Study On Properties Of Na-ion Batteries As Electrode Materials

With the development of science and technology,batteries have always played an extremely important role in our life.Li-ion batteries have been gradually integrated into all aspects of our life depend on the advantages of high capacity,high efficiency and long cycle life.However,lithium resources are very limited,the existing Li source could be only sustained for about 65 years with the current expending rate,so to find the substitution of Li is in urgent.As the same main-group element of Li,Na has similar physical and chemical properties with Li.But the stock of Na accounts for about 2.83% of the earth’s crust so the price of Na is much lower than Li.In present,the problems of low capacity and slow migration are common in Na-ion batteries need to be resolved.The appearance of two-dimensional(2D)carbon nanomaterials can improve these problems as a possible candidate.In this paper,two novel 2D carbon materials net-Y and cyclicgraphene are studied,based on density functional theory(DFT),the properties of the composite were studied by means of atom adsorption,molecular dynamics,CI-NEB and so on.The work of this paper is divided into the following two parts:(1)We build net-Y supper-cell model to search the optimal adsorption site of Na on net-Y.We find only four ring centers can stably absorb Na,and the maximum adsorption energy of-1.96 e V is obtained at the center of the octagon ring.The analysis of the density of state show that adsorbed Na can improve the metallic properties of the whole structure.Through CI-NEB,three possible diffusion paths of materials are found.The diffusion barrier is 0.35 e V for the most likely diffusive path.The maximum Na adsorption is 32 with considering the symmetry of the whole structure,and the average adsorption energy is-1.51 e V.The thermal stability of the structure at maximum adsorption were checked by molecular dynamics simulation,when the temperature changes dramatically,the structure keep stable.Finally,the maximum specific capacity is 1787 m Ah/g,and the average open-circuit voltage is 0.44 V,which is higher than ever reported in similar structure.(2)Similarly we build cyclicgraphene(CG)supper-cell model.To search the optimal adsorption site of Na on CG,we add a single Na atom on 12 potential adsorption sites and found only one stable adsorption point at the center of the largest ring with adsorption energy of-1.95 e V.Comparing the density of state of CG and Na-adsorbed CG,we found that the material endure a semiconductor-metal transition after adsorbing Na.One diffusion path was found by CI-NEB,and the diffusion barrier is 0.64 e V.Then we determine the maximum amount of Na adsorbed on thestructure is 16,and the average adsorption energy is-1.70 e V.When 16 Na adsorbed on CG,molecular dynamics simulation could be carried out,the result indicate the material has good thermal stability.Finally,the maximum theoretical capacity of CG is 1117 m Ah/g,the average open-circuit voltage is 0.58 V.