Environmental pollution and energy shortage become more serious. How to effectively store energy and reduce energy loss is a recent challenge. Dielectric ceramic capacitors have some advantages such as long lifespan, fast charging and discharging speed, high power density, and good temperature stability. Na_0.5Bi_0.5TiO₃ (NBT) is a ferroelectric material with an A-site composite ABO₃ perovskite structure. The Curie temperature of NBT is 320 ℃, with a large maximum polarization (P_max) (40-50 μC/cm²) and a wide sintering temperature range. NBT is one of the hot research topics in energy storage ceramics. However, a high remanent polarization (P_r) (~38 μC/cm²) results in low recyclable energy storage density (W_rec) and efficiency (η) for NBT. Introducing a low P_r component into NBT to form a solid solution can refine the polarization-electric field (P-E), and improve the material breakdown strength (BDS) to achieve a high W_rec. In this paper, Ti⁴⁺ was replaced in Sr_0.7Bi_0.2TiO₃ by Zr⁴⁺. The introduction of Sr_0.7Bi_0.2ZrO₃ (SBZ) could enhance the degree of disorder in the A-site and B-site in the NBT, respectively. The ferroelectric long-range ordered structure was effectively break, further enhancing the relaxor ferroelectricity of NBT ceramics. Zr⁴⁺ was chosen instead of Ti⁴⁺ because Zr⁴⁺ was more stable chemically, and the properties of the material were further improved via introducing ionic radius difference and oxygen vacancies, leading to the lattice distortion. Also, ZrO₂ with a larger band gap was conducive to improving BDS and achieving a high energy storage performance.

The (1-x) Na_0.5Bi_0.5TiO₃-xSr_0.7Bi_0.2ZrO₃ (NBT-SBZ, x=0.1, 0.2, 0.3, 0.4) ceramics were synthesized by a solid-phase method. Na₂CO₃ (99.8%, in mass fraction, the same below), Bi₂O₃ (99.9%), ZrO₂ (99.0%), SrCO₃ (99.0%), and TiO₂ (98.0%) (National Pharmaceutical Group Chemical Reagent Co., China) were used as raw materials. The raw materials were mixed with alcohol and ground in a ball mill for 24 h. After drying, the powder was calcined at 850 ℃ for 4 h. The green body with a diameter of 12 mm was pressed by cold isostatic pressing at 200 MPa for 3 min. The green body was sintered in air at 1 090-1 150 ℃ for 4 h.

The phase structure of NBT-SBZ ceramic samples was characterized by a model D8 Advance X-ray diffractometer (XRD, Bruker Co., Germany) at 20°-80°. The microstructure was determined by a model S4800 scanning electron microscope (SEM, RIGAKU Co., Japan). The dielectric properties were measured by a model 4980A precision LCR (Agilent Co., USA). The thickness of the dielectric properties sample was 0.6 mm, and a silver electrode was fired at 850 ℃. The P-E loops were tested by a model Premier II ferroelectric analyzer (Radiant Co., USA). The P-E test sample thickness was 0.1 mm and a diameter of 2.0 mm for a gold electrode. The charging and discharging characteristics were determined from a model CFD-003 charging/discharging system (Gogo Co., China). The sample thickness was 0.3 mm, and a silver electrode was fired at 850 ℃.

The XRD pattern of NBT-SBZ ceramics indicates that all the samples exhibit a NBT pseudocubic perovskite structure. ZrO₂ and Bi₂Ti₂O₇ second phase appear as the SBZ content increases. A peak at 46.5° (200) shifts towards a lower angle as the SBZ content increases, indicating an increase in the spacing between ceramic crystal planes. The SEM images indicate that the grain size distribution of all the samples is uniform and presents a normal distribution. The average grain size decreases from 1.57 μm for the sample with x of 0.10 μm to 1.06 μm for the sample with x of 0.4 with the increase of SBZ content. The substitution of larger radius ions increases the lattice strain energy and hinders the grain boundary movement, leading to a grain refinement. The grain refinement and reduction of pores are beneficial to obtaining a larger BDS. The dielectric constant (ε_r) decreases from 870.5 for the sample with x of 0.1 to 668.5 for the sample with x of 0.4 with increasing the frequency due to a decrease in polarization mechanism. As the SBZ content increases, the dielectric loss (tanδ) at 1 kHz decreases from 0.041 for the sample with x of 0.100 to 0.029 for the sample with x of 0.4. All the samples exhibit obvious relaxor ferroelectric characteristics. The P-E loops also change from a typical ferroelectric shape to a thin and elongated shape of a relaxor ferroelectric, and the current density (J)-E current peak becomes rectangular, indicating that the relaxor ferroelectricity of NBT-SBZ ceramics increases with the addition of SBZ. BDS increases from 189.7 kV/cm for the sample with x of 0.1 to 298.1 kV/cm for the sample with x of 0.4, which is consistent with the decrease in average grain size and leakage current density, indicating that small grain size and dense microstructure are important reasons for achieving a high BDS. 0.8NBT-0.2SBZ ceramic with x of 0.2 at 265 kV/cm obtains W_rec of 3.15 J/cm³ and η of 76.05%. 0.8NBT-0.2SBZ ceramics exhibit a superior frequency/temperature stability in energy storage performance. C_D and P_D of 0.8NBT-0.2SBZ ceramics are 188.54 A/cm² and 11.31 MW/cm³ at 120 kV/cm, with an extremely fast t_0.9 of 52.6 ns.

