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

Abstract

Two types of MnO₂ polymorphs, α-MnO₂ and β-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 α-MnO₂ and β-MnO₂ nanorods as cathode materials in Na-ion batteries were evaluated by galvanostatic charge/discharge testing. Both α-MnO₂ and β-MnO₂ nanorods achieved high initial sodium ion storage capacities of 278 mA h g⁻¹ and 298 mA h g⁻¹, respectively. β-MnO₂ nanorods exhibited a better electrochemical performance such as good rate capability and cyclability than that of α-MnO₂ nanorods, which could be ascribed to a more compact tunnel structure of β-MnO₂ nanorods. Furthermore, the one-dimensional architecture of nanorods could also contribute to facile sodium ion diffusion in the charge and discharge process.