Na-ion batteries are amongst the most appealing alternatives to Li-ion batteries due to a similar intercalation chemistry and lower cost. Novel Na-ion chemistries are often assessed in half cells that contain Na-metal counter and reference electrodes (Figure 1). These electrodes are highly reactive and require a passivation layer to work, which is formed from electrolyte decomposition products. Recently, however, the passivation layer of standard carbonate electrolytes on sodium metal was found to be rather instable, raising questions about the reliability of both Na-ion half cells and other low voltage anodes.^[1]

This work presents a dimethoxyethane (DME) / sodium bis(fluorosulfonyl)imide (NaFSI) (2/1 (n/n)) electrolyte that forms a stable and resilient passivation layer on sodium metal, as indicated by highly reversible sodium plating/stripping. Further investigation of the passivation layers by post-mortem FTIR and ¹H-NMR analysis reveals that conventional organic carbonate based electrolytes decompose to organic salts such as sodium ethyl dicarbonate. In contrast, the DME/NaFSI electrolyte forms ethereal and alkoxide species as well as inorganic salts derived from the FSI anion. These compounds ensure sodium metal passivation and thereby enable more reliable and efficient testing in Na-ion half cells.

^[1] Iermakova, D. I.; Dugas, R.; Palacin, M.R.; Ponrouch, A. J. Electrochem. Soc. 2015, 162, A7060-A7066

Figure 1: A typical half-cell setup for testing a novel electrode material in a Na-ion cell