Preparation of β-MnO2nanocrystal/acetylene black composites for lithium batteries

Abstract

Composites consisting of β-MnO₂ nanocrystals and acetylene black were synthesized by thermal decomposition of Mn(NO₃)₂ with acetylene black. The effects of heating temperature and specific surface area (S_BET) of the starting acetylene black on the formation and properties of the composite were investigated. The decomposition of Mn(NO₃)₂ mixed with acetylene black progressed at a lower temperature than that of Mn(NO₃)₂ alone. The formation and the size of β-MnO₂ nanocrystals depended strongly on the heating temperature (T) and S_BET of the starting acetylene black. The β-MnO₂/acetylene black composite could be produced for acetylene black of S_BET = 60 m² g⁻¹ at 160 °C ≤ T ≤ 320 °C and S_BET = 133 m² g⁻¹ at 160 °C ≤ T ≤ 300 °C. However, the composite could not be obtained for acetylene black with larger specific surface area (S_BET = 300 m² g⁻¹). The size of β-MnO₂ nanocrystals decreases and their dispersion in the composites increases with an increase in the S_BET or the heating temperature. The lithium insertion behavior and voltage feature in the first discharge process depend strongly on the size of the β-MnO₂ nanocrystals as well as their dispersion. The electrochemical lithium insertion progressed topotactically for well-dispersed β-MnO₂ nanocrystals, retaining the framework of their rutile structure to permit a large amount of lithium insertion (Li/Mn = 1.15 in the solid). The charge/discharge curves showed a flat voltage plateau around 2.8 V and a stable cycling feature up to the 20th cycle. It is likely that the easy lattice expansion of β-MnO₂ nanocrystals along the a crystal axis plays an important role in the topotactic lithium insertion/extraction reactions.