Electrolyte and SEI Decomposition Reactions of Transition Metal Ions Investigated by On-Line Electrochemical Mass Spectrometry

We use on-line electrochemical mass spectrometry (OEMS) to elucidate and quantify the electrolyte reduction on graphite caused by transition metal ions. To have a controlled system, we use ethylene carbonate (EC) with 1.5 M LiPF6 and representative amounts of Ni(TFSI)(2) or Mn(TFSI)(2) as model electrolytes, combined with a 2-compartment cell in which anolyte and catholyte are separated by an impermeable solid lithium ion conductor. Focusing on C2H4 evolution as a marker for EC reduction, we find that both Ni2+ and Mn2+ lead to enhanced gas evolution on pristine graphite electrodes once the potential is decreased to below the (TM2+/TM0) redox potential, demonstrating that the reduced transition metals are active toward electrolyte reduction. If the electrodes are preformed in a TM-free electrolyte and subsequently cycled in an electrolyte containing either Mn2+ or Ni2+, the activity of nickel toward electrolyte decomposition is greatly reduced, whereas the electrolyte with manganese still shows a strong ongoing C2H4 generation. The use of vinylene carbonate during formation partially suppresses the gas evolution from manganese. Using OEMS and post-mortem ATR-FTIR, we finally show that reduced manganese can decompose organic SEI components into Li2CO3, thereby compromising the integrity of the SEI and enabling the additional reduction of electrolyte. (c) The Author(s) 2018. Published by ECS.