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Ions at battery interfaces participate in both the solid-electrolyte interphase (SEI) formation and the subsequent energy storage mechanism. However, few in situ methods can directly track interfacial Li + dynamics. Herein, we report on scanning electrochemical microscopy with Li + sensitive probes for its in situ , localized tracking during SEI formation and intercalation. We followed the potential-dependent reactivity of edge plane graphite influenced by the interfacial consumption of Li + by competing processes. Cycling in the SEI formation region revealed reversible ionic processes ascribed to surface redox, as well as irreversible SEI formation. Cycling at more negative potentials activated reversible (de)intercalation. Modeling the ion-sensitive probe response yielded Li + intercalation rate constants between 10 −4 to 10 −5 cm s −1 . Our studies allow decoupling of charge-transfer steps at complex battery interfaces and create opportunities for interrogating reactivity at individual sites. 

Abstract The solid electrolyte interphase (SEI) is a dynamic, electronically insulating film that forms on the negative electrode of Li⁺batteries (LIBs) and enables ion movement to/from the interface while preventing electrolyte breakdown. However, there is limited comparative understanding of LIB SEIs with respect to those formed on Na⁺and K⁺electrolytes for emerging battery concepts. We used scanning electrochemical microscopy (SECM) for the in situ interfacial analysis of incipient SEIs in Li⁺, K⁺and Na⁺electrolytes formed on multi‐layer graphene. Feedback images using 300 nm SECM probes and ion‐sensitive measurements indicated a superior passivation and highest cation flux for a Li⁺‐SEI in contrast to Na⁺and K⁺‐SEIs. Ex situ X‐ray photoelectron spectroscopy indicated significant fluoride formation for only Li⁺and Na⁺‐SEIs, enabling correlation to in situ SECM measurements. While SEI chemistry remains complex, these electroanalytical methods reveal links between chemical variables and the interfacial properties of materials for energy storage.