Rechargeability of Manganese Dioxide-Zinc Batteries

Abstract

Primary alkaline batteries use electrolytic manganese dioxide (EMD) as the cathode and zinc as the anode and have been in commercial use for several decades. This chemistry is of great interest because the materials used are inexpensive, environmental friendly, and abundant. In addition, alkaline chemistry offers a better energy density and shelf life than some other aqueous batteries such as lead-acid technology. Rechargeability of EMD, however, is challenging and commercial rechargeable batteries based on this electrochemistry have faced limited success due to the limitation of cycling performance. We have investigated the causes of cycling failures using flatplate-type cathode (MnO2) and anode (Zinc) electrodes which are amenable to large scale production. Electrochemical and physiochemical characterizations were performed and will be presented. Flatplate-type electrodes were prepared by producing paste-like mixtures of cathode and anode materials and applying them onto expanded Ni (cathode) and expanded brass (anode) current collectors. The cathode mixture contained EMD, graphite, additives, SBR binder and CMC gelling agent. Anode mixtures were prepared from zinc, zinc oxide, hydrogen inhibitors, teflon binder and carbapol gelling agent. The alkaline electrolyte (9M) was prepared from DI-water and KOH salt. All the cell hardware used was developed in-house. For the cathode electrode, a number of chemical additives such as BaSO4, Sr(OH)2·8H2O, Ca(OH)2, and Bi2O3were investigated at 5 wt. %. Deep discharge cycling (to 1.1 V) reduces the initial specific capacity of 250 mAh g-1 by 50% after 20 cycles. The specific capacity reduces, although at a slower rate, to about 50 mAh g-1 after 100 cycles. Additives such as BaSO4show marginal improvement in the earlier cycles (cycles 10-50), but beyond cycle 50 no noticeable effect was observed. Deeper cycling can be combined with shallow depths of discharge (1.4 – 1.45 V) to extend the cycle life-time of the batteries. Figure 1 shows the cycling performance of RAM cells for combined deep/shallow depths of discharge at C/2 and C/10 rates. Post-cycling XRD analysis of the cathode electrode shows formation of a todorokite-like phase which could limit the cycling performance. Impedance studies show a rapid increase in the charge transfer resistance, indicating that formation of surface inactive phases also play a role in reducing the capacity upon cycling. Figure 1