Ultrahigh energy-storage dielectric ceramics via synergistic polymorphic nanodomain and defect design

Domain: biology
Published: 2026-03-20
Editorial outcome: major_revision

Abstract
Dielectric capacitors are critical for advanced energy storage due to their ultrahigh power density and rapid charge-discharge rates. However, their application is limited by the low energy density. Here, we design a BaTiO3-(K0.5Na0.5)NbO3-(Bi0.5Na0.5)Ti0.9Zr0.1O3 solid-solution system through synergistic polymorphic nanodomain engineering and defect optimization. By engineering polymorphic nanodomains with coexisting rhombohedral, orthorhombic, tetragonal, cubic nanodomain in 1.0–2.0 nm size and defect design with reduces oxygen vacancy concentration and forms defect complexes, we achieve an ultrahigh energy density of 18.7 J cm⁻3 with remarkable efficiency of 92.1% in multilayer ceramic capacitors, along with excellent cycling stability (>107 cycles) and thermal stability (<±5% from 25 to 150 °C). In this work, we provide a paradigm for designing high-performance dielectric capacitors through the synergistic manipulation of domain structures and defect engineering.

Introduction
High-performance energy storage devices with superior energy and power densities are crucial for next-generation electronic and electrical systems. Dielectric capacitors, as electrostatic energy storage devices, offer exceptional charging and discharging speeds along with ultrahigh power densities, which allow them to outperform electrochemical energy storage devices such as batteries and supercapacitors1-4. However, the widespread application of ceramic capacitors faces significant challenges due to their relatively low energy storage densities Ue and/or low energy efficiency η, which currently impede their miniaturization and integration into advanced systems. To address these limitations, extensive efforts have been made to develop novel dielectric materials with simultaneously enhanced energy storage densities and improved efficiency, striving to fulfill the escalating requirements of next-generation energy storage technologies5-7.

The energy-storage performance of ceramic capacitors is characterized as follows:
Ue =  Pr Pm E d P , η = Ue / ( Ue + Uloss ), where Ue, η, Pm, Pr, and Uloss are the recoverable energy density, energy efficiency, maximum polarization, remnant polarization, and energy loss, respectively8. To achieve an ultrahigh Ue and η, it is necessary to optimize three key parameters simultaneously: high Pm, small Pr, and large breakdown strength (Eb). However, these parameters exhibit strong intercorrelation, creating significant challenges for concurrent optimization9. For example, ever-increasing efforts have been made in typical lead-free ferroelectric-based dielectrics with intrinsically high Pm values, including BiFeO3 (BFO)10, 11 and (Bi0.5Na0.5)TiO3 (BNT)-based ceramics7, 12, 13. However, their high Pm values are typically accompanied with high Pr values, which causes significant hysteresis loss due to the irreversible motion of ferroelectric domain walls, leading to a decrease in η14, 15. Typically, the main approach to suppress hysteresis loss has been to develop relaxor ferroelectrics (RFEs) by disrupting the long-range-ordered micrometer-scale domains into short-range-ordered nanodomains or polar clusters of several unit cells through element doping and/or solid solution16-19, which lowers the domain switching barriers. Furthermore, the improved η directly correlates with reduced Joule heating from hysteretic losses during polarization switching, thereby minimizing localized thermal effects and enhancing Eb2. However, the main limitation of this approach lies in the significant reduction in polarization 20, 21. This inherent limitation underscores the need for innovative material design strategies that transcend current paradigms, paving the way for next-generation dielectric materials capable of meeting the rigorous demands of advanced energy storage applications.

To achieve optimal balance between polarization and breakdown strength in dielectric materials, researchers have developed several innovative strategies. Among these, polymorphic nanodomain engineering has demonstrated remarkable advantages to enhance the Pm values by 27% compared to that in a single rhombohedral (R) nanodomain2. Additionally, polymorphic nanodomains play a significant role in improving the Eb of the material while simultaneously reducing the polarization switching hysteresis. These combined effects result in a twofold increase in Ue. The evolution of microstructures, particularly the transition between amorphous and crystalline states, also plays a critical role in enabling simultaneous maintenance of high levels of polarization and superior breakdown strength. This principle is exemplified in Bi3NdTi4O12 microcrystalline-amorphous dual-phase films, where the synergistic interaction between amorphous and crystalline phases achieves an ideal balance between polarization and breakdown field strength, resulting in ultrahigh energy storage density9. In addition to structural modifications, defect engineering has emerged as a crucial strategy for further improving the performance of dielectric materials22-26. By transforming shallow single-point defects into defect complexes as effective charge carrier traps, this approach not only significantly improves material resistivity and breakdown field strength27, but also ensures the overall stability and reliability of high-performance energy storage applications. In addition to electrical resistivity and dielectric breakdown strength behaviors, defect complexes can also contribute to modifications in polarization properties. The polarizability of defect complexes exhibits three orders of magnitude higher than that caused by electron/ion displacement, providing a novel pathway for enhancing Pm28. However, the stabilization of defect complexes through defect engineering to achieve simultaneous ultrahigh energy storage density and efficiency in bulk ceramics remains a fundamental research challenge. Therefore, it is desirable to overcome this bottleneck for developing next-generation energy storage capacitors.

In this work, we designed a BaTiO3(BTO)-(K0.5Na0.5)NbO3(KNN)-(Bi0.5Na0.5)Ti0.9Zr0.1O3(BNTZ) solid-solution system to achieve optimal energy storage performance through synergistic polymorphic nanodomains and defect design. The dielectric BTO was selected as the matrix material due to its high intrinsic dielectric constant, low loss, and superior chemical stability. To further reduce domain switching barriers, we incorporated KNN to form a polymorphic nanodomain 0.55KNN-0.45BTO29. Furthermore, the introduction of BNTZ significantly enhances the polarization response of the material. Concurrently, it reduces oxygen vacancy concentrations, which contributes to an increased breakdown strength. Precise tuning of the Bi3+/Na+ ratio enabled further effective defect engineering, resulting in a higher breakdown strength without degrading polarization, which is critical for simultaneously boosting the breakdown strength and fine tuning of the polarization response. The composition series was systematically studied and the optimized 0.7(0.45BTO-0.55KNN)-0.3(Bi0.475Na0.525)Ti0.9Zr0.1O3 composition demonstrated exceptional energy storage performances, achieving an ultrahigh Ue of 18.7 J cm-3 and outstanding η of 92.1% under an applied electric field of 965 kV cm-1.

Results
We synthesized a series of compositions (1-x)(0.45BTO-0.55KNN)-x(Bi0.5-yNa0.5+y)Ti0.9Zr0.1O3 ceramics (x=0.0 to 0.4, y= 0 to 0.05) using the traditional solid-state method to explore their potential in dielectric and energy storage properties. X-ray diffraction (XRD, Fig. 1a) confirmed that all the samples exhibited a pure perovskite phase. The pristine BTO sample displayed characteristic peak splitting (002)T/(200)T, confirming its tetragonal (T) symmetry. With the incorporation of KNN, the split XRD peaks merge into a single peak in the x=0, y=0 samples, which showed the macroscopically nonpolar cubic symmetry29. The introduction of BNTZ into the matrix led to a noticeable shift of the peaks towards higher angles, due to the smaller ionic radius of the doped cations in comparison to those of the matrix cations, which in turn suggests a lattice shrinkage. This shift, coupled with peak broadening in Raman spectra (Supplementary Fig. S1), confirms successful solid solution formation and the introduction of local structural disorder through multi-element occupation at equivalent lattice sites30. To further elucidate the crystal structures of the ceramics, we have calculated the lattice parameter and unit-cell-volume of the samples (Supplementary Fig. S2). For pure BTO, the average A-site and B-site radii are 1.610 Å and 0.605 Å, respectively31. The addition of KNN, whose average A-site and B-site radii are 1.515 Å and 0.640 Å31, 32, leads to the initial cell volume shrinkage from 64.414 Å3 to 64.096 Å3 with a reduction of 0.49%. With the subsequent incorporation of 30 mol% BNTZ (average radii: 1.375 Å for A-site, 0.617 Å for B-site)4, 31, compared to the 0.45BTO-0.55KNN composition with average radii of 1.558 Å (A-site) and 0.624 Å (B-site), this reduction at the A-site and B-site with smaller ionic radius drives the more pronounced cell volume shrinkage to 62.665 Å3, representing a total volume decrease of 2.72% from pure BTO. Notably, adjusting the Bi/Na ratio within BNTZ results in a slight increase of the cell volume to 62.807 Å3.

Scanning Electron Microscopy (SEM) was further used to investigate the microstructure of the BTO-based ceramics, including the grain size distribution and overall morphology (Fig. 1b and Supplementary Fig. S3). The pristine BTO sample exhibits an average grain size of 1.86 μm with an approximate relative theoretical density of 94.5% (Supplementary Fig. S4). In the BTO-KNN sample (x=0, y=0), a noticeable reduction in average grain size to 0.35 μm with a relative density of 95.2% was realized, which can be induced by the addition of KNN that inhibits the grain size growth. Further evolution in the microstructure can be induced by the addition of BNTZ due to the sintering aid effect of Bi elements, the average grain size of the samples with x=0.3, y=0 and x=0.3, y=0.025 increases slightly to 0.51 and 0.56 μm (Fig. 2b), while the relative density rises to 96.1% and 97.6%, respectively (Supplementary Fig. S4), indicating further densification of the ceramics, which is essential for enhancing the dielectric breakdown strength.

To gain comprehensive atomic-scale insights into the structural evolution and domain characteristics, we conducted high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) along the [110] zone axis, as illustrated in Fig. 1c. By quantitative analyzing the projected displacement vectors of the B-site cations relative to the lattice center of their four nearest A-site cations, indicated by the yellow arrows, we were able to determine the domain structures15, 18. Utilizing 2D Gaussian peak fitting results, we delineated the domains with shaded boxes, where the projected displacements of B-site cations along the [001]c, [1-11]c, and [1-10]c directions signify the T, R and orthorhombic (O) nanodomain, respectively16. For the composition with x = 0.3, y = 0.025, the T, R, O, and cubic (C) nanodomain are clearly observed, with domain sizes ranging from approximately 1.0 to 2.0 nm. To further elucidate the local structural modifications induced by BNTZ incorporation, we conducted detailed analysis of local c/a ratio distributions at A-site sublattice positions derived from atomic-resolution STEM images (Fig. 1d and 1e). Although the average c/a ratio shows only a slight variation (from 0.999 to 0.994) in the BTO-based ceramics, the c/a ratio distribution for x=0.3, y=0.025 composition is more diffused than that for x=0, y=0 composition, with its standard deviation increasing from 0.028 to 0.038 (Fig. 1f). This can be attributed to the intensified local strain and random field induced by the incorporation of multiple ions with different ionic radii and valence states, respectively. These competing perturbations effectively maintain short-range polar nanoregions, leading to local distortions and fluctuations of the c/a ratio13, 16.

