Impedance measurements of low temperature mesophase carbon fibers during lithium insertion/extraction

Surface modifications with three different procedures were performed on an integrated felt of mesophase carbon fiber (MCF) fired at 600 °C. The surface modification procedures were (1) heating in acetylene black powder under a low pressure of oxygen (MOT), (2) vacuum deposition of Ag film on the fiber surface and (3) vacuum deposition of Ag film on the fiber, followed by heating under low pressure of oxygen. The performance of Li insertion/extraction reaction was found improved by any of these modification treatments. The cause of the improvement was examined mainly with impedance spectroscopy analysis (EIS). EIS spectra revealed that both the film resistance and the charge-transfer resistance were decreased remarkably by these surface modifications. Among the three modification procedures, MOT revealed the best improving effect. By comparing the EIS data with the results obtained from the double layer capacity measurement, we concluded that the major cause of the increase in the reaction rate is the activation of the reaction site.