Entropy-driven structure and dynamics in carbon nanocrystallites

New carbon composite materials are being developed that contain carbon nanocrystallites in the range of 5–17 A in radius dispersed within an amorphous carbon matrix. Evaluating the applicability of these materials for use in battery electrodes requires a molecular-level understanding of the thermodynamic, structural, and dynamic properties of the nanocrystallites. Herein, molecular dynamics simulations reveal the molecular-level mechanisms for such experimental observations as the increased spacing between carbon planes in nanocrystallites as a function of decreasing crystallite size. As the width of this spacing impacts Li-ion capacity, an explanation of the origin of this distance is relevant to understanding anode performance. It is thus shown that the structural configuration of these crystallites is a function of entropy. The magnitude of out-of-plane ripples, binding energy between layers, and frequency of characteristic planar modes are reported over a range of nanocrystallite sizes and temperatures. This fundamental information for layered carbon nanocrystallites may be used to explain enhanced lithium ion diffusion within the carbon composites.