Dielectric Temperature Stability and Enhanced Energy-Storage Performance of Sr_0.4Ba_0.6(Zr_0.2Ti_0.2Sn_0.2Ta_0.2Nb_0.2)₂O₆ High-Entropy Ferroelectric Ceramics

Abstract

In this research, we employed a high-entropy approach in tungsten-bronze-structured ferroelectric ceramics, preparing Sr_0.4Ba_0.6(Zr_0.2Ti_0.2Sn_0.2Ta_0.2Nb_0.2)₂ (denoted as SBN40-H) ceramics through the traditional solid-state reaction technique. By utilizing the high-entropy approach, the resulting SBN40-H ceramics demonstrated extremely fine grains, averaging 0.58 μm in size. Furthermore, these ceramics possessed a high bandgap of 3.35 eV, which, when combined with the small grain size, contributed to a remarkable breakdown strength of 570.01 kV/cm. The dielectric characteristics demonstrated typical relaxation behavior and outstanding temperature stability, with a capacitance temperature coefficient (TCC) of less than 5% within the temperature range of 111–317 °C. Additionally, the SBN40-H ceramics exhibit slim P–E loops with negligible hysteresis, which is considered to be related to the existence of weakly coupled relaxors. This results in exceptional overall energy-storage properties in the SBN40-H ceramics, exhibiting a notable recoverable energy density (W_rec) of 2.68 J/cm³ and an efficiency (η) of 93.7% at 390 kV/cm, and finally achieving a remarkable temperature stability in terms of energy-storage performance with variations in W_rec and η being less than 3.5% and 4.4% from 25 to 150 °C. It is worth noting that the high-entropy approach is highly effective in reducing grain size, increasing the breakdown field strength and enhancing the dielectric temperature stability of tungsten-bronze-structured ferroelectric ceramics.

1. Introduction

As traditional energy sources are consumed, the development of new energy sources and storage technologies has emerged as a crucial challenge [1,2,3,4]. As crucial components in energy-storage devices, ferroelectric ceramics have attracted considerable attention in the field of energy storage owing to their swift charging and discharging rates, extended operational lifespan, high power density, and impressive thermal stability [5,6,7,8,9,10]. However, their practical applications are limited by their relatively low energy-storage density and temperature stability. Consequently, the advancement of lead-free ferroelectric materials with outstanding energy-storage performance is increasingly important, driven by the trend of miniaturizing electronic devices and the growing emphasis on environmental protection [11,12,13]. The performance metrics for energy storage, including the recoverable energy density (W_rec) and energy-storage efficiency (η), can be derived using the formulas outlined in Equations (1)–(3) [14,15]:

$$W_{\mathrm{total}}={\int }_0^{P_{\mathrm{m}}}E\mathrm{d}P$$

(1)

$$W_{\mathrm{rec}}={\int }_{P_{\mathrm{r}}}^{P_{\mathrm{m}}}E\mathrm{d}P$$

(2)

$$\eta =W_{\mathrm{rec}}/W_{\mathrm{total}}$$

(3)

where P_m, P_r, and E represent the maximum polarization, remnant polarization, and applied electric field, respectively. Dielectric energy-storage materials are mainly grouped into four distinct categories: linear dielectric, normal ferroelectric, relaxor ferroelectric, and anti-ferroelectric materials [16,17]. Linear dielectrics exhibit a high η because they exhibit a direct, linear correlation between polarization and the applied electric field. However, their low dielectric constant leads to a correspondingly low recoverable energy density W_rec. The anti-ferroelectric materials own double ferroelectric polarization–electric field (P–E) loops, which result in a lower remnant polarization (P_r) and a high recoverable energy density W_rec [18,19]. Despite this advantage, anti-ferroelectric materials suffer from relatively large energy losses that dissipate as heat, posing potential risks for practical applications in energy-storage devices [20,21].

