A Brief Review of Sodium Bismuth Titanate-Based Lead-Free Materials for Energy Storage: Solid Solution Modification, Metal/metallic Oxide Doping, Defect Engineering and Process Optimizing

Abstract

With the ever-increasing demand for energy, research on energy storage materials is imperative. Thereinto, dielectric materials are regarded as one of the potential candidates for application in advanced pulsed capacitors by reason of their ultrahigh energy-storage density, low energy loss, and good thermal stability. Among the numerous dielectric materials for energy storage, sodium bismuth titanate (Bi_0.5Na_0.5TiO₃, BNT) with high saturation polarization, as one of the successful alternatives to lead-based materials, has been extensively studied. However, degraded dielectric and ferroelectric properties as a consequence of chemical alterations usually produced by inhomogeneity in microstructure and composition due to the ion volatilization during preparing, thus affecting performance of devices. Hence, this review served to encompass the current state and progress on the optimization of energy storage performance in lead-free BNT-based materials over the past few years, including ceramics, multilayer ceramics, thin films, and thick films, involved in solid solution modification, metal/metallic oxide doping, process optimization and other related aspects to optimize energy storage performance. Furthermore, some prospective approach in the improvement of energy storage performance for BNT-based materials were also provided in this work according to the existing theoretical and experimental results, to impel their practical application.

1. Introduction

In recent decades, with the ever-increasing energy demand, energy storage materials have drawn extensive attention. As one of the energy storage devices, ceramic-based dielectric capacitors have attracted more and more consideration on account of their excellent thermal properties, good mechanical properties, fast charging and discharging speed, and high power density [1,2] and thus can be widely employed in the area of hybrid electric cars, high-frequency inverters, and pulsed power systems [3,4,5]. Among the numerous materials, lead-based materials have drawn consideration because of their large saturation polarization (P_s) and dielectric breakdown strength (E_b), and a small remanent polarization (P_r) near the morphotropic phase boundary (MPB) [6], which gives them a high energy storage density and facilitates energy storage. For the moment, the most well-known lead-based antiferroelectric PLZS ceramics have achieved excellent energy storage properties (ESP) with a recoverable energy storage density (W_rec) of 10.4 J cm⁻³ and an energy storage efficiency ($\mathsf{\eta }$) of 87% [7]. Thus, thanks to the excellent ESP of lead-based ceramics, they have already been succesfully applied in actuators, sensors, and acoustic radiation force impulse imaging. However, the large amount of lead contained in lead-based ceramics causes damage to the environment and human health. Therefore, research and development of lead-free materials have been regarded as one of the effective ways to solve this problem.

Sodium bismuth titanate (Bi_0.5Na_0.5TiO₃, BNT) is one of the ABO₃ type ferroelectric perovskites with the rhombohedral crystallographic structure (space group: R3c) at room temperature, in which sodium (Na)/bismuth (Bi) ions occupy eight corners of the lattice in equal proportions, oxygen (O) ions occupy face-centered positions, and titanium (Ti) ions are in the center of the octahedron formed by oxygen ions (as shown in Figure 1a), meanwhile the Na, Bi and Ti ions are displaced along with the [111]_p direction, resulting in a polarized ferroelectric phase under the electric field [8]. Thanks to the high Curie temperature T_c (~320 °C) (in Figure 1b) and a large P_s (~45 μC/cm² in Figure 1c) BNT-based ceramics has drawn a lot of attention for energy storage [9,10,11]. Nevertheless, the high P_r (~35 μC/cm² in Figure 1c) due to the strong ferroelectricity and low breakdown strength (E_b) are unfavorable for energy storage. Therefore, researchers mainly focus on diminishing P_r and enhancing E_b of BNT ceramic materials to improve ESP.

2. Fundamental Principle of Energy Storage

According to the classical electromagnetic theory, the energy storage density refers to the electric energy contained in a unit volume, and the unit usually used is J cm^-3. Since BNT is a nonlinear dielectric, its energy storage capacity can be calculated from the following Equations (1)–(3) [5,9].

$$\mathrm{W}={\displaystyle {\int }_0^{{\mathrm{P}}_{\max }}\mathrm{EdP}}$$

(1)

$${\mathrm{W}}_{\mathrm{rec}}={\displaystyle {\int }_{{\mathrm{P}}_{\mathrm{r}}}^{{\mathrm{P}}_{\max }}\mathrm{EdP}}$$

(2)

$$\mathsf{\eta }=\frac{{\mathrm{W}}_{\mathrm{rec}}}{{\mathrm{W}}_{\mathrm{rec}}+{\mathrm{W}}_{\mathrm{loss}}}\times 100\%$$

(3)

where E, P_max, W, and W_loss is the applied electric field, maximum polarization, total energy storage density and loss energy density, respectively.