NBT-SBZ relaxor ferroelectric ceramics were prepared via introducing SBZ to improve the relaxor ferroelectricity and energy storage properties of NBT ceramics. 0.8NBT-0.2SBZ ceramic achieved the optimum energy storage properties (i.e., W_rec=3.15 J/cm³, η=76.05%) at 265 kV/cm, as well as a superior stability of the energy storage performance at 1-100 Hz) and 20-140 ℃. The C_D and P_D of 0.8NBT-0.2SBZ ceramic were 188.54 A/cm² and 11.31 MW/cm³ at 120 kV/cm, respectively, with a fast t_0.9 of 52.6 ns. The 0.8NBT-0.2SBZ ceramic had a superior temperature/frequency stability, a super high power density, and extremely fast discharge time, having a great potential for application in pulse power supply equipment.

The environmentally friendly dielectric capacitors based on lead-free relaxation ferroelectrics have attracted recent attention. The material (Bi_0.5Na_0.5)TiO₃ (BNT) exhibits an intrinsic high saturation polarization due to the hybridization between Bi 6s and O 2p orbitals, surpassing conventional lead-free relaxor ferroelectrics. However, a low breakdown strength (BDS) of BNT results in a relatively poor energy storage performance, restricting its further applications. Therefore, improving the BDS of BNT-based materials is a necessity towards achieving a high energy storage density, and one effective measure is a compositional modification. Sr_0.7Bi_0.2TiO₃ (SBT) as an emerging relaxor ferroelectric material has a higher dielectric constant due to the ion substitution-induced local charge imbalance and dipole fluctuations in nanoscale. It is anticipated that introducing SBT as a solid solution component into BNT matrix films can enhance the BDS of the films, optimizing their energy storage performance. In this paper, the impact of SBT/BNT solid solution ratio on phase structure, microstructure, dielectric, and energy storage properties of the SBT modified BNT-based film was investigated.

The precursor solutions for (1-x)(Bi_0.5Na_0.5)TiO₃-xSr_0.7Bi_0.2TiO₃ (BNT-xSBT, x=0.30, 0.35, 0.40, 0.45, and 0.50) were prepared by a sol-gel method. The precursor solutions prepared were aged via static aging for 24 h before spin-coating. Subsequently, the films were heat-treated on a hot plate and rapidly annealed at 630 ℃ for 2 min. The film preparation process, including spin-coating, heat treatment, and rapid annealing, was repeated for 8 times to complete the film fabrication.

The crystal structure of the films was analyzed by a model PANalytical X'Pert PRO X-ray diffractometer (XRD). The surface and cross-sectional morphologies of the samples were determined a model Ultra Plus scanning electron microscope (SEM). To evaluate the electrical properties of the samples, platinum electrodes with a diameter of 0.2 mm were deposited on the film surface using magnetron sputtering. The leakage current density of the films was obtained by a model 6517A electrostatic/high-resistance meter. The ferroelectric properties of the samples were tested by a model PolyK CPE1801 ferroelectric workstation. The dielectric properties of the samples were analyzed by a model 4294A precision impedance analyzer.

All the BNT-xSBT films exhibit a single pseudocubic phase structure. The unit cell volume initially increases and then decreases as the SBT content increases, which is affected by a combined effect of Sr²⁺ doping at the A-site and vacancies. The films demonstrate a well-crystallinity, clear grain boundaries, good adhesion to the substrate, and a thickness of approximately 140 nm. The introduction of SBT results in a reduction in the dielectric constant of the film, improving the temperature stability of the dielectrics. At 2 000 kV/cm, the maximum polarization (P_max) and remnant polarization (P_r) of the films both decrease as the SBT content increases, which is consistent with the dielectric performance. The incorporation of an appropriate amount of SBT increases the BDS of the films from 2 975 kV/cm for BNT-0.3SBT to 3 622 kV/cm for BNT-0.40SBT, which is consistent with the leakage current density. Therefore, the optimal composition BNT-0.40SBT film achieves a high recoverable energy storage density (W_rec) of 85.99 J/cm³ and an efficiency of 74% at the breakdown electric field (E_b). This film demonstrates some advantages, compared to other film capacitors. In addition, the BNT-0.40SBT film exhibits a superior high-frequency stability, a heat resistance, and a fatigue resistance, showing a promising potential for applications in dielectric capacitors.

All the films exhibited a well-crystallinity, having a single perovskite phase. With the introduction of SBT, the unit cell volume initially increased and the decreased, and the BDS firstly increased and then decreased. The dielectric constant and polarization intensity gradually decreased, while dielectric loss remained stable. The optimal component BNT-0.40SBT film achieved a high BDS of 3 622.25 kV/cm and a high energy storage density (W_rec) of 85.99 J/cm³. The superior energy storage density and stability indicated that BNT-0.40SBT thin films could be promising candidates for pulse power and power electronics applications in capacitors.

Dielectric ceramic capacitors are widely used in the field of high energy pulse because of their high power density, ultra-fast charge-discharge speed and good stability. However, compared with other capacitors, the low recoverable energy storage density (W_rec) affects its development in the field of energy storage. The typical ferroelectric BaTiO₃ (BT) with good dielectric and insulation properties is restricted in the development of energy storage field due to the low breakdown strength and large residual polarization. It is indicated that the combination of multi-component solid solution and ion doping induces the polar nanoregions (PNRs) and the transformation of BaTiO₃ ceramics from ferroelectrics to relaxation ferroelectrics due to the reduction of P_r and the improvement of energy storage performance. In addition, obtaining simutanously the high W_rec and energy storage efficiency (η) is a challenge. In this paper, (1-x){0.88(Ba_0.6Ca_0.4)TiO₃-0.12Bi[Zn_2/3(Nb_0.85Ta_0.15)_1/3]O₃}-xCeO₂ (abbreviated as BCT-BZNT-xCe, x = 0, 0.010, 0.015, 0.020, 0.025) ceramics were prepared by a conventional solid-state reaction method. The structure and energy storage properties of the ceramics with differet Ce⁺⁴ doping contents were investigated.

