The electrochemical permeation method was applied to the determination of the diffusion coefficient of Li+ ion (D
Li+) in a glassy carbon (GC) plate. The cell was composed of two compartments, which were separated by the GC plate. Li+ ions were inserted electrochemically from one face, and extracted from the other. The flux of the permeated Li+ ions was monitored as an oxidation current at the latter face. The diffusion coefficient was determined by fitting the transient current curve with a theoretical one derived from Fick's law. When the potential was stepped between two potentials in the range of 0 to 0.5 V, transient curves were well fitted with the theoretical one, which gave D
Li+ values on the order of 10−8 cm2 s−1. In contrast, when the potential was stepped between two potentials across 0.5 V, significant deviation was observed. The deviation indicated the presence of trap sites as well as diffusion sites for Li+ ions, the former of which is the origin of the irreversible capacity of GC.