We investigated the adsorption of Na on graphene and graphene oxide, which are used as anode materials in sodium ion batteries, using density functional theory. The adsorption energy for Na on graphene was -0.507 eV at the hollow sites, implying that adsorption was favorable. In the case of graphene oxide, Na atoms were separately adsorbed on the epoxide and hydroxyl functional groups. The adsorption of Na on graphene oxide-epoxide (adsorption energy of -1.024 eV) was found to be stronger than the adsorption of Na on pristine graphene. However, the adsorption of Na on graphene oxide-hydroxyl resulted in the generation of NaOH as a by-product. Using density of states (DOS) calculations, we found that the DOS of the Na-adsorbed graphene was shifted down more than that of the Na-adsorbed graphene oxide-epoxide. In addition, the intensity of the DOS around the Fermi level for the Na-adsorbed graphene was higher than that for the Na-adsorbed graphene oxide-epoxide.

Abstract
 1. Introduction
 2. Experimental
 3. Results and Discussion
  3.1. Adsorption energies and charge analysis
  3.2. Electronic properties
 4. Conclusions
 Acknowledgements
 References

• Hye Sook Moon(Department of Organic Material Science and Engineering, Pusan National University, Busan 609-735, Korea)
• Ji Hye Lee(Department of Organic Material Science and Engineering, Pusan National University, Busan 609-735, Korea)
• Soonchul Kwon(School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA)
• Il Tae Kim(Department of Chemical and Biological Engineering, Gachon University, Seongnam 461-701, Korea) Corresponding author
• Seung Geol Lee(Department of Organic Material Science and Engineering, Pusan National University, Busan 609-735, Korea) Corresponding author