To tether or not to tether? The merits of adding functionality to Li-O₂ cell redox mediators

The cycle life of lithium-oxygen (Li-O₂) cells is limited by the continual degradation of cell components by both potential-independent and high overpotential-induced parasitic reactions. Extending cell cycle life requires (i) driving electrochemical reactions at low overpotentials with high selectivity, (ii) stabilising the Li|electrolyte interface and (iii) mitigating reactive oxygen species (ROS)-induced electrolyte/electrode degradation. Charge redox mediators (RMs) present a promising strategy to facilitate low overpotential oxidation of Li₂O₂. By covalently linking or "tethering" the RM to motifs that can enhance cell discharge, quench ROS, and/or stabilise the Li|electrolyte interface, a single multifunctional species can be realised to improve cell lifetime. In this work, a series of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-tethered RMs based on ionic liquids/salts is reported. The effect of these additives on discharge capacities and cell cyclability was explored systematically in Li-O₂ cells. The tethered RMs gave superior performance relative to TEMPO when using Li_1-x FePO₄ as the counter electrode. Improved performance in Li metal-based cells with the tethered RMs was attained by substitution of the bis(trifluoromethanesulfonyl)imide ([TFSI]^-) anion of the RM with nitrate ([NO₃]^-), resulting in an exceptionally high Li₂O₂ yield close to 100% on discharge, almost 15% greater than for the analogous [TFSI]^--containing tethered RM. A marker for cell/RM failure was identified with operando gas evolution measurements to track charge mediation losses by the tethered RMs. Furthermore, degradation reactions with Li metal and superoxide, where superoxide-induced decomposition occurs at the TEMPO moiety, were identified as key RM degradation pathways, and the major bottleneck to achieving high cycling performance was established.