Enhancing energy storage performance in tungsten bronze ceramics utilizing site engineering
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Abstract
Advanced electronic systems and innovative pulsed power applications are driving the rapid development of high-energy-storage density and high-efficiency capacitors. In the present study, we have prepared
Keywords
INTRODUCTION
As a functional electronic material that can store and convert transform under an applied electric field, dielectric ceramics serve a crucial role in fields such as aerospace, pulsed power systems, electromagnetic catapults, hybrid vehicles, and electronic communications[1,2]. The growing demand for high-quality dielectric capacitors has given a great impetus to the study of dielectric ceramics with properties such as high efficiency, high energy storage density, large power density (PD), and rapid discharging times[3-5]. Dielectric ceramics as energy storage materials are not only able to withstand higher voltages, but also have high PD and fast charging and discharging characteristics. Importantly, dielectric ceramic capacitors offer the advantage of a longer service life than supercapacitors and batteries[6,7]. Compared to a range of dielectric materials such as ferroelectrics, anti-ferroelectrics and linear dielectrics, relaxation ferroelectric ceramics stand out for their excellent temperature stability, high-energy-storage density accompanying high efficiency[8-10]. Among these materials, tungsten-bronze (TB) ferroelectric ceramics have attracted significant attention owing to the flexibility in adjusting the structure and properties through multiple crystallographic positions[11-13]. These ceramics have the structure (A1)2(A2)4(C)4B10O30, connected by the common vertices of the oxygen octahedra resulting in three different crystal positions: pentagonal (A2), tetragonal (A1), and triangular (C), which gives TB ceramics a complex structure and a rich compositional modulation and leads to a wealth of electrical properties[14,15]. It has been shown that the improvement in the electrical properties of TB can be achieved by the ordered/disordered arrangement of cations with varying oxidation states and size, which arises from the coexistence of its A-site and B-site cations[16-18]. The smaller the difference in ionic radius between the A-sites of TB ceramics, the easier it is to induce relaxor ferroelectric behavior[18-20]. In addition, the main source of ferroelectricity is mainly due to the movement and deflection of the BO6 octahedron along the c-axis caused by the ions occupying the B-site[21,22]. Therefore, the ferroelectric properties of TB ceramics can be optimized by doping different elements in the A-site and B-site or by changing the radius of the dopant ions.
Ion doping substitution at the A-site and B-site crystallographic positions of TB ceramics to achieve high maximum polarization (Pmax) and low remanent polarization (Pr) is a key factor in obtaining excellent energy storage properties. Cao et al. investigated TB ceramics doped with Ca0.28Ba0.72Nb2O6 with different radius ions, tuning the relaxor behavior to significantly improve the energy storage performance. The final energy storage performance was obtained with a storage density of Wrec ~ 2.59 J·cm-3, an efficiency of
In this study, SBSRN ceramics were prepared using a site engineering approach. This method involved co-doping the A-site with Ba2+, Sr2+and Sm3+ ions to synthesize unfilled TB structures. Additionally, the B-site underwent substitution of Nb5+ with various tetravalent ions, including Mn4+, Ti4+, Sn4+, and Hf4+. The relaxor ferroelectric behavior of SrBa3.5Sm0.5R0.5Nb9.5O30 (R = Mn, Ti, Sn, Hf) (SBSRN) ceramics was induced by increasing the degree of system disorder and enhancing the degree of distortion of BO6 oxygen octahedra to obtain excellent energy storage performance[27,28]. A combination of XRD, Rietveld structure refinement, transmission electron microscopy (TEM), and Raman reveals that the SBSRN ceramics obtain high efficiency as the radius of the B-site dopant ions induces an increase in the crystallite spacing and the degree of octahedral distortion. Among them, the SBSTN ceramics have high energy storage density of approximately 4.31 J·cm-3 and high efficiency of about 91.3 % because SBSTN have high dielectric constants, and show excellent stability in ferroelectric temperature variation and frequency variation tests. In the charge/discharge test, the SBSTN ceramics demonstrate excellent performance, including a discharge energy density (Wdis) reaching 2.68 J·cm-3, a current density (CD) of up to 619.96 Acm-2, a PD of 65.1 MWcm-3 and a rapid discharge time (t0.9) of 62 ns.
