Ultrahigh energy-storage in lead-free ceramic capacitors via local structure design

Abstract

Dielectric capacitors are crucial energy-storage modules in pulsed- and high-power devices. However, the simultaneous enhancement in recoverable energy density (W_rec) and efficiency (η) still remains challenge owing to the restrictive relationship between the maximum polarization (P_max), remanent polarization (P_r) and electric breakdown strength (E_b). To address this, we propose a strategy of local polar structure design in BiFeO₃-based ceramics. By incorporating NaNbO₃ to create embedded, persistent polar nanoregions within a weakly polar matrix, we achieve a giant polarization difference ΔP (56.4 μC cm^-2), a low P_r (3.6 μC cm^-2) and a large E_b (66 kV mm^-1), endowing an ultrahigh W_rec of 14.5 J cm^-3 and a high η of 88%. The local structure, directly visualized via the 2D/3D atomic displacement mapping, enables a high P_max under an applied field and a low P_r at zero field, a mechanism explicitly validated by phase-field simulation. Meanwhile, the optimized microstructure and enhanced insulating property contribute to a giant E_b. This work provides insights to overcome the paradox between multiple parameters and paves a feasible route for the cutting-edge energy-storage applications.

The authors propose a local structure design to overcome the restrictive relationship between polarization behavior and electric field durability, yielding an ultrahigh Wrec of 14.5 J cm^-3, which offers a feasible route for the cutting-edge energy-storage applications.

Introduction

As one of the important functional materials, dielectric ceramics have provoked ever-increasing interest in capacitive energy-storage in recent years^1–3. Unlike chemical batteries and electrochemical capacitors, the electric energy stored and released by dielectric capacitors is based on the polarization response to applied electric field (E), allowing for ultrafast charge/discharge speed, giant power density, high working voltage and excellent reliability. Hence, the dielectric capacitors hold promising applications in pulsed-power and high-power fields, such as inverters and converters in hybrid vehicles^4–6. Generally, the energy-storage performance (ESP) of dielectric ceramics, characterized by recoverable energy density (W_rec) and efficiency (η), is closely associated with the polarization electric-field (P-E) curve. Specifically, a high maximum polarization (P_max), a small remanent polarization (P_r) (or a high polarization difference ΔP = P_max -P_r) and a large electric breakdown strength (E_b) are desirable for the high W_rec, while the low polarization hysteresis characterized by a small P_r is responsible for the high η⁴. Relaxor ferroelectric (RFE) ceramics are considered as a potential alternative for energy-storage, resulting from their slim P-E loop and unique polar nanoregions (PNRs). However, the prevailing W_rec < 10 J cm⁻³ poses challenges for their usage in advanced electronic devices requiring compact size and light weight^4–6.

Numerous strategies have been implemented to improve W_rec based on the regulation of electric field durability and polarization behavior. On the one hand, the E_b of RFE ceramics not only restricts their operating electric field, but also possesses a positive correlation with W_rec^6,7. Compositional optimizations are widely employed to reduce grain size, improve bandgap and insulating properties^3,8. Novel processing techniques, including tape-casting and hot-press sintering, are generally adopted to enhance the density and reduce the porosity, thereby improving E_b^9,10. Additionally, microstructural designs like core-shell structure, 0-3 type composite and multilayer structure, help prevent the development of local electric branch, further improving E_b^11–13. On the other hand, the element selection, polymorphic PNRs, chemical framework and entropy engineering are the popular and feasible approaches to regulate the polarization behavior^1,8,14–18. For example, Bi-based matrix or end-member, including BiFeO₃, Bi(Mg_0.5Zr_0.5)O₃, etc., is beneficial to retain a relatively high P_max owing to the strong hybridization between Bi³⁺ 6s² lone pair electrons and O²⁻ 2p orbitals^4,8,15. High-entropy assisted Bi_0.5Na_0.5TiO₃-based RFE ceramics exhibited ultra-small polymorphic domains, which weakened the polarization anisotropy and reduced the switching energy barriers. This polarization configuration finally resulted in a small P_r, delayed polarization saturation while maintaining high P_max, thereby endowing superior ESP^14,17.

