Solvate ionic liquids (SILs) are exemplified by the equimolar mixture of tetraglyme (G4) and LiTFSA, where-in the low Lewis basicity of the [TFSA] ̄ anion (weak interaction with Li⁺) allows the effective crown ether-like coordination of the Li⁺ cation by the glyme, yielding a relatively stable solvate cation, and thus forming a SIL, denoted [Li(G4)][TFSA] (Figure 1(a) inset). We have previously reported on some of the interesting physico-chemical properties of SILs and their applications to electrochemical energy storage, including systems incorporating a lithium metal anode.^1-3 Improved transport properties (in particular improved ionic conductivity) for SILs may be achieved with low polarity diluting solvents, which lower the viscosity without substantial disruption to the solvate cation structure.⁴ CO₂ solubility in conventional ILs is an area of intensive research, and pressurized CO₂ may be viewed as an unconventional example of a low polarity diluting solvent. We have also previously presented on our preliminary studies of the binary SIL/CO₂ system as a medium for electrochemistry, as applied to various typical battery electrode materials, and including the lithium anode. Aside from the possible implications for lithium battery devices, the fundamentals of electrodeposition studied under these unconventional conditions is intriguing, and in this work, we focus on the lithium deposition and dissolution process in this binary SIL/CO₂ medium.

Cycling of a [Li | [Li(G4)][TFSA]/CO₂(5 MPa) | Cu] cell (1 hr of 400 µA current application corresponding to 0.2 mA.cm^-2, 0.72 C.cm^-2 with 10 min rest periods, at 30 ºC) allows quantitative evaluation of the efficiency of the reversible electrochemical deposition and dissolution of Li from this medium, as shown in Figure 1(a). For comparison, analogous data recorded in a high pressure Ar rather than CO₂ medium are also shown. It is noteworthy that the data recorded in the presence of CO₂ provides a considerably improved performance in terms of cycle life, and thus indicates that the CO₂ plays an important role in the deposition process. This is assumed to be in part due to the improved conductivity of the binary medium, which may be varied as a function of pressure, as shown in Figure 1(b). Conductivity observed upon dilution of the SIL with conventional molecular solvents is shown for comparison.⁴

Microscopic (FE-SEM) investigation of the deposited lithium surface (deposition of lithium onto the clean Cu surface for 10 hours at 400 µA), see Figure 1(c), indicates the deposited lithium has a particle-like structure, and there are only minor structural differences apparent in the presence of CO₂ compared to Ar. Further analysis of the surface chemistry of these deposits has revealed that the Li deposited under CO₂ conditions is of higher purity, as will be elaborated further in the presentation.

References:

1. Kazuki Yoshida, Megumi Nakamura, Yuichi Kazue, Naoki Tachikawa, Seiji Tsuzuki, Shiro Seki, Kaoru Dokko, and Masayoshi Watanabe, J. Am. Chem. Soc., 2011, 133, 13121-13129.

2. Kaoru Dokko, Naoki Tachikawa, Kento Yamauchi, Mizuho Tsuchiya, Azusa Yamazaki, Eriko Takashima, Jun-Woo Park, Kazuhide Ueno, Shiro Seki, Nobuyuki Serizawa, and Masayoshi Watanabe, J. Electrochem. Soc., 2013, 160, A1304-A1310.

3. Morgan L. Thomas, Yoshiki Oda, Ryoichi Tatara, Hoi-Min Kwon, Kazuhide Ueno, Kaoru Dokko, and Masayoshi Watanabe, Adv. Energy Mater., 2017, 7, 1601753.

4. Kazuhide Ueno, Junichi Murai, Kohei Ikeda, Seiji Tsuzuki, Mizuho Tsuchiya, Ryoichi Tatara, Toshihiko Mandai, Yasuhiro Umebayashi, Kaoru Dokko, and Masayoshi Watanabe, J. Phys. Chem. C, 2016, 120, 15792-15802.

Figure caption

(a) Coulombic efficiency for deposition (1 hour) and dissolution of Li, under pressure, with cell configuration [Li | [Li(G4)][TFSA]/gas(5 MPa) | Cu], 30 °C, 0.4 mA (0.2 mA.cm^-2).

(b) Ionic conductivity as a function of Li+ concentration for [Li(G4)][TFSA]/CO₂ (mole fraction of CO₂ is varied with pressure). *Molecular solvents shown for comparison, from Ref.4.

(c) FE-SEM images of Li deposition from [Li(G4)][TFSA]/gas(5 MPa) on Cu (10 hours at 400 µA)