Enhanced energy storage performance in Pb-free Na_0.5Bi_0.5TiO₃–Sr_0.7Bi_0.2TiO₃-based relaxor ferroelectric ceramics through a stepwise optimization strategy

This study employed a stepwise optimization strategy, including doping Sr(Ga_0.5Nb_0.5)O₃ (SGN) into 0.65Na_0.5Bi_0.5TiO₃-0.35Sr_0.7Bi_0.2TiO₃ (NBT-SBT) ceramics to modify the composition and a further processing improvement. By introducing Ga³⁺ and Nb⁵⁺ ions with high ionic polarization rates into the B-site, the average polarization of the unit cell was maintained while further inducing local disordered fields and promoting polar nanoregions. The high activity of the polar nanoregions was confirmed using piezoelectric microscopy. The effective energy storage density (W_rec) of 2.73 J/cm³ was obtained when the SGN doping was 6.5 %. Subsequently, the densification of ceramics was improved through viscous polymer processing, thereby enhancing the breakdown strength (BDS). A combined approach of finite element simulation and experimental studies was further utilized to investigate the impact of grain size on the BDS. The stepwise optimization strategy enabled the NBT-SBT-6.5SGN_VPP ceramics to achieve the W_rec of 5.2 J/cm³ and efficiency (η) of 85 % under 350 kV/cm. Additionally, the ceramics developed by this strategy possess excellent temperature and frequency stability and pulse discharge performance, making them potential candidates for practical applications.