A new phenomenon in sodium batteries: Voltage step due to solvent interaction

The galvanostatic cycling of electrode materials in lithium or sodium batteries is assumed to arise just from the contribution of the working electrode, with the counter electrode seen as always operating at a fixed potential. In this manuscript, we demonstrate a hitherto unreported phenomenon which involves a voltage step seen in the discharge profiles at high rates of two-phase electrode materials in sodium batteries that is produced not from a change in potential of the working electrode, but from an increase in polarization of the counter electrode. The choice of solvent used is critical in this context, with the phenomenon existing for EC:PC and not for PC solvent. It arises due to a passivation layer formed on the sodium counter electrode. The voltage step is observed only in the presence of sodium metal as the counter electrode, as it is not seen in full sodium-ion cells.