Cationic Engineering of a Sr ²⁺ -Modified Na _0.5 Bi _0.5 TiO ₃ -Based Ferroelectric Material for Ultra-High Energy Efficiency and Energy Harvesting

Increasing breakdown strength (BDS) and reducing remnant polarization ( P r ) are key parameters to enhance recoverable energy storage density ( W rec ) and efficiency (η%) of a dielectric material. In this study, a lead-free (Na 0.5 Bi 0.5 ) 1– x Sr x Ti 0.8 (Mg 1/3 Nb 2/3 ) 0.2 O 3 ceramic ( x = 0.05–0.30, labeled MNSr100x) was synthesized to achieve high energy storage performance. Structural modifications induced by Sr 2+ substitution were confirmed through the Rietveld refinement of the X-ray diffraction patterns and Raman spectroscopy. The introduction of Sr 2+ effectively mitigated Bi 3+ volatility and promoted the introduction of a relaxor nature, which is confirmed with the slim polarization–electric field curve and broad type phase transition. MNSr30 exhibits a high W rec of 1.09 J/cm 3 with an ultra-high energy efficiency of 88%. Furthermore, MNSr30 demonstrated excellent thermal stability from 30 to 280 °C, making it a promising candidate for energy harvesting, pulsed power applications, and high-temperature dielectric systems. The MNSr30 ceramic with 5, 10, and 15 wt % was successfully integrated into a poly(vinylidene fluoride) (PVDF) polymer to develop a flexible film for energy harvesting applications. Under the mechanical stimulation through finger tapping, the composite film with 15 wt % demonstrated a peak-to-peak output voltage of 27.6 V. This study highlights a cationic approach for enhancing the energy storage efficiency, thermal stability, and energy harvester potential of dielectric materials.