Progress and perspectives in dielectric energy storage ceramics

Abstract: Dielectric ceramic capacitors, with the advantages of high power density, fast charge–discharge capability, excellent fatigue endurance, and good high temperature stability, have been acknowledged to be promising candidates for solid-state pulse power systems. This review investigates the energy storage performances of linear dielectric, relaxor ferroelectric, and antiferroelectric from the viewpoint of chemical modification, macro/microstructural design, and electrical property optimization. Research progress of ceramic bulks and films for Pb-based and/or Pb-free systems is summarized. Finally, we propose the perspectives on the development of energy storage ceramics for pulse power capacitors in the future. 

Keywords: energy storage ceramics; dielectric; relaxor ferroelectric; antiferroelectric; pulse power capacitor

1 Introduction

Electric energy, as secondary energy, plays a dominant role in human daily life, industrial manufacture, and scientific research owing to its cost-effectiveness, versatility, and convenient transportation. Compared with traditional fossil fuels, electrical energy generated from renewable resources can effectively cope with resource depletion and reduce environmental pollution. However, the characteristics of intermittence, fluctuation, and randomness result in a time and space difference between practicality and expected demand, and thereby seriously hinder its large-scale development and application [1]. It is urgent to develop advanced technologies to address the issue of electric energy storage and conversion. Currently, the researches of energy storage technologies are mainly concentrated on dielectric capacitors [2,3], electrochemical capacitors [4], batteries [5], and solid oxide fuel cells [6], whose corresponding characteristics are given in Fig. 1. 

Fig. 1 Ragone pattern of different energy storage technologies. 

Ceramic capacitor, as a passive component, possesses high power density (~GW/kg), fast charge–discharge speed (μs, or even ns), well fatigue endurance (≥ 106 cycles), and high temperature stability, playing an indispensable role in solid-state power systems [1,7]. Generally, ceramic capacitors with a physical power supply based on dipole orientation, have relatively lower energy density than lithium-ion batteries and solid oxide fuel cells. Therefore, it is critical to improve the energy density of ceramic capacitors for expanding their practical applications. 

Polarization behavior of dielectric materials under external electric field can be characterized by P–E loops (hysteresis loops) [8,9], as exhibited in Fig. 2. According to different P–E loop characteristics, dielectric materials can be classified into linear dielectric, ferroelectric, and antiferroelectric, of which ferroelectric includes normal ferroelectric and relaxor ferroelectric. Based on basic principle and reported literature, the polarization of linear dielectric is linearly proportional to the electric field, whereas its relatively low dielectric constant (ɛr) makes it difficult to achieve high energy density. Normal ferroelectric also possesses limited energy density because of its high remanent polarization (Pr). Relaxor ferroelectric and antiferroelectric could achieve both high energy density and efficiency, owing to their relatively high maximum polarization (Pmax), low remanent polarization (Pr), and moderate breakdown strength (Eb), and thus have been considered to be the most potential candidates for pulse power systems.

Fig. 2 Typical dependence of polarization and permittivity on electric field for (a) linear dielectric, (b) relaxor ferroelectric, and (c) antiferroelectric. Reproduced with permission from Ref. [2], © WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2017. 

Currently, the researches of energy storage ceramics are mainly concentrated on bulk (> 100 μm), thick film (1–100 μm), and thin film (< 1 μm). It should be noted that these three dielectric ceramics categories possess a big difference in actual energy storage capability, and thus one cannot treat them as one object in the same way. Meanwhile, the device application type also has different categories: ceramic bulk, multilayer structure capacitor, flexible electronic, integrated circuit, etc. This review combines the related work of authors, discusses the progress of energy storage performances of linear dielectric, relaxor ferroelectric, and antiferro-electric with emphasis on composition modification, macro/microstructural modulation, and electrical property optimization. 

2 Key parameters for evaluating energy storage properties 

2. 1 Energy storage density 

Generally, energy storage density is defined as energy in per unit volume (J/cm³), which is calculated by [2]

where W, E, Dmax, and dD are the total energy density, applied electric field, maximum electric displacement at E, and increment of electric displacement per unit of the electric field, respectively. For ceramic dielectric, D is an unmeasurable microscopic physical quantity, and is usually expressed by polarization as following:

D = ɛ0E+ P  (2)  

Meanwhile, P is dependent on E as follows: 

P = ɛ0χE   (3)  

where ɛ0 is the vacuum dielectric constant of 8.854 × 10−12 F/m, χ is the dielectric polarization coefficient, so that Eq. (2) can be expressed as 

D = ɛ0(χ+1)E=ɛ0ɛr(E)E   (4)

where ɛr(E) is the relative dielectric constant at E. Therefore, Eq. (1) after the change is as follows: 

It is obvious that high ɛr and high Eb are important factors for achieving high W [10]. However, these two factors are hard to obtain simultaneously in a given material due to the trade-off relationship. 

It should be noted that W is a sum of effective energy density (Wrec) and energy loss (Wloss) [8,11]. In practice, Wrec is more important than W in evaluating the energy storage performances of dielectric materials. As shown in Fig. 2, Wrec is determined by the area enclosed by the discharge curve of its P–E loops and the polarization axis. The equation is given as follows: 

Obviously, high Pmax, low Pr (i.e., large ΔP = Pmax–Pr), and high Eb are essential factors to achieve high Wrec.  

2. 2 Energy storage efficiency 

Energy storage efficiency (η) is another important parameter to evaluate energy storage performances of dielectric materials, which is expressed as 

where Wloss is the energy loss during the discharge process, which equals to the area enclosed by the P–E loop in number. Wloss is mainly dissipated as heat, and a higher value means a stronger negative effect on the service life of ceramic capacitors. Therefore, it is vital that P–E loops gradually go slim to enhance η, and then improve its practical application. 

According to Eqs. (6) and (7), P–E loops go slim accompanied with high Pmax, low Pr (i.e., high ΔP = Pmax–Pr), and high Eb, which become the key issues in optimizing the energy storage characteristics of dielectric materials. There are two strategies: For one, 
to optimize the polarization behavior and strengthen their relaxor characteristics, which means that P–E loops go slim; for the other, to improve the breakdown behavior of dielectric ceramics, i.e., enhancing its Eb. This paper chooses linear dielectric, relaxor ferroelectric, and antiferroelectric as targets, and discusses the influences of chemical modification and macro/microstructural design on polarization behavior and breakdown strength of dielectric materials.

2. 3 Rapid charging–discharging characteristics 

Generally, energy storage performances of ceramic materials can be reflected by P–E loops measured by a modified Sawyer–Tower circuit. Meanwhile, the energy storage characteristics of ceramic capacitors, including effective discharging time (t0.9) and power density (P), are more accurately reflected by the charging–discharging curve recorded at a specific RLC circuit [12]. The simple equivalent circuit model is exhibited in Fig. 3(a). In the charging process (“1” connected to “3”), the potential difference (Φ) between two surfaces of capacitor equals the applied voltage (V), representing the charging process is finished. And then, the vacuum switches automatically and quickly rotate (“2” connected to “3”) to achieve the discharging process. Current flows to the oscilloscope via a load (R0), which is recorded as wave function as a function of time. The discharging density is given by 

where φ is the effective volume, τ is the relaxation time, and V(t) and I(t) are the voltage and current as function of time, respectively. Discharging current (I) and energy density (J) versus time (t) are shown in Fig. 3(b).  

Fig. 3 (a) Circuit of charging–discharging. (b) Discharging current (I) and energy density (J) versus time. Reproduced with permission from Ref. [1], © Elsevier Ltd. 2018.

It is reported that the value of J calculated by Eq. (8) is generally smaller than that of Wrec. For the reasons, it may be closely related to two factors. Firstly, the domains cannot switch and orientate promptly due to fast discharge speed, and thus the energy is not completely released [13]. Secondly, the circuit has equivalent series resistance that generates Joule energy [14]. In order to acquire higher value of J and Wrec, reducing domain size (e.g., polar nanoregions) only from material itself may be an effective method [15]

Power density (P) is also an important parameter of dielectric ceramic capacitors, which is determined as follows:

where f is the testing frequency, and the others are the same as those discussed above. It can be seen that high εr, high Eb, and low tanδ are the basic requirements of achieving high P. Meanwhile, P has an obvious dependence on frequency (f), which means high f corresponds to high P. In addition, t0.9 is an important factor for influencing the practical application of ceramic capacitors. The smaller the value of t0.9, the stronger the pulse current can be generated in a short time. It is worth mentioning that the value of t0.9 is dependent on not only the intrinsic properties of the dielectric materials, but also some external factors. For example, Li et al. [15] found that t0.9 can be controlled by adjusting the load (R0) in the circuit. 

2. 4 Reliability of work

No matter with dielectric material or pulse capacitor, the reliability of work is a crucial factor to influence its application scenes. Generally, reliability of work requires keeping stability of electric properties of material and device under thermal, electric, mechanical, magnetic, etc., external field stimulus. 

