In Situ Investigation of the Electrochemical Reduction of Carbonate Electrolyte Solutions at Graphite Electrodes

The reductive decomposition of carbonate electrolyte solutions containing lithium ions was studied by differential electrochemical mass spectrometry (DEMS) and subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS). The influence of the composition of the negative electrode on its electrochemical behavior was investigated by varying the graphite/binder ratio and using nongraphitic materials. The effect of different electrolyte salts was studied in a 1:1 ethylene carbonate (EC)/dimethyl carbonate (DMC) mixture. In addition, EC/DMC-based electrolyte solutions containing different amounts of water were used to investigate the influence of H₂O on the performance of graphite electrodes. Ethylene and hydrogen were the volatile decomposition products monitored by DEMS. Ethylene evolution is restricted to a potential region between about 0.8 and 0.3 V vs Li/Li⁺ and occurs only during the first cycle. Hydrogen begins to evolve at about 1.3 V vs Li/Li⁺ in the first cycle and the amount of gas decreases with increasing cycle number. The experimental results are discussed in terms of proposed reaction mechanisms.