Bidirectional manipulation of iodine redox kinetics in aqueous Fe-I₂ electrochemistry

Catalyzing conversion is a promising approach to unlock the theoretical potentials of the I₂/I^- redox couple in aqueous Fe-I₂ electrochemistry. However, most reported results only obtain one-directional efficient iodine conversion and cannot realize a balance of full reduction and reoxidation, thereby resulting in rapid capacity decay and/or low coulombic efficiency. Herein, the concept of bidirectional catalysis based on a core-shell structured composite cathode design, which accelerates the formation and the decomposition of FeI₂ simultaneously during battery dynamic cycling, is proposed to regulate the Fe-I₂ electrochemical reactions. Notably, the functional matrix integrates N, P co-doping and FeP nanocrystals into a carbon shell to achieve bidirectional catalysis. More specifically, the carbon shell acts as a physical barrier to effectively capture active species within its confined environment, N, P heteroatoms function better in directing the iodine reduction and FeP facilitates the decomposition of FeI₂. As confirmed with in situ and ex situ analysis, the Fe-I₂ cell operates a one-step but reversible I₂/FeI₂ pair with enhanced kinetics. Consequently, the composite cathode exhibits a reversible Fe²⁺ storage capability of 202 mA h g^-1 with a capacity fading rate of 0.016% per cycle over 500 cycles. Further, a stable pouch cell was fabricated and yielded an energy density of 146 W h kg_iodine^-1. Moreover, postmortem analysis reveals that the capacity decay of the Fe-I₂ cell originates from anodic degradation rather than the accumulation of inactive iodine. This study represents a promising direction to manipulate iodine redox in rechargeable metal-iodine batteries.