Temperature stability usually requires that dielectric properties and polarization of material have a gentle fluctuation as a function of temperature. For example, dielectric constant needs to maintain a ±15% variation over a temperature range from −55 to 125 ℃ (X7R), to 150 ℃ (X8R), and even to 300 ℃, especially in high temperature working environments. Therefore, one should know the Joule heat categories: dielectric loss or leakage current, and to solve corresponding issues. Generally, different electric field conditions such as cycle number, frequency, voltage category, magnitude, etc., all require keeping good energy storage stability. Specially, fatigue endurance is a very important issue to influence capacitor working capability. In 2009, Lou [16] reviewed different fatigue mechanisms in ceramic bulks, films, and single crystalline, which help us better understand fatigue behavior of material. In addition, mechanic and magnetic fields both influnence its polarization behavior of material, and its importance may be playing a crucial role in the future multifunctional coupling requirement. Therefore, the evaluation criterion of reliability of working should be a complicated project. 

3 Dielectric ceramics for energy storage capacitors 

As given in Fig. 2, dielectric materials mainly include three categories, namely linear dielectric, relaxor ferroelectric, and antiferroelectric. Therefore, we here compare and analyze the energy storage properties of some representative dielectric ceramic bulks and films. 

3. 1 Linear dielectric ceramics 

Linear dielectric ceramics usually possess characteristics of low εr and tanδ, as well as moderate Eb. It is thereby hard to obtain high Wrec despite under high electric field. In this regard, the researches of linear dielectric ceramics are mainly concentrated on increasing εr or improving polarization behavior based on maintaining high Eb

3.1.1 TiO2 based ceramics 

TiO2 is a typical linear dielectric, with characteristics of moderate Eb (> 350 kV/cm), low tanδ (< 0.1%), dielectric constant (~110), and wide band gap (~3.2 eV), and thus receives wide use in contemporary electronic ceramic industries and photocatalysis field [17–20]. Generally, TiO2 has three crystal structures including orthorhombic brookite, tetragonal rutile, and anatase. The rutile phase is more stable and easier to be synthesized than others, and hence gains more attention in ceramic bulks and films [21,22]

In 2013, Hu et al. [23] firstly reported that (In, Nb) co-doped TiO2 ceramics displayed a giant dielectric constant (> 104) as well as low tan (< 5%), and possessed good temperature and frequency stabilities over a wide temperature range (80–450 K). The authors claim that this phenomenon should be closely related to defect clusters and localized electrons, and thus propose a “localized defect polarization” mechanism. However, the specified reason of giant dielectric constant appearing in donor/acceptor co-doped TiO2 ceramics still has some controversies. Li et al.[24,25] attempted to explain the phenomenon by using an internal barrier layer capacitance (IBLC) model, similar to CaCu3Ti4O12 (CCTO) ceramics from extrinsic factors influencing dielectric constant. Actually, most of subsequent discussion on the physical origin of different-type donor/acceptor co-doped TiO2 ceramics is basically around above two mechanisms [26–28], even though no significant progress has been achieved in Wrec for donor/acceptor co-doping TiO2 ceramics because of the expense of a rapid reduction in Eb

In another way, improving the sintering behavior of TiO2-based ceramics would be effectively enhancing Eb, such as refining grain size [17,29], adding glass phase [30], applying special sintering technology (SPS), and so on. Liu et al. obtained a Wrec of 1.15 J/cm³ for alkali-free glass modified TiO2 ceramics at 501.7 kV/cm. Unfortunately, Wrec of those ceramic bulks after different modification ways is still only at a scale of 1 J/cm³, which is less than other energy storage dielectric materials. It should be noticed that high quality TiO2 film is a good direction owing to their high Eb. For instance, Chao and Dogan [31] fabricated 0.1-mm thick TiO2 films using tape-casting method, which achieve Wrec of 14 J/cm³ at 1400 kV/cm. 

3.1.2 SrTiO3 based ceramics 

SrTiO3 (ST) is a cubic paraelectric phase with ABO3 type perovskite structure, accompanying to space group of Pm 3 m and lattice constant of a = 3.905 Å [32,33]. ST ceramics possess moderate εr of ~300, Eb of ~100 kV/cm, and low tanδ of ~10−3, as well as good temperature-, frequency-independent dielectric properties, bias voltage stability, and thermoelectric energy conversion efficiency [34–36]. Therefore, it is considered to be a potential energy storage and conversion candidate. Compared with TiO2 ceramic (~110) and polymer linear dielectric (< 10), ST has an advantage of relatively high εr, and thus is more suitable for pulse capacitor.

Utilizing Ca2+, Ba2+, and Pb2+ ions to replace Sr2+ on the A-site of ST could adjust the Curie temperature (TC) to room temperature, and thus dielectric constant could be enhanced [37,38]. Especially, BaxSr1−xTiO3 (BST) solid solutions combine the characteristics of high Eb of SrTiO3 and high εr of BaTiO3, and receive much more attention in recent years [39]. The structure and performance of BST can be adjusted over a wide range to meet the requirements of different applications. As the molar fraction of Ba increases from 0 to 1, phase composition of BST varies from cubic paraelectric (ST) to tetragonal ferroelectric (BT), accompanying by the TC increase from near absolute 0 to ~393 K. According to theoretical calculation of BST solid solution by Fletcher et al. [40], it is easier to obtain an ideal energy storage property if the TC of the ceramic composition is far away the working temperature. Thereby, BST compositions with x ≤ 0.4 would be more suitable pulse power capacitor candidates because P–E loops display linear or weak nonlinear characteristic at room temperature, as shown in Fig. 4. In 2015, Wang et al.[37] investigated energy storage performances of BaxSr1−xTiO3 (x ≤ 0.4) ceramics, and found that Ba0.4Sr0.6TiO3 achieved the highest Wrec, while relatively low and rapidly decreased η becomes a serious problem to hinder its application. By comparison, Ba0.3Sr0.7TiO3 possessed moderate Wrec, high η (≥ 95%), and very low dielectric loss (tanδ = 7.6×10−4 @ 1 kHz), making it more suitable for the fabrication of solid state compact portable pulse power electronics. 

Fig. 4 Polarization–electric field (P–E) hysteresis loops of BaxSr1−xTiO3 (BST, x ≤ 0.4) ceramics. The inset shows the effective energy storage and energy loss during the charge–discharge process. Reproduced with permission from Ref. [37], © Elsevier Ltd and Techna Group S.r.l. 2015. 

For aliovalent doping in A-site of ST, a similar phenomenon with giant dielectric TiO2 can be observed, and corresponding mechanisms are widely studied. Chen et al. [41–43] reported (Bi,Sr)TiO3 ceramics with giant dielectric constant, discussed the related physical mechanisms, and proposed that the first and second ionization of oxygen vacancies as well as corresponding thermal movement were the main reasons. To avoid the problem of Bi-containing oxides volatilizing at high temperature, Shen et al. [34,44–46] used trivalent nonvolatile rare earth ions (Re3+ = La, Sm, Gd, Er, Nd, etc.) to replace Sr2+, and designed three composition formulas based on three possible charge compensation mechanisms such as equimolar substitution, introducing Sr or Ti vacancy in advance, and successfully fabricated ceramics with perovskite structure. It is experimentally verified the feasibility of introducing ion vacancies in advance for charge compensation, and then the concept of “forced charge compensation mechanism” is summarized and proposed. For instance, Shen et al. [44] synthesized Re0.02Sr0.97TiO3 ceramics by introducing Sr vacancy in advance, displaying a high dielectric constant and good bias voltage stability, as shown in Figs. 5 and 6. Furthermore, multiple mechanisms such as Maxwell–Wagner interface polarization, variable charge of Ti element, defect clusters, etc., are all proposed to illustrate donor and/or acceptor doping ST ceramics. 

Fig. 5 εr and tanδ of the Re0.02Sr0.97TiO3 ceramics as a function of measuring temperature from −60 to 200 ℃. Reproduced with permission from Ref. [44], © The American Ceramic Society 2013. 

Fig. 6 εr of the Re0.02Sr0.97TiO3 ceramics as a function of bias electric field. Filled pattern: increasing bias. Open pattern: decreasing bias. Reproduced with permission from Ref. [44], © The American Ceramic Society 2013. 

Microstructure regulation plays an important role in enhancing Eb of ST ceramics. In 2014, Song et al. [47] prepared Ba0.4Sr0.6TiO3 ceramics with various grain sizes (0.5–5.6 μm), and observed that dielectric peak gradually depressed and broadened and Eb gradually increased with decreasing grain size, which should be closely related to the ratio of grain/grain boundary and polar nanoregions (PNRs). Ba0.4Sr0.6TiO3 ceramic bulk with grain size of 0.5 μm achieves a high Wrec = 1.28 J/cm³ measured at the highest Eb of 243 kV/cm. Wu et al. [48] compared microstructure and energy storage properties of spark plasma sintered (SPS) and conventionally sintered (CS) Ba0.3Sr0.7TiO3 ceramics. The SPS sintered ceramics consists of tetragonal and cubic phases with an average grain size of 880 nm, while CS ones are only of the tetragonal phase. The maximum Wrec of SPS samples is 1.13 J/cm³ at Eb = 230 kV/cm, which is approximately twice as much as that of CS samples (0.57 J/cm³). In addition, the addition of suitable glass compositions is also an effective method to enhance Eb and reduce the sintering temperature of ST based ceramics [49,50]. In 2019, Shen et al. [51] used a homemade glass frit to modify BST enhancing the Eb and reducing the high temperature resistivity, which expanded the working temperature range for energy storage ceramic capacitor applications. 

