In this study, we use on-line electrochemical mass spectrometry (OEMS) to investigate the electrolyte decomposition reactions associated with transition metals on graphite electrodes. In order to have defined amounts of transition metals in the system, we use model electrolytes containing EC + 1.5 M LiPF6 and small concentrations of Mn(TFSI)2, Co(TFSI)2 or Ni(TFSI)2. As ethylene is the major gaseous product of the reductive decomposition of EC, we can use it as an indicator for the quantitative analysis of electrolyte reduction. In this way, we can compare the extent of electrolyte decomposition during formation in the presence of different transition metal ions (see Figure 1). By using potential resolved OEMS, we determine the potential dependence of electrolyte decomposition and correlate this with the reduction potentials of the transition metal ions. Further, we investigate the effect of transition metal ion concentration per graphite surface area on the extent of electrolyte decomposition.
In real lithium-ion cells, however, transition metal dissolution typically occurs during extended cycling, i.e., long after the formation process is completed. Therefore, we investigate the effect of transition metal ions on graphite electrodes that have been pre-formed in transition metal free electrolyte. These electrodes are then transferred into cells containing the same transition metal spiked model electrolytes as before. Here, we also test the effect of different SEI forming additives, namely vinylene carbonate (VC) and fluoroethylene carbonate (FEC), on their ability to suppress electrolyte decomposition induced by transition metal ions, by performing the pre-formation in electrolytes containing VC or FEC.
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Acknowledgements:
This work is financially supported by the BASF SE Battery Research Network. Funding for R. J. was provided by BMW AG.
Figure 1: Ethylene evolution during the first (solid bars) and the second (dashed bars) cycle measured by OEMS during potentiodynamic formation (2 CVs between 0.1 and 2 V vs. Li/Li+ at 0.2 mV/s) of a graphite electrode (95% graphite, 5% PVDF) in an EC / 1.5 M LiPF6 electrolyte containing no transition metal ions, 10 mM Co(TFSI)2, 10 mM Ni(TFSI)2, or 10 mM Mn(TFSI)2. To avoid any deposition on the lithium counter electrode, the experiments were performed in a sealed 2-compartment9 cell.