Despite great advancements in dielectric capacitors, the intrinsic constraint between P_max, P_r and E_b has not yet been addressed, limiting the further enhancement of ESP. Typically, P_max and P_r typically increase or decrease synchronously, creating a challenge for obtaining a low P_r while maintaining a high P_max, which finally yields a trade-off between W_rec and η⁶. Additionally, a high P_max generally indicates that more induced charges emerge at two surfaces of a dielectric, leading to a large electrostatic force and premature electro-mechanical breakdown (i.e., a low E_b)^4,19. Therefore, it still remains a great challenge to concurrently realize high W_rec and η through breaking the interdependent limitations by P_max, P_r and E_b.

To address this issue, we propose the local structure design to overcome the restraint between different parameters in (0.6-x)BiFeO₃-0.4SrTiO₃-xNaNbO₃ + 0.1 wt% MnO (denoted as BF-ST-xNN-Mn) ceramics, thereby achieving high W_rec and η simultaneously. 0.6BF-0.4ST ceramic with a morphotropic phase boundary demonstrates significant potential for energy-storage capacitors, due to its high intrinsic polarization ~40 μC cm^−220,21. Yet, the strong ferroelectricity and high leakage of 0.6BF-0.4ST ceramics enable lossy square P-E loops and inferior ESP (Fig. S1a). Herein, the perovskite end-member NN was incorporated to disrupt the long-range ferroelectric orders and promote the formation of PNRs while retaining highly polarized local clusters^3,18, leading to a slim P-E loop with a relatively high P_max and a significantly reduced P_r. Meanwhile, NN possess a large band gap (E_g ~ 3.4 eV), which is favorable for improving resistivity and enhance E_b. Additionally, the synthesis of phase-pure BF-based ceramics is challenging due to volatilization of Bi₂O₃ and the reduction of Fe³⁺ (Fe³⁺ + e⁻ → Fe²⁺) during sintering, which typically leads to high dielectric loss and leakage. In this study, MnO was added as a sintering aid to mitigate these issues. By helping to maintain the charge balance, MnO suppresses the formation of oxygen vacancies and secondary phase, thereby reducing leakage current and enhancing E_b^4,8. Resultantly, the restrictive relationships between P_max, P_r and E_b were significantly mitigated, endowing an ultrahigh W_rec of 14.5 J cm⁻³ and a high η of 88% under 65 kV mm⁻¹ in 0.46BF-0.4ST-0.14NN-Mn ceramics.

Results and discussion

Energy-storage properties

A series of BF-ST-xNN-Mn (x = 0–0.16) ceramics were synthesized by solid-state reaction, and their P-E loops under 10 kV mm⁻¹ were depicted in Figs. S1, S2a. With increasing NN content, the P-E loops become gradually slimmer, accompanied by decreased P_max and P_r from 39.7 and 30.4 μC cm⁻² (x = 0) to 8.5 and 0.2 μC cm⁻² (x = 0.16), owing to the enhanced relaxor behavior (Fig. S3). Accordingly, the η progressively increases from 13.2% at x = 0 to 91.3% at x = 0.16 (Fig. S2b). Figure S2c exhibits the P-E loops of BF-ST-xNN-Mn ceramics under the respective maximum applied E. Samples were prepared with a thickness of ~80 μm and an electrode area of ~0.79 mm². This sample geometry, combined with the high relative permittivity (Fig. S3), suggests a relatively lower susceptibility to fringing-field effect and parasitic capacitance (Fig. S4)⁷. A high P_max exceeding 40 μC cm⁻² under applied E is maintained across compositions x = 0.05–0.16, due to the strong hybridization between Bi-O orbitals^8,15. Particularly, the composition x = 0.14 achieves a giant P_max of 60 μC cm⁻², a low P_r of 3.6 μC cm⁻², and the highest ΔP of 56.4 μC cm⁻² under 65 kV mm⁻¹, thereby delivering an optimal ESP with W_rec = 14.5 J cm⁻³ and η = 88% (Fig. S2d). These results suggest that x = 0.14 achieves a good balance between P_max and P_r, ΔP and E, and W_rec and η. It was therefore selected as the representative composition for further study.

