Insight into the Origin of the Rapid Charging Ability of Graphene-Like Graphite as a Lithium-Ion Battery Anode Material Using Electrochemical Impedance Spectroscopy

Graphene-like graphite (GLG), a carbon material synthesized by heat treatment of graphite oxide, can be charged at a high rate without deposition of lithium metal even when the potential of the GLG anode is stepped below the Li deposition potential. However, the factors contributing to this outstanding rapid chargeability of GLG have been unclear until now. In this study, to clarify the factors, the charge-transfer resistance and diffusion resistance of GLG were quantitatively evaluated by the electrochemical impedance method using GLG thin films as model electrodes. The activation energy of lithium-ion transfer at the GLG/electrolyte interface was found to be almost the same as that reported for graphite, and it was clear that this process was as slow as that in graphite. On the other hand, the lithium-ion diffusion coefficient in GLG calculated from the Warburg impedance was several orders higher than that of graphite, which clearly shows that this contributed greatly to the fast-charging characteristics of GLG. In addition, in the comparison among GLGs with different structural parameters, the lithium-ion diffusion coefficient was higher for those with less oxygen content and more pores formed in the graphene layer inside the GLG. Therefore, it was concluded that a high diffusion coefficient was ascribed to the pores in the GLG, which enabled lithium-ion diffusion in the c-axis direction.