Energy Storage Performance of Na_0.5Bi_0.5TiO₃–CaHfO₃ Lead-Free Ceramics Regulated by Defect Engineering

Abstract

Over the past decades, Na_0.5Bi_0.5TiO₃ (NBT)-based ceramics have received increasing attention in energy storage applications due to their high power density and relatively large maximum polarization. However, their high remnant polarization (P_r) and low breakdown field strength are detrimental for their practical applications. In this paper, a new solid solution (1−x)Na_0.5Bi_0.5TiO₃–xCaHfO₃ (x = 0.04, 0.08, 0.12, 0.16) was constructed by introducing CaHfO₃ into NBT, and thus was prepared using a conventional solid-state reaction. With the addition of CaHfO_3, the disorder of the structure increased, A-site vacancies formed, and thus oxygen vacancies were suppressed due to the replacement of the Na⁺ by Ca²⁺, resulting in the enhanced relaxation behavior and the reduced P_r, the refined grain, and improved breakdown strength. Furthermore, an optimal recoverable energy storage density (W_rec) of 1.2 J/cm³ was achieved in 0.92Na_0.5Bi_0.5TiO₃–0.08CaHfO₃ ceramics under the breakdown strength of 140 kV/cm, which is mainly attributed to the resultant defect of Na⁺ vacancy_.

1. Introduction

Due to their high power density and quick charge/discharge rate, dielectric capacitors have drawn more attention in recent decades in the field of energy and electricity, and electronic circuit systems [1,2]. Among them, ceramic-based dielectric materials have high breakdown field strength (E_b), low dielectric loss (tan(δ)), excellent fatigue resistance, and superior thermal stability, making them suitable in a variety of industries, including in oil exploration, aerospace, and military applications [3,4,5]. Usually, dielectric ceramic materials with large maximum polarization (P_max), small remnant polarization (P_r) and high E_b are perfect for creating energy storage capacitors, and energy storage performance can be evacuated by using the polarization-electric field (P-E) loop according to the subsequent Equations (1)–(3).

$${\mathrm{W}}_{\mathrm{t}\mathrm{o}\mathrm{t}}={\int }_0^{{\mathrm{P}}_{\mathrm{m}\mathrm{a}\mathrm{x}}}\mathrm{E}\mathrm{d}\mathrm{P}$$

(1)

$${\mathrm{W}}_{\mathrm{r}\mathrm{e}\mathrm{c}}={\int }_{{\mathrm{P}}_{\mathrm{r}}}^{{\mathrm{P}}_{\mathrm{m}\mathrm{a}\mathrm{x}}}\mathrm{E}\mathrm{d}\mathrm{P}$$

(2)

$$\mathsf{\eta }={\displaystyle \frac{{\mathrm{W}}_{\mathrm{r}\mathrm{e}\mathrm{c}}}{{\mathrm{W}}_{\mathrm{t}\mathrm{o}\mathrm{t}}}}\times 100\mathrm{\%}={\displaystyle \frac{{\mathrm{W}}_{\mathrm{r}\mathrm{e}\mathrm{c}}}{{\mathrm{W}}_{\mathrm{r}\mathrm{e}\mathrm{c}}+{\mathrm{W}}_{\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}}}}\times 100\mathrm{\%}$$

(3)

where E, W_tot, W_rec, W_loss, and η denote the applied electric field, total energy storage density, recoverable energy storage density, energy loss density, and energy storage efficiency, respectively.

Although Pb-based ceramics exhibit excellent energy storage performance due to their larger P_max and higher E_b, the resultant heavy metal Pb due to the volatilization during high-temperature preparation process and the subsequent scrap of products all pose risks to the ecological environment and people’s health [6,7]. Therefore, designing and researching lead-free materials are considered a feasible solution to solve this problem. In numerous lead-free dielectric ceramics, Na_0.5Bi_0.5TiO₃ (NBT) based ceramics have attracted much attention on account of high Curie temperature (T_c) (~320 °C) and large saturation polarization strength (P_s, 45 µC/cm²) [8]. However, the energy storage properties of NBT ceramics were dissatisfied because of the high Pr (~38 µC/cm²). In addition, the elements Bi and Na are easily volatile during the process of high-temperature sintering, which may lead to the generation of cationic vacancies and oxygen vacancies, and thus the E_b reduced [9,10]. As a consequence, many researchers have focused on the settlement of these issues [11,12,13,14,15,16,17,18,19,20,21]. Thereinto, Yan et al. [12] prepared 0.75Bi_(0.5+x)Na_(0.5−x)TiO₃–0.25SrTiO₃ ceramics by defect engineering and suppressed the formation of oxygen vacancies by adjusting the non-stoichiometric ratio of the A-site, as well as refined the grains size to increase E_b significantly. Eventually, the outstanding W_rec (~5.63 J/cm³) and the relatively high η (~94%) were achieved under 569 kV/cm as x = 0.08. The 0.9(Na_0.4Bi_0.4Ba_0.06Sr_0.14Ti_(1−x)Ta_xO₃)–0.1NaNbO₃ ceramics designed and prepared by Jiang et al. [13] effectively suppressed oxygen vacancies and increased the E_b through defect engineering and finally realized a relative high W_rec (~3.12 J/cm³) and excellent η (~87.68%). By replacing Bi³⁺ and Na⁺ at A-site with La³⁺ to increase the concentration of cation vacancies and decrease oxygen vacancies, the 0.85NBT–0.15CaTiO₃–0.75La ceramics showed the improved energy storage performance with W_rec of ~2.15 J/cm³ and η of ~81% [16]. Apparently, the defect engineering modulation is an effective way to improve the energy storage performance of NBT-based ceramics.

