Surface analysis of electrodes from cells containing electrolytes with stabilizing additives exposed to high temperature

We have conducted a detailed investigation of the effect of thermal stabilizing additives, including dimethyl acetamide (DMAc), N-methyl pyrrolidone, vinylene carbonate (VC), and vinylethylene carbonate (VEC), on the reactions of the electrolyte with the surface of the electrodes in lithium-ion cells. Cells were constructed with mesocarbon microbead anodes, LiNi0.8Co0.2O2 cathodes, and 1.0 M LiPF6 in 1:1:1 ethylene carbonate/diethyl carbonate/dimethyl carbonate electrolyte with and without electrolyte additives. The cells were stored sequentially at 55, 60, and 65 degrees C for 10 days at each temperature. The cells were then dismantled, and the surfaces of the electrodes were analyzed via a combination of infrared spectroscopy with attenuated total reflection, X-ray photoelectron spectroscopy, and scanning electron microscope-energy dispersive spectroscopy. The surface of the electrodes extracted from cells containing the baseline electrolyte contained thick surface films composed of electrolyte decomposition products. The addition of 1% DMAc inhibits the reaction of the electrolyte with surface of the electrodes, especially on the anode. The addition of 1.5% VC results in the formation of poly(vinylene carbonate) on both electrodes and inhibits the reaction of electrolyte with the electrodes, especially the cathode. The addition of 1.5% VEC or 10% DMAc did not significantly impede the reaction of the electrolyte with the electrodes. (C) 2008 The Electrochemical Society.