Study of the Effect of Electrolyte Concentration on Li-Air Battery Performance

Abstract

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, high charge overpotential and electrolyte decomposition by O2 - radical, have to be solved [1]. We previously reported that equimolar mixtures of glymes and Li salts behave as ionic liquids (ILs) rather than as ordinary concentrated electrolyte solutions [2]. For instance, triglyme (G3) can coordinate to a Li+ ion, forming a 1:1 solvate cation of [Li(G3)]+, which behaves as an independent cation similar to the cation of typical ionic liquids. Thus, the glyme-Li salt complex has been categorized as a new subclass of ionic liquids called “solvate ILs” [3]. Solvate ILs show negligible volatility and high oxidative stability which are important for stable operation of the Li-air battery system [4]. The concentration of Li salt in glyme-Li salt solvate ionic liquids are extremely high ~ 3 mol dm−3. In this extremely concentrated electrolyte, almost all the solvents (glyme) coordinate to Li+ cation and almost no free (uncoordinated) solvent exists, which is an unique nature of the “concentrated electrolyte” [5]. These days, highly concentrated electrolytes, including solvate ionic liquids, have been attracted much attention owing to their unique properties [6]. In this study, we investigated the relationship between solubility of the intermediate and reversibility of Li-air battery performance as a function of Li+ concentration by RRDE method. Although it has been already reported that solubility of the intermediate depends on donor number of the solvent species and affects morphology of the discharge compounds [7], our results revealed that high Li+ concentration suppresses the intermediate solubility even in high donor number solvents such as DMSO (Figure). This low intermediate solubility originates from the absence of free solvent molecules. For the dissolution of LiO2, LiO2 should be solvated. The absence of free solvent in the highly concentrated electrolytes results in insolubility of LiO2. Interestingly, reversibility of the disk electrode reaction in highly intermediate soluble electrolyte (0.1 mol dm-3 Li[TFSA]/DMSO) is much lower than that of the intermediate insoluble electrolyte (3 mol dm-3 Li[TFSA]/DMSO) (Figure) because of dissolution of the intermediate species into the electrolyte solution. Further discussion such as relationship between solubility of LiO2 and battery performance will also be discussed. Acknowledgement This study was partly supported by RISING program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.