Understanding Binding of Chitosan to Graphene in Li–Ion Battery Anodes from First-Principles

As a water-soluble, biodegradable, and abundant biopolymer, chitosan presents great advantages over the common PVDF as a possible binder to be used in Li-ion battery graphite anodes. Using accurate density functional theory, which includes a van der Waals long-range energy functional, we have determined that chitosan molecules physisorb on graphene/graphite and orient themselves horizontally, exposing the hydrogen atoms and the amino group to the surface. The binding energy is less than half that of PVDF. The binding is accompanied by the transfer of 0.021 e from graphene to chitosan and is dominated by the van der Waals long-range interactions. We have then investigated how the functionalization of graphene using point defects, oxygen atoms, and OH and LiF molecules as adsorbates affects the binding properties of chitosan. We have found that the presence of carbon vacancies and the functionalization with the OH and LiF molecular adsorbates are effective at increasing the binding. Room temperature significantly increases the chitosan binding energy on graphene functionalized with atomic oxygen or OH groups. The interaction with charged ions in low and high pH environments strongly increases chitosan adhesion to graphene, to which the charge is transferred. The changes of the binding properties of chitosan to the different surfaces are monitored through the analysis of the electronic charge redistribution, Bader charges, and changes in the electronic structure. Our results constitute a baseline study for further investigations of the interaction between the eco-friendly polymeric chitosan and the active material surfaces of electrodes in energy devices.