“Defect engineering” is an effective tool to enhance Wrec by strengthening relaxor characteristics for ST-based ceramic films. Yang et al. [52] fabricated (Sr1−1.5xBix)Ti0.99Mn0.01O3 (SBTM, x = 0.01, 0.05, 0.1) thin films with a thickness of 217 nm using sol–gel method. As x value increases, relaxor behaviors are gradually strengthened due to a slight rotation of the (TiO6) octahedra induced by the formation of Bi3+– Vsr'' defect complex. Under an electric field of 1982 kV/cm,(Sr0.85Bi0.1)Ti0.99Mn0.01O3 possess a Wrec of 24.4 J/cm³ accompanied by the largest ΔP (Pmax–Pr = 34.3 μC/cm²). Actually, the introduction of other Bi-contained compounds such as Bi0.5Na0.5TiO3 (BNT) [53], BiFeO3 (BF) [54] has a similar effect to strengthen relaxor characteristics. For example, Pan et al. [54] deposited 5 mol% Mn-doped 0.6SrTiO3–0.4BiFeO3 (0.6ST–0.4BF) thin film on Nb-doped SrTiO3 single crystal substrate using pulsed laser deposition (PLD), and acquired that Wrec and η were 51 J/cm³ and 64%, respectively. In addition, the energy storage performances exhibited good temperature stability over (−40)–140 ℃ and well fatigue endurance after 2×107 cycles. In the related mechanism studies, Hou et al. [55] investigated the influence of interface difference and thickness on the energy storage performances for ST thin films, and observed the existence of ionic diffusion layers and oxygen vacancies using high resolution transmission electron microscope (HR-TEM). Moreover, they observed that Eb and Pmax (up to 102 μC/cm²) along the positive direction were higher than the negative direction. Therefore, a maximum Wrec of ST thin films reach 307 J/cm³ for positive direction, which may be related to local electric field and redistribution of oxygen vacancy. 

Various meaningful and interesting works have been done in the optimization of macro/micro structures to improve energy storage properties of ST-based ceramic films. It is well known that the amorphous phase usually possesses higher Eb but lower εr than their crystalline counterparts. Gao et al. [56] studied energy storage behaviors of amorphous ST thin films with different top electrodes, proposed “self-healing” mechanism based on the anodic oxidation reaction in aluminum electrolytic capacitors. At a relative humidity of 60%, amorphous ST films with Al top electrode achieve the Wrec of 15.7 J/cm³ at 3500 kV/cm, which approaches to 8 times of the samples with Au electrode. Since then, Gao et al. [57] inserted insulating Al2O3 as a blocking layer to form heterostructure, and achieved a maximum Wrec of 39.49 J/cm³ at Eb = 7542.3 kV/cm when interface number equals 4. Recently, Chen et al. [58] used Ca0.2Zr0.8O1.8 (CSZ) as dead layer to enhance Eb of Ba0.3Sr0.7Zr0.18Ti0.82O3(BSZT) thin films. Due to the formation of a high electron injection barrier, Schottky electron emission is suppressed, and thus Eb and Wrec are enhanced from 5.4 to 6.3 MV/cm and 64.8 to 89.4 J/cm³, respectively. Energy storage properties of partially Pb-free linear dielectric ceramics are summarized and listed in Table 1. 

Table 1 Energy storage properties of Pb-free linear dielectric ceramic bulks and films

In summary, for linear dielectric ceramic bulks, giant dielectric constant can be observed in TiO2-based with donor/acceptor co-doping at B-site, and ST-based with donor/acceptor co-doping at B-site or aliovalent doping of at A-site bulks by chemical modification. Related physical mechanisms, some controversies, however, are still existed. Energy storage properties of ceramic bulks are limited at expense of a rapid decrease in Eb. Adding of suitable glass phase, special sintering technology and refining grain size are both able to enhance Eb of ceramic bulks. For ST-based ceramic films, adjusting suitable ratio of amorphous and crystalline or introducing a high insulating layer would be a good way to improve its breakdown behavior.  

3. 2 Relaxor ferroelectric ceramics 

Ferroelectric is a special dielectric material that possesses spontaneous polarization (Ps) at a certain temperature range and the direction of Ps can be changed with an external electric field. Compared with linear dielectric, ferroelectric displays an obvious nonlinear characteristic since the domain cannot fast respond to electric field stimulation. Generally, polarization behavior of dielectric material can be characterized by P–E loop, and thus ferroelectric can be classified into normal ferroelectric and relaxor ferroelectric [59], as illustrated in Fig. 7. Normal ferroelectric possesses high Pmax while its high Pr leads to that most of energy is dissipated during the discharge process. By contrast, relaxor ferroelectric exhibits slim P–E loop with high Pmax and low Pr (i.e., high ΔP = Pmax–Pr) meaning that electric energy can be effectively released, and thus obtains better energy storage performances [60]. Note that strengthening the relaxor characteristics and enhancing Eb have become important factors for enhancing Wrec. Furthermore, ferroelectric here discussed mainly refers to relaxor ferroelectric. 

Fig. 7 Schematic of hysteresis loop: (a) normal ferroelectric and (b) relaxor ferroelectric

3.2.1 Pb-based relaxor ferroelectric ceramics

PbZr1−xTixO3 (PZT, 0 ≤ x ≤ 1) ceramic located at morphotropic phase boundary (MPB) where Zr:Ti is of 52:48, possesses a high piezoelectric activity (d33 up to 300 pC/N) and good temperature stability, and becomes an extremely popular dielectric material [61–63]. In addition, other ceramic compositions such as PZT 65/35, 70/30 also received more attention and no limitation by piezoelectric properties [64,65]. For example, the researches of actuator in PZT ceramics are increasing due to a large electrostrain under low electric field. In 2019, Kumar et al. [66] reported (Pb0.89La0.11)(Zr0.70Ti0.30)0.9725O3 (PLZT 11/70/30) ceramics achieved a Wrec only of 0.85 J/cm³ due to low electric field. Generally speaking, current energy density of Pb-based relaxor ferroelectric ceramic bulks is less than 3 J/cm³. In 2017, Zhang et al. [67] investigated energy storage properties of PbZr0.52Ti0.48O3-based thin films, acquired a high Wrec = 28.2 J/cm³ for PbZrO3/PbZr0.52Ti0.48O3 bilayer thin films at 2410 kV/cm. In addition, they continued to design a sandwich structure of PbZr0.52Ti0.48O3/Al2O3/PbZr0.52Ti0.48O3 (PZT/AO/PZT) to enhance Eb [68]. Due to the formation of so-called “built-in electric field” at the interface and the high insulating characteristic of AO, PZT/AO/PZT annealed at 550 ℃ achieved a Wrec of 63.7 J/cm³ at 5.7 MV/cm. It should be mentioned that PZT 52/48 system still possesses relatively high Pr restricting its energy storage properties. 

Generally, a small amount of La3+ (about 7–10 mol%) replacing Pb2+ would effectively strengthen the relaxor characteristics of PZT-based ceramics owing to a disrupted long-range ferroelectric order and diffuse phase transition [71,72]. (Pb,La)(Zr,Ti)O3 (PLZT) based relaxor ferroelectric is therefore considered to be a promising energy storage ceramic capacitor candidate. Adjusting suitable La/Zr/Ti ratio generates an important influence on electric properties because of the complicated phase structure. For example, Hu et al. [73] investigated the effect of different Zr/Ti ratios on the energy storage properties of PLZT thin films at a fixed La content of 8 mol%. As Ti/(Zr+Ti) ratio gradually increases, εr enhances while tanδ shows an opposite trend indicating phase structure gradually transforms from relaxor ferroelectric into normal ferroelectric. At Eb = 2180 kV/cm, Wrec, η of the PLZT 8/52/48 relaxor ferroelectric thin films are 30 J/cm³ and 78%, respectively. In chemical doping, Liu et al. [74] used Mn as a dopant of (Pb0.91La0.09)(Zr0.65Ti0.35)O3 (PLZT 9/65/35) to enlarge the polarization difference of Pmax–Pr, measured a Wrec of 30.8 J/cm³ for 1 mol% Mn thick films. In addition, the addition of excess Pb is a common chemical compensation method to solve the problems of Pb volatilization during the annealing process, and of suppressing the formation of pyrochlore phase [75]

