Orbital coupling effect enables low-chloride concentration electrolyte for four-electron redox in aqueous zinc | |iodine batteries

The I-/I0/I+ redox chemistry is hindered by the inherent instability of the electrophilic I+, which undergoes hydrolysis in aqueous media or is rapidly consumed by comproportionation with triiodide ions. Here, we design a copper single-atom embedded nitrogen-doped carbon host that synergistically combines d-p orbital hybridization and local low-concentration chloride ions to stabilize the I+. The atomic Cu-Nx sites interact moderately with iodine species through Cu d- and I p-orbital hybridization, which enhances iodine species adsorption, accelerates redox kinetics, and increases the hydrolysis energy barrier of I+. Simultaneously, the Cu-Nx sites promote a localized I-Cl microenvironment through electrostatic interaction, enabling efficient I0/I+ conversion even in a low-concentration Cl- ion electrolyte. As a result, the assembled zinc | |iodine battery exhibits a distinct four-electron dual-voltage plateau, achieving a high capacity of ~408 mAh g−1 at 1 A g−1. Even at a high specific current of 10 A g−1, the cell exhibits a high initial capacity of ~251 mAh g−1 and maintains a capacity retention of 70% with a low decay rate of 0.0062‰ per cycle over 50,000 cycles. This work provides a rational design method for coupling single-atom catalysis with localized ion confinement to realize high-voltage and long-life iodine conversion chemistry. Aqueous zinc | |iodine batteries suffer from irreversible iodine conversion and hydrolysis. Here, the authors design a copper single-atom porous carbon host that enables reversible four-electron I⁻/I⁰/I⁺ conversion for stable cycling.