Scalable and Effectual Preparation of MnO₂ from Spent Alkaline Batteries for Zn-Ion Batteries

Recycling Mn compounds from spent alkaline batteries (SABs) not only follows the ongoing trend of circular economy but also conserves nature without disposal of hazardous battery wastes. Both pyrometallurgical and hydrometallurgical techniques were included to aim for a low-cost, environmental-contaminant-minimized, and mass-production process. The SAB-electrode powder mainly consisted of Mn(III) compounds by X-ray diffraction (XRD), together with ca. 6 wt % carbon by X-ray fluorescence (XRF). Heat treatment under N₂ gas was executed on the powder at a temperature from 650 to 950 °C for 2 h. At 950 °C, XRD revealed only crystalline MnO, yet other components were amorphous/nanocrystalline C- and Zn-based substances from XRF. Leaching the heated SAB powder was tried with stoichiometric volume +20 vol % excess of 2 M H₂SO₄ to receive MnSO₄ solution-the best leaching efficiency of 98%. The leachate was well-mixed with the stoichiometric KMnO₄ mass at 30, 60, or 90 °C for 1 h, giving the 99% yield SAB-MnO₂. SAB-MnO₂ was electrochemically tested (rate and long-cycling tests) as a Zn-ion battery (ZIB). Both tests show promising results: especially for the long cycling at 5 mA/g, good capacity retention of the ZIB using SAB-MnO₂ was recognized when the ZIB with commercial MnO₂ was a reference.