Effects of the Composition of the Solid Electrolyte Interphase on the Charge-Discharge Performance of a Li Metal Anode in Li[N (CF3SO2)2]-Sulfolane-Based Electrolyte
Authors: Nishikawa Mika , Uchiya Tomohiro , Matsumoto Takaaki , Serizawa Nobuyuki , Katayama Yasushi 
Published: December 2020

Lithium–sulfur (Li-S) batterry is a promising alternative energy storage device due to its high theoretical specific capacity, low density, and abundance of sulfur. However, lithium polysulfides (Li2S
x
) produced during discharge have been known to dissolve in conventional organic electrolytes, resulting in self-discharge due to the shuttle mechanism. Moreover, the improvement of the Li metal anode is also necessary to realize the Li-S battery. It has been found that the solubility of Li2S
x
is very low in the solvate ionic liquids (SILs) composed of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) and tetraglyme (G4, CH3(OCH2CH2)4OCH3).1 We have reported the deposition and dissolution of Li in the SILs with different compositions [2]. Formation of the solid electrolyte interphase (SEI) on the anode in the SILs was analyzed by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Recently, a highly concentrated LiTFSA/sulfolane (SL) is considered as a candidate for the electrolyte of Li-S battery because of the low solubility of Li2S
x
and fast Li+ conduction caused by the hopping mechanism [3]. In the present study, the charge-discharge performance of a Li metal anode was investigated in a SL-based electrolyte from a point of view of the SEI formation on Li.
The electrolyte was prepared by mixing LiTFSA, SL and 1,1,2,2,-tetrafluoroethl 2,2,3,3,-tetrafluoropropyl (HFE) at 20.0-40.0-40.0 mol%. HFE was added to reduce the viscosity of the electrolyte. The SILs were prepared by mixing LiTFSA or LiFSA and G4 at 50.0-50.0 mol%. The water contents in the SL-based electrolyte and SILs were less than 50 ppm, which was determined by Karl Fischer titration. The charge-discharge measurements were conducted using a 2032 coin-type Li|LiFePO4 cell. Celgard 3501 was used as a separator. Li electrode was immersed in LiTFSA-G4 or LiFSA-G4 SILs to form SEI before the measurements. Charge–discharge cycle test was performed at a current density of 0.371 mA cm–2 with a charge density of 1.0 C cm–2. The characterization of SEI on the anode was carried out by TEM and XPS using air-tight transfer holders without exposure to air.
The larger polarization and lower columbic efficiency for deposition and dissolution of Li (with the cut-off potential of 2.0 V vs. Li|Li(I)) were observed in the SL-based electrolyte compared with those in LiTFSA-G4 SILs probably due to the inhomogeneous and/or unstable SEI on the anode. On the other hand, the cyclability was improved even in the SL-based electrolyte using the Li electrode immersed in LiTFSA-G4 or LiFSA-G4 SILs for 24 hours before the measurements. In the case of Li immersed in LiFSA-G4 SIL, charge-discharge operation can be achieved for more than 500-cycle. The SEI formed on the anode in the SIL was suggested to be ticker than that in the SL-based electrolyte by TEM observation. The SEI formed in the SILs was considered to work even in the SL-based electrolyte, resulting in the improvement of the reversibility of the Li anode.
Acknowledgment
This study was supported by the Advanced Low Carbon Technology Research and Development of Program (ALCA) of the Japan Science and Technology Agency (JST).
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