In contrast, relaxor ferroelectric materials exhibit slim P–E loops with a small P_r, contributing significantly to their high η and W_rec values [22,23,24]. These materials have been engineered to exhibit polar nanoregions through doping, which disrupts the long-range ordered structure, increasing compositional disorder and resulting in slimmer P–E loops [25,26]. Recently, high-entropy ceramics have come to prominence as an innovative and promising strategy in the design and development of ferroelectric ceramics [27,28,29]. Generally, high-entropy ceramics are composed of five or more elements, each present in equal molar ratios, disrupting the original ordered state and enhancing atomic, lattice, and compositional disorder to achieve relaxor behavior [30,31]. The high-entropy strategy has been extensively applied in perovskite ferroelectric materials to boost their energy-storage capabilities [32,33,34,35]. Notably, perovskite-structured, BNT-based, high-entropy ceramics have demonstrated an increase in W_rec of nearly 10 times, attributed to the reduced nanodomain size and an enhanced random field resulting from increased entropy [36]. Similarly, in traditional BaTiO₃-based ceramics, the introduction of high entropy led to the formation of a local polymorphic distortion structure and further reduction in the size of polar nanoregions, resulting in an ultrahigh η [37]. However, research on high-entropy tungsten-bronze-structured ferroelectric materials remains relatively limited due to the complexity of their structure.

Strontium barium niobate (SBN), which possesses a tetragonal tungsten-bronze structure and exhibits exceptional ferroelectric and optical properties as a lead-free ferroelectric material, has garnered significant attention [38]. The entropy related to its atomic configuration, denoted as S_config, can be calculated as follows [39]:

$$S_{\mathrm{config}}= -R\left[{({\sum }_{i=1}^N{x}_i\ln x_i)}_{\mathrm{cation}-\mathrm{site}}+{({\sum }_{j=1}^M{x}_j\ln x_j)}_{\mathrm{anion}-\mathrm{site}}\right]$$

(4)

where R signifies the ideal gas constant, while N and M represent the number of atomic species occupying the cation and anion sites, respectively. The atomic proportions are indicated by $x_i$ and $x_j$. When the S_config value exceeds 1.5R, it is categorized as high entropy [40,41]. Therefore, in this work, we propose the incorporation of various elements of Zr⁴⁺, Ti⁴⁺, Sn²⁺, and Ta⁵⁺ into the B-site of Sr_0.4Ba_0.6Nb₂O₆ to elevate the entropy, and the S_config value designed in this work is 2.28R. A thorough investigation into the impacts of this high-entropy design on the microstructure, dielectric properties, and energy-storage capabilities were conducted. By adopting the high-entropy approach, the strontium-barium-niobate ferroelectric ceramics achieved a refined grain structure and remarkable dielectric temperature stability. Additionally, they exhibited a wide bandgap and exceptionally high breakdown field strength. Consequently, these ceramics possess superior energy-storage density and efficiency. This research offers valuable insights for the design and development of tungsten-bronze-structured ferroelectric materials with high energy-storage performance.

2. Experimental Procedures

The high-entropy ferroelectric Sr_0.4Ba_0.6(Zr_0.2Ti_0.2Sn_0.2Ta_0.2Nb_0.2)₂O₆ (abbreviated as SBN40-H) ceramics were synthesized using the conventional solid-state reaction method. High-purity starting materials, including BaCO₃, SrCO₃, Nb₂O₅, TiO₂, ZrO₂, SnO₂, and Ta₂O₅, with a purity exceeding 99%, obtained from Alfa Aesar company located at Ward Hill (MA, USA), were weighed in accordance with the stoichiometric ratio and mixed through ball milling with ethanol for a period of 12 h. Afterwards, the calcination of the dried powders was conducted at 1200 °C for 3 h in a muffle furnace. Then, a second ball-milling process was carried out for 12 h. Following that, the powders underwent another drying process and were subsequently compressed into pellets, each with a diameter of 10 mm, using isostatic pressing at a pressure of 200 MPa for a duration of 3 min. The pressed ceramics were sintered at 1350 °C for 3 h.

The sintered ceramics underwent polishing and were then thermally etched at a temperature that was approximately 80 °C lower than the sintering temperature for a period of 10 min to determine the surface microstructure, which was detected using a Field Emission Scanning Electron Microscope (FESEM, ZEISS, Sigma 300, Oberkochen, Germany). The composition uniformity was detected by energy dispersive spectroscopy (EDS) mapping conducted using an Oxford Xplore analyzer (Oxford, UK). The phase structures of the samples at room temperature were determined using X-ray diffraction (XRD) analysis, employing a Rigaku SmartLab SE instrument (Tokyo, Japan) with Cu Kα radiation of λ = 0.15406 nm. The sintered samples were polished to a thickness of 0.7 mm and subsequently coated with a Ag electrode for the purpose of evaluating their dielectric properties. The temperature-dependent dielectric performance was evaluated using an LCR meter (Agilent, 4980 A, Santa Clara, CA, USA) within a temperature range of 0–500 °C. For ferroelectric measurements, the sintered pellets were refined to a thickness of 0.1 mm and sputtered with gold electrodes on both surfaces. The ferroelectric P–E hysteresis loops were characterized using a Precision Premier II system from Radiant Company (Redmond, WA, USA), in conjunction with a high-voltage amplifier. Additionally, the charge–discharge characteristics were tested using a capacitor charge/discharge facility (CFD-003, TG Technology, Shanghai, China) with a load resistance of 240 Ω.

