High-capacity crystalline V2O5 thick films prepared by RF sputtering as positive electrodes for rechargeable lithium microbatteries

We demonstrate that crystalline V2O5 thick films up to 3.6 mu m thick can be synthesized by radio frequency (rf) sputtering using a vanadium target and without any postannealing treatment. Crystalline deposits in the range 0.6-3.6 mu m thick exhibit a rough surface morphology and a high porosity, allowing the electrolyte to penetrate the active material. The discharge-charge properties of the films are investigated in the two voltage windows 3.8/2.8 V and 3.8/2.15 V as a function of the thickness. We clearly show that such films allow one to get high specific capacities between 20 and 120 mu Ah/cm(2) in the 3.8/2.8 V range, depending on the thickness, even at a high rate of 8 C and between 40 and 250 mu Ah/cm(2) in the 3.8/2.15 V range. The influence of the current density indicates a little change in the capacity and the polarization, showing the homogeneous and efficient behavior of these sputtered films, in spite of their thickness, which can reach 3.6 mu m. The capacity is found to be a linear function of the thickness with a similar slope from the C/15 rate to the 8 C rate. The cycling behavior of these cathode film materials is attractive with, for instance, a high and stable capacity of 100 mu Ah/cm(2) after 90 cycles at 0.1 mA/cm(2) at high voltage for the 3.6 mu m thick film and a capacity of 150 mu Ah/cm(2) found to be stable from the 50th cycle in the 3.8/2.15 V range at 1 mA/cm(2) (8 C). The capacity fading observed after all of the first cycles in the potential limits 3.8/2.8 V and that found when the three insertion steps are involved is discussed in terms of structural data and impedance behavior. (c) 2006 The Electrochemical Society.