Film capacitors are the core electronic components of modern power devices and electronic equipment. However, due to the low dielectric constant, it is difficult to obtain high energy storage density (effective energy storage density or discharged energy density) for present film capacitors, leading to a large device size and high application cost. To improve the energy storage density of film capacitors, a nanocomposite approach is an effective strategy via combining high dielectric constant of the ceramic nanoparticles with high breakdown strength of the polymer matrix. Nevertheless, for single-layer structure of 0-3 polymer/ceramic composites, the dielectric constant and breakdown strength are difficult to be effectively enhanced at the same time, which limits the further improvement of energy storage density. To solve this contradiction, researchers have combined the composite film with high dielectric constant and high breakdown strength in a superposition to prepare 2-2 type multilayer composite dielectrics, which can achieve synergistic regulation of polarization strength and breakdown strength to obtain high energy storage density. The optimization of electric field distribution and the synergistic regulation of dielectric constant and breakdown strength can be achieved through mesoscopic and microstructural modulation of multilayer composite dielectrics. In this paper, the research progress of multilayer polymer-based composite dielectrics including ceramic/polymer multilayer structure and all-organic polymer multilayer structure in recent years is reviewed. Effect of multi-layer structure control strategy on the improvement of energy storage performance is emphasized. Moreover, enhancement mechanism of energy storage performance of polymer-based multilayer structure composite dielectric is summarized. Finally, challenges and development directions of multilayer composite dielectrics are discussed.

Ferroelectric superlattices are artificial film materials with layered periodic structure formed by an alternate growth of two or more ferroelectric materials or non-ferroelectric materials at unit cell scale. Ferroelectric superlattices can exhibit excellent ferroelectric, piezoelectric, dielectric, and pyroelectric properties due to the existence of a large number of heterogeneous interfaces and the remarkable interface effect, and even show new functional properties that are not available in their constituent materials. Therefore, ferroelectric superlattices not only provide an ideal platform for studying interactions between charges and lattices at the interface of complex oxide materials, but also play an indispensable role in the next generation of integrated ferroelectric devices. With the development of preparation and characterization methods, researchers can design and control the microstructure and chemical composition at atomic scale to improve the functional properties of ferroelectric superlattice thin films. Ferroelectric polarization is the most basic property of ferroelectric film materials. In addition to being used for information storage devices, ferroelectric polarization also plays an important role in regulating the energy conversion performance of integrated ferroelectric devices such as piezoelectric devices, photovoltaic devices and electrocaloric devices. Therefore, the ferroelectric polarization intensity of ferroelectric superlattices directly determines their functional characteristics and practical application value of integrated ferroelectric devices composed of them. In this short review paper, we firstly introduced the structural characteristics, classification and several typical functional characteristics of ferroelectric superlattices, and then focused on several factors affecting the polarization performance of ferroelectric superlattices based on recent research results, including strain effect, electrostatic coupling effect, defect effect, and period thickness. Finally, we looked forward to the future research directions in ferroelectric superlattices to provide reference for the research in this field.

Compared with other electric energy storage devices, dielectric capacitors made of dielectric composites have great advantages in fast charging and discharging capacity with high power density. A dilemma of improving the energy density of dielectric composites and synchronous optimizing their breakdown performance is becoming an intriguing research direction. To further adjust the contradiction between dielectric constant and dielectric breakdown performance, here a finite element numerical simulation based on dielectric breakdown model (DBM) was proposed to study the effect of the distribution of inorganic fillers on the electric field and breakdown damage morphology in flexible polydimethylsiloxane(PDMS) based dielectric composite system. The results show that a large dielectric difference is observed between filler and matrix, which indicates that polymer matrix with a large dielectric constant or inorganic filler with a small dielectric constant can realize reducing the size of the high electric field area at the interface and improving the breakdown resistance of the material. This study further reveals that the more dispersed structure of inorganic fillers, the more likely its dendritic damage channels tend to branch, indicating that this situation is conducive to the increase of damage sites of dielectric breakdown dendritic damage channels, the decrease of damage rate, and the improvement of breakdown resistance of materials. All above data demonstrate that this study provides certain guidance for the development of organic-inorganic dielectric composites with both high energy storage and excellent breakdown performance.

