Antiferroelectric capacitor for energy storage: a review from the development and perspective

With the rapid development of electronics industry, the demand for dielectric energy storage devices is becoming more and more urgent, such as hybrid electric vehicles, laser weapons, space vehicle power systems, and cardiac defibrillators as shown in Fig. 1^[1–3], because they have fast charging/discharging speed, high power density and excellent fatigue resistance^[4]. Various types of dielectric materials can be potential candidates for energy storage, including antiferroelectrics (AFEs)^[5–7], relaxor ferroelectrics (RFEs)^[8,9], normal ferroelectrics (FEs)^[10], and linear nonpolar dielectric materials^[11]. Among these dielectrics, AFE dielectrics, characterized by a double hysteresis loop, are favored for energy storage due to their relatively high maximum polarization (P_max) and particularly low remanent polarization (P_r) compared with other types of dielectrics. In this paper, we summarized the energy storage density (U_rec) and efficiency (η) of AFE dielectrics systems in recent 10 years as shown in Fig. 2, and reviewed their industrialization application potential from the properties, economics, and environment.

Fig. 1  The application of dielectric energy storage materials in pulsed-discharge and power conditioning electronic devices.

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Fig. 2  Comparison of energy-storage properties among four typical anti-ferroelectric ceramics in recent 10 years.

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Lead-based antiferroelectrics

Lead-based AFE ceramics that possess excellent U_rec and η, like (Pb,La)(Zr,Ti)O₃ system^{[5,6,12–14]}, have been the mainstay energy storage materials. Even so, the U_rec of lead-based AFE ceramics is just about 0.55 J cm⁻³ in the early days, and increases to 13.9 J cm⁻³ at present, along with η > 90 %. From the whole development process as shown in Fig. 2a^[15–26], the improvement of energy storage performances can be summarized in the following three aspects. Firstly, increasing the electric field of AFE-FE phase transition, such as phase transition electric field of AFE-to-FE (E_F) and FE-to-AFE (E_A)^[27]; Secondly, designing a core shell^[28] or laminated composite structure^[26] to improve the breakdown strength (BDS); Thirdly, building a multiple phase transition^[22,29,30]. All these measures not only improve the U_rec of lead-based AFE ceramics, but also make their η significantly increased. Except for above bulk ceramics, multilayer capacitors (MLCCs) technique also is a great way to achieve high U_rec and η. For example, Hao et al.^[31,32] achieved a variety of ultrahigh U_rec > 15 J cm⁻³ and excellent η > 90% in lead-based MLCCs. However, with increasing environmental concerns, the adverse effect of lead oxide on the environment and human health will gain more and more attention, and the development of lead-free AFE energy storage capacitors have necessitated.

Bi_0.5Na_0.5TiO₃ based antiferroelectrics

Lead-free antiferroelectric materials have attracted increasing attention for environmentally friendly energy-storage applications in recent years^[33]. As one of the earliest studied lead-free energy storage ceramics Bi_0.5Na_0.5TiO₃ (BNT)-based AFE have a series of attractive features, including relatively large and tunable spontaneous polarization and high Curie temperature, which enables BNT to maintain large polarization over a wide temperature range. Yet, the U_rec of pure BNT ceramic is limited because of their high P_r and relatively low BDS. Therefore, from 2011 to 2020, researchers have been working on reducing the P_r and increasing the BDS by increasing relaxor behavior and decreasing the grain size ^[34–37]. Especially the strategies, such as defect engineering,^[38] field-induced structure transition^[39]_, multiscale polymorphic domains^[41], were adopted in recent years, making U_rec and η of BNT system markedly increased as shown in Fig. 2b^[35,38–44]. Besides, the feature of thin dielectric layer thickness also makes BNT based MLCCs possess an ultrahigh BDS and U_rec, especially for the <111>-textured Na_0.5Bi_0.5TiO₃-Sr_0.7Bi_0.2TiO₃ (NBT-SBT) MLCCs, which possess an ultrahigh BDS ~ 103 MV m⁻¹ and U_rec~21.5 J cm⁻³ because it has a greatly low tensile electro-strain^[45]. However, it is still not mature to be used to marketization, due to the complicated chemistry structure and high process cost for the texture.

