The energy storage performance can be improved to a certain extent by means of phase structure regulation, micro-morphology optimization, doping modification, etc.. However, the harsher application environment requires a higher energy storage performance (i.e., W_rec>10 J/cm³, η>90%). The future development aspects of NN-based energy storage ceramics involve 1) raw material preparation, i.e., wet chemical method (such as hydrothermal method, sol-gel method, self-spreading method, etc.) and high-energy ball milling method are used to reduce the particle size of the powder to further improve the electrical performance; 2) sintering process, i.e., advanced preparation processes (such as SPS, flash sintering, cold sintering, hot press sintering, etc.) are used to improve the energy storage performance while greatly reducing energy consumption; 3) molding process, i.e., the process of rolling film, casting and cold isostatic pressing are used to further optimize the microstructure and improve the energy storage performance; 4) theoretical research, i.e., combined with advanced in-situ characterization techniques (such as in-situ high resolution TEM, micro-infrared, etc.) and theoretical calculations (such as first-principles, phase field simulation, etc.), a relationship between process-structure-performance is further explored, and new theories and mechanisms are proposed; and 5) device preparation, i.e., the device with superior energy storage performance and high stability dielectric materials needs to be developed.

Dielectric materials have critical applications in many fields. Compared with electrochemical energy storage batteries, ceramic capacitors show outstanding competitive power performance in actual energy storage applications, such as diesel engine starters, camera flashlights, spacecraft, pulsed power weapons, and medical devices due to their ultra-fast charge/discharge capability and high-power density. However, with the development of electronic device integration and miniaturization, capacitors are required to have a high effective energy storage density (W_rec) under low electric field. The existing dielectric ceramic capacitors are difficult to meet the corresponding requirements, so it is urgent and significant to develop dielectric ceramic capacitors that can obtain high energy storage density under low voltage. In recent years, the co-design strategy has been used as a typical and effective method to enhance the energy storage performance of BNT, which is achieved by adding different kinds of ABO₃ perovskites and various doping ions. Co-design strategy improves energy storage properties by compositional design to induce the formation of polar nanoregions (PNRs) as a result of A/B site ion disordering or structural phase transitions. In this paper, KNN was added to BNBST ceramics to modify the internal crystal structure, and the effect of KNN on the phase structure, microstructure, dielectric properties, energy storage properties, ferroelectric stability and fast charge-discharge characteristics of BNBST ceramics were investigated.

As we know, high P_max, low P_r and high breakdown strengthen E_b are required for dielectric ceramics to achieve high W_rec. To obtain high energy storage density under a low electric field, we chose Ba_0.105Na_0.325Bi_0.325Sr_0.245TiO₃ (BNBST) with large P_max and high Curie temperature as the research object, the co-design strategy with the introduction of K_0.5Na_0.5NbO₃ was adopted to further destroy the ordered arrangement of A/B position ions, reduce their residual polarization intensity P_r, and optimize their electric polarization behavior.

Using solid-state method prepared Ba_0.105Na_0.325Bi_0.325Sr_0.245TiO₃+x%K_0.5Na_0.5NbO₃ (BNBST-x% KNN, mole fraction, x=0, 2, 4, 6, 8, 10, 12) ceramic samples. Bi₂O₃ (99.999%), K₂CO₃ (99%), BaCO₃ (99%), SrCO₃ (99%), Na₂CO₃ (99.8%), TiO₂ (99%), Nb₂O₅ (99.95%) were used as synthetic raw materials to prepare samples according to the following process: 1) Accurately weigh the ingredients according to the corresponding stoichiometric ratio, put them in the nylon ball mill tank for 24 h, dry, grind, and pass 80 mesh sieve; 2) Pre-burning after pressing into large sections, heating up to 850 ℃ with a heating rate of 5 ℃/min and holding for 3 h to exclude CO₂ and pre-synthesize the powder; 3) The large pieces are ground into powder, passed through an 80-mesh sieve, ball milling again for 24 h, drying, adding 5% concentration of PVA adhesive for granulation, pressing into a small disc with thickness of about 1 mm, diameter of about 13 mm (pressure (120±10) MPa); 4) The small disc was heated to 650 ℃ for 3 h to remove organic matter, and then heated to 1 240 ℃ for 2 h to sintering; 5) After polished and silvered, heating up 30 min at 650 ℃, and then various electrical properties are tested.

All BNBST-x%KNN ceramics have a single perovskite pseudo-cubic phase structure, and no second phase is generated. The grain distribution of all ceramics is uniform and the density is good. When x=6, the optimized energy storage characteristics are obtained only at the low electric field of 140 kV/cm with W_rec=1.8 J/cm³ and η=86%. Whatmore, BNBST-6% KNN ceramics have TCC=±15% high dielectric constant (ε_r=3 128@125 ℃) and excellent temperature, frequency, cycle stability and fast charge-discharge characteristics in the temperature range of -8-215 ℃. The results show that BNBST-6%KNN ceramics can be used as a candidate material for pulsed power capacitors, and has obvious advantages in low electric field application.

The main innovation points of this paper are summarized as following. A collaborative design strategy with the introduction of K_0.5Na_0.5NbO₃ was used to further destroy the ordered arrangement of A/B ions and promote the formation of polar nanodomains (PNRs), under a certain electric field, PNRs can be transformed into long-range ordered ferroelectric domains, resulting in larger P_max. When the electric field is removed, the ferroelectric domains formed by PNRs will quickly break back to the initial state, resulting in smaller P_r. More importantly, "premature saturation" is delayed due to the appearance of PNRs, which can obtain hight larger P_max at low electric field, resulting in a high W_rec=1.8 J/cm³ at the low electric field of 140 kV/cm.

