Morphology Control Of Molybdenum Disulfide And Its Properties Of Dielectric Energy Storage Composites

Dielectric capacitors belong to a class of basic electronic components that can quickly store and release charge and energy,and they possess broad application prospects in pulsed power devices,hybrid vehicles,5G communications and other fields.Compared to traditional ceramic counterparts,polymer dielectric capacitors are gaining more and more attentions due to the advantages of polymer intrinsic feature,including high breakdown strength,low weight,easy to process,and good flexibility etc.However,polymer dielectric composites have a serious problem for their application,that is the low energy storage density due to the low dielectric constant of polymers.Thus,it is difficult for polymer composites to meet the requirement of electronic devices in various fields.Therefore,how to improve the dielectric constant of polymer dielectrics has become a hot research topic in the field of dielectrics.Introducing dielectric fillers into polymer matrix is one of the most common and effective methods to prepare dielectric polymer composites.Compared with traditional ceramic fillers and conductive fillers,semiconductor fillers are one of the optimized fillers for improvement of the dielectric properties due to their considerable band gap,high exciton binding energy,low AC conductivity and unique nonlinear electrical behavior.In this work,one of the representative semiconductors,molybdenum disulfide（MoS₂）,was selected as the filler to enhance dielectric properties of polymer through designing the MoS₂-based hybrid fillers with different structures.The effect of hybrid fillers on the dielectric properties of the composites was also systematically investigated by introducing hybrid fillers into the matrix of polyvinylidene fluoride/poly（methyl methacrylate）（VM）blends.The main results are as follows:（1）The"hydrangea-like"MoS₂-f filler was successfully prepared by the hydrothermal reaction method,and then the MoS₂-f/VM composite films were prepared by solution casting.The microscopic morphological characterization of the filler reveals that the prepared MoS₂-fs are formed by the tight accumulation of numerous petal-like MoS₂nanosheets.The crystal structure characterization and Raman spectroscopy show that the nanoflakes have the characteristic properties of ultrathin MoS₂ sheets.In order to investigate the effect of MoS₂-f on the dielectric properties of the MoS₂-fs/VM,the dielectric constant,dielectric loss,and breakdown strength of the composites were characterized via Broadband dielectric spectrometer.It is found that due to the unique structure of MoS₂-f with high specific surface area,a great amount of"filler/matrix"interfaces are formed inside the composites,resulting in polarization enhancement of the composites as well as the increasing dielectric constant of the composites with the increase of the filler content,the dielectric constant of the 7 wt.%MoS₂-f/VM composite is 15.73@1 k Hz.However,the breakdown strength of composites exhibits a decreasing trend with increasing the loading,resulting in low energy storage capacity.（2）A core-shell hybrid filler with gradient band gap structure was constructed by introducing silicon dioxide（SiO₂）with wide band gap and high insulation into MoS₂-f by hydrothermal reaction MoS₂@SiO₂.Through the morphological characterization of hybrid fillers,it is found that the hybrid filler still retains the flower structure,and SiO₂ as the core is arbitrarily distributed between the lamellae of MoS₂.The interaction between SiO₂ and MoS₂ is confirmed by the chemical structure characterization to originate from the interaction between SiO₂ and Mo-2S dipoles.The hybrid fillers are introduced into VM matrix by solution casting.BDS spectra and breakdown strength testing shows that the hybrid filler retains the high polarization capacity of MoS₂-f,thus MoS₂@SiO₂/VM composites still maintain excellent dielectric constant（15.5@1 k Hz）;On the other hand,due to the introduction of SiO₂ with wide band gap,MoS₂@SiO₂ hybrid fillers can suppress the carrier migration inside the composite,leading to improve the distribution of current density inside the matrix.Finally,the dielectric loss of composites is effectively inhibited whereas the breakdown strength of the composites is enhanced to 486.8 MV/m.Due to their excellent dielectric and breakdown properties,the energy storage performance of the composite has also been improved.The hysteresis loop testing shows that when the MoS₂@SiO₂ content is 1 wt.%,The discharge energy density of VM composite under 500MV/m electric field is 6.62 J/cm³.Using finite element simulation software,it was further demonstrated that the suppression of internal leakage currents by a hybrid filler with a gradient band gap structure has a very good effect on the dielectric properties of the composite.（3）Molybdenum disulfide quantum sheet（MoS₂ QS）with a lamellar size of only 5～10 nm was successfully prepared by dry ball milling and gradient centrifugation.It was introduced into the polymer matrix to obtain MoS₂QS/VM composite.The microscopic morphology of MoS₂ QS was characterized,and it is found that MoS₂ QS is uniform in size and has a good MoS₂ lattice structure;The microstructure characterization inside the composite shows good compatibility between MoS₂ QS filler and matrix,and the composite is dense and defect-free.The effect of small-size MoS₂ QS on the energy storage performance of composites was studied by broadband dielectric spectrometer.It is found that compared with the traditional large-size fillers,small-size MoS₂ QS can provide more interface polarization,so that it can greatly improve the dielectric constant of composites with ultra-low loading of MoS₂ QS.At the same time,the small-size filler can inhibit the migration of internal carriers by coulomb blocking effect in the polymer,reducing the dielectric loss and the improvement of breakdown strength.When the filler loading is 0.05wt.%,the dielectric loss of the composite is only 0.012@1 k Hz,the breakdown strength is506.9 MV/m,and the energy storage density is 5.4 J/cm³.