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

Abstract

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.