Based on the equation, it can be known that reducing P_r and increasing E_b are the crucial ways to improve the W_rec and η. P_r is related to the ferroelectric properties and can be suppressed through doping elements or forming a solid solution, by which the polar phase usually transforms into a non-polar or weak-polar phase with relaxation characteristics and achieves minimum P_r. E_b is usually determined by density, pores, and grain size of the materials [5], which can be usually improved by modifying or optimizing the preparation process.

3. Current Status of BNT-Based Energy Storage Materials

3.1. BNT-Based Energy Storage Ceramics

Up to now, the research of BNT-based energy storage ceramics is mainly focused on the following aspects: solid solution modification, metal/metallic oxide doping, and optimization of the process. Solid solution modification and metal/metallic oxide doping aim at decreasing P_r by forming a solid solution or elemental doping with other components/elements. With the purpose of enhancing E_b, uniformly distributed grain with fine grain size and few pores is essential for the ceramics, which can be obtained by process optimization.

3.1.1. Solid Solution Modification of BNT-Based Ceramics

It has been found that the formation of appropriate solid solution in BNT is conducive to interrupt the long-range ordered ferroelectric state and introduce relaxation behaviour, thus decreasing the P_r and improving the W_rec and η [1,2,4,12,13,14,15,16,17,18,19]. Li et al. [12] fabricated 0.90Bi_0.5Na_0.5TiO₃-0.10Bi(Mg_2/3Nb_1/3)O₃ binary ceramics by traditional solid-state reaction (SSR) and obtained the considerable W_rec of 1.405 J/cm³ at a low E of 140 kV/cm, which was due to the induced transformation from ferroelectric to ergodic relaxor phase. Based on this, Peng et al. [2] prepared 0.92Bi_0.5(Na_0.82K_0.18)_0.5TiO₃-0.08Bi(Mg_2/3Nb_1/3)O₃ binary ceramics and found that with the increase of BMN content, the P_r (~7.1 μC cm⁻²) and dielectric loss (tan δ) decreased significantly and the ESP (W_rec = 2.2 J/cm³, η = 55.7%) was obtained at 110 kV/cm. Zhao et al. [13] reported that the 0.6Na_0.5Bi_0.5TiO₃-0.4Sr_0.775Bi_0.15TiO₃ binary ceramics demonstrated slender polarization hysteresis (P-E) loops and good ESP (W_rec = 2.41 J/cm³, η = 87.5%), as well as medium temperature stability, excellent cycling reliability and frequency dependence. Besides that, Karakaya et al. [14] introduced Bi(Li_1/3Ti_2/3)O₃ into 0.92Bi_0.5Na_0.5TiO₃-0.08BaTiO₃ binary system to prepare 0.98(0.92Bi_0.5Na_0.5TiO₃-0.08BaTiO₃)-0.02Bi(Li_1/3Ti_2/3)O₃ ternary ceramics by traditional SSR method and obtained good ESP (W_rec = 0.88 J/cm³, η = 97%) at a low E of 65 kV/cm. Also, complex ion (Ta_0.24Sn_0.70)⁴⁺ was added to BNT-Based ceramics to form Bi_0.47Na_0.376K_0.094Ba_0.06Nb_0.024Ti_0.94(Ta_0.24Sn_0.7)_0.03O₃ lead-free ternary ceramics, and the ESP (W_rec = 1.65 J/cm³, η = 77.69%) obtained under E of 125 kV/cm, associated with good fatigue resistance [1]. Furthermore, Ye et al. [16] increased the ESP of Bi_0.4465Na_0.4465Ba_0.057La_0.05TiO₃-based ceramics significantly by incorporating Sr_0.85Bi_0.1TiO₃ to enhance the E_b. Ultimately, the high W_rec of 4.55 J/cm³ and excellent η of >90% were obtained. Li et al. [17] reported that doping AgNb_0.85Ta_0.15O₃ into 0.75Na_0.5Bi_0.5TiO₃-0.25SrTiO₃ could break the long-range ferroelectric order and formed the polar nano-regions (PNRs), which dramatically improved the ESP accompanied by the W_rec = 3.6 J/cm³, η = 80%. Wang et al. [4] prepared 0.92(0.6Na_0.5Bi_0.5TiO₃-0.4Sr_0.7Bi_0.2TiO₃)-0.08Ba(Mg_1/3Ta_2/3)O₃ ternary ceramics by traditional SSR method and obtained the optimal ESP (W_rec = 8.58 J/cm³, η = 93.5%) under 565 kV/cm. They also disclosed that the high E_b comes from the addition of tantalum ions and magnesium ions with a large energy band. The ESP of recent reported BNT-based ceramics by means of solid solution modification are also summarized in

Table 1. ESP of BNT-based ceramics by solid solution modification.

