Nitro-Induced Electronic Tuning and Intermediate Stabilization for Enhanced Solution-Phase Reactions in Li-O₂...

Utilizing redox mediators (RMs) in aprotic Li-O₂ batteries as catalysts provides promising solutions to several challenges, including high overpotential, cathode passivation, and electrolyte instability, while enabling the solution-phase catalysis. Despite these advantages, the fundamental origin of their electrochemical activity and how it governs the solution-phase pathway remain poorly understood. To bridge this gap, we systematically explored the stability of reactive intermediates and their influence on solution-phase Li₂O₂ formation using a series of anthraquinone-based RMs: anthraquinone (AQ), 1-nitroanthraquinone (MNAQ), 1,5-dinitroanthraquinone (1,5-DNAQ), and 1,8-dinitroanthraquinone (1,8-DNAQ). All the studied RMs facilitate the formation of stable intermediate complexes with Li⁺, O₂^•-, and LiO₂^•, thereby promoting the solution-phase Li₂O₂ formation pathway. Among them, 1,8-DNAQ exhibits the enhanced coordination with Li⁺ through cooperative participation of carbonyl and nitro oxygens, highlighting the role of the NO₂ functional group in dual-site binding. The introduction of electron-withdrawing NO₂ groups systematically raises the reduction potential (AQ < MNAQ < 1,5-DNAQ < 1,8-DNAQ), approaching the ideal value of 2.96 V, which is consistent with experimental observations. A correlation is observed between the NO₂ substitution, reduction potential, and the LUMO energy, unveiling the underlying origin of the potential shift. Interestingly, the rise in reduction potential is not solely dictated by LUMO energy tuning through functional group modification but also by the thermodynamic stabilization of the reduced species. The involvement of the NO₂ group enables electron delocalization, which stabilizes the reduced species and results in an enhanced redox performance compared to the unsubstituted AQ. The following study establishes a structure-property relationship linking the electronic structure, stability of reduced species, and redox activity. It demonstrates how NO₂ functionalization correlates with the tuning of reduction potential. These insights provide design principles for developing redox mediators to enhance the catalytic activity and reversibility in next-generation Li-O₂ batteries.