Sodium-ion batteries (SIBs) have recently gained significant attention as a cost-effective and sustainable alternative to lithium-ion batteries for large-scale energy storage applications and battery-electric vehicles. With their ability to be fully discharged to 0 V, they allow much safer handling and transport. The reversibility of such a complete discharge is, however, debated as the solid electrolyte interphase (SEI) formed by commonly used electrolytes in SIBs becomes unstable at low full-cell voltages [1].

In the present study, the effects of overdischarging SIBs during their early cycle life and the implications on their long-term impedance degradation are investigated. For this purpose, commercial SIB-cells, comprising sodium nickel manganese iron oxide (NaNi0.33Mn0.33Fe0.33O2) as cathode active material, were discharged to 0 V five consecutive times, followed by an extended cycle ageing phase to assess the long-term effects of the overdischarge.

During the study, extensive electrochemical impedance spectroscopy (EIS) measurements were performed at various stages of degradation. This allows the monitoring of the electrochemical characteristics of the tested SIBs. The corresponding impedance spectra are deconvoluted using a modified distribution of relaxation times (DRT) method, which employs a set of distributed basis functions for capturing electrode inhomogeneities [2].

Notably, severe impedance degradation was observed during the overdischarge cycles, implying a decomposition and (uncontrolled) re-formation of the SEI. Furthermore, the long-term degradation behavior of the cells was significantly affected, indicating lasting changes in the passivating properties of the altered SEI. Finally, the results show that overdischarging can indeed cause irreversible changes in sodium-ion batteries, emphasizing the need to enhance their stability at low full-cell voltages to make the best possible use of their potential safety features.

[1] Desai, P., et al. (2022). Zero volt storage of Na-ion batteries: Performance dependence on cell chemistry! Journal of Power Sources 551.
[2] Leonhardt, R., et al. (2024). Reconstructing the distribution of relaxation times with analytical basis functions. Under review