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

Abstract

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 LiPF6 in either 3:7 (w/w) EC:DEC or 3:7 (w/w) FEC:DEC. The results suggest that some CO2 formation is caused by reactive oxygen species originating from Li2MnO3 domain activation. The electrolyte composition has no effect on O2 evolution during Li2MnO3 domain activation and oxidative CO2 evolution in HE-NCM/Li half-cells. In contrast, decomposition of PF6− 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 CO2 and possibly other processes relevant for the stability and endurance of high voltage lithium-ion batteries.