Surface Film Formation on a Graphite Negative Electrode in Lithium-Ion Batteries: Atomic Force Microscopy Study on the Effects of Film-Forming Additives in Propylene Carbonate Solutions

In situ electrochemical atomic force microscopy (AFM) observation of the basal plane of highly oriented pyrolytic graphite (HOPG) was performed during cyclic voltammetry in 1 M LiClO4/propylene carbonate (PC) containing 3 wt % vinylene carbonate (VC), fluoroethylene carbonate (FEC), and ethylene sulfite (ES) in order to clarify the roles of these additives in the formation of a protective surface film on a graphite negative electrode in lithium-ion batteries. Particle-like precipitates appeared on the HOPG surface at the potentials 1.35, 1.15, and 1.05 V versus Li+/Li in PC + VC, PC + FEC, and PC + ES, respectively, and covered the whole surface at lower potentials. No evidence for cointercalation of solvent molecules was observed in the presence of each additive. It was concluded that the layer of the precipitates functions as a protective surface film, which suppresses cointercalation of PC molecules as well as direct solvent decomposition on the surface of the graphite negative electrode.