High-Performance Dielectric Ceramic for Energy Storage Capacitors

In recent years, due to the depletion of fossil fuels, air pollution, carbon emissions, and other issues, it has become urgent to seek the development of renewable, non-polluting, and clean energy. However, these renewable energy sources are generally intermittent, so they should be converted into electrical energy for utilization and collection. Therefore, the key to solving the utilization of renewable energy is to find efficient and reliable electrical energy storage methods [1]. Compared with other energy storage devices, such as solid oxide fuel cells (SOFC), electrochemical capacitors (EC), and chemical energy storage devices (batteries), dielectric capacitors realize energy storage via a physical charge-displacement mechanism, functioning with ultrahigh power density (MW/kg) and high voltages, which have been widely used in military, civil, and scientific applications [2].

Polymer-based and ceramic-based dielectric materials are two main kinds of dielectric materials commonly used in recent years. Although polymer-based dielectric material possesses a high breakdown strength, it exhibits low dielectric constant temperature-sensitive and large leakage currents under high electric fields, which has limited their further applications at high temperature and/or under high electric fields [3]. By contrast, the ceramic-based dielectric materials possess excellent temperature stability and a long life-time and have therefore received extensive attention in recent years [4,5].

The energy storage performance of dielectric ceramic materials is closely related to the crystal structure of the material itself. According to the existence of dipoles, energy storage dielectric ceramics are divided into two types: linear dielectrics and nonlinear dielectrics. Linear dielectrics have a stable crystal structure without the presence of dipoles; therefore, they have relatively low dielectric constants and high breakdown strengths. Since the dielectric constant of the material is independent of the electric field, the energy storage density is proportional to the square of the applied electric field. The maximum energy storage density can be obtained if the breakdown of the electric field of the material is increased. The energy is completely released from the dielectric during charging and discharging [6].

According to different types of domains, nonlinear dielectrics are divided into paraelectrics, ferroelectrics, and antiferroelectrics [7]. There are interacting permanent dipoles in paraelectrics, but no domains. The dielectric constant of paraelectric materials is generally between ferroelectric materials and linear dielectrics [8]. Strontium titanate (SrTiO₃) is a typical perovskite-based paraelectric material with a cubic structure at room temperature, which has a relatively high dielectric constant (~250) and low dielectric loss (~0.01). Therefore, the modification of SrTiO3 is expected to obtain high energy storage density.

Unlike paraelectric dielectric materials, the polarization of a ferroelectric does not disappear after removing the applied electric field, because dipoles in ferroelectric domains have the same polarization direction and can be switched by an external electric field. This characteristic endows ferroelectric materials with a high dielectric constant, dielectric loss, and remanent polarization, and thus a small recoverable energy density. Furthermore, most ferroelectric dielectric ceramics have relatively low breakdown strengths. Therefore, conventional ferroelectric dielectric materials are not suitable for energy storage applications. Currently, the new ferroelectrics have put forward four kinds of promising ferroelectric ceramics, namely barium titanate (BaTiO₃), potassium–sodium niobate [(K,Na)NbO₃], sodium–bismuth titanate [(Bi_0.5Na_0.5)TiO₃], and bismuth ferrite (BiFeO₃) [9]. To improve the energy storage density of ferroelectric materials, relaxor ferroelectrics have attracted the attention of researchers. It has been found that relaxor ferroelectrics possess short-range ordered polar nano-regions (PNRs). These PNRs are very sensitive to external stimuli, and the dipoles can quickly return to the initial state after removing the external electric field, significantly reducing the remanent polarization and broadening the Curie peak dispersion of the material [10]. For example, transforming ferroelectric BT to the relaxor state by diffusing the ferroelectric–paraelectric phase transition favors the optimization of its electrostrictive strain and dielectric energy storage performance. Relaxor ferroelectrics not only have good energy storage density and temperature stability, but also exhibit high electric field stability and conduction activation energy. Therefore, relaxor ferroelectrics are promising for high-temperature energy storage.