With the increasingly severe environmental pollution and global warming, developing green and sustainable energy storage devices with high power density, energy storage density, and good stability becomes a research hotspot. Compared with batteries and electrochemical capacitors, lead-free ceramic dielectric capacitors exhibit eco-friendly, high power density, fast charge/discharge speed and excellent reliability, which are promising candidates for advanced pulse power systems. In general, the energy storage density of ceramic dielectric capacitors can be evaluated via measuring the polarization versus electric field hysteresis loops (i.e., P-E loops). Large maximum polarization (P_max), small remnant polarization (P_r) and high breakdown strength (E_b) are critical parameters to achieve a high recoverable energy storage density (W_rec) and an energy storage efficiency (η). Although some previous work reported a higher energy storage density at higher electric field (>200 kV/cm), the high electric field could restrict the application in some fields. It is thus crucial to develop ceramic dielectric capacitors with a superior energy storage performance at a lower electric field (<200 kV/cm). In this paper, we designed and prepared a series of (1-x)(0.94Bi_0.55Na_0.45TiO₃-0.06BaTiO₃)-xSr_0.7Bi_0.2TiO₃ (x=0, 0.10, 0.20, 0.30, 0.40) lead-free relaxor ferroelectric ceramics to achieve a high energy storage performance and a superior stability at a low electric filed. In addition, the energy storage properties were also investigated.

TiO₂ (98%), SrCO₃ (99%), Bi₂O₃ (99%), Na₂CO₃ (99.8%) and BaCO₃ (99%) were used as raw materials for the synthesis of (1-x)BNBT-xSBT lead-free ceramics. According to the stoichiometric ratio, these raw materials were weighed. The materials were mixed with alcohol and ground in nylon jars for 12 h. After drying, the powders were calcined at 950 ℃ for 4 h. Subsequently, the calcined powders with alcohol were further ground in nylon jars for 12 h. Afterwards, the obtained powders were mixed with binders and pressed uniaxially into disk-shaped samples with 1 mm in thickness and 10 mm in diameter. Finally, the samples were sintered at 1 150-1 250 ℃ for 2 h after removing the binders at 600 ℃ for 8 h.

The phase structure of the ceramics was analyzed by a model D8-Advance X-ray diffractometer (XRD, Germany). The Raman spectra were collected by a model Jobin-Yvon HR800 Raman spectroscope (Horiba Co., France). The microstructure of the ceramics was determined by a model APERO HIVAC scanning electron microscope (SEM, USA). In order to test the electrical properties of the (1-x)BNBT-xSBT ceramics, the silver paste was coated on the surface of the samples and fired at 580 ℃ for 10 min to form silver electrodes. The temperature-dependent dielectric constant and loss were measured by a model Agilent E4980A impedance analyzer. The ferroelectric performance was determined by a model Precision Premier II ferroelectric analyzer (Radiant Co., USA) with 2 mm in electrode diameter.

The microstructure analysis indicates that all the ceramics samples have a slight porosity. Although some secondary phases appear in the ceramics due to the non-stoichiometric ratio of Bi and Na elements, the ceramics maintain a main perovskite structure. The difference of peak intensity between v₂, v₃ and v₄, v₅ reduces gradually with increasing the SBT content due to the enhancement of structural symmetry. Meanwhile, the peaks of dielectric constant shift towards a lower temperature along with the improvement of temperature stability of dielectric constant as the SBT content increases. At 25-343.9 ℃, the variation of dielectric constant is less than ±10% for (1-x)BNBT-xSBT ceramics when x=0.40. At 10 Hz and 120 kV/cm, the P_max decreases with increasing the SBT content, but the P-E loops become slimmer, which is beneficial to achieving a higher energy storage efficiency. Meanwhile, four peaks appear in current versus electric field curves (I-E curves) and the current peak shifts towards zero electric field along with the decrement of current intensity when the SBT content increases, indicating the enhancement of response speed for domains after removing the external voltage. The ceramic with the composition of x=0.30 exhibits slim P-E loops at different electric fields and a recoverable energy storage density of 2.16 J/cm³ along with a high energy storage efficiency of 90% at a low electric field of 192 kV/cm. In addition, the energy storage performance shows superior frequency and cycle stability. The variation of recoverable energy storage density and energy storage efficiency is less than ±8% within 1-200 Hz and 1-10⁵ cycles.

(1-x)BNBT-xSBT lead-free ceramics were prepared by a solid-state reaction method. The effect of SBT content on the microstructure, dielectric, ferroelectric, energy storage properties and stability of (1-x)BNBT-xSBT ceramics were investigated. The temperature stability of dielectric constant was improved and the variation of dielectric constant was less than ±10% at 25-343.9 ℃ for the composition of 0.60BNBT-0.40SBT. The superior energy storage performance with a recoverable energy storage density of 2.16 J/cm³ and an energy storage efficiency of 90% was achieved at a low electric field (<200 kV/cm) and an optimized SBT content. In addition, the energy storage performance of (1-x)BNBT-xSBT ceramics showed a superior frequency and cycle stability. This study demonstrated that the addition of SBT could be beneficial to optimizing the energy storage performance of BNT-based lead-free ceramics and the (1-x)BNBT-xSBT ceramics could be promising candidates for advanced pulse power systems application.

