Enhancing the Energy Storage Properties and Breakdown Strength of Lead-Free Bismuth-Sodium Titanate-Based Ceramics Through NaNbO₃ Doping

Abstract

Dielectric capacitors with a high density of recoverable energy storage are extremely desirable for a variety of uses. However, these capacitors often exhibit lower breakdown strengths and energy efficiency compared to other materials, which poses significant challenges for their practical use. We report on a novel antiferroelectric ceramic system in the present study, (1 − x){0.97[0.985(0.93Bi_0.5Na_0.5TiO₃–0.07BaTiO₃)–0.015Er)]–0.03AlN}–xNaNbO₃ (x = 0, 10 wt%, 20 wt%, 30 wt%, and 40 wt%), synthesized via a conventional solid-state reaction approach. Here, (Bi_0.5Na_0.5TiO₃–BaTiO₃) is denoted as BNT–BT. We observed that varying the NaNbO₃ (NN) content gradually refined the grain size of the ceramics, narrowed their hysteresis loops, and transformed their phase structure from antiferroelectric to relaxor ferroelectric. These changes enhanced breakdown strength (E_b), thus increasing the performance of energy storage. Specifically, the recoverable energy density (W_rec) and energy storage efficiency (η), respectively, reached 0.67–1.06 J/cm³ and 44–88% at electric fields of 110–155 kV/cm, with the highest performance observed at 30 wt% NN doping. Additionally, over a broad range of temperature and frequency, the 70 wt% {0.97[0.985(BNT–BT)–0.015Er]–0.03AlN}–30 wt% NN ceramic demonstrated exceptional stability in energy storage. These results demonstrate the significant potential of lead-free(1 − x)({0.97[0.985(BNT–BT)–0.015Er]–0.03AlN}–xNN ceramics for the applications of high-performance energy storage.

1. Introduction

In light of the recent explosive growth of the electronics industry, customers’ expectations for the performance of their electronic devices have steadily grown [1,2,3]. As a result, industrial demand is now more focused on integration and miniaturization, driving a need for advanced high-energy storage materials [4,5,6]. The ultrafast charging/discharging speed, ultrahigh power density, and high operating voltage of dielectric capacitors are clear benefits when compared to alternative energy storage technologies like super-capacitors or Li-ion batteries [7]. However, conventional dielectric ceramics have drawbacks, like poor dielectric stability and low energy storage capacity, which seriously hinder their large-scale development and application in microelectronics and portable devices [8,9]. Hence, there is an immediate demand for dielectric ceramic materials having high density along with efficiency of energy storage. The main performance parameters, namely, W_rec, the overall energy storage density W_L and the η, can be computed from P–E loops [10,11,12]:

$$W_{rec}={\displaystyle {\int }_{p_r}^{p_{\max }}E}dp$$

(1)

$$W={\displaystyle {\int }_0^{P_{\max }}Edp}$$

(2)

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

(3)

Here, E stands for the applied electric field, P_max and P_r denote the maximal (saturate) polarization and residual polarization, respectively, and W_L denotes the density of energy loss. As seen from the equation, to obtain an enhanced energy storage density, these individual parameters should be reasonably balanced.

In recent years, researchers have studied lead-free dielectric materials, encompassing Bi_0.5Na_0.5TiO₃(BNT)-, NaNbO₃ (NN)-, BiFeO₃(BF)-, K_0.5Na_0.5NbO₃(KNN)-, and BaTiO₃(BT)-based materials [13,14,15,16,17]. Among these, lead-free ceramics based on BNT are of interest because of their large P_max, high Curie temperature, and excellent dielectric properties. On the other hand, pure BNT ceramics have low E_b and a square P–E loop, which results in low η and W_rec. Composition design and ion doping have been found to be effective ways of improving the E_b and P–E curves of BNT-based systems [18]. However, most previous research has focused on improving energy storage performance under high E_b. As an example, it has been reported by Wang et al. [19] that the incorporation of Sr(Ta_0.5Sb_0.5)O₃ into 0.94BNT–0.06BT ceramics refined the size of grain, improved the relaxor degree, and displayed the best energy storage properties of 8.33 J/cm³ at 620 kV/cm E_b. Likewise, Li et al. [20] explored the impact of Sm₂O₃ on the characteristics of energy storage of 0.6Na_0.5Bi_0.5TiO₃–0.4Sr_0.7Bi_0.2TiO₃ ceramics and suggested the W_rec at E_b of 470 kV/cm was 6.1 J/cm³. However, owing to the high cost of utilizing insulation technology and the danger of employing energy storage equipment at high E_b [21], outstanding energy storage performance at relatively low E_b circumstances is essential for the wide-ranging use of dielectric ceramic capacitors [22].

