Enhancing Mg2+ intercalation in MnO2/carbon composites via vanadium doping and oxygen vacancies for high-performance aqueous magnesium-ion batteries

Abstract

MnO₂ is a promising cathode material for rechargeable aqueous magnesium ion batteries (AMIBs) due to its high specific capacity and high discharge voltage. However, MnO₂ suffers from structural instability and slow diffusion kinetics of Mg²⁺, which limit its cycle life and rate performance. In this study, a vanadium (V)-doped, oxygen vacancy (O_vac)-rich birnessite-type MnO₂ loaded on a conductive three-dimensional (3D) carbon network composed of carbon nanotubes (CNTs) and reduced graphene oxide (rGO), referred to as V-O_vac-MnO₂/CG, was designed to enhance the kinetic of electrochemical reaction. For V-O_vac-MnO₂/CG, V doping and O_vac greatly reduce the Mg²⁺ insertion/extraction energy barrier, provide abundant active sites, and improve its electrical conductivity. As a result, the V-O_vac-MnO₂/CG cathode exhibits excellent electrochemical properties, achieving a specific capacity up to 398 mAh g⁻¹ at a current density of 0.1 A g⁻¹. At the same time, it exhibits excellent rate capability, providing a capacity of 143 mAh g⁻¹ even at 2 A g⁻¹. In addition, after 500 cycles at a current density of 0.2 A g⁻¹, the capacity retention rate of V-O_vac-MnO₂/CG is still as high as 81 %. These properties make V-O_vac-MnO₂/CG a highly promising cathode material in the field of AMIBs.