An Equilibrium Potential Analysis of Bi and Cu Modified Rechargeable MnO2 Cathodes

There is an urgent societal need for safe, scalable, low-cost energy storage technologies capable of sustaining the power grid at times when renewable power sources cannot. Currently, the energy storage market is dominated by lithium-ion battery technologies which make use of flammable electrolytes that combust in response to thermal runaway. Rechargeable Zn-MnO₂ alkaline batteries have been suggested as a suitable fit for grid-scale storage. Zn-MnO₂ cells use safe, non-toxic, earth-abundant materials making for an inexpensive (<$100/kWh) and sustainable battery technology. Accessing the full capacity of an MnO₂ rechargeable cathode requires that the conversion of Mn⁴⁺ to Mn²⁺ be completely reversible. Intermediate formation of some forms of Mn³⁺ during battery operation can lead to an irreversible phase change of MnO₂ to the Mn₃O₄ spinel structure. Once the spinel forms, that active material can no longer engage in the desired electron transfer reaction that stores energy. Recent studies suggest that incorporating Bi³⁺ and Cu²⁺ ions into layered MnO₂ cathode materials suppresses spinel formation and increases cyclability. In this work, the galvanostatic intermittent titration technique (GITT) is used to monitor the equilibrium potential of Bi- and Cu-modified MnO₂ cathodes at different states of charge during cycling. By correlating these equilibrium potentials to different redox states formed within the electrode, we seek to better understand the role of Bi and Cu in enhanced performance.