Effect of Charge Transfer upon Li- and Na-Ion Insertion in Fine-Grained Graphitic Material as Probed by NMR.

We investigated the insertion-extraction behaviors of Li and Na ions in graphitic materials using solid-state 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 ¹³ C 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 formation are considered on the atomic scale. The knowledge gained in the present work can be applied to 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.