In the pursuit of achieving a high W_rec under a relatively low E_b, an in-depth exploration has been carried out. According to the well-established previous reports [23,24,25], it has been demonstrated that NN holds great potential. NN has the ability to strengthen the relaxor behavior within relevant materials. This strengthening effect is crucial as it directly contributes to improving the overall performance of energy storage. For instance, Qi et al. conducted a series of experiments on BNT–NN ceramics. They specifically modified these ceramics using NN. Through their meticulous experimental procedures, they managed to get an extremely high W_rec value of 7.02 J/cm³ at 390 kV/cm [26]. This result clearly showcases the positive impact of NN modification on improving the energy storage capacity. In another study, Wu et al. [27] took a different approach. They introduced NN along with Sr_0.85Bi_0.1TiO₃ into BNT ceramics. Their comprehensive research led to significant findings. At an electric field strength of 220 kV/cm, they reported an improved W_rec value of 3.08 J/cm³. Moreover, the efficiency of the energy storage system reached 81.4%. This suggests a rather high-efficiency operation in this particular situation, in addition to an improvement in the recovered energy density. Zhu et al. [28] further delved into the energy storage performance of (1 − x)(0.95BNT–0.05SZ)–xNN ceramics, exploring their performance at lower electric field strengths. Remarkably, they obtained a W_rec of 3.14 J/cm³ and an outstanding energy efficiency of 79% at a relatively low E_b of 120 kV/cm. This achievement highlights the potential of this specific ceramic composition in optimizing the characteristics of energy storage at low electric fields.

In our previous research [29], the 0.985(BNT–BT)–0.015Er ceramic exhibited several desirable properties, such as narrow hysteresis loops and a high P_max. However, its relatively low E_b limited its potential applications. To address this limitation, we introduced AlN, which has an exceptional breakdown strength (up to 12–15 MV/cm), into the system. Our investigations revealed that the addition of 0.03 mol of AlN increased the breakdown strength, with a maximum value reaching 110 kV/cm. However, this composition exhibited a low recoverable energy efficiency (η = 44%) and energy density (W_rec = 0.67 J/cm³), indicating the need for further improvement. Therefore, in this work, we introduce NN as a dopant for grain size refinement and improvement of the relaxor behavior of the 0.97[0.985(BNT–BT)–0.015Er]–0.03AlN ceramics. This approach aims to improve W_rec and η under low E_b conditions. Consequently, a series of lead-free ceramics with the composition (1 − x)({0.97[0.985(BNT–BT)–0.015Er]–0.03AlN}–xNN were prepared. The microstructure, relaxor behavior, dielectric properties, crystal structure, and energy storage performance of these ceramics were examined systematically.