3. Results and Discussion

Figure 1a illustrates the micro-morphology of the component, revealing a uniform grain distribution devoid of notable porosity. It is noteworthy that the grain shape of the high-entropy ceramics, composed of SBN40-H isometric crystals, contrasts with the columnar crystals observed in pure strontium-barium-niobate ceramics, as previously reported in our work [42,43]. Additionally, the average grain size, determined using Nano Measure software 1.2.0 is presented in the inset of Figure 1a. Interestingly, the average grain size is only 0.58 μm, markedly smaller than that of strontium-barium-niobate ceramics produced through the identical process. This outcome was expected, as the distortion in the lattice structure resulting from the multiple cations present in the TTB structure had been predicted to cause such a reduction. The smaller grain size is advantageous for achieving a high breakdown strength (BDS), as grain boundaries effectively hinder carrier transport due to the high potential barrier between them [44,45]. The significantly decreased grain size and the compact microstructure have been proven to be helpful in improving the breakdown strength. Moreover, the EDS analysis reveals elemental mapping for the Sr, Ba, Ti, Zr, Nb, Sn, Ta, and O elements through distribution visualization, which is displayed in Figure 1b. It can be obviously seen that all of the elements are evenly distributed and there is no agglomeration phenomenon. This confirms the successful incorporation and uniform distribution of doping elements within the B-site of the tungsten-bronze structure. The smaller grain size achieved through high-entropy design suggests a possible enhancement in breakdown strength, which exhibits an inverse relationship with grain size. Moreover, the Rietveld refinement of the XRD pattern of as-prepared SBN40-H high-entropy ceramics analyzed by Highscore plus software (ver 4.0) is shown in Figure 1c. The refined results exhibit excellent reliability, as evidenced by the low values of the refinement parameters (R_wp, R_p, and χ²) shown in Figure 1c. The lattice parameters have been determined to be a = b = 1.2552 nm, and c = 0.3977 nm. Additionally, two secondary phases, identified as SnO₂ and SrTi_0.45Zr_0.55O₃, are observable. These phases may result from exceeding their respective solid solubility thresholds, with the impurity concentrations estimated to be roughly 16.7% for SnO₂ and 19.8% for SrTi_0.45Zr_0.55O₃, respectively.

The temperature dependence of the dielectric constant and loss tangent (tanδ) of SBN40-H high-entropy ceramics measured across various frequencies within a temperature range of 0–500 °C are shown in Figure 2a,b, respectively. In addition, the enlarged view of dielectric loss over a temperature range of 0–300 °C is illustrated in the inset of Figure 2b. It can be seen that the temperature-dependence curves of the dielectric constant exhibit quite a wide peak, and it is observable that the temperature of this peak value (T_m) shifts to higher temperatures as the frequency increases. The temperature-dependent dielectric loss also presents a corresponding frequency dispersion phenomenon with the dielectric anomaly peak shifting towards to a higher temperature with the frequency increasing. It is worth noting that the dielectric loss is very low (<0.08), while it increases sharply at high temperatures, indicating the onset of the conduction processes. Compared to SBN ceramics, the dielectric constant of SBN40-H ceramics decreases significantly, indicating strong relaxation characteristics. The low dielectric constant of these high-entropy ceramics is attributed to their small grain size. This is due to the presence of multiple ion species, which enhances the system’s disorder and causes fluctuations in B-site ion charge, generating random electric fields (RFs) that amplify the ferroelectric relaxation behavior [22]. To assess the diffuseness degree of SBN40-H high-entropy ceramics, the modified Curie–Weiss law is employed, as illustrated below, to quantify their relaxor characteristics [43].

