Online detection of reductive CO2 development at graphite electrodes in the 1 M LiPF6, EC : DMC battery electrolyte

Differential electrochemical mass spectrometry (DEMS) was used to investigate in detail volatile reaction products generated during the first few charge/discharge cycles of TIMREX KS44 and KS6 graphite electrodes in the 1 M LiPF6, ethylene carbonate: dimethyl carbonate (EC:DMC) standard battery electrolyte, with focus on the region where the solid electrolyte interphase (SEI) layer is formed on the graphite surface. Due to the further improved sensitivity of the DEMS method, a clear signal of CO2 (m/z = 44) due to the electrochemical reduction of the electrolyte was detected. The reductive development of CO2 starts slightly positive to 1 V vs Li/Li+. Other gases such as ethylene and hydrogen were detected at more negative potentials. Both the ethylene and the CO2 signal intensities rapidly decreased below the noise level after the first electrochemical cycle, but a quite constant hydrogen signal was detected during each charge/discharge cycle in the potential region where Li+ intercalates into the graphite. This indicates that the reductive decomposition of the electrolyte is not completed after the formation of the primary SEI layer. (c) 2008 The Electrochemical Society.