Chemical composition and morphology of the elevated temperature SEI on graphite

The relationship between the elevated temperature performance of Li/graphite half-cells and the composition and morphology of the solid electrolyte interphase (SET) formed on the graphite surface has been investigated for two electrolyte systems: 1 M LiPF6 in ethylene carbonate/dimethyl carbonate EC/DMC (2:1) and 1 M LiBF4 in EC/DMC (2:1). Precycled cells were stored at different temperatures up to 80 degreesC, and the graphite electrodes were analyzed chemically (by X-ray photoelectron spectroscopy) and electrochemically under continued cycling. Loss of charge (for both salts) and of intercalation capacity (for LiBF4) occurred after elevated temperature storage. The charge loss could be coupled to disappearance of the R-OCO2Li phase from the surface, with subsequent exposure of the graphite surface. The amount of LiF increased with increased storage temperature, but the LiF morphology differed between the two electrolyte systems. A model for the morphological changes of the SEI layer on storage at elevated temperature is proposed. (C) 2001 The Electrochemical Society.