Enhanced Energy Storage Performance in Paraelectric–Ferroelectric Bipolymer-Based PMMA-P(VDF-HFP) Composite Films via Polydopamine-Coated KNb₃O₈ Fillers

Enhancing the breakdown strength (Eb) and polarization of the polymer-based composites through improving the effective interface bonding between the inorganic fillers and polymer matrix is always pursued to achieve high electrostatic energy storage performances. Along with this, paraelectric–ferroelectric (P–F) structured bipolymer PMMA-P(VDF-HFP) was copolymerized and blended to maintain the high Eb but reduce the residual polarization, and meanwhile, potassium triniobate (KNb3O8) rods synthesized by a molten salt (KCl and K2SO4) method were surface-modified using a polydopamine (PDA) shell to form fillers for the fabrication of the composite films that were prepared by a solution casting process. The fabricated PDA@KNb3O8/PMMA-P(VDF-HFP) composite film by filling only a tiny amount of PDA@KNb3O8 (1.0 wt %) exhibits a much enhanced dielectric constant (ε ∼ 9) and polarization (Pmax ∼ 6.9) and large Eb (∼600 MV m–1), therefore giving rise to a high energy density of 15.30 J cm–3 combined with a discharge energy efficiency of 65%. The results herein can be comparative and superior to those of many high-energy storage inorganic–polymer systems that may render this system much potential for dielectric energy storage capacitors.