Microelectrode techniques for in situ measurements on electrical conductance of a carbon particle and its composite film during electrochemical lithium insertion/extraction

Novel electrochemical techniques based on a filament-type and an array-type microelectrodes were developed to study in situ the conductance change of a carbon particle and its composite film during electrochemical lithium insertion/extraction reactions. Mesocarbon microbeads (MCMB, Osaka Gas Co.) heat treated at 1000 degrees C were investigated in an organic solution containing 1 M LiClO4 as the electrolyte. Measurements focusing on a single MCMB particle were achieved by attaching a molybdenum-filament microelectrode to the particle; they have shown that both the voltammogram and conductance profile of MCMB itself were stable in amplitudes and shapes for successive lithium insertion. Another kind of measurement using an array-type microelectrode was performed on a composite film consisting of MCMBs and poly(vinylidene fluoride), which is an actual form of MCMB in the use for lithium secondary batteries. The composite film was prepared on an interdigitated array of nickel microelectrodes to measure in situ its conductance change. It was found that the conductance of the composite film decreased rapidly, being accompanied by a decrease of redox capacity of the film. By considering the stable behavior of MCMB itself, we concluded that the electrical contact between MCMBs was broken due probably to the volume change of MCMB induced by the lithium insertion/extraction reactions. Addition of acetylene black to the composite greatly improved the interparticle connection. In addition to these practically important results, it is also suggested that there exist at least two different insertion sites within the MCMB. As demonstrated here, microelectrode-based techniques are unique and an effective approach to study battery active materials from both fundamental and practical standpoints.