UNIQUE CHARGE DISCHARGE PROPERTIES OF CARBON MATERIALS WITH DIFFERENTSTRUCTURES/

The electrochemical properties of isotropic and anisotropic carbon fib ers were studied using the two-electrode method in a 1 M LiPF6 electro lyte solution dissolved in a 1:1 volume mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). Isotropic carbon fiber had a larger irreversible capacity than that of the other carbon fibers and might have many sites for the electrolyte decompositon and residual lithium ions. Active carbon fibers prepared from isotropic carbon fibers by st eam gasiformation at 850 degrees C showed large specific surface areas . After steam gasiformation, the active carbon fibers changed into a m ore crystalline state and had electrochemical characteritics similar t o the anisotropic carbon fibers. Impedance profiles of the carbon fibe rs were fitted by equivalent analogs of a three-resistance-capacitance (RC) circuit in series and the thickness of passivation layers in the carbon fibers was calculated. (C) 1997 Published by Elsevier Science S.A.