Online Electrochemical Mass Spectrometry of High Energy Lithium Nickel Cobalt Manganese Oxide/Graphite Half- and Full-Cells with Ethylene Carbonate and Fluoroethylene Carbonate Based Electrolytes

Online electrochemical mass spectrometry (OEMS) was employed to investigate the behavior of ethylene carbonate (EC) and fluoroethylene carbonate (FEC) co-solvents in high voltage lithium-ion batteries based on high energy lithium nickel cobalt manganese oxide (HE-NCM) and SFG6 graphite (SFG6) electrodes. Gas evolution from HE-NCM and SFG6 electrodes was studied during two charge/discharge cycles in half-cell and full-cell configuration using electrolytes composed of 1 M LiPF 6 in either 3:7 (w/w) EC:DEC or 3:7 (w/w) FEC:DEC. The results suggest that some CO 2 formation is caused by reactive oxygen species originating from Li 2 MnO 3 domain activation. The electrolyte composition has no effect on O 2 evolution during Li 2 MnO 3 domain activation and oxidative CO 2 evolution in HE-NCM/Li half-cells. In contrast, decomposition of PF 6 − is significantly enhanced in electrolyte containing FEC and found to be strongly facilitated by the SuperC65 conductive carbon additive employed in the HE-NCM electrodes. EC and FEC clearly follow different reductive decomposition pathways. This leads to major differences in solid electrolyte interphase (SEI) formation and affects follow-up reactions involving CO 2 and possibly other processes relevant for the stability and endurance of high voltage lithium-ion batteries.