A strategy and mechanism of enhancing energy density for poly(vinylidene fluoride-hexafluoropropylene) composites with multi-layered structure

An effective strategy is developed to improve the energy density for poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) composites through building novel multi-layered structure with gradient distribution of a low content of SiO 2 -coated potassium sodium niobate (KNN@SiO 2 ), and the resultant composites were coded as G-K@S/P. To discuss the mechanism behind the unique strategy, the P(VDF-HFP) composites with randomly distributed potassium sodium niobate (KNN) or gradient-distributed KNN, designed as K/P or G-K/P, were prepared; moreover, the structures and integrated properties of the three kinds of composites were systematically studied using experimental method and finite element simulations to declare the influence of both layered structure with gradient distribution and core–shell structure of fillers. Results show that compared to K/P and G-K/P composites, with the same total loading of fillers, G-K@S/P has a greater breakdown strength and energy density. When the filler content is as low as 1 wt%, the energy density of G-K@S/P is about 1.35 and 2.37 times of that of G-K/P and K/P, respectively, the improvement degrees are obviously higher than those of multi-layered polymer composites (SCI database), proving the effectiveness of the strategy. The mechanism behind is that combining the multi-layered structure with gradient distribution of a small content of KNN@SiO 2 plays a synergistic role to simultaneously improve the dielectric properties and breakdown strength and thus enhancing the energy storage performance of the composites.