In situ Raman study of electrochemical lithium insertion into mesocarbon microbeads heat-treated at various temperature

In situ Raman spectra were measured to elucidate the electrochemical lithium insertion mechanism of mesocarbon microbeads (MCMBs) heat-treated at 700 to 2800 degrees C. The spectral changes of the Raman E(2g2) band of MCMBs heat-treated at 2800 degrees C showed that the lithium insertion mechanism into the microbeads is similar to that into graphite via the formation of staged graphite intercalation compounds, although a clear phase transition from dilute stage 1 to stage 4 was not observed. For MCMBs heat-treated at 1800 and 1000 degrees C the E(2g2) band shifted downward and upward upon charging and discharging, respectively. No discontinuous change suggesting the formation of staged phases was observed, which indicated that lithium is inserted randomly between graphene layers without the formation of staged phases. The charge and discharge profile of MCMBs heat-treated at 700 degrees C showed a large hysteresis. A potential plateau appeared at about 1 V on the discharge curve, which led to a high capacity of 710 mAh/g. In the case of MCMBs heat-treated at 700 degrees C, the peak wave number of the E(2g2) band did not shift al all in the whole potential range during charging and discharging. It was considered that the plateau region of MCMBs heat treated at 700 degrees C giving the high discharge capacity originates not from lithium species inserted between organized graphene layers, but from lithium doped into regions without organized graphitic structure.