Antiferroelectric ceramic and multilayer ceramic capacitors have the advantages of high energy storage density, fast discharge speed, high discharge current, etc., which can improve the energy storage density of pulse power device and effectively suppress ripple in power electronic system, etc. Unlike ferroelectrics, antiferroelectric materials adjacent to the lattice of dipoles reverse parallel, and therefore do not show a macroscopic spontaneous polarization phenomenon. In a certain external electric field, the dipoles redirect, the antiferroelectric phase converts to a ferroelectric phase, the polarization intensity increases abruptly after the withdrawal of the external electric field, and it returns to an antiferroelectric phase. This polarization/phase transition behavior can be characterized by the hysteresis loop. In the hysteresis loop, antiferroelectric material shows a double loop, and it is divided into two kinds, i.e., "Square" and "Slant" according to the different shapes of the antiferroelectric hysteresis loop. The polarization effect of the antiferroelectric capacitor gives it the better energy storage characteristics and discharge performance, thus having an advantage in practical applications. General capacitors are polarized under a certain period of time and voltage in the factory. However, changes of external conditions (i.e., temperature, voltage, time, etc.) can depolarize the antiferroelectric capacitor. Also, some antiferroelectric capacitors need to consider the effect of polarity. As a result, the practical application of antiferroelectric MLCCs is different from that of conventional Class I or Class II MLCCs. Therefore, in this paper, antiferroelectric ceramics with hysteresis loop shapes of "square" and "slant" were prepared by a conventional solid-phase reaction method to investigate the polarization effect of antiferroelectric ceramics and capacitors, and provide a guidance for the application of antiferroelectric MLCCs.

For the preparation of the antiferroelectric ceramic, the raw materials were weighed according to the chemical formula. The excess Pb₃O₄ was used to compensate for lead loss in sintering. The weighed raw materials were mixed in a mass ratio of 1.0∶1.0∶1.5 and ground in anhydrous alcohol by grinding in a ball mill with zirconium balls at 300 rpm for 20 h. After ball-milling, the slurry was pre-fired at 850 ℃ for 3 h. The film was formed, and heated at 1 280 ℃ for 2 h. The samples were prepared after polished with sandpaper to a certain thickness (less than or equal to 0.1 mm) and sputtered with gold electrodes. The micro-morphology of the samples was measured by a model JSM-6390A scanning electron microscope (SEM, JOEL Co., Japan). The crystalline phase composition was determined by a model D/Max-2400 X-ray diffractometer (XRD, Rigaku Co., Japan), using Cu K_α rays, with a scanning range of 20°-70°. The hysteresis loops and polarization current curves of the samples were examined by a model TF Analyzer 2000 ferroelectric test system (aix-ACCT Co., Germany). The basic principle of the test was based on the Sawyer-Tower circuit. The test frequency was 1 Hz, and the voltage waveform was a triangular wave. The pulse discharge test used a self-built platform.

From the SEM images of the samples, the grains are uniform and have a good densification. From the XRD patterns of the samples, the prepared samples have a quadrilateral phase structure. The prepared samples have a typical double hysteresis loop, and the shapes of hysteresis loops for sample S1 and S2 are "square" and "slant". The hysteresis loops of the samples after and before polarization indicate that for the unpolarized ceramic sample, the transition field is higher, the stored energy density and energy conversion efficiency are lower. This phenomenon is also related to the shape of the hysteresis loops, so the polarization has a certain impact on the practical application of antiferroelectric ceramics. The electric hysteresis loop nearby transition field shows that for capacitors with antiferroelectric ceramic dielectrics, the antiferroelectric capacitor is unpolarized (or is depolarized) if the operating voltage is set nearby the transition field, thus leading to a weakening of its performance. To analyze whether high temperature should have an effect on the degree of polarization of antiferroelectric ceramics, the capacitances of the antiferroelectric ceramics are tested after depolarization at different temperatures, and the pulse discharge currents of the antiferroelectric MLCCs after depolarization at different temperatures are tested. The results show that if the antiferroelectric capacitor is overheated and warmed up, and although the temperature is recovered after using, the temperature still has a certain effect on the performance of antiferroelectric capacitor, especially at a lower applied voltage. To study the effect of polarity on the antiferroelectric material, the hysteresis loops of antiferroelectric ceramics in positive and negative electric fields were tested, and the pulse discharge of antiferroelectric MLCCs in different polarity electric fields was tested. The results show that the antiferroelectric domains are preferentially oriented, and the antiferroelectric domains have a memory effect after a certain direction of the electric field polarization, preventing its entry into the opposite direction of the ferroelectric domains. And this effect is more serious on the antiferroelectric ceramics with a "square" hysteresis loop.

The antiferroelectric polarization and polarity characteristics had dominant effects on their applications. Those effects above were lower although antiferroelectric capacitors were generally made of materials with "slant" hysteresis loops. However, since antiferroelectric materials were in an electric field that was slightly higher than the E_AFE-FE phase transition field, their depolarization and polarity characteristics still affected their performance. In addition, a slight attenuation of the antiferroelectric MLCCs performance still adversely affected the reliability of the whole system since some applications required an extremely high reliability. In practice, antiferroelectric MLCCs were bound to depolarize after soldering, and their performance could be greatly degraded if the applied electric filed was lower than the transition field.

Flexible and stretchable electronics have attracted much attention due to their potential applications in wearable devices, robotics, and healthcare. Among various materials, barium titanate (BTO) is a promising candidate for flexible and stretchable electronics because of its high dielectric constant, piezoelectric property, and superior thermal stability. However, the brittle nature of BTO restricts its application in flexible and stretchable devices. Polymer composites such as polydimethylsiloxane (PDMS) can be used, providing a flexibility and a stretchability for BTO-based devices. In this paper, flexible BTO/PDMS composites with a hierarchical microstructure were prepared and their dielectric performance was optimized. The hierarchical microstructure could enhance the mechanical properties, electrical conductivity, and dielectric performance of the composites. This study could provide valuable insights into the design and fabrication of flexible and stretchable electronic devices based on BTO/PDMS composites.

