Highly-Ordered Structural Deposition of Li2O2 in Li-O2 Battery

Abstract

Non-aqueous Li-O2 battery is one of the possible alternatives to current lithium ion batteries due to its theoretical high energy density, however, it still presents unsolved fundamental issues related to poor cycle life and low current density. Li2O2, which has been accepted as a discharge product, shows inherently low electron conductivity and its deposition on cathodes leads the obstruction of continuing discharge reaction. Therefore, in many cases, nano-structural carbon materials with large surface area have widely been applied as cathode to obtain specific high discharge capacity. Herein we present an alternative strategy to enhance the discharge capacity in the Li-O2 batteries with controlling the morphology of the deposition. We anticipated that the continuous steady discharging could be obtained if the reaction active sites on the cathode were not clogged with discharge products which were deposited in highly-order structure. In this study, we focused on the effect of the combination of cathode materials and electrolytes on the discharge capacity and the morphology of Li2O2. Figure 1. shows that relatively high current density (0.2 mA/cm2) and high discharge capacity (> 600 mAh/g) were obtained with the carbon paper cathode and the acetonitrile (AN) electrolyte (0.5 M LiTFSI in AN). In contrast, tetraethylene glycol dimethyl ether (TEGDME) or dimethyl sulfoxide (DMSO), which have higher viscosity and lower O2 solubility than acetonitrile, 1 resulted in the lower discharge capacity (< 30 mAh/g) when they were used as a solvent of the electrolyte under the same condition. We figured out the interesting correlation between the characteristic structure of Li2O2formed in the AN electrolyte and the discharge behavior that showed the oscillatory phenomena in the galvanostatic discharge process. 1) Laoire, C. O.; Mukerjee, S.; Abraham, K. M.; Plichta, E. J.; Hendrickson, M. A. J. Phys.Chem. C 2010, 114, 9178-9186. Figure 1