Enhancement of Energy Storage Performance in NaNbO₃-Modified BNT-ST Ceramics

Abstract

Relaxor ferroelectrics based on sodium bismuth titanate (Bi_0.5Na_0.5TiO₃, BNT) have attracted more interest recently as potential ecologically acceptable materials for pulse power technology because of their excellent full-energy storage capabilities. This paper formed (1 − x){0.97[0.98(BNT-ST)-0.02CN]-0.03AlN}-xNN ceramics through a traditional solid-state reaction process. It was noted that the incorporation of NaNbO₃ enhances the property of energy storage by elevating the breakdown strength and causing the creation of an ergodic relaxation state. The effective energy storage density (W_rec) and the energy storage efficiency (η) are 1.09 J/cm³ and 85%, respectively. The breakdown field strength E_b reached 155 kV/cm at x = 40%. These ceramics have excellent temperatures and frequency stabilities from 0.5 to 50 Hz and 20 to 60 °C.

1. Introduction

Lead-free piezoelectric ceramics based on Bi_0.5Na_0.5TiO₃ (BNT) have been extensively exploited in aerospace, naval sonar, high-speed trains, and electronic products owing to their favorable piezoelectric and ferroelectric characteristics, together with their good environmental friendliness [1,2,3,4]. Relaxor ferroelectrics usually have a high dielectric constant and a slender hysteresis loop and have become a research hotspot in the field of energy storage materials [5,6,7,8].

Bi_0.5Na_0.5TiO₃ (BNT) is an encapsulated crystal structure (ABO₃) ferroelectric, where the A site is taken up by Na⁺¹ and Bi³⁺ in a 1:1 ratio. In 1962, Buhrer [9] considered that the hybridization between the 6p orbital of Bi³⁺ and the 2p orbital of O²⁻ in BNT gives it favorable ferroelectricity, which also makes the dielectric energy storage ceramics containing Bi have higher polarization. However, the hysteresis loop of an undoped BNT material is approximately rectangular, and the corresponding large leakage current and high residual polarization (P_r = 38 μC/cm²) are its fatal weaknesses in energy storage applications. The breakdown electric field has a direct correlation with the η, W_rec, as well as with the W of the dielectric constant of the dielectric material [10,11,12].

$$\eta ={\displaystyle \frac{W_{\mathrm{rec}}}{W}}\times 100\%$$

(1)

$$W_{\mathrm{rec}}=-{\displaystyle {\int }_{P_m}^{P_r}E\mathrm{dP}\left(\mathrm{upon}\mathrm{discharging}\right)}$$

(2)

$$W={\displaystyle {\int }_0^{P_m}E\mathrm{dP}\left(\mathrm{upon}\mathrm{charging}\right)}$$

(3)

