Theoretical Study On The Storage Mechanism Of Sodium On Graphene And Transition Metal Dichalcogenides

In recent years,the rapid consumption of fossil fuels has led to resource depletion and environmental problems such as Greenhouse Effect.The development of clean energy such as wind and solar energy and efficient energy storage devices have attracted widespread attention.Sodium ion capacitors have become a hot spot in the research of energy storage devices because of their high energy density and high power density.Layered graphene carbon materials and molybdenum disulfide are ideal electrode materials for sodium ion capacitors because of their large specific surface area,excellent conductivity and easy functionalization.China is rich in graphite and molybdenum ore in the world,so it has advantages in production technology to prepare graphene and molybdenum disulfide electrode materials using graphite and molybdate as raw materials respectively.However,the intercalation and detachment of sodium atoms with larger diameter can easily lead to poor cyclic stability of graphene carbon materials and molybdenum disulfide materials,which limits their large-scale commercial application in supercapacitors.Based on density functional theory,taking the sodium ion capacitance constructed by graphene carbon materials and MoS2 electrode materials as the research object,the deposition of Na atoms(clusters)on graphene and layered molybdenum disulfide and their derivative materials were investigated systematically by ab initio and firstprinciples calculation methods.The co-deposition behavior of Li/Na atoms(clusters)on graphene were also investigated.The changing rules of microstructure configuration,adsorption energy,cohesion energy,electronic structure and surface work function of Li/Na atoms(clusters)deposited on the electrode surface were expounded,and the energy storage mechanism of sodium ion capacitors was explained from the atomic level,which could provide a theoretical basis for improving the electrochemical performance of graphene and layered molybdenum disulfide materials.The specific research contents include:Based on the graphene model of carbon material structure unit,four paths of sodium storage on graphene were designed.When the number of Na atoms x<5,Na atoms are preferentially deposited on the graphene surface on both sides,and when x≥5,Na atoms are deposited on the graphene surface in the form of clusters.When x=5,the Na atom deposited on graphene reached the maximum capacity,which was 155mAh/g.The calculation of electronic properties shows that there is a weak ion bond between Na atom and graphene,and when x≥ 5,there is a metal bond between Na and Na atom in the Nax cluster.When Li and Na atoms are co-deposited on graphene,Li atoms are deposited more preferentially on graphene than Na atoms,and the deposition height is lower.When x+y≥ 3,LixNay is deposited on graphene in the form of clusters,in which Li atoms are usually at the bottom of LixNay clusters,while Na atoms are usually at the upper layer of clusters.The electrons of LixNay clusters are transferred to the antibonding π orbitals near graphene,and the 2s orbitals of Li atoms,2s and 2p orbitals of Na atoms are hybridized with the 2p orbitals of C atoms.There is ionic bond with partial covalent bond between Li or Na atom and C atoms,and covalent bond between Li atom and Na atom in LixNay clusters.Based on MoS2 layered materials,alloying modified MoSSe materials were constructed,and the deposition mechanism of Na atoms on 1H/1T-MoS2 and 1H/1TMoSSe surfaces were investigated.The layered materials of 1H-MoS2,1H-MoSSe,1TMoS2 and 1T-MoSSe are thermodynamically stable.Isolated Na atom is preferentially deposited on the top site of Mo atom in MoS2,while more Na atoms can be deposited on the hollow site composed of S/Se and Mo.After alloying,the electrical conductivity of MoSSe changes,and the band gap is reduced to facilitate the conductivity.In MoSSe,the work function on the side of S atom increases,while that on the side of Se atom decreases.When Na atoms are deposited on the MoSSe surface,the S-side interface transfer resistance increases slightly.With the continued deposition of Na atoms,the dxy orbitals located in the Mo-Mo plane in 1T-MoSSe would produce CDW phase after lattice distortion.The model of 1H-Na2MoS2/1H-Na2MoSSe and 1T-Na2MoS2/1T-Na2MoSSe were built and calculated its correlation properties under implicit solvent conditions.The calculation results show that the lattice constants of 1H-MoS2/1H-MoSSe and 1TMoS2/1T-MoSSe are basically unchanged at ε=40,60 and 78.The 1H-MoS2/1HMoSSe are lattice dynamics stable configurations,but 1T-MoS2/1T-MoSSe are not lattice dynamics stable configurations.After considering implicit solvation,the solvent facilitates the phase transition of the system by influencing enthalpy transformation.The deeper phase transition mechanism is due to the fact that the presence of an implicit solvent promotes the occupation of the e’ electronic state of Mo at the Fermi energy level.