Reasonable design and selection of heterostructure for Pb-based thin films, is an important step in optimizing energy storage properties. In 2013, Zhang et al. [76] prepared a compositionally gradient (Pb1−xLax)(Zr0.65Ti0.35)O3 (PLZT, x = 0.08, 0.09, 0.1) thick films using sol–gel method. Up-graded PLZT films possess a Wrec of 12.4 J/cm³ at 800 kV/cm, down-graded one of 8.9 J/cm³, and the lowest single composition one of 7.1 J/cm³. It is accepted that high texture quality and dense structure are both severely influencing breakdown behaviors and energy storage performances [77,78]. Nguyen et al. [77] deposited (Pb0.9La0.1)(Zr0.52Ti0.48)O3 (PLZT 10/52/48) thin films using PLD on Si substrate choosing Ca2Nb3O10 (CNOns) and Ti0.87O2 (TiOns) nanosheets as the template layer. Highly textured (001)-oriented PLZT 10/52/48 films grown on CNOns possess a high Wrec of 58.4 J/cm³, which exceeds Wrec of 44 J/cm³ for (110)-oriented PLZT 10/52/48 films grown on TiOns. Generally speaking, there are differences in lattice constants and thermal expansion coefficients in hetero-interfaces, which provide good conditions for stress. Figure 8(a) shows P–E loops of PLZT 8/52/48 thick films at different substrates. Ma et al. [69] utilized XRD to analyze residual stress for PLZT 8/52/48 thick films, and considered compressive stress can in a certain extent improve tunability of the polarization, enhance Eb and domain switch ability. In addition, note that Peng et al. [70] recently prepared Mn-doped Pb0.97La0.02(Zr0.905Sn0.015Ti0.08)O3 (PLZST) relaxor ferroelectric thin films, and innovatively proposed a “low-temperature poling” method to improve Eb, and called it “wake-up” mechanism. Figure 8(b) shows P–E loops of PLZST films before and after “awaken state”. Eb and Wrec at room temperature are greatly enhanced (nearly doubled) from 1286 to 2000 kV/cm, and from 16.6 to 31.2 J/cm³, respectively. Energy storage performances of Pb-based relaxor ferroelectric materials are summarized and listed in Table 2.

Fig. 8 (a) P–E loops of the PLZT films on different substrates. Reproduced with permission from Ref. [69], © Springer Science Business Media New York 2015. (b) P–E loops of the Pb0.97La0.02(Zr0.905Sn0.015Ti0.08)O3 films before and after “awaken state”, with the inset showing the I–E curve after “awaken state”. Reproduced with permission from Ref. [70], © Elsevier Ltd. 2020. 

Table 2 Energy storage properties of Pb-based relaxor ferroelectric ceramic bulks and films 

3.2.2 Lead-free relaxor ferroelectric ceramics 

Lead is a toxic metal, and its volatilization problem at high temperature results in serious environmental and human health concerns. Moreover, some legislation in countries and regions also promote researchers to explore a new lead-free ceramic substitute. Currently, lead-free relaxor ferroelectric ceramics mainly focused on Bi0.5Na0.5TiO3 (BNT), BaTiO3 (BT), BiFeO3 (BF), and K0.5Na0.5NbO3 (KNN) systems, which will be discussed in the following sections. 

(1) Bi0.5Na0.5TiO3 based ceramics 

Bi0.5Na0.5TiO3 (BNT) is a ferroelectric material firstly discovered by Smolenskii et al. [79], which possesses complicated phase structure and good dielectric, piezoelectric, and ferroelectric properties, especially high Pmax (~40 μC/cm²) [80–83]. And so it becomes a popular research topic on fundamental theories and practical studies for ferroelectric materials [84–86]. However, the characteristics of high Pr (~38 μC/cm²), high Ec (~73 kV/cm), and poor sintering behavior for pure BNT ceramics hinder its energy storage applications.

Bi0.5Na0.5TiO3−xBaTiO3 (x = 6%–7%) binary solid solution near MPB exhibits excellent electrical properties, and is a most promising candidate for replacing Pb-based ceramics [87–89]. Since then, extensive energy storage studies have been done on this system. It is particularly important that strengthening dynamic of polar nanoregions (PNRs) through disturbing long-range ferroelectric ordering or expanding nonergodic–ergodic phase transition range both could optimize polarization behavior to obtain high Wrec. In 2011, Gao et al. [90] firstly reported that 0.89Bi0.5Na0.5TiO3–0.06BaTiO3–0.05K0.5Na0.5NbO3 (0.89BNT–0.06BT–0.05KNN) ceramics possessed a Wrec of 0.46 J/cm³ at 56 kV/cm. In 2016, Cao et al. [91] used Mn2+ to modify 0.7(0.94BNT–0.06BT)–0.3ST ceramics to reduce Pr by forming MnTi''- VO defect complex, which can induce a local electric field influencing domain switch. A Wrec of 1.06 J/cm³ for 1.1 mol% Mn is obtained at 95 kV/cm owing to a large Pmax–Pr up to 36.8 μC/cm². Actually, similar phenomenon was already reported by Ren et al. [92]. In 2017, Li et al. [93] incorporated NaNbO3 into 0.8Bi0.5Na0.5TiO3– 0.2SrTiO3 relaxor ferroelectric ceramics, observed that P–E loop gradually goes slim together with vanished current peaks as NN content increases, which was attributed to the nonergodic–ergodic phase transition. 0.5 mol NaNbO3 modified ceramics exhibit a high Wrec of 0.74 J/cm³, accompanied by good high temperature energy storage stability and charging–discharging capability. Indeed, delaying the early saturation of polarization is also an effective method to enhance Wrec[94]. In our previous work [37,95], Ba0.3Sr0.7TiO3, which is suitable for pulse power systems, is selected to improve energy storage performances of BNT-based with an optimized ceramic composition. P–E loop of high εr (Ba0.3Sr0.7)0.35(Bi0.5Na0.5)0.65TiO3 (BS0.35BNT) relaxor ferroelectric ceramics originally presents an obvious clamped behavior, but its Pr is still high, as shown in Figs. 9(a) and 9(b). We therefore choose NaNbO3 (NN) antiferroelectric to continue to optimize its polarization behavior [96]. 0.94BSBNT–0.06NN relaxor ferroelectric ceramics achieve a high Wrec of 1.25 J/cm³ at room temperature. Besides, the system exhibits good high temperature stability and fatigue endurance, which may be closely related to the reduction in domain size. 

Fig. 9 (a) Temperature dependent dielectric constant and loss of BSxBNT ceramics. The inset is Tm as a function of x value. (b) P–E hysteresis loops of the BSxBNT ceramics with different x value at room temperature. Reproduced with permission from Ref. [95], © The Author(s) 2020. 

Besides doping modification, multilayer and miniaturization of BNT-based ceramic capacitors are also a significant research direction. In 2018, Li et al. [15] designed a series of (1−x)Bi0.5Na0.5TiO3−x(Sr0.7Bi0.2)TiO3 (NBT−xSBT, x = 0.3–0.5) ceramics, and corresponding multilayer ceramic capacitors (MLCC) with a single layer thickness of 20 μm were fabricated. A high Wrec of 9.5 J/cm³, together with η of 92%, is achieved in NBT–0.45SBT MLCC. Furthermore, energy storage properties of MLCC display good temperature stability, fatigue endurance, and charging–discharging capability. Recently, Li et al. [97] attempted to enhance Eb through controlling grain orientation, and investigated energy storage performances of NBT–0.35SBT MLCC under different stress states. By comparison, Yang et al. [98] reported energy storage properties of gradient structure (SrTiO3 + 0.5wt% Li2CO3) /(0.93Bi0.5Na0.5TiO3–0.07Ba0.94La0.04Zr0.02Ti0.98O3) (STL/(BNT–BLZT)) ceramics along thickness direction. A high Wrec of 2.72 J/cm³ for STL/(BNT–BLZT) ceramics is obtained at 294 kV/cm. Due to the strict requirements of the harsh working environment such as high temperature (> 200 ℃ or even 300 ℃), good insulation and antioxidant, MLCC would encounter many challenges in the future [99,100]. Meanwhile, material system selection, electrode design such as equivalent series resistance (ESR) and loss, cost control of fabrication, and so on, need further consideration. 

The volatilization of Bi and Na and variable valence of Ti are easy to generate oxygen vacancy for BNT-based thin films resulting in a large leakage current. Single mental oxides (MnO2 [101,102], Fe2O3[103], etc.) are used to modify BNT films, which would form different defect complexes to compensate charge balance and suppress oxygen vacancy migration. For example, Mn doped BNT thick films display a reduced leakage current due to the formation of MnTi''- VO . defect complex, which gives rise to a Wrec of 30.2 J/cm³ for x = 0.01 composition [102]. In addition, controlling annealing temperature also has a similar effect on reducing leakage current [104]. In addition, Peng et al. [105] deposited La/Zr modified 0.94BNT–0.06BT high epitaxial quality thin films using PLD, and Pmax can reach to 102 μC/cm² scale due to the complicated phase composition and great relaxor dispersion. (100) and (111) oriented (Bi1/2Na1/2)0.9118La0.02Ba0.0582(Ti0.97Zr0.03)O3 (BNLBTZ) thin films achieve maximum Wrec of 137 and 154 J/cm³, respectively, far exceeding other Pb-free even Pb-based systems. 