A piece of foam nickel was prepared, cleaned with acetone under ultrasound for 10 min and then washed with deionized water for several times to remove impurities on the surface. An isopropanol was used as a solvent, an appropriate amount of phenolic resin was used as a stabilizer, and an appropriate amount of aluminum nitrate nonahydrate was added to make the BTO positively charged. Little precipitation occurred after mechanical stirring and ultrasonic treatment. A uniform and stable barium titanate suspension was formed. A layer of BTO coating was deposited on the pore walls of the pretreated foam nickel by an electrophoretic deposition (EPD) process. Two copper plates were used as the poles of the electrophoretic deposition. A nickel metal foam template was fixed on a negative pole, immersing it into the EPD tank with the stable BTO suspension To prepare the samples with different BaTiO₃ contents, the diameter of the BTO framework pore was controlled via adjusting the electrophoresis time. The sample was sintered in an argon atmosphere at 1 200 ℃ for 2 h, and then immersed in a 1 mol/L FeCl₃ solution to completely remove the foam nickel. Finally, the sample was sintered in air at 700 ℃ for 2 h to remove carbon, forming a BTO framework. To determine the mass fraction of BTO, a barium titanate framework was weighed before and after penetration by epoxy resin. The PDMS (with curing agent) was filled into the porous structure and cured at room temperature for 24 h to obtain the BTO/PDMS dielectric composite material.

The results indicate that when the BTO content reaches 10.16% (in mass), the relative permittivity of the composite increases up to 85 at 1 000 Hz (compared to only 2.75 for pristine PDMS). Also, the dielectric permittivity increases as the mass fraction of BTO increases from 5% to 20% (in mass). For the BTO with an ordered morphology, the dielectric permittivity value increases, improving the dielectric strength performance in specific directions. Adding PVDF slightly increases the dielectric permittivity, while reducing the difference between the dielectric properties of filler and substrate, thereby enhancing the dielectric strength performance via combining PVDF with PDMS to form a core-shell structure and optimizing the dielectric properties by a finite element method. The hierarchical microstructure of the composites plays an important role in enhancing the dielectric properties. The uniform distribution of BTO particles and the interconnected porous structure of PDMS matrix provide a continuous path for the movement of charge carriers, which contributes to a high dielectric constant and a low dielectric loss of the composites. In addition, the flexible nature of PDMS matrix also makes the BTO/PDMS composites suitable for various applications, such as flexible electronic devices, sensors, and actuators.

The BTO/PDMS flexible dielectric composites with a hierarchical microstructure were prepared by a template method and an electrophoresis deposition method with BTO as a filling material and PDMS as a matrix. The influences of filler morphology and content on the dielectric constant and dielectric strength of the composites were investigated via finite element simulation analysis. Two kinds of ordered network structures of the composites, i.e., tetrahedral network structure and honeycomb network structure, were designed. This provides a theoretical guidance for the preparation of high-quality dielectric composite materials with superior energy storage density. The main conclusions were as follows: (1) Compared to the particle-filled composites, the BTO/PDMS composites with a hierarchical microstructure have an improved dielectric constant at a low filler content. For instance, the relative dielectric constant of the sample with 10.16% BTO could reach 85 at 1 kHz, which was 31 times higher than that of pure PDMS; (2) The morphology of filler had an impact on the dielectric performance of composites. In the case of a low filler content, the composites with a hierarchical microstructure had a higher dielectric constant rather than those with a solid network structure; (3) PVDF was used to form a nuclear shell structure (PVDF@BTO/PDMS) to improve the relative dielectric constant of the composites to a limited extent and the breakdown performance of composites; (4) The orderly network structures were designed. The ordered network structures with a tetrahedral network structure and a honeycomb network structure could improve the dielectric constant of composites to a certain extent and had the anisotropic characteristics.

With the development of power electronic systems, energy storage capacitors have the advantages of large discharge power, fast charging and discharging speed and stable performance, and play an important role in power systems, electronic devices, pulse power supplies, etc.. They are widely used in civilian and military fields. Compared with fuel cells, the energy storage capacitors do not need to convert the Gibbs free energy of the chemical energy of the fuel into electrical energy through electrochemical reactions, but store the charge by pressurizing between the two plates of the capacitor with higher safety and reliability as well as environmentally friendness. The existing pulsed dielectric materials are mainly divided into ceramics, glass ceramics, thin films and other composite materials. Glass ceramics have energy storage advantages, compared with other materials. Increasing the dielectric constant of dielectric materials plays a crucial role in increasing the energy density.

A glass with the composition of 13K₂O-21SrO-32Nb₂O₅-5B₂O₃-4Al₂O₃-25SiO₂-xBi₂O₃ (x=0.0%, 1.0%, 2.0%, and 4.0%, in mole) was produced by a conventional melt annealing process. First, 40 g of raw materials were weighed and ground in a polypropylene ball mill in ethanol for 12 h. The resulting mixture was dried at 100 ℃ and subsequently melted in a corundum crucible in a resistance furnace at 1 500 ℃ for 2 h. The molten liquid was pressed into a sheet on a preheated copper plate and then annealed at 500 ℃ for 6 h to eliminate residual internal stresses. Finally, a niobate glass was obtained. The glass slices were crystallized at 850 ℃ and 950 ℃ for 2 h to obtain glass ceramics. The glass ceramic samples were ground, polished, and plated with electrodes for the structural and performance tests.

