Preparation of Kevlar-derived carbon fibers and their anodic performances in Li secondary batteries

We report here the preparation of Kevlar-derived carbon fibers and the preliminary results on their anodic performances in Li secondary batteries. The single- or two-step carbonization process is employed for the preparation. In the single-step process, the Kevlar flocks are heat-treated under Ar atmosphere from 25 to 800°C with a heating rate of 5°C min^-1, but an additional intermediate heating at a fixed temperature between 330 and 510°C for 2 h is applied in the two-step preparation. The two-step carbonization allows a higher carbon yield than that achieved in the single-step one (ca. 36%). In particular, a maximum yield (ca. 50%) is observed with an intermediate heating at 400-410°C. The infrared study indicates that the higher carbon yield achieved in the two-step process is indebted to the formation of condensed/cross-linked polyaromatic molecules during the intermediate heating period, which seemingly alleviates the volatile loss in the subsequent higher temperature heating. The Kevlar-derived carbon fibers exhibit the charge-discharge behaviors that are commonly observed in non-graphitizable carbons. Here, the carbon made by the two-step process (intermediate heating at 410°C) delivers a higher discharge capacity than the single-step ones. In particular, a notably high capacity near zero volts is observed in the two-step carbon that is associated with the Li⁺ removal from the micropores. The higher population of micropores in the two-step carbons may be explained by the enriched formation of cross-linked polyaromatics during the intermediate heating.