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-CO 2 batteries offer the dual advantages of high-capacity energy storage and CO 2 utilization. However, a trace amount of O 2 in the test environment can significantly elevate the discharge platform, leaving the underlying CO 2 reduction mechanisms unclear. Thus, understanding the coupled Li-O 2 and Li-CO 2 reactions is crucial for their real-world applicability. Herein, soluble Mn-porphyrin (Mn-TMPP) was adopted as a catalyst in O 2 -mediated Li-CO 2 batteries (LOCBs), which substantially enhances the electrochemical performance and modulates the discharge products to a combination of Li 2 C 2 O 4 and low-crystallinity Li 2 CO 3 . 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 O 2 -catalyzed CO 2 reduction pathway was proposed during discharge. This route forms Li 2 C 2 O 4 through a C 2 O 6 2– intermediate, where the catalytic activity of the O 2 participates without being stoichiometrically consumed. The formed Li 2 C 2 O 4 subsequently either remains stable or disproportionate into Li 2 CO 3 and carbon. During charging, Mn-TMPP catalyst facilitates the codecomposition of Li 2 CO 3 and carbon, reducing the charge overpotential and preventing 1 O 2 formation. This O 2 -catalyzed route reconceptualizes the CO 2 reduction mechanisms in conventional Li-CO 2 battery systems and establishes reversible electrochemistry in LOCBs.