Unlocking Reversible Electrochemistry in O ₂ -Mediated Li-CO ₂ Batteries Using Soluble Mn-Porphyrin Catalyst through Reconceptualization of the CO ₂ Reduction Mechanism

Rechargeable Li-CO₂ batteries offer the dual advantages of high-capacity energy storage and CO₂ utilization. However, a trace amount of O₂ in the test environment can significantly elevate the discharge platform, leaving the underlying CO₂ reduction mechanisms unclear. Thus, understanding the coupled Li-O₂ and Li-CO₂ reactions is crucial for their real-world applicability. Herein, soluble Mn-porphyrin (Mn-TMPP) was adopted as a catalyst in O₂-mediated Li-CO₂ batteries (LOCBs), which substantially enhances the electrochemical performance and modulates the discharge products to a combination of Li₂C₂O₄ and low-crystallinity Li₂CO₃. Multiple ex situ and in situ techniques coupled with theoretical calculations were used to elucidate the exceptional reaction pathway regulation capability of Mn-TMPP. A novel O₂-catalyzed CO₂ reduction pathway was proposed during discharge. This route forms Li₂C₂O₄ through a C₂O₆²⁻ intermediate, where the catalytic activity of the O₂ participates without being stoichiometrically consumed. The formed Li₂C₂O₄ subsequently either remains stable or disproportionate into Li₂CO₃ and carbon. During charging, Mn-TMPP catalyst facilitates the codecomposition of Li₂CO₃ and carbon, reducing the charge overpotential and preventing ¹O₂ formation. This O₂-catalyzed route reconceptualizes the CO₂ reduction mechanisms in conventional Li-CO₂ battery systems and establishes reversible electrochemistry in LOCBs.