Free-Standing Type Mixed Oxide Oxygen Electrode for Rechargeable Li–O2 Batteries

The essential prerequisite for the successful operation of a rechargeable Li–O₂ battery is the formation of Li₂O₂ as reaction products during discharging and the decomposion of Li₂O₂ to Li and O₂ during charging. However, one of the critical problems which limit the practical use of the non-aqueous Li-O₂ batteries employing carbon based oxygen electrode is the huge polarization during the discharge/charge process. The high cell polarization may be due to the high activation energy required to produce Li₂O₂ during discharge and to decompose the Li₂O₂ during charge. It have been reported that the discharge/charge efficiencies can be improved by the addition of catalytic materials to the carbon supported oxygen electrodes. The cathode catalysts can affect the discharge/charge potentials and determine the rechargeability of the cells. Therefore, it is vital to develop an effectively catalyzed oxygen electrode for the oxygen reduction and oxygen evolution reactions in Li–O₂ batteries.

In this study, for the first time we introduce a free-standing type oxygen electrode design, where additional carbon and binders are no longer necessary in the oxygen electrode of Li–O₂ battery. A simple and cost effective solvo-thermal techniques was used to fabricate free standing carbon-free electrodes. The highly porous structure of the electrode allows electrolyte and oxygen to diffuse effectively into the catalytic active sites and hence improve the performance. The discharge products would precipitate and decompose on the pores of the free-standing catalyst nanorods. In this study, Mn-based composite oxide was selected as the catalyst candidate and directly deposited on the metallic nickel foam which was applied as the current collector. The electrochemical examination demonstrates that the free-standing electrode without carbon support gives almost the highest cell potential during discharge (2.85 V), the lowest cell potential during charge (3.54 V), almost the highest specific capacity (when considering all active mass in the cathode) and the minimum capacity fading among the rechargeable Li–O₂ batteries reported to date. The Li-O₂ battery demonstrated cyclability for more than 100 cycles while maintaining a moderate specific capacity of 1000 mAh g^-1. Further, the synergistic effect of the fast kinetics of electron transport provided by the free standing structure and the high electro-catalytic activity of the composite oxide enabled the excellent performance of the oxygen electrode for Li-O₂ batteries.