2. Experimental Procedure

2.1. Sample Fabrication

(1 − x)({0.97[0.985(BNT–BT)–0.015Er]–0.03AlN}–xNN(x = 0, 10 wt%, 20 wt%, 30 wt%, 40 wt%) lead-free dielectric materials were manufactured via a traditional solid-state reaction approach. First, TiO₂ (98%), Na₂CO₃ (99.5%), Bi₂O₃ (99%), Er₂O₃ (99.9%), BaTiO₃ (99.9%), Nb₂O₅ (99.9%), and AlN (99.5%) were milled for 4 h in ethanol and subsequently dried at a temperature of 100~120 °C. After being pre-sintered at 100~150 °C/hour, the dried material was heated to 800 °C and held for two hours before naturally cooling to room temperature (RT). Under the same conditions, a second cycle of milling and drying was conducted. The powder was then mixed with 8 wt% PVA as a binder and formed into 1 mm thick by 10 mm wide pellets. After covering the pellets with calcined powder of the same composition, the pellets were sintered at 1150 °C in air for 120 min to prevent volatilization of Bi and Na. Lastly, two surfaces of the ceramic particles were placed with silver electrodes and heated at 550 °C for 30 min. Ultimately, the specimens were subjected to polarization within silicone oil to facilitate dielectric property analysis.

2.2. Structure and Electrical Characterization

The crystallization phase structure of (1 − x)({0.97[0.985(BNT–BT)–0.015Er]–0.03AlN}–xNN was examined via XRD (Bruker D8 Advance diffractometer, Madison, Germany) with CuKα radiation at the ambient surrounding temperature. SEM (Quanta FEG 250, Frequency Electronics, Inc., Hillsboro, OR, USA) was applied to observe the samples’ morphology and microstructure. Using an energy dispersive spectrometer (EDS, Inca X-Max 50, Oxford Instrumental Analysis Co., Ltd., Oxford, UK), the energy spectrum was analyzed. The temperature stability, P–E hysteresis loops, and dielectric breakdown strength were assayed with a ferroelectric test system (RTI-LC Π, Radiant Technologies Inc., Burbank, CA, USA).

3. Results and Discussion

The XRD patterns of (1 − x)({0.97[0.985(BNT–BT)–0.015Er]–0.03AlN}–xNN ceramics in the 2θ ranges of (a) 20–80°, (b) 39–41°, and (c) 45.5–47.5°, ceramics at RT are revealed in Figure 1a–c. The development of a full solid solution is confirmed by the observation that all compositions exhibit a characteristic perovskite structure and are devoid of impurity phases. Meanwhile, Figure 1b,c illustrates that the 2θ ranges of 39–41° and 45.5–47.5° are zoomed in. In these regions, the splitting of the (111) peak at around 40.5° and the (200) peak at around 47° revealed that rhombohedral and tetragonal phases coexist in all (1 − x)({0.97[0.985(BNT–BT)–0.015Er]–0.03AlN}–xNN samples, which is a common feature in perovskite materials undergoing phase transitions.

Moreover, with the introduction of NN, these diffraction peaks moved to smaller angles. This shift is associated with the substitution of larger Nb⁵⁺ ions (0.64 Å ionic radius) for the smaller Ti⁴⁺ ions (0.605 Å ionic radius) on the B sites. However, the slight expansion of the single-cell volume is insufficient to significantly enhance the relative displacement and activity space of the B-site ions, even when the larger Nb⁵⁺ ions occupy these sites (please refer to references [30,31,32]). This structural change may lead to a reduction in the P_max, which in turn significantly affects the energy storage characteristics.

Figure 2a–e reveal the SEM images of the surface microstructure and the mean grain size measurements for all samples. All samples displayed dense microstructure and low porosity. Notably, when the addition of NN is increased to 30 wt%, the originally elongated bar-shaped grains disappear. This phenomenon can be attributed to the substitution of Ti⁴⁺ with Nb⁵⁺, which introduces significant lattice distortion since Ti⁴⁺ and Nb⁵⁺ have different valence states and ionic radii. The lattice distortion increases internal stress within the grains, reducing grain mobility and suppressing grain growth [33]. As a result, the elongated bar-shaped grains, which typically form under conditions of higher grain mobility, are no longer sustained and instead transform into smaller, more equiaxed grains. We used NaNo measuring software to estimate the particle size distribution with increasing NN concentration, and the results are displayed in Figure 3. The average particle size reduced from 2.82 μm to 2.14 μm as the NN concentration increased. This reduction in grain size further supports the idea that the introduction of NN suppresses grain growth. The decreased grain size and increased density can result in a rise in the E_b. The correlation between breakdown field strength and grain size can be expressed by the equation below [34]:

$$E_b\propto {\displaystyle \frac{1}{\sqrt{G}}}$$

(4)

Here, G represents the average grain size (AGS). It can be deduced from Equation (4) that smaller grain sizes in ceramics result in larger E_b, which is beneficial for achieving higher W_rec [35,36,37].

