High-Efficiency Lead-Free BNT-Based Relaxor Ferroelectrics via Synergistic A/B-Site Substitution for Enhanced Energy Storage and Stability

High-efficiency, lead-free dielectrics are sought for pulsed-power capacitors, yet pristine Bi_0.5Na_0.5TiO₃ (BNT) suffers from large remanence, high coercivity, and limited breakdown strength. Here, we report (1 - x)Bi_0.5Na_0.5Ti_0.97Nb_0.03O₃-xSr_0.85Ba_0.15Ta_0.5+0.02xAl_0.5-0.02xO₃ (BNTNb-SBTA, x = 0-0.15) ceramics synthesized via solid-state reaction, achieving enhanced relaxor ferroelectric behavior through multi-cation substitution at A- and B-sites. X-ray diffraction confirms a pure perovskite solid solution, while scanning electron microscopy reveals grain refinement, suppressing oxygen vacancies and boosting the breakdown strength. Raman and dielectric analyses evidence strengthened relaxor behavior, accompanied by loop slimming and a systematic rise in breakdown strength. The composition x = 0.10 achieves the best trade-off, delivering <i>W_rec</i> = 3.357 J cm^-3 and <i>η</i> = 90.5% at <i>E_b</i> = 240 kV cm^-1. Robust operational stability is demonstrated with small variations of <i>W_rec</i>/<i>η</i> over 0.1-200 Hz, 25-175 °C, and 10⁶ cycles. Pulsed tests show fast discharge (∼26 ns) with <i>W_d</i> = 0.826 J cm^-3 at ∼90% efficiency under moderate fields. These results indicate that synergistic A/B-site disorder (Sr/Ba on A-site; Ta/Al with Nb on B-site), combined with microstructural densification, effectively minimizes <i>P_r</i> while elevating <i>E_b</i>, enabling high-efficiency energy storage under practical operating conditions.