B NMR of boron-doped graphite as the negative electrode of a lithium secondary battery

Boron-doped graphite used as a negative electrode for a lithium rechargeable battery is known to have higher discharge capacity than undoped graphite. Herein, the graphites were mixed with 1, 2.5, 5, and 7 wt.% of boron carbide during the graphitizing process. The structural states of boron in those boron-doped graphites were successfully identified by solid-state ¹¹B NMR spectroscopy. For 1 wt.% sample, all boron atoms were at substitutional sites, as evident by the second-order quadrupole broadened ¹¹B NMR line having a quadrupole coupling constant, Q_CC=3.36(2) MHz. The NMR results show evidence of excess boron atoms recrystallizing as boron carbide during the graphitizing process which is in agreement with XRD data. Agreement of experimental results with computer simulated data indicate that the substitutional sites in boron-doped graphites were observed for the first time.