The effect of charge transfer upon Li- and Na-ion insertion in fine-grained graphitic material as probed by NMR.

We investigated the insertion/extraction behavior of Li and Na ions in graphitic materials using solid-state nuclear magnetic resonance (NMR). A unique advantage of high-degree 13 C isotope enrichment of graphitic material allowed sensitive and metastable graphite intercalation compounds to be measured in a short time. Ex situ 13 C magic angle spinning NMR spectra of 13C fine-grained graphite are presented as a function of state-of-charge. The observations are discussed with respect to graphite intercalation phenomena, which include the effects of charge transfer and the demagnetizing field. Dramatic narrowing of the 13 C NMR signal in metal-intercalated graphite evidences quasi-complete charge transfer occurring between lithium and graphite host material and resulting in reducing the macroscopic field effects. Upon Na insertion, incomplete charge transfer is observed and explained by inaccessibility of graphitic interlayer space for Na ions in our study. In addition, critical issues of reversibility of Li- and Na-ion electrochemical cells and solid electrolyte interphase (SEI) formation are considered on the atomic scale. The knowledge gained in the present work can be applied for advanced high-power-density electrode materials for safe and fast charging metal-ion batteries or for novel spintronic concepts with controlled spin-polarized charge carrier injection and transport combined with the possibility to manipulate magnetic anisotropy.