Core-shell structured PVDF-based copolymer fiber design for high energy storage performance

Polymer-based capacitors are very promising for high-power systems due to their high power density and ultrafast charge-discharge speed, yet reaching high dielectric constant and high breakdown strength simultaneously in dielectric polymers required by high-performance capacitors still remains a huge challenge. Herein, poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) were coaxial electrospun in core-shell structured fibers to create locally inhomogeneous microstructures deliberately. Through adjusting the functional group HFP/TrFE monomer ratio, P(VDF-HFP)/P(VDF-TrFE) hybrid polymer films with topological composition distribution have been elaborately designed, enabling gradient polarization distribution from core to shell. Compared with homogeneous hybrid films of the same composition, the core-shell structure significantly boosts breakdown strength, thus resulting in a significantly improved energy storage capacity. At an HFP/TrFE monomer ratio of 10:1, an optimal comprehensive energy storage performance has been achieved with Ue ∼20.7 J/cm3 and efficiency 67.8%; moreover, the film could maintain its energy storage performance after 106 charge/discharge cycles without reduction. Molecular dynamic simulation and finite element analysis have been employed in combination to reveal the dipole moments distribution at the molecular level and polarization distribution at the microscale, which further demonstrates that elaborate polarization distribution adjustment is an effective strategy toward high-performance electrostatic energy storage capacitors.