The primary approaches for the modification of energy storage ceramics based on BNT are as follows. On the one hand, doping boosts the breakdown field strength, raises the ∆P (P_m − P_r), reduces the grain size, and raises the dielectric constant. On the other hand, by refining the preparation process to optimize the ceramic microstructure, a high E_b value can be obtained. The fabrication of various ceramic solid solutions based on BNTs is a frequent approach to enhancing the performance of BNTs in terms of energy storage. For instance, Zhao et al. [13] prepared an energy storage ceramic of sodium bismuth titanate-barium strontium titanate (BNT-SBT) with high-density energy storage and charging and discharging performance in a low electric field. Because of the improved relaxation and dielectric properties, the ceramic that was prepared had an η value of 87.5% and a W_rec value of 2.41 J/cm³ at 190 kV/cm. By combining antiferroelectric and relaxation characteristics, dielectric ceramics can simultaneously realize high density and energy storage efficiency. With this concept in mind, Zuo Ruzhong’s team [14] combined NaNbO₃ (NN) with BNT to stabilize the high-temperature antiferroelectric P4bm phase in the relaxor ferroelectric BNT to RT. A large polarization difference (ΔP = 55 μC/cm²), an ultra-high Eb (680 kV/cm), and a record effective energy storage density (W_rec = 12.2 J/cm³) were acquired for 0.76NaNbO₃-0.24(Bi_0.5Na_0.5)TiO₃ bulk ceramics. Li et al. [15] chose antiferroelectric NaNbO₃ (NN) to modify composition-optimized BNT-BST relaxor ferroelectric ceramics, and the prepared BNT-BST-NN showed strong relaxation characteristics. Due to the incorporation of NN, the polarization behavior is optimized, and an effective W_rec of 1.25 J/cm³ with superior cycle reliability and temperature stability is obtained. Higher fields led to the occurrence of an electric field-induced antiferroelectric–ferroelectric phase transition when La3+ and Ta5+ ions were added to the antiferroelectric AgNbO₃ A/B sites by Li et al. [16]. Under an electric field of 540 kV/cm, the Ag_0.94La_0.02Nb_0.8Ta_0.2O₃ ceramic produced a W_rec and η of 6.73 J/cm³ and 74.1%. With an enhanced hot quenching approach, Sun et al. [17] created BF-BT-AN ceramics that exhibit high relaxation properties and have MPB. The produced ceramics exhibit a W_rec of 2.11 J/cm³ and an η of 87.5%, respectively. At 25–150 °C, the ceramics have exceptional thermal stability, and even after 8 × 10⁴ cycles of discharging and charging, they can still demonstrate strong energy storage capabilities.

In early works, BNT-ST was selected as the matrix, and the ion (Co_0.5Nb_0.5)⁴⁺ (CN) was introduced as the dopant to prepare lead-free BNT-ST-xCN ceramics. It has an η value of 72% and a maximum W_rec value of 0.80 J/cm³. Considering the typical antiferroelectric characteristics of NaNbO₃, its AFE behavior is usually stabilized by chemical substitution with other components. The role of the ferroelectrically active ions Bi³⁺, Na⁺, and Ti⁴⁺ is to generate electric dipoles that are spatially correlated. [AO₁₂] cube-octahedral primary frameworks can be expanded by the addition of Sr²⁺ with greater ionic radii, which displaces the ions and partially destroys the long-range structural ferroelectric domains [18,19]. Despite the high Pmax displayed by the BNT-ST ceramic, the large P_r still results in a low energy storage efficiency. Significant decreases in the coercive field (E_c) and P_r will result from the additional disruption of the macroscopic ferroelectric domains with the introduction of NN. The loss of polarization due to relaxation can be compensated via lone electrons in the Nb⁵⁺ and Na⁺ outer 5 s orbitals [20]. Moreover, NN’s large bandgap (approximately 3.58 eV) allows for improved insulation of the system, which raises the E_b [21]. According to the composition design mentioned above, we present NN as a dopant for (1 − x){0.97[0.98(BNT-ST)-0.02CN]-0.03AlN}-xNN ceramics in order to refine the grain size and improve relaxor behavior. The goal of this method is to increase η and W_rec in low-E_b situations. As a result, a number of ceramics free of lead were made with a composition of (1 − x){0.97[0.98(BNT-ST)-0.02CN]-0.03AlN}-xNN. These ceramics’ dielectric characteristics, relaxor behavior, microstructures, and energy storage capabilities, as well as their crystal structures, were all thoroughly investigated.

2. Materials and Methods

2.1. Synthesis of (1 − x){0.97[0.98(BNT-ST)-0.02CN]-0.03AlN}-xNN Ceramics

The (1 − x){0.97[0.98(BNT-ST)-0.02CN]-0.03AlN}-xNN (x = 0, 10%, 20%, 30%, and 40%) ceramics were produced by a conventional SSR process utilizing high-purity raw ingredients of TiO₂, Na₂CO₃, Bi₂O₃, SrCO₃, Co₂O₃, and Nb₂O₅. Following their weighing in line with the stoichiometric ratio of compositions, these raw materials were homogenized with ZrO₂ balls in ethanol for 6 h. The dehydrated slurries underwent a two-hour calcination process at 850 °C and then were subjected to ball milling for 12 h. Eventually, the obtained mixture was dried. Afterward, the obtained powder was subjected to compression under a pressure of about 100 MPa into disks 10 mm in diameter. The disks were sintered in air at 1120 °C for 120 min and were placed in a mixture of the same materials to minimize the possibility of volatile element loss. Lastly, the ceramic surface was covered with silver electrodes for 30 min at 580 °C.

