Neutron scattering studies of disordered carbon anodematerials

Abstract

Carbon-based anodes show many promising properties in lithium-ionrechargeable batteries. So-called `disordered carbons' arecharacterized by a substantial amount of residual hydrogen, andexhibit large Li uptake capacities. We have employed a variety ofneutron scattering techniques, coupled with computer simulations, tostudy the composition, local atomic structure, and vibrationaldynamics of such materials. Radial distribution function analysisof neutron diffraction data, and incoherent inelastic scatteringshow that the structural motif is a planar graphene fragment, withedge carbons terminated by single hydrogen atoms, and randomstacking between fragments. The vibrational spectra of thehydrogen-rich carbons are remarkably similar to the spectra of thepolycyclic aromatic hydrocarbon coronene in the medium-frequencyregion. At low frequencies, only a boson peak is observed,characteristic for glassy and disordered materials, and this featureshifts upon doping. The results are consistent with two proposedmechanisms for Li capacity, one analogous to conventionalintercalation but with Li on both sides of graphene fragments, theother involving bonding of Li to H-terminated edge carbons.