Atomic-scale investigation on lithium storage mechanism in TiNb2O7

Titanium niobium oxide (TiNb2O7) with a monoclinic layered structure has been synthesized by a solid state reaction method as an anode candidate for Li-ion batteries. The TiNb2O7 electrode shows a lithium storage capacity of 281 mAh g(-1) with an initial coulombic efficiency as high as 93% at a current density of 30mA g(-1) (ca. 0.1C). The average lithium insertion voltage is about 1.64 V vs. Li/Li+ at a voltage range of 0.8-3.0 V. The electrodes exhibit small voltage hysteresis (c. a. 0.1 V at 30 mA g(-1)) and good capacity retention. Such superior electrochemical performance of TiNb2O7 makes it one of the most promising anode materials to replace spinel Li4Ti5O12 for applications in hybrid vehicles and large scale stationary Li-ion batteries. In addition, we demonstrate crystal structures of TiNb2O7 and lithiated TiNb2O7 using advanced spherical-aberration-corrected scanning transmission electron microscopy (STEM), to picture the lattice sites occupied by the Li, Ti, Nb and O atoms at atomic-scale. Possible lithiation/delithiation processes and reaction mechanisms are revealed in consistence with first-principles prediction.