Hydrogenated defective graphene as an anode material for sodium and calcium ion batteries: A density functional theory study

Recent experimental studies indicated that hydrogenation improves the performance of graphitic materials used as anodes in lithium and sodium ion rechargeable batteries. Here, results of density functional theory calculations are presented to demonstrate that this is also effective for both sodium and calcium ion batteries. It is shown that this can be explained by the increase in binding strength of the metal adatom to the hydrogenated graphene, compared to its binding to graphene, and also an increase in the inter-layer spacing of the layered materials. According to our calculations, whereas Na and Ca bind weakly to the graphene sheet with binding energies of −0.763 and −0.817 eV, they bind more strongly to the single layer of the proposed hydrogenated graphene sheet (CH) with binding energies of −1.670 and −2.756 eV, respectively. Furthermore, although Na does not intercalate strongly in the layers of the CH material, up to 16 Ca can intercalate into the bulk layers of this material giving an electrical capacity of 591.2 mA h/g and a 29.3% expansion of the inter-layer distance. Thus, hydrogenated defective graphene provides an anode material that might enhance the performance of rechargeable batteries compared to graphene, using metals that are cheaper than lithium.