Metal-organic Framework/Polyimide Composite with Enhanced Breakdown Strength for Flexible Capacitor

Abstract Metal-organic frameworks (MOFs), as new-generation of “star” materials, are widely applied in electrochemistry, pharmaceutical biology, energy storage and other emerging fields. However, there are few reports about strengthening mechanical and insulation properties of polymer-based dielectric materials by using 3D topological framework of MOFs. Here, we found that the polyimide (PI)-based composites dielectrics using ZIF-8 nanoparticles (a typical MOFs) as fillers exhibit significantly enhanced Young's modulus and breakdown strength. The DC breakdown strength of the composites as high as 516.3 kV/mm with only loading 1 wt% ZIF-8, which is about 85% higher than those of PI (279.8 kV/mm). In-situ synchrotron radiation characterizations revealed that the unsaturated active sites of ZIF-8 could bind to PI molecules, via thermal activation, inducing 3D multi-site bonding networks, which improves the Young's modulus and reduces the dielectric loss of the composite, and further enhances the breakdown strength of the composite. Molecular dynamics (MD) calculations show that the multi-site bonding networks can effectively capture charges and raise the ionization threshold at high field. The discharge energy densities (Ue) of composites are significantly superior to that of PI and the state-of-the-art commercial biaxially oriented polypropylene BOPP (about 5 times of the latter), meanwhile the charge-discharge efficiencies (η) of all composites are maintained above 90%. This breakdown strength enhancement effect also exists in PI-based composites loaded with UiO-66 nanoparticles (another typical MOFs). We believe that this study would further broaden the flexible dielectric application opportunities of MOFs, as well as develop into a new pathway for structural designs of flexible thin-film capacitors.