The Mechanisms of Sodium Storage and the Kinetics Behaviors of Na ions on Reduced Graphene Oxide- A First Principles Study
Date Issued
2016
Date
2016
Author(s)
Lin, Yu-Chieh
Abstract
Sodium ion batteries (SIBs) are attractive alternatives of LIBs due to their chemical similarity and the natural abundance of Na resources. Although the energy density of SIBs is lower than LIBs, it is still a promising candidate for use in large-scale applications considering their low cost. However, Na ions cannot intercalate into graphite, the anodes of most commercial LIBs, for any appreciable extent, and most of the capacity turns out to be electrochemically irreversible. To further find appreciate materials for anodes on SIBs, great efforts have been made. In this study, we employed first-principles density functional theory calculations to investigate the sodiation process and the Na storage mechanism of reduced graphene oxide (RGO). Here we have applied various types of functional groups, which are located at edge and attached to basal plane, to investigate the Na storage and kinetic behaviors. There are two main parts in this thesis: In the first part of the thesis, we studied the effect of functional groups on the sodiation behaviors of nanoribbons (GNRs) in thermodynamic viewpoints. Our calculated results show that the adsorption is usually stronger for zigzag than armchair under the same edge groups, due to large number of states located at conduction band bottom. Furthermore, sodiation is almost unlikely to occur on pristine graphene and the GNRs terminated with OH and H groups. Only GNRs terminated with ketone, K-E pair, carboxylic acid and cyclic ether can effectively enhance the Na storage capacity. During the sodiation process, these edge-oxidized groups are always the most favorable sites for Na adsorption rather than the hollow sites on the basal plane. And most functional groups can physically adsorb Na except of cyclic ether, which was found to occur via a reductive ring-opening process instead of the general physical adsorption. The results of sodiation show that the Na/O atomic ratio for ketone-terminated GNRs is found to be around 1.0~2.0 depending on concentration of ketone and types of edge, while that for K-E pair is found to be around 0.25~0.5. This indicates that the ether group in K-E pair has no appreciable effect on Na storage enhancement. However, the Na/O atomic ratio for cyclic ether is found to be around 0.25~0.33, thereby, the capacity enhancement of cyclic ethers is only available when they are aggregated at the graphene edges. As for the in-plane functional groups, they were found to serve as the nucleation centers for Na clustering during the sodiation process, which can thus enhance the Na storage capacity of GNRs. And the Na/O atomic ratio for epoxy and hydroxyl is found to be 4 and 2 respectively. Compared the Na/O atomic ratio with edge functional groups, the in-plane functional groups appear to be more effective in enhancing the Na storage capacity than the edge-oxidized groups in terms of the calculated Na/O atomic ratios. Furthermore, the adsorption for NanO/Nan(OH) becomes stronger with cluster growth. In the second part of the thesis, we studied the effect of functional groups on the kinetic behaviors in reduced graphene oxide. Our calculated results show that the migration barrier of Na on the basal plane is similar to that on pristine graphene when Na is located away from the edge functional groups. As the edge sites are terminated with the ketone, K-E pair, or the carboxylic groups, Na atom on an edge hollow site is found to diffuse easily towards the edge terminations and then adsorb onto the edge functional groups with a very small energy barrier. As Na is in the vicinity of an epoxy/hydroxyl group, it can diffuse readily towards the functional group and then form a Na-O/Na-(OH) pair on the basal plane without any sizeable energy barrier. Furthermore, the Na-O/Na-(OH) pair can keep growing into a bigger compound cluster like the Na4O/Na2(OH) by overcoming a small energy barrier.
Subjects
Sodium-ion Battery
Reduced graphene oxides
First Principles calculations
Type
thesis
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