MATERIALS AND METHODS
The SBSRN compounds were synthesized using high-purity raw materials including SrCO3 (99.95%), BaCO3 (99.95%), Sm2O3 (99.99%), MnO2 (99.95%), TiO2 (99.99%), SnO2 (99.99%), HfO2 (99.99%), and Nb2O5 (99.99%) in stoichiometric ratios as determined by their respective compositions. Anhydrous ethanol and ZrO2 were used as the ball milling media in a high-energy ball milling tank. The raw materials, weighed according to their stoichiometric ratios, were mixed and subjected to high-energy ball milling for 6 h. The resulting ball-milled slurry was subsequently dried in an oven at 120 °C. The dried slurry was pressed into a 20 mm diameter column using a mold and calcined at 1,000 °C for 4 h to ensure that the main crystalline phase could be initially synthesized. In order to further improve homogeneity and reduce particle size, secondary ball milling using a high-energy ball mill was again used, and the drying of the secondary ball milled slurry was again placed in an oven at 120 °C. The slurry was then dried. Finally, the dried powder was subjected to cold isostatic pressing (256 MPa, 10 min) to form particles with a diameter of about 8 mm, and then sintered at temperatures ranging from 1,250~1,370 °C to obtain dense ceramic samples. Among them, in the electrical performance test, the thickness of the dielectric constant test sample was 1.2 mm
The physical phase of SBSRN ceramics was determined by X-ray diffractometry (PANalytical, Empyrean Cu Kα1). The microscopic surface morphology and energy dispersive X-ray spectroscopy (EDS) of the samples were acquired using an emission scanning electron microscope (Hitachi S-4800). Raman spectra were recorded using a DXR laser confocal Raman spectrometer (Thermo Fisher Scientific). The 8 mm diameter ceramic samples were polished and coated on both sides with silver paste electrodes and sintered at 650 °C for 30 min. X-ray photoelectron spectroscopy (XPS) data was obtained using Thermo Fisher ESCALAB 250Xi equipment on the powdered ceramic samples. Ferroelectric polarization hysteresis loops were acquired with a ferroelectric workstation at room temperature and a frequency of 10 Hz (TF 2000E Analyzer, Aix ACCT). The temperature characteristics of dielectric constant (εr) and dielectric loss (tanδ) were measured using a precision LCR meter (TZDM-200-300, Harbin Julang Technology). The charge/discharge performance was assessed using a dielectric charge test system (CFD-005, TG Technologies). TEM analysis was employed to examine the surface morphology of the powders by a transmission electron microscope (JEM 2100F, JEOL).
RESULTS AND DISCUSSION
Figure 1A shows the phase structure of SBSRN ceramics. The XRD diffraction peaks of the SBSRN ceramic samples are in excellent agreement with those of the standard cards (COD#96-400-1509); all the samples have a single tetragonal TB structure with a P4bm space group. Figure 1B shows a local zoom in from 31° to 33° where a shift of the XRD peaks towards lower angles can be clearly observed, indicating an increase in the crystal spacing d, which is attributed to the increase in the coordination radius of the B-site atoms.
Figure 1. (A) XRD patterns of SBSRN ceramics and (B) the enlarged patterns at the 2theta angle of 31°-33°.
The Rietveld refinement results of the XRD data of SBSRN ceramic powders are shown in Figure 2A-D and Table 1, where Rp < 10% and Rwp < 10%, indicating that the results are reliable[29,30]. Moreover, as the ionic radius of the dopant ions on the B-site increases leading to an increase in the cell volume, this is in agreement with the above analysis of the XRD results.
Figure 2. (A-D) Rietveld refinement plots of SBSRN ceramics, where Yobs is the experimental measurements value, Ycalc is the calculated measurement value, Yobs-Ycalc are the difference values, Rp is the profile factor, and Rwp is the weighted-profile factor.