Figure 1a displays the E-dependent P-E loops for x = 0.14. The P-E loop always retains a slender shape across all fields, with both P_max and P_r increasing steadily as the field rises (Fig. S5). This is attributed to the E-induced long-range ferroelectric orders, which rapidly return the initial disorganized state after removing E. Ultimately, the W_rec increases monotonically from 0.6 J cm⁻³ at 10 kV mm⁻¹ to 14.5 J cm⁻³ at 65 kV mm⁻¹, while the η remains largely constant (Fig. 1b). To appreciate the ESP of x = 0.14, the comparison in the key parameters, including ΔP, E_b, W_rec, and η, between BF-based bulk ceramics is presented in Figs. 1c, d, S6. A high W_rec is dominated by the collective effect of high ΔP and large E_b. However, these two parameters are typically in direct competition, fundamentally limiting the enhancement of W_rec in dielectric ceramics^4,6. Remarkably, the x = 0.14 composition mitigates this trade-off, achieving a giant ΔP of 56.4 μC cm⁻² simultaneously with a high E_b, resulting in a record-high W_rec of 14.5 J cm⁻³ among BF-based ceramics (Fig. 1c). Considering the inequality for W_rec obtained under different E, the factor of merit W_rec/E is adopted as another indicator to further evaluate the ESP. As shown in Fig. 1d, the simultaneous optimization of a high W_rec/E and a high η also remains a considerable challenge. For example, BF-BaHf_0.05Ti_0.95O₃-Sr(Al_0.5Nb_0.5)O₃ ceramic exhibits an ultrahigh W_rec of 12.2 J cm⁻³ and a high η of 90.1% under 83 kV mm⁻¹, but its W_rec/E is only 0.15¹⁶. And BF-BaTiO₃-(Na_0.7Sm_0.1)NbO₃ ceramic achieves a W_rec of 13.1 J cm⁻³ under 65 kV mm⁻¹ with a high W_rec/E of 0.20, but the η is only 78.3%²². However, x = 0.14 in this work overcomes the conventional restriction between P_max, P_r and E_b, thereby exhibiting a high W_rec/E of 0.22 and a high η (88%) simultaneously, which demonstrates a superior comprehensive ESP among BF-based ceramic capacitors.

Fig. 1. Energy-storage properties of x = 0.14.

E-dependent a unipolar P-E loops and b corresponding W_rec, η. Comparison of c ΔP and E_b, d W_rec/E and η between BF-based bulk ceramics. e W_d as a function of time under increasing E. f Comparison in W_d between BF-based bulk ceramics.

The E-dependent overdamped charge-discharge curves of x = 0.14 are depicted in Fig. S7, which is a direct route to obtain the discharge energy density (W_d). A sharp current peak under each E reveals the fast charge-discharge speed. W_d is determined by the time-dependent current curves according to the equation: $W_{\text{d}}=\frac{R\int I^2\left(t\right)\text{dt}}{V}$, where R, I, V are the load resistance (13 kΩ), current and active volume of sample^4–6, respectively, as shown in Fig. 1e. W_d increases monotonically from 0.4 J cm⁻³ at 10 kV mm⁻¹ to 10.3 J cm⁻³ at 65 kV mm⁻¹, far beyond currently reported BF-based bulk ceramics (Fig. 1f). Notably, W_d of x = 0.14 is smaller than W_rec (14.5 J cm⁻³) under the same E, which is ascribed to the clamping effect of domains and the viscous force in the direct charge-discharge process⁴. Additionally, the discharge time t_0.9 is 3.3 μs, which is normally utilized to evaluate the discharge time and extracted from the time to reach 90% of W_d, as marked in Fig. 1e.