High purity BaCO₃, CaCO₃, TiO₂, Bi₂O₃, ZnO, Nb₂O₅, Ta₂O₅ and CeO₂ were used as raw materials. The raw materials were weighted and mixed according to stoichometric ratio. The mixed raw materials were firstly ground in a ball mill, and then dried and calcined in air at 900 ℃ for 4 h. Afterwards, the calcined powder was further ground, dried and pressed into discs with 10 mm in diameter. The discs were sintered at 1 200 ℃ for 2 h to obtain the ceramic samples. The ceramic samples were thinned and polished to 0.05 mm, and sputtered with Au electrodes for the energy storage properties.

The phase structure of the samples was analyzed by a model SmartLab X-ray diffractometer (XRD, Rigaku Co., Japan) in the range of 20°-80°. The Raman spectra of the samples were analyzed by a model Renishaw inVia Laser Confocal Raman Microspectroscopy (Renishaw Co., UK). The microstructure and elemental analysis (EDS) were determined by a model Merlin Compact Field Emission Scanning Electron Microscope (FE-SEM, Zeiss Co., Germany). The dielectric properties were analyzed by a model HCT1821 High-Low Temperature Dielectric Testing System (Tongguo Technology Co., China). The unipolar P-E curves were measured by a model TF2000 ferroelectric tester (aixACCT Co., Germany) to investigate the ferroelectric performance. The pulsed performance of the samples was examined by a model CFD-003 plus high-voltage charge-discharge measuring instrument (Tongguo Technology Co., China).

All the samples are a multiphase structure with a tetragonal phase BaTiO₃ (BT) as a main phase and an orthorhombic phase CaTiO₃ (CT) as a second phase. The excessive Ce⁴⁺ causes the appearance of the diffraction peak at 28.5° of 2θwhen x=0.025. Also, the diffraction peaks move to a low angle from the magnified (200). The minimum average grain size (AGS) of both coarse and fine grains is obtained when x=0.02. The BCT-BZNT-0.02Ce ceramic obtains a high breakdown strength. The EDS analysis indicates that the coarse grains contain more Ba, while the fine grains contain more Ca.

The dielectric permittivity of samples decreases with increasing the frequency. A large valence difference betweent the A-site and B-site ions induces PNRs, having a typical relaxation behavior. Besides, the diffuseness parameter (γ) as x=0.02 is the maximum value (i.e., 1.917), which is similar to that of the ideal relaxation ferroelectric. The unipolar P-E curves of all the samples are slender and exhibit good relaxation characteristics. The breakdown strength and maximum polarization strength (P_max) increase with the increase of Ce content, and reach the maximum values as x=0.02. A high W_rec of 5.49 J/cm³ and a high η of 89.4% are obtained at 600 kV/cm in BCT-BZNT-0.02Ce ceramic, which is attributed to the increasse of breakdown strength and P_max. Moreover, the superior charge-discharge performance is obtained in BCT-BZNT-0.02Ce ceramic. The ultra-high current density (i.e., C_D=1 757.07 A/cm²) and power density (i.e., P_D=237.20 MW/cm³) are obtained at 270 kV/cm. Besides, a discharge energy density (W_D) of 1.55 J/cm³ and an ultra-fast charge-discharge speed (t_0.9) of 23.8 ns are obtained simultaneously. The W_D is smaller than the W_rec due to the unremarkable contribution of domain wall motion at higher frequencies from the perspective of dynamics. The BCT-BZNT-0.02Ce ceramic with ultra-high W_rec (5.49 J/cm³), η (89.4%), C_D (1 757.07 A/cm²), P_D (237.20 MW/cm³) and ultra-fast t_0.9 (23.8 ns) can be used in pulsed capacitors.

The grains size of BCT-BZNT-xCe ceramics was refined via the introduction of rare-earth element Ce. The γ of the BCT-BZNT-xCe ceramics was improved, and the BCT-BZNT-0.02Ce ceramic showed a typical relaxation ferroelectric performance (i.e., γ =1.917). The superior energy storage performance (i.e., W_rec=5.49 J/cm³, η=89.4%) of BCT-BZNT-0.02Ce ceramic was achieved at 600 kV/cm at room temperature due to the breakdown strength and P_max. Also, the BCT-BZNT-0.02Ce ceramic exhibited a superior charge-discharge performance (i.e., C_D=1 757.07 A/cm², P_D=237.20 MW/cm³, and t_0.9=23.8 ns). The BCT-BZNT-0.02Ce ceramic with the superior comprehensive properties could have a great application prospect in pulsed capacitors.

Lead-free sodium niobate (NaNbO₃) based ceramics with a superior energy storage density have attracted recent attention in high power dielectric energy storage applications. However, a pure NaNbO₃ (NN) ceramic exhibits a square-like square hysteresis loop associating with a large hysteresis and a high remnant polarization at room temperature, leading to a high energy dissipation. In general, stabilizing the antiferroelectricity in NN ceramic is one of the effective measures to solve the problems above. However, despite efforts are made on this measure, the large hysteresis is still kept due to the existence of antiferroelectric-ferroelectric phase transition. The compositions in the phase boundary region usually exhibit abnormally enhanced electrical properties and relaxation characteristics, thus constructing a phase boundary region in NN through composition modulation is another promising method. Moreover, sodium element volatilizes inevitably during the high-temperature and long-time sintering process, giving rise to a poor sintering quality in NN system, which is not beneficial to achieving superior energy storage performance. In this paper, (1-x)NaNbO₃-xCaTiO₃ (NN-CT100x, 0.08≤x≤0.18) ceramics were prepared by a conventional solid-state method with CuO as a sintering aid. The phase structure and polymorphic phase boundary in NN-CT100x system were investigated, and the energy storage performance of compositions in the phase boundary region was analyzed.

