Preparation of β-MnO2 nanocrystal/acetylene black composites for lithium batteries

Composites consisting of beta-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 beta-MnO2 nanocrystals depended strongly on the heating temperature (T) and S-BET of the starting acetylene black. The beta-MnO2/acetylene black composite could be produced for acetylene black of S-BET = 60 m(2) g(-1) at 160degreesC less than or equal to T less than or equal to 320degreesC and S-BET = 133 m(2) g(-1) at 160degreesC less than or equal to T less than or equal to 300degreesC. However, the composite could not be obtained for acetylene black with larger specific surface area (S-BET = 300 m(2) g(-1)). The size of beta-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 beta-MnO2 nanocrystals as well as their dispersion. The electrochemical lithium insertion progressed topotactically for well-dispersed beta-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 beta-MnO2 nanocrystals along the a crystal axis plays an important role in the topotactic lithium insertion/extraction reactions.