Progress and perspectives on the reaction mechanisms in mild-acidic aqueous zinc-manganese oxide batteries

The appeal of safe, energy-dense, and environmentally-friendly MnO2 as a cathode for rechargeable aqueous zinc-metal oxide batteries (AZMOBs) has attracted significant research attention, but unexpected complexities have resulted in a decade of confusion and conflicting claims. The literature base is near saturation with a mix of efforts to achieve practical, rechargeable Zn-ion batteries and to untangle the presented electrochemical mechanisms. We have summarized the respective mechanisms and contextualized the respective justifications. As new perspectives arise from in situ and operando techniques, renewed efforts must solidify mechanistic understandings and reconcile disparate data through judicial application of ab initio modelling. In light of a variety of MnO2 cathode phases and stable, meta-stable, and complex reaction products, this perspective emphasizes the need for greater supplementation of the in situ and operando characterization with modelling, such as density functional theory. Through the elucidation of key mechanisms under dynamic operating and characterization conditions, the body of previously contradictory research and routes to practical batteries may be unified, and guide the way to longevity and grid-scale applicable charge rates and capacity. The appeal of MnO2 as a cathode for rechargeable aqueous zinc-metal oxide batteries has attracted significant attention, but the complex mechanisms still require pairing of advanced characterization and computation techniques to overcome.