Silicon (Si) is a promising anode material for lithium ion batteries (LIB) due to its high specific capacity (3579 mAh g^-1). However, the large capacity of Si is accompanied by a large volume expansion which causes severe mechanical degradation within the electrode. Ultimately it leads to loss of electrical contact between the active material and current collector, causing capacity fading. Several approaches have been conducted to enhance the electrochemical performance of Si anode, for example, using nano-sized Si particles, limiting the charge and discharge voltage window, and incorporating additives to the electrolytes. Among these approaches, the use of electrolyte additive can be one of the most effective approaches to improve LIBs. In particular, fluoroethylene carbonate (FEC) is known to be a proper electrolyte additive for Si anode because it can form a stable solid electrolyte interface (SEI). Edstrom et al demonstrated that an SEI layer derived from FEC could improve the capacity retention and Coulombic efficiency of lithium/Silicon in half cell ⁽¹⁾. This is largely due to the decomposition of FEC, which mainly forms LiF. It plays an important role in enhancing the electrochemical cycling performance. However, Schroder et al. suggest that FEC eventually runs out. After long term cycling, a similar SEI composition as the non-FEC containing electrolyte is formed ⁽²⁾. In addition, the electrolyte used in LIB is generally carbonate type solvent with LiPF₆ as a salt. It has been reported that decomposition of the carbonate electrolyte leads to gas evolution in these cells during charge and discharge. Gas evolution is a significant issue because it affects not only the battery performance, but also the reliability as a safe industrial product. To prevent the electrolyte from gas evolution, gas evolution has to be improved for long cycle stability and safety of cells for in practical use. Therefore, one of the important solutions in the development of a long-life and safe LIB is to change the composition of electrolytes for long cycling.

Ionic liquids have been investigated for LIB as a promising electrolyte because of its low volatility, negligible vapor pressure, thermal stability, and high ionic conductivity. The electrochemical performance and interfacial phenomenon with ionic liquid and Si composite anode is not well understood. Herein, we focus on bis(fluorosulfonyl) imide (FSI)-based ionic liquid as a promising electrolyte for Si composite anode. Comparisons among ionic liquid electrolyte and conventional carbonate electrolytes are thoroughly investigated. Effects of different electrolytes including (1) ethylene carbonate/diethyl carbonate (EC/DEC), (2) FEC containing EC/DEC, (3) 1-Ethyl-3-methylimidazolium bis(fluorosulfonyl) imide (EMI-FSI) on cycling performance and the SEI formation mechanism on Si anodes have been investigated using electrochemical performances, scanning electron microscope (SEM), electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS). Electrochemical cycle performance (C/10) of Si anodes with FEC additive and EMI-FSI were better than that of EC/DEC without the FEC. Moreover, the electrochemical impedance measurement with FEC additive and with EMI-FSI after cycling demonstrated low interfacial and charge-transfer resistances compared to the corresponding resistances observed in the EC/DEC cycled electrode. Although EMI-FSI has a relatively high viscosity, it showed not only the stable cycling performance, but also the lower impedance for a Si anode. The difference of the SEI formation mechanism among each electrolyte has not yet been clearly understood yet, which is characterized through further studies.

1. Chao Xu, Fredrik Lindgren, Bertrand Philippe, Mihaela Gorgoi, Fredrik Björefors, Kristina Edström, and Torbjörn Gustafsso “Improved Performance of the Silicon Anode for Li-Ion Batteries: Understanding the Surface Modification Mechanism of Fluoroethylene Carbonate as an Effective Electrolyte Additive”, Chem. Mater., 2015, 27 (7), 2591

2. K. Schroder, J. Alvarado, T. A. Yersak, J. Li, N. Dudney, L. J. Webb, Y. S. Meng, and K. J. Stevenson. “The Effect of Fluoroethylene Carbonate as an Additive on the Solid Electrolyte Interphase on Silicon Lithium-Ion Electrolytes”, Chem. Mater., 2015, 27 (16), 5531