Functional cathodes with immobilized TEMPO(2,2,6,6-tetramethylpiperidinyloxyl) for Li–O2 batteries

Abstract

Non-aqueous Li–O₂ batteries offer an extremely high energy density, but suffer from high overvoltage on charge and poor cycle characteristics. In the past decade, soluble redox mediators (RMs) have been utilized to reduce the charge overvoltage. However, the use of RMs inhibits the effective decomposition of Li₂O₂ due to the shuttling of RMs between the cathode and anode. In this study, 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO), which was previously proposed as an RM, was immobilized on the electrode surface by immersing carbon paper in a solution of the synthesized 4-(N-(3-triethoxysilyl-propyl) carbamoyloxy)-2,2,6,6-tetramethyl-1-piperidinoxyl (TESPCP), followed by a heat treatment. Charge–discharge testing of Li–O₂ batteries using the TEMPO-immobilized cathode with a Li anode exhibited a charge plateau of about 3.7 V, indicating that the immobilized TEMPO could react electrochemically as a redox mediator. No overcharge behavior was observed in the cell, suggesting the RM shuttling effect was suppressed. Furthermore, SEM and XPS analyses of the cathode surface confirmed that no Li₂O₂ residues remained on the cathode after charging, unlike the control sample that utilized soluble RMs. These results indicate that a TEMPO-immobilized cathode can successfully mitigate RM shuttling while maintaining the benefits of RMs, allowing effective decomposition of Li₂O₂, during charging without leading to overcharging.