In Situ AFM Study of Surface Film Formation on the Edge Plane of HOPG for Lithium-Ion Batteries

Changes in the surface morphology of the edge planes of graphite during a potential sweep were studied using highly oriented pyrolytic graphite (HOPG) in an ethylene carbonate (EC) + diethyl carbonate (DEC)-based electrolyte solution by in situ atomic force microscopy (AFM). The effects of the microscopic structures of graphite, i.e., edge and basal planes, on surface film formation are discussed. The formation of fine particles and precipitates was observed depending on the electrode potential between 1.0 and 0 V. These were considered to be remnants of blisters that could be observed at the basal plane and decomposition products of the electrolyte solution. The surface films were 56 and 66 nm thick after the first and second cycles, respectively. The precipitate layer formed on the edge plane was thinner than that observed on the basal plane after the second cycle. These results enabled us to elucidate the difference in the formation of surface films on the edge and basal planes of HOPG.