The effect of Bi₂O₃ concentrations (x=0.0%, 1.0%, 2.0%, and 4.0% in mole) on the phase evolution, microstructure, dielectric and energy storage properties of K₂O-SrO-Nb₂O₅-B₂O₃-Al₂O₃-SiO₂ glass ceramics was investigated by a controlled crystallization process. Based on the XRD analysis, KSr₂Nb₅O₁₅ is a main precipitated crystalline phase, and the crystallization promotes due to the addition of bismuth oxide. The A-site ions are adjusted by Bi³⁺ with a smaller radius (i.e., 1.03 Å) into the crystal lattice instead of Sr²⁺ with a larger radius (i.e., 1.18 Å), thus reducing the unit cell volume and increasing the lattice distortion. From the Raman spectra, the change of A-site occupancy leads to the distortion of the structure and enhances the spontaneous polarization. The microstructure shows that the grains are dense and evenly distributed, and the grain size is fine. The dielectric temperature spectroscopy indicates that the relaxation characteristics correspond to the polar nanoregion of the microstructure, and the DC breakdown strength firstly increases and then decreases, which is consistent with the change of the dielectric constant. The polarized electric field curve is slender with a large spontaneous polarization and a small residual polarization, improving the energy storage efficiency. The charge-discharge curve shows that the energy density firstly increases and then decreases with the addition of bismuth oxide, and the glass-ceramic sample B2-9 obtains the maximum energy density, discharge power density and fast discharge time, having a great application prospect in the field of dielectric energy storage.

Strontium niobate potassium glass ceramics were prepared by a conventional melting method and a controlled crystallization technology. At a crystallization temperature of 950 ℃, the unit cell volume of the crystalline phase decreased from 611.83 ų to 610.19 ų with the increase of Bi₂O₃ doping content. The grain size of the microstructure firstly decreased and then increased, and the optimal grain size of glass ceramic sample B2-9 was 57.72 nm. The dielectric and energy storage properties also firstly increased and then decreased, and the maximum dielectric constant of glass-ceramic sample B2-9 was 342 times greater than that of glass-ceramic sample B2-8, which was 1.9 times greater than that of glass-ceramic sample B2-8. According to the P-E curve, the glass ceramic sample B2-9 had the optimum performance (i.e., P_max=9.33 μC/cm², P_r=0.73 μC/cm², W_rec=1.27 J/cm³, W_total=1.55 J/cm³, η=82% at 350 kV/cm). The energy density of the glass ceramic sample B2-9 was 1.33 J/cm³ at 400 kV/cm, the maximum discharge power density was 124.13 MW/cm³, and the fastest discharge time was 22 ns. The breakdown strength firstly increased and then decreased, which were 742, 886, 923 kV/cm and 622 kV/cm, respectively. Glass-ceramic materials could have great application prospects in the field of dielectric energy storage.

The pulse power capacitors can be used in military, medical, and aerospace fields due to their high power density and rapid charge-discharge capabilities. Despite these advantages, limited energy storage capacity and efficiency become challenges in achieving miniaturization and high-power performance. Among dielectric energy storage materials, antiferroelectric materials are regarded as promising for high-performance pulse power capacitors due to their distinctive double hysteresis loops, lower residual polarization, and higher energy storage efficiency. However, the breakdown strength decreases due to non-uniform electric field when the resistance of ceramic grain is equal to that of grain boundary, resulting in a decrease of energy storage density. Therefore, the enhancement of grain boundary impedance plays a key role in improving the dielectric breakdown strength of the system. In this paper, (Pb_(1-1.5x)Tm_x)(Zr_0.55Sn_0.44Ti_0.01)O₃ (x=0.00, 0.02, 0.04, 0.06) antiferroelectric ceramics were prepared via tape casting. The strategy of grain boundary impedance was constructed via enhancing the grain boundary quantity.

Powders of Pb₃O₄, Tm₃O₂, ZrO₂, SnO₂, and TiO₂ as raw materials were precisely weighed according to their chemical formulae after drying. The powders were mixed with alcohol and then ground in a ball mill for 24 h. Afterwards, the mixture was dried and pre-sintered at 850 ℃, and further ground in a ball mill for 24 h. The ground powders were mixed with toluene ethanol as a solvent, polyvinyl alcohol as an adhesive and butyl phthalate as a plasticizer. After 24 h milling, the mixture underwent degassing for 40 min, resulting in a slurry with an optimal fluidity. A flexible film with a thickness of 0.02 mm was produced via casting at a linear speed of 40 cm/min, and dried for 6 h. This film was then compressed into a block with the sizes of 10.0 mm×10.0 mm×0.5 mm. The samples were sintered in a muffle furnace via increasing the temperature from room temperature to 600 ℃ for 12 h to firstly remove the adhesive and then at 1 300 ℃. The sintered samples were cooled to room temperature.

The crystal structure of PTZST ceramics at room temperature was determined by X-ray diffraction. The ion vibration mode and the degree of disorder in the local structure were characterized by Raman spectroscopy. The surface morphology and grain size of the ceramic samples were analyzed by scanning electron microscopy. The particle sizes were determined by a software named Nano Measure. The dielectric temperature spectra were analyzed. The hysteresis curves were measured at various electric fields and temperatures as well as at a constant electric field intensity of 10 Hz, by a ferroelectric testing system. The test samples had a thickness of 0.1 mm and an electrode diameter of 2 mm. The impedance data were recorded by a high-temperature dielectric test system and subsequently analyzed by a software named ZView to evaluate the electrical performance of the samples.

All the PTZST ceramics exhibit a pronounced perovskite structure without elemental distortion, as evidenced by X-ray diffraction analysis. The orthogonal antiferroelectric phase of these ceramics is confirmed through superlattice diffraction and component selection analysis. The scanning electron microscopy images reveal that the grain size of the ceramics decreases with increasing Tm³⁺ content. This phenomenon is attributed to the formation of lead ion vacancies induced by the substitution of high valence Tm³⁺ for Pb²⁺, subsequently inhibiting oxygen vacancy formation and grain growth. Moreover, this modification increases the number of grain boundaries, enhancing the effectiveness of the grain boundary impedance strategy. However, excessive Tm³⁺ doping deteriorates the sample properties. The analysis of the P-E loops indicates a change in the energy storage density of PTZST ceramics. The PT4ZST ceramics demonstrate the maximum energy storage density (i.e., W_rec=9.37 J/cm³) and efficiency (i.e., η=77%). There is little variation in energy storage density at 30-120 ℃.

