Optimization of the dimensions of vapor-grown carbon fiber for use as negative electrodes in lithium-ion rechargeable cells

Electrochemical performance examinations of a new generation of carbon whisker were carried out in 1 M LiClO{sub 4} in a 50:50 mixture of ethylene carbonate and diethyl carbonate. The whisker has a high reversible capacity (363 mAh/g), and the coulombic efficiency was 100%. This capacity depends strongly on the diameter of the fiber and the preparation method. When the length of graphite whisker is 23 {micro}m, it gives the highest first cycle coulombic efficiency (96%). The authors investigated the charge-discharge characteristics with different current densities. When measuring the slow cyclic voltammogram of the graphite whiskers, five cathodic peaks could be seen. The kinetics of electrochemical intercalation of lithium ions into the graphite whisker was studied by impedance spectroscopy. The diffusion coefficient of lithium in the carbon is larger when the fibers were graphitized and then chopped, than when they were chopped first, and it decreased linearly as x in Li{sub x}C{sub 6} increased. This is related directly to the microstructure of fiber.