The transition toward sustainable and resource-abundant energy-storage systems has intensified research into sodium-ion battery (SIB) [1] and aluminum-ion battery (AlIB) [2] chemistries, where interfacial phenomena critically dictate efficiency, reversibility, and long-term cycling stability to replace the lithium-ion batteries (LIBs). Across these studies, a central theme emerges: controlling the electrode/electrolyte interface is essential for advancing alternative battery technologies.

Hard carbon (HC) is a highly promising negative electrode material for SIBs [1]. Its disordered graphene structure provides substantial porosity for Na-ion storage, whereas the formation of stage-wise intercalation compounds typical of graphite in lithium-ion batteries does not occur in defect-free HC [3]. Despite decades of research on the solid/electrolyte interphase (SEI) in LIBs, significantly less information is available regarding SEI formation on HC in SIBs, particularly concerning the influence of binders, electrolyte composition, and additives [4]. In this invited talk, I will present our recent efforts to characterize interphase formation on HC anodes using a correlative multimodal approach. Spray-coated HC electrodes were examined in 1 M NaPF₆/diglyme to assess how electrolyte formulation, cycling rate, and electrode microstructure influence SEI development. Scanning electrochemical microscopy (SECM) provided spatially resolved information on the electrochemical properties of the SEI layer [5,6], while atomic force microscopy (AFM) was used to probe morphology, roughness, and nanomechanical properties. Complementary X-ray photoelectron spectroscopy (XPS) [7] and time-of-flight secondary ion mass spectrometry (ToF-SIMS) analyses revealed changes in SEI composition as a function of cycling and current rate. Our results show that HC develops a relatively uniform SEI after the first cycle, followed by partial dissolution and increased chemical and mechanical heterogeneity during extended cycling, indicating a delicate balance between stabilization and degradation in ether-based electrolytes.

For AlIBs, we investigate how aluminum foil properties including thickness, surface finish, roughness, and the characteristics of the native Al₂O₃ layer influence corrosion behavior and interfacial stability in the highly acidic AlCl₃:[EMIm]Cl (1.5:1) ionic liquid. AFM studies after prolonged immersion and electrochemical cycling reveal progressive removal of the amorphous oxide layer and the development of a granular corrosion morphology, providing insight into the origins of performance variability between different aluminum sources.

Together, these results demonstrate how correlative microscopies and spectroscopies can uncover nanoscale mechanisms governing interphase formation and degradation in post-lithium battery systems. Such insights are essential for guiding electrolyte design, surface engineering strategies, and the development of durable sodium- and aluminum-based energy-storage technologies.

Key words: SEI, HC, SIBs, AlIBs, SECM/AFM