In this study, by incorporating CaHfO₃ into NBT ceramics from a defect engineering perspective, a binary solid solution Na_0.5Bi_0.5TiO₃–CaHfO₃ was designed. On the one hand, the introduction of Ca²⁺ at A-site may induce the cation vacancies and thus significantly suppress oxygen vacancies, along with Hf⁴⁺ replacing Ti⁴⁺ in the B-site with a relatively strong bond strength of Hf–O, ultimately all improving the E_b. On the other hand, due to the incorporation of non-equivalent cations as Na⁺, Ca²⁺, and Bi³⁺, the creation of a local random field may break the long-range order ferroelectric domain and facilitate the formation of polar nanoregions (PNRs), which contributed to the decrease of P_r [22,23,24]. On the basis of the above analysis, (1−x)Na_0.5Bi_0.5TiO₃–xCaHfO₃ (NBT–xCaHfO₃, x = 0.04, 0.08, 0.12, 0.16) solid solution was prepared using the conventional solid state method, and the phase structure, microstructure, dielectric properties and energy storage characteristics were investigated in detail. The ceramics designed and fabricated using this strategy are expected to be used in the field of pulsed power capacitors.

2. Experimental Procedures

(1−x)Na_0.5Bi_0.5TiO₃–xCaHfO₃ (NBT−xCaHfO₃, x = 0.04, 0.08, 0.12, 0.16) ceramics were prepared using a conventional solid phase method [25,26]. The starting materials including Na₂CO₃ (analytical reagent (AR), ≥ 99.8%), Bi₂O₃ (AR, ≥ 99.9%), TiO₂ (chemically pure, ≥ 98.0%), CaCO₃ (AR, ≥ 99.0%), and HfO₂ (AR, ≥ 98.0%), which were all from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China, were weighed according to the stoichiometric ratio of their components. The weighed powders were ball milled for 8 h by using the alcohol as medium. After that, the mixture was dried in an oven before calcination in a muffle furnace at 850 °C for 2 h. Following that, the powder was ball milled for another 6 h, and then was dried and sieved. The sieved powder was granulated with 6% polyvinyl alcohol (PVA) and then pressed into disks with a diameter of 13 mm under a pressure of 300 MPa. Finally, the disks were heated to 550 °C at the speed of 3 °C/min for 3 h to remove the PVA, and then sintered at 1100–1140 °C for 2 h at the rate of 5 °C/min to obtain the final ceramic samples.

X-ray diffraction (XRD, D8 Advanced, Bruker AXS Ltd., Karlsruhe, Germany) was used to analyze the crystal structure of the ceramic samples with diffraction angle 2θ in the range of 20–80°, and the accurate crystal structure of the ceramic materials were obtained using Rietveld full diffraction patterns fitting. The microstructures of the ceramic samples were observed using field emission scanning electron microscopy (SEM, S-4800, Hitachi, Japan). As for electrical testing, the ceramic samples were polished into a thickness of ~1 mm, covered with silver paste on the top and bottom surfaces, and then were fired at 850 °C for 10 min to form the electrodes. The LCR impedance meter (E4294A, Agilent, Santa Clara, USA) was used to measure the dielectric properties in the temperature range of 25–400 °C with a frequency in the range of 1 kHz to 1 MHz. In addition, the P-E curves of ceramic samples under different electric field strength were tested using a ferroelectric analyzer (TF2000, AixACCT, Aachen, Germany).

