High Efficient Charge of Li-O2 Batteries with TEMPO Derivatives

Li-O₂ batteries, in which Li₂O₂ is formed and decomposed on the cathode during discharge and charge, respectively, have attracted considerable attention as one of the future energy storage devices demanding high energy density. Recently, we demonstrated the Li-O₂ battery employing a ionic liquid, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI), as the solvent of the electrolyte. In this battery, little amount of parasitic reaction products were obtained after discharge because of specific stability of DEME-TFSI against O₂ radicals which is well known as one of the major causes decomposing electrolytes in Li-O₂ batteries. In addition, relatively lower over potential in charging (< 3.5 V vs. Li/Li⁺) was observed, whereas the charge capacity was limited to approximately 80% of discharge. According to the quantification analysis, the major causes of the irreversible capacity is Li₂O₂ remained in the cathode even after charge and parasitic products accompanied by charge process.

We, therefore, focused on redox mediators in order to accelerate the charge reaction. Recently, several redox compounds dissolved in the electrolyte of Li-O₂ batteries are reported as redox mediators which make formation or oxidation of Li₂O₂ more effectively. In this study, we applied 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MeO-TEMPO) as a mediator because its oxidized compound, MeO-TEMPO⁺,  stoichiometorically oxidizes Li₂O₂. In charge process, dissolved MeO-TEMPO was oxidized to MeO-TEMPO⁺ on the cathode that encouraged the oxidation of Li₂O₂ and obviously improved the rechargeability (Fig. 1). However, during the detailed investigation, we found that the mediator itself was deactivated along with the deterioration of the carbon cathode during charging; therefore, it is essential to avoid electrochemical oxidation of Li₂O₂ on the carbon for more efficient battery system.

In order to solve these problems, we have developed new strategy to decouple discharge and charge by chemical regeneration mechanism with mediator. In our new Li-air battery system, the charging process is totally different from an ordinary Li-air battery, and is a replacement of its electrolyte by that containing MeO-TEMPO⁺ oxidized (charged) separately in advance. Li₂O₂ in the discharged cathode is stoichiometorically decomposed by the MeO-TEMPO⁺ (chemical regeneration of cathode), while MeO-TEMPO⁺ is recovered to the original MeO-TEMPO after the reaction. This indirect charging mechanism promotes the decomposition of Li₂O₂ without potential electrochemical parasitic reactions and provides a significant improvement of the cycle life. At the same time, the decoupled charge/discharge processes makes more flexibility for the usage of the Li-air battery.