The impedance spectrum transformation from two semicircles to a single semicircle indicates the effective implementation of the grain boundary impedance strategy. This alteration enhances the dielectric breakdown strength of PTZST ceramics, thereby augmenting the energy storage density.

In exploring practical applications, charge-discharge tests were conducted. The discharge energy density of PT4ZST ceramics increases with increasing applied electric field, having the maximum value at a discharge energy density (i.e., W_dis=5.5 J/cm³) and a discharge rate (i.e., t_0.9=199.85 ns).

The original long-range ordered structure was disrupted via doping Tm³⁺ in the A-site to replace Pb²⁺, reducing a tolerance factor and stabilizing an antiferroelectric phase. Consequently, this led to an improved dielectric breakdown and an enhanced energy storage capability. The results indicated that the PT4ZST ceramics achieved a high dielectric breakdown (i.e., E_b=490 kV/cm) and a significant energy storage density (i.e., W_rec=9.37 J/cm³). These ceramics exhibited a high energy storage efficiency (i.e., 77%) and a substantial discharge energy density (i.e., W_dis=5.5 J/cm³), coupled with a rapid discharge rate (i.e., t_0.9=199.85 ns). Furthermore, the PT4ZST ceramic had a superior temperature stability at 30~120 ℃. The results indicated that the PT4ZST ceramic could be used as an effective material for high-temperature pulse power capacitors.

The common energy storage devices mainly include supercapacitors, lithium-ion batteries, fuel cells, and ceramic capacitors. Compared to other energy storage devices, ceramic capacitors have attracted much attention due to their ultra-high charge-discharge rates and power density. Antiferroelectric ceramics exhibit a large maximum polarization and close-to-zero residual polarization, displaying characteristics of a double hysteresis loop. Also, antiferroelectric ceramics have an advantage of low dielectric loss, making them one of the optimal energy storage materials. Lead-free NaNbO₃-based antiferroelectric ceramics become popular due to the environmental concern and cost reduction. However, NaNbO₃-based ceramics can transform from an antiferroelectric phase to a ferroelectric phase at a certain applied electric field. Even after removing the applied electric field, some ferroelectric phase remains, increasing the residual polarization strength and affecting the formation of the double hysteresis loop at room temperature. Furthermore, a transition between antiferroelectric phase and ferroelectric phase caused by the complex phase structure results in a loss of most energy in the form of heat, reducing the energy storage efficiency (η). To further regulate the energy storage characteristics of NN-based antiferroelectric ceramics, this paper introduced Bi³⁺ and Fe³⁺ into the NaNbO₃ matrix to enhance the stability of the antiferroelectric phase and the relaxor behavior of the ferroelectric phase, resulting in a relaxor antiferroelectric ceramic. Different molar fractions of Sm₂O₃ were doped into the NN-BF ceramics with Sm³⁺ solid solution into the NaNbO₃ lattice. In addition, the influence of doping amount of Sm³⁺ on the phase structure, microstructure, dielectric properties, and energy storage performance of NN-BF ceramics was also investigated.

Different Sm³⁺ doping amounts of (1-x)[0.9NaNbO₃-0.1BiFeO₃]-xSm₂O₃ (x=0.01, 0.02, 0.03, 0.04) relaxor antiferroelectric ceramics were prepared via conventional solid-state reaction. The ceramic green bodies with a diameter of 10 mm and a thickness of 1 mm were prepared. They were sintered in a furnace at a rate of 3 ℃/min at 1,150 ℃ for 3 h, and then cooled naturally to form ceramics. The ceramic samples were polished to a thickness of 0.1 mm, and silver electrodes with a radius of 2 mm were screen-printed on the upper and lower surfaces of the ceramic samples. The samples were heated at 500 ℃ for 1 h to connect the ceramics and electrodes tightly. The phase structures of different ceramic samples were analyzed by an X-ray diffractometer. The microstructure and elemental distribution of the ceramic samples were analyzed by a field emission scanning electron microscope. The dielectric loss and dielectric constant of the ceramic samples at different temperatures (i.e., 25~300 ℃) and frequencies (i.e., 1, 10, 100, 500, 1 000 kHz) were tested by an LCR meter. The electric hysteresis loops of the ceramic samples were measured at room temperature and 10 Hz by a ferroelectric tester, and the energy density and energy storage efficiency of the ceramic samples were calculated based on these measurements. The XPS spectra of Fe 2p in ceramic samples with different Sm³⁺ doping amounts (x=0.01, 0.02, and 0.03) were determined by an X-ray photoelectron spectrometer.

The XRD patterns show that the ceramic with the Sm³⁺ doping amount (x) of 0.01 or 0.02 has a homogeneous perovskite structure without the second phase. Bi³⁺, Fe³⁺, and Sm³⁺ are all solid-solutions into the NaNbO₃ lattice. When the Sm³⁺ doping amount (x) is 0.02, Sm³⁺ occupies the position of Na⁺ in the NaNbO₃ lattice. The lattice energy increases after solid solution since the ionic radius of Sm³⁺ is greater than that of Na⁺, inhibiting the growth of grains. Consequently, the microstructure of the ceramic becomes denser, with smaller and more uniformly distributed grains. The larger dielectric constant reduces the breakdown field strength of the ceramic, while the smaller dielectric constant decreases the polarization of the ceramic. Therefore, a moderate dielectric constant is conducive to achieving a larger energy storage density for the ceramic composition when the Sm³⁺ doping amount (x) is 0.02. The hysteresis loop of the NN-BF ceramic is relatively slender, resembling a "waist-like" shape typical of relaxor antiferroelectric. This is because the doping of Bi³⁺ and Fe³⁺ enhances the stability of the antiferroelectric phase and the relaxor behavior of the ferroelectric phase. The XPS spectra of Fe 2p indicate that an appropriate doping of Sm³⁺ can stabilize the valence state of Fe³⁺, further stabilizing the antiferroelectric phase of NN-BF and reducing the generation of oxygen vacancies in the ceramic. This leads to an increase in the breakdown field strength.

