Li mass transfer through a metallic Ag film covering on a carbon fiber anode

Ag was observed to be an effective additive to improve the anode performance of carbon for Li-ion secondary batteries. A graphitized carbon fiber entirely covered with an Ag film by vacuum deposition was examined by cyclic voltammetry (CV) in a 1:1 (v/v) mixed electrolyte solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) containing 1 M LiClO4. A large sharp peak due to the intercalation of Li into the carbon at about 0.15 V vs. Li/Li+ was observed to be enhanced by the deposition of the Ag film. The peak height due to the deintercalation of Li was also enhanced compared with that in the absence of the film. This seems to suggest that Li can be intercalated into the carbon fiber through the Ag film. Using a bipolar cell constructed with a thin Ag foil which separates an electrolyte containing 1 M LiClO4 in one compartment from water not containing Li ion in the counter compartment, a Li+ concentration increase was confirmed in the water during polarization. The Li permeation speed was estimated using the diffusion coefficient by the potential step (time lag) method. The diffusion coefficients of Li in Ag ranged from 10−6 to 10−10 cm2/s during Li extraction polarization. This experimental evidence supports the postulate that Li can move freely in the Ag film deposited on the surface of carbon.