Industrial pulse energy storage multilayer ceramic capacitors (MLCC) are important components for the development and production of electronic starting devices in China. In view of the shortcomings of large size, short life and low reliability of organic film capacitors, SrTiO₃ and CaTiO₃ based pulse energy storage dielectric ceramics were prepared by traditional solid-state reaction method in this study. The effects of sintering aid doping and Sr²⁺/Ca²⁺ mutual doping on the dielectric properties of ceramic materials were studied, and the property of MLCC based on (Sr,Ca)TiO₃ were further prepared and investigated. The results show that the dielectric constant of SrTiO₃ materials can be improved by adding the sintering aid with a mass ratio of 1.0%, such as the introduction of trace Bi³⁺, while Bi³⁺ has no obvious effect on the CaTiO₃ based materials. Doping of Mn element can effectively inhibit the reduction of Ti⁴⁺ during high-temperature sintering and reduce dielectric loss. Moreover, the addition of sintering aid can effectively reduce the sintering temperature of ceramic powder and improve the compactness of the material. The MLCC prepared from (Sr_xCa_1-x)TiO₃ material can maintain high dielectric constant and low dielectric loss, at x=0.4, the dielectric loss tanδ=1.8×10^-4, the breakdown strength is 59.38 V/μm, and the high and low temperature discharge current change rate is ±7%, which shows good discharge stability. In addition, no matter it is at room temperature or high temperature (125 ℃), the sample has no failure after 1000 cycles of discharge experiment. Therefore, the as-obtained (Sr, Ca)TiO₃ based ceramic dielectric material can be a promising pulse capacitor with relatively excellent capacity stability and high reliability under different electric field strength.

Antiferroelectric (AFE) materials exhibit great potential in the application of high-performance dielectric energy storage capacitors due to their electric field-induced AFE-ferroelectric (FE) phase transition. However, the large hysteresis of field-induced phase transition makes it difficult to simultaneously achieve high energy-storage density (W_rec) and efficiency (η) for AFEs. This work improved the energy-storage performance of NaNbO₃-based lead-free AFE ceramics by introducing the third group Bi(Mg_0.5Ti_0.5)O₃ into 0.76NaNbO₃-0.24(Bi_0.5Na_0.5) TiO₃ to regulate its relaxation characteristics. Novel lead-free AFE ceramics, (0.76-x)NaNbO₃-0.24(Bi_0.5Na_0.5)TiO₃-xBi(Mg_0.5Ti_0.5)O₃, were prepared by a traditional solid-state reaction method. Their phase structure and microstructure as well as dielectric, energy-storage, and charge-discharge characteristics were studied. The results indicated that introduction of Bi(Mg_0.5Ti_0.5)O₃ obviously enhanced the dielectric relaxor behavior of the matrix without changing its AFE R-phase structure, which led to the significantly reduced polarization hysteresis. Especially, a linear-like polarization-field hysteresis loop with extremely-low hysteresis was obtained in the composition of x=0.050. At the same time, microstructure of the ceramic was effectively optimized, its dielectric constant decreased, and its breakdown strength had significant enhanced. As a result, a high W_rec=3.5 J/cm³ and a high η=93% were simultaneously achieved under a moderate electric field of 30 kV/mm in the x=0.050 ceramic. Moreover, the x=0.050 ceramic also exhibited excellent charge-discharge characteristics with a high-power density P_D=131(1±1%) MW/cm³, a high discharge energy density W_D=1.66(1±6%) J/cm³ and a fast discharge rate t_0.9<290 ns at 20 kV/mm. The charge-discharge properties maintained good stability within a wide temperature range of 25-125 ℃. These results indicate that 0.71NaNbO₃-0.24(Bi_0.5Na_0.5)TiO₃-0.050Bi(Mg_0.5Ti_0.5)O₃ ceramics can be expected to be applied in high-power energy-storage capacitors.

Dielectric thin film, one of the materials of which storage energy in the form of electrostatic field via dielectric polarization, can be widely used in electric equipment, due to their high power density and high charge/ discharge efficiency. Currently, the dielectric energy storage films perform lower energy density and weak temperature stability. In this work, 0.9BaTiO₃-0.1Bi(Mg_1/2Ti_1/2) O₃(0.9BT-0.1BMT) ferroelectric thin films were prepared via a Sol-Gel method on Pt/Ti/SiO₂/Si substrates and annealed in the range of 700-900 ℃ to realize high energy storage density and wide-temperature stability by introducing BMT. The effect of annealing temperature on phase composition and microstructure was investigated. The results show that denseness of thin films reduce obviously when the annealing temperature is over 750 ℃ and their grain size increases gradually with the increase of treatment temperature. Additionally, the thin films annealed at 750 ℃ display optimized comprehensive feature: room-temperature dielectric constant of ~399, loss tangent of ~5.79% at 1 kHz, and ∆C/C_{25 ℃} ratio only within ±13.9%. Meanwhile, relaxor value, γ≈1.96 calculated according to Currie-Weiss law consolidates that the thin films possess obvious relaxor characteristics. Results of energy storage shows that the max value of W_rec is ~ 51.9 J/cm³, and the τ_0.9 is below 15 μs at pulse charge measure. Moreover, results of temperature stability measurement show W_rec>20 J/cm³, η>65% (1600 kV/cm) and τ_0.9<7.2 μs from room temperature to 200 ℃, demonstrating that the film still exists high and stable energy storage under high temperature. Therefore, the ferroelectric thin film 0.9BT-0.1BMT prepared in this work has a promising applications in energy storage under high temperature environment.