To comprehensively investigate the defect behavior of these ceramics, we conducted thermally stimulated depolarization current (TSDC) measurements across a range of poling electric fields (2-8 kV cm-1). The poling temperature (Tp) was 250 °C, the poling time was 30 minutes, followed by a controlled heating rate of 5 °C min-1 during measurement. The types of defects can be identified through systematic analysis of relaxation peak characteristics, including the peak positions (Tm, the temperature where the current density reaches its maximum absolute value) and the peak intensities (Jm, the maximum current density) as a function of the poling electric field (Ep)33-36. The TSDC results revealed significant composition-dependent variations, demonstrating that the incorporation of KNN and BNTZ, along with precise tuning of the Bi3+/Na+ ratio, profoundly influences the defect types and associated relaxation processes (Fig. 2). The TSDC curves of pure BTO ceramics exhibited four peaks, with peaks at -65°C, 23°C, and 124°C corresponding to the transitions from rhombohedral to orthorhombic to tetragonal to cubic, respectively, and an additional relaxation peak at 190°C. This high-temperature peak showed a characteristic field dependence, with increasing Jm coupled with decreasing Tm at higher Ep values, indicating the presence of trapped charges in the BTO ceramics37. With the introduction of KNN, the TSDC curve for the sample with x = 0 and y = 0 displays two prominent new relaxation peaks (labeled peak C1 and peak D1) above 150°C, both showing slightly increasing Tm and Jm with higher poling fields. These characteristics strongly suggest these peaks originate from oxygen vacancy migration within grain interiors and across grain boundaries, respectively38. Furthermore, the incorporation of BNTZ (x = 0.3, y=0) produced remarkable modifications in the defect landscape, as evidenced by several distinct changes in the TSDC curves. Most notably, oxygen vacancy-related peaks C2 and D2 were substantially suppressed, with Jm values decreasing by more than 60%, demonstrating effective inhibition of these oxygen vacancy defects since the defect concentration can be semi-quantitively represented by Jm39, 40. Simultaneously, new weak relaxation peaks emerged near the Curie temperature (peaks A2 at 0 °C versus Tc = 5 °C, Supplementary Fig. S5), corresponding to the ferroelectric-paraelectric phase transitions. Equally significant were the appearances of B2 peaks, which exhibited field-independent Tm values but linearly increasing Jm values as Ep increased. These observations suggested that the B2 peaks were related to the relaxation of defect complexes41, 42. These collective data unambiguously showed that BNTZ addition simultaneously reduced oxygen vacancy concentrations while establishing new defect complexes configurations. Further refinement through precise Bi3+/Na+ ratio adjustment led to dramatic intensification of peak B3, signaling a substantial increase in defect complexes concentrations. This carefully controlled composition modification facilitated the formation of defect complexes and further reduced the oxygen vacancy content (Supplementary Fig. S6). Further supporting evidence for the formation of defect complexes formation can be observed in the current loop measurements (Supplementary Fig. S7). In normal ferroelectric materials, such as pure BTO in Supplementary Fig. S7a, two sharp current peaks can be observed, corresponding to the switching of macroscopic ferroelectric domains. As the material transitions to a relaxor ferroelectric, as shown in Supplementary Fig. S7b-c, these current peaks become less pronounced, indicating a transition from long-range-ordered, micrometer-sized ferroelectric domains to short-range-ordered, nanometer-sized domains. In relaxor ferroelectrics containing defect complexes (Supplementary Fig. S7d), four current peaks are observed in the loop. These defect complexes, composed of paired positive and negative charges, which introduce a random field to hinder the PNRs switching and causes 4 peaks in the current loop27.

We systematically characterized the dielectric properties of the ceramics over a broad temperature range from -180 °C to 300 °C (Fig. 3a-c; Supplementary Fig. S5). Pure BTO ceramics exhibit characteristic dielectric peaks at -65 °C, 25 °C, and 125 °C, respectively, consistent with the phase transitions observed in TSDC measurements. The relaxor diffuseness factor γ, calculated based on modified Curie-Weiss law, was 1.10 (Supplementary Fig. S8), which suggests normal ferroelectric characteristics. For the ceramic with a composition of x = 0 and y = 0, we observed a clear evolution toward relaxor ferroelectric characteristics, as evidenced by a broadened dielectric peak and an increased γ value of 1.56. This relaxor behavior became more significant with the incorporation of BNTZ (x = 0.3, y = 0), where γ further increased to 1.97, and the temperature of maximum permittivity (Tm) shifted from -155°C to 7°C at 1 MHz. This enhanced relaxor feature led to significantly improved temperature stability. For example, the variation in permittivity over the temperature range from -55°C to 125°C was only 13.7% for the composition with x = 0.3, y = 0, meeting the X7R dielectric standard and significantly smaller than the 39.6% variation observed for the composition with x = 0, y = 0. Further optimization with y= 0.025 preserved the strong relaxor characteristics, as indicated by a γ value of 1.99, while achieving exceptional temperature stability, with only 9.6% variation in permittivity over an extended temperature range (-55°C to 150°C). This outstanding thermal stability makes the material particularly suitable for applications that demand reliable performance across a broad temperature range. Meanwhile, the room-temperature permittivity at 1 MHz gradually increased from 611 (x=0, y= 0) to 843 (x= 0.3, y= 0) and 841 (x= 0.3, y= 0.025), demonstrating that BNTZ incorporation effectively enhances both the dielectric constant and temperature stability, which are crucial for practical capacitor applications.

We measured the P-E loops of the ceramics under an electric field of 100 kV cm-1 at a frequency of 100 Hz (Fig. 3d and Supplementary Fig. S9, S10). The BTO ceramic exhibits FE features with a Pm of 32.2 μC cm-2 and a visible Pr of 11.5 μC cm-2. After introducing KNN, the ceramic with x = 0, y = 0 exhibited typical relaxor ferroelectrics with superparaelectric state, with a negligible Pr of 0.08 μC cm-2 and a moderate Pm of 8.60 μC cm-2. Following the addition of BNTZ, the Pm increased to 11.06 μC cm-2, which aligns well with the trend observed in the dielectric constant. Meanwhile, the Pr remained relatively small at 0.28 μC cm-2. As the value of y increased to 0.025, both Pm and Pr exhibited negligible changes. We also tested and calculated the statistical dielectric Eb of the ceramics with same dielectric layer thickness and area using the Weibull distribution (Fig. 3e). The Eb values increased with increasing x and y values—from ~120 kV cm-1 (BTO) to ~200 kV cm-1 (x = 0, y = 0) to ~380 kV cm-1 (x = 0.3, y = 0) to ~520 kV cm-1 (x = 0.3, y = 0.025). The Weibull modulus β also increased from 6.4 (BTO) to 8.5 (x = 0, y = 0) to 19.8 (x = 0.3, y = 0) and finally to 21.3 (x = 0.3, y = 0.025), indicating a narrower distribution of Eb data and improved ceramics uniformity. The enhanced Eb can be attributed to three key factors. First, defect engineering effectively reduces oxygen vacancy concentration and promotes the formation of defect complexes. It is recognized that during high-temperature sintering, element volatilization and valence variation occur, leading to the generation of oxygen vacancies. The defect engineering suppresses the long-distance migration of oxygen vacancies and enhances electrical resistivity. This is supported by the leakage current density, which was reduced by three orders of magnitude from ~10-6 A cm-2 to ~10-9 A cm-2 at 100 kV cm-1 (Supplementary Fig. S11). Second, the observed grain refinement and increased densification contribute to higher Eb by introducing abundant grain boundaries that act as barriers to impede the propagation of breakdown paths, as expressed by the formula: Eb  (G) -α, where G is grain size6. Third, the improved energy storage efficiency, evidenced by slimmer P-E loops and reduced hysteretic loss, minimizes joule heating and mitigates thermal breakdown risks, further enhancing the dielectric breakdown strength8.

To comprehensively evaluate the energy density Ue and efficiency η, the dipolar P-E loops were measured at electric fields up to the Eb (Fig. 3f and Supplementary Fig. S12-S14) and calculated the Ue and η (Supplementary Fig. S15a). Through a two-step optimization of BNTZ composition and the Bi3+/Na+ ratio, a simultaneous enhancement of maximum Pm and breakdown strength Eb was achieved, leading to a high Pm of 45.6 µC cm-2 and a superior Eb of 520 kV cm-1. Therefore, the energy density Ue reaches a maximum of 10.55 J cm-3 for the sample with x = 0.3, y=0.025. This value is nearly twice than that of the sample with x = 0.3, y=0 (5.66 J cm-3), and 19 times that of pure BTO ceramics. These high Ue values originate from the synergistic effects of the polymorphic nanodomain and defect design. This unique structure simultaneously promotes high Pm, maintains low Pr and hysteresis, and enables a large Eb. Additionally, we observed a significant improvement in energy efficiency η, which increased from 47.0% for BTO to 94.2% for the sample with x = 0.3, y = 0.025. As the electric field approached Eb, η typically degraded due to increased conduction loss. However, the sample with x = 0.3 and y = 0.025 maintained a high η of approximately 90%, which is ascribed to the suppressed leakage current achieved by the synergistic strategy. Owing to low hysteresis loss and significantly increased Ue, the Ue = Ue/(1-η), attained a high value of 181.9 (Supplementary Fig. S15b). These results strongly demonstrate the effectiveness of our strategy in suppressing hysteresis loss and increasing Eb, both of which are crucial factors in optimizing energy storage performance.

To evaluate the applicability of the proposed strategy for practical energy storage systems, MLCCs were produced via tape-casting using the optimized material composition (x=0.3, y=0.025). The fabricated MLCCs have 7 dielectric layers, each with a thickness of ~7.4 μm. The dielectric layers exhibit dense microstructures with a refined average grain size of 0.44 μm (Supplementary Fig. S16), which is slightly smaller than the grain size (0.56 μm) of the corresponding bulk ceramics. While the electrode layers maintain structural continuity, forming sharp interfaces without detectable elemental interdiffusion (Fig. 4a). In these high-quality MLCCs devices, we obtained slim P-E loops with Pm rising steadily to 50.0 μC cm-2, while Pr stays below 1.6 μC cm-2 under increasing electric fields (Supplementary Fig. S17, S18). In addition, we achieved a breakdown strength of ~965 kV cm⁻¹ (Fig. 4b, inset). These characteristics enable an outstanding Ue of 18.7 J cm⁻³ with a high η of 92.1%, substantially exceeding most recently reported benchmarks in key energy storage parameters4, 5, 7, 43-53 (Supplementary Fig. S19).

For practical energy storage capacitor applications, cycling reliability is a crucial factor for dielectric capacitors, which was evaluated through accelerated charge-discharge cycling tests conducted at a frequency of 1 kHz (Fig. 4c and Supplementary Fig. S20). The MLCCs devices withstood over 107 cycles under electric fields ranging from 500 to 700 kV cm-1, with less than 3% degradation in either Ue or η. This exceptional cycling stability is attributed to the relaxor characteristics of the material, where the absence of large-scale ferroelectric domains eliminates the energy dissipation and material fatigue associated with the pinning and de-pinning of macroscopic domain walls54. Instead, the highly dynamic response of the polymorphic nanodomains facilitates reversible polarization switching with minimal loss and degradation over millions of cycles. Besides, the thermal stability of the energy storage performance was also investigated, with energy density and efficiency plotted as functions of temperature (Fig. 4d and Supplementary Fig. S21). The ceramics exhibit good performance stability under electric fields of 500-600 kV cm-1 from room temperature to 150 °C. Even under a higher electric field of 700 kV cm-1 between 20 and 120 °C, the degradation in Ue and η remained below 8%. This temperature insensitivity originates from the strong relaxor ferroelectric behavior, which decouples polarization dynamics from thermal fluctuations, resulting in temperature-insensitive energy storage properties over a wide temperature range55. Additionally, their superior insulating properties suppress the thermal activation of charge carriers, thereby minimizing conduction losses at elevated temperatures. These combined attributes high cycling durability and thermal stability, which position these ceramics as competitive candidates for use in high-energy-density storage applications.

The charge-discharge characteristics of the MLCCs were systematically investigated at different electric fields using a 100 Ω load resistor (Fig. 4e and Supplementary Fig. S22, S23). With increasing electric field, the discharge energy density (WD) increases from 1.54 J cm-3 to 10.76 J cm-3. Simultaneously, the MLCCs exhibits rapid discharge rates, with a τ0.9 value of approximately 0.46 μs, which corresponds to the 90% stored energy release time. Remarkably, reliability testing under 500-700 kV cm-1 demonstrates exceptional cycling stability: after 100,000 cycles, the variation in energy density remains below 5%. Furthermore, thermal stability tests conducted between 20 and 140°C under 500-600 kV cm-1 show less than 7% variation in discharge performance, even the variation remaining below 4% under an elevated electric field of 700 kV cm-1 across a wide temperature range of 20-120 °C, confirming outstanding cycling reliability and temperature insensitivity, which is highly consistent with the P-E loop measurements. Moreover, the MLCCs maintain rapid discharge rates under long-term cycling and over a wide temperature range. These results reveal superior charge-discharge properties that meet the stringent requirements of modern energy storage applications.

