Fe³⁺-driven tunnel engineering for stabilizing metastable ramsdellite MnO₂ in high-performance zinc-ion batteries

Ramsdellite MnO2 (R -MnO2), with its expanded (1 × 2) tunnels, offers superior Zn2+ diffusion kinetics for aqueous zinc-ion batteries but suffers from metastability-induced phase collapse. Herein, Fe3+ doping is demonstrated as a critical strategy to thermodynamically stabilize R -MnO2 while optimizing its electrochemical functionality. Through a synergistic H+/Fe3+ hydrothermal process, spent ZnMn2O4 from alkaline batteries is converted into orthorhombic R -FexMn1-xO2 nanocrystals. Fe3+ incorporation enlarges the tunnel structure, reduces surface energy, and mitigates Jahn-Teller distortion by increasing the Mn4+/Mn3+ ratio. This yields a high specific surface area, enhanced ion diffusion kinetics, and exceptional cycling stability. The R -FexMn1-xO2 cathode achieves a 286.8 mAh g-1 capacity at 0.1 A g-1, outperforming β -MnO2 (30.9 mAh g-1 at 1.5 A g-1). This work establishes Fe3+ doping as an essential mechanism for stabilizing high-performance metastable cathodes, enabling sustainable upcycling of battery waste.