Manganese vacancies and tunnel pillars synergistically improve the electrochemical performance of MnO₂ in aqueous Zn ion batteries

High-oxidation niobium was used for the first time in manganese dioxide cation doping to reduce the diffusion resistance of zinc ions, in order to improve its kinetic and electrochemical properties. The results show that using a simple hydrothermal process, all niobium ions were doped into the manganese dioxide lattice. As niobium(v) was incorporated into the [2 x 2] tunnel of alpha-MnO2, it induced manganese vacancies, which reduced the diffusion resistance of Zn2+ in manganese dioxide, improving the migration kinetics. It acted as a tunnel pillar, avoiding the collapse of the tunnel structure during the repeated insertion/extraction of the Zn2+ process, and prevented a rapid degradation of the cycling performance. In particular, the sample with the Nb/Mn molar ratio of 0.003 exhibited the best kinetic reversibility and rate performance. After 400 cycles at 1C, the capacity retention of Nb-doped MnO2 significantly increased to 89%, which was only 55% for the undoped sample. Meanwhile, at a power density of 400 W kg(-1), it presented the highest energy density of 765 W h kg(-1) due to the existing doping of metal ions.