7Li ‐Nuclear Magnetic Resonance Observation of Lithium Insertion into Mesocarbon Microbeads

Abstract

The stacking order of graphite layers in mesocarbon microbeads (MCMBs) heat‐treated between 700 and 3000°C was examined by analyses of x‐ray diffraction measurements, and lithium insertion into the MCMBs has been observed using solid‐state ‐nuclear magnetic resonance (‐NMR) spectroscopy. In MCMBs heat‐treated above 2000°C, the fully lithiated MCMBs showed two bands at ca. 45 ppm (vs. LiCl) and ca. 27 ppm in their ‐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°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 and ca. , respectively. Although MCMB heat‐treated at 700°C gave a higher capacity than , the line shift in the ‐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 were also examined.