2.2. Characterization

These ceramics’ crystal structures were described employing XRD (D8-Advance, Bruker AXS Inc., Madison, Germany) with CuKα radiation. The microstructure and morphology were examined via SEM (Quanta FEG450, Frequency Electronics, Inc., Hillsboro, OR, USA). Energy spectrum analysis was carried out using an Inca X-Max 50 energy spectrometer produced by Oxford Instrumental Analysis Co., Ltd. A ferroelectric tester (RTI-LC Π, Radiant Technologies Inc., Burbank, CA, USA) coupled to a laser measuring system (MTI-2100,Hottinger Baldwin Messtechnik GmbH, Darmstadt, Germany) was employed to record the hysteresis loops for the polarization (P-E), along with strain (S-E) caused by an electric field.

3. Results and Discussion

The XRD patterns for the (1 − x){0.97[0.98(BNT-ST)-0.02CN]-0.03AlN}-xNN ceramics are demonstrated in Figure 1a. All of the ceramics present an encapsulated crystalline structure, and no second phases or impurities were found, which shows that NaNbO₃ has fully entered the lattice of the 0.97[0.98(BNT-ST)-0.02CN]-0.03AlN ceramics, resulting in the stabilization of the solid solution [22]. In Figure 1b,c, the magnified XRD patterns from 39° to 41° and from 46.5° to 47.5° are exhibited, respectively. The results show that the (111) peak and (200) peak of NaNbO₃-doped 0.97[0.98(BNT-ST)-0.02CN]-0.03AlN ceramics have obvious splitting, indicating that the tetragonal and rhombohedral phases coexist. It was also observed that the (200) and (111) diffraction peaks shifted to a lower angle with rising NaNbO₃ content, suggesting that the volume of the unit cell grows and the lattice expands, which is attributed to the replacement of Nb⁵⁺ (0.69Å) with a smaller B site and a larger ionic radius for Ti⁴⁺ (0.605Å) [23,24]. The introduction of NN induces lattice stress by B-site doping and enhances local structural distortion, effectively decreasing coercive fields and residual polarization. Meanwhile, it suppresses the creation of ferroelectric ordered phases in the long range, broadens the dielectric peak temperature region, weakens the frequency dispersion effect, and considerably enhances the material’s dielectric stability over a wide range of temperatures. Moreover, the inclusion of NN facilitates the thinning of the polarization–electric field loop and boosts the breakdown field intensity by enhancing the polarization inversion barrier and regulating the oxygen vacancy concentration. Finally, it realizes the synergistic optimization of relaxor ferroelectricity and energy storage performance.

Figure 2 shows the surface morphology of the (1 − x){0.97[0.98(BNT-ST)-0.02CN]-0.03AlN}-xNN ceramics. Notably, when the content of NaNbO₃ is 0, there are holes on the surface of the ceramic, but with the increase in NaNbO₃, the holes on the surface of the ceramic gradually decrease and gradually show a dense microstructure. It is commonly recognized that E_b can be improved through declining the size of the grains and elevating the density [11,25]. It demonstrates how significantly the sintering characteristics of (1 − x){0.97[0.98(BNT-ST)-0.02CN]-0.03AlN}-xNN ceramics benefit with the addition of NaNbO₃ [26,27,28,29,30].

To investigate the distribution of different elements in ceramics, EDS surface scanning was performed on 0.6{0.97[0.98(BNT-ST)-0.02CN]-0.03AlN}-0.4NN ceramics, and the findings are presented in Figure 3. From the figure, it can be observed that all of the elements are uniformly distributed.

