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 kgiodine−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.