An Electrochemical Impedance Study of the Capacity Limitations in Na-O2 Cells

Electrochemical impedance spectroscopy, pressure change measurements, and scanning electron microscopy were used to investigate the nonaqueous Na-O-2 cell potential decrease and rise (sudden deaths) on discharge and charge, respectively. To fit the impedance spectra from operating cells, an equivalent circuit model was used that takes into account the porous nature of the positive electrode and is able to distinguish between the electrolyte resistance in the pores and the charge-transfer resistance of the pore walls. The results obtained indicate that sudden death on discharge is caused by, depending on the current density, either accumulation of large NaO2 crystals that eventually block the electrode surface and/or a thin film of NaO2 forming on the cathode surface at the end of discharge, which limits charge-transfer. The commonly observed sudden rise in potential toward the end of charge may be caused by a concentration depletion of NaO2 dissolved in the electrolyte near the cathode surface and/or an accumulation of degradation products on the cathode surface.