Ion radius and Rietveld refinement results for SBSRN ceramics
| R = Mn | R = Ti | R = Sn | R = Hf | |
| Radii (Å) | 0.53 | 0.605 | 0.69 | 0.71 |
| Rp (%) | 4.46 | 4.02 | 4.30 | 4.11 |
| Rwp (%) | 6.60 | 5.76 | 6.18 | 5.91 |
| a (Å) | 12.4861(1) | 12.4945(6) | 12.4949(4) | 12.5014(0) |
| b (Å) | 12.4861(1) | 12.4945(6) | 12.4949(4) | 12.5014(0) |
| c (Å) | 3.9483(0) | 3.9467(9) | 3.9468(2) | 3.9493(3) |
| V (Å3) | 613.778(1) | 616.149(3) | 616.191(5) | 617.221(0) |
Due to the large difference in the radius of the B-site ions shown in Table 1, we further take the EDS energy spectrum analysis to support the rationality of the doping. A uniform distribution of all elements can be seen in Figure 3. In addition, the microscopic morphology of the grains demonstrated by the SEM patterns can be observed to be relatively homogeneous and dense and without obvious pore formation for each ceramic sample. The average grain sizes of the SBSRN ceramic samples were 2.50, 2.08, 2.09, and 2.36 μm, respectively.
Figure 3. SEM and EDS spectra of SBSRN ceramics, (A) R = Mn, (B) R = Ti, (C) R = Sn, (D) R = Hf.
The feasibility of doping elements with large differences in ionic radii at the B-site was analyzed by XRD and EDS energy spectroscopy, and then we performed Raman scattering spectroscopy tests on the SBSRN ceramic samples to analyze the structure and phase of the SBSRN ceramic as shown in Figure 4. The characteristic peak shape associated with the tetragonal phase of TB is evident. Here, ν1 and ν2 correspond to the R/Nb-O stretching modes, while ν5 represents the O-R/Nb-O bending mode. Vibrations resulting from the movement of the A-site cations relative to the BO6 octahedron occur at wave numbers below
Figure 4. (A) Raman spectra of SBSRN ceramics at room temperature, (B) Results of Gaussian fitting of Raman peaks, (C) FWHMs of
The high-resolution TEM (HRTEM) images and electron diffraction of selected regions of the SBSRN ceramic are shown in Figure 5, in which the diffraction spots were able to complete the calibration of the spatial group at P4bm, which further verified that the synthesized ceramic samples were TTB structure.
Figure 5. (A-D) HRTEM image and (E-H) Electron diffraction patterns of selected regions of SBSRN (R = Mn,Ti,Sn,Hf) ceramics.
As illustrated in Figure 5A, D, E, H, SBSMN and SBSHN ceramics were analyzed using HRTEM and electron diffraction techniques in selected regions along the [110] zone axis. The lattice spacings at the (110) crystal plane were determined to be 0.947 and 0.950 nm, respectively. For SBSTN and SBSSN ceramics, HRTEM and electron diffraction analyses were conducted on selected regions along the [220] zone axis, as depicted in Figure 5B, C, F, G. These analyses confirmed the corresponding lattice spacings of the (220) crystal plane to be 0.429 and 0.430 nm, respectively. TEM testing revealed that the substitution of large-radius ions at the B-site can lead to an increase in the crystal plane spacing, which aligns well the finding from XRD analysis Rietveld refinement.
The variation of dielectric properties of SBSRN ceramics with temperature was measured at -160~250 °C, as shown in Figure 6A-D. The increase in temperature results in a frequency dispersion of the dielectric constant, which exhibits typical dielectric relaxor properties[35]. In addition to the low-temperature diffuse phase transition, SBSMN ceramics also have a ferroelectric phase to paraelectric phase transition at high temperatures[36], due to the combined effect of both of which results in SBSMN ceramics having excellent temperature stability at -43~165 °C, as shown in Figure 6A. In addition, SBSHN ceramics comply with the X7R capacitor standard from -80 to 130 °C due to having a low dielectric constant, as shown in Figure 6E.
Figure 6. The temperature-dependent behavior of dielectric properties, (A) R = Mn, (B) R = Ti, (C) R = Sn, (D) R = Hf, (E) variation rate of dielectric constant of SBSRN ceramics at 1 kHz.
In the comparison of SBSRN ceramics, SBSTN ceramics have the highest dielectric constant, with a peak dielectric constant of over 2,400 (εm). A high dielectric constant is usually accompanied by a high polarization, a property that is advantageous for the improvement of energy storage density. On the other hand, SBSSN and SBSHN ceramics have lower Curie peak temperatures and exhibit a paraelectric phase above room temperature, which may be detrimental to energy storage density.
