Controllable synthesis of hollow bipyramid β-MnO(2) and its high electrochemical performance for lithium storage

Three types of MnO2 nanostructures, viz., α-MnO2 nanotubes, hollow β-MnO2 bipyramids, and solid β-MnO2 bipyramids, have been synthesized via a simple template-free hydrothermal method. Cyclic voltammetry and galvanostatic charge/discharge measurements demonstrate that the hollow β-MnO2 bipyramids exhibit the highest specific capacity and the best cyclability; the capacity retains 213 mAh g(-1) at a current density of 100 mA g(-1) after 150 cycles. XRD patterns of the lithiated β-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 β-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.

Nitrogen-doped porous carbon nanofiber webs as anodes for lithium ion batteries with a superhigh capacity and rate capability

Nitrogen-doped carbon nanofiber webs (CNFWs) with high surface areas are successfully prepared by carbonization-activation of polypyrrole nanofiber webs with KOH. The as-obtained CNFWs exhibit a superhigh reversible capacity of 943 mAh g(-1) at a current density of 2 A g(-1) even after 600 cycles, which is ascribed to the novel porous nanostructure and high-level nitrogen doping.