Characterization of Iron Anodes for Rechargeable Aqueous Batteries

Metallic iron is an attractive anode material for aqueous batteries, particularly if the full 3-electron redox between Fe and Fe (III) can be accessed reversibly. However, oxidation of Fe (II) to Fe (III) causes drastic morphology change and irreversible formation of highly resistive phases. Yet, it remains unclear how these phases form at the nanoscale and, crucially, how they might be avoided. To achieve fully reversible Fe anodes, it is critical to obtain a mechanistic understanding of nanoscale morphology and phase evolution during electrode cycling. Here we investigated the electrochemical transformation pathways between Fe (II) and Fe (III) oxides using both ex-situ and operando techniques, including SEM, Raman, AFM, Infrared Nanospectroscopy (nano-FTIR) and Scanning Transmission X-ray Microscopy (STXM). Correlating the morphology evolution during this transformation to local Fe oxidation state and phase would offer fundamental insight into Fe (II)/(III) conversion and inspire novel engineering of the Fe anode to achieve higher capacity and cyclability.