Enhanced Lithium-Ion Intercalation Properties of V[subscript 2]O[subscript 5] Xerogel Electrodes with Surface Defects
V{sub 2}O{sub 5} xerogel films were fabricated by casting V{sub 2}O{sub 5} sols onto fluorine-doped tin oxide (FTO) glass substrates and annealing at 300 C for 3 h in different annealing atmospheres of air and nitrogen. Films prepared in different annealing conditions possess different grain sizes and crystallinity, while the vanadium ion oxidation state also varies, as identified by X-ray absorption spectroscopy. A nitrogen annealing atmosphere induces the presence of defects, such as V{sup 4+} ions, and associated oxygen vacancies. Thus, the presence of defects, whether on the film surface or in the bulk, can be controlled by using air and nitrogen annealing atmospheres in the proper order. Electrochemical impedance analyses reveal enhanced charge-transfer conductivity in films with more V{sup 4+} and oxygen vacancies on the film surface, that is, a film annealed, first, for 0.5 h in air and then for 2.5 h in nitrogen. Lithium-ion intercalation measurements show that, at a charge/discharge current density of 600 mA g{sup -1}, this film possesses a noticeably better lithium-ion storage capability than films without surface defects. This sample starts with an initial discharge capacity of 139 mA h g{sup -1}, and the capacity increases slowly to a maximum value of 156 mA h g{sup -1} in the 15th cycle, followed by a mild capacity degradation in later cycles. After 50 cycles, the discharge capacity is still as high as 136 mA h g{sup -1}. A much improved lithium-ion intercalation capacity and cyclic stability are attributed to V{sup 4+} surface defects and associated oxygen vacancies introduced by N{sub 2} annealing.