Determination of Diffusion Rate and Accommodation State of Li in Mesophase Carbon for Anode Materials by NMR Spectroscopy

Abstract

Diffusion coefficients of Li in mesophase carbon of anode materials, independent of the interfacial transport effect, were directly measured first using the pulsed gradient spin echo NMR method. Disordered carbons fired at temperatures lower than 1500 °C consist of two phases: one is a crystalline phase comprising a number of roughly oriented microcrystalline of graphene layers; the other is a disordered phase comprising a number of cavities caused by mismatching space and edge regions among crystalline phases. A single component was shown by NMR spectra and the diffusion coefficient of Li in disordered carbon at 100 °C, indicating that Li accommodated in the phases can move between each other rapidly compared with measuring NMR frequency at that temperature. The observed diffusion coefficient increased with the increase in charged Li content in disordered carbons, indicating that Li is first accommodated into the disordered phase; then, with increasing charging level, Li would dominate the ordered phase in which Li diffusivity is more rapid than that in the disordered phase. The observed diffusion value of Li of ca. 3 x 10 - 7 cm 2 s - 1 confirmed the feasibility of disordered carbons for anode materials, which have large Li storage capacity, by improvement of interfacial reaction sites with electrolyte materials.