High Performance Vanadium Oxide Thin Film Electrodes for Rechargeable Lithium Batteries
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microstructure can be obtained which leads to a high capacity and highly reversible thin film cathode. EXPERIMENTAL Vanadium oxide films were prepared using a 13.56 MHz capacitively coupled PECVD process with 24 cm diameter parallel-plate electrodes. Vanadium oxytrichloride (VOCl3 ) was used as the primary reactant precursor, with argon as the carrier gas. The precursor temperature was controlled at 10°C, and the vapor pressure of the precursor was -9 torr. The pressure of the argon carrier gas was 15 psig (1406 torr in Golden, Colorado), so that the partial pressure of the vanadium oxytrichloride was --0.64 % of the Ar/VOCl3 mixture. This mixture was then combined with hydrogen and oxygen before entering the plasma region of the reaction chamber. The flow rates of the reactants were controlled using mass flow controllers, and partial pressures of hydrogen, oxygen, and VOCI3 vapor in the chamber were varied to optimize film properties. The weights of the substrates (SnO2) were measured before and after film deposition using a high-precision microbalance to determine the net weight of the vanadium oxide films. Substrate temperature (during film deposition) was varied between 300 C and 3000 C. Film thicknesses were measured with a DekTak profilometer after deposition. The films were stored and tested inside a controlled-environment glove box under argon atmosphere. The water and oxygen concentrations in the dry-box were measured to be less than I ppm and 1.5 ppm, respectively. The samples were then tested in an electrochemical cell with a three-electrode configuration. Lithium metal was used as both the counter and reference electrodes, and a solution of 1M of LiC10 4 in propylene carbonate (Merck & Co., Inc.) was used as the electrolyte. Samples with active areas of-1 cm 2 were cycled between 4.0 volts and 1.8 (or 1.5) volts at 25 °C and tested at discharge/charge rates between C/0. 1 to C/1 .0 (where the C/I rate is defined as the insertion of one mole lithium in one mole V20 5 per hour). The electrochemical experiments were performed using a computer-controlled battery-testing system (BT2043, Arbin Instruments Corp.). Film crystallinity was characterized by X-ray diffraction (XRD) on a 4-circle Scintag X-1 diffractometer with a Cu-Kct anode source. Film stoichiometry was measured by X-ray photoelectron spectroscopy (XPS). RESULTS AND DISCUSSION We started our systematic approach with the optimization of the H2 flow rate, using the deposition rate as the criterion for guidance. The flow rates of vanadium oxytrichloride (VOC13) and oxygen (02) were held constant at 3.2 and 3.8 sccm, respectively, while the hydrogen flow rate was varied between 14 and 42 sccm. All of the films were deposited at a substrate temperature of 30°C for 10 min, and the RF power was controlled at 50 W. The chamber pressure during deposition was maintained at 0.6 torr. The film deposition rate exhibits a maximum of 11 A/s at a H2 flow rate of 28 sccm and is more than 5 times higher than those reported for vanadium oxide films prepared by the
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