Special Two-Dimensional Structure of Strontium Bismuth Titanate Nanosheets Enhancing Energy Storage Performance and High-Temperature Stability of Polymer Dielectrics

In advanced electronics and power systems, flexible dielectric materials with a high energy density are crucial. To compensate for the low dielectric constant (εr) and insufficient energy storage density of polymer materials, inorganic fillers with high εr, typically zero- or one-dimensional, are commonly introduced. However, 0D fillers, like nanoparticles, tend to agglomerate in the polymer matrix, increasing interfacial defects and exacerbating polarization losses, whereas 1D fillers, such as nanotubes and nanowires, can disrupt flexibility and mechanical properties due to their high aspect ratios and poor interfacial compatibility. In contrast, two-dimensional (2D) nanosheets, with excellent thickness-to-diameter ratios, can interact more effectively with the polymer matrix, reducing interfacial polarization, enhancing dispersion, and improving interface compatibility. Furthermore, these 2D nanosheets can serve as conductive barriers, limiting charge migration and hindering the expansion of electrical trees, thereby significantly improving the stability and service life of composite materials. This study synthesized bismuth strontium titanate (SrBi4Ti4O15, abbreviated as SBT) nanosheets with distinct two-dimensional structure, excellent temperature stability, and low dielectric loss using a simple and cost-effective molten salt method. SBT/poly(ether imide) (PEI) composite films were then prepared by an in situ polymerization process. Experimental results show that the 0.5 vol % SBT/PEI composite films exhibit an energy storage density of 6.69 J/cm3 and a charge–discharge efficiency of 94.09% at room temperature while maintaining stable dielectric properties across the 0–200 °C temperature range. To further enhance interfacial compatibility, we synthesized SBT@SiO2 core–shell nanoflakes. With the same filler loading, the energy storage density of the composite films increased to 8.37 J/cm3, with a charge–discharge efficiency of 92.46%. This work provides an idea for the fabrication of high-energy-density polymer nanocomposites.