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

Abstract

Rechargeable sodium-iodine (Na-I-2) 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-I-2 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-I-2 batteries is demonstrated. The Fe-2[(2,3,9,10,16,17,23,24-octahydroxy phthalocyaninato)Cu] MOF composited with I-2 (Fe-2-O-8-PcCu/I-2) serves as a cathode for a Na-I-2 battery exhibiting a stable specific capacity of 150 mAh g(-1) after 3200 cycles and outperforming the state-of-the-art cathodes for Na-I-2 batteries. Operando spectroelectrochemical and electrochemical kinetics analyses together with density functional theory calculations reveal that the square planar iron-bis(dihydroxy) (Fe-O-4) species in Fe-2-O-8-PcCu are responsible for the binding of polyiodide to restrain its dissolution into electrolyte. Besides the monovalent Na-I-2 batteries in organic electrolytes, the Fe-2-O-8-PcCu/I-2 cathode also operates stably in other metal-I-2 batteries like aqueous multivalent Zn-I-2 batteries. Thus, this work offers a new strategy for designing stable cathode materials toward high-performance metal-iodine batteries.