Carbon fibers prepared at 1000 °C from mesophase pitch were treated with trimethyl silazane for modifying surface -OH group into trimethyl silyl group. The irreversible reduction current on the first cathodic sweep was diminished. Low-temperature carbon fibers prepared at 700°C and active carbon fibers were evaluated for electrochemical lithium doping and undoping reaction in propylenecarbonate(PC)/ethylene carbonate or PC/dimethyl carbonate mixtures(l:l) containing 1 M LiClO4 at room temperature.
Pristine samples prepared at 700 °C gave very poor reversibility and capacity on cyclic voltammograms(cv), but a constant current charge/discharge with a very low current gave a capacity over 400 mAh/g. After prolonged heating at 700°C, the cv characteristics were gradually improved. Partial oxidation in a mild oxidizing atmosphere at 700 °C for a few minutes was found to be quite effective in improving the cv characteristics remarkably. The charge/discharge capacity, however, was very similar to that obtained for the pristine sample at a very low consatant current. This indicates that the heat treatments effectively increase the electrochemical reaction rate of lithium doping and undoping, but have no effect on the intrinsic capacity.
Active carbon fibers were found to have considerable capacity after repeated oxidation/reduction cycles in solution. Partial oxidation treatment was also effective in increasing the charge/discharge current. The capacity after the treatment was over 400mAh/g, which is far larger than that of the double layer capacity of the sample.
The improvement due to the heat treatment is elucidated based on the removal of a surface inhibiting layer through the treatment.