Solvate Ionic Liquids as Electrolyte for Li-Oxygen Battery

Rechargeable Li-air (Li-O₂) batteries have recently attracted enormous attention due to their prospective high energy density. To realize reversible and stable operation of Li-air batteries, however, many serious problems, such as volatilization of electrolyte and electrolyte decomposition by O₂^- radical, have to be solved.^[1]

 Our group recently reported that equimolar mixtures of glymes and Li salts behave as ionic liquids (ILs) rather than as ordinary concentrated electrolyte solutions.^[2] For instance, as shown in Fig. 1, triglyme (G3) can coordinate to a Li⁺ ion, forming a 1:1 complex cation, [Li(G3)₁]⁺; it can also behave as an independent cation similar to the cations of typical ILs. Thus, the glyme-Li salt complex has been categorized as a new subclass of ILs called “solvate ILs”.^[3] The solvate ILs have many desirable properties as lithium conducting electrolyte^[4]-[5] as well as low volatility and thermal- and electrochemical stabilities, which are crucial for Li-air batteries. In the present study, we applied solvate ILs to electrolyte of Li-air battery.^[6]

 The solvate IL, [Li(G3)₁][TFSA], was prepared by simply mixing purified triglyme (G3) and lithiumbis(trifluoromethanesulfonyl)amide (LiTFSA) in a 1:1 molar ratio. The electrochemical properties of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the solvate ILs were investigated by cyclic voltammetry (CV), rotating ring-disk electrode voltammetry (RRDE) and galvanostatic charge-discharge measurement.

 Fig. 2 shows CVs for a glassy carbon (GC) electrode in [Li(G3)₁][TFSA] saturated with O₂ or Ar (as background). A large reduction peak emerges when the electrode is polarized to negative potential under the O₂ atmosphere, and a clear anodic peak can be seen on reversing the sweep from 1.5 to 4.2 V, although no appreciable current is observed under the Ar atmosphere. It suggests that reversible ORR-OER takes place on the GC electrode in [Li(G3)₁][TFSA]. The results of RRDE and charge-discharge test in this solvate ILs will also be reported.

Acknowledgement

This study was partly supported by RISING program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan, for which the authors are grateful.

References

[1] P. G. Bruce et al. Nat. Mater. 11 (2012) 19. [2] T. Tamura et al. Chem. Lett. 39 (2010)  753. [3]C. A. Angell et al. Faraday Discuss. 2012, 154, 9. [4] K. Yoshida et al. J. Am. Chem. Soc. 133 (2011) 13121. [5] K. Dokko et al. J. Electrochem. Soc. 160 (2013) A1304. [6] R. Tatara et al. Chem. Lett. 42, (2013) 1053.