Ultrahigh energy density and greatly enhanced discharged efficiency of sandwich-structured polymer nanocomposites with optimized spatial organization

Sandwich-structured polymer nanocomposites that provide a pathway to overcome the paradox between permittivity and breakdown strength ever existing in dielectric materials are receiving increasing attentions for their superior energy storage performance. Despite certain advances obtained in previous effort, further enhancement of the energy density by structure optimizing is still a challenge. Herein, we present a newly designed sandwich-structured barium titanate/poly(vinylidene fluoride-co-hexafluoropropylene) (BaTiO₃/P(VDF-HFP)) nanocomposite via layer-by-layer tape casting process, where high contents of BaTiO₃ nanoparticles are dispersed in the middle layer to offer high permittivity, while two outer layers containing small amounts of BaTiO₃ provide favorable breakdown strength. The solution-processed nanocomposites with an optimal composition exhibits an ultrahigh discharged energy density of 26.4 J cm⁻³ and a superior discharged efficiency of 72%, which are by far the highest values ever achieved in sandwich-structured dielectric polymer composites. It is revealed that the designed structure can enhance the breakdown strength and discharged efficiency by preventing the charge injection from electrodes and impeding the development of electrical tress during breakdown process, as confirmed by the leakage current and thermally stimulated depolarization current measurements, as well as the finite element simulations. This work represents a new design paradigm to exploit advanced dielectric materials for electrical energy storage applications.