Significantly Improved Dielectric Breakdown Strength and Energy Density in P(VDF-TrFE-CTFE) Polymer via a Facile Uniaxial Drawing Process

In the past decade, poly(vinylidenefluoride) (PVDF)-based polymers have attracted increasing attention in energy storage applications due to the advantages of high breakdown strength, flexibility, processability, and low cost. Compared with the PVDF homopolymer and binary copolymer, terpolymer poly(vinylidenefluoride-trifluoroethylene-chlorotrifluoroethylene) (P(VDF-TrFE-CTFE)) possesses higher permittivity due to a strong polar group and its relaxor characteristic. However, its breakdown strength is limited due to the premature saturated polarization and low Young's modulus. In this work, a facile drawing strategy has been employed to overcome this problem. With the increase in the stretching ratio from 0 to 10, the crystalline grain size is declined from 8.9 to 6.7 nm with the increase in crystallinity, all of which are favorable to improving the breakdown strength and energy storage density, and α/γ-phase tends to transform to β-phase as well. It turns out that the optimal property could be obtained with the stretching ratio of 6, where the breakdown strength could reach 620 kV/mm and a discharge energy density could reach up to 18.3 J/cm3. The optimized performance is significantly larger than that of the original terpolymer P(VDF-TrFE-CTFE) with breakdown strength and energy density of 280 kV/mm and 6.9 J/cm3, respectively. This work demonstrates that the uniaxial drawing is an effective strategy to improve the performance of P(VDF-TrFE-CTFE).