AgNbO₃ based antiferroelectrics

The characteristic of double P-E hysteresis loops for AgNbO₃ (AN) was first reported in 2007, suggesting the practical possibility of its application in energy storage^[46]. However, the U_rec of pure AN ceramic is only about 2 J cm^{−3 [47,48]}, far lower than that of lead-based AFE systems ^[45,49,50]. The main reasons are that there have a non-zero P_r value and poor breakdown strength (BDS) at room temperature for pure AN ceramic. Enormous efforts ^[51–58] have been made as to solve these questions as following three aspects. One is using oxide dopants for compositional modification to suppress the ferroelectricity characteristic and boost the AFE one. Typical examples include AN+0.1wt%MnO₂ ^[51] and AN+0.1wt%WO₃^[47] systems, where the U_rec reaches 2.5 J cm⁻³ and 3.3 J cm⁻³, respectively. Another is ion substitutions, e.g., replacement of Ag⁺ by La^3+[48], Sm^3+[59], Ba^2+[60], Lu^3+[61], Gd^3+[62], etc., and/or Nb⁵⁺ by Ta^{5+ [52]}. The U_rec can be effectively increased to 3.2 J cm⁻³ in Ag_1-3xLa_xNbO₃ system at x=0.02^[48], 4.5 J·cm⁻³ in Ag_1−3xSm_xNbO₃ system at x=0.02^[59], 2.3 J cm⁻³ in Ag_1-2xBa_xNbO₃ system at x=0.02^[60], and 4.2 J cm⁻³ in Ag(Nb_1-xTa_x)O₃ system at x=0.15^[52], and so on. Reducing the thickness of the dielectric layer is the other efficacious strategy to enhance the BDS and U_rec of the AN system. For example, Zhu et al.^[63] indicates that an ultrahigh U_rec > 14 J cm⁻³ and excellent η > 85 % were achieved in Sm_0.05Ag_0.85Nb_0.7Ta_0.3 MLCCs. All these results suggest that AN AFE materials have a great application potential in pulsed-discharge and power conditioning electronic devices. However, it is difficult to meet the low-cost market requirement because of its high price for raw materials in AN system.

NaNbO₃ based antiferroelectrics

Sodium niobate (NaNbO₃, NN) system has also been attracted intensive interest because it has similar with AN system’s AFE perovskite structure. Different with the AN system, lead-free NN AFE ceramics has a raw material with low cost, simple sintering process, high band gap, and so on, which was considered the most promising dielectric capacitors. However, it is difficult to get double P-E hysteresis loops at room temperature, because its metastable FE phase (P21ma) and AFE phase (Pbma) can coexist at room temperature and the electric field induced FE phase can still be preserved after the electric field is removed, exhibiting a ferroelectric P-E hysteresis loop^[64–71]. An idea which was to stabilize the antiferroelectric phase and exploits its potential for AFE energy storage was achieved in NaNbO₃-CaZrO₃^[72,73], NaNbO₃-CaHfO₃^[74], and NaNbO₃-CaSnO₃^[75] systems, and so on with applying the strategy of reducing the tolerance factor and average B-site polarization. However, their U_rec are still very low because of low BDS and large P_r. For example, the U_rec and η of AFE NaNbO₃-CaZrO₃ system are just about 0.55 J cm⁻³ and 63% ^[76]. To further improve the U_rec and η, many of the strategies were adopted, such as breaking long-range ferroelectric order which is the benefit to build relaxation characteristics and improve BDS, decreasing the grain size or reducing the dielectric properties to enhance the breakdown field, and so on ^[77–87]. The U_rec and η of NN-based ceramics have gotten an obviously improvement as shown in Fig. 2d ^{[76–79,84,88–90]}. For example Qi et al.^[77] have reported that an 0.76NaNbO₃-0.24(Bi_0.5Na_0.5)TiO₃ relaxor antiferroelectric (AFE) ceramic was designed, which shown an ultrahigh energy-storage density U_rec = 12.2 J cm⁻³, which is higher that of other lead-free system and most of lead ceramic. Xie et al.^[78] have shown that the fine grain for 0.83NaNbO₃-0.17SrTiO₃ ceramics prepared by the two-step sintering method is the benefit to improve the U_rec and η, which are 1.60 J cm⁻³ and 50%, respectively. Besides, Shi et al.^[48] have reported that a PC-phase 0.78NN-0.22Bi(Mg_2/3Ta_1/3)O₃ ceramics has ultrahigh breakdown field about 627 kV cm⁻¹, which conduce to achieving high U_re= 5.01 J cm⁻³. Although the U_rec and η of NN-based ceramics have achieved a big breakthrough, they still have some shortcomings, such as the volatilization of Na ions at high temperature, complex phase transitions and sample poor stability, which is not conducive to its practical application.