To evaluate the distribution of various elements in the ceramics, energy dispersive spectroscopy (EDS) was applied to conduct an elemental scan of 60 wt% {0.97[0.985(BNT–BT)–0.015Er]–0.03AlN}–40 wt% NN, as presented in Figure 4. The findings indicate that the elements are uniformly distributed within the ceramic grains. This uniform distribution is in line with the results of XRD, which confirm the formation of a homogeneous solid solution. Additionally, the proportions of the various elements closely match the theoretical values, validating the stoichiometric composition of the samples.

Figure 5a exhibits the P–E loops, (b) the remanent polarization (P_r) and P_max, and (c) the η, total energy density (W_tot), together with W_rec as a function of x for (1 − x)({0.97[0.985(BNT–BT)–0.015Er]–0.03AlN}–xNN ceramics at RT and 110 kV/cm. Figure 5a demonstrates that the P–E loops become narrower as NN content increases. At x = 0, P_max reduces from 28.13 μC/cm² to 11.06 μC/cm² at x = 40 wt%, while P_r reduces from 9.38 μC/cm² to 1.22 μC/cm², as exhibited in Figure 5b. The generation of polar nanoregions (PNRs), along with the disturbance of the long-range ferroelectric order caused by the presence of NNs, may be the cause of this phenomenon. Under the condition of the applied electric field, PNRs can achieve a saturation value of P_max by polarizing into ferroelectric domains [38], while the complete elimination of the random distribution of PNRs by the applied electric field results in a return to a local ergodic relaxor state that minimizes P_r [39]. Consequently, the creation of PNRs strengthens the characteristics of the ferroelectric relaxor, elongates the P–E loop, and enhances the performance of W_rec.

Figure 5c illustrates the performance of energy storage of the ceramic samples identified from P–E loops. With increasing NN content, W_rec decreases slightly, primarily caused by the gradual decrease in P_max. In comparison, η shows an increasing trend, reaching a peak value of 81.96% at x = 40 wt%. This trend indicates that while the P_max decreases, the reduction in hysteresis (as evidenced by the lower P_r) leads to improved energy storage efficiency.

E_b is among the critical parameters of energy storage ceramics. For examining the E_b performance of (1 − x)({0.97[0.985(BNT–BT)–0.015Er]–0.03AlN}–xNN ceramics, Figure 6a–c illustrate the P–E loops, P_max, P_r, E_b, together with the corresponding W_tot, W_rec, and η of the ceramics at the maximal applied electric field. Figure 6a reveals that as the NN content elevates, polarization saturation of the P–E ring occurs and narrows considerably. As NN content increases, E_b increases considerably, from 110 kV/cm at x = 0 to 155 kV/cm at x = 40 wt%. Nevertheless, P_max and P_r also decrease significantly, as depicted in Figure 6b. In comparison with the values at 110 kV/cm presented in Figure 5b, P_max is somewhat improved. When x exceeds 10 wt%, P_r shows little change. Smaller values of P_r and the continuous enhancement of E_b are conducive to achieving higher densities of energy storage, as seen in Figure 6c. However, W_rec does not show a consistent increasing trend. Instead, it first elevates from 0.67 J/cm³ to 1.02 J/cm³, reaches a maximum value of 1.06 J/cm³, and then slightly reduces to 1.05 J/cm³. The primary reason for this situation is the insufficient applied electric field. W_rec and P_max of the dielectric material are significantly higher than those at lower electric fields as the applied electric field is close to the breakdown threshold. Despite this, the η is considerably elevated from 44% to 88%, making it superior to the undoped 0.97[0.985(BNT–BT)–0.015Er]–0.03AlN ceramics.