In summary, we demonstrate a breakthrough in energy storage performance by synergistic polymorphic nanodomain engineering and defect optimization. Thus, a high energy density of 18.7 J cm⁻³ with efficiency of 92.1% were obtained in the BTO-based MLCCs. Moreover, the MLCCs exhibit outstanding reliability and thermal stability, owing to their relaxor characteristics and enhanced resistivity. This study highlights the effectiveness of polymorphic nanodomains coupled with defect engineering, facilitating the development of next-generation high-power energy storage applications.

Methods
Ceramic Preparation
The ceramic systems (1-x)(0.45BTO-0.55KNN)-x(Bi0.5-yNa0.5+y)Ti0.9Zr0.1O3 (x=0.0 to 0.4, y=0 to 0.05) were synthesized using the solid-state reaction method. High-purity precursors, including BaCO3 (99.8%), Bi2O3 (99.99%), K2CO3 (99.99%), TiO2 (99.8%), Na2CO3 (99.0%), Nb2O5 (99.99%), and ZrO2 (99.5%) were stoichiometrically weighed. These powders were ball-milled (18 h), dried, and calcined (750°C, 3 h). The calcined powders were further milled for 24 hours, with 0.5 wt% MnO2 (Aladdin, 99.95%) added as a sintering aid. The dried mixture was mixed with polyvinyl alcohol (PVA), and then pressed into 10 mm pellets. The pellets were then heated to 600 °C for 3 hours to remove the binder and sintered at 1000–1200 °C for 3 hours. The pellets were polished to a thickness of 50-60 μm, and Au electrodes with an area of 0.00785 cm2 were sputtered for electrical measurement.

MLCCs Preparation
For multilayer ceramic capacitors (MLCCs), a similar process was used to synthesize the powders. The matrix powders were mixed with the polyvinyl butyral binder, dioctyl phthalate plasticizer, and a polyester/polyamide copolymer dispersant in a toluene-ethanol solvent. The mixtures were ball-milled for 48 hours to achieve a homogeneous pseudoplastic slurry, and then tape cast using a roll-to-roll tape-casting machine with a slot-die. After drying, the tapes were precision-cut and screen-printed with 70Ag/30Pd as the internal electrodes. Seven dielectric layers were precisely stacked, followed by consolidation through warm isostatic pressing to ensure uniform and adherent multilayer integration. The MLCCs were heated at 280 °C for 6 h for binder burnout and then sintered at 1000-1200 °C for 3 h. Silver paste was used as the termination electrode for electrical measurement.

