Electrochemical Properties of Amorphous Nb2O5 Thin Film and Its Application to Rechargeable Thin Film Lithium Ion Batteries

Nb2O5 thin films were prepared using RF magnetron sputtering system. By X-ray diffraction and SEM cross-section analysis, Nb2O5 thin film was in an amorphous state and its growth rate is 0.013 nm/s. The electrochemical behaviors of Nb2O5 thin films were characterized by capacity measurement and cycling performance. The average capacity of Nb2O5 thin films of 630 nm is about 41.8 μAh/cm2 for cut-off voltage range of 3.0 and 1.0V (vs. Li/Li+) at a rate of 10 μAh/cm2 and the coulombic efficiency reaches to over 98% after the first cycles. In addition, electrochemical characteristics of flexible thin film rechargeable lithium ion batteries composed of LiMn2O4 thin films of 720 nm, LiPON electrolyte thin films of ca. 1.0 μm and Nb2O5 thin films of 630 nm, fabricated on 30 μm SUS304 substrates, were also investigated. At a discharge current of 50 μA, the maximium discharge capacity of 24.7 μAh/cm2 was obtained, and the coulombic efficiency was all over 95% except for the first few cycles, which is due to a large internal resistance of the battery when no Li ion was inserted into Nb2O5 thin films. Even at a high discharge current of 500 μA, the thin film battery can still deliver reversible capacity of 18.4 μAh/cm2. The long-term cycle performance of the thin film battery showed steady cycling behaviors with a small decreasing ratio of 0.1% after 20 cycles, and the coulombic efficiency was all over 95% except for the first few cycles.