Other properties

Except the U_rec and η, the cost of raw materials for dielectric capacitors is also another critical factor in whether it can be used to industrialization application. Herein, the prices of main four typical antiferroelectric raw materials (data from McLean and Aladdin website) are listed in Fig. 3a. Ag₂O and HfO₂ are the most expensive than others in these raw materials, which lead to the AgNbO₃-based and PbHfO₃-based AFE ceramics exhibited high raw material cost as shown in Fig. 3b ^{[22–24,32,37,40,43,55,56,77,91–95]} that is not conducive to their practical application. Besides, the temperature stability is also the crucial parameters to make sure the practical application of dielectric capacitor. The temperature stability of Bi_0.5Na_0.5TiO₃-based and AgNbO₃-based systems is better than that of NaNbO₃-based and Pb-based capacitors as shown in Fig. 3c ^{[30,40,58,92,94,96,97]}. This result suggests that Bi_0.5Na_0.5TiO₃-based and AgNbO₃-based ceramics can be used in more complex environments conditions. Fig. 3d presents the comparison of U_rec and η of these typical AFE ceramics ^{[19–24,26,27,29,30,35,37,39,40,48,54–56,59,77,88–90,92,94,98–127]}. The Pb-based capacitor possesses the highest U_rec than others, and an excellent η. In addition, the U_rec and η of Bi_0.5Na_0.5TiO₃-based and NaNbO₃ ceramics are higher than that of AgNbO₃-based systems.

Fig. 3  a The prices of commonly used oxides. Comparison of b raw material price, c temperature stability and d energy storage performance of four typical antiferroelectric ceramics reported recently.

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Outlook and perspective

To meet the practical applications, the energy storage capacitors are needed to possess not only the high U_rec and η values, but also the excellent temperature stability and the low cost as well as environment friendly, and so on. The advantages and disadvantages of four typic antiferroelectric systems were shown in Table 1. Although the Pb-based AFE capacitors have a significant advantage in U_rec and η, its widespread adoption will be restricted because the hazardous element exists. In addition, AgNbO₃-based ceramic has an ultrahigh BDS, high U_rec and η, as well as excellent temperature stability, but it is also difficult to be used to marketization due to high cost for raw materials. However, for NaNbO₃-based and Bi_0.5Na_0.5TiO₃-based AFE systems, due to low cost for the raw materials, eco-friendly, high U_rec > 10 J cm⁻³ and η > 85 %, as well as excellent temperature stability, they have a great potential in pulsed-discharge and power conditioning electronic devices as the energy storage capacitors.

Table 1.  The advantages and disadvantages of four typic antiferroelectric systems.

	Pb-based	Bi0.5Na0.5TiO3-based	AgNbO3-based	NaNbO3-based
Urec	excellent	poor	well	well
η	well	excellent	poor	well
Materials price	low	low	high	midium
Human health	poor	excellent	excellent	well

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