Characterization of lithiated natural graphite before and after mild oxidation

Partial oxidation of natural graphite utilized in lithium ion batteries was found to increase its reversible capacity, while decreasing the irreversible capacity. Several chemically distinct Li sites in lithiated graphite were identified by solid state 7Li nuclear magnetic resonance (NMR): intercalated Li, and Li chemically bonded within the surface passivation layer or solid electrolyte interface (SEI). The partially oxidized graphite exhibited a third site, attributed to Li bonded to armchair, zigzag, or other edge sites in the carbon. In addition, the NMR signal from the SEI in the partially oxidized graphite is consistent with earlier work suggesting that oxidation lays the foundation for a chemically bonded SEI that is implicated in improved electrochemical performance. Electron paramagnetic resonance (EPR) signals observed in lithiated graphite are attributed to conduction electrons, as noted by other authors. EPR in unlithiated graphite, however, failed to detect a correlation between possible radical sites to which Li could bond and excess Li capacity in the partially oxidized graphite.