Lithium insertion in hard carbon as observed by 7Li NMR and XRD. The local and mesoscopic order and their relevance for lithium storage and diffusion

Abstract

We investigate hard carbon fibers in different states of charge by a combination of ⁷Li-NMR and 2D-XRD. In particular, we record the quadrupole-split ⁷Li-NMR spectra and ⁷Li longitudinal relaxation over a wide temperature range, and determine lithium self-diffusion both parallel and perpendicular to the fiber axis. Recording the temperature dependence permits us to interpret the presence of motional averaging of spin couplings for mobile Li. The joint analysis shows that at low Li content, Li occupies sites that lack ordered coordination and delocalized electrons and are collected in disordered spatial domains. Upon increasing the Li content, ordered sites collected in ordered domains become populated. Both disordered and ordered domains have a high inherent heterogeneity with a typical spatial extension of a few nanometers. The disordered domains exhibit a continuous topology that permits unhindered diffusion within it. At high Li content we also observe the presence of very small (∼nm) particles of metallic lithium. The joint analysis of XRD in combination with diffusion anisotropy, and anisotropy from the ⁷Li-NMR spectrum (with samples oriented differently with regard to the applied magnetic field), shows that the mesoscopic structure is made by ordered domains arranged in a cylindrically rolled-up manner with the mesoscopic axis parallel to the fiber axis.