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.