Ceramic Compositions	E (kV/cm)	Wrec (J/cm3)	Wloss (J/cm3)	η (%)	Ref
Bi0.47Na0.376K0.094Ba0.06Nb0.024Ti0.94(Ta0.24Sn0.7)0.03O3	125	1.65	0.47	77.69	[1]
0.92Bi0.5(Na0.82K0.18)0.5TiO3-0.08Bi(Mg2/3Nb1/3)O3	110	2.20	1.75	55.7	[2]
0.92(0.6Na0.5Bi0.5TiO3-0.4Sr0.7Bi0.2TiO3)-0.08Ba(Mg1/3Ta2/3)O3	565	8.58	0.60	93.5	[4]
0.90Bi0.5Na0.5TiO3-0.10Bi(Mg2/3Nb1/3)O3	140	1.405	-	-	[12]
0.6Na0.5Bi0.5TiO3-0.4 Sr0.775Bi0.15TiO3	190	2.41	0.34	87.5	[13]
0.98(0.92Bi0.5Na0.5TiO3-0.08BaTiO3)-0.02Bi(Li1/3Ti2/3)O3	65	0.88	0.03	97	[14]
0.98(0.66Bi0.5Na0.5TiO3-0.34Sr0.7Bi0.2TiO3)-0.02K0.5Nd0.5TiO3	110	1.20	0.36	76.9	[15]
Bi0.4465Na0.4465Ba0.057La0.05TiO3-0.25Sr0.85Bi0.1TiO3	320	4.55	-	>90	[16]
0.9(0.75Na0.5Bi0.5TiO3-0.25SrTiO3)-0.1Ag(Nb0.85Ta0.15)O3	290	3.6	-	80	[17]
0.98(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-0.02La0.33NbO3	214	1.66	-	-	[18]

. According to the further investigation, the elements with low ion mobility such as La³⁺, Ta⁵⁺, Nb⁵⁺, etc. introduced into BNT-based ceramics could facilitate the decrease of grain size, thus improving the E_b [4,18,19]. All these results indicate that the ESP of BNT-based ceramics can be enchanted via introducing other components to form the appropriate solid solution.

Based on the investigation above, the long-range ordered ferroelectric state in BNT ceramics is broken due to the local random electric field induced by the heterogeneous charges distribution and the difference of the ionic radii, thus ferroelectric domains can transform into PNRs, leading to reduced P_r and obvious relaxation properties. Besides, PNRs are profitable for the stability, frequency stability, and fatigue resistance of the materials [20].

3.1.2. Metal/Metallic Oxide Doping of BNT-Based Ceramics

Metal elemental or metallic oxide component doping in BNT ceramics is an effective modification method for improving ESP. Ma et al. [21] added SiO₂ to 0.95(0.76Na_0.5Bi_0.5TiO₃-0.24SrTiO₃)-0.05AgNbO₃ ternary ceramics and found that the movement of grain boundary became difficult due to the low sintering temperature and diffusivity, resulting in the significant decreases of grain size and increase of E_b and W_rec effectively. Zhu et al. [22] doped MgO into BNT-based ceramics and found that adding small amount of MgO could inhibit the space charge migration, diminish the leakage current, and increase E_b, which showed excellent ESP as well as good temperature stability, excellent frequency, and fatigue stability. Yao et al. [23] introduced ZnO into 0.9(0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃)-0.1NaNbO₃ and revealed that both the dielectric constant (ε_r) and ΔP (P_max − P_r ) increase. Also, ZnO doping in BNT-based ceramics not only enhanced the densification but also reduced the sintering temperature [24], which were all favourable for promoting the W_rec. Zhang et al. [25] reported that doping semiconducting ZnO into 0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃ could significantly optimise electric properties while enhance thermal stability, contributing to boost the ESP. Yang et al. [26] synthesized La³⁺/Nb⁵⁺ co-doped [(Bi_1-xLa_x)_0.5Na_0.5]_0.94Ba_0.06(Ti_1-5y/4Nb_y)O₃ ceramics by traditional SSR method and disclosed that adding La³⁺/Nb⁵⁺ was beneficial to reduce thePr and improve the E_b. As x/y = 0.07/0.02, the ceramics showed good ESP. Relevant data is summarized in

Table 2. ESP of BNT-based ceramics by oxides doping.

