High Li storage capacity of poorly crystalline porous δ-MnO ₂ prepared by hydrothermal route

Poorly crystalline porous δ-MnO₂ is synthesized by hydrothermal route from a neutral aqueous solution of KMnO₄ at 180 C and the reaction time of 24 h. The as-synthesized sample and also the sample heated at 300 C have nanopetals morphology with large surface area. On heating at temperatures ≥400 C, there is a decrease in BET surface area and also a change in morphology from nanopetals to clusters of nanorods. Furthermore, the poorly crystalline δ-MnO₂ converts into well crystalline α-MnO₂ phase. The electrochemical lithium intercalation and de-intercalation studies in a non-aqueous electrolyte provide a high discharge specific capacity (275 mAh g^-1) at a specific current of 40 mA g ^-1 for the poorly crystalline δ-MnO₂ samples. The rate capability is also high. There is a decrease in capacity on repeated charge-discharge cycling. The specific capacity values of the crystalline α-MnO₂ samples are considerably less than the values of poorly crystalline δ-MnO₂ samples. Thus, the hydrothermal route facilitates preparation of poorly crystalline electrochemically active porous MnO₂.