Figure 4a gives the P-E curves of (1 − x){0.97[0.98(BNT-ST)-0.02CN]-0.03AlN}-xNN ceramics at room temperature, 110kV/cm, and 10Hz. Obviously, all (1 − x){0.97[0.98(BNT-ST)-0.02CN]-0.03AlN}-xNN ceramics exhibit obvious ergodic relaxation characteristics and typical relaxation hysteresis loops. At x = 10%, the remanent polarization P_r and the coercive field E_c drop, and thereafter, they undergo slight changes as the concentration of NaNbO₃ rises. The P_max derived from the P-E curve is revealed in Figure 4b. The P_max intensity reduces gradually as the content of NaNbO₃ rises, from 27.85 μC/cm² when x = 0 to 14.16 μC/cm² when x = 40%. PNRs can polarize into ferroelectric domains to reach a saturation value of P_max when an electric field is applied [31], but the PNR returns to a locally traversed relaxation state that minimizes P_r as the applied electric field totally removes the stochastic distribution of the PNRs [32]. Hence, the generation of PNRs elongates the P-E loop, amplifies the properties of the ferroelectric relaxor, and boosts the properties of W_rec.

The P-E curves for (1 − x){0.97[0.98(BNT-ST)-0.02CN]-0.03AlN}-xNN ceramics at the breakdown field strength, 10 Hz, and RT are illustrated in Figure 5a. The figure shows that with the addition of NN, the ferroelectric phase gradually transforms into the relaxation ferroelectric phase, the double hysteresis loop becomes thinner, and the residual polarization P_r decreases [33]. Figure 5b shows the breakdown field strength (E_b) at RT. It is evident from the figure that the E_b value of NaNbO₃ is gradually elevated from 110 kV/cm for x = 0 to 155 kV/cm for x = 40%.

Figure 6a gives the unipolar P-E curve of (1 − x){0.97[0.98(BNT-ST)-0.02CN]-0.03AlN}-xNN ceramics at room temperature, 10 Hz, and the breakdown field strength. The rate of change of the ceramic polarization strength varied markedly in the electric field around 20 kV/cm. With the inclusion of NN, the P-E curve of the ceramic becomes more elongated. The total η, W, energy loss W_L, and W_rec, which are determined by the unipolar hysteresis loop, are displayed in Figure 6b. The figure suggests that W first reduces and later enhances as the content of NaNbO₃ rises then subsequently reduces. When x = 0, the maximum is 1.41 J/cm³, and when x = 10%, the minimum is 1.17 J/cm³, which is affected by the maximum polarization intensity and residual polarization intensity. The W_L first decreases and then increases, and then decreases again. The maximum is 0.63 J/cm³ when x = 0, and the minimum is 0.18 J/cm³ when x = 10%. The η gradually enhances and subsequently reduces as the NaNbO₃ content rises. The maximum is 85% when x = 20%, and the minimum is 56% when x = 0%. The W_rec is influenced by the polarization strength and hysteresis loop shape [34,35,36,37,38]. When x = 0, the minimum is 0.78 J/cm³, and the maximum is 1.09 J/cm³ at x = 20%.

In addition, the (1 − x){0.97[0.98(BNT-ST)-0.02CN]-0.03AlN}-xNN ceramic demonstrates improved η and W_rec values in contrast to other ceramics for lead-free energy storage when x = 20%, as illustrated in Figure 7. In particular, ceramics based on AN [39,40,41,42] and KNN [43,44] have relatively low η values but high W_rec values. However, while having relatively high η values, ceramics based on BNT [45,46,47] and BF [48,49,50,51] are still not able to reach the performance exhibited by the (1 − x){0.97[0.98(BNT-ST)-0.02CN]-0.03AlN}-xNN (x = 20%) ceramic. The (1 − x){0.97[0.98(BNT-ST)-0.02CN]-0.03AlN}-xNN (x = 20%) ceramic in this study outperforms the performance of previously reported high-performance systems based on BNT, with a remarkable η of 85% and a W_rec of 1.09 J/cm³.

