Anion-guided interphase formation stabilizes reversible iron electrodeposition in aqueous electrolytes

The reversible electrodeposition of iron metal in aqueous electrolytes is a promising strategy for enabling cost-effective, large-scale aqueous rechargeable batteries. However, its practical viability is hindered by parasitic side reactions, particularly the hydrogen evolution reaction (HER), which lowers coulombic efficiency, and by the instability of deposited iron, leading to corrosion, capacity loss, and reduced cycle life. This study investigates the impact of three distinct ferrous-based electrolytes—sulfate (FeSO₄), chloride (FeCl₂), and trifluoromethane sulfonate (Fe(OTf)₂)—on the reversible deposition behavior and passivation dynamics of iron metal anodes. Surface analysis reveals that electrolyte composition critically influences passivation layer formation, directly affecting stability and efficiency. FeSO₄ and Fe(OTf)₂ generate compact, iron-oxide/hydroxide-rich films that suppress hydrogen evolution, resist corrosion, and deliver high coulombic efficiencies during cycling. Notably, Fe(OTf)₂ is especially effective at stabilizing the electrodeposited iron metal during long-term storage, exhibiting minimal self-discharge behavior. Conversely, FeCl₂ leads to inadequate passivation, resulting in lower efficiency of electrodeposition and rapid loss of plated iron due to self-discharge. While increasing current density and electrolyte concentration can reduce water activity and improve deposition efficiency through kinetic regulation, we show that a stable, hydrated solid-electrolyte interphase is crucial for long-term corrosion protection and the durability of iron anodes in aqueous batteries.