Graphene-based materials have been widely used in Li-ion batteries. However, their perspective for Na-ion batteries is not well studied. We present a first ab initio study of Na adsorption and diffusion on ideal and defected graphene that considered Na clustering, graphene defects, and dispersion forces. From density functional theory (DFT) calculations using GGA, Na atoms prefer to adsorb at the Hex sites and diffuse via the bridge site. The adsorption energy of -0.75 eV vs. the vacuum state is larger than the cohesive energy of Na metal (EcohDFT=-1.07 eV). The adsorption energy becomes comparable with Na metal cohesive energy (EcohDFT-D=-1.21 eV) when dispersion corrections are included (DFT-D), with Eads= -1.14 eV. Both DFT and DFT-D predict that the increase of Na concentration at graphene surface results in Na clustering. This is evidenced by smaller charge donation to graphene, 0.55e (0.48e for DFT) compared to 0.86e (for both DFT and DFT-D) for single atom adsorption as well as by the formation of a Na-Na bond identified by analysis of the charge density. These results suggest that ideal graphene is not a promising anode material for Na-ion batteries. They also highlight the necessity to include dispersion forces when modelling alkali atom interaction with graphene derivatives. Finally, we study the effects of points defect and B/N doping on the Na-graphene interaction and show that Na storage can be controlled by appropriate design of graphene-based materials.

Na adsorption and diffusion on graphene: Clustering, role of dispersion forces, and defects

247th National Meeting (2014)

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