The First Electrochemical Cycle: State‐of‐Charge Dependent Formation and Evolution of the Solid Electrolyte Interphase on Hard Carbon Anodes in Sodium‐Ion Batteries

ABSTRACT Understanding the solid electrolyte interphase (SEI) is central to improving the performance and longevity of sodium‐ion batteries. Here, we combine scanning electron microscopy (SEM), conductive atomic force microscopy (c‐AFM), energy‐dispersive x‐ray spectroscopy (EDS), time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS), and scanning electrochemical microscopy (SECM) to investigate SEI formation on Hard Carbon (HC) composite electrodes. Unlike most literature, which analyses the SEI after multiple cycles, we examine different states of charge during the first galvanostatic cycles in sodium half‐cells using EC:PC with FEC additive and NaPF 6 as the electrolyte. SEM reveals early surface film formation becoming continuous at full sodiation, while c‐AFM shows surface conductivity is strongly suppressed below 0.2 V, with only isolated conductive domains recovering after desodiation. EDS confirms accumulation of Na and O species on HC particles alongside persistent F components, identified as NaF by XPS. Depth‐resolved ToF‐SIMS reveals an SEI evolving from an oxide/hydroxide‐rich outer layer to a NaF‐dominated interphase, with Na 2 F + signals increasing steadily through desodiation. SECM provides a functional link to these changes, as the apparent rate constant κ decreases sharply during sodiation and only partly recovers upon desodiation. These findings offer an in‐depth understanding of initial SEI dynamics in HC composite electrodes for sodium‐ion batteries.