In Situ Electrochemical Atomic Force Microscopy Study of Interfacial Reactions on a Graphite Negative Electrode for Magnesium-Ion Batteries

The cointercalation of solvated Mg2+ ions into graphite has typically been considered challenging because of concerns regarding the instability of the electrolyte and the potential for structural degradation. However, recent developments in electrolyte design suggest that this process may be reversible under appropriate conditions. In this study, the interfacial behavior of graphite in a magnesium-ion system was investigated using in situ electrochemical atomic force microscopy. Electrochemical tests in a triglyme-based electrolyte revealed a reversible capacity of 158 mAh g(-1), attributed to the insertion of triglyme-solvated Mg2+ ions. Real-time surface imaging of highly oriented pyrolytic graphite revealed the formation of a passivating surface film during the initial cycle, along with nanoscale hill-like (similar to 1 nm) and blister-like (similar to 5 nm) structures, which were partially reversible and showed good correlation with the redox peaks observed in the cyclic voltammetry experiments, suggesting that the surface film enables Mg2+ transport while mitigating electrolyte decomposition. These findings demonstrate that stable co-intercalation of solvated Mg2+ ions is achievable in the early cycles in graphite and highlight the importance of interfacial engineering and solvation structures in the development of magnesium-ion batteries.