Distinguished from single composition of MLCC, macrostructure modification of BNT-based films mainly focuses on gradient composition. It is widely accepted that BNT-based ceramic film is p-type conductivity due to many vacancies generating acceptor states in the band gap, and thus p–n junctions and block layers are applied to inhibit charge transportation. For instance, Guo et al. [108] reported the introduction of Bi3.25La0.75Ti3O12 (BLT) and Pb(Zr0.4Ti0.6)O3 (PZT) dielectric layer on pyroelectric and ferroelectric properties of 0.94BNT–0.06BT ceramic films, and observed leakage current reduce about 3 orders of magnitude. Besides, Chen et al. [106] studied the effect of interface number on energy storage properties of 0.94(Bi0.5Na0.5)TiOη–0.06BaTiOη/BiFeOη(abbreviated as BNBT/nBFO) multilayer film capacitors under a given total thickness. BNBT/2BF thin films exhibit a Wrec of 31.96 J/cm³ and a η of 61% at 2400 kV/cm owing to enhanced insulating characteristic and high polarization. With the rapid development of flexible wearable materials in recent years, related researches on their energy storage performances have gradually increased. Qian et al. [107] prepared a multilayer (Na0.8K0.2)0.5Bi0.5TiO3/0.6(Na0.8K0.2)0.5Bi0.5TiO3–0.4SrTiO3 (NKBT/NKBT–ST)N (N = 2, 3, 6, 8) films on the F-Mica substrate, with measured Wrec of 73.7 J/cm³ at 3077 kV/cm for N = 6. Under different conditions such as (−50)–200 ℃, 108 cycle numbers, 104 bending tests, energy storage performances of (NKBT/NKBT–ST)6 ceramic films both maintain good stability. Energy storage performances of BNT-based relaxor ferroelectric materials are summarized and listed in Table 3. 

Table 3 Energy storage characteristics of BNT-based relaxor ferroelectric ceramic bulks and films 

(2) BaTiO3-based ceramics 

BaTiO3 (BT) with simple perovskite structure possesses moderate Curie temperature (TC) of 120 ℃ and corresponding high εr of 104 [109,110], and becomes a common dielectric material in passive capacitor. BT-based materials also received more attention in memory and memristor for information storage and transfer. In addition, the structure and physical property research of BT-based materials still attract more attentions [111–113]. However, pure BT ceramic has some problems such as high Pr, the reduction from Ti4+ to Ti3+ at high temperature, which restrict it to achieve high Wrec [114]

In 2009, Ogihara et al. [115] synthesized a temperature stable 0.7BaTiO3–0.3BiScO3 (0.7BT–0.3BS) relaxor ferroelectric ceramics with a thickness of 0.2 mm, and obtained Wrec of 2.3 J/cm³ at 225 kV/cm. When thickness reduces to 15 μm, Wrec enhances to 6.1 J/cm³ at 730 kV/cm. By comparison, Wu et al. [116] utilized BiScO3 (BS) as shell material to coat BT, and Wrec only of 0.68 J/cm3 was measured at Eb = 120 kV/cm for BT@3 mol% BS ceramics. Yuan et al. [117] prepared Bi(Mg1/2Zr1/2)O3 (BMZ) modified BT-based relaxor ferroelectric, and achieved a high Wrec = 2.9 J/cm³ for 0.85BT–0.15BMZ ceramics higher than pure BT ceramics (0.4 J/cm³), and corresponding P–E loop is exhibited in Fig. 10(a). The authors thought the enhancement of Wrec is related to PNRs, as evidenced by piezoelectric force microscope (PFM) and transmission electron microscope (TEM). It should be mentioned that other systems such as BaTiO3–Bi(Mg,Ti)O3 (BT–BMT) also display good energy storage capabilities due to the low tolerance factor of Bi-based compound [118,119]. In recent years, Ba0.85Ca0.15Zr0.10Ti0.90O3 (BCZT) based relaxor ferroelectric gains more attention in ferroelectric and pyroelectric properties because of high εr at room temperature [120–123]. However, Wrec of BCZT-based ceramic bulks is commonly less than 3 J/cm³ limited to a low Eb. If the trade-off relationship between εr and Eb gets a good balance, BCZT-based ceramics would be a promising energy storage candidate. 

Fig. 10 (a) P–E loop of BaTiO3−xBi(Mg1/2Zr1/2)O3 at room temperature. Reproduced with permission from Ref. [117], © Elsevier Ltd. 2018. (b) EDS line scanning analysis of Sr in the BT@ST ceramic. Reproduced with permission from Ref. [126], © The Royal Society of Chemistry 2015. (c) Schematic of the microstructure evolution with film thickness for Ba(Zr0.2,Ti0.8)O3 thin films. Reproduced with permission from Ref. [127], © The Author(s) 2017. 

“Core–shell” structure is a common way of modifying BT-based ceramics, especially in enhancing temperature stability for ceramic capacitor applications. In 2014, Su et al. [124] coated BT nanocrystals with 65PbO–20B2O3–15SiO2 and 65Bi2O3–20B2O3–15SiO2 glass phases, reducing sintering temperature to 900 ℃. Meanwhile, the authors estimated Wrec approaching to 10 J/cm³ by equation for Bi-based glass phase. Similarly, high Eb materials such as SiO2 [125] and SrTiO3 (ST) [126] are both used as shell materials to improve breakdown behavior. For example, Wu et al. [126] fabricated BT@ST relaxor ferroelectric ceramics using the sol-precipitation approach, and EDS analysis illustrated BT@ST ceramics in Fig. 10(b). However, with a Wrec only of 0.22 J/cm³ at 47 kV/cm, η approaches to 90%. It should be pointed that high Ebmaterials usually possess low εr, and so Wrec of coated BT ceramics cannot be effectively enhanced at the sacrifice of εr. Bi-based materials such as BiScO3 as shell material also suffer from the same problem of obtaining high Wrec. The reason needs to be further explored due to the complex composition gradient. 

Ba(Zr,Ti)O3 (BZT) based ceramic films are considered to be a potential energy storage material since Zr4+ has a stable valence and similar ion radius of Ti4+, as well as good relaxor characteristics. As Zr4+content increases, relaxor degree gradually enhances, and a weak diffuse phase transition accompanying slim P–E loop can be founded when Zr4+ near 15 mol%, which is attributed to the inhomogeneous distribution of Zr and mechanical stress [128,129]. Therefore, energy storage performances of BZT-based ceramic systems receive more attention. Instan et al. [130] prepared 400 nm <100>-oriented Ba(ZrxTi1−x)O3 (x = 0.3, 0.4, 0.5) relaxor ferroelectric thin films using PLD on La0.7Sr0.3MnO3 /MgO substrates. Importantly, Wrec of all ceramics can reach a scale of 102 around Eb ≈ 3 MV/cm, and a maximum Wrec of 156 J/cm³ is obtained at x = 0.3 composition. In 2017, Cheng et al. [127] reported the influence of thickness and substrate categories on domain/phase of Ba(Zr0.2Ti0.8)O3 thin films. With increasing thickness, mismatch stress gradually releases and rhombohedral phase content increases, as well as twined domain structure is formed, as shown in Fig. 10(c). A high Wrec of 166 J/cm³ for Ba(Zr0.2Ti0.8)O3 thin films is achieved at Eb ≈ 5.7 MV/cm. In addition, oxygen pressure is found to generate a positive effect on energy storage properties of BT-based thin films [131]

“Interface engineering”, including interface compatibility, periodic number, and space charge, plays a critical role in enhancing Eb and optimizing polarization behavior of BT-based ceramic films. In 2012, Ortega et al. [132] deposited BaTiO3/Ba0.3Sr0.7TiO3 superlattice thin films on MgO single substrate using PLD, and acquired a Wrec of 12.24 J/cm³ measured by P–E loop. It should be noted that a theoretical value of Wrec = 46 J/cm³ at Eb reach to 5.8–6 MV/cm. Sun et al. [133] studied energy storage properties of laminated Ba0.7Ca0.3TiO3/BaZr0.2Ti0.8O3 (BCT/BZT) thin films with two layers as a period and the number is 2, 4, 8. With increasing period, Eb enhances from 3 to 4.5 MV/cm, and a maximum Wrec is 52.4 J/cm³ for N = 8. Meanwhile, a series of multilayer structures consisting of two materials or a stack of different dielectric layers, have become the most charming model systems since some unique properties can be enhanced. Due to the difference of electric properties in stacked dielectric material, some physical mechanisms such as current leakage mechanism and charge distribution in heterostructure interface, still need to be further investigated [134,135]. Energy storage performances of the BT based relaxor ferroelectric materials are summarized and listed in Table 4. 

