Fluorene- and Xanthene-Based Redox Mediators for Li–O2 Batteries: From Spiro-Linkage to Solo Moieties

Soluble charge redox mediators (RMs) are key to improving the reversibility and efficiency of lithium–oxygen (Li–O2) batteries by facilitating the oxidation of discharge products and reducing charge overpotentials. In this work, we report on two prospective RMs, F-OMeTAD and X-OMeTAD, based on fluorene and xanthene cores, respectively. These compounds model the molecular halves of recently introduced spiro-configured RMs, where two fluorenes, or a fluorene and a xanthene unit, are connected through a sp3 carbon atom. Electrochemical studies revealed that F-OMeTAD exhibits better electronic delocalization and faster redox kinetics with respect to X-OMeTAD, which is also oxidized at a higher potential due to a localized disruption of π-conjugation resulting from the bridging oxygen heteroatom. Spectroscopic and photochemical analyses confirmed the stability of both compounds toward reactive oxygen species such as singlet oxygen, although, compared to their spiro counterparts, some degradation was observed under forcing conditions. When tested in Li–O2 prototype cells, both proposed RMs successfully reduced the charge overpotential and promoted the oxidation of the oxygen reduction reaction (ORR) deposits over the electrode surface. F-OMeTAD effectively reduced the charge overpotential even at low concentrations, extending battery life, whereas X-OMeTAD did not show a comparable mediation effect but instead primarily contributed to improved solid electrolyte interphase (SEI) formation on lithium metal. This comparative study, combined with DFT modeling, allowed a better understanding of the functioning mode of the parent spiro-configured RMs and offers valuable insights into structure–function relationships that can guide the future design of efficient, stable redox mediators for high-performance Li–O2 batteries.