Characterizations
The crystal structure was analyzed via X-ray diffraction (Malvern PANalytical Empyrean) with a Cu Kα X-ray source (wavelength λ = 0.1542 nm) and Rietveld refinement in GSAS-II. Morphology and elemental mapping were characterized using SEM and EDS tests (Zeiss Merlin Compact). The statistical data, including average size and distribution, were obtained using NanoMeasurer. For atomic-scale insights, TEM studies were conducted by dispersing crushed fragments on microgrids, followed by HAADF-STEM (JEOL ARM200F). Cation displacements (δ) and lattice distortions (c/a ratios) were quantified using Gaussian fitting on filtered HRTEM images. Raman spectroscopy (Horiba HR800, 532 nm excitation) was used to further investigate local structural features. Dielectric properties versus temperature and thermally stimulated depolarization current measurements were measured by Novocontrol broadband dielectric/impedance spectrometer (Novocontrol Technologies GmbH& Co. KG.). The polarization loop, current density, leakage current density, and breakdown strength were evaluated using a ferroelectric test system (Radiant Technologies Inc.). Charge-discharge performance was tested using a PolyK Technologies system (PK-CPR1901-20012), with energy release monitored across a 100 Ω load.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript reports a lead-free dielectric ceramic system based on a BTO–KNN–BNTZ. The authors aim to exploit polymorphic nanodomains and defect engineering to achieve high energy density and efficiency While the reported performance metrics (Ue ≈ 18.7 J cm⁻³, η ≈ 92%) are impressive, I find that the scientific novelty, rigor, and interpretation are insufficient for publication in Nature Communications. My specific concerns are:\n1. The combination of a BTO matrix with KNN, Bi-based components, and Zr is a well-established approach in relaxor ferroelectrics. Although the methods state a “series” (x=0–0.4, y=−0.05–0.05), only a few points are actually synthesized and analyzed, and the claimed optimum (x=0.3, y=0.025) lacks a systematic study (no trends across 10%, 20%, 40%, etc.). This does not convincingly demonstrate a design breakthrough or a true optimization.\n2. XRD is discussed mainly by peak splitting/merging and shifts; there is no Rietveld refinement or quantitative phase fraction to support “polymorphic” claims. The STEM-based domain assignments are used as evidence of four phases but without corroborating diffraction/symmetry proof. As presented, “four phases coexisting” reads as over-interpretation of local polarization variance typical of relaxors.\n3. The paper reports average c/a changing from ~1.004 (x=0,y=0) to ~0.997 (x=0.3,y=0.025) and a very large standard deviation rising from 0.044 to 0.055. A ~5% spread in local c/a is very high for perovskites and likely reflects measurement/analysis artefacts; these absolute values should not be used to argue “enhanced local distortion.”\n5. The TSDC results are used to support the defect complex hypothesis, but some interpretations are presented without adequate explanation. For instance, the authors identify multiple peaks and assign them to various phenomena. The appearance of new “B2” and “B3” peaks upon doping is attributed to relaxations of defect complexes, and the D2 peak is said to be almost eliminated. These assignments need clearer justification.\n6.In the I–E loops, the authors observe that pure BTO shows two sharp current peaks, the relaxor (KNN-doped) sample shows much more diffuse peaks, and the relaxor with defect complexes shows four current peaks. They explain the four peaks as arising from defect-complex-mediated domain pinning. In the I–E loops, the authors ascribe the appearance of four current peaks in the relaxor sample with defect complexes to defect-complex-mediated domain pinning. This interpretation seems questionable. In relaxor ferroelectrics, long-range ferroelectric domains are largely absent, and the electrical response is governed by polar nano regions (PNRs) rather than classical domain structures. The occurrence of four current peaks has been widely reported in relaxor systems and is more consistently attributed to reversible switching and reorientation of PNRs under high electric field, rather than to domain wall pinning/unpinning. While defect complexes may influence the local field environment and modify PNR dynamics, the current data do not support a domain-based explanation.\n5. The high energy densities reported in this work in MLCCs, are primarily enabled by the significant enhancement in breakdown strength, rather than dramatic changes in polarization behaviour. The optimized composition (x = 0.3, y = 0.025) achieves Eb of 520kV/cm in bulk, compared to ~120 kV/cm for pure BTO and intermediate values of ~200–380 kV/cm for partially modified compositions. While maximum polarization value shows only modest variation across samples for the those relaxor at 100 kV/cm, the increase in Eb provides a much greater impact on recoverable energy, given its quadratic influence on energy density. Even under high fields, the final stored energy is not unusually high for ferroelectrics, and the slim-loop relaxor behavior contributes more to improved efficiency and thermal/cycling stability than to energy density itself. The manuscript occasionally presents the relaxor nanodomain structure and defect engineering as equally responsible for the energy performance, but the data suggest that through its effect on insulation, grain refinement, and breakdown resistance, which plays a larger role.\nI therefore do not recommend publication in Nature Communications in its current form.", "reviewer_2": "The manuscript proposes the design of BaTiO3-(K0.5Na0.5)TiO3-(Bi0.5Na0.5)Ti0.9Zr0.1O3 solid-solution system for high-energy-density applications. The article presents an actual and interesting topic regarding high-performance dielectric capacitors. The motivation for the present study is well argued, and the current state of research is adequately represented in the introduction.\nThe work can be of significance to the field and related fields, but the manuscript needs improvements in several aspects, as detailed below:\n1. Introduction: The novelty factor must be better highlighted. The selection of specific compositions (1-x)(0.45BTO-0.55KNN)-x(Bi0.5-yNa0.5+y)Ti0.9Zr0.1O3 ceramics (x=0.0 to 0.4, y=-0.05 to 0.05) and Bi3+/Na+ must be better argued.\n2. Main part of the manuscript (Experimental Procedure / Results and Discussions):\n- The relative theoretical density for pure BT is relatively low, 94.5%, for high-energy-density applications. The authors consider that the addition of KNN induces an increase in density up to 95.2%. The 0.7% increase cannot necessarily be attributed to the addition of KNN. The same discussion related to the addition of Bi, which is followed by a slight increase in GS. Preferably an expression like \"can be induced by the addition...\". What value of the theoretical density was considered for the calculation of the relative densities of the presented compositions? Please detail and introduce the density values for each composition in the manuscript. From Fig. S2 (SEM images), it is evident that BT has a denser microstructure than x=0.3, y=0.\n- The data presentation method in Figure S3 needs to be changed.\n- Missing fitting factor R for relaxor diffuseness?\n- Why was a high frequency of 100 Hz used for P-E loops of the ceramics under an electric field of 100 kV/cm? Normally, f=1Hz is used.\n- With a γ value of 1.56, the composition x=0 and y=0 is not in superparalectric state. So, how can it be correlated with the drastic reduction in saturation and remnant polarization (Fig. 9b) observed? Maybe the fit is not perfect (Fig 6b).\n- How long did the reliability test last (100000 cycles)? What about charge-discharge cycling tests (107 cycles)? Why was the frequency of 1 kHz used? Is an accelerated charge-discharge cycling test proper?\n- Another aspect that is not well correlated: the characterization of the compositions is done on ceramic pellets obtained through specific preparation steps, followed by pressing and sintering. In contrast, the application part is performed on MLCCs obtained by tape casting with powders mixed with various additives and then hot isostatically pressed. Different methods can generate different properties. Therefore, this transition needs to be better supported. Preferably, the introduction of detailed SEM images of the MLCC microstructure to highlight GS and porosity.\n3. The discussion part can be improved.\nGeneral presentation: The authors obtained complex data using appropriate techniques and analyzed and interpreted it carefully; however, some assertions are speculative and not fully justified (especially the high field analysis). The comparison between the results obtained in this study and those reported in the literature can be improved. These presented results can be important to the field.\nThe manuscript text requires careful review. Some grammatical errors are presented below, including some expressions:\n- Weijia guo1 – missing capital letter\n- “611(x=0, y=0)” – missing space\n- Line 364, missing a comma before the word \"and\"; some extra spaces.\n- Etc.\nTo increase visibility, I recommend using a larger font and symbol dimensions in all S4 to S13 figures from the SI, as well as in some figures from the manuscript (Figures 3 and 4).\nIn conclusion, the reviewer considers that the presented version of the manuscript can be considered for publication in Nature Communications only with a major revision of the manuscript."}, "author_response": "A Point-by-point Response Letter for Manuscript #NCOMMS-25-71549\nSubject: Manuscript #NCOMMS-25-71549 entitled “Ultrahigh energy-storage\ndielectric ceramics via synergistic polymorphic nanodomain and defect design” by M.\nZhang et al.\nDear Editor and Reviewers,\nWe greatly appreciate the constructive comments and suggestions from all reviewers. Accordingly, we have carried out extended experiments & analysis, and made suitable revisions, which we believe have greatly improved our manuscript. The following are our point-by-point responses to the reviewers’ comments, with the revisions highlighted in the revised manuscript.\nREVIEWER COMMENTS\nReviewer #1:\nThis manuscript reports a lead-free dielectric ceramic system based on a BTO–KNN–\nBNTZ. The authors aim to exploit polymorphic nanodomains and defect engineering to\nachieve high energy density and efficiency While the reported performance metrics (U\ne\n≈ 18.7 J cm⁻³, η ≈ 92%) are impressive, I find that the scientific novelty, rigor, and\ninterpretation are insufficient for publication in Nature Communications. My specific\nconcerns are...:\nResponse: We thank the reviewer for his/her recognition of our results (the reported\nperformance metrics (U ≈ 18.7 J cm⁻³, η ≈ 92%)) as “impressive”. Meantime, we have\ne\ncarefully added extended experiments and revised the discussions corresponding to the\nreviewer’s concerns, so as to enhance the scientific novelty, rigor, and interpretation of\nthe manuscript. We hope that these revisions now make this manuscript suitable for\npublication in Nature communications.\n1. The combination of a BTO matrix with KNN, Bi-based components, and Zr is a well-\nestablished approach in relaxor ferroelectrics. Although the methods state a “series”\n(x=0–0.4, y=−0.05–0.05), only a few points are actually synthesized and analyzed, and\nthe claimed optimum (x=0.3, y=0.025) lacks a systematic study (no trends across 10%,\n20%, 40%, etc.). This does not convincingly demonstrate a design breakthrough or a\ntrue optimization.\nResponse: We appreciate the reviewer’s insightful comments, yet we politely disagree\nwith the idea that this “is a well-estabilished approach in relaxor ferroelectrics”. Here\nwe would like to emphasize that the specific ternary solid-solution system BaTiO\n3\n(BTO)-(K Na )NbO (KNN)-(Bi Na )Ti Zr O (BNTZ) is designed based on\n0.5 0.5 3 0.5 0.5 0.9 0.1 3\nthe new strategy of “synergistic optimization of polymorphic nanodomains and defect\ndesign”, which has not been estabilished (to the best of our knowledge) before. We used\nthe polymorphic relaxor phases (PRP) formed by 0.45BTO-0.55KNN as the foundation\n(to realize low loss and high efficiency); subsequently, BNTZ was introduced as a\nmultifunctional modifier, where Bi3+ ions (with 6S2 lone pair electrons) enhance the\npolarization response, while precise tuning of the Bi3+/Na+ ratio (y = -0.05 to 0.05)\nenables defect engineering that effectively suppresses leakage current and significantly\nimproves the breakdown strength. Together, those combinations give rise to the\noutstanding overall energy-storage performances in this work.\nFor the material series, we are sorry for the misunderstanding: we had actually\nconducted systematic optimizations of the (1-x)(0.45BTO-0.55KNN)-x(Bi\n0.5-\nNa )Ti Zr O ceramics; first with a series of x (0.0, 0.1, 0.2, 0.3 and 0.4; with\ny 0.5+y 0.9 0.1 3\ny=0) and then a series of y (-0.05, -0.025, 0, 0.025, and 0.05; with optimized x = 0.3).\nThe detailed process was not elaborated on due to article length limit. The (di)electric\nproperties (P-E loops, polarization, breakdown strength, etc) of these ceramics have\nnow been added into the supplementary materials (Supplementary Fig. S11, S12, page\n12, lines 286-289; also shown below as Figs. R1 and R2).\nFirst, we systematically optimized the (Bi Na )Ti Zr O content (x) to\n0.5 0.5 0.9 0.1 3\nenhance the maximum polarization (P ). As shown in Fig. R1, P was successfully\nm m\nincreased from 16.0 to 23.0 µC cm-2 at 200 kV cm-1 with increasing x, which is\nattributed to the incorporation of Bi3+ ions with highly polarizable 6s2 lone pair\nelectrons. While the remnant polarization (P) remained minimal (0.34-0.53 µC cm-2),\nr\nleading to large values of P -P and P /P. The breakdown strength E first increases\nm r m r b\nand then decreases, reaching a maximum of 380 kV cm-1 at x = 0.3. Subsequently, we\nfurther optimized the Bi/Na ratio (y) to maximize the breakdown strength (E ) via\nb\ndefect design (Fig. R2). At the same electric field, both P and P exhibit negligible\nm r\nchanges with varying y. In contrast, the E values are significantly influenced, reaching\nb\na maximum value of 520 kV cm-1 at y = 0.025. This optimal y ratio effectively reduces\noxygen vacancy concentration and promotes the formation of defect complexes, which\nsuppress oxygen vacancy migration and enhance electrical resistivity. This is supported\nby the leakage current density data, which was reduced by three orders of magnitude\nfrom ~10-6 A cm-2 to ~10-9 A cm-2 at 100 kV cm-1 (Supplementary Fig. S10).