A.c. impedance analysis of electrochemical lithium intercalation into highly oriented pyrolytic graphite

Electrochemical lithium intercalation into graphite was studied by cyclic voltammetry and a.c. impedance spectroscopy. Highly oriented pyrolytic graphite was used as a model graphite material to distinguish the difference in electrochemical behavior between the basal and the edge planes at graphite. A comparison between cyclic voltammograms of the basal plane and the whole surface of highly oriented pyrolytic graphite revealed that electrochemical lithium intercalation proceeds predominantly at the edge plane/electrolyte interface. The charge-transfer resistance changed continuously with electrode potential, and no significant change was observed at stage transition potentials (210, 120, and 90 mV versus Li/Li+). From the variations of the Warburg impedance of samples of different sizes, it was concluded that lithium diffuses from the edge plane to the interior in the direction parralel to the basal plane and that its diffusivity changes with the stage structure of the bulk lithium—graphite intercalation compound.