Compared with fuel cells and electrochemical capacitors, dielectric capacitors are regarded as promising devices to store electrical energy for pulsed power systems due to their fast charge/discharge rates and ultrahigh power density. Dielectric materials are core components of dielectric capacitors and directly determine their performance. Over the past decade, extensive efforts have been devoted to develop high-performance dielectric materials for electrical energy storage applications and great progress has been achieved. Here, we present an overview on the current state-of-the-art lead-free bulk ceramics for electrical energy storage applications, including SrTiO3, CaTiO3, BaTiO3, (Bi0.5Na0.5)TiO3, (K0.5Na0.5)NbO3, BiFeO3, AgNbO3 and NaNbO3-based ceramics. This review starts with a brief introduction of the research background, the development history and the basic fundamentals of dielectric materials for energy storage applications as well as the universal strategies to optimize their energy storage performance. Emphases are placed on the design strategies for each type of dielectric ceramic based on their special physical properties with a summary of their respective advantages and disadvantages. Challenges along with future prospects are presented at the end of this review. This review will not only accelerate the exploration of higher performance lead-free dielectric materials, but also provides a deeper understanding of the relationship among chemical composition, physical properties and energy storage performance.

High-performance lead-free bulk ceramics for electrical energy storage applications: design strategies and challenges

Abstract Next-generation advanced high/pulsed power capacitors rely heavily on dielectric ceramics with high energy storage performance. However, thus far, the huge challenge of realizing ultrahigh recoverable energy storage density ( W rec ) accompanied by ultrahigh efficiency ( η ) still existed and has become a key bottleneck restricting the development of dielectric materials in cutting-edge energy storage applications. Here, we propose a high-entropy strategy to design “local polymorphic distortion” including rhombohedral-orthorhombic-tetragonal-cubic multiphase nanoclusters and random oxygen octahedral tilt, resulting in ultrasmall polar nanoregions, an enhanced breakdown electric field, and delayed polarization saturation. A giant W rec ~10.06 J cm −3 is realized in lead-free relaxor ferroelectrics, especially with an ultrahigh η ~90.8%, showing breakthrough progress in the comprehensive energy storage performance for lead-free bulk ceramics. This work opens up an effective avenue to design dielectric materials with ultrahigh comprehensive energy storage performance to meet the demanding requirements of advanced energy storage applications. 

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Giant energy-storage density with ultrahigh efficiency in lead-free relaxors via high-entropy design

The growing demand for high-power-density electric and electronic systems has encouraged the development of energy-storage capacitors with attributes such as high energy density, high capacitance density, high voltage and frequency, low weight, high-temperature operability, and environmental friendliness. Compared with their electrolytic and film counterparts, energy-storage multilayer ceramic capacitors (MLCCs) stand out for their extremely low equivalent series resistance
and equivalent series inductance, high current handling capability, and high-temperature stability. These characteristics are important for applications including fast-switching third-generation wide-bandgap semiconductors in electric vehicles, 5G base stations, clean energy generation, and smart grids. There have been numerous reports on state-of-the-art MLCC energy-storage solutions. However, lead-free capacitors generally have a low-energy density, and high-energy density
capacitors frequently contain lead, which is a key issue that hinders their broad application. In this review, we present perspectives and challenges for lead-free energy-storage MLCCs. Initially, the energy-storage mechanism and device characterization are introduced; then, dielectric ceramics for energy-storage applications with aspects of composition and structural optimization are summarized. Progress on state-of-the-art energy-storage MLCCs is discussed after elaboration of the fabrication process and structural design of the electrode. Emerging applications of energy-storage MLCCs are then discussed in terms of advanced pulsed power sources and high-density power converters from a theoretical and technological point of view. Finally, the challenges and future prospects for industrialization of lab-scale lead-free energy-storage MLCCs are discussed.