Fig. S8a presents the cumulative cycle-dependent P-E loops of x = 0.14 ceramic measured at 25 kV mm⁻¹. No significant variation in P-E loop can be observed with increasing cycling numbers, demonstrating excellent fatigue resistant. Accordingly, the W_rec of 3.25 ± 0.02 J cm⁻³ and the η of 88 ± 1% are achieved over 1–10⁶ cycles (Fig. S8b). Figure S8c displays the unipolar P-E loops measured at various temperatures under 25 kV mm⁻¹. As the temperature elevates from 30 to 120 °C, the P_max and P_r slightly increase owing to the enhanced leakage loss and/or ionic conduction at elevated temperature. Consequently, W_rec slightly decreases from 3.3 to 3.1 J cm⁻³ while η shows a more pronounced reduction from 88.5 to 81.5% (Fig. S8d), revealing a satisfactory thermal stability.

Characterization of local structure

To probe the underlying mechanism for the outstanding ESP, transmission electron microscopy (TEM) was employed to directly explore the 2D local microstructure of x = 0.14. Conventional ferroelectric materials typically exhibit lamellar domains visible in dark-field TEM imaging^8,23. However, such long-range domains are absent in the x = 0.14 sample (Fig. 2a). Instead, x = 0.14 displays a slim P-E loop (Fig. S2), characteristic of relaxor behavior. This combination suggests the presence of PNRs or fine-scale nanodomains. Moreover, x = 0.14 exhibits a long-range average cubic structure (Fig. 2b, c), consistent with X-ray diffraction (XRD) patterns (Fig. S9). The high-angle annular dark-field (HAADF) image recorded by atomic-resolution STEM along the [001]_C is shown in Fig. 2d. The larger and brighter dots are A-site atoms while the smaller and dimmer atoms are B-site atoms, due to their different ionic radius and atomic masses¹¹. The diffused c/a ratio ranging from 0.97 to 1.06 indicates the fluctuating local lattice distortion (Fig. 2e). The intrinsic ferroelectric polarization stems from the structural distortion. So, this fluctuating lattice distortion yields various local polarizations^24,25. The atomic displacement vectors, determined by calculating the offset of A/B-site atoms from the center of the nearest four B/A-site atoms via 2D Gaussian function fitting^1,24, are illustrated in Fig. 2f. The length and color of arrows represent the magnitude and direction of these vectors, respectively. The atomic displacement vectors reveal regions with polar orientations corresponding to rhombohedral (R), orthorhombic (O), tetragonal (T), and cubic (C)-like symmetries (Fig. 2g). This local symmetry frustration and the coexistence of polymorphic polar configurations are characteristic of relaxor systems with compositional disorder and are consistent with the formation of PNRs. A wide distribution of polar direction suggests that these polar clusters are connected by the monoclinic (M) symmetry, low symmetry in the transitional regions (Fig. 2h), which facilitates reducing the polarization anisotropy and polarization switching barrier, thereby resulting in a decreased polarization hysteresis, a low P_r and an enhanced η. Moreover, 1-4 nm sized polar clusters with polar magnitude up to 18 pm are embedded into the matrix (Fig. 2i), demonstrating the strong local polar clusters, which does favor to retain a high P_max/ΔP under applied E, and thus enhance W_rec.

Fig. 2. Microstructures of x = 0.14 probed by electron microscopes.

a The typical low-resolution, b high-resolution dark-field image and corresponding c SAED image. d HAADF-STEM image recorded along the [001]_C, and corresponding e c/a ratio, f atomic displacement vectors. g Magnified image of selected areas from f. Mapping of h polar angle and i polar magnitude of atomic displacement vectors.

