Lead-free dielectric capacitors have attacted recent attention due to the super power density, fast charge-discharge rate and outstanding thermal stability. However, the recoverable energy density of lead-free ferroelectric ceramic is relatively lower, compared to ceramics containing lead. The introduction of strong relaxor ferroelectrics can realize the excellent energy storage performance via decreasing remanent polarization as well as enhancing impedance. Bi_0.5K_0.5TiO₃ (BKT) is a promising candidate for dielectric capacitor due to the unique lone pair electronic 6s² configuration about Bi, leading to a large spontaneous polarization (i.e., P = 52 μC/cm²). Nevertheless, the energy storage properties of BKT-based ceramics are poor (i.e., W_rec < 3 J/cm³, η < 80%) because of the large remanent polarization and negative breakdown strength. Chen et al. reported that the recoverable energy storage density reached 7.5 J/cm³ through adding the BaTiO₃ and NaNbO₃ into BKT, indicating that BKT-based ceramics had a substantial prospect. The long-range ferroelectric order is broken to obtain a slim hysteresis loop, which can improve the energy storage performance. Simultaneously, the uniform small grain and compact microstructure of ceramic are essential, which can upgrade the breakdown strength for ferroelectric material. Thus, reducing grain size and breaking ferroelectric long-range order to generate polar nano-regions (PNRs) could improve the BKT-based specimens energy storage properties. In this paper, a relaxor ferroelectric material of Na_0.73Bi_0.09NbO₃ (NBN) was induced into 0.75Bi_0.5K_0.5TiO₃-0.25BiFeO₃(0.75BKT-0.25BF) to reinforce the ESP of ceramics. BF and NBN were selected to optimize ESP. The morphotropic phase boundary (MPB) of BKT-BF binary system was constructed.

For the synthesis of BKT-BF-xNBN (x=0, 0.1, 0.2, 0.3 and 0.4) ceramics, Bi₂O₃ (99.5%, in mass fraction, the same below), K₂CO₃ (99.5%), TiO₂ (99.5%), Fe₂O₃ (99.5%), Na₂CO₃ (99.8%) and Nb₂O₅ (99.5%) were used as raw materials. The dried oxides were weighted and mixed according to stoichiometric ratios in a polyethylene container, and then the mixed materials with alcohol were ground in a ball mill for 12 h. After drying, the mixture was calcined at 850 ℃ for 5 h, and the powder was further ground under the same condition. Sequentially, the dried powder with 0.5% (in mass fraction) polyvinyl alcohol (PVA) was pressed into discs with a thickness of 0.8 mm and a diameter of 12 mm. Finally, the green plates were sintered at 1 000-1 035 ℃ for 3 h.

The results show that an optimum recoverable energy storage density (W_rec) of 4.23 J/cm³ and an improved energy storage efficiency (η) of 81.2% at 380 kV/cm are achieved. The microstructure of the specimens is compact due to the refined size of grain, leading to a boosted breakdown strength (E_b). In addition, an enhanced efficiency of the specimen with x of 0.3 can be obtained, which arose from the negligible remanent polarization (P_r). Based on the analysis of dielectric relaxation, the dispersion coefficient (γ) of the BKT-BF-0.3NBN is 2.2, implying a short-range ferroelectric order, which can increase the polar nanometer regions. To better understand the reason for the decreased P_r, the first order reversal curve (FORC) distribution of ceramics was measured. The optimized composition has a distinguished dielectric thermal stability and a superior charge-discharge performance (i.e., t_0.9 (the time for releasing 90% of all storage energy) < 75 ns, current density (C_D) of 757.9 A/cm² and power density (P_D) of 94.7 MW/cm³), indicating a promising prospect in the applications for high-power dielectric capacitors. These results illustrate that the composition design for BKT-BF-xNBN is effective to refine the energy storage performance of ceramic.

Lead-free BKT-BF-xNBN ceramics with x of 0, 0.1, 0.2, 0.3 and 0.4 were synthesized. The introduction of Na_0.73Bi_0.09NbO₃ improved the microstructure of ceramics, boosted the breakdown strength, enhanced the ceramics relaxtion and resulted in the appearance of PNRs. As a result, the BKT-based ceramic with x of 0.3 had the optimum properties (i.e., W_rec of 4.23 J/cm³ and η of 81.2%). Moreover, the superior dielectric thermal stability as well as charge-discharge performance (i.e., t_0.9<75 ns, C_D=757.9 A/cm², P_D=94.7 MW/cm³) were achieved. This work indicated that BKT-BF-0.3NBN ceramic could have a promising application potential in cutting-edge performance dielectric capacitor.