Energy Storage Characteristics Of Multilayer P(VDF-TrFE-CTFE) Composite Dielectric Based On Functionally Graded Structure

Among the currently available electrical energy storage technologies,dielectric capacitors play a significant role in advanced electronic and power systems such as portable electronic equipment and hybrid electric vehicles with their higher power density.Thin film capacitor is more favored for its superhigh power density and fast charge-discharge efficiency.However,their energy density is much lower than that of electrochemical energy storage devices such as batteries,which greatly limits the application of film capacitors.According to previous studies,there is a negative coupling relationship between polarization and breakdown strength.How to improve the material electric displacement while maintaining a high breakdown strength and energy storage density is the focus of this paper.In this paper,poly(vinylidene fluoride trifluoroethylene chlorotrifluoroethylene)(P(VDF-Tr FE-CTFE)),which possesses high dielectric constant was selected as the energy storage basic unit.The polymer films were prepared by high-speed electrospinning technology and hot pressed at different temperatures.After testing,it is found that the best electrical properties were obtained at 110°C.On this basis,a series of single-layer uniform composite dielectric with two-dimensional boron nitride nanosheets(BNNSs)with high insulation properties as fillers were prepared.The electrical properties test shows that compared with pristine P(VDF-Tr FE-CTFE)polymer films,the breakdown strength and energy storage density of 4vol.% BNNSs doped single-layer uniform composite dielectric are increased by 20.46% and 66.7%,respectively.Further,in order to improve the energy storage density,this paper designed and prepared 10 layers of composite dielectric with gradient distribution of fillers.The overall spatial content of fillers was based on the optimal performance content of single-layer uniform composite dielectric of 4 vol.%,and the concentration gradient between adjacent layers was adjusted to be 2vol.%,1.5vol.%,1vol.%,0.5vol.%,respectively.Eight groups of composite dielectric films with a positive gradient distribution(PGD)from low to high filler content and an inverse gradient distribution(IGD)from high to low were prepared,which are abbreviated as 0-8-0,1-7-1,2-6-2,3-5-3,8-0-8,7-1-7,6-2-6 and 5-3-5,respectively.After testing,it is found that the best performance parameters of the PGD and IGD are obtained in the smallest component of the concentration gradient.Among them,the maximum energy storage density of the 3-5-3 composite dielectric is 12.93J/cm~3,which is higher than the 10.38J/cm~3 of the 5-3-5,the main reason is that 5-3-5 outermost filler content is larger,and the aggregation of BNNSs causes the insulation performance of the composite dielectric to decrease.The above research results show that the gradient structure design plays an important role in optimizing the breakdown strength and energy storage characteristics of composite dielectric.