Antiferroelectrics have unique double P-E loops and anti-parallel dipoles. At low electric fields, the antiferroelectric ceramics often possess negligible P_r and small hysteresis loss and transform into the ferroelectric phase after applying a high electric field and transform into the initial antiferroelectric state after removal. That is, antiferroelectrics can be observed accompanying a high P_max at higher electric fields because their anti-parallel dipoles are aligned to form the ferroelectric phase. Thus, the antiferroelectric ceramics are beneficial to obtaining high energy storage performance at high electric fields [11]. AgNbO₃ and NaNbO₃-based ceramic systems are considered as potential energy storage materials. A series of chemical modifications further increased the recoverable energy density (U_rec) values of AgNbO₃-based ceramics to a range of 2–4.5 J/cm³. An energy storage density (U_tot) of 7.35 J/cm³, and recoverable energy density (Urec) of 5.00 J/cm³, were achieved in NaNbO₃-based antiferroelectrics modified by Bi(Ni_0.5Sn_0.5)O₃ [12]. In conclusion, due to the emergence of double hysteresis loops, antiferroelectric dielectrics are promising materials for achieving high energy storage performance.

In addition to the structure of the material itself, the microscopic morphology, defect structure, and phase composition of ceramics also affect the energy storage performance of dielectric materials. For example, there are many interfaces (such as grain boundaries, phase boundaries, etc.) in the ceramics, and the interface structure will also affect the polarization, breakdown, and even domain motion of the ceramic. Many methods have been proposed for significantly optimizing the ESP of dielectric ceramics, including the introduction of a polymorphic phase boundary [13], doping with a relaxor ferroelectric phase [14], the formation of a fine-grain structure [15], the combination of the phases with different electric characteristics [16], and the construction of a core–shell structure [17], as well as adopting different sintering techniques [18].

The solid-phase method is the most common ceramic preparation method. It can realize element doping, multi-phase composite, and prepare ceramic materials with a uniform structure. However, the solid-phase method has difficulty realizing the interface design and control of ceramic materials. In addition, the large particle size synthesized by the solid-phase method cannot meet the demand for the miniaturization of modern electronic devices. As a result, the chemical preparation method attracts more and more attention, especially in the synthesis of ceramic materials with a core–shell structure. This method can suppress the growth of powder particles, generate heterostructures in ceramics, and avoid the introduction of impurities and uneven mixing problems during the preparation process. Nano-powders with a core–shell structure obtained by the chemical preparation method can combine the performance and function of both core material and shell material. Therefore, these particles can be used to control and design the properties of materials, such as controlling the surface chemical composition, stability and tuning of specific surface area, insulation, thermal properties, and so on [19,20].

The energy storage properties, i.e., total energy storage density (W_tot), recoverable energy storage density (W_rec), energy loss density (W_loss), and the energy storage efficiency (η), can be evaluated via ferroelectric hysteresis loops [4]. It is evident that an optimal energy density for dielectrics can be achieved in samples with small remnant polarization, large maximum polarization, and large breakdown strength. Generally, the polarization of dielectric ceramics is associated with the leakage current and defects of the systems, which can be improved by the appropriate doping and enhancement of the microstructure. Breakdown strength is related to band gap, electrical homogeneity, and conductivity, which can be regulated by decreasing the dielectric layer thickness, reducing porosity, decreasing grain size, and introducing a core–shell structure [21].

The energy storage densities obtained by calculating the integral area of the P-E hysteresis loop are generally higher than those calculated by the charge-discharge method [22]. This difference can be attributed to different mechanisms and their respective measurement frequencies. P-E hysteresis loops are usually measured at a low frequency and at the millisecond level, while the discharge process in charge-discharge measurements is done at the microsecond level. At high electric fields and high measurement frequencies, the dipoles are bound and aligned and cannot be fully switched and oriented. Furthermore, the rapid reorientation of domain walls leads to high viscous forces, resulting in more energy losses. Therefore, the recoverable energy storage density calculated from the charge-discharge method is lower than the value obtained from P-E loops. For pulsed power applications, where the capacitor is required to release the stored energy as quickly as possible, the charge-discharge method is more reasonable for evaluating the performance of the capacitor. The maximum current I_peak during discharge and the 90% discharge time t_0.9 are often used to characterize the energy storage capacity of capacitors under high pulse power.

High energy storage density dielectrics significantly reduce device volume (increase volumetric efficiency), and play a crucial role in realizing device miniaturization, lightening, integration, and reducing production costs. It is worth noting that if the energy storage density of dielectric capacitors can be made comparable to that of electrochemical capacitors, the application of dielectric capacitors in the field of energy storage will be greatly expanded.