Preparation of β-MnO ₂ nanocrystal/acetylene black composites for lithium batteries

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 (SBET) 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 SBET of the starting acetylene black. The β-MnO2/acetylene black composite could be produced for acetylene black of SBET = 60 m2 g−1 at 160 °C ≤ T ≤ 320 °C and SBET = 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 (SBET = 300 m2 g−1). The size of β-MnO2 nanocrystals decreases and their dispersion in the composites increases with an increase in the SBET 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.