Fully Conjugated Phthalocyanine Copper Metal-Organic Frameworks for Sodium-Iodine Batteries with Long-Time-Cycling Durability

Rechargeable sodium-iodine (Na-I₂ ) batteries are attracting growing attention for grid-scale energy storage due to their abundant resources, low cost, environmental friendliness, high theoretical capacity (211 mAh g^-1 ), and excellent electrochemical reversibility. Nevertheless, the practical application of Na-I₂ batteries is severely hindered by their poor cycle stability owing to the serious dissolution of polyiodide in the electrolyte during charge/discharge processes. Herein, the atomic modulation of metal-bis(dihydroxy) species in a fully conjugated phthalocyanine copper metal-organic framework (MOF) for suppression of polyiodide dissolution toward long-time cycling Na-I₂ batteries is demonstrated. The Fe₂ [(2,3,9,10,16,17,23,24-octahydroxy phthalocyaninato)Cu] MOF composited with I₂ (Fe₂ -O₈ -PcCu/I₂ ) serves as a cathode for a Na-I₂ battery exhibiting a stable specific capacity of 150 mAh g^-1 after 3200 cycles and outperforming the state-of-the-art cathodes for Na-I₂ batteries. Operando spectroelectrochemical and electrochemical kinetics analyses together with density functional theory calculations reveal that the square planar iron-bis(dihydroxy) (Fe-O₄ ) species in Fe₂ -O₈ -PcCu are responsible for the binding of polyiodide to restrain its dissolution into electrolyte. Besides the monovalent Na-I₂ batteries in organic electrolytes, the Fe₂ -O₈ -PcCu/I₂ cathode also operates stably in other metal-I₂ batteries like aqueous multivalent Zn-I₂ batteries. Thus, this work offers a new strategy for designing stable cathode materials toward high-performance metal-iodine batteries.