Ceramic Compositions	E (kV/cm)	Wrec (J/cm3)	Wloss (J/cm3)	η (%)	Refs
0.95(0.76Na0.5Bi0.5TiO3-0.24SrTiO3)-0.05AgNbO3: SiO2	316	3.22	-	-	[21]
0.76Bi0.5Na0.5TiO3-0.2NaNbO3-0.04SrZrO3: 0.01MgO	370	5.2	0.92	85	[22]
0.9(0.94Na0.5Bi0.5TiO3-0.06BaTiO3)-0.1NaNbO3: ZnO	100	1.27	0.63	67	[23,24]
[(Bi1-xLax)0.5Na0.5]0.94Ba0.06(Ti1-5y/4Nby)O3: x/y = 0.07/0.02	135	1.83	-	70	[26]

.

3.1.3. Defect Engineering of BNT-Based Ceramics

It is well known that the elements of bismuth (Bi) and sodium (Na) in BNT-based ceramics were significantly volatile during the high temperature sintering process. In order to maintain charge neutrality, oxygen vacancies were inevitably generated in BNT-based ceramics. However, it has been proved that the generation of oxygen vacancies could facilitate the ionic conductivity and result in a drastic decrease in E_b [27,28]. Thus, it has been proposed to suppress the formation of oxygen vacancies by defect engineering. Yan et al. [29] fabricated 0.75Bi_(0.5+x)Na _(0.5−x)TiO₃-0.25SrTiO₃ ceramics using the defect engineering, which significantly suppressed the formation of oxygen vacancies. When x = 0.08, the E_b reached 569 kV cm⁻¹, and demonstrated excellent ESP with a W_rec of 5.63 J cm³ and η of 94%. Jiang et al. [30] reported an ultrahigh W_rec of 3.12 J cm⁻³ along with excellent η of 87.86% in 0.9(Na_0.4Bi_0.4Ba_0.06Sr_0.14Ti_(1−x)Ta_xO₃)-0.1NaNbO₃ ceramics via the defect engineering. Yang [31] adopted a defective engineering strategy by doping the multivalent element Mn to avoid the reduction of Ti⁴⁺. Thus excellent W_rec of 7.05 J cm³ was achieved at 387 kV/cm. Zhang et al. [32] designed a defect engineering strategy by introducing the La³⁺ to partially replace Bi/Na/K in the 0.6(Bi_0.5Na_0.4K_0.1)_1–1.5xLa_xTiO₃-0.4[2/3SrTiO₃-1/3Bi(Mg_2/3Ni_1/3)O₃]. When the doping content of La³⁺ was 3 at.%, the optimum ESP was achieved with W_rec of 8.58 J cm³ accompanied by η of 94.5%. A summary of the relevant data is given in

Table 3. ESP of BNT-based ceramics modified by defect engineering.

Ceramic Compositions	E (kV/cm)	Wrec (J/cm3)	η (%)	Ref
0.75Bi0.58 Na0.42 TiO3-0.25SrTiO3	535	5.63	94	[29]
0.9(Na0.4Bi0.4Ba0.06Sr0.14Ti(1−x)TaxO3)-0.1NaNbO3	270	3.12	87.86	[30]
0.76Bi0.5Na0.5TiO3-0.04SrZrO3-0.2NaNbO3-0.15MnO2	387	7.05	65	[31]
0.6(Bi0.5Na0.4K0.1)0.955La0.03TiO3-0.4[2/3SrTiO3 -1/3Bi(Mg2/3Ni1/3)O3]	420	8.58	94.5	[32]
0.85Bi0.5Na0.5TiO3-0.15AgNb0.5Ta0.5O3	510	6.6	72	[33]

.

3.1.4. Process Optimization of BNT-Based Ceramics

At present, the traditional SSR method is dominant in the preparation of BNT-based ceramics, whereas the ceramics fabricated via this process expose some deficiencies such as nonuniform grain size, pores, microscopic cracks, and comparatively low density, which are all unfavourable for the E_b and W_rec.

Recent investigations have manifested that two-step reaction sintering (TRS) technology displayed remarkable capacity in inhibiting the movement of grain boundaries, making it easier to obtain micron- or even nano-sized grains. Yu et al. [34] employed TRS method to prepare 0.94(0.75Bi_0.5Na_0.5TiO3-0.25Bi_0.5K_0.5TiO₃)-0.06BiAlO₃ ceramics with fine grains and dense microstructure. As the E was 105 kV/cm, the W_rec and η reached 1.15 J/cm³ and 72.3% respectively. Ding et al. [35] found that 0.89Bi_0.5Na_0.5TiO₃-0.06BaTiO₃-0.05K_0.5Na_0.5NbO₃ ternary ceramics prepared by the TRS method displayed three times higher W_rec than that of prepared via the conventional SSR method. Chu et al. [36] also found by the TRS method, [(Bi_0.5Na_0.5)_0.94Ba_0.06]_0.82La_0.12TiO₃(BNT-BT-0.12La) ceramics exhibited smaller grain size and dense microstructure. Also, the W_rec was 6.69 J/cm³ with the η of 87% under the high E_b of 440 kV/cm.