3. Results and Discussions

The XRD patterns of the NBT–xCaHfO₃ (x = 0.04, 0.08, 0.12, 0.16) ceramics sintered at the optimum temperature are shown in Figure 1a. As can be seen from this Figure, the X-ray diffraction patterns of all ceramic component exhibited a single perovskite structure, and no second phases were detected, which indicated that CaHfO₃ was completely diffused into the NBT lattice and formed a perfect solid solution. Figure 1b,c present the magnified plots of the diffraction peaks near 40.2° and 46.8° to further investigate the structural evolution of the ceramics. It can be seen that a significant splitting presented in both peaks near 40.2° and 46.8°, confirming that all ceramics were in the coexistence of the rhombohedral phase (R3c) and the tetragonal phase (P4bm) at room temperature [27]. The two split diffraction peaks weakened and merged gradually when CaHfO₃ content increased, which demonstrated that the coexistence of R3c and P4bm phases had evolved into a pseudo-cubic phase [28,29]. In addition, as CaHfO₃ doping content increased, the diffraction peaks were first shifted to higher angles (in particular, x = 0.12 is the most prominent), confirming the decreased cell volumes owing to the smaller radius Ca²⁺ (1.34 Å) replacing the larger radius Na⁺ (1.39 Å). However, due to the limited solid solubility of the A-site, when CaHfO₃ doping content exceeds the threshold, the Ti⁴⁺ (0.605 Å) with a smaller radius at the B-site could be replaced by Ca²⁺ (1.34 Å) [30], and at the same time, more Hf⁴⁺ (0.71 Å) with a larger radius enters the B-site to replace Ti⁴⁺, resulting in lattice volume expansion and diffraction peaks shifting towards small angles. In addition, the bulk density of NBT–xCaHfO₃ (x = 0.04, 0.08, 0.12, 0.16) ceramics sintered at different temperatures was plotted in Figure 1d. The bulk density of NBT–xCaHfO₃ increased initially with increasing sintering temperatures and then decreased after reaching the maximum value. It should be noted that the sintering temperature of the maximum bulk density was 1120 °C for all the different compositions.

In order to further analyze the effect of CaHfO₃ doping on the phase structure and lattice parameters of the NBT–xCaHfO₃ ceramics, Figure 2 presents the Rietveld refinement of the full-scan XRD diffraction patterns based on the R3c and P4bm phase, and the corresponding phase proportion, lattice parameters, and reliability factors are shown in

Table 1. The refinement results from full-diffraction pattern fitting of NBT–xCaHfO₃ ceramics.

x	Phase Structure	Relative Content (%)	Lattice Parameters			Reliability Factor
			a(Å)	b(Å)	c(Å)	Rwp (%)	χ2
0.04	R3c	37.30%	5.497	5.497	13.478	8.4%	1.76
	P4bm	62.70%	5.500	5.500	3.887
0.08	R3c	7.47%	5.497	5.497	13.494	8.7%	1.83
	P4bm	92.53%	5.503	5.503	3.889
0.12	R3c	28.26%	5.495	5.495	13.502	8.8%	2.26
	P4bm	71.74%	5.500	5.500	3.892
0.16	R3c	22.85%	5.494	5.494	13.520	9.4%	2.28
	P4bm	77.15%	5.507	5.507	3.895

. The reliability factors of weighted patterns (R_wp) and the goodness-of-fit indicator (χ²) of all component ceramics are within the confidence interval, confirming that the results of XRD refinement are relatively reliable. Clearly, the fraction of the P4bm phase first increased and then decreased with the increase in CaHfO₃ content, as x = 0.08 reached the maximum. Usually, P4bm as the weakly polar phase could be converted into a polar ferroelectric state under an electric field and return to the original weakly polar state when the electric field was removed, hence the enhancement proportion of the P4bm phase could contribute to the reduction in P_r and thus promote the energy storage properties [31,32]. It can be assumed that the NBT–0.08CaHfO₃ ceramics may have optimal energy storage performance accompanied with relatively low P_r.

