7Li nuclear magnetic resonance studies of hard carbon and graphite/hard carbon hybrid anode for Li ion battery

The hard carbon is attractive for the Li ion battery because of its higher capacity than the theoretical value of 372 Ah kg−1 based on the composition of stage 1 Li-intercalated graphite, LiC6. However, since the Li-doping reaction occurs at the potential of around 0 V versus Li/Li+ reference electrode, it is often pointed out the possibility of Li metal deposition on the surface of anode. From the viewpoint of the safety, it may be a moot point. In the present study, 7Li NMR measurement was performed to estimate the degree of Li metal deposition on the surface of graphite and hard carbon anode. As a result, it is clarified that the Li metal deposition does not occur up to 110% over-discharge of the reversible capacity of hard carbon, whereas in the case of graphite anode, Li metal deposition occurred above 105% over-discharge of the capacity. From the 7Li NMR spectroscopy, the safety limit of hard carbon is rather superior to that of graphite.