To elucidate the role of distinct ions in the formation of polar clusters, the atomic pair-distribution function (PDF) based on the neutron total scattering is performed, which reveals the local structure information from subnanometre to nanoscale²⁶. Figure 3a depicts the refinement results of real-space data G(r) and reciprocal-space S(Q) data of x = 0.14 derived from the reverse Monte Carlo (RMC) algorithm using a 20 × 20 × 20 perovskite supercell (~80 × 80 × 80 Å)^27,28. The perfect matching between experimental and calculated data suggests high reliability. Figure 3b exhibits the PDF G(r) pattern in the low-r range, manifesting the first coordination environment for different atomic bands. The extracted partial A/B-O PDFs further indicate a large difference in the local distribution of different atomic pairs (Fig. 3c). The obviously separated peaks of Bi-O, Sr-O and Na-O atomic pairs suggest the strong local polar displacement of Bi, Sr and Na²⁹. The specific atomic polar displacement vector can be obtained from the deviation between the A/B-site ion and the center of the AO₁₂ and BO₆ polyhedron. The 3D polar projections of Bi³⁺ along [001]_C are shown in Fig. 3d. The random distribution of polar vectors indicates the coexisting of R, T and O symmetry. In addition, the Bi³⁺ achieves the largest average polar displacement of 0.38 Å among all ions (Fig. 3e), ascribing to its unique electronic structure^3,30. And the average polar displacement of Nb⁵⁺ is larger than that of Fe³⁺ and Ti⁴⁺, indicating the improvement of local polar displacement at the B-site. The reconstructed 3D atomic model and polar vector distributions (Fig. 3f–i) reveal significant A-site positional disorder and local chemical clustering. These features, commonly observed in multi-component perovskites, contribute to the nanoscale polar heterogeneity that underpins the relaxor behavior. The introduced Na⁺ ions are randomly distributed in the matrix consisted by Bi³⁺ and Sr²⁺, which tend to gather together to form nanoscale clustering. Similar phenomenon is also found in the distribution of different oxygen octahedron. Accordingly, a wide polar direction distribution, as well as several 1–4 nm polar clusters with a high polarization as high as 50 μC cm⁻³, is observed, consistent with STEM results. This result suggests that the engineered local structure is induced by the heterogeneous distribution of atoms with differing ionic radii, electronic configurations, and polarizabilities^3,18, that collectively facilitate a high P_max/ΔP, a low P_r, and superior ESP.

Fig. 3. Local structures of x = 0.14 based on the neutron total scattering.

a Fitting results of neutron PDFs. b Enlarged G(r) pattern in the r range of 1.5-4.5 Å. c A/B-O partial PDFs. d Polar vector stereograph along the [001]_C. e Average magnitude of the atomic polar displacements. f 3D A-site atomic and g oxygen octahedron configuration. h 3D polar magnitude and i polar angle distribution.

Phase-field simulation for the role of polar structure

To further elucidate the role of local polar structure, phase-field simulations were performed to visualize polarization vectors and their evolution under external E. The simulated configuration, shown in Fig. 4a, exhibits polarization vectors with diverse orientations, representing polymorphic nanodomains. Figure 4b shows polar fluctuations with spontaneous polarization ~0.3 C m⁻² randomly embedded in the weak-polar matrix, closely resembling the polarization configurations of x = 0.14 composition. With increasing E, the polarization of the matrix rapidly aligns with E. In contrast, the embedded local polar fluctuations retain their initial orientation. As E is further increased, these local polar fluctuations gradually align along the E, promoting the development of enhanced long-range ferroelectric orders and a further increase in overall polarization. This behavior indicates that the existence of strong local fluctuations delays the polarization saturation and facilitates a higher P_max under high E³¹, a trend consistent with the E-dependent P-E loops observed for x = 0.14 (Fig. 1a). Upon removal of the external E, the polarization vectors largely revert to their initial randomized and low magnitude state, suggesting a low energy barrier for domain switching and minimal energy dissipation during charging/discharging process, thereby leading to a low P_r. This mechanism illustrates how relaxors with pronounced local polar structure can circumvent the conventional trade-off between P_max and P_r under an applied field, which concurrently enhances the W_rec and η.

Fig. 4. Phase-field simulation for the role of local structure.

Evolution of a polarization angle and b magnitude of relaxors with strong local polar structure during loading and unloading E.

