In Situ Electrochemical Measurements of Deposition and Dissolution of Lithium in Li[N(CF3SO2)2]-Glyme Solvate Ionic Liquids

Abstract

The equimolar mixture of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA)-tetraglyme (G4) has been known as a solvate ionic liquid consisting of [Li(G4)]+ and TFSA–. This solvate ionic liquid contains lithium ions in high concentration (ca. 3 mol dm–3) and has negligible vapor pressure at ambient temperature. It has been reported that the solvate ionic liquid is compatible with sulfur cathode because of the low solubility of lithium polysulfides [1]. To achieve the high energy density of the rechargeable Li-S batteries, it is required to investigate anode materials having high specific capacity such as a lithium anode. In this study, the initial stage of deposition of lithium was investigated in the solvate ionic liquids with different compositions using electrochemical quartz crystal microbalance (EQCM) and optical microscope. LiTFSA-G4 solvate ionic liquids were prepared by mixing LiTFSA and G4 at different molar ratios of LiTFSA (X LiTFSA) in an Ar filled glove box. The potentials of Li(I)|Li were evaluated using a Ag|Ag(I) reference electrode, which consisted of a silver wire immersed in 50.0-50.0 mol% LiTFSA-G4 containing 0.1 mol dm–3 AgCF3SO3. EQCM measurement was performed with an airtight three-electrode cell. A Cu coated quartz crystal electrode was used as a working electrode. Lithium was used as a reference and counter electrode. [Li(G4)]+ is considered a major lithium species in LiTFSA-G4 solvate ionic liquid at X LiTFSA = 50 mol%. On the other hand, both [Li(G4)]+ and [Li(TFSA)2]– are expected to exist in LiTFSA-G4 solvate ionic liquid at X LiTFSA > 50 mol%. It was found that the potential of Li(I)|Li in the LiTFSA-G4 solvate ionic liquid at X LiTFSA > 50 mol% was given by an equilibrium, [Li(TFSA)2]– + e– = Li + 2TFSA–, indicating the active lithium species is [Li(TFSA)2]– rather than [Li(G4)]+. Li was deposited at a current density of –0.01 mA cm–2. The electric charge for deposition was –0.05 C cm–2. In the case of 54.5-45.5 mol% LiTFSA-G4 solvate ionic liquid, Li was deposited sparsely on the Cu electrode surface and grew as a whisker-like shape. On the other hand, whisker-like Li deposit was not observed in 50.0-50.0 mol% LiTFSA-G4 solvate ionic liquid where Li nuclei were uniformly generated on the electrode surface. These results suggest that the nucleation and crystal growth of Li are affected by the composition of the solvate ionic liquids, resulting in the morphology change of Li deposits. 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). Reference [1] K. Dokko et al., J. Electrochem. Soc., 160, A1304 (2013).