Apart from the TRS technology, the microwave sintering method has been proved a fast-heating speed, controllable grain growth, low sintering temperature, short holding time, high densification, and significant energy-saving effect [37]. In addition, compared with the conventional sintering technology, the microwave sintering process helps to promote the P_max of ceramics. Pu et al. [38] prepared (Na_0.5Bi_0.5)_0.8Ba_0.2Ti_0.8Sn_0.2O₃ ceramics by microwave sintering and obtained excellent ESP with W_rec of 2.347 J/cm³ under 120 kV/cm in comparison with the ones fabricated using SSR method.

In addition, other novel processes have been employed to obtain BNT-based ceramics with considerable performance. Uddin et al. [39]. prepared 0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃ ceramics by a sol-gel method, and under the low E of 90 kV/cm at 150 ℃, the W_rec was 0.6 J/cm³. Based on this, Xu et al. [40] employed Bi_0.5Na_0.5TiO₃-BaTiO₃ powders fabricated via the sol-gel method and KNbO₃ nano-scale powders obtained by the hydrothermal method as an initial material to fabricate 0.95(0.93Bi_0.5Na_0.5TiO₃-0.07BaTiO₃)-0.05KNbO₃ ceramics successfully. The results showed that the maximum W_rec reached 1.72 J/cm³, which was even better than some lead-based ceramic materials. Zhang et al. [41] successfully prepared 0.95Na_0.4K_0.1Bi_0.5TiO₃-0.05CaZrO₃-0.10NaNbO₃ lead-free ternary ceramics by a sol-gel method associated with TRS method, which displayed favourable ESP (W_rec = 2.63 J/cm³, η = 83.5%) at the E_b of 240 kV/cm. Kornphom et al. [42] found 0.722Bi_0.5Na_0.5TiO₃-0.228SrTiO₃-0.05AgNbO₃ ternary ceramics fabricated via the solid-state combustion method displayed a dense microstructure and thus improving E_b. The W_rec was 2.25 J/cm³ with η of 75.88%, which was better than the reported perovskite ceramic materials at the identical E of 130 kV/cm. These results indicate that compared with the traditional SSR method, BNT-based ceramics prepared by wet chemical method, such as sol-gel, hydrothermal method, et.al. generally produce fine grains and more grain boundary preventing the movement of charges effectively, which is extremely helpful to improve E_b and ESP.

3.2. BNT-Based Multilayer Ceramic Capacitors

With the increasing development of ceramic preparation technology, multilayer ceramic capacitors (MLCCs) have attracted comprehensive attention due to the characteristics of small equivalent series resistance (ESR), high-rated ripple current, complete varieties and specifications, small size, and low leakage current [43]. Jia et al. [44] studied 0.9[0.94(Bi_0.5Na_0.5TiO₃-0.25NaNbO₃)-0.06BaTiO₃]-0.1CaZrO₃ MLCCs, which displayed the excellent temperature stability in W_rec at the E of 120 kV/cm in the temperature range of −55–175 ℃, suggesting that the MLCCs can be used as a high-temperature ceramic capacitor. Generally, as for the energy storage of MLCCs, more uniformed fine grain in microstructure and electrical homogeneity of the composition facilitates the reduction of dielectric layer thickness and conductive pathways so as to enhance E_b and W_rec. Also, excellent energy storage is highlighted as shown in

Table 4. ESP of BNT-based multilayer ceramics.

MLCCs	E (kV/cm)	Wrec (J/cm3)	Wloss (J/cm3)	η (%)	Ref.
0.9[0.94(Bi0.5Na0.5TiO3-0.25NaNbO3)-0.06BaTiO3]-0.1CaZrO3	120	0.31	<0.09	>77	[44]
Na0.5Bi0.5TiO3-0.50Sr0.85Bi0.10TiO3	420	4.9	-	-	[45]
0.62Na0.5Bi0.5TiO3-0.3Sr0.7Bi0.2TiO3-0.08BiMg2/3Nb1/3O3	1013	18	1.35	93	[46]
0.4((Bi0.5Na0.5)TiO3)-0.6(0.87BaTiO3-0.13Bi(Zn2/3(Nb0.85Ta0.15)1/3)O3)	123.2	14.49	2.58	84.9	[47]
(Na0.5Bi0.5)TiO3-(Sr0.7Bi0.2)TiO3	1000	21.5	-	-	[48]

