Electrochemical and infrared studies of the reduction of organic carbonates

The reduction potentials of five organic carbonates commonly employed in li thium battery electrolytes, ethylene carbonate (EC), propylene carbonate (P C), diethyl carbonate (DEC), dimethyl carbonate (DMC), and vinylene carbona te (VC) were determined by cyclic voltammetry using inert (Au or glassy car bon) electrodes in tetrahydrofuran/LiClO4 supporting electrolyte. The reduc tion potentials for all five organic carbonates were above 1 V (vs. Li/Li+) . PC reduction was observed to have a significant kinetic hindrance. The me asured reduction potentials for EC, DEC, and PC were consistent with thermo dynamic values calculated using density functional theory (DFT) assuming on e-electron reduction to the radical anion. The experimental values for VC a nd DMC were, however, much more positive than the calculated values, which we attribute to different reaction pathways. The role of VC as an additive in a PC-based electrolyte was investigated using conventional constant-curr ent cycling combined with ex situ infrared spectroscopy and in situ atomic force microscopy (AFM). We confirmed stable cycling of a commercial li-ion battery carbon anode in a PC-based electrolyte with 5 mol % VC added. The p referential reduction of VC and the solid electrolyte interphase layer form ation therefrom appears to inhibit PC cointercalation and subsequent graphi te exfoliation. (C) 2001 The Electrochemical Society.