In recent years, non-aqueous type rechargeable Li-air (O₂) batteries (LABs) have attracted much attention as large-scale energy storage devices for electric vehicles because of the high energy density over 5 times larger than that of the conventional Li-ion batteries (LIBs) [1]. However, there are some problems to be solved for the practical use such as smooth deposition/decomposition reaction of Li₂O₂ at air electrode and suppression of Li dendrite growth at Li metal negative electrode (NE). To address these problems, we selected 1.0 M LiNO₃/tetraglyme(G4) electrolyte and added acetonitrile (AN) or dimethyl sulfoxide (DMSO) with both high dielectric constant ε and low viscosity η to enhance the Li salt dissociation and lowering the electrolyte viscosity. Namely, the NO₃^- anion was reported to work as a mediator to decompose the Li₂O₂ product at air electrode [2] and to form Li₂O layer on the surface of Li metal NE to suppress the Li dendrite growth [3,4] and extra electrolyte decomposition. In this study, we investigated the bifunctional effects of NO₃^- anion and its enhancement by using the dual solvent system for electrolyte. As a reference, 1.0 M LiOTf/G4 was also examined in the same way.

Figure 1 shows the η values for 1.0 M LiNO₃/G4+X and 1.0 M LiOTf/G4+X (X= DMSO, AN) electrolytes. The both η values decreased with an increase in the content of mixed solvents especially for AN because of one tenth lower η (0.37 mPas) than G4. As a result, the σ value was drastically improved (Fig. 2). For the addition of DMSO, the decrease in the η was not so big. However, the σ value effectively increased as well as those for the AN. This indicates that the relative high ε (47) of DMSO enhanced to dissociate the Li salts and improved the σ value by increasing the number of carrier ions. In fact, the effect was confirmed by Raman spectra and Walden plots. Figure 3 shows the discharge/charge curves for the LAB cells using the dual solvent electrolytes at the applied current of 0.20 mA cm^-2. By mixing DMSO, the overpotential was drastically reduced especially during discharge process. This effect was enhanced at a higher rate operation. In addition, Li deposition/dissolution tests using Li foil | Cu mesh cell also exhibited improved performances for the 1.0 M LiNO₃/G4 electrolytes. The effects for the dual solvent system will be reported in more detail at the meeting.

This study was supported by JST Project “ALCA-SPRING”, Japan.

[1] P. G. Bruce et al., Nature materials, 11, 19 (2012).

[2] D. Sharon, et al., ACS App. Mater. & Int., 7, 16590 (2015).

[3] J. Uddin, et al., J. Phy. Chem. Lett., 4, 3760 (2013).

[4] M. Saito et al., J. Electrochem. Soc., 164(12), A2872 (2017).