(1/ε − 1/ε_m) = (T − T_m)^γ/C

(5)

Here, ε_m denotes the maximum dielectric permittivity at T_m, and C is assumed to be constant. The parameter γ, which ranges from 1 to 2, describes the diffuseness degree. Specifically, γ = 1 characterizes the Curie–Weiss behavior typical of normal ferroelectric materials, whereas γ = 2 is indicative of ideal relaxor ferroelectric materials. The fitting result, depicted in Figure S1, yields a γ value of 1.70, indicative of typical relaxor behavior. Furthermore, the relaxation behavior of ferroelectric materials constitutes a thermally activated process, often likened to a spin or dipole glass that only exhibits dynamic activity above a specific freezing temperature. In such scenarios, the Vogel–Fulcher (V–F) law expressed in Equation (6) is usually utilized to delineate this behavior [46,47,48].

f = f₀exp[−E_a/k_B(T_m − T_f)]

(6)

Here, f denotes the probing frequency, f₀ is related to the attempt frequency, E_a represents the activation energy of the relaxation process, k_B denotes the Boltzmann’s constant, T_m is the temperature at which the dielectric permittivity reaches its maximum, and T_f is the freezing temperature. A nonlinear fitting analysis was conducted using the V–F law, with the results presented in Figure 2c. It is evident that the measured data align closely with the V–F law, and the fitted parameters f₀, E_a, T_f are also depicted in Figure 2c. Notably, the activation energy E_a is approximately one order higher than that observed in classical relaxor ferroelectric behavior. This intriguing phenomenon has also been observed in perovskite systems, such as BaTiO₃-BiScO₃ (E_a $\approx$ 0.24–0.26 eV) [49], BaTiO₃-Bi(Mg_1/2Ti_1/2)O₃ (E_a $\approx$ 0.17–0.22 eV) [50], BaTiO₃-Bi(Zn_1/2Ti_1/2)O₃-BiScO₃ (E_a $\approx$ 0.37–0.50 eV) [51], and BaTiO₃-Bi(Zn_2/3Nb_1/3)O₃ (E_a $\approx$ 0.20–0.22 eV) [52]. High activation energies suggest that achieving long-range dipole alignment under field-cooled conditions is challenging, resulting in isolated and frustrated polar clusters with minimal coupling between adjacent clusters. However, macroscopic switching of the polarization in these so-called “weakly coupled relaxors” becomes feasible at low temperatures with the application of high fields [49]. This behavior is particularly advantageous for energy-storage applications, as it is typically associated with low dielectric nonlinearity (or a high polarization saturation field) and minimal hysteresis (or low energy loss).

Moreover, to assess the stability of dielectric performance across various temperatures, the temperature coefficient of capacitance (TCC) is defined as follows:

$$\mathrm{TCC}={\displaystyle \frac{C_{\mathrm{T}} -C_{\mathrm{base}}}{C_{\mathrm{base}}}}$$

(7)

where C_T denotes the capacitance over the measured temperature range, and the base capacitance is referred to as C_base. In the case of high-temperature capacitors, the chosen base temperature is typically 150 °C. As is illustrated in Figure 2d, the TCC value of SBN40-H high-entropy ceramics initially decreases, then increases, and finally decreases again. Notably, these ceramics exhibit excellent TCC stability, with values below 5% over a wide temperature range of 111–317 °C. Given the temperature range where tanδ is less than 0.08 (0–286 °C), the combined range of both low loss and high capacitance stability extends up to 175 °C. Overall, the SBN40-H high-entropy ceramics demonstrate impressive dielectric properties, characterized by a broad temperature range where both the temperature coefficient of capacitance (TCC) is less than 5% and tanδ is less than 0.08.

As far as is known, the breakdown strength (BDS), which exhibits a close correlation with the bandgap, is crucial for the energy-storage capabilities of ferroelectric materials. Generally, a larger bandgap (E_g) indicates that promoting electrons from the valence band to the conduction band is more demanding, leading to reduced conductivity and a higher BDS value. Consequently, Figure 3a displays the UV–vis absorption spectra of the SBN40-H high-entropy ceramics, while Figure 3b illustrates the deduced bandgap (E_g) calculated using the equation listed as follows:

$${\left(\alpha h\upsilon \right)}^2=A(h\upsilon -E_{\mathrm{g}})$$

(8)

where A refers to a proportionality constant, α denotes the absorption coefficient, and $h\upsilon$ represents the photon energy. From Figure 3b, it is evident that the SBN40-H high-entropy ceramics possess a notably high E_g of 3.35 eV, contributing to their enhanced breakdown strength. Dielectric ceramics’ breakdown strength (BDS) is typically assessed using the two-parameter Weibull distribution, owing to the statistical characteristics associated with failure phenomena. Generally speaking, the formulation of the Weibull distribution for evaluating BDS is articulated as follows [36,53,54,55,56]:

$$P(E_i)=1-\exp \left[-{\left({\displaystyle \frac{E_i}{E_{\mathrm{b}}}}\right)}^{\beta }\right]$$

