First-principles Study On Energy Storage Preformance Of Nb₂C Based Mxene

It is one of the key challenges for the development of metal ion batteries to find efficient electrode materials with high power density and high energy density.The surface environment of Nb₂C Mxene is complex,and most of Nb₂C Mxene samples in the experiment have lamellar structure.Therefore,its energy storage performance is largely controlled by the surface terminal groups and the layer thickness of the nanosheet.In this paper,we demonstrate that two-dimensional Nb₂C MXene is a promising anode material for metal ion batteries by first-principle calculations.The progress has been made in the following areas:1 The monolayers of Nb₂C and Nb₂CT₂（T=O,F,OH）were constructed,and the formation energies of each configuration were calculated.It was confirmed that the introduction of terminal groups would promote the formation of monolayers by chemical stripping method.Then we analyze the density of state and energy band structure of each configuration.It indicates that the presence of terminal groups changes the intrinsic physical properties of bare Nb₂C,and the electron transfer exists between surface terminal groups and Nb₂C.All the monolayers are metal,and the d orbitals of Nb atoms are dominant near the Fermi level,and the presence of terminal groups reduces the conductivity of Nb₂C monolayers.The structures of Nb₂C（OH）₂-Bsite and Nb₂C（OH）₂-Csite show magnetic properties near the Fermi level.2 The monolayers configurations of Nb₂C and Nb₂CT₂（T=F,O,OH）adsorbed Li atoms were established,and the theoretical capacities of different configurations were calculated.The Nb₂C monolayer showed a promising future as an electrode material due to its high capacitance and low diffusion energy barrier.Through the characterization of the adsorption properties of metal atoms on the T terminal Nb₂C surface,it is found that the surface terminal groups have a considerable influence on the monolayer Nb₂C.For example,the F-terminal or O-terminal surfaces facilitate charge transfer between lithium ion atoms and the surface.However,these terminations significantly reduce the electronic conductivity and ionic conductivity performance during charge and discharge.In addition,F and OH functional groups on Nb₂C surface can be removed by metal ion adsorption.3 The monolayers Nb₂C and Nb₂CO₂structures were constructed and their surface Na storage properties were studied.Moreover,Nb₂C and Nb₂CO₂nanosheets with different symmetry groups were constructed,and their lattice constants were similar to those of experimental samples.The results show that the O-terminal groups have different effects on the capacity performance of Nb₂C monolayer and Nb₂C nanosheet,and it is more feasible to adjust the energy storage performance of nanosheet by increasing the interlayer space than by adjusting the surface chemical properties.The symmetry group has little effect on the energy storage of nanosheets.