Morphology and Phase Changes in Iron Anodes Affecting their Capacity and Stability in Rechargeable Alkaline Batteries

Rechargeable alkaline batteries may become attractive nonflammable alternatives to lithium-ion (Li-ion) batteries for applications where achieving the highest energy density is less critical than safety, environmental friendliness, and low cost of energy storage. The broad abundance and low price of iron (Fe) make it attractive as a rechargeable anode material for aqueous batteries. Through cyclic voltammetry and post-mortem analysis, we revealed four distinct stages of Fe anode evolution: development, retention, fading, and failure, where each stage is associated with very specific changes in the morphology and phase of Fe anodes. We observed the Fe particle fragmentation resulted in the capacity increase during the initial cycles of charge–discharge. Most importantly, we discovered the irreversible formation of maghemite (γ-Fe2O3) with low reactivity is responsible for the eventual Fe anode capacity fading. This unexpected discovery changes the paradigm on possible routes to stabilize Fe anodes and contributes to future development of low-cost alkaline cells.