Deciphering the structure, dielectric, and energy-storage performances of A-site stoichiometric/nonstoichiometric (Na0.5Bi0.5)TiO3-based ceramics via a two-step optimization design

Herein, the stoichiometric ((Na0.5Bi0.5)TiO3 (S-0(N=B)) and (Na0.36Bi0.36Sr0.28)TiO3 (S-28(N=B))) and nonstoichiometric ((Na0.3Bi0.38Sr0.28)TiO3 (S-28(N < B)) and (Na0.42Bi0.34Sr0.28)TiO3 (S-28(N > B))) ceramics were fabricated by a conventional solid-state reaction method. Subsequently, the effects of A-site stoichiometry/nonstoichiometry on their structural, dielectric, and energy-storage performances were investigated. All of them exhibit a single perovskite phase without any impurity phase. S-0(N=B) shows rhombohedral with R3c symmetry, while the other three feature the coexistence of rhombohedral with R3c symmetry and tetragonal with P4bm symmetry. Interestingly, there are significant microstructure and dielectric differences among them. The average grain sizes of S-28(N=B) and S-28(N < B) are smaller than those of S-0(N=B) and S-28(N > B). Notably, S-28(N < B) achieved a significant improvement in dielectric properties. It also exhibited a slim and pinched polarization-electric field hysteresis loop with the largest polarization difference and the smallest electric hysteresis loss, leading to superior energy-storage performance compared to the other three. Subsequently, a tape-casting method (TCM) was employed to prepare S-28(N < B,S- TCM) with improved microstructure and restrained electrical conduction behavior, thereby strengthening the electric breakdown strength from 145 to 200 kV/cm. Consequently, a large recoverable energy density of 2.53 J/cm(3) and a high energy conversion efficiency of 83.71 % were realized in S-28(N < B,S- TCM). Additionally, excellent frequency- and temperature-dependent energy-storage and/or charge-discharge stabilities were confirmed. These results indicate that the structure and electrical properties of (Na0.5Bi0.5)TiO3-based ceramics are sensitive to A-site stoichiometry/nonstoichiometry. They also highlight the superiority of S-28(N < B,S- TCM) over the others, suggesting its promising potential in the dielectric energy-storage field. Furthermore, this work would provide valuable insights into the design and performance optimization of dielectric energy-storage materials/capacitors.