7Li nuclear magnetic resonance study of lithium insertion in pristine and partially oxidized graphite

Pristine and partially oxidized graphite were lithiated electrochemically and investigated by solid state 7Li nuclear magnetic resonance (NMR) methods. Both untreated and partially oxidized graphite exhibit three kinds of reversible Li sites, one, at an average position of 43 ppm for both stage 1 and 2 intercalation compounds (LiC6 and LiC12) at intermediate and higher lithiation levels, a stage 4 intercalation compound at 3 ppm (LiC24), at low Li content, and Li associated with both edge sites and possibly stage 2L at intermediate Li content (at 11–13 ppm). The edge site population is enhanced in the partially oxidized graphite. Both treated and untreated graphite undergo reconstructive transitions in which the edge site lithium transforms into stage 1 intercalated lithium at higher levels of lithiation. Changes in both the NMR spectra and in the electrochemical parameters of stages 1, 2 and 4 have been noticed in the first two charge–discharge cycles. We attribute the changes in the NMR spectra and in the electrochemical parameters to graphite structural changes taking place in the first few cycles.