Electrochemical and in situ XAFS-XRD investigation of Nb2O5 for rechargeable lithium batteries

Nb2O5 exhibits various crystal systems, such as orthorhombic (o), tetragonal (t), and monoclinic (m), among which Nb2O5 synthesized at 900-1000 degrees C is commercially used as a cathode material of the 2-V lithium ion battery. The battery performances depended on the structure of Nb2O5, and the t-Nb2O5 synthesized at 1000 C exhibited an excellent cycling performance with a large discharge capacity of 190 mAh (g oxide)(-1). The structural variations of Nb2O5 during electrochemical reaction were examined. The in situ synchrotron radiation-X-ray diffraction (XRD) measurement indicated that o- and t-Nb2O5 maintain their original crystal lattices, accompanying a small change in the cell volume even after the Li intercalation. The in situ X-ray absorption fine structure (XAFS) analysis of o- and t-Nb2O5 revealed that the continuous variation from Nb5+ to Nb4+ took place during the intercalation process. A significant rearrangement of the Nb-O octahedra accompanied by the change of Nb-O and Nb-Nb interactions occurred in both structures with Li intercalation. XRD and XAFS data suggests that the two-dimensional layer structure of t-Nb2O5 seems to be more flexible regarding the Li intercalation compared with the three-dimensional structure of o- Nb2O5. This may account for the better cyclic performance of the former material as the electrode material. (c) 2006 The Electrochemical Society.