Electrochemical impedance studies of a decade-aged magnesium/manganese dioxide primary cell

Abstract

A Mg/MnO2 primary cell yields specific energy higher than a conventional Zn/MnO2 cell. Additionally, the shelf life of the Mg/MnO2 cell is extremely high. These cells, which were more than a decade old, were investigated for their discharge capacity, delay-time behaviour and impedance characteristics. The values of discharge capacity and the delay-time of an aged Mg/MnO2 cell were comparable to those of a fresh cell. The voltage dip on initiation of a galvanostatic current, however, was rather large. This was attributed to the presence of a thick, and highly resistive, surface passive film on theMg anode. The complex plane electrochemical impedance spectrum of a partially discharged cell consisted of two semicircles whose sizes decreased with decrease of state-of-charge of the cell. The a.c. frequency corresponding to the maximum value of the imaginary part of the high frequency semicircle was shown to be a useful parameter for estimation of the state-of-charge of the cell. The resistance parameters of a partially discharged Mg/MnO2 cell increased linearly with open circuit ageing time. This feature was attributed to growth of a passive surface layer on the Mg anode.