Environmental pollution and energy shortage become more serious. How to effectively store energy and reduce energy loss is a recent challenge. Dielectric ceramic capacitors have some advantages such as long lifespan, fast charging and discharging speed, high power density, and good temperature stability. Na_0.5Bi_0.5TiO₃ (NBT) is a ferroelectric material with an A-site composite ABO₃ perovskite structure. The Curie temperature of NBT is 320 ℃, with a large maximum polarization (P_max) (40-50 μC/cm²) and a wide sintering temperature range. NBT is one of the hot research topics in energy storage ceramics. However, a high remanent polarization (P_r) (~38 μC/cm²) results in low recyclable energy storage density (W_rec) and efficiency (η) for NBT. Introducing a low P_r component into NBT to form a solid solution can refine the polarization-electric field (P-E), and improve the material breakdown strength (BDS) to achieve a high W_rec. In this paper, Ti⁴⁺ was replaced in Sr_0.7Bi_0.2TiO₃ by Zr⁴⁺. The introduction of Sr_0.7Bi_0.2ZrO₃ (SBZ) could enhance the degree of disorder in the A-site and B-site in the NBT, respectively. The ferroelectric long-range ordered structure was effectively break, further enhancing the relaxor ferroelectricity of NBT ceramics. Zr⁴⁺ was chosen instead of Ti⁴⁺ because Zr⁴⁺ was more stable chemically, and the properties of the material were further improved via introducing ionic radius difference and oxygen vacancies, leading to the lattice distortion. Also, ZrO₂ with a larger band gap was conducive to improving BDS and achieving a high energy storage performance.

The (1-x) Na_0.5Bi_0.5TiO₃-xSr_0.7Bi_0.2ZrO₃ (NBT-SBZ, x=0.1, 0.2, 0.3, 0.4) ceramics were synthesized by a solid-phase method. Na₂CO₃ (99.8%, in mass fraction, the same below), Bi₂O₃ (99.9%), ZrO₂ (99.0%), SrCO₃ (99.0%), and TiO₂ (98.0%) (National Pharmaceutical Group Chemical Reagent Co., China) were used as raw materials. The raw materials were mixed with alcohol and ground in a ball mill for 24 h. After drying, the powder was calcined at 850 ℃ for 4 h. The green body with a diameter of 12 mm was pressed by cold isostatic pressing at 200 MPa for 3 min. The green body was sintered in air at 1 090-1 150 ℃ for 4 h.

The phase structure of NBT-SBZ ceramic samples was characterized by a model D8 Advance X-ray diffractometer (XRD, Bruker Co., Germany) at 20°-80°. The microstructure was determined by a model S4800 scanning electron microscope (SEM, RIGAKU Co., Japan). The dielectric properties were measured by a model 4980A precision LCR (Agilent Co., USA). The thickness of the dielectric properties sample was 0.6 mm, and a silver electrode was fired at 850 ℃. The P-E loops were tested by a model Premier II ferroelectric analyzer (Radiant Co., USA). The P-E test sample thickness was 0.1 mm and a diameter of 2.0 mm for a gold electrode. The charging and discharging characteristics were determined from a model CFD-003 charging/discharging system (Gogo Co., China). The sample thickness was 0.3 mm, and a silver electrode was fired at 850 ℃.

The XRD pattern of NBT-SBZ ceramics indicates that all the samples exhibit a NBT pseudocubic perovskite structure. ZrO₂ and Bi₂Ti₂O₇ second phase appear as the SBZ content increases. A peak at 46.5° (200) shifts towards a lower angle as the SBZ content increases, indicating an increase in the spacing between ceramic crystal planes. The SEM images indicate that the grain size distribution of all the samples is uniform and presents a normal distribution. The average grain size decreases from 1.57 μm for the sample with x of 0.10 μm to 1.06 μm for the sample with x of 0.4 with the increase of SBZ content. The substitution of larger radius ions increases the lattice strain energy and hinders the grain boundary movement, leading to a grain refinement. The grain refinement and reduction of pores are beneficial to obtaining a larger BDS. The dielectric constant (ε_r) decreases from 870.5 for the sample with x of 0.1 to 668.5 for the sample with x of 0.4 with increasing the frequency due to a decrease in polarization mechanism. As the SBZ content increases, the dielectric loss (tanδ) at 1 kHz decreases from 0.041 for the sample with x of 0.100 to 0.029 for the sample with x of 0.4. All the samples exhibit obvious relaxor ferroelectric characteristics. The P-E loops also change from a typical ferroelectric shape to a thin and elongated shape of a relaxor ferroelectric, and the current density (J)-E current peak becomes rectangular, indicating that the relaxor ferroelectricity of NBT-SBZ ceramics increases with the addition of SBZ. BDS increases from 189.7 kV/cm for the sample with x of 0.1 to 298.1 kV/cm for the sample with x of 0.4, which is consistent with the decrease in average grain size and leakage current density, indicating that small grain size and dense microstructure are important reasons for achieving a high BDS. 0.8NBT-0.2SBZ ceramic with x of 0.2 at 265 kV/cm obtains W_rec of 3.15 J/cm³ and η of 76.05%. 0.8NBT-0.2SBZ ceramics exhibit a superior frequency/temperature stability in energy storage performance. C_D and P_D of 0.8NBT-0.2SBZ ceramics are 188.54 A/cm² and 11.31 MW/cm³ at 120 kV/cm, with an extremely fast t_0.9 of 52.6 ns.

NBT-SBZ relaxor ferroelectric ceramics were prepared via introducing SBZ to improve the relaxor ferroelectricity and energy storage properties of NBT ceramics. 0.8NBT-0.2SBZ ceramic achieved the optimum energy storage properties (i.e., W_rec=3.15 J/cm³, η=76.05%) at 265 kV/cm, as well as a superior stability of the energy storage performance at 1-100 Hz) and 20-140 ℃. The C_D and P_D of 0.8NBT-0.2SBZ ceramic were 188.54 A/cm² and 11.31 MW/cm³ at 120 kV/cm, respectively, with a fast t_0.9 of 52.6 ns. The 0.8NBT-0.2SBZ ceramic had a superior temperature/frequency stability, a super high power density, and extremely fast discharge time, having a great potential for application in pulse power supply equipment.