Lithiation mechanisms and lithium storage capacity of reduced graphene oxide nanoribbons: a first-principles study

We employed first-principles calculations to investigate the lithiation mechanisms of functionalized graphene nanoribbons (GNRs) and to examine the effect of various functional groups on the electrochemical performance of graphene-based nanomaterials. In this work, we have extensively explored the Li storage behaviors of various types of functional groups located on the basal plane and those terminating the edge sites within different levels of lithiation and functionalization on GNRs. For functional groups terminating the edge sites, only ketone and its related derivatives (pyrone/quinone) can effectively enhance Li adsorption on GNRs, and the most favorable sites for Li adsorption turn out to be these edge-oxidized groups rather than the hollow sites on the basal plane. In addition, as the ketone-terminated GNRs were fully lithiated, the Li/O atomic ratio was found to be similar to 1.0 and that for the ketone-ether pair (pyrone) was similar to 0.5, indicating that these edge-oxidized groups can effectively enhance the Li capacity of GNRs as compared with that of graphite (Li1/6C). As regards the in-plane functional groups, the epoxy and hydroxyl groups were shown to have multiple Li uptakes on the basal plane and appeared to serve as the nucleation centers for Li clustering, thereby resulting in the great enhancement of the Li capacity of GNRs. Our calculations showed that the achievable Li/O atomic ratio was 4 for the epoxy group (Li4O pyramid cluster) and 3 for the hydroxyl group (Li-3(OH) cluster), which suggests that these in-plane functional groups can be more effective in enhancing the Li storage capacity than those terminating the edge sites of graphene-based nanomaterials.