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

• A new phenomenon in sodium batteries is reported
• Voltage step seen in cycling of electrode material is influenced by solvent
• It occurs due to increased polarization of the sodium counter electrode
• A passivation layer on sodium counter electrode is responsible. The voltage step is not seen in full sodium-ion cells

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.

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