Gradient core–shell structure enabling high energy storage performances in PVDF-based copolymers

Abstract

Polymer-based capacitors are essential components in modern electronics and power systems. The long-standing challenge that is the contradiction between the breakdown strength and permittivity of dielectric materials has severely impeded their development for high-power capacitors. Polymer blends have recently been demonstrated as promising candidates with remarkably enhanced energy storage capability, and our previous study has indicated that the construction of a core–shell structure is an effective strategy. Herein, blends of poly(vinylidene fluoride)-based copolymers with large discrepancy in polarization, i.e., poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) have been elaborately designed with a gradient composition distribution in a core–shell structure by employing a coaxial electrospinning technique. By adjusting the monomer ratio of functional group HFP/TrFE, a polarization distribution which strongly correlates with the crystallization of the polar phase and its topological distribution has been subtly regulated to decrease from core to shell. Therefore, at the optimal HFP/TrFE ratio of 2/1, a high breakdown strength of 694.8 kV mm⁻¹ and discharged energy density of U_e of ∼23.6 J cm⁻³ have been achieved, with a high energy density of 27.8 J cm⁻³ and power of 10.7 MW cm⁻³ delivered to a 20 kΩ load. Moreover, the film shows robust long-term reliability, enduring up to 10⁷ charge–discharge cycles, highly competitive with currently reported polymer-based dielectrics. The underlying relationship between the HFP/TrFE ratio and polar phase and its topological distribution has been investigated from molecular to microstructural scale by combining molecular dynamic simulation and finite element analysis. Our study thus provides a promising alternative route for boosting the comprehensive energy storage performance of polymer dielectrics.