Secondary manganese dioxide electrodes for grid-scale batteries

Manganese dioxide (MnO ₂) is well-established as aprimary battery cathode, but irreversibility has generally excluded its use insecondary batteries. Repurposingthis chemistry as a rechargeable electrical storage medium make possible a newera of MW-scale advanced batteries to add universal storage capability to theelectrical grid. This would bepossible due to low cost, high availability, and safety of the materials. There is no universal consensus on the strategy forextending the cycle life of MnO ₂ electrodes, which are typicallyelectrolyte-filled porous electrodes also containing carbon and a binder. Adding dopant atoms to the MnO ₂ crystal structure is one. Anotheris low depth of discharge. Thefirst of these renders electrochemically irreversible products (Mn ₂O ₃)reversible, the second avoids their formation. These methods are not mutually exclusive, and a combinationof them may prove successful. Inany case, the problem is not only one of manganese electrochemistry, but alsocurrent distribution in the electrode and interactions with the conductivecarbon matrix. Any one of many processes may become limiting in a porousMnO ₂-carbon electrode: ionic conduction in pores, electronic conductionin the carbon matrix, interfacial contacts, or charge transfer of the MnO ₂reaction itself. Parallel studiesof battery cycling, half-cell cycling, ex situ material characterization, andfailure analysis must be used to identify the limiting processes in such porouselectrodes. For this study, porous manganese dioxide cathodes werepaired with non-limiting cadmium anodes with a base cell size of ∼4 Ah. The electrolyte was quiescent 12 M KOH. Electrochemical impedancespectroscopy (EIS) revealed a steady increase in cell impedance with cyclenumber, although battery failure was relatively sudden, usually after 100-200cycles. The EIS results weremodeled to pinpoint which phenomena in the porous electrode were responsiblefor increase in overall impedance. Ex situ methods such as XRD were used to track materialchanges in the electrodes. Recent battery efforts at the CUNY Energy Institute haveincorporated parallel experimentation across many cell sizes, with the targetbeing large cell stacks.