Chemical diffusion coefficient of lithium in carbon fiber

Electrochemical investigations on coal pitch-based carbon fiber (heat-treated at 2800 degrees C) were carried out. The open-circuit voltages of the Li\LixC6 cell were lower than 0.15 V vs. Li/Li+ in the range of 0.15 < x < 0.65. The open-circuit voltage profile vs. x showed no distinct two-phase region in the present Li-carbon system in 0 < x < 0.65. Almost constant capacity of 220 mAh/g was observed until the 140th cycle in the cycling tests of the Li\carbon fiber cell (current density 25 mA/g). The compositional variation of the chemical diffusion coefficient of lithium <(D)over tilde(Li)> at ambient temperature was measured by two different methods, i.e., the current pulse relaxation method and the potential step chronoamperometric method. Excellent agreement within one order of magnitude was observed between the two sets of <(D)over tilde(Li)> values obtained from these two methods. The values were around 10(-9.5) cm(2)/s at x is-approximately-equal-to 0 and decreased with increasing x. The <(D)over tilde(Li)> values lay between 10(-11) and 10(-12) cm(2)/s in x > 0.2.