Improved High-Temperature Polarization and Charge–Discharge Efficiency in Fluorinated Polyimide Copolymers

Abstract

Dielectric film capacitors have received extensive attention because of their fast release rate and great electrical reliability. A polymer film with large energy density and charge−discharge efficiency is required to expand the application of film capacitors at high temperatures. In this work, a series of ternary fluorinated polyimides (FPI) with different fluorine-containing ratios have been copolymerized to investigate the high-temperature electrical displacements. The fluoro-dianhydride monomer was introduced into the regular 4,4′-diaminodiphenyl ether polymerization system with accompanied thermal imidization. The FPI film exhibits large energy capability with intrinsic breakdown strength compared with pristine PI, which is ascribed to the strong electronegative effect and hydrophobic feature of −CF 3 as well as the increase of free volume in bulk. The energy density of optimal FPI film reaches 20.2 J/cm 3 at 659 MV/m, and the high-temperature energy storage property is improved, e.g., the energy density up to 11.5 J/cm 3 with an external field of 525 MV/m at 100 °C. Low molar polarizability between C−F bonds also contributes to the decreased leakage current, which results in the large charge−discharge efficiency under high temperatures. The PI film with tailored fluorostructure demonstrates the competing candidate for the application of a high-temperature polymer film capacitor.

Cross-Linked Poly(vinylidene fluoride-hexafluoropropylene)-graft-poly(glycidyl methacrylate) Achieved Enhanced Dielectric Properties and Charge–Discharge Efficiency

“…Dielectric film capacitors with ultrahigh power density have attracted widespread attention in energy storage devices, such as pulsed power systems, mixed-power vehicles, and electromechanical actuators. − Polymer dielectrics are considered to be the promising candidate for capacitors due to their advantages of flexibility, lightweight, high breakdown strength ( E b ), and easy processing. − Dielectric constant ( ε r ), dielectric loss ( tan δ ), breakdown strength ( E b ), and electrical conductivity ( σ ) play crucial roles in the capacitive performance of energy density ( U e ) and charge–discharge efficiency ( η ). − The U e can be described as U e = ∫ E b d D , and E b indicates the maximum applied electric field without the loss of insulation. Thus, U e is determined by electric field ( E ) and electric displacement ( D ). , Currently, the most widely used commercially available dielectric polymer is biaxially oriented polypropylene (BOPP), which has an extremely high E b of ∼600 MV/m and ultralow tan δ (∼0.02). However, the low ε r (∼2) offsets its merits of high E b , leading to a low U e . , The development of polymer dielectrics with excellent capacitor performance is a concern in both academia and industry.…”