To get the dielectric material with a high dielectric constant and low dielectric loss, the modified multiwalled carbon nanotube (MWNT-S) and ferroferric oxide (Fe3O4) particles were embedded into polyvinylidene fluoride (PVDF) to fabricate the Fe3O4/MWNT-S/PVDF ternary composites. The maximum dielectric constant of these composites can be up to 3490 at a very low filler fraction, and dielectric loss can be suppressed below 0.5. The small amount of the second filler (Fe3O4) can accelerate the formation of a percolation conductive network and improve the interfacial polarization. Therefore, the excellent dielectric properties can be achieved at low loading of fillers.

To improve the dielectric performance of polyvinylidene fluoride (PVDF), BaTiO 3 /MWNTs/PVDF ternary composites were prepared by the solution casting method. The percolation threshold (fraction of MWNTs) has dropped greatly below 0.4 vol %, with the enhancement of dielectric constant and breakdown field. For the BaTiO 3 /MWNTs/PVDF (11.5/0.35/88.15) composite, the dielectric constant is 59, the loss is below 0.055, and the maximum operating electric field is 324 MV/m, so the discharged energy density can be of up to 10.3 J/cm 3 with the efficiency of above 77.2%. The reason of improvement was revealed by the scanning electron microscope images and the X-ray diffraction data. It is found that uniform distribution of filler in the composites and the increase of the β phase of polymers result in the enhancement of polarization and improvement of dielectric constant of PVDF. The third-phase spherical inorganic particles prevent the formation of conductive networks and improve the uniformity of local electric field, so the breakdown strength of composites can be enhanced greatly. Here, this paper provides a method to get the composites with high energy storage density for supercapacitors.

In order to seek for the single-phase multiferroic metal-organic frameworks (MOFs) materials, we prepared a multiferroic material [NH2-CH+-NH2]Co(HCOO)3 (FMDCo) by solvothermal method. We found that it had weak ferromagnetism below 12.5 K with the coercive fields (Hc) of 560 Oe, remnant magnetization (Mr) of 7.67 emu/g and saturation magnetization (Ms) of 10.3 emu/g and exhibited obvious dielectric relaxation. The octahedral metal ions (Co2+) were linked by formate (HCOO-) ligands. The AmineH+ cation (NH2-CH+-NH2) were located within the cube-like cavities of the framework and formed extensive hydrogen bonds with the framework. This improved the phase transition temperature and thermal stability. This finding helps to understand the nature of magnetic and electric ordering in the MOFs.

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An enhanced energy storage ability, under a low operating electric field, was achieved in Fe3O4@TiO2-P(VDF-HFP) composite films. The low conductivity TiO2 layer was coated onto the high polarization Fe3O4 to construct Fe3O4@TiO2 core–shell fillers for decreasing filler fraction and alleviating conductivity contrast. For instance, the 2 vol.% Fe3O4@TiO2-P(VDF-HFP) film shows a discharged energy density and energy efficiency of 8.6 J cm−3 and 61.7%, respectively, under a low operating electric field of 261.9 kV mm−1. The coated TiO2 and modified –OH groups not only restrict the adverse effects (such as high conductivity, easy agglomeration, etc) caused by Fe3O4, but also contribute greatly to the improvement of polarization and breakdown strength, leading to a significantly improved energy storage performance. Additionally, the present work might possess great potential applications for energy storage owing to the low filler fraction, simple, and low electric filed operation.