Li mass transfer through a metallic copper film on a carbon fiber during the electrochemical insertion/extraction reaction

Copper is known to form no alloys with Li, implying that a free mass transfer of Li in copper film is improbable at an ambient temperature. To examine the movement of Li in copper, a copper film was vacuum-deposited over the entire surface of a graphitized carbon fiber and cyclic voltammograms (CV) were measured in a 1:1 (v/v) mixed electrolyte solution of ethylene carbonate and dimethyl carbonate containing 1 M LiClO4. A strong sharp peak due to the intercalation of Li into carbon appeared at about 0.15 V vs. Li/Li+. A strong peak due to the deintercalation of Li from the carbon sample was also revealed on the anodic branch of the CV as in the case of bare carbon fiber. This is indicative of insertion of Li into carbon through the copper film. When the film thickness was thin enough, i.e., 100 Angstrom, the peak height was enhanced as compared with that in the absence of the film, although the peak height was somewhat unstable. The peak height decreased with an increase in the film thickness, but a fiber with a 1000 Angstrom thick film still revealed a sufficiently observable peak. By the use of a bipolar cell constructed with a 30 mum thick copper foil which separates an electrolyte containing 1 M LiClO4 in one compartment from that containing 1 M NaClO4 in the counter compartment, Li+ concentration increase was confirmed in the NaClO4 compartment during polarization. This experimental evidence supports the postulate that Li moves freely in the film of copper deposited on the surface of carbon. (C) 2000 The Electrochemical Society. S1099-0062(00)05-063-X. All rights reserved.