Analysis of the chemical composition of the passive film on Li-ion battery anodes using attentuated total reflection infrared Spectroscopy

We examined the surfaces of graphite anodes extracted from Li-ion cells with Fourier transform infrared spectroscopy using attenuated total reflection geometry. The cells were of the 18650-type and subjected to calender aging (60% state of charge) at 55 degreesC. The composition of the film on an anode from a control cell (not aged) is composed of lithium oxalate (Li2C2O4), lithium carboxylate (RCOOLi), and lithium methoxide (LiOCH3). After aging, there is also lithium hydroxide (LiOH) and methanol (CH3OH), and in some cases lithium hydrogen carbonate (LiHCO3), probably due to the reaction of water with the methoxide and oxalate. There is substantial variation in the relative amounts of the five compounds over the surfaces of the electrodes. Alkyl carbonates may form early on, but decompose to more "inorganic'' compounds with aging. The multicomponent composition reflects the complex chemistry of passive film formation in real Li-ion cells. (C) 2003 The Electrochemical Society.