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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.

 

Interphases formed at battery electrodes are key to enabling energy dense charge storage by acting as protection layers and gatekeeping ion flux into and out of the electrodes. However, our current understanding of these structures and how to control their properties is still limited due to their heterogenous structure, dynamic nature, and lack of analytical techniques to probe their electronic and ionic properties in situ . In this study, we used a multi-functional scanning electrochemical microscopy (SECM) technique based on an amperometric ion-selective mercury disc-well (HgDW) probe for spatially-resolving changes in interfacial Li + during solid electrolyte interphase (SEI) formation and for tracking its relationship to the electronic passivation of the interphase. We focused on multi-layer graphene (MLG) as a model graphitic system and developed a method for ion-flux mapping based on pulsing the substrate at multiple potentials with distinct behavior ( e.g. insertion–deinsertion). By using a pulsed protocol, we captured the localized uptake of Li + at the forming SEI and during intercalation, creating activity maps along the edge of the MLG electrode. On the other hand, a redox probe showed passivation by the interphase at the same locations, thus enabling correlations between ion and electron transfer. Our analytical method provided direct insight into the interphase formation process and could be used for evaluating dynamic interfacial phenomena and improving future energy storage technologies. 

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.