Hydrothermal synthesis of alpha-MnO2 and beta-MnO2 nanorods as high capacity cathode materials for sodium ion batteries

Two types of MnO2 polymorphs, alpha-MnO2 and beta-MnO2 nanorods, have been synthesized by a hydrothermal method. Their crystallographic phases, morphologies, and crystal structures were characterized by XRD, FESEM and TEM analysis. Different exposed crystal planes have been identified by TEM. The electrochemical properties of alpha-MnO2 and beta-MnO2 nanorods as cathode materials in Na-ion batteries were evaluated by galvanostatic charge/discharge testing. Both alpha-MnO2 and beta-MnO2 nanorods achieved high initial sodium ion storage capacities of 278 mA h g(-1) and 298 mA h g(-1), respectively. beta-MnO2 nanorods exhibited a better electrochemical performance such as good rate capability and cyclability than that of alpha-MnO2 nanorods, which could be ascribed to a more compact tunnel structure of beta-MnO2 nanorods. Furthermore, the one-dimensional architecture of nanorods could also contribute to facile sodium ion diffusion in the charge and discharge process.