Functionalized metal–organic framework modified membranes with ultralong cyclability and superior capacity for zinc/bromine flowless batteries

Abstract

Aqueous zinc/bromine flowless batteries (ZBFLBs) are gaining attention as a next-generation energy storage system with the advantages of a cost-effective active material, simple system configuration, high output, high energy density, and high safety. However, the unrestricted crossovers of Br₂ and polybromide (Br_n⁻) seriously cause self-discharge and Zn electrode corrosion, limiting the coulombic efficiency (CE) and battery life. An amidated and sulfonated-functional UiO-66 (U-AS) composite membrane is developed to enhance the ZBFLB performance. U-AS contributes to the balanced ion transport of Zn²⁺ and Br⁻ ions, while the U-AS internal cage and amine group capture Br₂/Br_n⁻, suppressing the crossover. The proper hydrophilic domains in the U-AS membrane form well-connected water channels, facilitating ion transport. Thus, the modified membrane induces homogenous Zn deposition and greatly extends the life span of the ZBFLB over 2600 h at 9.7 mA h and 10 mA cm^—2 with an excellent CE of 98%. At a higher depth of charge (DOC) of 96.5 mA h, it exhibits CE of 93.6% from complete suppression of Br₂/Br_n⁻ crossover. Furthermore, we demonstrate the stable operation of a practical ZBFLB even at a high DOC of 40% for 1200 h with 79% CE. This work provides promising strategies for membrane design toward high-performance ZBFLBs.