For the synthesis of NN-CT100x powder, Na₂CO₃ (≥99.8%, in mass fraction, the same below), Nb₂O₅ (≥99.9%), CaCO₃ (≥99.0%), TiO₂ (≥98.0%) and CuO (≥99.0%) were used as raw materials (Shanghai Sinopharm Chemical Reagent Co., Ltd., China). The raw materials were weighed/mixed according to the stoichiometric ratio and ground with anhydrous ethanol in a ball mill with zirconia balls for 12 h. After dried and sieved, the powder was calcined at 900 ℃ for 6 h. The calcined powders were mixed with 1.5% (in mole fraction) of CuO as a sintering aid and further ground for 12 h. The ground powder was hand-pressed into discs with the diameter of 7 mm and the thickness of 1.2 mm and then further densified by cold isostatic pressing at 300 MPa for 30 min. The NN-CT100x discs were sintered at 1 050-1 250 ℃ for 2 h.

Based on the results by X-ray diffraction, Raman spectroscopy and dielectric behavior, an 'antiferroelectric P phase (orthorhombic, Pbma)-ferroelectric INC phase-paraelectric CT phase (orthorhombic, Pbnm)' polymorphic phase boundary (PPB) region exists as 0.09≤x≤0.16 when CaTiO₃ content increases. The polarization exhibits an enhancement in compositions within this PPB region. The optimum comprehensive energy storage performance is obtained as x=0.16, having the energy storage density (W_rec) of 3.04 J/cm³ and efficiency (η) of 84.4% at 300 kV/cm, respectively. The difference between the maximum polarization (P_m) and the remnant polarization (P_r) of the NN-CT16 ceramic increases as the electric field increases, thus obtaining an admirable energy storage performance. This composition is in the PPB region and contains a few antiferroelectric phases. These antiferroelectric phases are easily transformed into ferroelectric phases at high electric fields, which η reduces from 92.4% to 77.0%. The NN-CT16 composition has a high P_m of 42.2 μC/cm², a high W_rec of 6.1 J/cm³ and η of 77% at 590 kV/cm. For the NN-CT16 ceramic, W_rec and η both exhibit a frequency stability at 1-300 Hz, with minimal variations of <10% and <8%, respectively. Furthermore, the W_rec and η also demonstrate temperature-insensitive characteristics at 25-160 ℃, with small variations of <10% and <5%, respectively. The discharge energy density (W_dis) of the NN-CT16 ceramic increases and reaches a plateau in a short time. The t_0.9 (time at which 90% of the stored energy is released) is less than 8 μs. Moreover, the t_0.9 decreases with the increasing electric field or temperature, indicating the discharge rate accelerates, which is probably ascribed to domains respond quickly and switch more easily at high electric fields or high temperatures.

Based on the phase boundary modulation strategy, the NN-CT100x (0.08≤x≤0.18) ceramics were designed and prepared via introducing CaTiO₃ into NN with CuO as a sintering aid. The polarization of the compositions in the phase boundary region was enhanced. The NN-CT16 ceramic exhibited a high maximum polarization of 42.2 μC/cm², a high energy storage density of 6.1 J/cm³ and an energy efficiency of 77% at 590 kV/cm. It also exhibited a good frequency/temperature stability and an excellent discharge performance. This ceramic could be used as a promising dielectric energy storage material.

As one of the important electronic components in pulsed power devices, dielectric capacitors have wide applications in hybrid vehicles, and distributed power system. However, a relatively low energy density affects their application in portable and light-weight devices. The dielectric capacitors store and release energy in form of electrostatic polarization and depolarization of dielectric materials. Hence, achieving the maximum polarization (P_max), small remanent polarization (P_r) and large electric breakdown strength (E_b) in dielectric materials is an effective approach to improve the energy storage properties. The antiferroelectric ceramics are one of the most prospect materials for energy storage due to their unique antiferroelectric-ferroelectric (AFE-FE) phase transition during charging and discharging process. As one of the classic antiferroelectric ceramics, Pb(Zr,Ti)O₃ antiferroelectric ceramics are widely investigated. However, the content of Ti should be lower than 10% to stabilize the antiferroelectric phase, leading to the narrow windows for compositional regulation. In recent years, Pb(Zr,Sn)O₃ antiferroelectric ceramics become popular because of the electric field induced multistage phase transition. In this paper, (Pb_0.97La_0.02)(Zr_0.6Sn_0.4)O₃ (PLZS) antiferroelectric ceramic was synthesized via solid-state sintering at different temperatures. In addition, the crystal structures, microstructures, dielectric properties and energy storage properties of the ceramic were also investigated.

PbO, La₂O₃, ZrO₂ and SnO₂ were used as raw materials, and weighted and mixed according to the stoichiometry. The mixed materials with alcohol were ground in a ball mill for 12 h. Afterwards, the suspension was dried, the powders were calcined at 900 ℃ for 3 h, and then ground with alcohol for another 12 h. The dried powders with polyvinyl alcohol were pressed into disks with the diameter of 10 mm and the thickness of 1 mm. Finally, the green disks were sintered at 1 200-1 250 ℃ for 2 h to obtain the densified ceramics.

The crystal structures of ceramics were determined by X-ray diffraction (XRD). The surface microstructure of ceramics was characterized by scanning electron microscopy (SEM). For the measurement of dielectric properties, the ceramics were polished and covered with a silver paste, which was dried at 550 ℃ for 10 min. The dielectric properties were analyzed by a dielectric test system at room temperature to 450 ℃, and the P-E loops of ceramics were determined by a ferroelectric analyzer.

