Photo-Driven Acceleration of Oxidation Kinetics in Aqueous Zinc–Sulfur Batteries

Aqueous zinc-sulfur (Zn-S) batteries offer high theoretical energy density and safety but suffer from sluggish solid-solid conversion kinetics during charging (ZnS→S), resulting in high polarization. To overcome this limitation, we introduce a novel photodriven iodine-mediated strategy decoupled from the traditional solid-state transformation energy barrier. By coupling an N-doped anatase/rutile TiO₂ heterojunction (A-TO(N)@R-TO) photoanode with a ZnI₂ electrolyte additive, we achieve ultralow charging voltages and accelerated reaction kinetics. Under illumination, photogenerated holes oxidize I⁻ to I₃⁻, which acts as a redox mediator to efficiently oxidize ZnS to S. Simultaneously, photogenerated electrons migrate via the external circuit to reduce Zn²⁺ to Zn at the anode. The photovoltage compensates the charging overpotential, reducing the charging voltage from 1.72 to 0.44 V. Energy efficiency increases dramatically from 26% to 125% (output electric energy/input electric energy, not including solar energy). Characterization (PL, XRD, XPS, in situ UV-vis) confirms that the N-doped heterojunction enhances charge separation, while I₃⁻ promotes ZnS decomposition. The system maintains stable cycling for 80 h at 0.44 V, demonstrating a photochemical cascade mechanism that enables high-efficiency, low-polarization metal-sulfur batteries.