Difec the Rescue? Evaluating Di-Fluoroethylene Carbonate As Additive for Silicon Electrodes
Authors: Wetjen Morten , Pritzl Daniel , Hong Gloria , Solchenbach Sophie , Gasteiger Hubert A. 
Published: February 2020

Fluoroethylene carbonate (FEC) is one of the most commonly used electrolyte additives for silicon-based electrodes in Li-ion batteries.1 Several studies demonstrated an improved cyclability with FEC-containing electrolytes and ascribed the effect to the formation of a kinetically stable SEI with an increased LiF and Li2O content.2 The SEI morphology also revealed less fractures and a better passivation of the silicon particles upon cycling.3 Nonetheless, recently it was reported that FEC only suppresses the decomposition of other electrolyte compounds, such as ethylene carbonate (EC), as long as FEC is present in the electrolyte.4,5 As a consequence, the lifetime of silicon-based electrodes in Li-ion batteries is limited by the total amount of FEC in the electrolyte solution.
In the present study, we evaluate di-fluoroethylene carbonate (DiFEC) as alternative electrolyte additive for silicon-based electrodes. Taking into account the beneficial effect of FEC on the cycling performance, we seek to enhance the additive’s properties by addition of a second fluorine atom. Recently, Dahn and co-workers investigated DiFEC in regard to its oxidative decomposition in NMC442/graphite cells.6 In the same study, they also demonstrated that DiFEC is a SEI forming additive on the negative electrode.
To benchmark the electrochemical behavior of DiFEC on silicon electrodes, we prepared three electrolyte solutions, consisting of (i) 1 M LiPF6 in EC:ethyl methyl carbonate (3:7 w:w) – also referred to as LP57; (ii) 5 wt% FEC in LP57, and (iii) 5 wt% DiFEC in LP57. As working electrode, we prepared silicon-graphite (SiG) composite electrodes with 35 wt% silicon and 45 wt% graphite with an areal capacity of 2.1±0.2 mAh cm-2 (~1.5 mgelectrode cm-2). Lithium poly(acrylate) binder and vapor grown carbon fibers accounted for the remaining 20 wt%.

Figure
1
. Differential capacity of Li//SiG coin-cells, incorporating different (fluorinated) EC derivatives (each 5 wt%) in LP57, obtained from a galvanostatic scan at 0.02 h-1.

By use of dQdV analysis of galvanostatic scans in Li//SiG cells, as shown in Figure 1, we demonstrate that DiFEC reductively decomposes at more positive potentials compared to FEC and EC. Further, we evaluate the cycling stability in SiG//LiFePO4 cells with capacitively oversized cathodes and quantify the (Di)FEC consumption upon cycling by post-mortem 19F-NMR analysis. Our results reveal a trade-off between capacity retention and irreversible capacity losses within 300 cycles. To understand the corresponding processes at the SiG electrode-electrolyte interface, we conduct impedance measurements to monitor the impedance evolution upon cycling, using a gold-wire reference electrode that allows to separate the impedance contributions from the negative and positive electrode.7 Finally, we discuss our findings with the aid of on-line electrochemical mass spectrometry (OEMS) and propose a reduction mechanism for DiFEC on silicon-based electrodes.

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Acknowledgements: The German Federal Ministry for Economic Affairs and Energy is acknowledged for funding (funding number 03ET6045D).

Figure 1