Synthesis of the Hollandite-type MnO<sub>2</sub> by Calcinating the Birnessite-type MnO<sub>2</sub> and its Electrochemical Properties as Electrodes for Rechargeable Lithium Batteries

The thermal property and heat-treatment products of the K-birnessite-type MnO2 were examined in the temperature range from 80 to 700℃ by means of TG-DTA, XRD, and TEM measurements. The birnessite phase was transformed gradually to the hollandite MnO2 phase on calcinating in the temperature range from 200 to 500℃, resulting in the formation of a single phase of high crystalline hollandite having a (2ラ2) tunnel structure at 550-600℃. The XRD and high-resolution TEM measurements have revealed that the nano-composite products consisting of the birnessite with scale-like crystals and the hollandite with needle-like crystals are formed at 300-400℃ in the course of calcinating the birnessite oxide. The electrochemical properties of the birnessite and its calcined products, including the discharge and cycling characteristics, were examined as cathodes for rechargeable lithium batteries. The birnessite/hollandite composite formed at around 300℃ showed a S-type smooth discharge curve with an average potential of 2.6 V vs. Li/LI+ and better performances with the initial discharge capacity of 210 mAh/g and the cycling capacity of about 170-200 mAh/g during 10 cyclings.