Table 4 Energy storage properties of BT-based relaxor ferroelectric ceramic bulks and films

(3) BiFeO3-based ceramics 
BiFeO3 (BF) as a multiferroic material with perovskite structure exhibits high TC (~850 ℃), high Ps (~100 μC/cm²), and large Smax (~0.4 %), and gains extensive studies in different cross fields such as piezoelectric, magnetic, and quantum [136–139]. The volatile nature of Bi and multiple valence variation of Fe during sintering cause large dielectric loss and leakage current [140]. Nevertheless, BF relaxor ferroelectric is still considered as a potential candidate for energy storage capacitors because of high Ps

BiFeO3–xBaTiO3 (BF–xBT, 0 ≤ x ≤ 0.5) ceramics maintain high TC, good ferroelectric and piezoelectric properties resulted from complicated phase structure evolution. With increasing x value, BF–xBT ceramics possess a high Pmax near MPB of BT ≈ 0.33 mol [141–143]. Energy storage properties of BF-based ceramics, therefore, basically are around BF–0.33BT system to reduce Pr. For example, Liu et al. [144] added Ba(Zn1/3Ta2/3)O3 (BZT) into BT–0.34BT relaxor ferroelectric ceramics obtaining a Wrec of 2.56 J/cm³ at 160 kV/cm. To enhance breakdown strength of BF-based ceramics, Qi et al. [145] introduced NaNbO3 (NN) combined with 0.1 wt% MnO2 and 2 wt% BaCu(B2O5) (BCB) as sintering aids to modify 0.67BF–0.33BT relaxor ferroelectric ceramics, and corresponding Weibull distribution of Eb is given in Fig. 11(a). At 360 kV/cm, x = 0.1 sample possesses a maximum Wrec of 8.12 J/cm³ accompanied to η of 90%, which is related to the existence of nanodomain and high density. In addition, Wang et al. [146] reported that Wrec of 0.62BF–0.3BT–0.08Nd(Zr0.5Zn0.5)O3 ceramic bulks is 2.45 J/cm³ at 240 kV/cm. When an MLCC device based on this system is made, Wrec is enhanced to 10.5 J/cm³. 

Fig. 11 (a) Weibull distribution and calculated Eb values of (0.67−x)BF–0.33BT–xNN ceramics. Reproduced with permission from Ref. [145], © WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2019. (b) P–E loops from two film capacitor structures before and after the introduction of an alumina layer at the electrode–0.6BF–0.4ST interface. Reproduced with permission from Ref. [150], © AIP Publishing 2012. 

Similar to BF–xBT systems, ST modified BF-based energy storage ceramics are mainly concentrated on films due to thickness limitation. In 2013, Correia et al. [147] prepared 0.4BF–0.6ST thin films using PLD, obtained a high Wrec of 18.6 J/cm³ at 972 kV/cm. Recently, Pan et al. [148] reported (0.55–x)BF–xBT–0.45ST (x = 0–0.4) thin films with polymorphic nanodomain structure are grown on Nb-doped SrTiO3 single crystal substrate. Under an electric field of 4.9 MV/cm, x = 0.3 composition achieves a maximum Wrec =112 J/cm³ more than 53 J/cm³ of pure 0.55BF–0.45ST thin films, which is owing to the enhanced relaxor behaviors and breakdown strengthen. Moreover, the energy storage performances of this system maintain good stability in a wide temperature range of (−100)–150 ℃ and 108 cycles, respectively. In addition, an important and interesting double P–E loop can be observed in rare earth substituted BF thin films due to the transition from orthorhombic to rhombohedral phase [149]

In heterostructure design, McMillen et al. [150] firstly proposed to use “artificial dead layer” to enhance Eb, deposited Al2O3 layer with 6 nm between 0.6BF–0.4ST film and ST substrate, and corresponding P–E loops before and after inserting Al2O3 layer were illustrated in Fig. 11(b). Interface polarization behavior to some degree increases hysteresis loss while Al2O3 insulating layer enhances Eb, and thus Wrec enhances from 13 to 17 J/cm³. Since then, a sandwich structure of “soft layer” with high polarization and “hard layer” with high Eb are constructed to prevent “electric tree” growth and achieve high Wrec [151]. It is particularly important that strong interface coupling factors should be considered in designing structure and analyzing properties [152]. Energy storage performances of the BF-based relaxor ferroelectric materials are summarized and listed in Table 5. 

Table 5 Energy storage properties of BF-based relaxor ferroelectric ceramic bulks and films 

(4) K0.5Na0.5NbO3-based ceramics 

K0.5Na0.5NbO3 (KNN) is a binary solid solution of KNbO3 ferroelectric and NaNbO3 antiferroelectric, and possesses a moderate d33 around 80 pC/N, high TC of 420 ℃, and complicated phase structure [153,154]. Since the 1950s at latest century, the researches of KNN materials concentrate on piezoelectric properties due to MPB located at KNbO3 content of 47.5% [154]. The ratio of K/Na has a slight variation while mainly concentrated on 0.5/0.5. However, narrow sintering temperature range, easy volatilization characteristic of K, Na at high temperature both hinder its applications. 

In 2016, Du et al. studied KNN-based energy storage ceramics, and achieved a high Wrec of about 4 J/cm³ for SrTiO3 (ST) [155] and Bi(Mg1/3Nb2/3)O3 (BMN) [156] modified KNN ceramics. And they further used CuO [157], ZnO [158], etc., as fruit to improve sintering behavior of KNN-based ceramics. It should be noted that modified KNN-based energy storage ceramics with superfine grain size possess not only high Eb but also good transparency. In addition, utilize “phase boundary engineering” of KNN ceramics to enhance electric properties is a useful method. Recently, Yang et al. [159] proposed “morphotropic relaxor boundary (MRB)” in BT modified KNN ceramics to illustrate an obvious enhancement of electrostrain and dielectric permittivity. At the MRB the electrostrain increases by ~3 times and the permittivity increases by ~1.5 times over a wide temperature range of more than 100 K, as compared with off-MRB compositions. In comparison, the studies of KNN-based ceramic films for energy storage are relatively less, which should be related to insolubility of Nb and volatilization of K and Na. In 2017, Won et al. [160] reported 6 mol% BiFeO3-doped (K0.5,Na0.5)(Mn0.005,Nb0.995)O3 (KNMN) thick film possessed a slim P–E loop, and achieved a Wrec of 28 J/cm³, η of 90.3%. Recently, Huang et al. [161] also used MnO2 to reduce leakage current of 0.95(K0.49Na0.49Li0.02)(Nb0.8Ta0.2)O3–0.05CaZrO3−x mol% Mn (KNN–LT–CZ5−x mol% Mn) thin films prepared by sol–gel methods, and achieved a high Wrec = 64.6 J/cm³ under an electric field of 3080 kV/cm at x = 0.5 composition. Energy storage properties of KNN-based relaxor ferroelectric ceramics and films are summarized and listed in Table 6. 

Table 6 Energy storage properties of KNN-based relaxor ferroelectric ceramic bulks and films

In summary, for relaxor ferroelectric ceramics, the formation of PNRs due to disturbed long-range ferroelectric order or nonergodic–ergodic phase transition can strengthen relaxor characteristics. Reflecting on macroscopic ferroelectric properties, P–E loop goes slim. In addition, delay saturated polarization is also used to optimize polarization behavior. Designing multilayer ceramic capacitor (MLCC), “core–shell” structure with non-ferroelectric phase as shell material is commonly accepted method to improve electric breakdown behavior for ceramic bulks. By comparison, chemical modification of ceramic films functions two roles: For one, suppress the generation and transportation of vacancy defect to reduce leakage current; for the other one, strengthen its relaxor characteristics and optimize polarization behavior. In the heterostructure of ceramic films, match degree of physical paraments such as lattice constant, thermal expansion coefficient, etc., gradient sequence, template or new inert layers are all important factors to influence energy storage performances. 

3. 3 Antiferroelectric ceramics 

As a special group of ferroelectric material, antiferro-electric has many similarities with ferroelectric whereas still exists obvious differences. In 1951, Kittel [162] originally proposed the concept of antiferroelectric, predicted the existence, and gave some basic characteristics. Generally speaking, antiferroelectric materials possess Ps in a unit cell, but the direction of Ps is antiparallel to that of neighboring unit cell. Antiferroelectric, therefore, does not exhibit polarization in macroscopic characteristics. It is particularly important that antiferroelectric exists a unique feature: double hysteresis loop under an external field. P is linearly proportional to E at a low electric field. When E exceeds the forward switching (AFE-to-FE) field EA–F, antiferroelectric displays an obvious ferroelectric behavior, and P fastly increases and gradually reaches Pmax. Note that antiferroelectric can undergo a ferroelectric–antiferroelectric phase transition field (EF–A) after removing E. Consequently, P gradually reduces and returns to the initial state (that is, E = 0, Pr = 0). A similar variation in P can be observed as E continuously increases in an opposite direction. 

It is hard to see a double hysteresis loop for most pure antiferroelectric materials at a low electric field, which usually requires special conditions of high temperature and strong electric field to stimulate. Thereby, it is a crucial challenge for antiferroelectric to obtain double P–E loop especially for ceramic bulks. In addition, a typical high squareness P–E loop, and corresponding internal strain induced by antiferroelectric–ferroelectric phase transition both result in low Wrec and reduced device life [12]. In this regard, how to stabilize and relax the antiferroelectric phase (corresponding to enhancing EA–F, reducing the difference of EA–F–EF–A) and enhance Eb has become a particularly important issue to acquire good energy storage performances.

