Charge-Discharge Characteristics of Lithium and Silicon Anodes in Li[N(CF3SO2)2]-Glyme Solvate Ionic Liquids

The equimolar mixture of lithium bis(trifluoromethylsulfonyl)amide (Li[N(CF₃SO₂)₂] or LiTFSA) and triglyme (G3) or tetraglyme (G4) has been known to give a solvate ionic liquid, which is composed of Li⁺ solvated by glyme and TFSA^–. Since Li⁺is strongly coordinated by the glyme molecule, the vapor pressure at ambient temperature is negligible. In addition, the high concentration of Li(I) species in the solvate ionic liquids is considered favorable for the electrolytes of rechargeable lithium batteries. Furthermore, the solvate ionic liquids have been expected to be applied for the electrolytes of rechargeable sulfur batteries because of the low solubility of lithium polysulfides [1]. In order to make the best use of the high specific capacity of the sulfur cathode, it is favorable to employ lithium or silicon as the anode material. In the present study, the charge-discharge characteristics of lithium and silicon anodes were investigated in LiTFSA-glyme (G3 or G4) solvate ionic liquids containing excess LiTFSA.

Galvanostatic charge-discharge experiments were conducted with 2032 coin-type cells. Copper and silicon thin film prepared by RF magnetron sputtering on a copper foil were used as a working electrode. Lithium was used as a counter electrode. A polypropylene-based porous membrane was used as a separator. The solvate ionic liquids prepared by mixing LiTFSA and glyme (G3 or G4) at the molar ratios of 1 : 1 and 1.2 : 1 were used as the electrolytes, which are denoted by 50.0-50.0 and 54.5-45.5 mol% LiTFSA-glyme, respectively.

The coulombic efficiency for deposition and dissolution of lithium on the copper substrate was examined at ±0.1 mA cm^–2 with the cut-off voltage of 2 V. The electric charge for deposition was 0.2 C cm^–2. The coulombic efficiency in both 50.0-50.0 and 54.5-45.5 mol% LiTFSA-glyme increased within the first ten cycles and exceeded more than 90% until 50 cycles. The efficiency in 54.5-45.5 mol% LiTFSA-glyme was greater than that in 50.0-50.0 mol% one, suggesting addition of excess LiTFSA to the solvate ionic liquids improves the coulombic efficiency. The coulombic efficiency for alloying and dealloying of silicon and lithium was also examined in the same electrolytes at ±0.05 mA cm^–2 within the voltage range from 0.01 to 2 V. Reversible cycling was possible with the specific discharge capacity of more than 2400 mAh g^–1-Si in both 50.0-50.0 and 54.5-45.5 mol% LiTFSA-G4. The coulombic efficiency of more than 99% was achieved with the silicon thin film anode within 30 cycles while significant capacity fading was observed within a few cycles in LiTFSA-G3 solution at the molar ratio of 1 : 4. High coulombic efficiency of lithium and silicon anodes in the solvate ionic liquids containing excess LiTFSA may be due to the low activity of free glymes in the electrolyte. Addition of LiTFSA to 50.0-50.0 mol% LiTFSA-glyme is considered to introduce a Li⁺ species, [Li(TFSA)₂]^–, which is known to form in other TFSA^–-based ionic liquids, and decrease the activity (or life-time) of free glyme, which may react with lithium and lithiated silicon and cause capacity fading.

References

1. K. Dokko, N. Tachikawa, K. Yamauchi, M. Tsuchiya, A. Yamazaki, E. Takashima, J.-W. Park, K. Ueno, S. Seki, N. Serizawa, and M. Watanabe, J. Electrochem. Soc., 160(8), A1304 (2013).