Li-7-nuclear magnetic resonance observation of lithium insertion into mesocarbon microbeads

The stacking order of graphite layers in mesocarbon microbeads (MCMBs) heat-treated between 700 and 3000 degrees C was examined by analyses of x-ray diffraction measurements, and lithium insertion into the MCMBs has been observed using solid-state Li-7-nuclear magnetic resonance (Li-7-NMR) spectroscopy. In MCMBs heat-treated above 2000 degrees C, the fully lithiated MCMBs showed two bands at ca. 45 ppm (vs. LiCl) and ca. 27 ppm in their Li-7 NMR spectra. The profile of the band at 45 ppm was very close to that for the first-stage lithium graphite intercalation compound (Li-GIC), though the other band at 27 ppm could not be assigned to any phases of Li-GICs. From these results, it is suggested that the structures of the MCMBs heat-treated above 2000 degrees C for lithium insertion are classified as graphitic structure, which has the AB stacking order of graphite layers, and turbostratic structure with a random stacking sequence of graphite layers; the fully lithiated compositions of both structures were estimated as LiC6 and ca. Li0.2C6, respectively. Although MCMB heat-treated at 700 degrees C gave a higher capacity than LiC6, the line shift in the Li-7-NMR spectra indicated that lithium stored in the MCMB displayed an ionic character. Capacity change of the MCMBs during charge-discharge cycling up to 20 cycles and capacity loss at higher current densities (<200 mA g(-1)) were also examined.