Crown ether-based hamburger POMOF combined with polypyrrole as a highly reversible lithium-ion battery anode material

The incorporation of polyoxometalates (POMs) with metal-organic frameworks (MOFs) to construct POMOFs can improve the stability of POM-based materials, making POMOFs suitable as anode materials for lithium-ion batteries. In this study, a novel hybrid hamburger crystalline sample, namely, CuI4(L1)₂(Mo₈O₂₆) (NAU6), was successfully synthesized by stabilizing POM clusters through the introduction of [aza-crown ether-Cu] units (L1 = 12,25-dimethyl-3,8,16,21-tetraaza-tricyclo[21,3,11,14]octacosa-1(27),10,12,14(28),23,25-hexaene-27,28-diol). NAU6 was characterized using single-crystal X-ray diffraction, revealing that each [β-Mo₈O₂₆]^4- anion was sandwiched between two [aza-crown ether-Cu] units to form two discrete hamburgers, which were further extended to a 3D supramolecular framework. The intrinsic structural features of NAU6 enabled it to act as an anode material, particularly when composited with polypyrrole to form NAU6@PPy, which was employed in lithium-ion batteries (LIBs); NAU6@PPy exhibited excellent electrochemical performance with a reversible capacity of 1079.2 mA h g^-1 after 100 cycles at a current density of 100 mA g^-1. Results from density functional theory (DFT) simulations further revealed that the synergistic effect between Mo₈O₂₆^4- anions and the suitable cavity of the aza-crown ether were the dominant factors in the lithium storage process. This work paves the way for POM cluster stabilization through host-guest roles based on crown ether in the structural design of electrode materials.