Photo‐Driven Cu‐I Coordination Stabilizes I ₃ ⁻ for Ultralow‐Overpotential Mg‐CO ₂ Batteries

ABSTRACT Mg‐CO 2 batteries present a compelling opportunity for next‐generation energy storage by combining high energy density with CO 2 valorization. Among various strategies, the use of redox mediators has emerged as a powerful approach to dynamically direct electrochemical pathways and boost CO 2 conversion efficiency. However, conventional redox mediators suffer from poor stability under operating conditions, which severely undermines long‐term performance. Here, we report a light‐activated 2,3,6,7,10,11‐hexahydroxytriphenylene‐Cu‐I 3 (HHTP‐Cu‐I 3 ) charge‐transfer complex that addresses the intrinsic instability of conventional redox mediators by forming robust Cu‐I coordination to stabilize I 3 − . Upon light illumination, Cu(II) undergoes photoreduction to Cu(I) releasing electrons that accelerate CO 2 activation, whereas I − is simultaneously oxidized to I 3 − , facilitating efficient electron shuttling and regulating product formation. This dynamic modulation switches the reaction pathway from thermodynamically favored MgCO 3 to electrochemically preferred MgC 2 O 4 by lowering the energy barrier for C 2 O 4 2− incorporation into MgC 2 O 4 . As a result, the system achieves an ultralow overpotential of 0.027 V and a record‐high energy conversion efficiency of 97.2%. Furthermore, the versatility of this strategy is demonstrated by its successful application in photoassisted Li‐O 2 batteries, delivering enhanced performance.