Surface Electron Affinity Modulation: Balancing Iodine Oxidation and Reduction Reaction Kinetics on Mo ₂ C Cathode Host Catalyst toward Efficient Zn‐I ₂ Batteries

Abstract The energy efficiency of aqueous Zn‐I ₂ batteries (AZIBs) is traditionally enhanced by cathode host catalysts with high electron affinity, based on the consensus that stronger electron affinity improves electron‐rich polyiodides adsorption and prevents shuttle effects, thus promoting the I ₂ /I⁻ conversion reaction. Herein, carbon‐coated Mo ₂ C nanocrystals supported on carbon spheres (CS/Mo ₂ C@C) as an iodine cathode catalyst is developed. Interestingly, a deviation from the expectation is observed: moderate electron affinity on CS/Mo ₂ C@C, rather than higher on CS/Mo ₂ C, actually leads to faster reaction kinetics, while maintaining stable adsorption of iodine species. This phenomenon can be attributed to the optimal electron‐donating properties and charge transfer dynamics associated with lower electron affinity. Through systematic electrochemical and spectroscopic analysis, it is uncovered that the conformal carbon layer covering the Mo ₂ C nanocrystals is key to adjusting the electron‐donating capability, enhancing charge transfer, and improving interfacial reaction kinetics of the CS/Mo ₂ C@C cathode catalyst. Consequently, the assembled AZIBs employing CS/Mo ₂ C@C/I ₂ cathode demonstrate a smaller overpotential gap (0.06 V) and superior cyclic stability (89.6% capacity retention after 25 000 cycles at 5 A g⁻ ¹ ) compared to the CS/Mo ₂ C/I ₂ cathode (0.09 V/40.6% after 5000 cycles). This study highlights the significance of modulating surface electron affinity in cathode catalyst design for high‐efficiency AZIBs.