Consumption of Fluoroethylene Carbonate (FEC) on Si-C Composite Electrodes for Li-Ion Batteries

In the emerging market of electric vehicles (EVs), the development of batteries with higher energy density and improved cycle-life is essential.1 Silicon-based anodes can significantly improve the energy density due to the huge specific capacity of ~3600 mAh/gSi, (corresponding to the Li15Si4 phase2) which is roughly ten times larger compared to conventional graphite electrodes (372 mAh/gC 3). However, the alloying of silicon with lithium is accompanied by large structural changes, resulting in a volume increase by 310% upon full lithiation.2 This causes fast degradation of the capacity due to particle cracking, loss of electric and ionic conductivity, increasing electrode polarization and continuous electrolyte consumption.4-6 In this work we will investigate the consumption of the electrolyte additive fluoroethylene carbonate (FEC) which is known to significantly improve the lifetime of Li-ion batteries with silicon anodes.7 Fig. 1 shows the cycling stability and coulombic efficiency of Si/C composites (40%wt nano-Si, 40%wt VGCF fibers, and 20%wt LiPAA binder; 1.2 mgSi/cm2) cycled in coin cells with metallic lithium counter electrodes and LP57 electrolyte containing various amounts of FEC. A striking feature is a rapid capacity drop which is delayed to higher and higher cycle numbers as more FEC is added to the electrolyte. By the use of 19F-NMR spectroscopy we demonstrate that the rapid capacity drop is due to the total consumptions of FEC. Its depletion causes a significant increase of the cell polarization, leading to the rapid capacity drop. We show with On-line Electrochemical Mass Spectrometry (OEMS) that the presence of FEC in the electrolyte prohibits the reduction of other electrolyte components almost entirely. Consequently, the cumulative irreversible capacity until the rapid capacity drop correlates linearly with the specific amount of FEC (in units of µmolFEC/mgelectrode) in the cell. The lifetime of silicon anodes is therefore determined by the specific amount of FEC rather than by the FEC concentration. By correlating the cumulative irreversible capacity and the specific amount of FEC in the cell, we present an easy tool to predict how much cumulative irreversible capacity can be tolerated until all FEC will be consumed in either half-cells or full-cells. We further demonstrate that four electrons are consumed for the reduction of one FEC molecule and that one carbon dioxide molecule is released for every FEC molecule that is reduced. Using all information from this study and combining it with previous reports in the literature, a new reductive decomposition mechanism for FEC is proposed yielding CO2, LiF, Li2O, Li2CO3, H2 and a partially cross-linked polymer. Fig. 1. (a) Coulombic efficiency and (b) specific lithiation capacity vs. cycle number of Si-Li coin cells with 75 µL LP57 electrolyte containing different amounts of FEC. The first three cycles are conducted at C/10 followed by cycling at C/3. The theoretical capacity is 1440 mAh/gelectrode and the specific amount of FEC (in µmolFEC/mgelectrode) is specified in the figure. References: McDowell, M. T.; Lee, S. W.; Nix, W. D.; Cui, Y., Adv. Mater. (Weinheim, Ger.), 25, 4966 (2013). Obrovac, M. N.; Christensen, L., Electrochem. Solid-State Lett., 7, A93 (2004). Kasavajjula, U.; Wang, C.; Appleby, A. J., J. Power Sources, 163, 1003 (2007). Fuchsbichler, B.; Stangl, C.; Kren, H.; Uhlig, F.; Koller S.;, J. Power Sources, 196, 2889 (2011). Christensen, J.; Newman, J., J. Electrochem. Soc., 151, A1977 (2004). Christensen, J.; Newman, J., J. Electrochem. Soc., 152, A818 (2005). Etacheri, V.; Haik, O.; Goffer, Y.; Roberts, G. A.; Stefan, I. C.; Fasching, R.; Aurbach, D., Langmuir, 28, 965 (2012). Acknowledgements: BMW is gratefully acknowledged for financial support. The authors kindly thank Uta Schwenke and Morten Wetjen from our group for helpful advices and great contributions to this work. Figure 1