Electrochemical insertion behavior of lithium ions into mesophase pitch carbon films prepared by electrostatic spray deposition

A carbon thin film (thickness, ca. 1μm) was prepared by the electrostatic spray deposition onto a Ni-substrate at 400°C and heat-treated at 500°C in vacuum. Electrochemical lithium ion insertion / extraction properties of the resulting carbon film electrodes were studied by cyclic voltammetry and electrochemical impedance spectroscopy in IM LiClO₄ / (PC + EC). In the case of slow scan rates less than 10mV / 22sec in step potential electrochemical spectroscopy, two anodic peaks appeared, indicating that there are two different insertion / extraction mechanisms. Cole-Cole plots of impedance spectra showed two well-separated semicircles and a linear locus. The first semicircle at higher frequency range seems to correspond to the migration of lithium ions through passivating film at carbon surface, and the second semicircle and linear locus arise from carbon thin film itself.