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-Fe_xMn_1-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-Fe_xMn_1-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.