Local Polarity Engineering via Unsaturated Cu-N₃ Sites for Enhanced Iodine Redox Chemistry in Zinc-Iodine Batteries

Rational engineering of the local microenvironment in catalytic host materials is pivotal for high-performance zinc-iodine batteries, as it governs iodine species adsorption, accelerates redox kinetics, and suppresses polyiodides shuttling. Herein, we propose a local polarity engineering strategy by incorporating unsaturated Cu-N₃ sites into carbon matrix to construct polarized microenvironments and promote iodine redox chemistry. Combined theoretical and experimental analyses reveal that the unsaturated coordination of Cu atoms induces intrinsic local polarity, which enhances charge redistribution, lowers the activation barrier of the I₂/I^- redox reaction, and strengthens electronic coupling with polyiodide intermediates. In situ UV-vis and Raman spectroscopies corroborate that the Cu-N₃ sites effectively immobilize polyiodides, thus mitigating the shuttle effect. As cathode host, the Cu-N₃ sites-rich carbon electrode achieves high discharge capacity of 232.2 mAh g^-1 at 0.2 A g^-1 and exceptional long-term stability with 94.02% capacity retention after 50,000 cycles at 10 A g^-1. More importantly, benefiting from its superior catalytic activity toward iodine redox reaction, the Cu-N₃ sites-rich carbon enables solar cells to achieve a remarkable power conversion efficiency of 9.14%. This work elucidates a novel design principle for regulating local polarity to propel iodine electrochemistry, offering new insights into the development of advanced iodine-based energy devices.