Crowding Agent Stabilizes Aqueous Electrolyte for Reversible Iron Metal Anode

Aqueous batteries with Fe metal anodes are a promising technology for safe, low-cost stationary storage. Fe can deliver high gravimetric and volumetric capacities, but its commercial viability is hindered by competing side reactions and low Coulombic efficiency. This work explores the feasibility of employing polyethylene glycol (PEG) as a crowding agent in Fe electrolyte to suppress the competing hydrogen evolution reaction (HER) and improve Fe plating/stripping efficiency. We demonstrate that in a 1 molal Fe electrolyte, employing 60% PEG/40% H2O as the solvent yields higher Coulombic efficiency in Fe||Cu half cells and enables over two times the cycle life in Fe||Fe symmetric cells compared to the control composed of 100% H2O solvent. Further, we find that the 60% PEG electrolyte decreases H2 generation by almost 10 times during cycling. This electrolyte engineering approach also yields more crystalline Fe nuclei and denser growth during plating, leading to highly compact and reversible Fe anodes. Our results encourage further development of molecular crowding electrolytes for high efficiency Fe metal anodes.