As shown in Figure 7A-D, a macroscopic phenomenological statistical model was used to match the dielectric constant of SBSRN ceramics at 1 kHz. This approach aimed to elucidate how doping at the B-site with different ionic coordination radii influences the dielectric behavior of tetragonal-phase TB ceramics, specifically in terms of dipole thermodynamic behavior. The fitting results, represented by the red line, exhibit a strong correlation with the temperature-dependent dielectric constant curve, denoted by the black line. This model employs an average potential well depth (Eb) to elucidate the mechanism by which PNR dipole contributes to polarization. The Eb value is strongly associated with the volume of the PNR, and the dipole energy follows the Maxwell-Boltzmann distribution. The relationship between the number of activated state dipoles, N1 (Eb, T), and the number of dipoles confined within the potential well, N2 (Eb, T), is determined by[33,37]
Figure 7. Macroscopic phenomenological statistical models fitting of the dielectric constant of SBSRN ceramics at 1 kHz, (A) R = Mn, (B) R = Ti, (C) R = Sn, and (D) R = Hf.
where N stands for the total number of dipoles in the system, kB is the Boltzmann constant, and T indicates absolute temperature. ε1, ε2, b, and θ are constants, and P1 and P2 are the variations in the dipole with the temperature at a frequency of 1 kHz. while the dipoles are also considered. The relationship between dielectric constant and temperature is given in
The volume of the PNR is determined using the depth of the potential well (Eb), which is derived from the fitting of the model and the Landau-Devonshire free energy equation:
Here, ΔGv ranges from 105 to 107 J m-3, and V represents the volume of each PNR. The results of the calculations are shown in Table 2.
The dielectric constant parameters of SBSRN ceramics
| R = Mn | R = Ti | R = Sn | R = Hf | |
| Eb (eV) | 0.095 | 0.204 | 0.079 | 0.039 |
| V (×10-27 m3) | 1.52 | 3.27 | 1.27 | 0.63 |
As shown in Table 2, the PNR volume is progressively declining; however, an anomaly occurs in SBSTN ceramics, when its PNRs are significantly higher than the other three compounds.
By comparing the dielectric constant versus temperature in Figure 6A-D, we found that the dielectric constant peak of SBSTN is relatively sharp, so we fitted the dielectric constant at 1 kHz according to the modified Curie-Weiss law and evaluated the relaxor properties of SBSRN ceramics through the dispersion coefficient, as defined by the law in[38,39].
where Tm, εm and γ represent the peak dielectric temperature, the maximum dielectric constant, and the dispersion coefficient, respectively. The value of γ typically falls between 1 and 2 for the relaxor ferroelectric. As illustrated in Figure 8A-D, the fitted values of γ are all greater than 1 and less than 2, indicating dielectric relaxor behavior.
Figure 8. The 1 kHz dielectric constant fitting using a modified Curie-Weiss law, (A) R = Mn, (B) R = Ti, (C) R = Sn, and (D) R = Hf.
Based on the fitting results, it was found that SBSTN ceramics have the lowest γ, and the reason for this may be due to the larger volume of PNRs of SBSTN ceramics compared to that of other SBSRN ceramics, which results in a relatively sharp dielectric peak.
Figure 9A illustrates the P-E hysteresis loops of the SBSRN ceramic measured at the electric strength corresponding to the breakdown field, whereas Figure 9B presents the specific energy storage density and energy storage efficiency. It was observed that the doping with tetravalent cations at the B-site causes a distortion in the BO6 octahedral structure, leading to local structural order/disorder. This disruption affects the long-range ferroelectric ordering, resulting in the formation of PNRs and imparting relaxor ferroelectric properties to the SBSRN ceramics. These PNRs help maintain low Pr values in the ferroelectric ceramics. The energy storage characteristics of SBSRN ceramics were determined using[40-42].
Figure 9. (A) Hysteresis loops of SBSRN ceramics, (B) The specific energy storage density and energy storage efficiency of SBSRN ceramics.