Frequency stability is very important for dielectric energy storage ceramics. Good frequency stability can make energy storage ceramics work stably at different frequencies. The P-E curves and unipolar P-E curves of 0.7{0.97[0.98(BNT-ST)-0.02CN]-0.03AlN}-0.3NN ceramics at room temperature, 120 kV/cm, and different frequencies (0.5~50 Hz) are given in Figure 8a and Figure 8b, separately. It can be seen from the diagram that the maximum polarization intensity P_max decreases slightly as the frequency rises, while the intensity of the residual polarization P_r remains almost unchanged. The total η, W, W_L, and W_rec, which are determined by the unipolar hysteresis loop, are displayed in Figure 8c. The figure displays that there are only minor variations in the charging W, the W_rec, the W_L, and the η at different frequencies. For example, W is 1.07 J/cm³ at 0.5 Hz and 0.88 J/cm³ at 50 Hz, and W_rec is 0.84 J/cm³ at 0.5 Hz and 0.74 J/cm³ at 50 Hz. The energy storage efficiency η is 78% at 0.5 Hz and 83% at 50 Hz. This indicates that 0.7{0.97[0.98(BNT-ST)-0.02CN]-0.03AlN}-0.3NN ceramics exhibit good frequency stability between 0.5 and 50 Hz.

Figure 9a,b present the P-E curves together with the monopole P-E curves of 0.7{0.97[0.98(BNT-ST)-0.02CN]-0.03AlN}-0.3NN ceramics at various temperatures (20~70 °C), 120 kV/cm, and 10 Hz, separately. The diagram presents that the maximum polarization intensity Pmax elevates slightly as the temperature rises, and the residual polarization intensity P_r remains almost unchanged. The total η, W, W_L, and W_rec, which are determined by the unipolar hysteresis loop, are displayed in Figure 9c. The figure reveals that there are only minor variations in the W, W_rec, W_L, and η at different temperatures. For example, at 20 °C and 70 °C, W is 0.90 and 1.06 J/cm³, separately. At 20 °C, W_rec is 0.74 J/cm³, while at 70 °C, it is 0.73 J/cm³. At 20 °C, the η is 82%, whereas at 60 °C, it is 72%, which changes within 10%, but decreases to 68% at 70 °C.

4. Conclusions

(1 − x){0.97[0.98(BNT-ST)-0.02CN]-0.03AlN}-xNN ceramics were produced via the solid-state reaction. A thorough investigation was conducted into the surface morphology, phase structure, energy storage characteristics, and ferroelectricity of these ceramics. The residual polarization of the BNT-ST is reduced by CN doping. In combination with AlN’s high breakdown field strength, the heterogeneous interface between the ternary composite system (BNT-ST-CN) and AlN forms a nanoscale stress field, which promotes the dynamic reconstruction of PNRs and realizes the simultaneous optimization of efficiency and energy storage density. The introduction of gradients in NN further strengthens the relaxation properties so that the polarization–electric field loop exhibits improved and excellent slender characteristics and effectively suppresses energy loss. The breakdown field strength E_b progressively rises from 110 kV/cm when x = 0 to 155 kV/cm when x = 40% as the NaNbO₃ concentration increases. At the strength of the breakdown field, as the concentration of NaNbO₃ grows, the η progressively rises and finally falls. The maximum is 85% when x = 20%, and the minimum is 56% when x = 0%. The W_rec is influenced by the polarization strength and hysteresis loop’s shape. When x = 0, the minimum is 0.78 J/cm³, and the maximum is 1.09 J/cm³ at x = 20%. The ceramics show good frequency stability and temperature stability between 0.5~50 Hz and 20~60 °C.

This work is currently in the experimental stage, and its key innovation is to synergistically regulate the polarization, temperature adaptability, frequency applicability, and other behaviors of relaxor ferroelectrics through multi-scale coupling effects, which presents a novel solution for high-power-density capacitors, enabling both a wide range of frequency and temperature adaptability and high-performance energy storage.