Electrochemical impedance studies of Na/MnO2 primary cells

Primary Na/MnO2 cells are expected to gain importance for replacement of primary Li/MnO2 cells in future akin to the Na-ion cells as alternative for Li-ion cells. Amorphous MnO2 is prepared by a drop-wise addition of KMnO4 solution to MnSO4 solution, and coin cells of Na/MnO2 are fabricated in a non-aqueous electrolyte of Na salt. The electrochemical impedance spectroscopy (EIS) data provides a high resistance of Na metal due to the surface passive film. On subjecting the cell for discharge, the surface film causes a delay response of the cell voltage and the closed circuit voltage reaches the normal discharge level following dielectric breakdown of the film. The EIS data measured at different stages of cell discharge are subjected to non-linear least square fitting with the aid of an appropriate equivalent circuit. The impedance parameters are examined to throw light on state-of-charge of Na/MnO2 primary cells.