Microscopic and Spectroscopic Analysis of the Solid Electrolyte Interphase at Hard Carbon Composite Anodes in 1 M NaPF6/Diglyme

Abstract The formation of the solid electrolyte interphase (SEI) on HC composite electrodes plays a crucial role in enhancing the performance and operational stability of sodium (Na + ) ion batteries. It has been demonstrated that for HC anodes improved electrochemical performance, e. g., increase in coulombic efficiency (CE) and improved rate performance have been achieved in ether‐based electrolytes. Here, we investigate spray‐coated HC composite electrodes charged at low and high current rates in 1 M sodium hexafluorophosphate (NaPF 6 ) in diglyme using half‐cell experiments. The pristine and cycled HC anodes were examined in terms of conductivity and their electrochemical properties after cycling. In 1 M NaPF 6 ether‐based electrolyte, the spray‐coated HC composite electrodes (film thickness approx. 22.0 μm with an active mass loading of approx. 2.0 mg cm −2 ) reached a discharge capacity of 431 mA h g −1 at 0.1 C that stays constant for 40 cycles, which is substantially higher than that obtained in carbonate‐based electrolytes. We investigated the formed interphase using conductive atomic force microscopy (c‐AFM), scanning electrochemical microscopy (SECM), X‐ray photoelectron spectroscopy (XPS) and time‐of‐flight secondary‐ion mass spectrometry (ToF‐SIMS), revealing distinct differences for longer cycling and at varying current rates which indicates that the properties of the formed SEI layers are influenced by the formation conditions.