All the prepared ceramics show an orthorhombic perovskite structure without secondary phase. Moreover, some weak diffractions appear due to an antiparallel aligned adjacent Pb²⁺ in antiferroelectric ceramics. The microstructure of ceramics becomes denser with decreasing holes and increasing grain size as sintering temperature increases, which is beneficial to enhancing E_b. All the samples show three dielectric anomalies in a temperature-dependent dielectric constant (ε_r), indicating a successive phase transition between orthorhombic antiferroelectric phase (AFE_O), tetragonal antiferroelectric phase (AFE_T), multicell cubic phase (MCC) and cubic paraelectric phase (PE_C) as the temperature increases. In addition, the dielectric loss (tanδ) for all the samples is quite low (<0.2%) at a low temperature, which is also helpful to enhance E_b. All the samples sintered at different temperatures show double P-E loops and I-E curves, having the typical antiferroelectric properties. Furthermore, the polarization increases slowly at a relatively low electric field, and increases quickly at a moderate electric field due to the electric field induced AFE-FE phase transition., the polarization increases more quickly with further increasing electric field because of the electric field induced FE-FE phase transition. Hence, the corresponding I-E curves exhibit electric field induced current peaks. As a result, the PLZS ceramic sintered at 1 225 ℃ has the optimal energy storage properties (i.e., W_rec of 13.3 J/cm³ and η of 83.6%) at 480 kV/cm, indicating great potential applications in pulsed power devices. Note that a large E_b plays an important role in achieving superior energy storage properties, and a large E_b can be attributed to dense microstructure, small thickness and electrode area, moderate ε_r and low tanδ.

PLZS antiferroelectric ceramics were fabricated via solid-state reaction. The crystal structure, microstructure morphologies, antiferroelectric properties and energy storage behavior of PLZS ceramics sintered at different temperatures were investigated. All the samples exhibited an orthorhombic antiferroelectric phase. Also, the density of prepared ceramics improved, and grain size increased with increasing sintering temperature. Therefore, the sample sintered at 1 225 ℃ achieved the optimum energy storage properties (i.e., recoverable energy density of 13.3 J/cm³ and efficiency of 83.6%).

Energy storage technology plays a vital role in advanced electronic and power systems. Ceramic dielectric capacitors show a great potential in electronics, pulse power, and defibrillators due to their ultrahigh power density, fast charging/discharging speed, and superior reliability. Increasing the energy storage density of dielectric capacitors will boost the development of powder devices towards miniaturization, lightweight, and low cost. It is thus crucial to improve the energy storage density of dielectric capacitor. In general, high energy storage properties (ESP) are closely related to a larger maximum polarization (P_m), a smaller remnant polarization (P_r), and a larger breakdown strength (E_b). The recoverable energy storage density (W_rec) and energy storage efficiency (η) are the crucial parameters to evaluate their ESP.

This paper designed and prepared a series of (Bi_0.5Na_0.5)_0.65(Sr_1-xCa_x)_0.35Ti_0.96(Al_0.5Nb_0.5)_0.04O₃ (BNST-xC) ceramics (x=0.05, 0.10, 0.15, 0.20). The effect of relaxation characteristics on the energy storage characteristics of BNT ferroelectric ceramics was investigated.

The BNST-xC ceramics were fabricated via conventional solid-state reaction and subsequent tape-casting. The oxides and carbonates as raw materials were weighed according to a stoichiometric ratio and ground in a planetary mill with a nylon grinding chamber for 12 h. After drying, the powders were calcined at 850 ℃ for 6 h and then ground for 12 h. Subsequently, the synthesized BNST-xC powders were mixed with glycerol trioleate, ethanol absolute, butanone, dibutyl phthalate, polyvinyl butyral, and polyethylene glycol to obtain a slurry. The uniformly mixed slurry was tape-casted on the surface of vacuum adsorbed PET release film using a model MSK-AFA-Ⅳ automatic thick film coater at a casting speed of 0.6 cm/s. After stacking and pressing, the films were placed in a model PTC LT₀₈₀₀₁ isostatic pressing machine (EASEN) and pressed at 75 ℃. Finally, the green samples were firstly heated at 600 ℃ for 8 h to remove the organics, and then sintered at 1 180-1 220 ℃ for 2 h.

The phase composition of ceramic was characterized by a model D/max 2550V X-ray diffractometer (XRD, Rigaku Co., Japan). The grain size of the sample was determined by a model TM4000 desktop scanning electron microscope (SEM, Hitachi Co., Japan). The Raman spectra were collected by a model Jobin Yvon HR800 Raman scattering spectrometer (Horiba Co., France) under 532 nm laser excitation. The hysteresis loop was determined by a model Premier Ⅱ and radiant ferroelectric analyzer (Precision Co., USA) at 10 Hz; The dielectric performance and frequency dependence were analyzed by a model Agilent E4980A LCR meter and heating equipment (Tongguo Technology Co., China). A specially designed RLC circuit was used to test the charging and discharging performance of the capacitor.

From the analysis of phase composition, Sr and Ca ions at a position A, and Al and Nb ions at a position B completely enter the interior of the lattice, forming a solid solution. The improvement of grain size by B-site (Al_0.5Nb_0.5)⁴⁺ ion doping is beneficial to enhancing the breakdown field strength. The average grain size of the ceramic gradually increases from 1.36 μm to 1.09 μm as the Ca doping content increases. From the Raman spectroscopy analysis, the substitution of A-site ions increases an overall local structural disorder. Furthermore, the transformation of their internal phase structure can be investigated based on the dielectric temperature spectra of BNST-xC ceramics. The substitution of Ca causes T_s and T_m moving towards a high-temperature region. Also, the dispersion of the peak shape in the dielectric temperature spectrum further proves that the relaxation degree of the overall material system continuously increases with the increase of Ca ion doping content. To more accurately demonstrate the law of material relaxation degree, the Curie-Weiss law is used to calculate the relaxation coefficient of the dielectric temperature spectrum. The flipping of the internal domain structure at the electric field was tested by PFM, further showing the contribution of Ca ions to the enhancement of relaxation properties.