Based on these calculations, the SBSMN ceramic has an energy storage density of 1.34 J·cm-3 and an efficiency of 82.3% when subjected to a breakdown electric field of 200 kV·cm-1. In comparison, the breakdown electric fields for SBSTN, SBSSN, and SBSHN ceramics are approximately 350 kV·cm-1, with corresponding energy storage densities of 4.31, 3.19, and 2.84 J·cm-3, respectively. These materials also demonstrate high efficiencies, specifically 91.3%, 91.3%, and 95.2%, respectively. The elevated high energy storage density in SBSTN ceramics can be attributed to their high dielectric constant (εm > 2400), whereas the superior efficiency in SBSTN, SBSSN and SBSHN ceramics is due to the reduced volume of the PNRs.
In order to explore the intrinsic factors that make the breakdown electric field of SBSMN ceramics lower than that of the other three groups of SBSRN ceramics, we tested the X-ray electron spectra of SBSMN ceramics.
Figure 10A shows the X-ray electron spectra of Mn 2p3/2 and Mn 2p1/2 of SBSMN ceramics near 642.0 and 653.3 eV, indicating the presence of Mn ions. The successful fitting of the two characteristic peaks of Mn2+ and Mn3+ in the Mn-2p orbitals indicates the presence of a valence change behavior of Mn4+ which induces a ferroelectric decay effect, which may be responsible for the dielectric anomalies at low temperatures of the SBSMN ceramics, which therefore have excellent dielectric temperature stability. The valorization of Mn4+ causes an increase in oxygen vacancies with the reactions:
Figure 10. (A) X-ray electron spectroscopy mapping of SBSMN ceramics, (A) Mn, (B) O.
Figure 10B illustrates four distinct peaks at binding energies of approximately 529.5, 532, 532.5 and
Variable-temperature ferroelectric measurements were performed on SBSRN ceramics, as shown in Figure 11A-D, using electric fields of 90, 150, 150, and 200 kV·cm-1, respectively. All these tests were conducted at a frequency of 10 Hz. The presence of valence-changing behavior results in an increase in dipole defects, leading to the loss of ferroelectric properties at elevated temperatures and a significant reduction in the energy storage density and efficiency of SBSMN ceramics during variable-temperature ferroelectric tests (see Figure 11A and E). As illustrated in Figure 11B, for SBSTN ceramics, the polarization value exhibits a fluctuating trend across different temperatures, peaking at 17.5 μC/cm2 at 50 °C during the variable temperature ferroelectric characterization. Furthermore, as shown in Figure 11F, the energy storage density of the SBSTN ceramics reaches its highest point of 1.02 J·cm-3 at 90 °C, with an efficiency of 95.6%. Additionally, the ferroelectric properties of SBSTN ceramics demonstrate excellent temperature stability, varying by no more than 5% throughout the tested temperature range. From Figure 11C, it is observed that as the temperature increases, the hysteresis loop of SBSSN ceramics initially rises and then subsequently declines at the point of Pmax. Meanwhile, the Pr shows a steady increase, resulting in a significant decrease in both its energy storage density and efficiency. In conjunction with the relationship between the variation of dielectric constant and temperature, it is evident that SBSSN ceramics remain in the ferroelectrics phase throughout the variable-temperature ferroelectric testing range at a frequency of 10 Hz. In addition, the sharp decrease in dielectric constant leads to a significant decrease in its energy storage density and efficiency, so that the storage density of SBSSN ceramics decreases from 0.95 J·cm-3 at 30 °C to 0.74 J·cm-3 at 120 °C, corresponding to a decrease in efficiency from 95.2% to 62.5%, as shown in Figure 11G. Similar to SBSSN ceramics, SBSHN ceramics were also in the paraelectric phase during the temperature interval tested, with the Pmax decreasing with increasing temperature, but the SBSHN ceramics obtained the same excellent ferroelectric temperature stability as the SBSTN ceramics due to the lower rate of change of the dielectric constant with increasing temperature. The energy storage density of SBSHN ceramics reduces from 1.62 to 1.55 J·cm-3 with a rate of change that does not exceed 5% and the efficiency drops from 95.38% to 92.4% (see Figure 11H).