\nIn summary, this work is not a simple combination of established components.\nWe provide a paradigm for designing high-performance dielectric capacitors through\nthe synergistic manipulation of domain structures and defect engineering, which\nshould be applicable to a wide variety of material compositions and structures.\nFig. R1 Ferroelectric properties of BTO-based ceramics with different x. (a) P-E loops\nas a function of electric field up to E . (b) Comparison of maximum polarization P at\nb m\n200 kV cm-1 and breakdown fields E .\nb\nFig. R2 Ferroelectric properties of BTO-based ceramics with different y. (a) P-E loops\nas a function of electric field up to E . (b) Comparison of maximum polarization P at\nb m\n200 kV cm-1 and breakdown fields E .\nb\n2. XRD is discussed mainly by peak splitting/merging and shifts; there is no Rietveld\nrefinement or quantitative phase fraction to support “polymorphic” claims. The STEM-\nbased domain assignments are used as evidence of four phases but without\ncorroborating diffraction/symmetry proof. As presented, “four phases coexisting” reads\nas over-interpretation of local polarization variance typical of relaxors.\nResponse: Thanks for pointing out this critical issue. Based on the suggestion, we have\nperformed Rietveld refinement on the XRD data to provide extra evidence for the\nexistence of the 4 phases (rhombohedral R3c, tetragonal P4mm, orthorhombic Amm2,\nand cubic Pm-3m). As shown below as Fig. R3, the excellent goodness-of-fit\nparameters (R = 2.20-2.93%, R = 3.50-4.11%, χ² = 1.32-1.53) validates the four-\np wp\nphase structural model. This quantitative diffraction analysis confirms that the different\ndomains observed in STEM are not merely local polarization variations but\ncorresponding to distinct crystalline phases. Based on the above analysis, we have\nadded a detailed description of the Rietveld refinement in the manuscript (please see\npage 6, line 124-131), including the refinement methodology, goodness-of-fit\nparameters, quantitative phase fractions, and the refinement figure (Supplementary Fig.\nS2).\nFig. R3 Rietveld refinement of XRD patterns of the BTO-based ceramics with (a) x=0.3,\ny=0 and (b) x=0.3, y=0.025.\n3. The paper reports average c/a changing from ~1.004 (x=0,y=0) to ~0.997\n(x=0.3,y=0.025) and a very large standard deviation rising from 0.044 to 0.055. A ~5%\nspread in local c/a is very high for perovskites and likely reflects measurement/analysis\nartefacts; these absolute values should not be used to argue “enhanced local distortion.”\nResponse: Thank you very much for pointing out this critical question. We rechecked\nthe data and found that the previous c/a results for each composition were derived from\na single STEM image; the large standard deviation (~5%) is thus likely originated from\nscanning distortions in STEM1-3. To address this problem, we have re-analyzed the data\nby acquiring and aligning multiple STEM images of the same region. This process\neffectively eliminated most of the scanning distortions. The standard deviation of the\nprocessed c/a data are now 0.028 for the composition of x = 0, y = 0; and 0.038 for x =\n0.3, y = 0.025, which falls within the reasonable range for perovskite material. We have\nupdated the relevant figures (Fig. 1d-e) and discussion in the manuscript (Please see\nPage 7, line 157-160).\nHere we would like to emphasize that, while the the macroscopic c/a ratio measured\nby XRD are close to 1 (meaning an average pseudocubic structure), the local c/a ratio\nexhibits an enhancing trend of local distortion with its standard deviation increasing\nfrom 0.028 to 0.038. This should be related to the enhanced local strain and random\nfield induced by the incorporation of multiple ions with different ionic radii and valence\nstates, leading to enhanced local distortions and fluctuations of the local c/a ratio, as\nsimilarly observed in previous reports4-6.\nThe references are as follows:\n1. Kang, M. C. et al. Registration-based method for correcting nonlinear drift and\nrandom jitter in STEM imaging and spectroscopic mapping. Micron, 200, 103926\n(2026).\n2. Fujinaka, S. et al. Understanding of scanning-system distortions of atomic-scale\nscanning transmission electron microscopy images for accurate lattice parameter\nmeasurements. J. Mater. Sci. 55, 8123 (2020).\n3. Ning, S. et al. Scanning distortion correction in STEM images. Ultramicroscopy, 184,\n274 (2018).\n4. Zhang, Y. et al. High‐performance ferroelectric based materials via high‐entropy\nstrategy: design, properties, and mechanism. InfoMat, 5, e12488 (2023).\n5. Zhang, M. et al. Ultrahigh energy storage in high-entropy ceramic capacitors with\npolymorphic relaxor phase. Science, 384, 185 (2024).\n6. Chen, L. et al. Giant energy-storage density with ultrahigh efficiency in lead-free\nrelaxors via high-entropy design. Nat. Commun. 13, 3089 (2022).\n4. Performance appears driven mainly by breakdown strength, not new polarization\nphysics. The large Uₑ improvement tracks the strong increase of E (to ~520 kV cm⁻¹\nb\nin bulk; ~965 kV cm⁻¹ in MLCCs) rather than a fundamentally higher P . Grain\nm\nsize/densification and reduced leakage (three orders of magnitude) seems able to\nexplain much of E .\nb\nResponse: We agree with the reviewer that the significant enhancement in E is one of\nb\nthe primary contributors to the superior energy storage performance. Yet, the slim\npolarization loop with high efficiency should be of the same, if not greater, importance\nfor the high performance, which originates from new polarization design of\npolymorphic nanodomains. As shown in our manuscript, the polymorphic nanodomains\nreduce polarization anisotropy and energy barriers, resulting in a flatter energy profile\nthan those of relaxor ferroelectrics with only single type of nanodomains. This\nfacilitates easier polarization rotation and switching with minimal energy loss,\neffectively addressing the longstanding challenge of simultaneously achieving high\npolarization and low energy loss in relaxor ferroelectrics for energy storage applications.\nThis mechanism is distinct from conventional ferroelectrics and is a key feature of our\nmaterial design.\nFurthermore, our defect design, which involves the introduction of highly\npolarizable BNTZ and optimization the Bi/Na ratios, not only enhances the P value\nm\nunder the same electric field (Figs. R1, R2), but also reduces oxygen vacancy\nconcentration and promotes the formation of new defect complexes. As a result, this\napproach effectively improves the breakdown field while simultaneously increasing the\nmaximum polarization. In contrast to most reported studies were enhancing E typically\nb\nsacrifices P (Fig. R4), our method enables a synchronous enhancement of both\nm\nproperties, demonstrating significant potential for further improving energy storage\nperformance.\nFig. R4 Comparison of maximum polarization P at 200 kV cm-1 and breakdown fields\nm\nE of our BTO-based ceramics with representative dielectrics.\nb\nAgain, we agree with the reviewer that the significant enhancement in E is a key\nb\ncontributor to the superior energy storage performance. However, we would like to\nclarify that this high E , along with the desired polarization response (high P and low\nb m\nP), is not merely a result of grain refinement/densification and reduced leakage, but a\nr\ndirect consequence of our synergistic design of polymorphic nanodomains and defect\ndesign. Firstly, regarding the role of grain size. While the introduction of KNN\nsignificantly reduces grain size from 1.86 μm (pristine BTO) to 0.35 μm (x=0, y=0),\nthe optimal compositions (x=0.3, y=0 and x=0.3, y=0.025) actually showed a slight\nincrease to 0.51-0.56 μm. Given the established inverse relationship between\nbreakdown strength (E ) and grain size (G) ( , the enhancement of E\nb b\n−𝛼\ncannot be attributed to grain refinement alone. Secondly, the reviewer correctly notes\n𝐸𝑏 ∝ (𝐺) )\nthe critical role of reduced leakage current. We emphasize that this three-order-of-\nmagnitude reduction in leakage current (from ~10-6 to ~10-9 A cm-2) is not a generic\noutcome of processing but is specifically achieved by our defect engineering strategy,\nwhich effectively pins oxygen vacancies and suppress their migration. Therefore, the\nhigh electrical resistivity and the high E were achieved.\nb\nIn conclusion, we agree that the high E is a major driver for the high U . However,\nb e\nwe respectfully argue that this E enhancement with the high P and low P is\nb m r\nintrinsically linked to the synergistic effect of polymorphic nanodomains and defect\ndesign, which together enable the breakthrough in the overall energy storage\nperformance.\nThe references are as follows:\n1. Gao, Y. et al. Optimizing high-temperature energy storage in tungsten bronze-\nstructured ceramics via high-entropy strategy and bandgap engineering. Nat.\nCommun. 15, 5869 (2024).\n2. Kong, X. et al. High-entropy engineered BaTiO -based ceramic capacitors with\n3\ngreatly enhanced high-temperature energy storage performance. Nat. Commun. 16,\n885 (2025).\n3. Li, D. et al. Global-optimized energy storage performance in multilayer ferroelectric\nceramic capacitors. Nat. Commun. 16, 188 (2025).\n4. Cao, W. et al. Breaking polarization-breakdown strength paradox for ultrahigh energy\nstorage density in NBT-based ceramics. Nat. Commun. 16, 6228 (2025).\n5. Liu, J. et al. Ferroelectric tungsten bronze-based ceramics with high-energy storage\nperformance via weakly coupled relaxor design and grain boundary optimization.\nNat. Commun. 15, 8651 (2024).\n5. The TSDC results are used to support the defect complex hypothesis, but some\ninterpretations are presented without adequate explanation. For instance, the authors\nidentify multiple peaks and assign them to various phenomena. The appearance of new\n“B2” and “B3” peaks upon doping is attributed to relaxations of defect complexes,\nand the D2 peak is said to be almost eliminated. These assignments need clearer justification.\nResponse: To address the reviewer’s concerns, below we provide clearer justification\nand explanations regarding the B2, B3, and D2 peaks.\n1) About the identification of B2 and B3 peaks, the methodology of using TSDC\ntechniques for defect type identification has been well-established, as the theoretical\norigins of TSDC defect complexes have been systematically studied in many\nliteratures1-6. Generally, three potential types of defects (trapped charge, defect\ncomplexes, and oxygen vacancy) can be determined through J and T dependence of\nm m\nE . If the depolarization current is caused by trapped charge, T decreases with\np m\nincreasing E . This indicates that the dielectric thermal release of trap charge during\np\ndepolarization begins at an earlier stage when defect ions acquire higher energy under\nhigher E ; T of TSDC related to oxygen vacancy will show an increase with increasing\np m\nE , which is a sign of polarization due to the movement of oxygen vacancies; T of\np m\nTSDC associated with defect complexes will exhibit no dependence on E due it is\np\ndetermined by the internal characteristics of the defect complexes itself 7-10. In this\npaper, B2 and B3 peaks exhibited field-independent T values but linearly increasing\nm\nJ values as E increased, suggesting that they were related to the relaxation of defect\nm p\ncomplexes.\n2) About D2 peaks, due to the equipment limitation of testing temperature, the\nTSDC above 300 °C were unable to be fully obtained. However, J of D2 and C2 peaks\nm\nwere significantly smaller than J of D1 and C1 peaks, which could be certainly\nm\nconfirmed in the test results below 300 °C shown in Fig. 2. Since the defect\nconcentration can be semi-quantitively represented by J 11, 12, the collective data\nm\nunambiguously showed that BNTZ addition simultaneously reduced oxygen vacancy\nconcentrations while establishing new defect complexes configurations.\nThe discussion about TSDC results, with the references mentioned above, have\nbeen added to the revised manuscript (Please see Page 8, line 197-200).\nThe references are as follows:\n1. Bräunlich, P. Topics in Applied Physics: Thermally Stimulated Relaxation in Solids.\nBerlin, Heidelberg, Germany: Springer, 1979.\n2. Chen, I. Theory of thermally stimulated current in hopping systems. J. Appl. Phys.\n47, 2988 (1976).\n3. Hino, T. Thermally stimulated characteristics in solid dielectrics. IEEE Trans. Elect.\nInsulation. 3, 301 (2007).\n4. Liu, W. et al. Thermally stimulated relaxation in Fe‐doped SrTiO systems: I. Single\n3\ncrystals. J. Am. Ceram. Soc. 91, 3245 (2008).\n5. Liu, W. et al. Thermally stimulated relaxation in Fe‐doped SrTiO systems: II.\n3\n6. Zhang, J. et al. Microwave dielectric properties and thermally stimulated\ndepolarization currents of MgF ‐doped diopside ceramics. J. Am. Ceram. Soc. 97, 3537\n2\n(2014).\n7. Song, H. et al. Review of the thermally stimulated depolarization current (TSDC)\ntechnique for characterizing dielectric materials. J. Korean Ceram. Soc. 60, 747 (2023).\n8. Yoon, S.H. et al. Effect of acceptor (Mg) concentration on the resistance degradation\nbehavior in acceptor (Mg)‐doped BaTiO bulk ceramics: II. Thermally stimulated\n3\ndepolarization current analysis. J. Am. Ceram. Soc. 92, 1766 (2009).\n9. Guo, W, et al. Microwave dielectric properties and thermally stimulated\ndepolarization of Al‐doped Ba (Sm, Nd) Ti O ceramics. J. Am. Ceram. Soc. 102,\n4 9.33 18 54\n5494 (2019).\n10. Guo, W. et al. Defect-related broadband dielectric loss mechanisms of\nNa Sm Ti (Al Nb ) O ceramics. Acta Mater. 255, 119093 (2023).\n1/2 1/2 1–z 1/2 1/2 z 3\n11. Zhang, J. et al. Understanding the thermally stimulated relaxation and defect\nbehavior of Ti-containing microwave dielectrics: A case study of BaTi O . Mater. Des.\n4 9\n130, 479 (2017).\n12. Guo, W. et al. Lattice dynamics and terahertz response of microwave dielectrics: a\ncase study of Al-doped Ca Sm TiO ceramics. J. Eur. Ceram. Soc. 42, 4953 (2022).