Structural Disorder-Order Transition Driven by Valence Variation: Iodine Trapping via a Chemo-Physical Synergistic...

To avoid iodine dissolution in electrolytes and the resultant shuttle effect in Na-I₂ batteries, a chemo-physical synergistic strategy for iodine immobilization is proposed based on a 3D porous compound formulated as Cu₃(OH)₂V₂O₇·2H₂O (CuVO). After low-temperature annealing, the dehydrated sample, D-CuVO, becomes amorphous, but maintains the original host skeleton. After H₂O₂-treatment, D-CuVO recovers to the crystalline phase, and the order-disorder conversion is associated with valence variation. After incorporation of iodine, CuI is detected in D-CuVO@I₂, and the cycled D-CuVO@I₂ can restore to the original crystalline D-CuVO after H₂O₂-treatment, indicating the reversible transformation of D-CuVO + 1/2 I₂ ↔ CuI + Cu-deficient D-CuVO during cycling. This is related to abundant Cu-O-Cu linkages and short Cu ··· Cu distances in D-CuVO, which can stabilize the framework of D-CuVO in the presence of a Cu defect. Furthermore, I^- can be reversibly adsorbed/desorbed on the CuI (111) surface. Additionally, a new NaVO₃ host phase appears in the discharge process, which originates from partial irreversible intercalation of Na⁺ into D-CuVO. The inner channel of the NaVO₃ host phase can physically accommodate iodine. D-CuVO@I₂ shows excellent electrochemical performance in Na-I₂ batteries.