Comparison between Dissolved and Coated LiBr As Redox Mediator for Li-Air Batteries

Abstract

Lithium-air battery (LAB) is one of promising next-generation batteries because of its theoretical energy density as high as 3500 Wh kg-1. However, the battery still has some difficulties to be challenged, among which large overpotential during charging is the most important one for the air electrode (AE). Application of redox mediator (RM) is a common strategy to lower the overpotential. Although RM is usually dissolved into electrolyte solution to make it react uniformly on the surface of the AE, the oxidized product of RM, RM+, diffuses to the Li electrode to be reduced into RM, which is a kind of short-circuit reaction, shuttle effect [1]. Therefore, we fixed LiBr as the RM on the surface of the AE by coating it as a mixture with carbon powder (Ketjen black, KB) and a binder (PVDF) not only to suppress the shuttle effect but also to concentrate the RM on the surface of the AE where it works [2].Three types of LAB cells were used in this study. The first one is “ no RM ” cell as a reference consisting of an air electrode coated on a carbon paper with a slurry of KB and PVDF, an electrolyte solution of 0.2 M lithium bis(trifuluoromethanesulfonyl)imide/diethyleneglycol dimehtyl ether, 0.2 M LiTFSI/G2, and a Li metal negative electrode. The other two contain LiBr as a RM in different ways. The second one is “ RM in EL (electrolyte)” cell which contains 50 mM of LiBr in addition to 0.2 M LiTFSI/G2. The third one is “ RM on AE” cell which contains LiBr in the slurry of KB-PVDF coated on the carbon paper with the same amount as that in the electrolyte solution of the RM in EL cell. Discharge/charge cycle tests were carried out at a current density of 200 mA (g-KB)−1 with a constant capacity of 500 mAh (g-KB)−1 in the range of 2.0 – 4.5 V at 30oC.Figure 1 shows the cycle performance of three types of LAB cells, which presents a better cyclability of the RM on AE cell than the others while that for the RM in EL cell is almost the same as the no RM cell. Figure 2 shows the charge and discharge voltage evolution during cycles as the voltages at midpoint, 250 mAh (g-KB)−1, for each discharge/charge cycle. The RM on AE cell shows the highest discharge voltages, the lowest charge voltages and the longest cycle life between the voltage range of 4.5 to 2.0 V among the three cells. However, the RM in EL cell shows slightly suppressed charge voltage merely early stage of the cycle test, and lower discharge voltages than the others probably due to the shuttle effect of RM, i.e. Br3 -. Therefore, coating of LiBr on AE, RM on AE , is proved to be a better strategy than dissolving LiBr into the electrolyte solution.At the presentation, we will discuss the detail of LiBr reactions as RM and compare the effect of LiBr between RM in EL and RM on AE cells with some analytical data.This study was supported by JST Projest ALCA-SPRING (JPMJAL1301) and NIMS Joint Research Hub Program, Japan.[1] X. G. Wang et al., ChemElectroChem, 4, 2145 (2017).[2] Y. Hayashi et al., J. Electrochem. Soc., 167, 020542 (2020). Figure 1