(9)

$$P_i= {\displaystyle \frac{i}{n+1}}$$

(10)

Here, P(E_i) denotes the probability of dielectric breakdown for each sample, with E_i representing the breakdown field strength (E_b) of each sample. The variable i serves as the serial number of the sample, and n indicates the total count of samples. The term E_b corresponds to the field strength at which the cumulative failure probability reaches 63.2%. The parameter β is associated with the distribution of breakdown strength (BDS), where a higher β value suggests a more tightly concentrated BDS. Equations (9) and (10) can be converted to the following:

$$\ln (-\ln (1-i/(n+1\left)\right))=\beta \ln E_i-\beta \ln E_b$$

(11)

Hence, Equations (12) and (13), derived from Equation (11), incorporate two parameters, X_i and Y_i, which exhibit a linear relationship as referenced in [57,58,59].

$${\mathrm{X}}_i=\ln (E_i)$$

(12)

$${\mathrm{Y}}_i=\ln (-\ln (1-i/(n+1\left)\right))$$

(13)

The graph in Figure 3c illustrates X_i and Y_i values based on these equations. As is depicted in Figure 3c, when X_i is plotted as a function of Y_i, the resulting linear fit yields a slope (β) of 6.22 (significantly greater than 1). This indicates a high degree of reliability for the Weibull distribution and suggests minimal variation in the quality of the samples studied. The SBN40-H high-entropy ceramics exhibit a remarkable BDS value of 570.01 kV/cm, which is advantageous for enhancing energy-storage density. This high BDS can be ascribed to the small grain size and the high bandgap.

To assess the energy-storage characteristics, Figure 4a displays the unipolar P–E hysteresis loops of the SBN40-H high-entropy ceramics measured at a frequency of 100 Hz and across a range of electric field intensities. And, the unipolar P–E hysteresis loops plotted separately and the bipolar P–E loops measured at 100 Hz under different electric fields are also shown in Figures S2 and S3 in the Supporting Information. It can be observed that all of the unipolar and bipolar P–E loops at various electric fields exhibit distinctive slim P–E loops with neglectable hysteresis, which can be attributed to the increased B-site disorder and strong relaxation induced by the high-entropy strategy. Additionally, Figure 4b illustrates the variations in P_m, P_r, and P_m − P_r with respect to the electric field. The P_m − P_r values are almost identical to P_m, which is technologically meaningful because large P_m − P_r values are often favorable for high discharged energy densities. The nearly linear P–E loops with almost the same value between with P_m − P_r and P_m are similar to those of the relaxor ceramics (1 − x)BaTiO₃−xBi(Zn_2/3Nb_1/3)O₃ (x > 0.05) reported in ref. [52]. This is considered to be related to the weakly coupled behavior of these ceramics. Additionally, using Equations (1)–(3), the calculated recoverable energy density (W_rec) and efficiency (η) were then plotted in Figure 4c. Due to the small P_r value, a polarization difference of 14.14 μC/cm² is achieved at 390 kV/cm, resulting in exceptional energy-storage performance with a W_rec of 2.68 J/cm³ and η of 93.7%. Additionally, the stability of the energy-storage performance across varying temperatures is crucial for the practical utilization of ferroelectric ceramics. Therefore, Figure 4d–f displays the unipolar hysteresis loop, W_rec, and η of the sample as they vary across a temperature range of 25–150 °C, when measured at an electric field strength of 200 kV/cm and a frequency of 100 Hz. Over this temperature range, the hysteresis loops remain slim and P_m, P_r, and P_m − P_r exhibit minimal temperature-dependent changes. As is displayed in Figure 4f, although W_rec and η decrease slightly within the temperature range of 25–150 °C, the energy-storage performance of SBN40-H high-entropy ceramics exhibits a good temperature stability and the variation range of W_rec and η is less than 3.5% and 4.4%, respectively.

