Electrochemical Characteristics of Fluorinated Activated Carbon Fibers‐Lithium Cells

Abstract

Because of their characteristic structural properties such as microcrystallinity with monodispersive micropores and their ability to resist graphitization, activated carbon fibers (ACF's) readily react with elemental fluorine at significantly low temperatures ranging from 20° to 200°C under fluorine pressures of 100–760 torr. The electrochemical characteristics of fluorinated ACF's (F‐ACF's) obtained at the above conditions have been investigated as cathode materials for lithium cells in carbonate solutions. OCV's of F‐ACF cathodes decrease from 3.68 to 3.35V as fluorination temperatures increase from 20° to 200°C. The dependence of OCV's on fluorination temperatures is ascribed to the change in the nature of the C‒F bond which varies from chemisorption through charge transfer to covalent bond, depending on fluorination temperatures. Discharge performance, particularly the stability of the discharge potential and cathode utility, are significantly improved with increasing fluorination temperature. The relation between utilization and x‐ray diffraction patterns of F‐ACF's suggests that to a certain extent the three‐dimensional structure in a F‐ACF is required to be a cathode material with satisfactory discharge performances. F‐ACF's obtained at 200°C but with reduced fluorine pressures have stable discharge potentials (for current densities of 0.5 mA cm−2) which are 0.5V higher than that of conventional graphite fluoride . Cathode utilization is higher than 70%.