Capacity degradation analysis of the rechargeable iron ion batteries using post-mortem analysis and the impedance spectroscopy

We fabricated rechargeable iron ion batteries (RIIBs) using mild steel (MS) as a negative electrode, vanadium penta oxide as a positive electrode, and ferrous perchlorate hydrate in tetra-ethylene glycol dimethyl ether (TGDME)-based solvent under ambient conditions without any inert atmosphere and investigated the performance degradation. The cell exhibits ~ 120 mAh g−1 specific capacity at a 33 mAg−1 current density. The initial capacity fades to about 41% in 30 cycles. The cell was disassembled, and post-mortem characterizations were carried out for both electrodes. The performance degradation is attributed to the corrosion of the mild steel and the formation of multiphase iron–vanadium-based oxides at the cathode end, reducing the iron ion concentration. Thus, the present studies provide a microscopic understanding of capacity fading in iron ion batteries and may assist in designing suitable electrode materials for the improved electrochemical response.