Surface film formation on a graphite negative electrode in lithium-ion batteries: AFM study on the effects of co-solvents in ethylene carbonate-based solutions

In situ AFM observation of the basal plane of highly oriented pyrolytic graphite (HOPG) was performed before and after cyclic voltammetry in 1 mol dm⁻³ LiClO₄ dissolved in ethylene carbonate (EC), EC+diethyl carbonate (DEC), and EC+dimethyl carbonate (DMC) to clarify the effects of co-solvents in EC-based solutions on surface film formation on graphite negative electrodes in lithium-ion cells. In each solution, surface film formation involved the following two different processes: (i) intercalation of solvated lithium ions and their decomposition beneath the surface; and (ii) direct decomposition of solvent molecules on the basal plane to form a precipitate layer. The most remarkable difference among these solvent systems was that solvent co-intercalation took place more extensively in EC+DEC than in EC+DMC or EC. Raman analysis of ion-solvent interactions revealed that a lithium ion is solvated by three EC molecules and one DEC molecule in EC+DEC, whereas it is solvated exclusively by EC in EC+DMC and in EC, which suggested that the presence of linear alkyl carbonates in the solvation shell of lithium ion enhance the degree of solvent co-intercalation that occurs in the initial stage of the surface film formation.