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

Abstract

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, Cu^I₄(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 Mo₈O₂₆⁴⁻ 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.