Rechargeable alkaline batteries may become promising non-flammable alternatives to Li-ion batteries for the applications where achieving the highest energy density is less critical than safety, environmental friendliness and low cost of energy storage systems. A renewed interest on Ni-Fe aqueous batteries awakes due to the aforementioned advantages as well as long life and robustness against harsh conditions. The challenges for improved Ni-Fe batteries, however, remain limiting their utilization for large-scale energy storage systems: electrolyte decomposition, high self-discharge and poor cell efficiency. Such drawbacks can be overcome by sound understanding on the cell reactions happening at the electrodes.

By conducting systematic studies on Fe anodes using cyclic voltammetry and a broad range of characterization tools, four distinct stages of Fe anode evolution were revealed: development, retention, fading and failure, where each stage is associated with very specific changes in the morphology and phase of Fe anodes. The particle fragmentation with the consequent gain in the surface area resulted in the increase in the Fe anode capacity during the initial cycles of deep charge/discharge. Most importantly, it was discovered that the irreversible formation of monocrystalline maghemites (γ-Fe₂O₃) with low reactivity is responsible for the eventual Fe anode capacity fading. We confirmed that the microstructure change takes over during early charge-discharge cycles of Fe anodes while their phase change is dominant at later stage which is the key for the rationale explaining the failure mechanism of Fe anodes in Ni-Fe batteries. Ultimately, our study correlates the changes in the electrochemical behavior of micro-scale Fe upon deep cycling in alkaline conditions with the changes in its morphology and phase.

* DOI: 10.1021/acsenergylett.8b00063