Proximity Matters: Interfacial Solvation Dictates Solid Electrolyte Interphase Composition

The composition of the solid electrolyte interphase (SEI) plays a crucial role in controlling lithium-electrolyte reactions, yet the underlying cause of SEI composition differences between electrolytes remains unclear. Many studies have correlated SEI composition with the bulk solvation of Li ions in electrolytes, but this correlation does not fully capture the interfacial phenomenon of SEI formation. In our work, we present a direct mechanistic relationship between SEI composition and Li-ion solvation by forming SEIs using polar substrates that modify interfacial solvation structures. We avoid the deposition of Li metal by forming the SEI above the Li⁺/Li redox potential. Using theory and spectroscopy, we demonstrate that an increase in the probability density of anions near a polar substrate enhances anion incorporation within the SEI, establishing a direct correlation between interfacial solvation and SEI composition. We demonstrate the generalizability of this SEI-solvation correlation using three distinct classes of electrolytes. Finally, we employ this concept to generate stable, anion-rich SEIs, leading to high-performance lithium metal batteries. In summary, our work clarifies the mechanistic relationship between electrolyte solvation and SEI composition, resulting in a discovery that can be applied to the design of improved lithium metal batteries.