Sodium-ion-batteries (NIBs) are promising energy storage devices with advantages
like low cost and highly abundant raw materials. In order to probe the electrochemical
properties of NIBs, sodium metal is most frequently applied as 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 our study, we demonstrate spontaneous reactions of sodium metal with organic
electrolytes and emphasize the importance of critical interpretation of electrochemical
experiments. When using sodium-metal half-cells, decomposition products
contaminate the electrolyte during the electrochemical measurement and can easily
lead to wrong conclusions on the stability of active materials. We prove that the cycling
stability is highly affected by these electrolyte contaminations by comparing sodium
metal free with sodium metal containing cells. Interestingly, a more stable cycling
performance of the Li4Ti5O12 half-cells can be observed when replacing the Na metal
counter and reference electrodes by activated carbon electrodes. We attribute this
difference to the altered properties of the electrolyte due to contaminations and to
different surface chemistries.