A series of BNST-xC ceramics were prepared by a solid-state reaction method. The relationship between the relaxation characteristics and energy storage characteristics of BNT based ceramics was analyzed via adjusting the ratio of Sr and Ca ions. The optimal energy storage performance (i.e., E_b of 630 kV/cm, and W_rec of 6.4 J/cm³) was achieved as x=0.15. The energy storage efficiency reached 85%. At E_b of 300 kV/cm, the ceramic exhibited a good stability at different temperatures and frequencies as well as a high power density and an operating current density. It was indicated that BNT based relaxor ferroelectrics could have a promising application potential.

Dielectric capacitors are used in pulse power systems due to their advantages of high energy storage efficiency, long storage lifetime, and fast charge/discharge speed. Among the major dielectric materials, ceramic-based dielectrics have a superior temperature stability and a longer lifetime rather than polymer-based dielectrics. Dielectric energy storage ceramics have attracted recent attention. Among various dielectric energy storage ceramics, SrTiO₃ ceramic with a linear dielectric behavior at room temperature is a promising dielectric energy storage material with a high breakdown strength and a low dielectric loss. However, SrTiO₃ ceramic has a relatively low dielectric constant and lacks a spontaneous polarization, resulting in a limited energy storage density. Therefore, SrTiO₃ ceramic cannot meet requirements for specific energy storage application. Improving the polarization strength while maintaining the high breakdown strength through effective structural adjustment can be an effective method to enhance its energy storage performance. In this paper, we fabricated high-performance (Sr_0.42Na_0.15Bi_0.29Ca_0.07□_0.07)TiO₃ (SNBCT) energy storage ceramics with a small amount of ZnO as doping species. In addition, the effect of ZnO doping on the structure and energy storage properties of the ceramics were also investigated.

SNBCT-x%ZnO energy storage ceramics were prepared by a conventional ceramic preparation process. The raw materials were weighed and then mixed in alcohol by grinding in a ball mill for 24 h. The slurry was dried and calcined at 950 ℃ for 4 h to obtain the powders. After further grinding for 24 h, the powders were mixed with polyvinyl alcohol (PVA) binder solution and then pressed into pellets with 10 mm in diameter and 0.5 mm in thickness. After burning off the PVA, the disk samples were sintered at 1 140 ℃ for 2 h. The samples used for dielectric performance tests were polished and coated with silver electrodes, and the samples used for energy storage and charge/discharge performance testing were thinned to a thickness of 0.1 mm, and gold electrodes with a diameter of 2 mm were sputtered on their surfaces.

The structure of the ceramic was analyzed by a model D8 advance X-ray diffractometer (Bruker Co., Germany). The microstructure of the ceramics was determined by a model Merlin compact field emission scanning electron microscope (Carl Zeiss Co., Germany). The ferroelectric properties of the ceramics were analyzed by a model TF Analyzer 3000 ferroelectric analyzer (aixACCT Co., Germany). The temperature-dependent dielectric properties were characterized by a model Wayne Kerr 6500B impedance analyzer (WayneKerr Co., UK). The charge-discharge characteristics were analyzed by a model CFD-003 plus pulsed charge-discharge system (Tongguo Technology Co., China).

ZnO-modified SNBCT samples show a perovskite structure with a cubic phase. A trace of impurity peak appears in the XRD pattern as ZnO doping amount increases to 1.0%, corresponding to a small amount of the secondary Bi₂Ti₂O₇ phase (JCPDS 032-0118). All the ceramics show a uniform and dense microstructure.

The ZnO doping has little influence on the microstructure of the ceramics as the average grain size in these compositions changes slightly.

For the electrical properties, the doping of ZnO weakens the ferroelectric properties of SNBCT ceramics, which is manifested by a decreased maximum polarization strength P_max. In addition, the doping of ZnO improves the breakdown strength, thus enhancing the energy storage performance of SNBCT ceramics. For the sample doped with 0.5% ZnO, a high recoverable energy-storage density (W_rec) of 5.16 J/cm³ and a high energy-storage efficiency (η) of 88.2% in a large breakdown electric field of 468 kV/cm are obtained. Compared with the SrTiO₃-based energy storage ceramics, a great energy storage performance of the studied sample is obtained. A simulation model containing grains and grain boundaries is proposed to explain the enhanced breakdown strength, and the results show that fine grains with many grain boundaries hinder the propagation of breakdown cracks. For the charge-discharge performance, the sample doped with 0.5% ZnO shows a discharge energy density (W_d) of 2.61 J/cm³ and a discharge rate (t_0.9) of 227 ns in an electric field of 280 kV/cm.

All the samples exhibited a perovskite structure dominated by a cubic phase, and provided a slim P-E loops. ZnO doping effectively improved the breakdown strength of the SNBCT matrix, thus enhancing the energy storage performance. The effective energy storage density (W_rec) of the sample reached 5.16 J/cm³ and the energy storage efficiency (η) was 88.2% in the electric field of 468 kV/cm for the sample doped with 0.5% ZnO. A simulation model containing grains and grain boundaries was established and the extension of breakdown cracks in an external electric field was simulated, which explained the enhanced breakdown strength. In addition, the sample doped with 0.5% ZnO had a good charge-discharge performance, showing a discharge energy density (W_d) of 2.61 J/cm³ and a discharge rate (t_0.9) of 227 ns in an electric field of 280 kV/cm. Meanwhile, the sample exhibited a good temperature stability. These results indicated that the SNBCT-x%ZnO ceramics could be promising materials for pulse power capacitor applications.

Ceramic-based dielectric capacitors with a high power density are one of the most prospective energy-storage components, compared with Li-ion batteries and super-capacitors. Unfortunately, the energy-storage density of ceramic-based dielectric capacitors is one or two orders of magnitude lower than that of the batteries, which is restricted by the property of dielectric materials. Therefore, developing reliable and high-performance dielectric ceramic is a challenge in the field of dielectric capacitors. Among dielectric ceramics, antiferroelectic (AFE) material is one of the most promising candidates for energy-storage devices since they show a high maximum polarization (P_max), low remnant polarization (P_r), and a regulated AFE-ferroelectric (FE) phase transition electric field. Recent efforts are made to enhance energy-storage properties of ceramic by component design, ion-doping and material form construct. However, a superior η is always gained at the cost of a high W_rec, becoming a challenge for synergistically achieving a ultrahigh W_rec and a superior η.

