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

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 h igh. These cells, which were more than a decade old, were investigated for their discharge capacity, delay-time behaviour and impedance characteristic s. The values of discharge capacity and the delay-time of an aged Mg/MnO2 c ell 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 the Mg anode. The complex plane electrochemical impedance spectrum of a par tially discharged cell consisted of two semicircles whose sizes decreased w ith decrease of state-of-charge of the cell. The a.c. frequency correspondi ng to the maximum value of the imaginary part of the high frequency semicir cle was shown to be a useful parameter for estimation of the state-of-charg e of the cell. The resistance parameters of a partially discharged Mg/MnO2 cell increased linearly with open circuit ageing time. This feature was att ributed to growth of a passive surface layer on the Mg anode.