\n0.6 0.27 3\n6. In the I–E loops, the authors observe that pure BTO shows two sharp current peaks,\nthe relaxor (KNN-doped) sample shows much more diffuse peaks, and the relaxor with\ndefect complexes shows four current peaks. They explain the four peaks as arising from\ndefect-complex-mediated domain pinning. In the I–E loops, the authors ascribe the\nappearance of four current peaks in the relaxor sample with defect complexes to defect-\ncomplex-mediated domain pinning. This interpretation seems questionable. In relaxor\nferroelectrics, long-range ferroelectric domains are largely absent, and the electrical\nresponse is governed by polar nano regions (PNRs) rather than classical domain\nstructures. The occurrence of four current peaks has been widely reported in relaxor\nsystems and is more consistently attributed to reversible switching and reorientation of\nPNRs under high electric field, rather than to domain wall pinning/unpinning. While\ndefect complexes may influence the local field environment and modify PNR dynamics,\nthe current data do not support a domain-based explanation.\nResponse: We thank the reviewer for the question regarding the current peaks in\nthe I-E loops. As the reviewer pointed out, the occurrence of four current peaks has\nindeed been widely reported in relaxor ferroelectric systems, where the electric field\ndriven transition between weakly polar regions and long-range polar regions is\nconsidered the key mechanism1-3. However, our sample (x = 0.3, y = 0.025, Fig. 1c)\nexhibits a polymorphic nanodomains with coexisting rhombohedral, orthorhombic,\ntetragonal, and cubic nanodomains in the size range of 1.0–2.0 nm. Meanwhile, the\nincorporation of multiple ions with different ionic radii and valence states in this sample\nenhances local distortion. The coexistence of PNRs with different local symmetries\nalong with the large local distortion tends to hinder their merging, which has been\nreported in the literature4, 5. We further note that the x = 0.3, y = 0 and x = 0.3, y = 0.025\nceramics exhibit similar relaxor behavior (with a diffuseness factor γ close to 2),\nhowever, their I–E loops significantly differ from each other. This suggests that the\nintroduced defect complexes in the x = 0.3, y = 0.025 ceramics are indeed playing a\nfunctional role. Rather than acting through traditional domain wall pinning, we propose\nthat these defect complexes are equivalent to dipoles consisting of positive and negative\ncharges, which introduce a random field to hinder the PNRs switching and causes 4\npeaks in the current loop. The discussion about I–E loops have been added to the revised\nmanuscript (Please see Page 9, line 221-223).\nThe references are as follows:\n1. Wu, J. et al. Perovskite Sr (Bi Na Li ) TiO ceramics with polar nano\nx 1−x 0.97−x 0.03 0.5 3\nregions for high power energy storage. Nano Energ. 50, 723 (2018).\n2. Liu, N. et al. Designing lead-free bismuth ferrite-based ceramics learning from\nrelaxor ferroelectric behavior for simultaneous high energy density and efficiency under\nlow electric field. J. Mater. Chem. C 6, 10211 (2018).\n3. Viola, G. et al. Lithium-induced phase transitions in lead-free Bi Na TiO based\n0.5 0.5 3\nceramics. J. Phys. Chem. C 118, 8564 (2014).\n4. Xie, A. et al. Supercritical relaxor nanograined ferroelectrics for ultrahigh‐energy‐\nstorage capacitors. Adv. Mater. 34, 2204356 (2022).\n5. Kong, X. et al. High-entropy engineered BaTiO -based ceramic capacitors with\n3\ngreatly enhanced high-temperature energy storage performance. Nat. Commun. 16, 885\n(2025).\n7. The high energy densities reported in this work in MLCCs, are primarily enabled by\nthe significant enhancement in breakdown strength, rather than dramatic changes in\npolarization behaviour. The optimized composition (x = 0.3, y = 0.025) achieves E of\nb\n520kV/cm in bulk, compared to ~120 kV/cm for pure BTO and intermediate values of\n~200–380 kV/cm for partially modified compositions. While maximum polarization\nvalue shows only modest variation across samples for the those relaxor at 100 kV/cm,\nthe increase in E provides a much greater impact on recoverable energy, given its\nb\nquadratic influence on energy density. Even under high fields, the final stored energy is\nnot unusually high for ferroelectrics, and the slim-loop relaxor behavior contributes\nmore to improved efficiency and thermal/cycling stability than to energy density itself.\nThe manuscript occasionally presents the relaxor nanodomain structure and defect\nengineering as equally responsible for the energy performance, but the data suggest that\nthrough its effect on insulation, grain refinement, and breakdown resistance, which\nplays a larger role.\nResponse: We thank the reviewer for his/her deep understanding of our work and\nfor acknowledging the remarkable E and energy-storage performances achieved in our\nb\nMLCCs. We agree that the dramatic enhancement in E is a primary driver of the\nb\nsuperior energy storage performance. However, we wish to emphasize that the\nconcurrent improvement in polarization properties plays an equally indispensable role.\nCrucially, we need to highlight the new polarization physics enabled by the\npolymorphic nanodomains. As shown in our manuscript, the polymorphic nanodomains\nreduce polarization anisotropy and energy barriers, facilitating easy polarization\nrotation and switching with minimal loss. This mechanism, combined with the highly\npolarizable BNTZ introduced, collectively leads to a large P -P differenceand a high\nm r\nP /P ratio, both of which are crucial for achieving high energy storage and high\nm r\nefficiency. Specifically, high efficiency realized by such polymorphic nanodomain\ndesign is the fundamental precondition for achieving high recoverable energy density\nespecially under high field. Otherwise, a significant portion of the stored energy would\nbe dissipated as waste heat, thereby severely reducing recoverable energy density even\nif the material possesses high E .\nb\nMeantime, the reviewer also notes that the slim-loop relaxor behavior benefits\nefficiency and stability more than energy density itself. This perspective may miss a\nkey link: the ultra-high efficiency (η>90%) enabled by the polymorphic nanodomains\nis precisely what prevents thermal breakdown under extreme electric fields. Therefore,\nit is an essential factor of the high E and energy density, not merely an additional\nb\nbenefit.\nWe believe that this synergistic strategy, which unlocks a new performance\nparadigm by concurrently optimizing polarization behavior and breakdown strength,\nrepresents a significant conceptual advance worthy of publication in Nature\nCommunications.\nReviewer #2 (Remarks to the Author):\nThe manuscript proposes the design of BaTiO -(K Na )NbO -(Bi Na )Ti Zr O\n3 0.5 0.5 3 0.5 0.5 0.9 0.1 3\nsolid-solution system for high-energy-density applications. The article presents an\nactual and interesting topic regarding high-performance dielectric capacitors. The\nmotivation for the present study is well argued, and the current state of research is\nadequately represented in the introduction. The work can be of significance to the field\nand related fields, but the manuscript needs improvements in several aspects, as detailed\nbelow:\nResponse: We greatly appreciate the reviewer’s positive recommendation to our\nmanuscript. We hope that all the concerns from the reviewer have been thoroughly\naddressed as presented below.\n1. Introduction: The novelty factor must be better highlighted. The selection of specific\ncompositions (1-x)(0.45BTO-0.55KNN)-x(Bi Na )Ti Zr O ceramics (x=0.0\n0.5-y 0.5+y 0.9 0.1 3\nto 0.4, y=-0.05 to 0.05) and Bi3+/Na+ must be better argued.\nResponse: We sincerely thank the reviewer for this insightful comment. Following the\nsuggestion, we have revised the introduction to more clearly express our synergistic\ndesign strategy and precise composition design (Please see pages 5, lines 97-119). In\nthe following we provide some more detailed explanations:\n1) Highlighting Core Novelty:\nThe core innovation of our work lies in the synergistic optimization of polymorphic\nnanodomains and defect design within a BTO-based matrix. Considering the synergy\neffects of polymorphic nanodomains (facilitating polarization reorientation with\ncoexistence of local polar tetragonal, orthorhombic, and rhombohedral nanodomains in\n1-2 nm size) and defect design (increasing the breakdown strength with reduces oxygen\nvacancy concentration and forms defect complexes), this work successfully addresses\nthe typically conflicting challenges of high polarization, low remnant polarization, and\nhigh breakdown strength. This synergistic manipulation establishes a new paradigm for\nachieving performance breakthroughs in energy-storage dielectrics.\n2) As regards choosing BTO+KNN system:\nWe used BTO as the main component because of its wide utilization for commercial\ndielectric applications. The introduction of KNN was to form the 0.45BTO-0.55KNN\nsolid solution with polymorphic relaxor phase (PRP)1. In such a PRP structure, the\npolarization anisotropy and domain switching barriers could be further weakened, thus\nleading to an even more flattened domain-switching pathway and minimizing hysteresis\nloss.\n3) As regards choosing the third component BNTZ with precise Bi3+/Na+ ratio control:\nThe introduction of the BNTZ component served two purposes: first, to enhance\npolarization by utilizing the 6s² lone pair electrons of Bi3+ to boost the intrinsic\npolarizability of the dielectric, thereby compensating for the typically lower\npolarization in relaxor ferroelectrics. Second, to implement defect engineering.\nThrough precise designing the Bi3+/Na+ ratio, we effectively reduced oxygen vacancy\nconcentration and promoted the formation of defect complexes. These defect\ncomplexes act as effective charge traps, suppressing electrical conduction and\nenhancing the E .\nb\nIn summary, this synergistic design successfully addresses the inverse coupling\nbetween polarization, hysteresis, and breakdown strength, establishing a new paradigm\nfor achieving high-performance energy-storage dielectrics.\nThe references are as follows:\n1. Yang, Y. et al. Morphotropic relaxor boundary in a relaxor system showing\nenhancement of electrostrain and dielectric permittivity. Phys. Rev. Lett. 123, 137601\n(2019).\n2. Main part of the manuscript (Experimental Procedure / Results and Discussions):\n- The relative theoretical density for pure BT is relatively low, 94.5%, for high-energy-\ndensity applications. The authors consider that the addition of KNN induces an increase\nin density up to 95.2%. The 0.7% increase cannot necessarily be attributed to the\naddition of KNN. The same discussion related to the addition of Bi, which is followed\nby a slight increase in GS. Preferably an expression like \"can be induced by the\naddition...\". What value of the theoretical density was considered for the calculation of\nthe relative densities of the presented compositions? Please detail and introduce the\ndensity values for each composition in the manuscript. From Fig. S2 (SEM images),\nit is evident that BT has a denser microstructure than x=0.3, y=0.\nResponse: We thank the reviewer for this insightful comment. As suggested, we have\nrevised the manuscript to use more cautious language to better reflect the correlative\nrelationship between the dopant addition and the observed microstructural changes\n(Please see page 6, lines 137-139), which reads as “In the BTO-KNN sample (x=0,\ny=0), a noticeable reduction in average grain size to 0.35 μm with a relative density of\n95.2% was realized, which can be induced by the addition of KNN that inhibits the\ngrain size growth. Further evolution in the microstructure can be induced by the\naddition of BNTZ due to the sintering aid effect of Bi elements”.\nWe have calculated the theoretical densities for these ceramics based on the XRD\nrefinement results (Table S1, also shown below as Table R1), The theoretical density\nfor each ceramic was calculated using the formula , where Z is the number of\n𝑍∗𝑀\nformula units per unit cell; M is the molar mass of th𝜌e =for𝑁mA∗u𝑉la unit; N\nA\nis the Avogadro\nconstant; V is the unit cell volume. The values were calculated for each composition\nbased on their respective lattice parameters and phase fractions. The resulting values\nare 6.01, 5.17, 5.39, and 5.33 g cm-3 for samples of pure BTO, the x = 0, y = 0 one,\nthe x = 0.3, y = 0 one, and the x = 0.3, y = 0.025 one, respectively. The bulk densities were\nmeasured at room temperature using the Archimedes method. The measured densities\nwere 5.68, 4.92, 5.18, and 5.20 g cm-3 for pure BTO, the x = 0, y = 0 one, the x = 0.3,\ny = 0 one, and the x = 0.3, y = 0.025 one, respectively, corresponding to relative\ndensities of 94.5%, 95.2%, 96.1%, and 97.6%. A detailed summary of these\nmeasurements has been added to the revised Supporting Information (Supplementary\nFig. S4, also shown below as Fig. R5a).\nTable. R1. Refined structural parameters of BTO-based ceramics.\nLattice Paremeter (Å)\nSpace Fraction\nSample Volume Rp Rwp χ2\na b c (%)\ngroup\nP4mm 3.9936(3) 3.9936(3) 4.0325(7) 75.83 64.31(6)\nBTO 4.41 5.83 2.31\nPm-3m 4.0103(4) 4.0103(4) 4.0103(4) 24.17 64.49(7)\nP4mm 3.9994(7) 3.9994(7) 4.0042(4) 35.33 64.05(0)\nx=0,\nPm-3m 4.0036(5) 4.0036(5) 4.0036(5) 28.89 64.17(2) 3.63 4.22 2.33\ny=0\nR3c 5.6561(0) 5.6561(0) 13.8583(6) 35.78 383.95(3)\nP4mm 3.9803(1) 3.9803(1) 4.0021(1) 25.14 63.40(5)\nx=0.3, Pm-3m 3.9738(6) 3.9738(6) 3.9738(6) 25.70 62.75(4)\n2.20 3.50 1.32\ny=0 R3c 5.6201(0) 5.6201(0) 13.7659(4) 23.79 376.55(3)\nAmm2 5.6459(1) 3.9519(6) 5.6685(2) 25.37 126.47(6)\nP4mm 3.9731(3) 3.9731(3) 4.0097(3) 24.48 63.29(7)\nx=0.3, Pm-3m 3.9789(9) 3.9789(9) 3.9789(9) 26.89 62.99(7)\n2.93 4.11 1.53\ny=0.025 R3c 5.6411(1) 5.6411(1) 13.8183(2) 25.81 380.66(5)\nAmm2 5.6474(0) 3.9494(1) 5.6813(0) 22.82 126.72(7)\nWe have remeasured the density of both samples using the Archimedes method.\nThe results confirm that the x=0.3, y=0 sample (~96.1%) is slightly denser than the pure\nBT ceramic (~94.5%). A relative density of 94.5% for BT is reasonable and consistent\nwith literature reports1, which serves as a reliable benchmark for our experimental data.\nSubsequent compositional modifications with KNN and BNTZ, where the presence of\nNa+ and Bi3+ ions facilitate the formation of a liquid phase, thereby enhancing the\ndensification2-4.\nThe references are as follows:\n1. Deshpande, S.B. et al. Characterization of barium titanate: BaTiO (BT) ceramics\n3\nprepared from sol-gel derived BT powders. J. electroceram. 15, 103 (2005).\n2. Ahn, C. W. et al. Sintering behavior of lead‐free (K, Na)NbO ‐based piezoelectric\n3\nceramics. J. Am. Ceram. Soc. 92, 2033 (2009).\n3. Rubio, F. et al. Role of sintering time, crystalline phases and symmetry in the\npiezoelectric properties of lead-free KNN-modified ceramics. Mater. Chem. Phys. 123,\n91 (2010).\n4. Gil, V. et al. Rapid densification by using Bi O as an aid for sintering of gadolinia-\n2 3\ndoped ceria ceramics. Ceram. Int. 33, 471 (2007).