Bi³⁺ in the A-site of the 0.9NaNbO₃-0.1BiFeO₃ lattice is gradually replaced by Sm³⁺, resulting in the appearance of the second phase Bi₂O₃. The low-melting-point Bi₂O₃ plays a role in promoting sintering during the firing process, thereby facilitating the grain growth. However, the second phase Bi₂O₃ is not resistant to breakdown, leading to a decrease in the breakdown field strength of the ceramic.

The stability of the antiferroelectric phase and the relaxor behavior of the ferroelectric phase in NN-BF ceramics with the Sm³⁺ doping amount of 0.02 were enhanced. An appropriate doping of Sm³⁺ could stabilize the valence state of Fe³⁺, further stabilizing the antiferroelectric phase of NN-BF ceramics. In an applied electric field of 300 kV/cm, the maximum polarization (P_max) of NN-BF ceramics (i.e., 17.83 μC/cm²), the minimum residual polarization (P_r) (i.e., 2.87 μC/cm²), and the maximized P_max-P_r (i.e., 14.96 μC/cm²) were obtained. The energy density and energy storage efficiency of NN-BF ceramics both reached their maximum values (i.e., 3.67 J/cm³ and 71.63%), respectively. The breakdown field strength of the ceramic was calculated to be 438 kV/cm through the Weibull distribution. These results indicated that NN-BF relaxor antiferroelectric ceramics with an appropriate Sm³⁺ doping could have promising potential applications in energy storage.

Dielectric ceramic capacitors are essential components in next-generation advanced pulse power systems due to their ultra-fast charging/discharging rates and remarkable power density. As a typical lead-free perovskite material, Sr_0.7Bi_0.2TiO₃ (SBT) has a larger permittivity rather than SrTiO₃ because Bi modification favors producing a high polarization due to the intense hybridization between the 6s² orbitals of Bi³⁺ and O 2p orbitals. Moreover, it also has a superior ferroelectric relaxor behavior with a diffused maximum dielectric constant in a wide temperature range, benefiting from Bi³⁺ off centering and Sr site vacancies. However, SBT ceramic still has a critical challenge for improving dielectric breakdown strength (E_b). Some previous studies indicate that adding some oxides can effectively inhibit grain growth and increase E_b. In this paper, the SBT ceramics were synthesized by a conventional solid-state sintering method with ZrO₂ as an additive. A part of Zr⁴⁺ was deposited on the grain boundary in the form of oxides to inhibit grain growth. The breakdown strength and the energy storage density of the ceramics with different ZrO₂ contents were analyzed.

Bi₂O₃(99.5%, in mass. The same below), SrCO₃(99.5%), TiO₂(99.8%), and ZrO₂(99.9%) were used as raw materials. The raw materials were weighed according to the stoichiometric ratio. The materials were mixed with anhydrous ethanol and ground in a ball mill with zirconium balls for 12 h. The slurry was dried in an oven. After pre-firing at 850 ℃ for 5 h, the powder was further ground for 24 h according to the corresponding mass ratio Sr_0.7Bi_0.2TiO₃+xZrO₂ (SBT-xZr, x= 0, 0.5%, 1.0%, 2.0%, 5.0%, in mass fraction). The dried ground powder was pressed into sheets with a thickness of 0.8 mm and a diameter of 12 mm, and then sintered at a rate of 4 ℃/min at 1 200-1 250 ℃ for 3 h, and then cooled down naturally.

Based on the XRD patterns and mapping energy spectra, element Zr partially dissolves in the SBT lattice in the form of Zr⁴⁺, and the rest exists in the form of ZrO₂ on the grain boundary and surface, showing the formation of a 0-3 type composite structure. The dielectric constant of SBT-2%Zr ceramic consistently exhibits a high value, while the low dielectric loss mitigates excess waste heat, which is beneficial to the energy storage performance. The heightened energy storage density primarily stems from the increased E_b value. With the augmentation of ZrO₂ content, the E_b value of the SBT-2%Zr increases, reaching a peak of 580 kV/cm, which is 27.6% higher than that of pure SBT ceramic. Consequently, the recoverable energy storage density (W_rec) experiences an elevation from 4.07 J/cm³ for the ceramic without x to 5.57 J/cm³ for the ceramic with x of 2%, while energy efficiency (η) maintains 88.97%. However, for the ceramic with x of 5%, despite achieving a high E_b value, the introduction of excessive ZrO₂ leads to a notable reduction in maximum polarization (P_max), preventing the attainment of the maximum W_rec value. The average grain size decreases from 2.30 μm to 1.31 μm, facilitating the attainment of more highly insulated grain boundaries. Also, the band gap energy (E_g) increases from 2.94 eV for the SBT ceramic to 3.02 eV for the SBT-5%Zr ceramic. This signifies that electrons require a larger applied electric field to achieve the valence band and conduction band transition. The expansion of the impedance circle indicates a higher resistivity of the material, and the elevated activation energy (E_a) effectively restrains the transmission of charged carriers. These interrelated mechanisms are crucial factors contributing to the substantial enhancement of E_b. In addition, SBT-2%Zr ceramic also shows an excellent fatigue stability (i.e, 5×10⁴ cycles, W_rec change <1%) and a good charge-discharge performance (i.e., discharge energy density W_d=2.15 J/cm³, current density C_D=1 060.51 A/cm² and power density P_D=169.68 MW/cm³).

