The Charge/Discharge Characteristics of Amorphous Li_xV_2O_(5-y) Thin Film Cathodes Deposited by R.F.-sputtering and Lithium Ion Diffusivity Determined by GITT Method

摘要

The electrochemical properties of amorphous vanadium pentoxide films fabricated by reactive r.f.-sputtering have been investigated by galvanostatic charge/discharge cycling experiment, AC impedance spectroscopy, and galvanostatic intermittent titration technique (GITT). As x in Li_xV_2O_(5-y) increased (x = 0 approx 2.0), the electromotive force of Li | 1 M LiClO_4 - propylene carbonate | V_2O_(5-y) cells decreased gradually without potential plateau or potential step, demonstrating that an abrupt structural change did not occur in the whole lithium content. Impedance spectra of the cells consisted of a high frequency arc, a low frequency straight line, and a middle frequency Warburg impedance line. Equivalent circuit deduced from Nyquist plot suggests that lithium intercalation reaction consists of three consecutive steps of charge transfer at the electrolyte/cathode interface, diffusion into the film and charge saturation. Chemical diffusivity of Li~+ in Li_xV_2O_(5-y) film, measured using GITT was determined to be 4 X 10~(-13) to 7 X 10~(-14) cm~2/ sec in the lithium content range investigated. The component diffusivity of lithium decreased linearly with lithium content from 9 X 10~(-14) to 5 X 10~(-15) cm~2/sec, owing to the decrease in the number of vacant sites available for jumping of lithium atom.