Engineering of graphene layer orientation to attain high rate capability and anisotropic properties in li-ion battery electrodes

Abstract

Novel carbon films with different graphene layer orientations are investigated as electrode materials for Li-ion batteries. It is demonstrated that engineering the crystallographic orientation with graphene layers oriented perpendicular to the surface substantially alters stress evolution during Li insertion. With this crystallographic orientation the intercalating/de- intercalating Li-ions also have direct access to the graphene interlayer spaces, resulting in higher capacity at faster electrochemical cycling, compared to carbon films with graphene layers parallel to the film surface. Electrodes with perpendicular alignment are prepared by supramolecular synthesis using either spin coating or bar coating of chromonic liquid crystal precursors into precursor organic films followed by in situ carbonization. These materials are compared with in situ stress measurements during lithiation/delithiation cycles, and the bar-coated films exhibit a highly anisotropic stress which is consistent with long-range alignment of the graphene layers. In contrast, the in-plane stresses in the spin-coated films are isotropic, which is consistent with the presence of randomly oriented domains (still with graphene layers oriented perpendicular to the surface). Overall, the use of thin film graphitic materials with controlled crystallographic orientations provides a valuable platform for investigating the impact of graphene structure on the properties of Li-ion battery electrode materials. Engineering of graphene layer orientations to obtain vertically aligned graphene layer based thin-film electrodes (a-axis oriented graphitic carbon film) for Li-ion batteries leads to considerably improved specific capacity at higher electrochemical cycling rates (improved rate capability), as compared to the more usual c-axis oriented graphitic carbon films. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.