Controllable Synthesis of Hollow Bipyramid β-MnO2 and Its High Electrochemical Performance for Lithium Storage

Three types of MnO2 nanostructures, viz., alpha-MnO2 nanotubes, hollow beta-MnO2 bipyramids, and solid beta-MnO2 bipyramids, have been synthesized via a simple template-free hydrothermal method. Cyclic voltammetry and galvanostatic charge/discharge measurements demonstrate that the hollow beta-MnO2 bipyramids exhibit the highest specific capacity and the best cyclability; the capacity retains 213 mAh at a current density of 100 mA g(-1) after 150 cycles. XRD patterns of the lithiated beta-MnO2 electrodes clearly show the expansion of lattice volume caused by lithiation, but the structure keeps stable during lithium insertion/extraction process. We suggest that the excellent performance for beta-MnO2 can be attributed to its unique electrochemical reaction, compact tunnel-structure and hollow architecture. The hollow architecture can accommodate the volume change during charge/discharge process and improve effective diffusion paths for both lithium ions and electrons.