Todorokite-type MnO2 as a zinc-ion intercalating material

The structural and electrochemical properties of manganese dioxide with 3 × 3 tunnels bearing the crystallographic structure of the mineral todorokite were investigated for use as a new cathode material for zinc ion cells. The large 3 × 3 tunnel of todorokite facilitates fast Zn2+ diffusion to the inner region of a particle compared to the conventional medium-sized 2 × 2 tunnels of cryptomelane. Todorokite was synthesized by the hydrothermal treatment of layered Na-birnessite in a concentrated Mg-containing solution according to a known method. In electrochemical studies, a promising discharge capacity of 108 mA h/g at C/2 and a good rate performance were observed in the potential range of 0.7 V-2.0 V. Structural and morphological analyses of the discharged and charged electrodes indicate that a reversible intercalation of zinc ions into the tunnel occurs during the discharge-charge process.