Computational Study of the Origins of the Enhanced Li Storage Capacity and the Kinetic Behaviors of Li Ions on Reduced Graphene Oxide
Date Issued
2015
Date
2015
Author(s)
Lin, Kun-Han
Abstract
We employed first-principles density functional theory calculations to investigate the lithiation mechanism of the reduced graphene oxides (RGO) and explore the origins of their enhanced storage capacity for the anode of Li-ion batteries. Here we have investigated the Li storage and kinetic behaviors of various types of functional groups located at the edge as well as those lying on the basal plane within different levels of lithiation and functionalization of RGO. In the first part of the thesis, we investigated the effect of functional groups on the lithaition behaviors of graphene narrow ribbons (GNRs) in terms of the thermodynamic viewpoints. Our result shows that lithiation is almost unlikely to happen in pristine graphene and GNRs terminated with OH and H groups. For functional groups terminating the edge, only ketone and K-E pair can effectively enhance Li adsorption on GNRs, and the most favorable sites for Li adsorption turn out to be these edged-oxidized groups rather than the hollow sites on the basal plane. Furthermore, as the ketone-terminated GNRs were fully lithiated, the Li/O atomic ratio was found to be mostly around 1.0~1.25 depending on the concentration of ketone and types of edge, while that for the K-E pair was found to be 0.5 irrespective of the K-E pair concentration and the orientations of graphene edge. This indicates that these edge-oxidized functional groups can indeed enhance Li storage capacity of GNRs. As for the functional groups located on the basal plane, they can serve as the nucleation centers for Li clustering, thereby enhancing the Li storage capacity of GNRs. The Li/O atomic ratio was found to be 4 for the epoxy and 3 (2 for armchair GNR and graphene) for the hydroxyl group. These results indicate that the epoxy and hydroxyl groups on the basal plane tend to be more effective in enhancing the Li storage capacity than the edge functional groups. In the second part of the thesis, we studied the effect of functional groups on the kinetic and dynamic behaviors of the lithaition process in RGO. Our results show that as Li atom is not located in the vicinity of the edge functional groups, the migration energy barriers of Li on the basal plane are mostly comparable to that on pristine graphene. As a Li atom was located near the edge ketone /K-E pair, it was found to diffuse easily toward the edge sites and then adsorb onto the edge functional groups without any sizable energy barrier. As for the functional groups on the basal plane, their effect on the kinetics and dynamics of the lithiation process is much more complicated. The migration energy barrier for Li away from the epoxy/hydroxyl group is nearly identical to that on pristine graphene. However, as Li is in the vicinity of the epoxy/hydroxyl group, Li can diffuse readily towards the functional group and form Li-O pair/Li(OH) cluster on the basal plane without any sizeable energy barrier. Furthermore, these small clusters can grow bigger through the interaction with other Li atoms at the cost of a very low energy barrier. Very interestingly, theses Li clusters can undergo diffusion with a much smaller energy barrier than that for Li diffusion on pristine graphene. Accordingly, these LinO/Lin(OH) clusters are very likely to diffuse towards the neighboring sites of other nanoclusters and then coalesce into an even bigger one as evidenced in our ab initio MD simulations. On the other hand, our MD simulations also show that some of these nanoclusters containing OH groups may tend to desorb from the RGO surface, which can thus lead to the irreversible loss of the Li storage capacity for RGO.
Subjects
Li-ion battery
anode materials
reduced graphene oxide
first-principles calculations
kinetics behavior
Type
thesis
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