Enhanced Dielectric Property and Energy Storage Capacity in a Poly(ether sulfone) Composite via Core–Shell Engineering of Barium Strontium Titanate

High-performance dielectric energy storage materials are crucial for advancing new energy and material fields. Consequently, enhancing the energy storage performance is essential to meeting the growing energy demand. This study selected BaTiO3–SrTiO3 (BST) and ZnO-coated BaTiO3–SrTiO3 nanofibers (BST@ZnO) as inorganic fillers for incorporation into a poly(ether sulfone) (PESU) matrix to improve the dielectric property of the polymer-based composite. The microstructure, dielectric property, breakdown strength, and energy storage performance of the composite were systematically investigated with varying filler content (2.5–15 wt % BST and BST@ZnO). The results showed that the 7.5 wt % BST/U composite achieved a maximum energy storage density of 2.1 J/cm3 at 260 kV/mm, which is a 20.2% improvement compared to the PESU under the same electric field strength. Furthermore, the core–shell-structured inorganic filler provided a notable enhancement in composite performance, as evidenced by the 10 wt % BST@ZnO/U composite, which exhibited a maximum energy storage density of 2.2 J/cm3 at 240 kV/mm and a 42.5% increase compared to the BST/U under the same electric field strength. This study demonstrates an effective strategy for the design and fabrication of a polymer-based dielectric composite with enhanced and balanced performance, offering valuable insight for their practical application in an emerging field such as the new energy system and power electronic.