High-purity Pb₃O₄ (≥95.0%, Sinopharm Chemical Reagent Co., Ltd., China), La₂O₃ (≥99.9%, Sinopharm Chemical Reagent Co., Ltd., China), Nd₂O₃ (>99.99%, Aladdin Biochemical Technology Co., Ltd., China), ZrO₂ (≥99.0%, Sinopharm Chemical Reagent Co., Ltd., China), and SnO₂ (≥99.5%, Sinopharm Chemical Reagent Co., Ltd.) were used as raw materials. Pb_0.98-1.5xLa_0.02Nd_x(Zr_0.60Sn_0.40)_0.995O₃ (PL100xNZS) was selected as a basic system. Nd³⁺ with a small ionic radius (i.e., 1.29 Å) was introduced in PLZS to replace Pb²⁺ (i.e., 1.49 Å) for enhancing electric filed-induced E_FE-AFE. A Pb_0.98-1.5xLa_0.02Nd_x(Zr_0.60Sn_0.40)_0.995O₃ component was designed and the ceramics were fabricated by a tape-casting method.

The crystallinity, phase structure and microstructure of PL100xNZS ceramics were analyzed by an X-ray diffractometer (Rigaku Co., Ltd., Japan) and a model TESCAN-MIRA3 field-emission scanning electron microscope (TESCAN Co., Czech). The dielectric properties of the PL100xNZS ceramics were measured by a computer-controlled Agilent E4980A LCR analyzer. The displace-electric field (D-E) hysteresis loops and current density-electric field curve were determined by a ferroelectric measurement system (Radiant Technologies Co., USA). The pulsed charge-discharge behaviors were determined by a model CFD-001 resistance-inductance-capacitance (R-L-C) electric circuit (Guoguo Technology Co., China).

The XRD patterns show that all the samples exhibit a perovskite structure with an orthorhombic phase. The SEM images show that a dense and compact microstructure appears in PL100xNZS ceramics. The average grain size is decreased by doping of Nd³⁺. The dielectric behavior of PL100xNZS ceramics as a function of temperature measured at 10 kHz and 25-400 ℃ indicates that there are three stages of dielectric constant with the increase of temperature, i.e., the rising, the plateau and the falling regions, which correspond to the AFE, multicell cubic (MCC) and single-cell cubic (PE_C) phases. The maximum polarization decreases at the same electric field, and the breakdown strength (BDS) increases with the increase of Nd³⁺ content due to the decreased grain size. The antiferroelectric-ferroelectric phase transition electric field (E_AFE-FE) increases as the Nd³⁺ content increases, because of a stable antiferroelectric phase. The enhanced BDS and increased E_AFE-FE imply a high energy-storage density. The recoverable energy-storage density and efficiency are critical parameters to characterize the material performance. Herein, the PL4NZS ceramic can be used as a promising energy-storage material because it exhibits a linear-like D-E loop with a large P_max (i.e., 60 μC/cm²) and a high BDS (i.e., 490 kV/cm). A large energy storage density of 14.8 J/cm³ is obtained in PL4NZS ceramic with a high efficiency of 85%. The calculated values of W_rec and η fluctuate slightly in the range of 6.18-4.88 J/cm³ with increasing the frequency. The discharge waveforms of PL4NZS ceramic at different electric fields are investigated at room temperature with an overdamped R-L-C electric circuit. The maximum value of W_dis is 4.4 J/cm³ at 300 kV/cm, and the t_0.9 is 28 ns. A large C_Dmax of 3 360 A/cm² and a high P_Dmax of 504 MW/cm³ are achieved at 300 kV/cm in PL4NZS ceramic, indicating that the PL4NZS ceramic has a great competitiveness in high power system applications.

The high energy-storage density and efficiency were realized in designed PL100xNZS ceramics fabricated by a tape-casting technique. The effects of Nd³⁺ doping on the microstructure, dielectric properties, ferroelectric properties and energy storage properties of the ceramics were investigated. The W_rec was 14.8 J/cm³ with a giant η of 85% at 490 kV/cm for PL4NZS ceramic. As the temperature increases, W_rec and η of PL4NZS ceramic increased slightly. Moreover, a high W_dis of 4.4 J/cm³, a large C_Dmax of 3 360 A/cm² and a high P_Dmax of 504 MW/cm³ were obtained in PL4NZS ceramic. It was indicated that PL4NZS ceramic could be used as a prospective dielectric material for energy-storage applications.

