Electrolyte Decomposition on Graphite Anodes in the Presence of Transition Metal Ions
Authors: Solchenbach Sophie , Hong Gloria , Freiberg Anna Teresa Sophie , Jung Roland , Gasteiger Hubert A. 
Published: February 2020

The dissolution of transition metals from cathode active materials is a major aging mechanism in lithium-ion batteries. Manganese dissolution has long been known for lithium manganese oxide spinel cathodes cycled at high voltages or temperatures.1,2 As layered oxide cathodes, i.e., lithium nickel manganese cobalt oxide (NMC), are cycled to higher cut-off potentials to enhance specific energy and capacity, transition metal dissolution also appears here and becomes significant not only for manganese, but also nickel and cobalt.3 The detrimental effect of transition metal dissolution does not lie so much in the actual destruction of the cathode active material, but rather in the deposition of transition metal ions on the anode.4 There, deposited transition metal ions lead to a decrease in capacity, lower coulombic efficiency, and increased impedance.5,6 The mechanism behind this deterioration is not fully understood and has been a subject of debate in the literature. While many reports indicate that the presence of transition metal ions might be related to an enhanced consumption of electrolyte on the anode,7,8previous works mostly focus on the oxidation state or chemical surrounding of transition metals in the SEI.
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.

Figure 1