Revealing Redox-Mediated CO₂ Reduction Reaction Mechanisms in Aprotic Li-CO₂ Batteries

Redox-mediated electrocatalysis represents an innovative strategy to unlock the energy capabilities of aprotic Li-CO₂ batteries by enabling solution-mediated CO₂ reduction reaction (CO₂RR). However, the underlying reaction pathways remain incompletely understood due to the lack of direct molecular evidence. Herein, multimodal in situ spectroscopic techniques are integrated with theoretical calculations to interrogate a model 9,10-phenanthrenequinone (PQ)-mediated CO₂RR. Direct spectroscopic evidence reveals a current-density-dependent CO₂RR pathway: the reduced PQ reacts with CO₂ to form metastable Li₂(PQ-CO₂) adduct via ECE and EEC pathways at low and high current densities, respectively. Subsequently, the metastable Li₂(PQ-CO₂) adduct dissociates to form the LiCO₂ intermediate and regenerate Li_nPQ (n = 0 and 1 at low and high current densities, respectively). Two LiCO₂ intermediates dimerize to produce the final discharge products of Li₂CO₃ and CO in bulk solution. Therefore, the operation of Li-CO₂ batteries at low-current densities reduces the activation barrier of CO₂RR and regenerates PQ for sustained redox cycling, enabling significantly minimized overpotential and enhanced discharge capacity. Additionally, the suppression effects of weakly acidic cations (e.g., K⁺, TBA⁺) are elucidated for the redox-mediated CO₂RR. This work highlights the pivotal chemical dissociation step in PQ-mediated CO₂RR and provides a mechanistic framework for designing better metal-CO₂ batteries.