<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mmultiscripts><mml:mi mathvariant="normal">C</mml:mi><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>13</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:math> NMR study of the electronic structure of lithiated graphite
Dylan C. Maxwell, Christopher A. O’Keefe, Chao Xu, Clare P. Grey
Abstract
The change in the electronic structure of graphite during electrochemical lithiation was investigated using solid-state NMR. The $^{13}\mathrm{C}$ NMR peak positions for the lithiated graphite compounds stages 3L - 1 were assigned. The dense stage ${\mathrm{LiC}}_{12}$ and ${\mathrm{LiC}}_{6}$ compounds were shown via $^{13}\mathrm{C}$ and $^{7}\mathrm{Li}$ to be metallic in nature, with Korringa relaxation being the dominant relaxation mechanism for both nuclei. The $^{7}\mathrm{Li}$ NMR shifts in graphite can be described by a direct Fermi-contact Knight shift, while the $^{13}\mathrm{C}$ NMR shifts can be described by the spin-dipolar Knight shift and isotropic Knight shift. The isotropic $^{13}\mathrm{C}$ NMR Knight shift of ${\mathrm{LiC}}_{6}$ was smaller than expected for a Knight shift caused purely by the core polarization of $1s$ orbitals. This showed that there must be a contribution of $2s$ orbitals at the Fermi level. The density of states at the Fermi level $\ensuremath{\rho}({E}_{F})$ for ${\mathrm{LiC}}_{6}$ was estimated and the values, which are dependent on the Stoner enhancement factor $\ensuremath{\alpha}$, agree reasonably well with those found in literature.