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 Cu^IINi^II(4,4'-bpy) (Mo^VIO₄)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 + (<i>x</i>/2) I₂ = Cu_1<i>-x</i>Ni(4,4'-bpy)(MoO₄)₂ + <i>x</i> 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 O^2- 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.