Synthesis And Energy Storage Performance Study Of Polymer-Based Dielectric Materials

With the development of modern electronic equipment toward light weight and high integration,dielectric materials with high energy density have attracted increasing attention.Dielectric capacitors with solid dielectric layers and ultrahigh power densities have been widely used in many fields,such as hybrid electric vehicles and pulse power devices.Polymer-based dielectric capacitors have become important energy storage components in electronic power systems due to their advantages,such as high breakdown strength,low dielectric loss,good self-healing property and nice processability.However,the low dielectric constant of polymers severely limits futher miniaturization of polymer-based film capacitors.In addition,compared with inorganic ceramic materials,the poor heat resistance of polymers leads to significant degradation of the breakdown strength and energy storage efficiency when the temperature rises.Therefore,it is urgent to enhance the dielectric constant,breakdown strength and working temperature of polymers and to develop polymer-based capacitors with high energy density and high energy storage efficiency at high temperatures.In this dissertation,we systematically investigate flexible polymer-based dielectric materials and focus on their dielectric energy storage performance,and the main research contents and innovative results are as follows:1.High-performance poly(vinylidene fluoride-hexafluoropropylene)(P(VDFHFP))nanocomposites with Ca2Nb3O10(CNO)nanosheets were prepared by solution casting.P(VDF-HFP)is a ferroelectric polymer.CNO has a high dielectric constant and a large aspect ratio and carries a great number of negative charges on the surface.The CNO nanosheets inside the nanocomposites tend to block charge transport and hinder the formation and development of breakdown paths,leading to a significant improvement in the energy storage performance of the nanocomposites.The dielectric constant of the nanocomposite can reach 12.3 at 1 kHz for only～0.3 wt%nanosheets,which is enhanced by 23%compared with the matrix(～10.0).Moreover,the breakdown strength of the nanocomposite increases from 650 MV/m for the matrix to 853 MV/m.Due to the distinct increase in both the dielectric constant and breakdown strength,the nanocomposite delivers an ultrahigh discharged energy density of 35.9 J/cm3,which is 94%higher than that of the matrix.To verify the universality of this strategy,linear polymer polystyrene(PS)-based nanocomposites containing CNO nanosheets were also prepared.At very low nanosheets content(0.2 wt%),the dielectric constant and breakdown strength of the nanocomposites are significantly enhanced compared to the matrix.The discharged energy density of the nanocomposite reaches～4.09 J/cm3,which is 78.6%higher than that of the matrix.The above results demonstrate that the two-dimensional nanosheet CNO is useful in improving the energy storage performance of polymer materials.2.Al2O3/BOPP/Al2O3 sandwich structures were fabricated by growing Al2O3 layers with a high dielectric constant,a wide band gap and high thermal conductivity on both sides of biaxially oriented polypropylene(BOPP)films via magnetron sputtering.Compared with the BOPP matrix,the dielectric constant of the composite film is improved.When the thickness of the Al2O3 layer is 270 nm,the dielectric constant of the composite film increases from 2.21 for the pristine film to 2.39 at room temperature and 1 kHz.In addition,the working temperature of the composite film is also greatly enhanced.The composite film obtains a high discharged energy density of～0.45 J/cm3 and a high energy storage efficiency of～97.7%at 125℃ and an electric field of 200 MV/m.It is noteworthy that the discharged energy density is comparable to that of BOPP at room temperature and under the same electric field.These results demonstrate that inorganic coatings can significantly improve the high-temperature energy storage performance of polymer dielectrics.3.High-temperature resistant cyclic olefin copolymer(COC)films were prepared by the solution casting method and subjected to ultraviolet(UV)irradiation.Compared with the pristine film,the all-organic films after a moderate amount of UV irradiation have a higher dielectric constant.After irradiation for 10 min,the dielectric constant of the irradiated film increases from 2.42 for the pristine film to 2.58 at room temperature and 1 kHz.In addition,the breakdown strength of the irradiated film at 150℃ is markedly enhanced from 520.8 MV/m for the pristine film to 644.8 MV/m.Benefiting from the simultaneous improvement in dielectric constant and breakdown strength,the irradiated film has an excellent discharged energy density of～3.34 J/cm3 at 150℃ with a charge-discharge efficiency above 95%,surpassing all reported all-organic polymers and polymer-based nanocomposites to date.The formation of carbonyl groups and cross-linking network during UV irradiation may be responsible for the excellent hightemperature energy storage performance of the irradiated films.Under operating conditions(200 MV/m at 150℃)in hybrid electric vehicles,the energy storage performance of the irradiated films remains highly stable over 20000 cycles.To verify the universality of the strategy,similar distinct improvements in energy storage performance are achieved in other grades of COC films.