Ultra-thin passivating film induced by vinylene carbonate on highly oriented pyrolytic graphite negative electrode in lithium-ion cell

We investigated the influence of vinylene carbonate, as an additive molecule, on the decomposition phenomena of electrolyte solution [ethylene carbonate (EC)-ethyl methyl carbonate (EMC) (1:2 by volume) containing 1 M LiPF6] on a highly oriented pyrolytic graphite (HOPG) negative electrode by using cyclic voltammetry (CV) and atomic force microscopy (AFM). Vinylene carbonate deactivated reactive sites (e.g. radicals and oxides at the defects and the edge of carbon layer) on the cleaved surface of the HOPG negative electrode, and prevented further decomposition of the other solvents there. Further, vinylene carbonate induced an ultra-thin film (less than 1.0 nm in thickness) on the terrace of the basal plane of the HOPG negative electrode, and this film suppressed the decomposition of electrolyte solution on the terraces of the basal plane. We consider that this ultra-thin passivating film is composed of a reduction product of vinylene carbonate (VC). and might have a polymer structure. These induced effects might explain how VC improves the life performance of lithium-ion cells. (C) 2002 Elsevier Science B.V. All rights reserved.