. Na_0.5Bi_0.5TiO₃-0.50Sr_0.85Bi_0.10TiO₃ (NBT-0.5SBT) layered lead-free ceramic capacitor fabricated via the tape-casting technology, which showed excellent ESP with W_rec of 4.9 J/cm³ under the E of 420 kV/cm, almost three times more than NBT-0.5SBT ceramics [45]. Ji et al. [46] prepared 0.62Na_0.5Bi_0.5TiO₃-0.3Sr_0.7Bi_0.2TiO₃-0.08BiMg_2/3Nb_1/3O₃ (NBT-SBT-0.08BMN) MLCCs with excellent ESP (W_rec = 18 J/cm³, η = 93%) at the E_b of 1013 kV/cm, which is almost three times of that for NBT-SBT-0.08BMN ceramics. Zhao et al. [47] fabricated 0.4(Bi_0.5Na_0.5)TiO₃)-0.6((0.87BaTiO₃-0.13Bi(Zn_2/3(Nb0.85Ta0.15)_1/3)O₃) (0.4BNT-0.6BTBZNT) MLCCs with more uniform, fine grain size and fewer pores in microstructure (as shown in Figure 2) via tape-casting combined with two-step sintering method, which found outstanding ESP (W_rec = 14.49 J/cm³, η = 84.9%) under 123.2 kV/cm, as well as good temperature stability and cycling stability. The group of Xu and Li also successfully prepared a high-quality <111> oriented BNT-based textured MLCCs with a texture degree of 91% by the casting-template method and achieved the high breakdown electric field of 100 MV/m and the highest W_rec of 21.5 J/cm³ up to now as shown in Figure 3 [48].

In summary, the ESP of BNT-based ceramics may be dramatically increased for the following reasons. First, the incorporation of non-covalent cations may interrupt the long-range ordered ferroelectric domain and strengthen its relaxation properties, thus decreasing the P_r. Secondly, adding the element with a large energy band makes it difficult for electrons to jump from the valence band into the conduction band, thus increasing the E_b of the ceramics. Alternatively, the suppression of space charge migrating by decreasing the sintering temperature may diminish the leakage current and enhance E_b. Meanwhile, using the defect engineering strategy would suppress the formation of oxygen vacancies and increase the E_b. Optimizing the process of the ceramics would lead to refine the grain size and obtain dense microstructure, so as to improve E_b. Third, fabricating MLCCs could significantly increase E_b, therefore optimizing the energy storage performance of BNT-based ceramics.

3.3. BNT-Based Energy Storage Thin Film

Compared with bulk materials, the most outstanding feature of the ferroelectric thin film is that considerable high E_b can be achieved, thus prompting the ESP as shown in

Table 5. ESP of BNT-based thin films.

Composition of Thin Films	E (kV/cm)	Wrec (J/cm3)	Wloss (J/cm3)	η (%)	Ref
0.4Bi0.5Na0.5TiO3-0.6Bi3.25La0.75Ti3O12 (sol-gel method)	2663	42.41	13.90	75.32	[49]
0.92(Bi0.54Na0.46)TiO3-0.08BaTiO3 (pulsed laser deposition)	4500	30	6.14	83	[50]
0.94Bi0.5Na0.5TiO3-0.06BaTiO3-0.05ST (sol-gel/spin-coating method)	1125	22.5	-	-	[51]
0.5Na0.5Bi0.5TiO3-0.5Sr0.7Bi0.2TiO3 (sol-gel method)	3200	35.041	12.44	73.8	[52]
0.5(Bi0.5Na0.5)TiO3-0.5Bi(Zn0.5Zr0.5)O3 (sol-gel method)	1500	40.8	-	-	[53]
0.9Na0.5Bi0.5TiO3-0.1BiFeO3 (magnetron sputtering and pulsed laser deposition)	1250	44	-	-	[54]

