Microscopic and Spectroscopic Analysis of the Solid Electrolyte Interphase at Hard Carbon Composite Anodes in 1 M NaPF$_6$ /Diglyme

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.