Composition of Oxygen Functional Groups on Graphite Surfaces

The types and compositions of oxygen functional groups on graphite surfaces are heavily subjected to the method in which the graphite is synthesized and processed in experiments, which makes the characterization difficult. The challenge even extends to the modeling of oxygenated graphite surfaces in computational studies. However, the determination of both the types and compositions of oxygen functional groups on graphite surfaces is of paramount importance as it plays a significantly important role in dictating the behaviors and performances of electrochemical systems. For example, the surface structure and composition of the graphitic anode used in lithium-ion batteries (LIBs) determine the quality of a solid electrolyte interphase (SEI) that forms at the electrode/electrolyte interface, which, in turn, substantially affects the stability and lifetime of the devices. To help predict the structure and the composition of the surface oxygen functional groups on graphite surfaces resulting from solution-based syntheses and modification processes, we analyze the adsorption of different oxygen functional groups at both edge and basal sites of graphite as a function of pH under which the solution-based processes may take place. A series of density functional theory (DFT) calculations reveal that at room temperature and for a pH range from 0 to 14, the (112̅0) edge surface of graphite will be fully oxygenated, while the basal sites remain unsaturated. The oxygen functional groups at the edge sites are comprised of mostly hydroxyl and ketonic groups, with carboxyl and carbonyl groups present only in small amounts. Furthermore, we observe the transformation of the carbonyl group into the ketonic group in the presence of empty surface carbon sites, which further stabilize the graphite surface. Meanwhile, carboxyl groups are more stable when all surface sites within a carboxyl layer are all populated. We conclude that the population of oxygen groups that can be found at the edge surface of a graphite in the ascending order is carboxyl < carbonyl < hydroxyl < ketonic. On the contrary to the edge plane, a small amount of oxygen functional groups may be forced to adsorb on the basal surface upon the application of an external potential. The adsorbed groups are found to prefer to cluster together on basal sites in a highly ordered fashion, while the edge surface does not show this preference for adsorption sites.