First-principles study of Na2+xTi7O15 as anode materials for sodium-ion batteries

Abstract Sodium titanate (Na 2 Ti 7 O 15 ) with tunnel structure has been experimentally proposed to be a good candidate for anode material of sodium ion batteries because of its high reversible capacity and excellent stability. In this work, the first principle calculations have been performed to investigate the variations of the structures and electronic properties of Na 2 Ti 7 O 15 after inserting different amount of Na atoms. Based upon ab initio molecular dynamics simulations, the most stable structures of various Na 2+x Ti 7 O 15 compounds have been determined. From a thermodynamical point of view, Na 2 Ti 7 O 15 can be sodiated to Na 3.5 Ti 7 O 15 with a theoretical capacity of 64.6 mA h/g, while further addition of sodium results in a negative intercalation potential due to the significant destruction of the tunnel structure. During the sodium insertion, the small lattice expansion indicates the good structural retaining ability of Na 2+x Ti 7 O 15 compounds, however, the configurations of four bridging TiO 6 octahedra in the unit cell that connect different zigzag edge-sharing octahedra are distorted obviously. Consequently, the intercalation of Na atoms has remarkable influences on the distributions of 3d states of those Ti atoms belonged to above bridging octahedra. Furthermore, compared to Na 2 Ti 7 O 15 , the introducing of Na atoms leads to the upward movement of the Fermi level, and correspondingly the metallic state is predicted for the sodiated Na 2+x Ti 7 O 15 , suggesting the enhancement of the electronic conductivity of the system.