3.3.1 Pb-based antiferroelectric ceramics 

(1) PbZrO3

PbZrO3 (PZ) is the prototype antiferroelectric, and serves as a model system to be thoroughly studied so far. The origin of the phase transition mechanism, however, is not well understood [163]. Despite this, a characteristic double P–E loop with high Pmax and low Pr makes it suitable for energy storage capacitors. PZ-based energy storage ceramics gain increasing attention since Chen et al. [164] reported Wrec of 7.1 J/cm³ for PZ thin films during phase transition, but basically around ceramic films. 

It is widely accepted that tolerance factor t of perovskite structure is closely related to phase stability, and the equation is represented as 

where rA, rB, rO denote ion radius of A, B, and O, respectively. In general, AFE phase can be stabilized for t < 1, and reducing t can enhance in some degree the stability of AFE phase. For example, Hao et al. [165] substituted Pb2+ (1.20 Å) with smaller Sr2+(1.12 Å) to reduce t, and P–E loop goes slim together with increased EA–F and decreased ΔE. As a consequence, (Pb0.95Sr0.05)ZrO3 (PSZ5) thin films obtain a Wrec of 14.5 J/cm³. Choosing smaller ion radius while aliovalent of La3+ as donor dopant has a similar effect [166]. In addition, controlling the orientation of PZ thin film also achieves the purpose of stabilizing the antiferroelectric phase. PZ thin film with (100) orientation needs a higher electric field than with (111) one to finish antiferroelectric–ferroelectric phase transition [167]

Tailoring local electric field and stress to enhance Wrec is hard but interesting work. As is well known, introducing a new nanoparticle in materials often brings novel properties and improved properties. For example, Sa et al. [169] used α-Fe2O3 nanoparticles to modify PZ thin films, obtained a Wrec of 17.4 J/cm³ and Pmax as high as 78 μC/cm² attributing to the local field effect. By comparison, Chen et al. [170] fabricated a self-assembled PZ:NiO nano-columnar composite using PLD, and proposed that tensile stress mainly comes from the interface of two phases. At Eb = 1000 kV/cm, 5 vol% NiO thin films possess a high Pmax of ~91 μC/cm², and Wrec of 24.6 J/cm³. Meanwhile, Ge et al. [171,172] also did a series of works on optimizing polarization behavior mainly such as enhancing Pmax, reducing EA–F–EF–A using the stress engineering method. To our best knowledge, it is difficult to directly measure and characterize local electric field and stress, and thus corresponding methods need to be developed. Furthermore, the gradient sequence of thin film is significant for enhancing Wrec. Ye et al. [168] reported a Wrec of 16.3 J/cm³ for down-graded higher than that of 9.7 J/cm³ for up-graded PZ-based thin films, and corresponding P–E loops are shown in Fig. 12. 

Fig. 12 P–E loops of up- and down-graded Eu-doped PZ thin films. Inset: the differentiated P–E hysteresis loops of down- and up-graded thin films, respectively. Reproduced with permission from Ref. [168], © The American Ceramic Society 2012.

It should be mentioned that PbHfO3 (PHO) antiferroelectric ceramics have some similarity with PZO, and thus the related studies still concentrate on phase structure [173–175]. Nevertheless, the energy storage properties of PHO-based ceramics have rarely been reported. In 2020, Chao et al. [176] prepared Pb0.98La0.02(HfxSn1−x)0.995O3 antiferroelectric ceramics, and achieved a good energy storage performance: Wrec and η are of 7.63 J/cm³ and 94% for x = 0.45 composition, respectively. Recently, Huang et al. [177] reported pure PHO ceramic films by sol–gel method at 650 ℃ annealing temperature, and achieved a Wrec of 24.9 J/cm³.

(2) (Pb, La)(Zr, Sn, Ti)O3

As discussed in Section 3.2.1, Pb(Zr1−xTix)O3 (PZT, x ≈ 0.05) solid solutions located at rich Zr regions of ferroelectric–antiferroelectric (FE–AFE) phase boundary, exhibit rich phase structures and well electrical properties [61,62]. It is worth noting that PZT systems at FE–AFE phase boundary differ from MPB in electric properties. For instance, FE and AFE cannot be transformed into each other under force or electric field, while temperature, stress, or other factors induced by FE–AFE phase transition can occur. Meanwhile, the region of the antiferroelectric phase for PZT 95/5 is relatively narrow, and a slight composition fluctuation would easily cause deviation from FE–AFE phase boundary. 

La3+ and Sn4+ are very popular A/B site dopants in PZT 95/5 ceramics where their functions are similar [178–182]. La3+, as a donor dopant, substitutes Pb2+ to disturb long-range ordering of domain by vacancy defect, which would strengthen the relaxor characteristic and expand the stabilized region of the antiferroelectric phase. Sn4+ as an equal valence dopant of Ti4+functions not only expands the stabilized region of antiferroelectric, but also adjusts Zr/Ti ratio to enable Ti up to 10 mol%. Currently, the related works of stabilizing the Pb-based antiferroelectric phase mainly focus on adjusting suitable Zr/Sn/Ti ratio [183–185]. Liu et al. [186] found that EA–F linearly increased and the squareness of P–E loop slightly improved when Ti content reduces from 0.11 to 0.07 mol at a fixed Zr of 0.58 mol. Wrec enhances, thereby, from 0.28 to 2.35 J/cm³ for Pb0.97La0.02(Zr0.58Sn0.35Ti0.07)O3 antiferroelectric ceramic bulks. In addition, it should be particularly noted that a high Wrec is difficult to obtain when EA–F exceeds Eb. 

In improving the breakdown behavior of Pb-based antiferroelectric ceramic bulks, Zhang et al. [187–189] did a series of works using some special sintering technology. For instance, Zhang et al. [188] reported that (Pb0.87Ba0.1La0.02)(Zr0.68Sn0.24Ti0.08)O3 (PBLZST) ceramics using hot-press (HP) possessed a high Wrec of 3.2 J/cm³ at Eb = 180 kV/cm due to smaller grain size and well insulation. Considering that special sintering technology requires expensive equipment, this method is not suitable for large-scale production. Some low-cost and convenient solutions are proposed, such as Bian et al. [190] used amorphous SiO2 to coat Pb0.97La0.02(Zr0.33Sn0.55Ti0.12)O3 (PLZST 2/33/55/12), and measured a high Wrec of 2.68 J/cm³ due to Eb enhancing from 12.2 to 23.8 kV/mm. It should be noticed that the introduction of non-antiferroelectric phase will reduce the content of the original antiferroelectric phase, even that EA–F would disappear. In a physical method, Wang et al. [191] used a rolling process to enhance the mechanical strength of (Pb0.98La0.02)(Zr0.55Sn0.45)0.995O3 (PLZS) antiferroelectric ceramics. At 400 kV/cm, PLZS ceramics obtain a high Wrec of 10.4 J/cm³ and a η of 87%. Similarly, Zhang et al. [192] and Liu et al. [193] both utilized tape-casting method to fabricate antiferroelectric thick film. For example, Liu et al. [193] utilized tape-casting method to fabricate (Pb0.98−xLa0.02Srx)(Zr0.9Sn0.1)0.995O3 (PLSZS) antiferroelectric thick films, and achieved Wrec and η of 11.18 J/cm³ and 82.2% for x = 0.04, respectively.  

Due to the phase structure complexity of PZT-based ceramics near FE–AFE boundary, it usually displays different polarization behavior especially for ceramic films. Gao et al. [194] reported different oriented (Pb0.98La0.02)(Zr0.95Ti0.05)O3 (PLZT) thin films using PLD, obtained a Wrec of ~40 J/cm³ while η only of ~50% for (111) PLZT, which may be related to ferroelectric-like behavior (i.e., high Pr) under high electric field. Despite this condition, the combination of relaxor ferroelectric and antiferroelectric still receives more attention to enhance Wrec. In addition, note that antiferroelectric behavior can be observed at different formulas such as PLZT at Zr/Ti ≈ 52:48 [196] or PLZST at Zr/(Sn+Ti) ≈ 65:55 [197]. With the development of micro-electric devices, flexible substrate such as Ti, Si, Ni foils, etc., is required to meet future application scenes. Ma et al. [195,198] did a series of meaningful works in the low-cost integration development of PLZT-based thin films. For instance, Pb0.92La0.08Zr0.95Ti0.05O3 (PLZT 8/95/5) antiferroelectric ceramic films possess a high Wrec of 53 J/cm³ at mental foil by CSD, and effective work time would maintain 5000 h at room temperature [195]. Energy storage properties of Pb-based antiferroelectric ceramics and films are summarized and listed in Table 7. 

Table 7 Energy storage properties of Pb-based antiferroelectric ceramic bulks and films

3.3.2 Lead-free antiferroelectric ceramics 

(1) NaNbO3 

NaNbO3 (NN) is a well-documented nonpolar antiferroelectric phase and possesses a complicated crystal structure due to the rotation of oxygen octahedron and off-centered displacement of Nb5+ [199,200]. In addition to temperature-induced transition, antiferroelectric–ferroelectric phase transition can also be induced by mechanical stress [201], grain size [202], and electric field [203]. Note that an electric field-induced antiferroelectric–ferroelectric phase transition is usually irreversible at room temperature because of the small free energy difference between antiferroelectric and ferroelectric. 

