Enhanced energy storage and fast discharge properties of BaTiO3-Bi(Ni0.5Zr0.5)O3 ceramics modified by (Bi0.5Na0.5)0.7Sr0.3TiO3

ABSTRACT Relaxed ferroelectric ceramics with good energy storage stability, high energy storage density and efficiency, and high charge/discharge rates have shown great potential for commercial applications. In this paper, 0.85[0.9BaTiO3 −0.1Bi(Ni0.5Zr0.5)O3]−0.15(Bi0.5Na0.5)0.7Sr0.3TiO3 ceramic was prepared via a solid – phase reaction method by doping (Bi0.5Na0.5)0.7Sr0.3TiO3 into 0.9BaTiO3 > −0.1Bi(Ni0.5Zr0.5)O3; it exhibited excellent energy storage performance values: Wrec = 2.87 J/cm3 and η = 88.10%. Charging and discharging tests were conducted to evaluate the feasibility of the ceramic for practical applications in energy – storage devices; it displayed an ultrafast discharge rate of 1.16 μs. In addition, the developed ceramic has good frequency stability (1–120 Hz) and high temperature stability (40–160°C). This excellent energy storage performance indicates that the ceramic is a suitable candidate for pulse power devices.


Introduction
With the worldwide development of information and communication technologies, pulsed power systems have become more widely used in military and civilian applications. However, the performance of their core energy storage components does not meet the growing demand for current applications [1,2], which limits their further development in commerce. Currently, lead -based ceramic capacitors are usually doped and modified to meet the corresponding application requirements [3]. However, with the sustainable development of the human living environment, current development in energy storage ceramics tends to explore lead -free ceramics [4].
Generally, the energy storage characteristics of ceramics can be calculated using the following equation: where W rec is the recoverable energy storage density, E is the electric field intensity, P max and P r are the maximum and residual polarization intensities, respectively, and η is the energy storage efficiency [5][6][7]. It can be observed that large ΔP (ΔP = P max -P r ) is extremely important for obtaining high energy storage density and efficiency at high magnetic fields [8]. Currently, mainstream lead -free energy storage ceramics mainly include NaNbO 3 -, BaTiO 3 -, (Bi 0.5 Na 0.5 ) 0.7 Sr 0.3 TiO 3 -, and (K 0.5 Na 0.5 )NbO 3 -based systems. Dong et al. [9] reported that NaNbO 3 -Bi(Ni 1/2 Sn 1/ 2 )O 3 ceramics have excellent properties with a high W rec of 5 J/cm 3 , good temperature (20-140°C) and frequency (1-100 Hz) stability; this was determined by replacing Na ions with Bi ions in the A -site and Nb ions with Ni and Sn ions in the B -site of the ceramics. Meng 3 , into (Bi 0.5 Na 0.5 ) 0.7 Sr 0.3 TiO 3 ceramic and obtained a W rec of 5.5 J/cm 3 and η of 90.1% [6]. Zhang et al. designed and prepared 0.85(K 0.5 Na 0.5 )NbO 3 −0.15 (K 0.7 Bi 0.3 )NbO 3 ceramics by replacing K and Na ions in the A -site of (K 0.5 Na 0.5 )NbO 3 ceramics with Bi ions, obtaining a W rec of 3.39 J/cm3 and η of 51.7% [11].
In contrast, BaTiO 3 -based energy storage ceramics have a lower residual polarization and easier doping modifications. More importantly, their lower prices render them more suitable for commercial production. Bi -based composites are typically added to modify the properties of BaTiO 3 -based ceramics. Dong et al. reported that 0.84BaTiO 3 −0.16Bi(Ni 2/3 Ta 1/3 )O 3 ceramic have good energy storage stability at 1-200 Hz and 20-100°C and good energy storage performance: W rec = 2.63 J/cm 3 and η = 90% [12]. Hu et al. prepared 0.6BaTiO 3 −0.4Bi(Mg 1/2 Ti 1/2 )O 3 ceramic via the solidphase reaction method, W rec and η are less than 5%, with good temperature stability (30-170°C). In addition, the ceramic has a W rec of 4.49 J/cm 3 and an η of 93% [13]. Jiang et al. designed and prepared 0.85BaTiO 3 −0.15Bi(Mg 1/2 Zr 1/2 )O 3 ((1-x)BT -xBMZ) ceramics by replacing Ba ions with Bi ions in the Asite of BaTiO 3 ceramics and Ti ions with Mg and Zr ions in the B -site of the ceramic, obtaining W rec and η values of 1.25 J/cm 3 and 95%, respectively, at 185 kV/ cm field strength [14].
