Dynamic Order–Disorder Conversions of Specific Facets in a Metal–Organic Framework: Chemically Reversible Iodine-Immobilization for Sodium–Iodine Batteries

Sodium–iodine batteries offer high voltage and theoretical capacity but suffer from iodine dissolution, shuttle effect, and sluggish kinetics. This work develops a porous Cu/Ni–Mo metal–organic framework (MOF) formulated as CuIINiII(4,4′-bpy) (MoVIO4)2 (4,4′-bpy = 4,4′-bipyridine) that addresses these challenges through a synergistic mechanism. The material’s channels can physically confine iodine via hydrogen bonds, while copper sites facilitate chemical immobilization via the reversible conversion CuNi(4,4′-bpy)(MoO4)2 + (x/2) I2 = Cu1-xNi(4,4′-bpy)(MoO4)2 + x CuI, with an energy barrier of 0.45 eV. The structural stability of the Cu-vacant MOF is associated with its rigid multinuclear {Cu2Ni2} clusters and abundant Mo–O–M (M= Ni, Cu) linkages. Impressively, some specific crystalline planes of the MOF undergo reversible order–disorder transitions during charge–discharge processes. These dynamic transformations are related to the adsorption of I2 on the Mo/Ni centers and the lattice O2– of these surfaces, promoting I–I bond elongation and cleavage, significantly enhancing iodine redox kinetics. Consequently, the as-fabricated Na–I2 battery shows a capacity of ∼250 mAh g–1 at 0.3 A g–1 with capacity retention (∼100 mAh g–1) over 1000 cycles at 2 A g–1.