Solvation Engineering via Metal-Phenanthroline Redox Mediators for Efficient Li-O₂ Batteries

Li-O₂ batteries are a potential next-generation high-energy-density stationary energy storage system. The mechanisms of solution-based discharge processes remain unclear, including superoxide transformation, diffusion of Li⁺, O₂/O₂ ^-, and the redox mediation in saturated coordinated systems like iron and cobalt phenanthroline-based (≡ redox mediators, RMs) model systems. Here, in situ and ex situ spectroscopy combined with density functional calculations reveal that saturated coordinated redox mediation involves reversible bond-breaking and formation via structural coordination changes. In the presence of RM, the crown-like structure of Li-TEGDME solvation undergoes synergistic transformations via strong coordination with superoxide (Li-Oxo-RM complex pathway), thereby stabilizing intermediates and discharge products via contact ion-pairs in the solution-based discharge process. The interactions between the cations and anions in the solvation sphere, altered by the RM, lead to enhanced ion diffusion and reaction kinetics. This process promotes the capture of LiO₂ intermediates, enabling solution-phase growth of discharge products while minimizing solvent reactivity and superoxide-related side reactions. Batteries utilizing RMs exhibit significantly reduced polarization (0.28-0.41 V), a sevenfold increase in specific capacity, and a cycle life of nearly 100 cycles compared to non-RM electrolytes. This work provides valuable insights into electrolyte design with precise solvation properties and the chemical influence of advancing metal-air batteries.