Figure 3 shows the natural surface micrographs of NBT–xCaHfO₃ ceramics. All samples present clear grain morphology and a relatively dense microstructure. In order to further investigate the effect of CaHfO₃ doping on the microscopic morphology of NBT ceramics, the average grain (AG) sizes of each component ceramics were analyzed, as shown in Figure 4. The average grain size of the ceramics decreased from 3.14 μm for x = 0.04 to 1.35 μm for x = 0.12, verifying that the addition of CaHfO₃ could refine grain size to a certain extent. Since Ca²⁺ tended to displace Na⁺ at A-site due to the similar ionic radii (R_Ca²⁺ = 1.34 Å, R_Na⁺ = 1.39 Å), and thus produced cation defects with positive charges, denoted as ${\mathrm{C}\mathrm{a}}_{\mathrm{N}\mathrm{a}}^{•}$ [32,33], and Na⁺ vacancies (expressed as V′_Na) for charge compensation and for maintaining electrical neutrality. Consequently, the oxygen vacancy (${\mathrm{V}}_{\mathrm{O}}^{••}$), created by loss of oxygen from the crystal lattice at low oxygen partial pressure during sintering at high temperatures had to be inhabited [13,34,35], which further suppressed the conductivity and refined the grain size as well as beneficial to the enhancement of the E_b [36,37]. Furthermore, with the CaHfO₃ increasing, more V′_Na favor the mass transfer in ceramics during sintering [13,38], and as a consequence, the grain size increased up to 1.40 μm when x = 0.16. The equation for the defect is as follows (Equations (4) and (5)), in which the Kröger–Vink notation was employed.

$$\mathrm{C}\mathrm{a}\mathrm{O}\stackrel{{\mathrm{N}\mathrm{a}}_2\mathrm{O}}{\to }{\mathrm{C}\mathrm{a}}_{\mathrm{N}\mathrm{a}}^{•}+{\mathrm{V}}_{\mathrm{N}\mathrm{a}}^{\prime }+{\mathrm{O}}_{\mathrm{O}}$$

(4)

$${\mathrm{O}}_{\mathrm{O}}\to {\mathrm{V}}_{\mathrm{O}}^{••}+{\mathrm{e}}^{\prime }$$

(5)

Figure 5 shows the variation curves of dielectric constant (ε_r) and dielectric loss (tan(δ)) under different frequency for NBT–xCaHfO₃ ceramics. The ε_r of all ceramics presents a decreasing trend to a different extent with the frequency increased, which may be due to the inability of defect dipoles to respond at high frequencies [39,40]. Besides that, when the CaHfO₃ content increased, ε_r at the frequency of 10 kHz decreased from 811 for the composition of x = 0.04 to 606 (x = 0.08) and then increased sharply to 1066 (x = 0.12), accomplished by the minimum dielectric loss (tan(δ) less than 0.022) emerging in the composition of x = 0.08 in a wide range of frequencies from 1k Hz to 1 MHz, implying that the appropriate amount of CaHfO₃ can reduce the concentration of oxygen vacancies and decrease the dielectric loss, which is conducive to the enhancement of the breakdown field strength. Figure 6 illustrates the temperature-dependent dielectric properties of NBT–xCaHfO₃ (x = 0.04, 0. 08, 0.12, and 0.16) ceramics at various frequencies. The dielectric anomaly peak (T_m) clearly observed refers to the ferro-paraelectric phase transition [41,42]. It is noteworthy that with the increase of CaHfO₃ content, the dielectric peak became flatter and broader, accompanied by the T_m shifting to the lower temperature, which may be caused by the diminished coupling effect of [TiO₆] octahedron and weakened structure stability, since A-site vacancies (V′_Na) formed as excessively Ca²⁺ doped [43,44]. In the meantime, as the frequency increased from 1 kHz to 1 MHz, ε_r decreased steadily and T_m shifted to the higher temperature, manifesting a relaxed behavior to a certain extent [45]. To further analyze the effect of CaHfO₃ content on the degree of relaxation, the diffuseness parameter γ between 1 (normal ferroelectric) and 2 (ideal relaxer) was calculated using the modified Curie–Weiss equation (Equation (6)) [46,47,48]:

$${\displaystyle \frac{1}{{\mathsf{\epsilon }}_{\mathrm{r}}}}-{\displaystyle \frac{1}{{\mathsf{\epsilon }}_{\mathrm{m}}}}={\displaystyle \frac{{\left(\mathrm{T}-{\mathrm{T}}_{\mathrm{m}}\right)}^{\mathsf{\gamma }}}{\mathrm{C}}}$$

(6)

where C represents the Curie–Weiss constant and ε_m is the dielectric constant corresponding to T_m. The γ obtained by fitting the curves between ln(T−T_m) and ln((1/ε_r)−(1/ε_m)) at 1 kHz, as shown in Figure 7, increased from 1.44 (x = 0.04) to 1.63 (x = 0.08) and then decreased to 1.57 (x = 0.12) and 1.54 (x = 0.16), hinting that the optimal relaxation characteristics can be obtained in the composition of x = 0.08. The optimizing relaxation performance may be related to the increased ionic disorder in the microstructure, where the non-equivalent substitution of Ca²⁺ for Na⁺ and Bi³⁺ broke the long-range ferroelectric domains and facilitated the formation of polar nanoregions [49,50,51].