Pulse power technology has important applications in national defense industry. Dielectric capacitor is one of the energy storage components of pulse power devices. Ceramic dielectric capacitors have some advantages like fast charging-discharging speed, good electromechanical performance, anti-aging, and resistance to extreme conditions, which can be used in dielectric energy storage. The existing energy storage properties of BaTiO₃-based dielectric ceramics are generally low at a high electric field, and lead-based ceramic capacitors are increasingly limited because of the presence of lead as a highly toxic element. It is thus necessary for pulse power technology to develop lead-free ceramic capacitors with superior comprehensive energy storage properties. NaNbO₃ (NN) ceramics have a rich structural phase transition and a low theoretical density, which makes their electrical properties highly adjustable and is conducive to the development of lightweight capacitors. Recent work on relaxor ferroelectric, anti-ferroelectric and relaxor antiferroelectric NaNbO₃-based ceramics are performed. Compared with the typical lead-free antiferroelectric AgNbO₃, NN as one of of lead-free antiferroelectric materials has some advantages like low theoretical density (i.e., 4.55 g/cm³), high polarization intensity (i.e., 40 μC/cm²), simple preparation process, and relatively low raw material cost. Developing NN-based relaxor antiferroelectric ceramics with superior comprehensive energy storage properties is still a challenge because of the contradiction between polarization and breakdown strength. Therefore, exploring a relationship between the structure and energy storage properties of NN-based relaxor antiferroelectric ceramics is still a research aspect for NN-based ceramics. In this paper, a strategy of reducing tolerance factor was adopted to stabilize the antiferroelectric phase of NN-based ceramics. 10% (in mole fraction) BiFeO₃(BF) with a low tolerance factor (i.e., 0.954 3) was incorporated into pure NN as a second end member, and the antiferroelectric R phase (Pbnm) of NN ceramics was stabilized at room temperature. The third end member Sr(Ti_0.85Zr_0.15)O₃(STZ) was introduced to further improve the relaxor characteristic of the ceramics, and then the relaxor antiferroelectric phase was constructed in NN-based ceramics. In addition, the phase structure, microstructure, energy storage properties, stability and charging-discharging performance of the ceramics were also analyzed.

The chemical composition of the ceramics was NaNbO₃, 0.9NaNbO₃-0.1BiFeO₃, and (1-x)[0.9 NaNbO₃-0.1BiFeO₃]-xSr(Ti_0.85Zr_0.15)O₃, where x=0.05, 0.10, 0.15. All the ceramics were prepared by a conventional solid phase method. The ceramics were prepared via sintering at 1 230-1 260 ℃ for 2 h. The crystal structure of ceramics was analyzed by a model DX-2700BH X-ray diffractometer (Haoyuan Instrument Co., China) and a model Lab-Ram HR800 spectrometer (HORIBA Co., France) with a laser wavelength of 532 nm. The in-situ temperature test for Raman spectra was performed on a model Linkam THMS600 temperature control platform. The sample used for in-situ electric field Raman spectroscopy test was plated with an electrode by an ion sputtering method. The sputtering current was set to 6 mA and the sputtering time to 2 min to ensure that the sample had both electrical conductivity and the penetration in Raman spectroscopic laser. The natural surface morphology of the samples was determined by a scanning electron microscope, and the grain size of the ceramics was determined using a software named Nano Measurer. After the sample was thinned, the domain structure of the ceramics was characterized by a model JEM-2100F field emission transmission electron microscope. The sample was polished to 0.5 mm thickness, cleaned, and coated with silver electrodes on the upper and lower surfaces of the sample. The dielectric properties of the ceramics were measured at -120 -450 ℃ by a model E4980A precision LCR bridge (Agilent Co., USA). The hysteresis loops of the ceramics with the thickness of 150 μm were tested by a model Precision Premier II ferroelectric tester (Radiant Co., USA).

Based on the XRD patterns, NN, NN-BF and NN-BF-xSTZ ceramics are a pure perovskite structure. NN ceramics show diffraction peaks for phases (110) and (200), manifesting that they are antiferroelectric orthogonal P(Pbcm) phases at room temperature. The orthogonal phase features disappear, and the diffraction peaks of phases (110) and (200) change from split state to single peak with the addition of BF and STZ, indicating that the addition of BF and STZ improves the symmetry of the ceramic and the ceramic transforms into a pseudo-cubic phase. NN ceramics have a dielectric anomaly peak at 400 ℃, which represents a transition from the antiferroelectric P phase to the antiferroelectric R phase. For NN-BF and NN-BF-xSTZ ceramics, the P-R phase transition peak disappears, and a diffuse dielectric anomaly peak appears at -100 ℃. Compared with NN ceramics, the room temperature dielectric constant of NN-BF ceramics increases, which is mainly due to the presence of anti-ferroelectric R phase in the ceramics. The in-situ XRD and Raman measurements show that the symmetry of the local structure of NN-BF-10STZ ceramics has a good stability. After BF and STZ doping, the grain size of the ceramics is reduced and the grain distribution uniformity is improved. The breakdown strength of ceramics can be enhanced due to the effect of decreasing grain size and improving distribution uniformity. STZ doping reduces the size of antiferroelectric domains. They can respond quickly to an external electric field due to the high activity and low energy barrier of nano-domains, thus reducing the remanent polarization, which is conducive to improving the energy storage properties. The transformation from antiferroelectric phase to ferroelectric phase of NN ceramics only occurs in the presence of the electric field. After the introduction of BF, the antiferroelectric R phase of the ceramics is stabilized, and the P-E curve shows a double hysteresis loop. The relaxation characteristics of the ceramics are further enhanced with the introduction of STZ. The P-E loops of NN-BF-10STZ ceramics become more elongated, and the polarization hysteresis is suppressed. The NN-BF-10STZ achieves the optimal energy storage properties (i.e., W_rec of 5.22 J/cm³ and η of 83.92%). Also, NN-BF-10STZ ceramics have a superior stability at 25-120 ℃ and 10-150 Hz.

Pure NN ceramics exhibited a typical square hysteresis loop with low energy storage density and efficiency (i.e., W_rec of 0.14 J/cm³, and η of 6.69%). After the introduction of the second component of BF, the ceramics had a double hysteresis loop like antiferroelectric, and the energy storage characteristics (i.e., W_rec~3.55 J/cm³, η~70.61%) were optimized. Furthermore, the introduction of STZ enhanced the relaxation of the ceramics, and reduced the polarization hysteresis of the hysteresis loop. The energy storage properties were optimized (i.e., W_rec~5.22 J/cm³, η~83.92%). 0.9[0.9NaNbO₃-0.1BiFeO₃]-0.1Sr(Ti_0.85Zr_0.15)O₃ ceramics with the optimal energy storage performance had a stability at different temperatures and frequencies, which could be used as a promising dielectric energy storage material with application prospects.