Antiferroelectric materials have been extensively studied in the field of dielectric energy storage due to their ultra-high power density. Lead zirconate (PbZrO₃, PZO), as a prototype of antiferroelectric material, has been one of the most studied antiferroelectric materials, and research on enhancing energy storage performance of PZO-based materials is a hotspot of the current study. In this work, further improvement of the energy storage performance of PZO-based antiferroelectric thin films was realized by further doping small-radius Sr²⁺ into the A-site of the PZO perovskite structure on the basis of La³⁺-doped PZO. A series of antiferroelectric thin films of A-site La/Sr co-doped Pb_0.94-xLa_0.04Sr_xZrO₃ (Sr-PLZ-x, x = 0, 0.03, 0.06, 0.09, 0.12) were prepared by Sol-Gel method, and the effects of Sr²⁺ doping on the crystal structure and electrical properties such as ferroelectricity, energy storage, and fatigue properties of Sr-PLZ-x antiferroelectric films were systematically investigated. The results show that with the doping of Sr²⁺, the lattice constants are gradually reduced, and the saturation polarization of the films is first slightly increased and then maintained, but finally gradually decreased. The tolerance factors of Sr-PLZ-x films are reduced with increasing Sr²⁺ doping content, while the antiferroelectricities of the films are enhanced. Both the switching field and the breakdown strength are increased, resulting in an improved energy storage performance of Sr-PLZ-x films. At x=0.03, the energy storage performance of Sr-PLZ-x antiferroelectric film reaches the highest, with the energy storage density and efficiency are 31.7 J/cm³ and 71%, respectively. Meanwhile, the doping of Sr²⁺ also makes the fatigue characteristics of Sr-PLZ-x antiferroelectric films further improved. The x=0.12 antiferroelectric film exhibits only 3.4% and 2.7% degradation in energy storage density and energy storage efficiency after 10⁷ cycles. In summary, the method of A-site La/Sr co-doping can effectively improve the energy storage performance of PZO-based antiferroelectric films.

Potassium sodium niobate (K_0.5Na_0.5NbO₃, KNN) based ceramics can be widely used for pulsed power systems due to their fast charge-discharge rate, high transparency, wide range of working temperature, and long cycle life. Improving the electrical and optical property of KNN-based ceramics through modification is a research hotspot in this field. 0.825(K_0.5Na_0.5)NbO₃-0.175Sr_1-3x/2La_x(Sc_0.5Nb_0.5)O₃ (x=0, 0.1, 0.2, 0.3) (0.825KNN- 0.175SLSN) ceramics were synthesized by solid state method. The effect of La₂O₃ doping on the phase structure, microstructure, optical property, dielectric property, ferroelectric property and energy storage property of the ceramic was studied. The results indicated that the structure of 0.825KNN-0.175SLSN ceramics is pseudo-cubic phase with high symmetry. With increment of La₂O₃ content, the average grain size of 0.825KNN-0.175SLSN ceramics decreased, and the phase transition temperature (T_m) and saturation polarization intensity (P_max) increased and then decreased. 0.825KNN-0.175SLSN ceramics exhibit excellent transparency at x=0.3, the transmittance in the visible wavelength (780 nm) and near-infrared wavelength (1200 nm) ranges reaches 65.2% and 71.5%, respectively. The dielectric breakdown strength of 310 kV/cm and a recoverable energy density of 1.85 J/cm ³ are achieved at x=0.3.

0.96NaNbO₃-0.04CaZrO₃(NNCZ) ceramic shows stable double hysteresis loops at room temperature, but the property of energy density, energy storage efficiency and breakdown strength of NNCZ are terrible, which limit NNCZ to be used as energy storage materials. In this work, Fe₂O₃ was chosen to modify the energy storage property of NNCZ. (0.96NaNbO₃-0.04CaZrO₃)-xFe₂O₃ (NNCZ-xFe) antiferroelectric ceramics were prepared by traditional solid reaction method. The phase, morphology, dielectric property and energy storage property of NNCZ-xFe were characterized. The results indicated that the crystal structures of NNCZ-xFe ceramics were pure perovskite structure. The sintering temperature of NNCZ ceramic was decreased with addition of Fe₂O₃. With the increase of Fe₂O₃ content, the grain size of NNCZ-xFe were decreased firstly and then raised. The NNCZ-0.02Fe ceramic obtained the smallest grain size (5.04 μm) and the best energy storage property. The breakdown strength of NNCZ-0.02Fe was 230 kV/cm at room temperature (RT). The recoverable energy density and energy storage efficiency before breakdown were 1.57 J/cm ³and 55.74% respectively. At 125 ℃ and 180 kV/cm, the energy density of NNCZ- 0.02Fe was 4.53 J/cm ³. Fe₂O₃ doping decreased the sintering temperature of NNCZ ceramics, reduced the the migration rate of oxygen vacancies and inhibited the growth of grains. At the same time, it reduced the dielectric loss and improved the breakdown strength. The oxygen vacancies pinning made antiferroelectric phase switch to ferroelectric phase harder, avoided appearance dumbbell-shaped double hysteresis loops, so the energy storage efficiency was improved. This research shows that NNCZ-xFe has a good potential application in the field of dielectric energy storage.