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

Abstract

Doping and compositing with conductive frameworks are adopted to increase the capacity and stability of δ-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 δ-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 δ-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 ZnSO₄·3Zn(OH)₂·5H₂O, MnOOH and ZnMn₂O₄ detected during the discharged state and ZnMn₃O₇·3H₂O identified during the charged state.