In-situ FTIR investigations on the reduction of vinylene electrolyte additives suitable for use in lithium-ion batteries

Lithium-ion batteries operate beyond the thermodynamic stability of the aprotic organic electrolyte used and electrolyte decomposition occurs at both electrodes. The electrolyte must therefore be composed in a way that its decomposition products form a film on the electrodes which stops the decomposition reactions but is still permeable to the Li+ cations which are the charge carriers. At the graphite anode, this film is commonly referred to as a solid electrolyte interphase (SEI). Aprotic organic compounds containing vinylene groups can form an effective SEI on a graphitic anode. As examples, vinyl acetate (VA) and acrylonitrile (AN) have been investigated by in-situ Fourier transform infrared (FTIR) spectroscopy in a specially developed IR cell. The measurements focus on electrolyte decomposition and the mechanism of SEI formation in the presence of VA and AN. We conclude that cathodic reduction of the vinylene groups (i.e., via reduction of the double bond) in the electrolyte additives is the initiating and thus a most important step of the SEI-formation process, even in an electrolyte which contains only a few percent (i.e. electrolyte additive amounts) of the compound. The possibility of electropolymerization of the vinylene monomers in the battery electrolytes used is critically discussed on the basis of the IR data obtained.