Introduction The past few years have seen an increasing interest in the development of wireless sensor networks. But the limited available energy source is one of the major bottlenecks, which are limiting the wireless sensor technology from mass deployment. Micro energy harvesting is the most promising solution towards autonomous sensor nodes by providing low cost, permanent, and maintenance-free energy source to wireless sensor nodes. Because many harvesting devices could only capture low levels of ambient energy, very small batteries are required for energy storage and intermittent use. Compared to liquid electrochemical batteries with limitation of life span, thin film batteries with flexible charging models and excellent cycle life are very promising for paring with a variety of energy harvesting devices. Thin-film rechargeable lithium or Li-ion batteries have been investigated extensively. So far, positive-negative electrode combinations of LiCoO₂-Li, LiMn₂O₄-Li, TiS₂-Li, LiCoO₂-SnO and LiMn₂O₄-V₂O₅ have been reported. Active Li requires an extra-protective layer to exclude entry of open air. Further, the voltage of the batteries containing Li electrode during charge-discharge processes is restricted to prevent degradation by over-charge or over-discharge. In the electrochemical conversion of oxides system, the initial Li₂O formation is irreversible and a significant initial capacity loss is observed. On the other hand, V₂O₅ is toxic and moisture-sensitive. For practical applications, Nb₂O₅ displayed relatively good charge-discharge performance and stability although the poor crystallinity of films fabricated through low-temperature processes. In this study, we report on electrochemical properties of amorphous Nb₂O₅thin film and its application to rechargeable thin film batteries.

Experimental.  Nb₂O₅ thin films are deposited using an RF magnetron sputtering equipment (ISP-400, ULVAC) and the thicknesses of these thin films were controlled by the sputtering time. During the deposition, the substrate temperature, the sputtering pressure and the RF power were kept at room temperature, 1 Pa and 100 W, and the sputtered gas (O₂/Ar) composition of 1:9 was maintained to minimize the loss of Oxygen. The electrochemical properties of  Nb₂O₅ thin film electrodes were investigated in a liquid electrolyte (1 M LiPF₆ in EC + DMC (EC/DMC = 1/1 v/v) )using head-type cells.  A Li foil and 20 μm PBS separator were used as the counter electrode and separator. These half-cells were assembled in an Ar-filled glove box. The charge and discharge measurements were carried out using a source-meter (Keithley 2400) with the applied various charge-discharge currents and the cutoff voltages of between 1.0 and 3.0 V.  In addition, thin-film batteries were fabricated on 30 μm SUS304 substrates. Charge–discharge properties of these thin-film batteries were measured at room temperature with charge-discharge currents of 50 μA and cutoff voltages of between 3.2 V and 0.5V by using a charge–discharge measuring instrument (ACD-01, Asuka). The surface area of thin film batteries was 4.2 x 4.2 cm².

Results and discussion. Figure 1 shows charge-discharge curves of Nb₂O₅ thin films in the voltage window 1.0-3.0 V (vs. Li/Li⁺) at a rate of 5 μAh/cm² up to 10 cycles. The open circuit voltage of the cell lies at 2.46 V. The discharge capacities in the 1st, 3rd, 5th, and 10th cycles are 59.4, 56.8, 56.1, and 56.8 μAh/cm²μm, respectively; the corresponding charge capacities are 79.2, 56.1, 55.4, and 56.2 μAh/cm²μm for 1st, 3rd, 5th, and 10th cycles, respectively. After first few cycles, the coulombic efficiency reaches to 100%. Such irreversible capacity loss during first few cycles was also observed with other nanoparticle. Figure 2a shows charge-discharge cycling curves of a flexible thin-film battery based on Li₂Mn₂O₄ thin films of 720 nm and Nb₂O₅ thin films of 800 nm at a charge-discharge current of 50 μA. The charge-discharge curve consists of smooth S-shape profiles at any cycle without any plateau having average voltage of about 1.7 V and the voltage may vary in broad voltage range of 3.2-0.5V with cycling. As shown in Fig. 2b, an initial irreversible capacity was observed, which is due to irreversible Li ions trapped in the negative electrode. The coulombic efficiency at the first cycle was only 26.3%, which is due to a large internal resistance of the battery when no Li ion was inserted into Nb₂O₅ thin films. After 20 cycles, the cycle performance of the thin-film battery showed steady cycling behaviors and the coulombic efficiency was all over 95% except for the first few cycles. The maximium discharge capacity (30.9 μAh/cm²μm) of the thin-film battery based on the negative electrode volume is about half of the negative capacity of the liquid battery (56.8 μAh/cm²μm), which may originate from Li ions partly being inserted into the negative electrode.