To further evaluate the practical applicability of SBN40-H high-entropy ceramics, we examined underdamped and overdamped charge–discharge curves measured at different electric fields, which are presented in Figure 5a,c, respectively. The energy-storage property, denoted as W_d, can be calculated using Equation (14) derived from the overdamped discharge curves.

$$W_{\mathrm{d}}={\displaystyle \frac{R\int i^2\left(t\right)\mathit{dt}}{V}}$$

(14)

where R represents the load resistor (240 Ω), i stands for the discharge current, and V signifies the volume of the sample. As observed, $W_{\mathrm{d}}$ increases monotonically as the electric field increases, and the discharge time τ_0.9 is extremely short at 13.3 ns. The $W_{\mathrm{d}}$ value obtained from this method is lower compared to that derived from the quasi-static P–E loop, primarily owing to the fundamental differences between the measurement principles of the two techniques [12,60]. Distinct and well-defined underdamped discharge curves are observable. The current density (C_D) and power density (P_D) shown in Figure 5b can be calculated using the following specified formulas [61]:

$$C_{\mathrm{D}}={\displaystyle \frac{I_{\max }}{S}}$$

(15)

$$P_{\mathrm{D}}={\displaystyle \frac{E{I}_{\max }}{2S}}$$

(16)

where S is the electrode area. With an increase in the electric field, there is a notable rise in I_max, C_D, and P_D. The charge–discharge test results underscore the considerable potential of SBN40-H high-entropy ceramics in energy-storage applications.

Additionally, in order to confirm the advancement of this work, the performance comparison between the current system and some other reports is shown in

Table 1. Comparison of the energy-storage performance and dielectric stability of this work and others.

Compositions	Wrec (J/cm3)	H (%)	BDS (kV/cm)	Temperature Range (°C)	Article
Sr0.4Ba0.6(Zr0.2Ti0.2Sn0.2Ta0.2Nb0.2)2	2.68	93.7	570.1	±5%, 111–317 °C	This work
Sr2Ag0.2Na0.8Nb5−xTaxO15	1.44	82	175	$\pm 15\%,$ −9.6–232.7 °C	[62]
Sr2Na0.85Bi0.05Nb5−xTaxO15	1.6	80	200	$\pm 15\%,$ −97.3–165.2 °C	[63]
NBT–BT–KNN	2.81	94.4	180	--	[64]
(1−x)BNT-xBCZT	0.665	83.2	70	±15%, 81–330 °C	[65]
(1−x)BNT-0.09BA-xST	0.84	48.3	90	±5%, 132.8–391.8 °C	[66]
0.93Na0.5Bi0.5TiO3-0.07LiTaO3	2.3	74.2	200	±15%, 15–600 °C	[67]
(1−x)BNBT-xBS	1.2	86.7	115	±15%, 45–338 °C	[68]
(Na0.5Bi0.5)0.7Sr0.3TiO3	3.13	91.14	262	--	[69]
Sr1.88La0.12NaNb4.88−xTaxTi0.12O15	2.03	94	35	--	[70]
Sr(Zr0.2Sn0.2Hf0.2Ti0.2Nb0.2)O3	3.52	86.5	283	±5%, 52.4–362 °C	[8]

. It can be seen that Sr_0.4Ba_0.6(Zr_0.2Ti_0.2Sn_0.2Ta_0.2Nb_0.2)₂ high-entropy ceramics developed in this study exhibit an exceptionally high η value coupled with a moderate W_rec, when compared to other lead-free systems that are especially attractive in TTB systems. Furthermore, our work has achieved a remarkably high breakdown strength along with superb dielectric stability, suggesting promising prospects for their application in energy-storage devices.

4. Conclusions

In conclusion, we introduce elements with different valences and radii into the B-site of strontium-barium-niobate ceramics to increase entropy. The Sr_0.4Ba_0.6(Zr_0.2Ti_0.2Sn_0.2Ta_0.2Nb_0.2)₂ high-entropy ceramics were synthesized successfully using the conventional solid-state reaction method. The results demonstrate that the high-entropy strategy leads to a significantly reduced grain size and increasing relaxation. The dielectric curves show an obvious relaxor characteristic and excellent temperature stability with the temperature coefficient of capacitance (TCC) <5% over a wide temperature range of 111–317 °C. The SBN40-H high-entropy ceramics exhibit a high breakdown strength of 570.01 kV/cm, which is ascribed to the fine grain achieved through the high-entropy design and the high bandgap of 3.35 eV. Finally, a superior overall energy-storage performance was achieved in SBN40-H high-entropy ceramics with the W_rec of 2.68 J/cm³ and η of 93.7% at 390 kV/cm, and an extraordinary temperature stability with W_rec and η varying less than 3.5% and 4.4% from 25 to 150 °C. This study once again demonstrates the important potential of a high-entropy strategy on enhancing the energy-storage performance of tungsten-bronze-structured ferroelectric ceramics.