Furthermore, the 70 wt% {0.97[0.985(BNT–BT)–0.015Er]–0.03AlN}–30 wt% NN ceramic exhibits superior W_rec and η values compared to other lead-free energy storage ceramics, as presented in Figure 7. Specifically, AN-based ceramics [40,41,42,43] and KNN-based ceramics [32,44,45] achieve high W_rec values but have relatively low η values. Meanwhile, BNT-based [46,47,48] and BF-based ceramics [49,50,51,52] exhibit relatively high η values, yet they still fall short of the performance demonstrated by the 70 wt% {0.97[0.985(BNT–BT)–0.015Er]–0.03AlN}–30 wt% NN ceramic. In this work, the 70 wt% {0.97[0.985(BNT–BT)–0.015Er]–0.03AlN}–30 wt% NN ceramic achieves an impressive W_rec of 1.06 J/cm³ and an η of 88%, surpassing the performance of previously documented high-performance BNT-based systems.

The practical use of energy storage capacitors requires stability in temperature along with frequency besides large η and high W_rec. Figure 8a,b exhibit that the unipolar P–E and P–E loops of the 70 wt% {0.97[0.985(BNT–BT)–0.015Er]–0.03AlN}–30 wt% NN ceramic are stable across both ambient temperature and the 0.5–50 Hz frequency range. Panels (a) and (b) indicate that P_max progressively declines from 12.57 μC/cm² to 11.48 μC/cm² with increasing frequency, while P_r remains essentially unchanged at 0.70 μC/cm². The corresponding values of W, W_rec, and η exhibit only slight fluctuations at different frequencies, as illustrated in Figure 8c. Specifically, the changes in W_rec (from 0.68 J/cm³ to 0.61 J/cm³) and η (from 85% to 86%) are less than 10% and 1% of their respective initial values, respectively.

The relationships between the unipolar P–E and P–E loops of the 70 wt% {0.97[0.985(BNT–BT)–0.015Er]–0.03AlN}–30 wt% ceramic at 10 Hz and temperatures ranging from 20 to 120 °C, as well as the corresponding W, W_rec, and η values, are displayed in Figure 8d–f. With increasing temperature, Pmax and Pr slowly increase from 12.56 μC/cm² and 0.58 μC/cm² at 20 °C to 14.30 μC/cm² and 2.55 μC/cm² at 120 °C, respectively. Meanwhile, W_rec decreases from 0.67 J/cm³ to 0.63 J/cm³, representing a change of 5.97%, while η decreases from 87.8% to 70.3%. These results demonstrate that the 70 wt% [0.97(BNT–BT–0.015Er)–0.03AN]–30 wt% NN ceramic exhibits outstanding stability in frequency and temperature for energy storage characteristics.

4. Conclusions

To sum up, dense (1 − x){0.97[0.985(BNT–BT)–0.015Er]–0.03AlN}–xNN ceramics with varying amounts of NN were fabricated in this work via employing a traditional solid-state reaction approach. In the current work, the effects of NN content on the ceramic microstructure, dielectric characteristics, and energy storage capacities were comprehensively examined. The induction of NN effectively improved the properties of energy storage by elevating the relaxor degree and refining the grain size. Compared with the 0.97[0.985(BNT–BT)–0.015Er]–0.03AlN ceramic (E_b = 110 kV/cm, η = 44%), the (1 − x){0.97[0.985(BNT–BT)–0.015Er]–0.03AlN}–xNN ceramic achieved a significant improvement in η = 88% at a moderate E_b = 155 kV/cm. Moreover, the W_rec was considerably raised from 0.67 J/cm³ to a maximal value of 1.06 J/cm³ when x = 30 wt%. Further investigation revealed that the 70 wt% {0.97[0.985(BNT–BT)–0.015Er]–0.03AlN}–30 wt% NN ceramic exhibited superior stability: W_rec varied by only 10% and η by 1% at 5–50 Hz, while the variations of both η and W_rec are less than 8% at 20–100 °C. These properties emphasize the potential of NaNbO₃-doped ceramics for the applications of high-performance energy storage.