First-principles Calculation Study On Na Storage Properties Of Structure-modified Bilayer Graphene

Limited Li resources restrict the application of lithium-ion batteries in the fields of power batteries and large-scale energy storage systems,and abundant Na resources reduce the cost of sodium-ion batteries,which have a broad application prospect.The graphene materials prepared by low-cost natural graphite usually have structural defects and few-layer graphenes.Bilayer graphene(BLG)possesses surface-adsorption properties similar to that of monolayer graphene,and has a intercalation structure similar to graphite.However,it is difficult to reveal the influence of structural modification(including vacancy defects and doping)on the storage and diffusion behavior of Na ions on the surface and in the interlayer of graphene materials by experimental methods.Using the first-principles theory based on the density functional theory(DFT),sodium storage performance for Na storage in mono-vacancy(MV),double-vacancy(DV)defects and nonmetallic(B/N/Si/P)doped BLG structures have been studied for providing theoretical guidance for the design and development of double layer,less layer and structure-modified graphene anode materials with excellent sodium storage performanceThe sodium-storage model of BLG is established,the stable position of Na atoms storaged in intrinsic BLG is determined at the central position of hexagonal carbon rings,and the storage is more stable in interlayer.About 0.71 e electrons transfer from Na atom to the substrate BLG exhibiting ion binding.With the increase of Na inserted concentration,AB stacking BLG gradually transforms into AA stacking with increasing interlayer distances.The calculation method of the surface reference state of Na has been given,and the stable sodium-storage capacity is 123.97 mAh/g for the intrinsic BLG.The average potential of the surface reference state based on Na is 0.260 V,which is 0.191 V higher than that of the metal reference state.The energy barrier(0.32 eV)of Na ions diffusion in interlayer is larger than that(0.17 eV)on the surface.Na adsorbed and inserted BLG shows metallic properties enhencing electrons conductivity.For MV defective BLG,Na atoms can be more stably storaged at the center of the MV defects,and Na tends to insert in the interlayer.MV defective BLG exhibits metallic properties with or without Na.MV defects can improve the binding between Na and BLG and the ionization;decline the transformation from AB stacking intercalation compounds into AA stacking ones;increase Na stable-storage capacity to be 382.54 mAh/g.The potential decreases to be less than 0.5 V leading to formation of Na clusters or dendrites.Adsorbed and inserted Na ions can migrate fastly to MV defects and be escaped.For DV defective BLG,the Na atoms can be more stably storaged at the center of the DV defects.Formation energies and transfer electrons(0.895 e)for Na adsorption and intercalation structures are close to ones of MV defects.Na storage increases the electronic conductivity.The storage stabilities for Na on the surface and in the interlayer are increased by a higher defect concentration,which declines the transformation from AB stacking intercalation compounds into AA stacking ones and increases the energy barrier on surface diffusion obviously.The stable storage capacity increases to be 262.75 mAh/g,and the Na clusters or dendrites are easy to generate with the potential of less than 0.7 V.The Na atoms in interlayer can enhance capturing Na on the surface of DV defects.For nonmetal doped BLG,B-and N-doped BLG still maintain the planar structures,while the Si-and P-doped atoms are more stable in the interlayer and out of surface,respectively.The binding between Na and B-doped-BLG surface is the most stable and slightly weaker than that for vacancy-defect BLG.The band gaps decrease for the structures with Na storage exhibiting metallic property,and enhencing electron conduct.For P-doped BLG,the energy barrier is the smallest for Na ions diffusion on the surface and the highest for the case in the interlayer.Na has a strong binding and a moderate diffusion barrier in B-doped BLG resulting in a better comprehensive performance.