High-Field Dielectric Properties of Oriented Poly(vinylidene fluoride-<i>co</i>-hexafluoropropylene): Structure–Dielectric Property Relationship and Implications for Energy Storage Applications
Mengxue Yuan, Bo Li, Shihai Zhang, Ramakrishnan Rajagopalan, Michael T. Lanagan
Abstract
Polyvinylidene fluoride (PVDF)-based fluoropolymers have generated interest in electrical energy storage due to their high dielectric constant. The dielectric properties of these fluoropolymers can be significantly improved by uniaxial/biaxial orientation, a common practice adopted in industrial manufacturing, but the underlying molecular origins still remain unclear. In this paper, we explore a series of stretched poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) and investigate the stretching-induced structure development and its correlation with dielectric properties. With increasing strain, P(VDF-HFP) exhibits the expected paraelectric to ferroelectric transition. Increased crystal orientation and reduced crystallite size facilitate dipole orientation, resulting in elevated polarization. Breakdown strength is improved with stretching because of enhanced barrier effects, and recoverable energy density is also improved arising from the formation of reversible ferroelectric nanodomains. Contrary to expectations, the β-phase is not unfavorable for energy storage under DC conditions. The energy density of stretched P(VDF-HFP) can reach ∼20 J/cm3, indicating the great potential of optimizing the crystalline morphology of ferroelectric polymers for high-energy storage applications.