Bimetallic Anionic Organic Frameworks with Solid-State Cation Conduction for Charge Storage Applications.

A new phosphonate-based anionic bimetallic organic framework, with the general formula of A4-Zn-DOBDP (wherein A is Li+ or Na+, and DOBDP6- is the 2,5-dioxido-1,4-benzenediphosphate ligand) is prepared and characterized for energy storage applications. With four alkali cations per formula unit, the A4-Zn-DOBDP MOF is found to be the first example of non-solvated cation conducting MOF with measured conductivities of 5.4 × 10-8 S cm-1 and 3.4 × 10-8 S cm-1 for Li4- and Na4- phases, indicating phase and composition effects of Li+ and Na+ shuttling through the channels. Three orders of magnitude increase in ionic conductivity is further attained upon solvation with propylene carbonate, placing this system among the best MOF ionic conductors at room temperature. As positive electrode material, Li4-Zn-DOBDP delivers a specific capacity of 140 mAh g-1 at a high average discharge potential of 3.2 V (vs. Li+/Li) with 90% of capacity retention over 100 cycles. The significance of this research extends from the development of a new family of electroactive phosphonate-based MOFs with inherent ionic conductivity and reversible cation storage, to providing elementary insights into the development of highly sought yet still evasive MOFs with mixed-ion and electron conduction for energy storage applications.