Zero volt storage of Na-ion batteries: Performance dependence on cell chemistry!

Sodium-ion batteries (NIBs) are regaining their importance in recent years as a sustainable complementary energy storage device for Li-ion batteries. Although, they cannot compete in terms of energy density with respect to Li-ion, they present a few advantages, namely the 0 V stability that makes them safe during external short and/or over-discharge. When the cell is discharged to 0 V, the negative electrode potential shoots up high during which the copper current collector in use for Li-ion cells could oxidize, dissolve and leads to internal short. In contrast, Na-ion cells utilize an aluminium current collector that is strongly resistant against oxidation, hence enabling their 0 V stability. However, apart from the current collector stability, the negative electrode potential rise could cause interphase instability which is not well elucidated. Hence, herein, we explored two different Na-ion chemistries, namely polyanionic Na3V2(PO4)2F3-hard carbon and sodium layered oxide-hard carbon using different electrolyte formulations. Combined impedance analyses, ex-situ X-ray photoelectron spectroscopy (XPS) and operando optical sensing indicate the 0 V discharge involves SEI degradation thereby deteriorating the cell performance, the extent of which depends on the positive electrode potential and the electrolyte in use. Overall, the 0 V stability is not an in-built property of Na-ion cells and a careful selection of cell chemistry is mandatory to achieve 0 V stable Na-ion cells.