. Yue et al. [49] prepared 0.4Bi_0.5Na_0.5TiO₃-0.6Bi_3.25La_0.75Ti₃O₁₂ thin film by a sol-gel method and excellent ESP (W_rec = 42.41 J/cm³, η = 75.32%) could be obtained under the E of 2663 kV/cm. Serralta-Macías et al. [50] deposited 0.92(Bi_0.54Na_0.46)TiO₃-0.08BaTiO₃ thin film on highly boron-doped silicon (p-Si) by a pulsed laser deposition method (PLD). The optimal ESP was obtained under 4500 kV/cm compared to the bulk ceramics [39] with W_rec only 0.6 J/cm³ when the E was 90 kV/cm. Based on this, Xie et al. [51] prepared 0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃-0.05ST thin film on Pt/Ti/SiO₂/Si substrates by a sol-gel/spin-coating method, which demonstrated the good ESP under the E_b of 1125 kV/cm at the frequency of 1 kHz. Besides, 0.5Na_0.5Bi_0.5TiO₃-0.5Sr_0.7Bi_0.2TiO₃ (0.5NBT-0.5SBT) relaxor ferroelectric thin films grown on Pt/Si/SiO₂ substrates by a sol-gel method has been investigated by Ding et al. [52] and under the E of 3200 kV/cm, the W_rec reached 35.014 J/cm³ with η of 73.8% as well as excellent thermal stability, frequency stability, and fatigue resistance. Sun et al. [53] employed the sol-gel technique to prepare 0.5(Bi_0.5Na_0.5)TiO₃-0.5Bi(Zn_0.5Zr_0.5)O₃ thin film on LaNiO₃(100)/Pt(111)/TiO₂/SiO₂/Si substrate and found that adding Bi(Zn_0.5Zr_0.5)O₃ in BNT matrix could deform the [TiO₆] octahedron in the lattice, resulting in the coexistence of ferroelectric domains and PNRs and increase of ΔP (P_max − P_r). Meanwhile, the thin film exhibited homogenous, dense and flat microstructure (as shown in Figure 4), which was beneficial to the improvement of E_b. The result showed that W_rec reached 40.8 J/cm³ under the E of 1500 kV/cm, which was 1.62 times higher than that of BNT thin film. Zhang et al. [54] prepared 0.9Na_0.5Bi_0.5TiO₃-0.1BiFeO₃ thin films on (001) oriented SrTiO₃ substrates by a hybrid technique of magnetron sputtering and PLD. The W_rec of 44 J/cm³ under the E of 1250 kV/cm was 1.8 times of BNT thin film.

In a word, the BNT-based thin film has excellent ESP due to its exceptionally high E_b accompanied by dense microstructure with few defects. In addition, the thin films also possess excellent thermal stability, frequency stability, and fatigue resistance.

3.4. BNT-Based Energy Storage Thick Film

Based on the discussed above, the bulk material has a low W_rec due to the low E_b, which is inconsistent with the trend of miniaturization of electronic devices. As for the thin film, it could obtain high E_b, however the W can be limited due to their relatively restricted size. Comparatively speaking, thick-film materials with relatively high E_b can cover the shortage of bulk ceramics and thin film, thus meeting the requirements of high W_rec [55].

Consequently, BNT-based thick films have been investigated and the ESP of some BNT-based thick films are displayed in

Table 6. ESP of BNT-based thick films.

Thin Film System	E (kV/cm)	Wrec (J/cm3)	Wloss (J/cm3)	η (%)	Ref
BNT: BaO-B2O3-SiO2 (screen printing method)	600	2.0	2.55	44.1	[56]
BNT (PVP-modified sol-gel method)	1200	12.4	16.44	43	[57]
0.95(Na0.5Bi0.5)TiO3-0.05SrTiO3	1965	36.1	52.38	40.8	[58]
6Bi0.5Na0.5TiO3-4Sr0.7Bi0.2TiO3 (aerosol deposition method)	900	10.4	5.72	64.5	[59]
Na0.5Bi0.5Ti0.99Mn0.01O3 (PVP-modified sol-gel method)	2310	30.2	33.11	47.7	[60]
(Na0.85K0.15)0.5Bi0.5TiO3-0.02Fe (PVP modified sol-gel method)	2296	33.3	31.61	51.3	[61]
0.6(Na0.5Bi0.5)TiO3-0.4Bi(Ni0.5Zr0.5)O3 (water-based sol-gel method)	2200	50.1	28.30	63.9	[62]

