Electrolyte Additives as Pathway Selectors in Early SEI Formation

Electrolyte additives such as fluoroethylene carbonate (FEC) and vinylene carbonate (VC) improve lithium-ion battery performance, yet whether they shift product distributions along common pathways or select fundamentally different decomposition channels remains debated. Here, ensemble ab initio molecular dynamics simulations coupled with machine learning structural analysis quantify how electrolyte composition controls access to configuration space during early solid-electrolyte interphase (SEI) formation. Analysis of 15 independent trajectories across baseline, FEC-rich, and VC-rich formulations using SOAP descriptors, UMAP, and HDBSCAN reveals that each composition occupies nonoverlapping structural regions characterized by distinct Li-O coordination environments and composition-dependent Li-F signatures. Diagnostic comparison of initial and evolved configurations, including Li-centered SOAP descriptors that isolate emergent structural changes from compositional encoding, and supported by time-resolved SOAP analysis, provides evidence that this separation reflects emergent structural divergence during decomposition. These results suggest that electrolyte additives function as pathway selectors during early interfacial reaction, offering a framework for rational electrolyte design.