Effects of Dual Solvents on the Electrolyte Properties of LiNO3/G4-Based Electrolyte for Li-Air Batteries

Abstract

Lithium (Li)-air (O2) batteries (LABs) have been expected to be used for electric vehicles because of the high theoretical energy density of ca. 3500 Wh kg-1. However, there are some problems in the practical use, i.e. Li dendrite growth at Li metal negative electrode (NE), high overpotential during discharge process, etc. Recently, to solve these problems, LiNO3/ tetraglyme (G4)-based electrolytes were studied because of the bi-functional effects of surface oxidation for Li metal NE [1] and Li2O2 decomposition mediator at air electrode [2] by the NO3 -. However, the LiNO3 salt is quite low dissociation degree in G4 solvent, which causes to low ionic conductivity. In this study, we mixed dimethyl sulfoxide (DMSO) or acetonitrile (AN) with high dielectric constant e and low viscosity h as binary solvent to 1.0 M LiNO3/G4 electrolyte and investigated the effects on the electrolyte properties and LAB cell performance. G4 (< 30 ppm H2O) and AN or DMSO were mixed at the volume ratio of 9:1, 7:3 and 5:5, and 1.0 M LiNO3/G4+X (X =DMSO, AN) was prepared by dissolution of LiNO3 as supporting salt in an Ar filled dry box. The viscosity η and ionic conductivity σ were measured. The self-diffusion coefficient D of ions and solvents were evaluated by a PGSE-NMR [3]. The dissociation degree of Li salt was evaluated by using a Raman spectroscopy and Walden plot. Discharge/charge properties were also tested by using LAB cells using the electrolytes to discuss the effect of dual solvent. The η values of 1.0 M LiNO3/G4+X (DMSO, AN) decreased with increase in the content of dual solvents especially for AN because of the lower viscosity (0.37 mPa s) than that of DMSO (2.0 mPa s). However, DMSO-mixed electrolytes exhibited a similar increase in the σ value as well as AN mixed ones. Namely, the Li salt dissociation was assumed to be enhanced by the high e value (47) of DMSO. Fig. 1 shows the Raman spectra for the dual solvent electrolytes. In fact, the peak corresponding [Li+-(G4)] complex, i.e. Li+ solvation structure of G4, at 870 cm-1 disappeared for the DMSO-mixed electrolyte, indicating the strong interaction with DMSO solvent molecules compared with G4 ones. Walden plots exhibited and supported the enhancement of Li salt dissociation. Therefore, the DMSO and AN mixing as binary solvent was effective to increase in the number and mobility of carrier ions, respectively. Fig. 2 shows the discharge/charge properties of LAB cells using the 1.0 M LiNO3/G4+X (X = DMSO, AN, vol. ratio = 5:5). The both dual electrolytes successfully reduced the overpotential during discharge process. Especially for the DMSO-mixing a significant effect was achieved owing to the good supplying rate of NO3 - as mediator to the air-electrode. The effects for the dual solvent system in more detail, e.g. stability of the DMSO and AN in the dual electrolytes will be also reported at the meeting. This study was supported by JST Project “ALCA-SPRING”, Japan. [1] J. Uddin et al., J. Phys. Chem., 4, 3760 (2013). [2] D. Sharon et al., ACS Appl. Mater. Interfaces, 7, 16590, (2015). [3] M. Saito et al., RSC Adv., 7, 49031-49040 ,(2017). Figure 1