Ramsdellite-MnO₂ Regeneration via Acid-Mediated Redox Tuning toward Rechargeable Aqueous Zinc-Ion Batteries

The mounting accumulation of spent alkaline batteries (SABs) elicits concerns over both environmental threats and the recycling industry's profitability, closely tied to the chemical reactions in manganese-based waste treatment. Herein, we design an acid-modulated phase-reconstruction strategy for sustainable recovery of manganese oxides from SABs, where moderate proton participation facilitates the preformation of MnOOH intermediates before the initial transformation to <i>ramsdellite</i>-MnO₂ (RM-<i>R,</i> orthorhombic) and the final conversion to <i>pyrolusite</i>-MnO₂ (RM-β<i>,</i> tetragonal) nanomaterials. This rarely reported metastable RM-<i>R</i> phase features a unique tunneled framework (1 × 2 edge-shared MnO₆ octahedra) enabling reversible H⁺/Zn²⁺ (de)intercalation, though its traditional synthesis remains challenging due to thermodynamic instability. First-principles calculations reveal that RM-<i>R</i> possesses lower Zn²⁺ diffusion barriers (0.44 eV) than RM-β (0.99 eV), consistent with its superior Zn²⁺ storage performance. Moreover, the higher specific surface area enables the RM-<i>R</i> cathode with a battery-supercapacitor hybrid storage behavior, which delivers remarkable capacity (214.9 mA h g^-1 at 0.1 A g^-1) and long cycling stability (98% retention after 1000 cycles), outperforming most reported MnO₂-based cathodes. This low-acid regeneration protocol (4 mL of HCl/1.85 g of waste) paves a sustainable way for closed-loop battery recycling and clarifies the structure-property relationships in metastable manganese oxides.