Li Storage Feasibility of Defected Single- and Bi-Layer Graphene

The limited capacity of the intercalation-type graphitic materials has lead to much effort to explore new electrode materials. A natural choice among the carbonaceous materials, graphene, has attracted increased attention for obtaining higher Li storage capacity. The outstanding mechanical strength, electronic conductivity, and ready availability of extra storage sites for Li, has suggested that this material could be an ideal platform for storing more Li. However, for wide range of Li coverages, the calculations predict that defect-free single layer graphene is not thermodynamically favorable for lithiation compared to bulk metallic Li. Thus, exploring ways with which graphene surfaces can be activated such that Li storage capacity can be improved has become a significant focus of research in the field. One such strategy is to activate graphene surface with structural defects. Generally speaking, structural defects can be found nearly in all carbonaceous materials. The presence of such defects can be detrimental for the performance of graphene-based devices. In contrast, however, deviations from the perfection of atomic lattice can also be useful for some applications, as they make it possible to tailor the properties leading to new functionalities.

Following this approach, we performed an extensive series of density functional theory (DFT) calculations for Li adsorption and intercalation in single and bi-layer graphene, which are activated by topological defects for improving Li adsorption. The results confirm that Li adsorption on defect-free single layer graphene is not thermodynamically favorable compared to bulk metallic Li. However, graphene surfaces activated by structural defects are generally found to bind Li more strongly, with the interaction strength sensitive to both the nature of the defects and their densities. The interaction of Li with a one-dimensional extended defect is additionally found to be strong, improving Li storage capacity. The theoretical Li storage capacities of the defected single-layer graphene structures are evaluated with a rigorous thermodynamic analysis, which establishes, in some cases, that these capacities approach, although not exceed, those of graphite. Li storage in bi-layer graphene with and without defects are evaluated in a similar way, and indicate more favorable interaction, and promise for higher Li storage than that of single layer graphene. We will present performance comparisons between defected single- and bi-layer graphene structures, and bulk-graphite for Li storage capacities. A detailed analysis of the effect of the van der Walls (vdW) interactions will also be presented.