Generally, reducing tolerance factor t is still a widely accepted method to stabilize antiferroelectric phase of NN ceramics. For instance, Shimizu et al. [204] used CaZrO3 (CZ), with similar electronegativity but smaller tolerance factor t, to modify NN ceramics. As the content of CZ increases, an obvious double P–E loop accompanying to EA–F gradually increases while Pmax decreases, as shown in Fig. 13. Since then, other compounds such as SrZrO3 (SZ) [205], CaHfO3 (CH) [206], BiScO3 (BS) [207], are both utilized to stabilize antiferroelectric phase, but few reports their energy storage performances. It should be pointed that double P–E loop of NN-based ceramics still has more hysteresis loop at room temperature resulting in low Wrec. Moreover, Zhou et al. [208] firstly reported Bi2O3 modified NN energy storage ceramic bulks, obtained a high Wrec of 4.03 J/cm³ for Na0.7Bi0.1NbO3 ceramics at Eb = 250 kV/cm attributed to the disrupted random electric field and reduced domain size. Energy storage properties of Na0.7Bi0.1NbO3 system show good temperature stability (20–100 ℃), fatigue endurance (105 cycles), and charge–discharge properties. After that, more Bi-based compounds including Bi(Mg1/3Nb2/3)O3(BMN) [209], Bi(Ni1/2Sn1/2)O3 (BNS) [210], and so on, are used to improve the relaxor characteristics and acquire high Wrec. It is particularly important that P–E loop of BiMeO3 modified system becomes slimmer than pure NN ceramic, but characteristic double hysteresis loop fails to be observed. 

Fig. 13 (a) Tolerance factor versus averaged electronegativity difference for (Na1−xA2+x)(Nb1−xB4+x)O3 composition, where (A,B) = (Ca,Zr), (Ca,Hf), (Sr,Zr), and (Sr,Hf); (b) P–E loops in the CZNN ceramics at 120 ℃. Reproduced with permission from Ref. [204], © Royal Society of Chemistry 2015. 

A double P–E hysteresis loop can be observed in high quality NN single crystal and ceramic film compared with ceramic bulk. However, a few studies reported energy storage properties of NN-based ceramic films, and concentrated on piezoelectric, dielectric tunability. In 2018, Fujii et al. [211] deposited 0.92NaNbO3–0.08SrZrO3 (0.92NN–0.08SZ) antiferroelectric thin films on SrRuO3 buffered ST substrates with different orientations by using PLD. 0.92NN–0.08SZ thin films with (110) oriented ST substrate exhibit antiferroelectric behavior, while that with (001) oriented substrate is still ferroelectric. Recently, Beppu et al. [212] obtained Wrec only of 2.9 J/cm³ for 0.92NN–0.08SZ thin films at Eb ≈400 kV/cm. In addition, Luo et al. [213] fabricated Mn-doped 0.96NaNbO3–0.04CaZrO3 (0.96NN–0.04CZ) thin films using sol–gel method, and leakage current of 1 mol% Mn reduces the magnitude of 103–104 compared to pure compositions. Wrec and η of 1 mol% Mn thin films are19.64 J/cm³ and 64.5%, respectively. Nevertheless, simple composition and no toxic metal elements make it have broad prospects for energy storage research in the future. 

(2) AgNbO3

AgNbO3 (AN) has a complicated phase structure and relatively low bandgap (~2.8 eV), and still has difficulty in observing double hysteresis loop at room temperature. Early studies of AN ceramics therefore concentrated on microwave communication and photocatalysis [214,215]. In addition, Ag2O decomposition at a high temperature requires the fabrication of AN-based ceramics at the oxygen-rich environment. Similar to NN, AN-based ceramic bulk is a main research direction for energy storage capacitors. 

In 2007, Fu et al. [216] successfully fabricated AN ceramics with a double hysteresis loop, and Pmax can up to 52 μC/cm² at 220 kV/cm exceeding other dielectric materials at the same electric field. The phenomenon strongly stimulates the studies of AN ceramics on energy storage applications. Tian et al. [217] synthesized pure AN antiferroelectric ceramic bulks, found two polarization structures by TEM and variable temperature P–I–E loops, and attained a high Wrec of 2.1 J/cm³. In general, AN ceramics experience a series phase transition [218]

where M1, M2, and M3 denote orthorhombic phases in rhombic orientation, O1 and O2 are the orthorhombic phases in a parallel orientation, while T and C denote the tetragonal and cubic phases, respectively. It is known that the stability of antiferroelectric phase of AN ceramic is closely related to phase transition among M1, M2, and M3. Therefore, reducing phase transition temperature among M1, M2, and M3 to low temperature would enhance Wrec of AN-based ceramics [218–221]

Chemical doping is a simple and effective method to enhance Wrec of AN-based ceramic bulks. For example, Ta5+ [218,222] and W3+ [223] substitute Nb5+ of B site, and Bi3+ [224], Ca2+ [225], and La3+ [226] replace Ag+ of A site. In 2017, Zhao et al. [218] fabricated Ag(Nb1−xTax)O3 antiferroelectric ceramics, and Pr decreases from 3.6 to 1.2 μC/cm² whereas Eb enhances from 175 to 242 kV/cm with increasing Ta content. Ag(Nb0.85Ta0.15)O3 exhibits a high Wrec of 4.2 J/cm³, together with η of 69%, as shown in Figs. 14(a) and 14(b). The authors attribute the enhancement of Wrec to the low polarizability of Ta5+ because of a closed t value. Since then, Zhao et al. [223] have done some works to verify the possibility. Besides, electronegativity difference is also playing a crucial role in influencing the function behavior of antiferroelectric materials [227].

Fig. 14 (a) P–E loops of AgNbO3 and Ag(Nb0.85Ta0.15)O3 ceramic; (b) energy storage performances of Ag(Nb1−xTax)O3 ceramics prior to their breakdown. Reproduced with permission from Ref. [218], © WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2017. 

Note that fine-grain ceramics is a good choice to enhance Eb due to an inverse relationship between Eb and grain size. Recently, Wang et al. [228] synthesized AN ceramics using hydrothermal process, and obtained a maximum Eb = 250 kV/cm among pure AN ceramics so far. However, Pmax has an obvious decrease compared with other AN ceramics using solid-state method, and thus Wrec and η are only of 1.8 J/cm³ and 40%, respectively. It is noteworthy that little attention has been shared on enhancing Eb of AN ceramics by other different methods apart from ion-doping. Energy storage behaviors of NN and AN antiferroelectric ceramic bulks and films are summarized and listed in Table 8. 

Table 8 Energy storage properties of NN and AN-based antiferroelectric ceramic bulks and films 

In summary, for antiferroelectric ceramics, tolerance factor t, electronegative difference, and polarizability are all influencing the stability of antiferroelectric phase. Note that the characteristic double P–E loop of pure antiferroelectric phase is difficult to be observed, which needs to further explore the related reasons. Physical (special sintering technology, rolling process, etc.) and chemical (coating, hydrothermal process, etc.) methods can enhance Eb by improving sintering behavior or mechanical strength. 

4 Conclusions and perspectives 

From the perspectives of composition modification, structural design, and electrical performance optimization, this paper briefly compares the research progress of energy storage ceramic bulks and films. Currently, Wrec of ceramic bulks is generally less than 10 J/cm³, while that of films can reach 102 J/cm³. Except for ceramic composition, Wrec is also closely related to other factors such as sample thickness, preparation, and testing methods. Although giant εr linear dielectric attracts more attention, it may be not suitable for energy storage capacitors. In contrast, relaxor ferroelectric and antiferroelectric with high Pmax and low Pr are easier to obtain high Wrec than linear dielectric, so they are relatively ideal energy storage dielectric materials. Whether for bulks or films, Eb is a decisive factor for affecting the upper limit of Wrec. In addition, the development from material to device still has a large gap, and thus needs to make more efforts to solve this problem. Judging from the existing researches, the authors believe that the following aspects need further exploration and improvement: 

1) It is still a long process to select the ideal energy storage ceramics through a single experiment. If relevant predictions and screenings can be combined with theoretical calculations or machine learning methods, work efficiency will be greatly improved. 
2) In order to better understand the variation in domain and phase structures as functions of the electric field, thermal, force, magnetic, etc., external fields, in-situ observation technique would be an important direction, which would help us to comprehensively understand the microscopic evolution mechanism of polarization. 
3) For materials with crystalline/amorphous phase and multilayer structure, the issues of generation reason and corresponding mechanisms of interface behaviors (such as interface polarization, fatigue, etc.) and stress still need to be deeply investigated. 
4) Considering many factors to influence Wrec and η, it is important to standardize the test parameters such as sample thickness, area of electrode, testing frequency, and AC/DC conditions, and establish the relevant test standard for performance evaluation of energy storage materials.

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