Double enhanced energy storage density via polarization gradient design in ferroelectric poly(vinylidene fluoride)-based nanocomposites

Encapsulating ferroelectric ceramic nanofillers into polymer matrix with various architectures has been demonstrated as an effective strategy for ultrahigh energy storage density. Nevertheless, large discrepancy in permittivities between ceramic fillers and polymer requires sophisticated interfacial modification to alleviate local charge concentration. The idea to design graded permittivity from the filler center to surrounding matrix in composites is promising, yet remains a big challenge. Here coaxial electrospinning technique has been employed to control the polarization gradient distribution in TiO₂ nanofibers (TO_nf) and poly(vinylidene fluoride)/poly(vinylidene fluoride-co-trifluoroethylene) [PVDF/P(VDF-TrFE)] core-shell structured hybrid nanocomposites system where TO_nf is fixed at volume fraction 2%, and the polarization gradient of the polymer is adjusted via altering the volume ratios of PVDF and P(VDF-TrFE) in the coaxial electrospun polymer blends in two layers, respectively. Compared with nanocomposites made from mixed solution electrospun randomly and oriented TO_nf-PVDF/P(VDF-TrFE) at the same constituent volume fraction, the dielectric losses of coaxial-spun films decrease more than twice, from 0.087 for random nanocomposite film to 0.028, and more intriguingly, the discharged energy storage density (U_e) doubled increasing from 6.5 J cm⁻³ for random nanocomposite film to 12.7 J cm⁻³ for coaxial electrospun film with medium polarization gradient. Finite element analyses reveal the polarization and local electric field distribution in the nanocomposites with various polarization gradient design, and the results unambiguously demonstrate that U_e can be significantly boosted via controlled polarization gradient design.