Photo-Driven Cu-I Coordination Stabilizes I₃ ^- for Ultralow-Overpotential Mg-CO₂ Batteries

Mg-CO₂ batteries present a compelling opportunity for next-generation energy storage by combining high energy density with CO₂ valorization. Among various strategies, the use of redox mediators has emerged as a powerful approach to dynamically direct electrochemical pathways and boost CO₂ 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-I₃ (HHTP-Cu-I₃) charge-transfer complex that addresses the intrinsic instability of conventional redox mediators by forming robust Cu-I coordination to stabilize I₃ ^-. Upon light illumination, Cu(II) undergoes photoreduction to Cu(I) releasing electrons that accelerate CO₂ activation, whereas I^- is simultaneously oxidized to I₃ ^-, facilitating efficient electron shuttling and regulating product formation. This dynamic modulation switches the reaction pathway from thermodynamically favored MgCO₃ to electrochemically preferred MgC₂O₄ by lowering the energy barrier for C₂O₄ ^2- incorporation into MgC₂O₄. 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-O₂ batteries, delivering enhanced performance.