Mechanism for electric breakdown

The large E_b also plays an indispensable role in achieving excellent ESP of x = 0.14. As shown in Fig. 5a, the distribution of actual E_b for each sample matches well with the Weibull distribution analysis^1,16. The average E_b elevates from 13 kV mm⁻¹ for x = 0 to 66 kV mm⁻¹ for x = 0.14 (Fig. 5b), resulting from complicated factors, including but not limited to microstructure, insulating properties⁴. The dense microstructure with homogeneous element distribution is observed (Figs. S10, S11). The average grain size decreases significantly from micron- to submicron-scale with increasing NN content (Figs. 5b, S12). The smaller grain size corresponds to more grain boundaries, acting as barriers for the electric breakdown. To demonstrate this, the finite element simulation on the distribution of the local E, electric potential distribution, and electric tree is implemented (Figs. 5c, 5d, S13, and S14). It is well-documented that the ε_r of grain is much higher than that of the grain boundary³², leading to the higher E distribution in the grain boundary. Additionally, the reduced grain size enables a more uniform distribution of local E and electric potential. The starting electric breakdown stems from the weak areas with concentrated E. With increasing time duration, the electric branches gradually propagate through the grains and the grain boundary. Particularly, the grain boundary is more easily to attract the electric branches, dispersing and dissipating the externally loaded electrostatic energy. Accordingly, the propagation speed of the electric tree slows down in x = 0.14 with higher grain boundary density, leading to an enhanced E_b.

Fig. 5. Analysis for the E_b of BF-ST-xNN-Mn ceramics.

a Weibull distribution. b Composition-dependent E_b and grain size. Finite element simulation of the breakdown path of c x = 0 and d x = 0.14. Composition-dependent e impedance spectra, and f E_g and E_a.

The insulating properties can be analyzed from two aspects. On the one hand, Figs. 5e, S15 depicts the impedance spectra of BF-ST-xNN-Mn ceramics, where Zʹ/Z" is the real/imaginary part of complex impedance. The composition-dependent total resistivity, extracted from the low frequency intercept of the arc on the Zʹ axis, steadily elevates from x = 0 to x = 0.14 (Fig. S16a). The activation energy E_a, calculated from the Arrhenius law (Fig. S16b), achieves a peak value at x = 0.14 (Fig. 5e), suggesting the optimized insulating performance. On the other hand, the band gap E_g represents the energy to excite electrons jumping from the top of the valence band to the bottom of the conduction band⁴. As plotted in Figs. 5f, S17, the improved E_g also suggests the enhanced insulating properties, leading to an improved E_b of x = 0.14.

In summary, BF-ST-xNN-Mn ceramics with an ultrahigh W_rec of 14.5 J cm⁻³ and a high η of 88% were simultaneously obtained via local structure design. Experimentally, the atomic displacement vectors mapping derived from STEM images and a 3D atomic model from neutron total scattering using the RMC method were performed to identify the local polar structure. Theoretically, the positive effect of local structure on the polarization behavior was justified by phase-field simulation. Moreover, the large E_b originated from the synergistic effect of multiple factors, including reduced grain size, enhanced insulating properties, and so on. Accordingly, a giant ΔP of 56.4 μC cm⁻² under a giant E of 65 kV mm⁻¹ was achieved, indicating the delayed restrictive relationship between P_max, P_r and E_b. These results establish a viable approach to designing high-performance dielectric ceramics for the cutting-edge capacitive energy-storage.

Methods

The detail in the experimental procedure is provided in the Supplementary Information.

Author contributions

J.Z. and Y.J.W. conceived the idea. Z.Q.L. fabricated the ceramic samples. Z.Q.L. and H.J.L. conducted structural and electrical characterization. S.H.W. performed phase field simulations and finite element simulations. J.Z. wrote the original manuscript. S.J.Z. and Y.J.W. commented on the results and revised the manuscript. Y.J.W. guided the project. All authors discussed the results and commented on the manuscript.

Peer review

Peer review information

Nature Communications thanks Hangfeng Zhang and the other anonymous reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-026-71276-4.