. Clearly, glass phase BaO-B₂O₃-SiO₂ added BNT thick film prepared by a screen-printing technology displayed good ESP compared with the pure BNT thick film and the W_rec was 1.5 times that of BNT thick film [56]. In addition, Zhao et al. [57] employed the polyvinylpyrrolidone (PVP)-modified sol-gel method to prepare BNT thick film on LaNiO₃/Si(100) substrate and found that it had excellent ESP with the maximum W_rec of 12.4 J/cm³ under the E of 1200 kV/cm. Based on this, Xu et al. [58] fabricated (1−x%)(Na_0.5Bi_0.5)TiO₃−x%SrTiO₃ binary thick film and obtained the optimum ESP with W_rec = 36.1 J/cm³ and good thermal stability when x = 5. Kim et al. [59] prepared 6Bi_0.5Na_0.5TiO₃-4Sr_0.7Bi_0.2TiO₃ binary thick film by the aerosol deposition (AD) method and obtained good ESP under the low E of 900 kV/cm as well as excellent temperature stability (ΔW_rec < ~9% and Δη < ~30%) up to 140 °C. Wang et al. [60] doped Mn into BNT to prepare Na_0.5Bi_0.5Ti_0.99Mn_0.01O₃ lead-free ferroelectric thick films by a PVP-modified sol-gel method. The W_rec reached 30.2 J/cm³ under the E of 2310 kV/cm, which was twice than that of pure BNT thick film, along with good temperature and frequency stability. Besides that, Fe was introduced into (Na_0.85K_0.15)_0.5Bi_0.5TiO₃ to obtain NKBT-Fex thick films through the PVP-modified sol-gel method and at the E of 2296 kV/cm, the W_rec is 33.3 J/cm³ with η of 51.3%, also displaying a good frequency and temperature stability when x = 0.02 [61]. Upon further investigation, Sun et al. [62] introduced Bi(Ni_0.5Zr_0.5)O₃ into BNT to induce PNRs in 0.6BNT-0.4Bi(Ni_0.5Zr_0.5)O₃ ferroelectric thick film via a water-based sol-gel method. The thick displayed a dense microstructure (as shown in Figure 5) and the W_rec reached 50.1 J/cm³ under an E of 2200 kV/cm, making it great potential to replace lead-based thick films. However, the large leakage current (about 1 × 10⁻⁵ A/cm², at room temperature) in BNT-based thick films is harmful to practical applications in the field of energy storage [57]. Accordingly, seeking effective measures to reduce leakage current in thick film and then achieving excellent ESP has been focused.

In a nutshell, the thick films of the BNT-based ceramics were performed with excellent energy storage properties owing to their incorporation of the high E_b of the thin film and relatively large volumes of the bulk ceramics.

4. Summary and Perspectives

Here, we briefly summarized the present state and progress of optimizing energy storage performance in lead-free BNT-based materials, including ceramics, MLCCs, thin films, and thick films. Evidently, reducing the P_r and increasing the E_b are crucial tools to improve the ESP of BNT-based materials. Accordingly, several methods involved in solid solution modification, metal/metallic oxide doping, defect construction and optimizing processes have been reviewed for improving ESP of BNT-based ceramics in detail. For BNT-based energy storage ceramics, solid solution modification and metal/metallic oxide doping can be extremely effective in decreasing P_r, by means of interrupting the long-range ordered ferroelectric domain and strengthening its relaxation properties as a result of the incorporation of non-covalent cations. On the other hand, it is favourable for the increasing E_b due to adding the element with a large energy band. Also, by defect engineering strategy, suppression in the formation of oxygen vacancies or impeding the space charge migrating could reduce the ionic conductivity and cause a drastic increase in E_b. Besides, by optimizing the process, a dense microstructure with uniformly distributed grains of fine size and few pores can be obtained in the ceramics, which is conducive to the enhancement of E_b. For energy storage MLCCs, more uniform and fine grains in microstructure and electrical homogeneity of the composition facilitates the increment of E_b, therefore optimizing the ESP of BNT-based ceramics. As for BNT- based thin film, excellent ESP can be obtained mainly due to its exceptionally high E_b accompanied by dense microstructure with fine grain size. Compared with ceramics, MLCCs and thin films, BNT-based thick films can incorporate the high E_b of the thin film and relatively large volumes of the bulk ceramics. However, compared with lead-based energy storage materials, there is still a great gap, which made BNT-based materials difficult to meet industrial requirements completely.

Therefore, based on the theory and research summarized above, some key aspects related to the improvement of ESP have also been presented: (1) Novel preparation processes are expected to be considered for the ceramics, such as low-temperature and fast sintering (spark plasma sintering or flash sintering), which can accomplish the densification of ceramics in a very short time, thus suppressing the grain growth and preventing ion (Na⁺ and Bi³⁺) volatilization, then increasing E_b. (2) Composite modification is recommended. BNT-based ceramic materials can be combined with other inorganic materials owning large band gap and small P_r so as to obtain relaxor-type antiferroelectric/ferroelectric composite materials and enhance the W_rec. On the other, considering that one of the main advantages of organic materials is their extremely high E_b, BNT-based ceramic materials can be compounded with polymers, which is beneficial for maintaining the high dielectric constant and enhancing the E_b. (3) Exploring and developing the advanced thin/thick film preparation technique is encouraged to obtain the novel BNT-based film with optimal ESP.