AFM Investigation of Solid Electrolyte Interphase on HOPG Anode in Sodium Ion Battery

Chemical and morphological structures of solid electrolyte interphase (SEI) play a vital role in sodium-ion battery (NIB). Up to date, SEI remains the least understood component in NIB due to its trace presence, delicate chemical nature, heterogeneity in morphology, elusive formation mechanism, and lack of reliable in situ quantitative characterization tools. SEI morphological evolution during the first cyclic voltammetry and the thickness of SEI after first discharging-charging cycle on highly oriented pyrolytic graphite (HOPG) surface were investigated by using in situ electrochemical atomic force microscopy (EC-AFM). Complemented by an ex situ XPS analysis, the differences in interfacial features and fundamental constitution of SEI formation in ethylene carbonate (EC) and fluoroethylene carbonate (FEC)-based electrolytes on HOPG electrode surface were revealed. In 1 mol/L NaClO₄/EC/DMC electrolyte, dense films were formed at the step edges, while thinner films were observed on the basal plane. In 1 mol/L NaClO₄/EC/DMC electrolyte, stable films with doubled layer structures were formed on HOPG electrode after initial electrochemical cycling. The upper layer was composed of large particles, which was soft and easy to be scrapped off by AFM tip while the under layer was composed of dense small particles.