title: Li-Ion Localization and Energetics as a Function of Anode Structure.
abstract: In this work, we study the effect of carbon composite anode structure on the localization and energetics of Li-ions. A computational molecular dynamics study is combined with experimental results from neutron scattering experiments to understand the effect of composite density, crystallite size, volume fraction of crystalline carbon, and ion loading on the nature of ion storage in novel, lignin-derived composite materials. In a recent work, we demonstrated that these carbon composites display a fundamentally different mechanism for Li-ion storage than traditional graphitic anodes. The edges of the crystalline and amorphous fragments of aromatic carbon that exist in these composites are terminated by hydrogen atoms, which play a crucial role in adsorption. In this work, we demonstrate how differences in composite structure due to changes in the processing conditions alter the type and extent of the interface between the amorphous and crystalline domains, thus impacting the nature of Li-ion storage. The effects of structural properties are evaluated using a suite of pair distribution functions as well as an original technique to extract archetypal structures, in the form of three-dimensional atomic density distributions, from highly disordered systems. The energetics of Li-ion binding are understood by relating changes in the energy and charge distributions to changes in structural properties. The distribution of Li-ion energies reveals that some structures lead to greater chemisorption, while others have greater physisorption. Carbon composites with a high volume fraction of small crystallites demonstrate the highest ion storage capacity because of the high interfacial area between the crystalline and amorphous domains. At these interfaces, stable H atoms, terminating the graphitic crystallites, provide favorable sites for reversible Li adsorption.
authors.affiliation: Department of Chemical and Biomolecular Engineering, University of Tennessee , Knoxville, Tennessee 37996, United States.
authors.forename: Nicholas W
authors.identifier: http://orcid.org/0000-0002-0372-0092
authors.initials: NW
authors.lastname: McNutt
authors.affiliation: Department of Chemical and Biomolecular Engineering, University of Tennessee , Knoxville, Tennessee 37996, United States.
authors.forename: Marshall
authors.initials: M
authors.lastname: McDonnell
authors.affiliation: Materials Science and Technology Division, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37830-8026, United States.
authors.forename: Orlando
authors.initials: O
authors.lastname: Rios
authors.affiliation: Department of Materials Science and Engineering, University of Tennessee , Knoxville, Tennessee 37996, United States.
authors.forename: David J
authors.identifier: http://orcid.org/0000-0002-6246-0286
authors.initials: DJ
authors.lastname: Keffer
keywords: Li-ion; anode; battery; carbon; composite; energetics; molecular dynamics; neutron diffraction