In this study, we prepared  After the ball mill slurry was poured out and dried, the mixed raw material powder was calcined at 900°C for 5 h to enable it to fully react. Ball milling was repeated at 250 rpm for 5 h. The resulting material was thoroughly mixed with polyvinyl alcohol to prepare uniform -sized pellets, and the small particles were pressed into discs with φ = 8 mm and h = 2 mm using a mold. Finally, the samples were sintered at 1275°C for 2 h. For further investigate the energy storage properties, the sintered sample was polished to thickness of 0.15 ± 0.01 mm and coated with silver paste with a diameter of 2 mm (area is 3.14 mm 2 ). Figure 1 shows the XRD patterns of the BT -BNZ-BNST ceramics at the optimum sintering temperature. It can be seen that Bi 2 O 3 impurity phase appear in the ceramics when x = 0. When BNST diffuses into the lattice of 0.9BaTiO 3 −0.1Bi(Ni 0.5 Zr 0.5 )O 3 ceramic, there is no secondary phase. All the components of the BT -BNZ-BNST ceramics have a chalcogenide structure and form a stable solid solution [15]. With the increasing x value, a gradual shift of the (101) and (200) diffraction peak is observed in the XRD pattern from the 2θ range of 45° −46° to a higher angle, as shown in Figure 1(b-c). This indicates that the lattice volume of the ceramics gradually decreases, which may be due to the substitution of the smaller ionic radius Bi 3+ (1.38 Å), Na + (1.39 Å) and Sr 2+ (1.44 Å) ion for the larger ionic radius Ba 2+ (1.61Å) [13,14-16-17-18]. The Bi 2 O 3 impurity phase in the ceramics disappeared after doping BNST. The (101) and (200) diffraction peaks of the ceramics changed from left high to right low, and the peak intensity ratio of 2 : 1 evolved into a single peak. This indicates that the ceramics evolve from the two -phase coexistence structure of tetragonal phase and pseudo -cubic phase to the pseudo -cubic phase structure [5]. The sharp diffraction peaks indicate that the BT -BNZ-BNST ceramic grains grew well [19]. of all components exhibit uniform and dense microstructures. The average grain size of the ceramics slightly increases with increasing BNST content. Because an increase in the Bi 2 O 3 content decreases the sintering temperature of the ceramics and refines the grains, the maximum value of the average grain size of the ceramics is <1.7 μm [20,21]. A smaller grain size leads to more grain boundaries, which are extremely important for improving the breakdown field strength of the ceramics [22]. However, in the P-E test, the = 0 ceramics did not obtain the ideal breakdown strength, which can be attributed to the influence of the defects of the pores in the ceramics [23,24].

Results and discussion
To determine the energy storage performance of the samples, P -E loops test was conducted on the BT -BNZ-BNST ceramics (electrode area : 0.0314 cm 2 ), and the results are shown in Figure 3. The maximum polarization intensity P max of the ceramics remains at approximately 22 μC/cm 2 and gradually increases with increasing BNST content. With an increase in the doping amount, the P r of the ceramics first decreases and then increases and remains between 0.8 and 2.8 μC/cm 2 . As shown in Figure 3(d-e), P max remains at approximately 19 μC/cm 2 , and P r remains between 0.64 and 2.8 μC/cm 2 at a field strength of 280 kV/cm. This results in a high ΔP, which can be attributed to the fact that the long -range ordered structure of the ceramics is destroyed by the doping of A/B heterovalent ions, causing the formation of polar nano -microregions (PNRs) [21,25,26]. The PNRs respond extremely rapidly to changes in the electric field, reducing the P r while enhancing the P max of the ceramic to improve its energy storage performance [27,28]. At x = 0, 0.05, 0.10, 0.15, 0.20 the BT -BNZ-BNST ceramics obtained a W rec of 1.83 J/cm 3 , 2.60 J/ cm 3 , 2.59 J/cm 3 , 2.84 J/cm 3 , 2.46 J/cm 3 and η of 77.94%, 92.18%, 94.68%, 90.87%, 91.60% at a field strength of 280 kV/cm, 320 kV/cm, 350 kV/cm, 340 kV/cm, 385 kV/ cm, respectively. With the increasing x, the breakdown field strength of the ceramics increased from 280 to 380 kV/cm, which may be due to the smaller average grain size and higher density of the ceramics [29].
