Sodium hydride inspired the clarification of the ether-carbonate solvent disparity in sodium metal anodes

Ether-based electrolytes enable highly reversible sodium (Na) plating/stripping behavior while conventional carbonate-based electrolytes deliver rather poor electrochemical compatibility with Na metal anodes. Yet, this ether-carbonate mystery in Na metal anodes is still not fully elucidated. Sodium hydride (NaH) has been recently demonstrated to be a key chemical component on the cycled Na metal anodes. In this work, via the combination of time of flight-second ion mass spectroscopy (ToF-SIMS) and on-line D2O titration mass spectroscopy, we disclose the spatial distribution of NaH and quantify its amount in Na metal anodes cycled in ether-based and carbonate-based electrolytes. A systematic study reveals that NaH accumulates upon continuous Na plating-stripping in conventional carbonate electrolytes, and the NaH formation is proved to be highly correlated with the evolution of H2 during cell operation. The huge performance disparity of Na metal anodes in ether-based and carbonate-based electrolytes is ascribed to the fact that much less NaH and H2 is produced in the case of ether-based electrolytes. This work enriches the understanding of interfacial chemistries of Na metal anodes and highlights the importance of minimizing the generation of NaH and H2 in protecting Na metal anodes.