Photo-driven Cu-I coordination stabilizes I(3)(-) for ultralow-overpotential Mg-CO(2) batteries

Abstract

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-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 CO2 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 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-O-2 batteries, delivering enhanced performance.