Can Metallic Sodium Electrodes Affect the Electrochemistry of Sodium-Ion Batteries? Reactivity Issues and Perspectives.

Sodium-ion batteries (NIBs) are promising energy-storage devices with advantages such as low cost and highly abundant raw materials. To probe the electrochemical properties of NIBs, sodium metal is most frequently applied as the reference and/or counter electrode in state-of-the-art literature. However, the high reactivity of the sodium metal and its impact on the electrochemical performance is usually neglected. In this study, it is shown that spontaneous reactions of sodium metal with organic electrolytes and the importance of critical interpretation of electrochemical experiments is emphasized. When using sodium-metal half-cells, decomposition products contaminate the electrolyte during the electrochemical measurement and can easily lead to wrong conclusions about the stability of the active materials. The cycling stability is highly affected by these electrolyte contaminations, which is proven by comparing sodium-metal-free cell with sodium-metal-containing cells. Interestingly, a more stable cycling performance of the Li4 Ti5 O12 half-cells can be observed when replacing the Na metal counter and reference electrodes with activated carbon electrodes. This difference is attributed to the altered properties of the electrolyte as a result of contamination and to different surface chemistries.