\n- The data presentation method in Figure S3 needs to be changed.\nResponse: We thank the reviewer for this suggestion. We have split the original figure\ninto two clearer panels (also shown below as Fig. R5). Furthermore, we have\nincorporated the data of the theoretical and measured density into the figure. The\nupdated figure has been included in the revised supporting information (Supplementary\nFig. S4, also shown below as Fig. R5a).\nFig. R5 The microstructural properties of BTO-based ceramics. (a) Relative density, (b)\naverage grain size.\n- Missing fitting factor R for relaxor diffuseness?\nResponse: We thank the reviewer for raising this important point regarding the relaxor\ndiffuseness factor. We would like to kindly draw the reviewer's attention to\nSupplementary Information Fig. S7, where we have provided the calculated fitting\nfactor for the relaxor diffuseness, as derived from the modified Curie-Weiss law.\n- Why was a high frequency of 100 Hz used for P-E loops of the ceramics under an\nelectric field of 100 kV/cm? Normally, f=1Hz is used.\nResponse: We thank the reviewer for this question. The measurement frequency for P-\nE loops in the literature typically ranges from 1 Hz to 1 kHz1-5. To test the influence of\nfrequency on the P-E loops, we have made the frequency-dependent P-E measurements\n(See below in Fig. R6, also included in supplementary materials as Fig. S9). It can be\nobserved that P-E loop has very small dependence on frequency, which provides\nimportant support for the frequency stability in energy storage performance of the x =\n0.3, y = 0.025 ceramics.\nIn detail, at lower frequencies (1–10 Hz), it has sufficient time for domain wall\nswitching, facilitating better switched hysteresis loops with higher P (11.07-10.88 μC\nm\ncm-2). However, a significant drawback of low-frequency measurement is the amplified\ninfluence of leakage current, which slightly degrades the energy storage efficiency\n(89.1%-91.7%). Conversely, at high frequencies (1 kHz), the effects of Joule heating\nand leakage current are substantially mitigated. Nevertheless, at these frequencies, the\ndomain wall motion during nanodomain flip is resisted by viscous-like forces between\nthe nanodomains, leading to a slight increase in the coercive field and again reduction\nin the energy storage efficiency. Simultaneously, the rapid electric field reversal may\nimpede the complete switching of slower domains, leading to an underestimation of P\nm\n(10.49-10.28 μC cm-2). Based on this rationale, a frequency of 100 Hz was selected for\nall measurements in this work. This intermediate value achieves an effective balance,\nminimizing leakage-related effects while still capturing the contributions from most\ndomain-switching.\nFig. R6 Ferroelectric properties of BTO-based ceramics (x=0.3, y=0.025) with different\nfrequency. (a) P-E loops, (b) Comparison of P and efficiency.\nm\nThe references are as follows:\n1. Liu, J. et al. Ferroelectric tungsten bronze-based ceramics with high-energy storage\nperformance via weakly coupled relaxor design and grain boundary optimization. Nat.\nCommun. 15, 8651 (2024).\n2. Zhao, W. et al. Advanced stability and energy storage capacity in hierarchically\nengineered Bi Na TiO -based multilayer capacitors. Nat. Commun. 16, 16549 (2025).\n0.5 0.5 3\n3. Fu, J. et al. A highly polarizable concentrated dipole glass for ultrahigh energy\nstorage. Nat. Commun. 15, 7338 (2024).\n4. Zhao, X. et al. Excellent high-temperature dielectric energy storage performance in\nbilayer nanocomposites with high-entropy ferroelectric oxide fillers. Nat. Commun. 16,\n5570, (2025).\n5. Li, W. et al. Generative learning facilitated discovery of high-entropy ceramic\ndielectrics for capacitive energy storage. Nat. Commun. 15, 4940 (2024).\n- With a γ value of 1.56, the composition x=0 and y=0 is not in superparalectric state.\nSo, how can it be correlated with the drastic reduction in saturation and remnant\npolarization (Fig. 9b) observed? Maybe the fit is not perfect (Fig 6b).\nResponse: We apologize for this misunderstanding. The superparaelectric is a specific\nstate of relaxor ferroelectrics defined by its temperature range. It exists in the\ntemperature range above the dielectric maximum temperature (T ) and below the Burns\nm\ntemperature (T ), i.e., T < T < T . As shown in Fig. 3, the temperature of the T and\nB m B m\nT for x=0 and y=0 is approximately -155°C and 45°C, respectively. Our electrical\nB\ncharacterization, particularly the P-E loops presented in Fig. S13b, was conducted at\nroom temperature (approximately 25°C). Since this temperature lies between T (-\nm\n155°C) and T (45°C), the material is indeed in the superparaelectric state. In the\nB\nsuperparaelectric state, the size of PNRs further decreases, and the interaction between\nthem weakens significantly, leading to minimal hysteresis. This directly explains the\nsharply decreased maximum polarization (P ) and remnant polarization (P) observed\nm r\nfor this composition. we have added a few sentences to clarify this point (Please see\npages 11, lines 261-262).\n- How long did the reliability test last (100000 cycles)? What about charge-discharge\ncycling tests (107 cycles)? Why was the frequency of 1 kHz used? Is an accelerated\ncharge-discharge cycling test proper?\nResponse: Thanks for the questions. In charge-discharge cycling testing, 100,000\ncycles require approximately 60 hours with the cycling frequency of 1kHz. If the\nnumber of test cycles is increased to 107, while frequency is still 1kHz, the testing time\nwould extend to approximately 6000 hours! The primary reason for selecting a\nfrequency of 1 kHz was to reduce the cycling tests to a reasonable time.\nThe use of high frequencies for accelerated cycling tests is a commonly used\nmethod in the field of dielectric capacitor reliability research1-3. Our frequency-\ndependent P-E tests (shown above in Fig. R6) demonstrate great stabilility, indicating\nthat the accelerated cycling test is proper in which the polarization switching still\nhappens at higher frequencies. Consequently, a material that demonstrates superior\nfatigue endurance at 1 kHz is highly likely to perform equally well at lower operating\nfrequencies.\nThe references are as follows:\n1. Zhao, W. et al. Advanced stability and energy storage capacity in hierarchically\nengineered Bi Na TiO -based multilayer capacitors. Nat. Commun. 16, 6549 (2025).\n0.5 0.5 3\n2. Zhang, M. et al. Ultrahigh energy storage in high-entropy ceramic capacitors with\npolymorphic relaxor phase. Science, 384, 185 (2024).\n3. Pan, H. et al. Ultrahigh energy storage in superparaelectric relaxor ferroelectrics.\nScience, 374, 100 (2021).\n- Another aspect that is not well correlated: the characterization of the compositions is\ndone on ceramic pellets obtained through specific preparation steps, followed by\npressing and sintering. In contrast, the application part is performed on MLCCs\nobtained by tape casting with powders mixed with various additives and then hot\nisostatically pressed. Different methods can generate different properties. Therefore,\nthis transition needs to be better supported. Preferably, the introduction of detailed SEM\nimages of the MLCC microstructure to highlight GS and porosity.\nResponse: We thank the reviewer for this valuable suggestion. As recommended, we\nhave now added the cross-sectional SEM images of the fabricated MLCC in the revised\nmanuscript as Supplementary Fig. S15. Compared to the bulk ceramic (as shown in the\nmanuscript Fig. 1b), the MLCC also exhibits dense microstructure, with a refined\naverage grain size of 0.44 μm that is slightly smaller than the grain size (0.56 μm) of\nthe corresponding ceramics. This optimized microstructure is crucial for the MLCCs\nperformance with superior E and energy storage performance.\nb\nFig. R7 Cross-sectional SEM images and grain size distribution of the BTO-based\nMLCC. (a) SEM images. (b) Grain size distribution.\n3. The discussion part can be improved.\nGeneral presentation: The authors obtained complex data using appropriate techniques\nand analyzed and interpreted it carefully; however, some assertions are speculative and\nnot fully justified (especially the high field analysis).\nResponse: We sincerely thank the reviewer for the positive evaluation (of the results&\ndiscussion part) of this work. To strengthen the discussion, we have thoroughly revised\nthe discussion section by replacing the potentially speculative statements with more\nevidence and logically structured arguments. The key modifications are summarized as\nfollows:\n1) In the section discussing the enhanced E , we now link the proposed mechanism\nb\nto the experimental evidence of the defect engineering with three-order-of-magnitude\nreduction in leakage current (Supplementary Fig. S10), which directly supports the\nsuppression of charge carrier migration and enhanced resistivity, and is a primary factor\nfor high E . We also carefully present the role of grain refinement, directly citing the\nb\nestablished relationship ( , where G is grain size) from the literature.\n−𝛼\n2) Regarding the origin of the high η under high electric fields, we have now added\n𝐸𝑏 ∝ (𝐺) \nbrief discussions to acknowledge the common challenge of efficiency degradation near\nthe E due to increased conduction loss. This establishes a crucial contrast and\nb\nhighlights the exceptional performance of our material system (x=0.3, y=0.025), which\nmaintains a high η of ~90% near the E . Furthermore, we explicitly attribute this\nb\nstability to the suppressed leakage current resulting from our synergistic design strategy,\nthereby directly linking the performance to our material’s innovation.\n3) In the discussion of cycling stability, we have added that the cycling stability\noriginates from the intrinsic relaxor characteristics of the material, and this\ncharacteristic is directly attributed to the nanodomain structure observed in our Fig. 1c.\nThe absence of large-scale ferroelectric domains eliminates energy dissipation and\nmaterial fatigue associated with the pinning and de-pinning of macroscopic domain\nwalls.\nWe have added/revised the related explanations in the manuscript (Please see page\n12, lines 273-281; page 13, lines 296-300; page 14, lines 330-336).\nThe comparison between the results obtained in this study and those reported in the\nliterature can be improved. These presented results can be important to the field.\nResponse: We thank the reviewer for this suggestion. We have added a new figure into\nthe revised supplementary materials (Supplementary Fig. S18) that provides a direct\ncomparison of the performance of our MLCCs with the state-of-the-art MLCCs. The\nanalysis demonstrated that our MLCCs outperform most recently reported benchmarks\nin key energy storage parameters.\nFig. R8 Comparison of energy density and efficiency of our BTO-based MLCCs with\nstate-of-the-art dielectric MLCCs.\nThe manuscript text requires careful review. Some grammatical errors are presented\nbelow, including some expressions:\n- Weijia guo1 – missing capital letter\n- “611(x=0, y=0)” – missing space\n- Line 364, missing a comma before the word \"and\"; some extra spaces.\n- Etc.\nResponse: We thank the reviewer for these valuable suggestions. We have made\nrevisions to the typos/errors. Furthermore, we have conducted a thorough check of the\nentire manuscript to correct grammatical and formatting issues.\nTo increase visibility, I recommend using a larger font and symbol dimensions in all S4\nto S13 figures from the SI, as well as in some figures from the manuscript (Figures 3\nand 4).\nResponse: We thank the reviewer for these constructive suggestions. We have\nincreased the font sizes and symbol dimensions in all figures from the Supplementary\nInformation (Figures S4 to S13) and the main manuscript (Figures 3 and 4). We believe\nthese changes have significantly enhanced the clarity and readability of the data\npresentation.\nIn conclusion, the reviewer considers that the presented version of the manuscript can\nbe considered for publication in Nature Communications only with a major revision of\nthe manuscript.\nResponse: We sincerely thank the reviewer for their insightful comments and\nconstructive suggestions. The manuscript has been significantly improved through this\nrevision process. We believe the revised version now fully meets the high standards of\nNature Communications.\nBest regards!\nProf. Yuan-Hua Lin\nSchool of Materials Science and Engineering,\nTsinghua University, Beijing, 100084, P. R. China\nPhone: +86-10-62773741\nFax: +86-10-62771160\nE-mail: linyh@tsinghua.edu.cn"}, {"round": 2, "reviewer_comments": {"reviewer_1": "I have carefully examined the revised manuscript, the authors’ response. Although additional data and discussion have been provided, the central scientific and conceptual concerns raised in my original review remain unresolved. In several cases, the revisions introduce new inconsistencies or unsupported interpretations, further weakening the manuscript. I therefore cannot support publication of this work in Nature Communications.\n1. The justification for choosing BaTiO3 as the matrix, because it is a “commercial dielectric with well-established properties” is scientifically weak and does not constitute a design rationale. Likewise, referring to 0.45BTO–0.55KNN as a “polymorphic relaxor phase composite” is chemically incorrect; this system is a solid solution. The claim that KNN is introduced to reduce polarization anisotropy is also problematic, since BaTiO3 does not intrinsically exhibit unusually high polarization anisotropy.\n2. The authors argue that the revised data justify the optimized composition (x = 0.3, y = 0.025), claiming that polarization enhancement is controlled by BNTZ content (x) while breakdown strength is governed by the Bi/Na ratio (y). However, this design logic remains incremental rather than conceptually innovative, and it is not internally consistent.The authors attribute polarization enhancement to Bi³⁺ lone-pair electrons. However, Figs. R1 and R2 show that polarization increases monotonically with increasing total BNTZ content (x). Because BNTZ simultaneously introduces Bi, Na, Ti, and Zr, it is not accurate to attribute the polarization increase to Bi alone. Importantly, polarization does not peak at x = 0.3; higher x values exhibit higher polarization. If BNTZ is introduced primarily to enhance polarization, the choice of x = 0.3 rather than x = 0.4 is not convincingly justified. The linkage between breakdown strength and Bi/Na ratio is not adequately supported. Significant changes in breakdown strength already occur when x is varied at a fixed Bi/Na ratio, indicating that Bi/Na ratio cannot be the dominant controlling factor. Conversely, when y is varied at fixed x, the authors state that both Pm and Pr remain unchanged, which directly contradicts their repeated claim that higher Bi content enhances polarization response. These statements are internally inconsistent.