Cobalt-doped δ-MnO2/CNT composites as cathode material for aqueous zinc-ion batteries

Doping and compositing with conductive frameworks are adopted to increase the capacity and stability of & delta;-MnO2 used as electrode material for zinc-ion batteries (ZIB). Co-doped MnO2/carbon nanotubes (CNTs) are prepared by hydrothermally reacting MnO4- and Mn2+ in the presence of Co2+, with acidified CNTs as the framework for crystallization. Characterization studies indicate that the doping with Co2+ does not alter the morphology of the & delta;-MnO2, but increased interplanar lattice spacing is achieved. A ZIB with Co-MnO2/CNTs can deliver a high specific capacity of 365 mA h g(-1) at a current density of 0.2 A g(-1), and only 5.4% of specific capacity is lost after 1500 charging and discharging cycles at 1 A g(-1), which is better than ZIBs with & delta;-MnO2 and Co-MnO2. For ZIBs with Co-MnO2/CNTs, diffusion-controlled processes dominate. The enhanced capacity and stability originate from the increased specific surface area induced by the CNT framework, decreased charge transfer resistances, and increased ion diffusion kinetics. The insertion and extraction of both H+ and Zn2+ are involved for charge storage, with ZnSO4 & BULL;3Zn(OH)(2)& BULL;5H(2)O, MnOOH and ZnMn2O4 detected during the discharged state and ZnMn3O7 & BULL;3H(2)O identified during the charged state.