Excessive sodiation and desodiation of commercial NFM-hard carbon sodium-ion batteries induced by low-rate cycling

Experiments reveal that the electrodes of a commercial NFM | | hard carbon sodium-ion battery undergo excessive sodiation and desodiation at low charge and discharge rates. These effects are observed during cycling, despite operating strictly within the manufacturer’s recommended voltage limits of 1.5 V to 4.1 V. The resulting increase in charge capacity originates from an additional phase transition in the cathode active material, which manifests itself electrochemically as a voltage plateau that delays reaching the cut-off voltage during charging. The occurrence of the phase transition at high cell voltages is verified by operando X-ray diffraction measurements. During the subsequent discharge, the reverse transition takes place. Unusual deep discharge is enabled by the combination of low overpotentials and the characteristic potential profile of the cell. These phenomena are reproducibly observed across multiple test sequences. To separate the effects of the individual electrodes on this full cell behavior, harvested electrodes from a commercial cell are assembled into an experimental three-electrode setup and analyzed by differential voltage analysis. Since the high-voltage phase transition is reported to involve irreversible processes, adapting characterization and cycling protocols (especially voltage limits) may improve long-term performance and facilitate future analyses. • Observation of an additional phase transition in commercial sodium-ion batteries triggered by slow charging rates. • Access to unintended sodiation regions despite staying in nominal voltage window. • Separation of anode and cathode contributions using harvested electrodes in 3-electrode cell setup. • Differential voltage analysis of charge and discharge voltage curves to identify unintended electrode processes. • Findings offer new insights for optimizing cycling protocols to enhance sodium-ion battery longevity.