ availability.