Abstract
To clarify the lithium storage states in disordered carbon materials for Li-ion secondary batteries 7 Li-nuclear magnetic resonance (NMR) spectra and cyclic voltammograms (CVs) of mesophase carbon fibers were measured. The CV obtained with mesophase carbon fibers fired at 800°C showed two peaks positioned in different potential regions. A sharp peak appearing at about 0.03-0.2 V vs. Li/Li + is attributed to lithium accommodation in the crystallites having a graphite structure and a broad one in the range of 0.2 to 1.2 V is attributed to the disorder site. In contrast, 7 Li-NMR spectra of the 800°C sample gave only a single peak, which shifted from 0 ppm at 0.8 V to 16 ppm at 30 mV depending on the charging level of lithium. Similar aspects of the spectra shift were also observed in a series of fully charged mesophase carbon fibers prepared at different firing temperatures. These results indicate that the rate of the exchange of lithium between the two different sites, the edge/surface site and intercalated site, is too fast to differentiate the different states. This suggests that there is a possibility of realizing a high capacity and high power anode for Li-ion secondary batteries based on a low-temperature mesophase carbon.
Published Version
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