Difec the Rescue? Evaluating Di-Fluoroethylene Carbonate As Additive for Silicon Electrodes

Wetjen, Morten

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 mg_electrode 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.

References:

(1) Schroder, K.; Alvarado, J.; Yersak, T. A.; Li, J.; Dudney, N.; Webb, L. J.; Meng, Y. S.; Stevenson, K. J. Chem. Mater. 2015, 27 (16), 5531–5542.

(2) Markevich, E.; Fridman, K.; Sharabi, R.; Elazari, R.; Salitra, G.; Gottlieb, H. E.; Gershinsky, G.; Garsuch, A.; Semrau, G.; Schmidt, M. A.; Aurbach, D. J. Electrochem. Soc. 2013, 160 (10), A1824–A1833.

(3) Xu, C.; Lindgren, F.; Philippe, B.; Gorgoi, M.; Björefors, F.; Edström, K.; Gustafsson, T. Chem. Mater. 2015, 27 (7), 2591–2599.

(4) Jung, R.; Metzger, M.; Haering, D.; Solchenbach, S.; Marino, C.; Tsiouvaras, N.; Stinner, C.; Gasteiger, H. A. J. Electrochem. Soc. 2016, 163 (8), A1705–A1716.

(5) Petibon, R.; Chevrier, V. L.; Aiken, C. P.; Hall, D. S.; Hyatt, S. R.; Shunmugasundaram, R.; Dahn, J. R. J. Electrochem. Soc. 2016, 163 (7), A1146–A1156.

(6) Xia, J.; Petibon, R.; Xiao, A.; Lamanna, W. M.; Dahn, J. R. J. Electrochem. Soc. 2016, 163 (8), A1637–A1645.

(7) Solchenbach, S.; Pritzl, D.; Kong, E. J. Y.; Landesfeind, J.; Gasteiger, H. A. J. Electrochem. Soc. 2016, 163 (10), A2265–A2272.

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

213 Difec the Rescue? Evaluating Di-Fluoroethylene Carbonate As Additive for Silicon Electrodes

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Difec the Rescue? Evaluating Di-Fluoroethylene Carbonate As Additive for Silicon Electrodes