Effect of a vacuum-deposited metal film on the CV of the Li insertion/extraction reaction at a graphitized carbon fiber electrode

Abstract

A metal film of silver, copper, or gold was vacuum-deposited on the surface of mesophase pitch-based carbon fiber prepared at 3100 °C, and the Li insertion/extraction behaviour was examined by measuring the cyclic voltammogram (CV) for various film thicknesses in a non-aqueous electrolyte containing 1 M LiClO 4. The peak height of the CV was examined in detail for the deposited silver film. The peak height increased at first with increasing film thickness up to 150 Å, but then began to decrease. At around 200 Å, a minimum peak height which was nearly the same as that of the pristine sample was observed. The peak height then increased again up to a film thickness of 400 Å, where it was two times higher than that of pristine sample, then decreased gradually. Such a complicated CV could not be observed with a gold or copper film where the enhancing effect was monotonous and more remarkable with a thinner film. X-ray diffraction patterns of the silver film deposited on the carbon fiber showed a strong crystal orientation to the carbon substrate like epitaxial growth, which was found to be dependent on both the thickness and the deposition rate. The CV peak height correlated well with the silver crystal orientation, implying that the electrochemical reaction rate differs on different crystal faces. Such could not be found in the case of gold or copper. The peak enhancing effect and the cycle behaviour were elucidated in view of the nature of solid electrolyte interphase (SEI), conductivity, and the effect of alloy formation.