Perspectives and challenges for lead-free energy-storage multilayer ceramic capacitors

Sr0.7Bi0.2TiO3 (SBT) is a promising pulse energy storage material due to minor hysteresis, but its low maximum polarization (Pmax) is bad for energy storage. K+-Bi3+ defect pairs were introduced into the A-site of SBT to obtain Sr0.35Bi0.35K0.25TiO3 (SBKT) with larger Pmax. Through first-principles calculations, we determined that the introduction of defect pairs destroys the paraelectric order phase and increases local polarization, resulting in more and larger polar nanoregion (PNR) formation. On this basis, doping NaNbO3 (NN) in A- and B-sites of SBKT increases the cationic disorder and ferroelectric destabilization, further destroying the long-range order structure and forming more PNRs with smaller sizes. This enhances relaxation and decreases remnant polarization, and the broadened dielectric peak enables 0.85SBKT-0.15NN to meet the X7R specification. Furthermore, the decreased grain size and oxygen vacancy, increased thermal conductivity, and weakened local electric field (simulated by COMSOL) increase the dielectric breakdown strength (BDS). As a result, 0.95SBKT-0.05NN exhibits a high energy storage density (W) of 2.45 J/cm3 with a high efficiency of 93.1%, a high pulsed discharge energy density of 2.1 J/cm3, and a high power density of 54.1 MW/cm3 at 220 kV/cm. The energy storage properties show excellent stability of temperature (-55 to 150 °C), frequency (10-500 Hz), and cycling (105 cycles). Notably, for the pulse charge-discharge properties, 0.95SBKT-0.05NN shows great fatigue resistance during 105 cycles under 25 and 150 °C, accompanied by excellent thermal stability. Moreover, the BDS and Pmax of 0.95SBKT-0.05NN sintered in O2 further enhance. A higher W of 2.92 J/cm3 with a high efficiency of 89% at 250 kV/cm is achieved. Therefore, 0.95SBKT-0.05NN shows great application potential for pulse energy storage. In this work, we provide a novel strategy and systematic in-depth study for improving the energy storage properties of SBT.

Aliovalent Doping Engineering for A- and B-Sites with Multiple Regulatory Mechanisms: A Strategy to Improve Energy Storage Properties of Sr0.7Bi0.2TiO3-Based Lead-Free Relaxor Ferroelectric Ceramics.

Enhanced breakdown strength and energy storage density of lead-free Bi0.5Na0.5TiO3-based ceramic by reducing the oxygen vacancy concentration

Energy storage performance of Bi0.5Na0.5TiO3-based relaxor ferroelectric ceramics with superior temperature stability under low electric fields

Enhanced energy storage density of Sr0.7BixTiO3 lead-free relaxor ceramics via A-site defect and grain size tuning

The urgent requirement of environment-friendly materials with excellent energy storage performance for pulse power systems has sparked considerable research on lead-free ceramics. In this work, a new lead-free 0.90(0.80NaNbO3–0.20Sr0.7Bi0.2TiO3)–0.10BaSnO3 ceramic with high recoverable energy storage density (Wr = 3.51 J/cm3) and decent energy storage efficiency (η = 70.85%) has been obtained. In particular, these ceramics exhibit an ultrahigh breakdown strength of 402 kV/cm due to the dense microstructure and small grain size. The impedance analysis also reveals that the incorporation of BaSnO3 is conducive to the enhancement of insulation ability and breakdown strength. Additionally, great thermal stability (ΔWr < 10% over 20–120 °C at 200 kV/cm) and fatigue resistance (ΔWr < 1% after 120,000 electrical cycles at 200 kV/cm) are observed, indicating that the 0.90(0.80NaNbO3–0.20Sr0.7Bi0.2TiO3)–0.10BaSnO3 ceramics have promising application prospect for high-temperature energy storage devices in pulse power applications.

Siyi Li

Papers

Energy storage performance of Bi0.5Na0.5TiO3-based relaxor ferroelectric ceramics with superior temperature stability under low electric fields

Enhanced energy storage density of Sr0.7BixTiO3 lead-free relaxor ceramics via A-site defect and grain size tuning

Enhanced energy storage properties in lead-free NaNbO3–Sr0.7Bi0.2TiO3–BaSnO3 ternary ceramic