Bidirectional Confined Redox Catalysis Manipulated Quasi‐Solid Iodine Conversion for Shuttle‐Free Solid‐State Zn‐I₂ Battery

Abstract

Electrochemically reversible conversion of I₂/I^- redox couple in a controllable iodine speciation manner is the eternal target for practical metal-iodine batteries. This contribution demonstrates an advanced polyiodide-free Zn-I₂ battery achieved by the bidirectional confined redox catalysis-directed quasi-solid iodine conversion. A core-shell structured iodine cathode is fabricated by integrating multiporous Prussian blue nanocubes as a catalytic mediator, and the polypyrrole sheath afforded a confinement environment that favored the iodine redox. The zincate Zn_x+1Fe^III/II[Fe(CN)₆]_y has substantially faster zinc-ion intercalation kinetics and overlapping kinetic voltage profiles compared with the I₂/ZnI₂ redox, and behave as a redox mediator that catalyze reduction of polyiodides via chemical redox reactions during battery discharging and an exemplary reaction is Zn(I₃)₂+2Zn_x+1Fe^II[Fe(CN)₆]_y=3ZnI₂+2Zn_xFe^III[Fe(CN)₆]_y,ΔG=-19.3 kJ mol^-1). During the following recharging process, the electrodeposited ZnI₂ can be facially activated by iron redox hotspots, and the Zn_xFe[Fe^III/II(CN)₆]_y served as a cation-transfer mediator and spontaneously catalyze polyiodides oxidation (Zn(I₃)₂+2Zn_xFe[Fe^III(CN)₆]_y=3I₂+2Zn_x+1Fe[Fe^II(CN)₆]_y,ΔG = -7.72 kJ mol^-1), manipulating the reversible one-step conversion of ZnI₂ back to I₂. Accordingly, a flexible solid-state battery employing the designed cathode can deliver an energy density of 215 Wh kg_iodine ^-1.