Nonmonotonic Filler Content Effects on Breakdown Strength and Energy Storage in P(VDF-HFP)-Based Nanocomposites

Dielectric polymer nanocomposites with low ceramic filler loadings are generally regarded as beneficial for suppressing conduction loss and enhancing breakdown strength. However, this conventional assumption overlooks the critical role of filler spatial distribution in governing microscopic electric field uniformity. In this work, 0.6(0.82Bi 0.5 Na 0.5 TiO 3 -0.18Bi 0.5 K 0.5 TiO 3 )-0.4SrTiO 3 (BNKT-ST) lead-free ceramics were synthesized and surface-modified with the silane coupling agent γ-aminopropyltriethoxysilane (KH550). The modified nanoparticles were then incorporated into a poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) matrix to fabricate a series of dielectric nanocomposite films with systematically varied filler contents (0.5–3 wt%). X-ray diffraction (XRD) and Fourier transform infrared (FTIR) confirmed the target perovskite phase of BNKT-ST and the coexistence of α and β phases in the polymer matrix. By combining quantitative image-based analysis of filler dispersion and microscopic electric field simulations, the spatial distribution of KH550@BNKT-ST nanoparticles in the P(VDF-HFP) matrix was systematically clarified: at excessively low filler loadings (≤ 1 wt%), nanoparticles were sparsely and unevenly dispersed; at a critical filler concentration of ~1 wt%, a continuous and spatially uniform insulating network was formed by the nanoparticles; while at higher filler loadings (> 1 wt%), slight agglomeration of nanoparticles occurred. A nonmonotonic dependence of breakdown strength on filler concentration was revealed. The continuous network at ~1 wt% effectively homogenized the local electric field, maximizing breakdown strength at 440 MV/m and an energy density of 8.97 J/cm 3 . This study demonstrates that low ceramic filler content does not inherently guarantee superior dielectric performance, and highlights the necessity of a critical filler threshold for achieving electric field homogenization in polymer-based dielectric nanocomposites. Highlights A nonmonotonic effect of filler content on breakdown strength is discovered. Quantitative analysis of filler dispersion is conducted to explain the mechanism. The underlying mechanism is revealed via phase-field simulation/analysis. The 1 wt% BNKT-ST/P(VDF-HFP) nanocomposite shows superior performance. This work provides guidance for designing high-energy-density dielectrics.