The escalating demand for advanced energy storage systems in modern electronics, electric vehicles, and renewable power grids has propelled the development of high-performance dielectric capacitors. Among various types of capacitors, those based on polymer dielectrics are highly prized for their ultrahigh power density, low loss, and ability to operate at high voltages. However, a fundamental limitation of pure polymers is their relatively low dielectric constant, which constrains their energy storage density. A promising strategy to overcome this is to create composite materials that combine the high breakdown strength of polymers with the high dielectric constant of ceramic fillers. Sandwich-structured composites, in particular, have emerged as an ideal architecture. By strategically layering materials with complementary properties, it is possible to synergistically enhance both the dielectric constant and the breakdown strength, the two key parameters determining energy storage performance. Nevertheless, the design space for such multi-layered composites is vast and complex. Relying solely on experimental trial-and-error to optimize parameters like filler concentration and distribution is inefficient and costly. This study, therefore, aims to systematically design and theoretically investigate a novel sandwich-structured composite film. Using a phase-field model, we seek to elucidate the impact of critical microstructural parameters on the overall energy storage properties, providing a foundational guide for the experimental fabrication of next-generation high-energy-density capacitors.

A comprehensive phase-field model was developed to simulate the polarization response and dielectric breakdown behavior of the proposed sandwich-structured composite films. The modeled structure consisted of a three-layer film: two outer layers of polyetherimide (PEI), a polymer known for its high breakdown strength and thermal stability, and a central layer of polyvinylidene fluoride (PVDF), a polymer with a relatively high intrinsic dielectric constant. Barium titanate (BaTiO₃, BT) nanoparticles, a classic ferroelectric ceramic with a very high dielectric constant, were incorporated as fillers exclusively within the central PVDF layer. This design intentionally confines the high-dielectric-constant ceramic phase to the middle layer, shielding it with the high-breakdown-strength PEI outer layers. The phase-field simulations accounted for key physical phenomena, including electric field distribution and the evolution of electrical treeing during breakdown. The primary variables investigated were the volume fraction of BT nanoparticles and their spatial dispersion uniformity within the PVDF matrix. The energy storage density of the materials was subsequently obtained through calculation of the simulation data.

Simulation results revealed a non-monotonic relationship between the energy storage density and the volume fraction of BT nanoparticles. Initially, the energy storage performance enhanced with the addition of BT fillers. This improvement is attributed to the significant increase in the overall dielectric constant of the composite material due to the high dielectric constant of BT nanoparticles, enabling the storage of more charge under a given applied electric field. However, further increase in filler content led to deterioration in energy storage performance. This decline is primarily due to the formation of excessive interfacial areas between the polymer and ceramic phases and the increased likelihood of nanoparticle agglomeration. These interfaces create local field concentrations, which act as hotspots for premature electrical breakdown, thereby limiting the maximum applicable electric field. The trade-off between enhanced permittivity and reduced breakdown strength defines this optimal loading.

Furthermore, the study unequivocally demonstrated that the dispersion uniformity of the BT nanoparticles is a critical factor, often as important as the filler content itself. A heterogeneous, agglomerated distribution of nanoparticles resulted in severe local field distortion around the large clusters. These high-field regions became preferential paths for the initiation and propagation of breakdown channels, drastically reducing the overall dielectric strength of the composite. In contrast, a highly homogeneous dispersion of nanoparticles minimized local field concentrations, leading to a more uniform electric field distribution throughout the material. This uniformity allowed the composite to withstand much higher global electric fields before breakdown occurred, thereby maximizing the energy storage density. The results underscore that achieving a fine and uniform dispersion of nanofillers is paramount in material processing to realize the full potential of the composite design.

This work utilizes an advanced phase-field modeling framework to successfully design and analyze the energy storage performance of a PEI/PVDF-BT/PEI sandwich-structured composite film. The key conclusion is that both the volume fraction and the spatial distribution of ceramic nanofillers are decisive in determining the energy storage characteristics. An optimal BT nanoparticle volume fraction exists that balances the benefit of increased permittivity against the detriment of reduced breakdown strength. More significantly, a homogeneous dispersion of nanoparticles is crucial for mitigating local electric field concentrations and achieving a high breakdown strength. The findings provide profound insights and valuable quantitative guidance for the material-by-design of multilayer polymeric composites for high-energy-density capacitor applications.