Abstract
A comprehensive understanding of the solid electrolyte interphase (SEI) is crucial for ensuring long-term battery stability. This is particularly pertinent in sodium-ion batteries (NIBs), where the SEI remains poorly understood, and investigations are typically undertaken in half-cell configurations with sodium metal as the counter electrode. Na metal is known to be highly reactive with common carbonate-based electrolytes; nevertheless, its effects on SEI formation at the working electrode are largely unexplored. This work investigates the evolution of the SEI in NIBs during cycling, with an emphasis on the consequences of using a sodium metal counter electrode. Advanced analytical techniques, including hard X-ray photoelectron spectroscopy (HAXPES) and time-of-flight secondary ion mass spectrometry (ToF-SIMS), are used to obtain depth-resolved insights into the chemical composition and structural changes of the SEI on hard carbon anodes during cycling. The findings demonstrate that the cell configuration has a significant impact on SEI evolution and, by extension, battery performance. These findings suggest that full-cell studies are necessary to better simulate practical operating conditions, challenging traditional half-cell experiments.