Figure 11. Variable temperature ferroelectric properties of SBSRN ceramics, (A) R = Mn, (B) R = Ti, (C) R = Sn, (D) R = Hf; Energy storage density as well as efficiency of variable temperature ferroelectrics based on SBSRN ceramics, (E) R = Mn, (F) R = Ti, (G) R = Sn, (H) R = Hf.
As shown in Figure 12A, when the frequency is increased from 1 to 200 Hz, the motion of the defective dipole is difficult to follow the frequency change, resulting in a decrease in the Pmax and Pr of the SBSMN ceramics at high frequencies, but the elongated electric hysteresis return line at high frequencies leads to a significant increase in the energy storage density and efficiency of the SBSMN ceramics (see Figure 12B). Fortunately, the change in frequency does not significantly affect the polarization characteristics of the SBSTN ceramics, SBSSN ceramics, and SBSHN ceramics, which is confirmed by the slender P-E curves (see Figure 12C-E). Correspondingly, in Figure 12F-H, the energy storage density and efficiency are maintained at a high level, showing excellent frequency stability. The maximum energy storage density of SBSTN ceramics is 0.98 J·cm-3 at 20 Hz, while at 1 Hz, it is 0.95 J·cm-3, and the rate of change can still be maintained below 5%.
Figure 12. Variable frequency ferroelectric properties of SBSRN ceramics, (A) R = Mn, (C) R = Ti, (D) R = Sn, (E) R = Hf, Energy storage density as well as efficiency of variable temperature ferroelectrics based on SBSRN ceramics, (B) R = Mn, (F) R = Ti, (G) R = Sn,
After a comprehensive comparison of the electrical properties of SBSRN ceramics, it was found that SBSTN ceramics have superior energy storage properties. To assess their practical application potentials, a range of charge/discharge tests were performed. These tests included measurements of t0.9, PD, Wdis, and CD. The aim is to evaluate how effectively SBSTN ceramics could perform in practical applications.
Therefore, the Wdis in the overdamping test can be calculated using [26,45]:
where R represents the load resistance, with an overdamping value of 140 Ω and an underdamping value of 3 Ω; i(t) denotes the discharge current; V stands for the sample volume.
From Figure 13A, it is evident that the peak current of the overdamped charge and discharge of the SBSTN ceramics rises as the electric field density increases. As observed in Figure 13B, the corresponding Wdis also exhibits an upward trend, ultimately achieving a high Wdis of 2.68 J·cm-3 and a fast charge and t0.9 = 62 ns at an electric field strength of 300 kV·cm-1.
Figure 13. Charge/discharge characteristics of SBSTN ceramics: (A) the overdamping behavior in charge/discharge current curves, (B) the discharge energy density and t0.9, (C) the underdamping characteristics of the discharge current.
It should be highlighted that, for the underdamping test, the CD and PD of the SBSTN ceramic capacitors were determined using
where Imax represents the peak current, while S denotes the electrode area[26,45].
The underdamping test of SBSTN ceramics presents a sinusoidal decay waveform for the discharge waveform under each electric field, as illustrated in Figure 13C. Notably, when the electric field reaches
CONCLUSIONS
In this study, SBSRN ceramics were prepared by site engineering utilizing tetravalent elements to replace Nb5+ at the B-site of tetragonal phase TB in conjunction with a solid-phase reaction method. XRD, TEM and Raman tests and Rietveld structure refinement have shown that the crystal plane spacing increases with the coordination radius of TB B-site doped ions, and this strategy is conducive to breaking the ferroelectric long-range ordering. Finally, the SBSTN ceramics, which possess a high dielectric constant, achieved a high energy storage density of 4.31 J·cm-3 and an efficiency of 91.3 % under an electric field strength of
DECLARATIONS
Authors’ contributions
Design: Hu, C.; Fang, L.
Experiments and data collection: Dan, Y.; Tang, L.; Ning, W.
Data analysis: Dan, Y.; Tang, L.
Manuscript writing: Dan, Y.
Manuscript revision and supervising: Hu, C.; Liu, L.
All authors have read and agreed to the published version of the manuscript.
Availability of data and materials
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Financial support and sponsorship
This work was supported by the National Natural Science Foundation of China (12164012), the Science and Technology Plan of Guangxi (AA25069001), and the Science and Technology Plan of Guilin (ZY20220101).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2025.
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