Figure 8 displays the P-E hysteresis loops of NBT–xCaHfO₃ ceramics under the electric fields of 120 kV/cm. Evidently, as CaHfO₃ content increases, the P-E loops become slimmer, especially for the NBT–0.08CaHfO₃ ceramics, which is in agreement with the result of diffuseness parameter γ. The slender P-E loops can be mainly ascribed to the broken long-range ferroelectric domain as the non-equivalent substitution of Na⁺ and Bi³⁺ by Ca²⁺. Thus, the formed polar microregions suppressed the early polarization saturation of the ceramics and reduced the P_r, which facilitated the achievement of excellent energy storage performance [24,52]. In addition, Figure 9 displays the P-E hysteresis loops of NBT–xCaHfO₃ ceramics under various electric fields. Apparently, P-E loops of all composition turn more slender with increasing applied electric filed, among which P_max and ∆P (∆P = P_max−P_r) of the composition with x = 0.08 increases significantly, while P_r increases slightly. Ultimately, under an electric fields of 140 kV/cm, P_max and ∆P reach their maximums, as displayed in the inset of Figure 9b.

The improvement of W_rec requires not only high P_max and low P_r, but also high applied field and, subsequently, the P-E hysteresis loops of NBT–xCaHfO₃ ceramics at 20 Hz under critical electric fields are displayed in Figure 10a. Compared to NBT–xCaHfO₃ ceramics with x = 0.04, 0.12, and 0.16, the NBT–0.08CaHfO₃ ceramics demonstrated the maximum E_b value of 140 kV/cm because of the dense microstructure and relatively small grain size (in Figure 3). The variation of W_rec, η and P_max, P_r, and ∆P for each ceramic under a critical electric field are shown in Figure 10b,c, and the relevant parameters are listed in

Table 2. P_max, P_r, W_rec, and η of NBT–xCaHfO₃ ceramics under the critical electric field.

x	Eb (kV/cm)	Pmax (μC/cm2)	Pr (μC/cm2)	Wrec (J/cm3)	η (%)
0.04	130	55.39	33.30	0.95	24%
0.08	140	32.10	7.88	1.20	45%
0.12	130	27.93	7.31	1.03	48%
0.16	120	23.66	3.09	1.07	69%

. Overall, with the increase in the CaHfO₃ content, the P_max decreased from 55.39 μC/cm² (x = 0.04) to 23.66 μC/cm² (x = 0.16) together with P_r maintaining in a relatively low value. Finally, the ∆P attained its maximum for NBT–0.08CaHfO₃ ceramics, achieving the optimizing energy storage performance with W_rec of 1.2 J/cm³ and η of 45%. Also, the energy storage performance and polarization properties of NBT–0.08CaHfO₃ ceramics under various electric fields from 20 kV/cm to 140 kV/cm are demonstrated in the inset of Figure 9b. Apparently, ∆P enhanced approximately linearly from 2.33 μC/cm² to 24.12 μC/cm², with the increase in applied electric field as a result of the significant increase of P_max along with a slight increase of P_r. The comparatively large ΔP and E_b facilitated excellent energy storage properties. Hence, the W_rec of 1.20 J/cm³ was highlighted in 0.92NBT–0.08CaHfO₃ ceramics under the maximum electric field of 140 kV/cm, indicating certain potential in energy storage application.

4. Conclusions

NBT–xCaHfO₃ (x = 0.04, 0.08, 0.12, 0.16) ceramics were prepared using a conventional solid phase method. The introduction of Ca²⁺ substituting for Na⁺ and Bi³⁺ at the A-site induced the cation vacancies and then suppressed the concentration of oxygen vacancies as well as refined the grain size, associated with Hf ⁴⁺ replacing Ti⁴⁺ in the B-site with a relatively strong bond strength of Hf–O, eventually fulfilling the improvement of the E_b. Besides that, a local random field created by the incorporation of non-equivalent cations as Na⁺, Ca²⁺, Bi³⁺, and cation vacancies, as well as bond strength of Hf–O, broke the long-range order ferroelectric domain and facilitated the formation of PNRs, resulting in an enhancement of relaxation properties and a decrease in P_r. Combined with enhanced relaxation behavior and increased E_b, an optimizing W_rec of 1.2 J/cm³ was achieved in NBT–0.08CaHfO₃ ceramics under the E_b of 140 kV/cm. All of these demonstrated that the NBT–0.08CaHfO₃ ceramics have an attractive potential in the field of energy storage ceramics.