A lead-free 0-3 type relaxation ferroelectric ceramic based on SBT was prepared by a stable solid-state reaction method. The ceramics doped with different concentrations of ZrO₂ exhibited an original crystal structure with some Zr⁴⁺ in the lattice. The remaining Zr⁴⁺ existed in the form of a second phase, predominantly appearing at the boundaries and surfaces of grains. The increase of energy storage density mainly depended on the increase of E_b, and the possible physical mechanisms (i.e., grain size, band gap, impedance and activation energy) cooperated to promote the increase of E_b. SBT-2%Zr ceramic had the optimum energy storage performance, i.e., recoverable energy storage density (W_rec=5.57 J/cm³) and energy efficiency (η=88.97%). After 5×10⁴ cycles, SBT-2%Zr ceramic with a good performance stability (i.e., W_rec change amplitude <1%) could be used as a promising candidate material for pulse power energy storage capacitors.

Polymer dielectrics play a crucial role in contemporary electronics because of their flexibility, low cost, high operating voltage, corrosion resistance, self-healing, etc., which are widely used in oil and gas exploration, and electric vehicles at operating temperatures (>105 ℃). However, polymers necessitate the maintenance of a substantial volume during usage due to their low dielectric constant, thereby resulting in a diminished energy storage density. Furthermore, polymers demonstrate a relatively low thermal conductivity, and are susceptible to failure when exposed to high-temperature conditions. To function effectively in high-temperature conditions exceeding 105 ℃, an extra refrigeration system must be incorporated into an electronic power system. This undoubtedly results in an increase in both energy consumption and overall weight of the system. Consequently, developing a new generation of polymer-based film capacitors with superior high-temperature energy storage capabilities becomes a necessity. In this paper, we utilized a multidimensional synergistic concept to comprehensively improve the high-temperature energy storage performance of PI-based polymer dielectrics via combining the electrical and thermal advantages of inorganic materials and polyimide (PI) in different dimensions.

(BNNS-TiO₂-BNNS)/PI sandwich-structured composite films (denoted as BTB/PI film) were prepared by a casting method based on a concept of multidimensional synergy. Titanium dioxide (TiO₂) with a high dielectric constant was prepared into a one-dimensional nanofiber structure by an electrostatic spinning method with PI as an interlayer of BTB/PI film. Electrically insulating and thermally conductive boron nitride was prepared into a two-dimensional nanosheet structure by a liquid phase exfoliation method with PI as an insulating and thermally conductive outer layer of the BTB/PI film. The frequency-/temperature-dependence of the films were examined by a model LRS-003 high and low temperature cooling and heating system and a model E4980 precision LCR meter. The PE loops for the films were determined by a linked test rig consisting of a model ZJ-6A quasi-static d33 tester, a model MODEL 610C Trek HV amplifier and a model BWT-001 variable temperature test rig at 25 ℃ and 200 ℃, respectively.

The (BNNS-TiO₂-BNNS)/PI sandwich composite films with the thicknesses of 22-25 μm prepared are dense and uniform, without obvious holes and obvious delamination, having superior dielectric properties, breakdown strength and high-temperature energy storage properties. The results by frequency-/temperature-dependent tests indicate that the dielectric constant of BTB/PI films increases with the increase of BNNS content. Compared with the dielectric constant of pure PI film at 1kHz (i.e., 3.12), BTB/PI-3, BTB/PI-5, 2BTB/PI-7 and BTB/PI-10 improve to 4.20, 4.37, 4.67 and 4.92, respectively. The dielectric loss of the composite film at 10³-10⁶ Hz and 25-200 ℃ is <0.025. From the breakdown strength of the material, the carriers inside the composite film have greater energy and concentration at high temperatures, making the PI chain more vulnerable to attack and breakage. The E_b of pure PI film thus decreases from 380 MV/m to 337 MV/m and the β reduces from 14.60 to 8.04 when the temperature increases from room temperature to 200 ℃. However, the E_b and β of BTB/PI-5 and BTB/PI-7 are higher than those of the pure PI films at the tested temperature. At an optimal BNNS volume ratio, BTB/PI-7 achieves the maximum E_b and β at 25, 100, 150 ℃, and 200 ℃. This can be attributed to the incorporation of wide-band, high-thermal-conductivity BNNS, preventing the composite films from experiencing breakdown caused by electrical or thermal runaway. At 25, 100, 150 ℃ and 200 ℃, the energy storage performance of BTB/PI film is higher than that of pure PI film. At an optimum BNNS volume ratio, BTB/PI-7 film achieves the maximum energy density and maintains a stable high efficiency in the tested temperature range. The energy density and efficiency of BTB/PI-7 film are 4.61 J/cm³ and 88.75% (at 465 MV/m) at 25 ℃, 3.95 J/cm³ and 88.08% (at 450 MV/m) at 100 ℃, 3.65 J/cm³ and 88.53% (at 440 MV/m) at 150 ℃, 3.04 J/cm³ and 88.00% (at 380 MV/m) at 200 ℃.

The polymer-based composite films were obtained to enhance the high temperature resistant energy storage performance based on the multidimensional synergistic design. The BTB/PI film exhibited stable frequency/temperature-dependence with a dielectric loss of lower than 0.025 at 10³-10⁶ Hz and 25-200 ℃. At 200 ℃, BTB/PI-7 achieved a high energy density of 3.04 J/cm³. Moreover, the energy efficiency of BTB/PI-7 was higher than 88% at 25, 100, 150 ℃ and 200 ℃. It was demonstrated that different materials with advantages (i.e., high polarization, great breakdown strength, and high temperature resistance) were obtained via incorporating various dimensional fillers into the polymer and implementing rational structural design. Consequently, the high temperature-resistant energy storage performance of PI-based composite films could be significantly enhanced.