Domain polarization in core–shell composites: Simultaneous high permittivity and low loss via filler cluster electron migration

Dielectric polymer composites with high permittivity (ε) and low dissipation factor (tan δ) are critical for advanced energy storage and capacitive applications, yet achieving both properties simultaneously remains challenging due to the typical trade-off driven by interfacial polarization and long-range charge transport. Herein, we propose a domain-type polarization mechanism mediated by electron migration across filler clusters to decouple this interdependence. A series of barium strontium titanate@titanium dioxide (BST@TiO2) core–shell particles with systematically tuned shell thicknesses is synthesized and incorporated into a poly(vinylidene fluoride) (PVDF) matrix. We find that the ε and tan δ exhibit a non-monotonic dependence on the TiO2 shell thickness, rather than following conventional interfacial polarization models. At an optimal shell thickness, the composite achieves decoupled regulation of a maximized ε with a minimized tan δ. This anomaly is attributed to the balanced contribution of intra-particle and inter-particle polarization, enabled by the formation of enlarged yet insulated polarizable filler clusters. The cluster-based mechanism is quantitatively supported by scaling laws linking relaxation strength and time, offering a new design paradigm for high-performance dielectric composites. The optimized BST@TiO2/PVDF composite demonstrates superior dielectric properties and breakdown strength, providing a viable pathway toward high-energy-density capacitors.