The frequency and temperature stabilities of the BT -BNZ-BNST ceramics are important parameters for evaluating their superior performance.   (a-c) illustrate the P -E loops of the 0.15BT -BNZ-BNST ceramic at different frequencies and 210 kV/cm. The W rec is stable between 1.52 and 1.55 J/cm 3 , and no change is evident with an increase in the frequency. The η value gradually decreases from 88.99% to 85.35%, and the P r rises from 1.15 to 2.08 μC/cm 2 . Figure 4(d-f) show the P -E loops of 0.15BT -BNZ-BNST ceramic at 210 kV/cm in the temperature range of 40-160°C. The η decreases from 88.56% to 86.6% with increasing x, the P r gradually increases from 1.26 to 1.39 μC/cm 2 , the P max decreases from 18.74 to 17.22 μC/cm 2 , and the W rec is stable at approximately 1.65 J/ cm 3 without significant changes. The P r increases and P max decreases at high temperatures owing to the increase in the oxygen vacancy concentration and conductivity [30][31][32]. The 0.15BT -BNZ-BNST ceramic exhibits excellent energy storage characteristics, and its energy storage performance remains stable over wide temperature and frequency ranges, which is beneficial for its commercial application.   The dielectric peaks for all the components of the ceramics gradually broaden with increasing frequency. With an increase in the BNST content, the dielectric constant and dielectric loss show obvious dispersive phase changes, exhibiting strong frequency dispersion behavior with typical relaxation characteristics. In addition, with the increase in BNST content, the ceramic gradually changes from the paraelectric to the ferroelectric phase [33]. This can be attributed to the fact that the increase in the content of the ferroelectric enhancer BNT breaks the long -range ordered structure of the ceramics and promotes the formation of PNRs within the ceramics [34,35], thus facilitating an increase in the working temperature range of the ceramics. Figure 5(f) shows the dielectric spectrum of the BT -BNZ-BNST ceramics in the frequency range of 100 Hz to 1 MHz. When the test frequency exceeds 100 KHz, the dielectric constant of the ceramic slightly decreases. The dielectric constant is essentially unchanged below 100 Hz, which is because of the relaxation behavior of the BT -BNZ-BNST ceramics [36]. These results indicate that the ceramics exhibits good frequency stability. Figure 6(a-d) a is the impedance diagram of the BT -BNZ-BNST ceramics at 500-580°C. The impedance data are fitted by two sets of equivalent circuits. The results show that all the fitted data show good correlation. The activation energy of grain boundary (E gb ) and activation energy of grain (E g ) of ceramics were analyzed by Formula : constant, Boltzmann constant, activation energy, respectively). The activation energy (E a ) can reflect the energy required for ions to cross the barrier. The higher E a , the stronger the inhibition of carriers, indicating that the ceramics are easier to obtain higher E b values. It can be seen from the calculation results that when x = 0.05, 0.10, 0.15, 0.20 E a (E a = E g + E gb ) of the ceramics are 2.67 eV, 3.47 eV, 3.36 eV, 3.13 eV, respectively. However, the ideal E b value was not obtained in the P -E test, which can be attributed to the defects such as pores in the ceramics. Figure 7(a-b) show the underdamped charge/discharge curves of the 0.15BT -BNZ-BNST ceramic. With the increase in the test electric field, the current density (C D ) and power density (P D ) of the ceramic increase from 128.50 to 910.13 A/cm 2 and 1.29 to 72.81 MW/cm 3 , respectively. Among these, C D = I max /S and P D = E -I max /2S are the basic parameters for measuring the pulse power system. To reflect the performance stability of ceramics at high temperatures, the underdamped charge/discharge test was performed at 40-160°C with the field strength set at 14 kV/mm. The results are shown in Figure 7(c-d). With the increasing temperature, the P D of the ceramic gradually increases from 59.91 to 65.85 MW/cm 3 and then decreases to 63.87 MW/cm 3 (with a change rate of < 9.02%), and the C D gradually increases from 855.89 to 940.78 A/cm 2 and then decreases to 912.48 A/cm 2 (with a change rate of < 9.02%). The results show that the ceramic exhibits good temperature stability. Figure 8   ceramics under different electric fields. The discharge current varies from 1.85 to 13.93 A. Figure 8(b) displays the variation between the discharge energy density (W dis ) and the discharge time of the ceramic. The graph visualizes the relationship between the discharge times and the performance of the ceramic. The t 0.9 indicates the time required to release 90% of the energy of the ceramic. As the test electric field increases, the W dis of the ceramic gradually increases from 0.02 to 0.94 J/cm 3 , and the t 0.9 stabilizes at approximately 1.16 μs with a variation rate of less than 4.2%. Figure 8(c) demonstrates the overdamped charge/discharge curves of the ceramics in the temperature range of 40-160°C. The discharge current of the ceramic at 14 kV/mm gradually increases from 12.54 to 12.90 A with a rate of change of 2.79%, and the t 0.9 is stable at approximately 1.12 μs. The test results indicate that the 0.15BT -BNZ-BNST ceramic exhibits good temperature stability and high discharge time and has the potential to be applied as an environmentally friendly material for pulsed power systems [37].