NiSO₄-Driven In Situ Alloy Formation To Unlock Highly Reversible Iron Electrochemistry in Aqueous Batteries

Grid-scale stationary energy storage requires technologies that are both safe and economically viable. Iron (Fe) metal-based aqueous batteries offer an attractive option owing to the abundance, low cost, and environmental benignity of iron, but their development has been hampered by uncontrolled hydrogen evolution and poor reversibility of iron plating and stripping. Here, we report using nickel sulfate (NiSO₄) as an electrolyte additive to induce the in situ formation of a FeNi₃ alloy interphase during early cycling. This alloy lowers the Fe nucleation barrier and promotes uniform iron deposition. Moreover, dynamic codeposition and stripping of Ni with Fe sustains fast reaction kinetics and stabilizes the alloy interphase during long-term cycling. As a result, Fe||Fe symmetric cells achieve over 3000 h of stable cycling, nearly an order of magnitude improvement over the baseline electrolyte. Fe||Cu cells with NiSO₄ additives enable stable long-term cycling with a high average Coulombic efficiency (CE) of ∼99.4%, while the control electrolyte rapidly fails with an average CE of 82.9%. These findings demonstrate that functional electrolyte additives and the controlled alloying interphase provide a viable pathway to high-performance, cost-effective iron metal-based aqueous batteries for large-scale energy storage.