Role of edge orientation in kinetics of electrochemical intercalation of lithium-ion at graphite.

A relation between the amount of edge orientations and the kinetics of lithium-ion intercalation at the basal plane of highly oriented pyrolytic graphite (HOPG) was studied. The fraction of edge orientations (f(edge)) exposed on the basal plane of HOPG was evaluated from the kinetics of heterogeneous electron transfer of Ru(NH(3))(6)(2+/3+). Obtained f(edge) values ranged from 0.025 to 0.067, which were in good agreement with those previously reported. The charge-transfer resistance (R(ct)) for lithium-ion intercalation at the same HOPG samples was evaluated by ac impedance spectroscopy in an electrolyte consisting of 1 mol dm(-3) LiClO(4) dissolved in a mixture of ethylene carbonate/dimethyl carbonate (1:1 by vol). A clear correlation was observed between the f(edge) and R(ct) values at the basal plane of 15 HOPG samples, and the edge-area specific resistance was evaluated to be ca. 200 Ω cm(2) at the electrode potential of 0.2 V versus Li/Li(+). These results indicate that the amount of edge orientations is one of the determining factors in the kinetics of lithium-ion intercalation at graphite.