\n3. Changing the Bi/Na ratio necessarily introduces charge imbalance, potentially acting as donor or acceptor doping, yet no rigorous defect-chemistry framework is presented. Claims of reduced oxygen vacancy concentration rely primarily on TSDC and leakage current data; direct chemical-state evidence (e.g., XPS or EELS) is needed to support these conclusions. In addition, Weibull statistics should be presented and compared under identical sample geometry to substantiate claims regarding breakdown reliability.\n4. Structural interpretation raises further concerns. The use of multi-phase Rietveld refinement to claim coexistence of multiple crystallographic phases is not well justified. Pure BTO is well known to be tetragonal at room temperature, yet is fitted with a mixed cubic and tetragonal models. The other three compositions exhibit pseudocubic XRD patterns without clear peak splitting. Since XRD cannot resolve nanometer-scale local symmetry variations typical of relaxor systems, multi-phase fitting in this context constitutes overfitting rather than proof of phase coexistence. And no clear lattice-parameter or unit-cell-volume trends are presented to correlate with composition.\n5.several mechanistic explanations are inconsistent with the data. Grain refinement is used to explain enhanced breakdown strength, yet the smallest grain size occurs at x = 0, y = 0, not in the optimized composition. Similarly, slimmer P–E loops are cited as mitigating Joule heating and enhancing breakdown strength, but all modified compositions (except pure BTO) exhibit slim loops while showing very different breakdown strengths.", "reviewer_2": "The manuscript proposes the design of BaTiO3-(K0.5Na0.5)TiO3-(Bi0.5Na0.5)Ti0.9Zr0.1O3 solid-solution system for high-energy-density applications. The article presents an interesting and current topic on high-performance dielectric capacitors. The motivation for the present study is well-argued, and the current state of research is adequately represented in the introduction. The work can be of significance to the field and related fields.\nThe manuscript was improved, and the answers and corrections fully addressed the questions raised in the previous review.\nIn conclusion, the reviewer considers the revised manuscript suitable for publication in Nature Communications."}, "author_response": "A Point-by-point Response Letter for Manuscript #NCOMMS-25-71549A\nSubject: Manuscript #NCOMMS-25-71549A entitled “Ultrahigh energy-storage\ndielectric ceramics via synergistic polymorphic nanodomain and defect design” by M.\nZhang et al.\nDear Editor and Reviewers,\nWe greatly appreciate the constructive comments and suggestions from all\nreviewers. Accordingly, we have carried out extended experiments & analysis, and\nmade suitable revisions, which we believe have greatly improved our manuscript. The\nfollowing are our point-by-point responses to the reviewers’ comments, with the\nrevisions highlighted in the revised manuscript.\nReviewer #1 (Remarks to the Author):\nI have carefully examined the revised manuscript, the authors’ response. Although\nadditional data and discussion have been provided, the central scientific and conceptual\nconcerns raised in my original review remain unresolved. In several cases, the revisions\nintroduce new inconsistencies or unsupported interpretations, further weakening the\nmanuscript.\n1. The justification for choosing BaTiO as the matrix, because it is a “commercial\n3\ndielectric with well-established properties” is scientifically weak and does not\nconstitute a design rationale. Likewise, referring to 0.45BTO–0.55KNN as a\n“polymorphic relaxor phase composite” is chemically incorrect; this system is a solid\nsolution. The claim that KNN is introduced to reduce polarization anisotropy is also\nproblematic, since BaTiO does not intrinsically exhibit unusually high polarization\n3\nanisotropy.\nResponse: We appreciate the reviewer’s insightful comments. Per the reviewer’s\nsuggestions, we have added more detailed discussions to address the selection of\nmaterials system (Please see pages 5, lines 98-102). The following are more detailed\nexplanations:\n1) As regards choosing BaTiO as the matrix:\nrevised this part with more specific scientific strategy: BTO has a high intrinsic\ndielectric constant and low dielectric loss, making it beneficial for device\nminiaturization. Moreover, its absence of volatile elements leads to superior chemical\nstability. However, its application in energy storage is limited by its intrinsic low\ndielectric breakdown strength (E ) and high remanent polarization (Pᵣ), leading to a low\nb\nrecoverable energy density (U ) and energy efficiency (η).\ne\nTherefore, we aim to\novercome these limitations through our synergistic strategy.\n2) As regards the “polymorphic relaxor phase composite” and the relevant\npolarization anisotropy:\nWe agree that the 0.45BTO–0.55KNN is a solid solution, and we apologize for the\nincorrect use of “composite” in our previous description. We have made comprehensive\ncorrections in the manuscript and supplementary information. Regarding the properties\nof pure BaTiO , it is indeed accurate that it does not exhibit unusually high polarization\n3\nanisotropy. However, to engineer materials with even lower anisotropy for minimal\ndomain switching barriers and enhanced energy storage performances, our intended\nstrategy is inspired by the mechanism at morphotropic phase boundaries in classical\nferroelectrics, where the coexistence of phases with different macroscopic symmetries\n(e.g., polar tetragonal and polar rhombohedral in PbZr Ti O ) minimizes\n0.52 0.48 3\npolarization anisotropy and facilitates rotation, therefore enhancing piezoelectric\nproperties. We hypothesize that a similar principle can be applied to relaxor\nferroelectrics1, 2. By forming a solid solution with coexisting nanodomains of different\nlocal symmetries in RFEs, the polarization anisotropy and domain switching barriers in\nsuch a polymorphic nanodomain structure could be further weakened, thus leading to\nan even more flattened domain-switching pathway and minimizing hysteresis loss.\nThe references are as follows:\n1. Yang, Y. et al. Morphotropic relaxor boundary in a relaxor system showing enhancement\nof electrostrain and dielectric permittivity. Phys. Rev. Lett. 123, 137601 (2019).\n2. Zhang, M. et al. Ultrahigh energy storage in high-entropy ceramic capacitors with\npolymorphic relaxor phase. Science 384, 185-189 (2024).\n2. The authors argue that the revised data justify the optimized composition (x = 0.3, y\n= 0.025), claiming that polarization enhancement is controlled by BNTZ content (x)\nwhile breakdown strength is governed by the Bi/Na ratio (y). However, this design logic\nremains incremental rather than conceptually innovative, and it is not internally\nconsistent. The authors attribute polarization enhancement to Bi³⁺ lone-pair electrons.\nHowever, Figs. R1 and R2 show that polarization increases monotonically with\nincreasing total BNTZ content (x). Because BNTZ simultaneously introduces Bi, Na,\nTi, and Zr, it is not accurate to attribute the polarization increase to Bi alone. Importantly,\npolarization does not peak at x = 0.3; higher x values exhibit higher polarization. If\nBNTZ is introduced primarily to enhance polarization, the choice of x = 0.3 rather than\nx = 0.4 is not convincingly justified. The linkage between breakdown strength and\nBi/Na ratio is not adequately supported. Significant changes in breakdown strength\nalready occur when x is varied at a fixed Bi/Na ratio, indicating that Bi/Na ratio cannot\nbe the dominant controlling factor. Conversely, when y is varied at fixed x, the authors\nstate that both P and P remain unchanged, which directly contradicts their repeated\nm r\nclaim that higher Bi content enhances polarization response. These statements are\ninternally inconsistent.\nResponse: Thank you for your thorough review of our manuscript and your valuable\nfeedback. We appreciate the opportunity to clarify this point and have added more\ndetailed discussions to highlight the innovative of our work (Please see pages 5, lines\n98-106; Supplementary Page 13, line 84-85 and 89-93; Page 14, line 98-100 and 102-\n103). In the following we provide detailed explanations:\n1) Our conceptual innovation and design logic:\nWe respectfully clarify that the core innovation of this work lies in the synergistic\noptimization of polymorphic nanodomains and defect engineering. We used the\npolymorphic nanodomains formed from 0.45BTO-0.55KNN matrix as the foundation\nto achieve low loss and high efficiency. On this basis, defect-design engineering was\nconducted through controlled BNTZ addition and adjustment of the Bi3+/Na+ ratio to\nsimultaneously enhance polarization and improve breakdown strength. We combined\nBNTZ content (x) to enhance the spontaneous polarization (BNT~40 C cm−2, much\nhigher than other lead-free FEs, e.g., ~26 C cm−2 for BTO and ~30 C cm−2 for KNN),\nwhile simultaneously reducing oxygen vacancy concentrations and increasing the\nbreakdown strength. The fine-tuning of y (the Bi/Na ratio) at an optimal x value enabled\nfurther effective defect engineering, resulting in a higher breakdown strength without\ndegrading polarization. This precise tuning of x and y allows the system to overcome\nthe conventional trade-off between polarization and breakdown strength, demonstrating\nsignificant potential for improving energy storage performance. Most significantly, this\nwork demonstrates a crucial step from materials synthesis to industrial application: we\nhave synthesized high-quality MLCCs devices through industrially compatible tape-\ncasting and co-firing techniques. This successful transition from material to a functional\ndevice demonstrates the material system not only as a promising candidate for\nfundamental research but also as a competitive candidate for commercial production.\n2) On the attribution of polarization enhancement:\nWe agree with the Reviewer that the polarization enhancement is a collective effect\nof all ions (Bi, Na, Ti, Zr) introduced by the BNTZ component. We have revised the\nmanuscript to state more accurately that the increase in polarization with x is due to the\nsynergistic contribution of the entire BNTZ perovskite.\n3) On the choice of optimal composition x= 0.3 and its function in breakdown\nstrength:\nIn accordance with your suggestions, we have revised the expressions with more\nprecise descriptions: the polarization enhancement is controlled by the BNTZ content\n(x), while the breakdown strength is co-controlled by both x and the Bi/Na ratio (y). As\npointed out by the Reviewer, the P value increases monotonically with the x content\nm\n(Fig. S12), and the highest P is achieved at x=0.4. This precisely demonstrates the\nm\neffectiveness of BNTZ addition in enhancing polarization. However, the selection of\nthe optimal composition (x=0.3 rather than 0.4) was based on the core challenge of\nbalancing polarization and breakdown strength for comprehensive optimization of\nenergy storage performance. As shown in Fig. S12, the breakdown strength E peaks at\nb\nx=0.3 (380 kV cm-1) and decreases at x=0.4. This trend is consistent with the leakage\ncurrent measurements, where the x=0.3 composition exhibits the lowest leakage current\n(Fig. R1), indicating its superior insulating properties and thus the highest E .\nThe\nb\noptimized E at x=0.3 is attributed to the role of Bi3+ as a sintering aid, which enhances\nb\ndensification and promotes uniform grain growth, thereby improving insulating\nproperties1-3. However, at higher concentrations (x=0.4), excessive Bi3+ may segregates\nat grain boundaries, hindering mass transfer during sintering and deteriorating the\ninsulation performance, thus lowering the E 1. Although the sample with x=0.4 exhibits\nb\na slightly higher P , an inferior energy density U 4.32 J cm-3 is achieved compared to\nm e\nthe ceramic sample with x=0.3 (5.66 J cm-3) due to its relatively lower E . Therefore,\nb\nx=0.3 represents optimal balance between P and E . Further adjustment of y with x\nm b\nfixed leads to a further enhancement of the E . Consequently, polarization enhancement\nb\nis governed by the x content, while the breakdown strength E is co-controlled by both\nb\nx and y. The polarization does vary with changes in y. However, since the variation in\ny is minimal, its impact on polarization is also very small. Thus, our focus is primarily\non its impact on the E .\nb\nFig. R1 Leakage current densities as a function of electric field of BTO-based\nceramics with different x.\nThe references are as follows:\n1. Qin, W. et al. High energy storage and thermal stability under low electric field in\nBi Na TiO -modified BaTiO -Bi(Zn Ta )O ceramics. Chem. Eng. J. 443, 136505 (2022).\n0.5 0.5 3 3 0.25 0.5 3\n2. Yan, Y. et al. Enhanced energy storage in temperature stable Bi Na TiO -modified\n0.5 0.5 3\nBaTiO -Bi(Zn Nb )O ceramics. Ceram. Int. 48, 36478 (2022).\n3 2/3 1/3 3\n3. Yin, S. et al. Improved energy storage performance in Bi Na TiO ‐modified\n0.5 0.5 3\nSr Ba Nb O tetragonal tungsten bronze ceramics. J. Am. Ceram. Soc. 109, e70334 (2026).\n0.6 0.4 2 6\n3. Changing the Bi/Na ratio necessarily introduces charge imbalance, potentially acting\nas donor or acceptor doping, yet no rigorous defect-chemistry framework is presented.\nClaims of reduced oxygen vacancy concentration rely primarily on TSDC and leakage\ncurrent data; direct chemical-state evidence (e.g., XPS or EELS) is needed to support\nthese conclusions. In addition, Weibull statistics should be presented and compared\nunder identical sample geometry to substantiate claims regarding breakdown reliability.\nResponse: We thank the reviewer for this question. As suggested, we performed XPS\nto analyze the high-resolution O 1s spectra of samples with different Bi/Na ratios. Three\nXPS peaks can be identified, corresponding to lattice oxygen (O ), oxygen vacancy\nL\n(O ) and adsorbed oxygen (O ). It is recognized that during high-temperature\nV Abs\nsintering, element volatilization and valence variation occur, leading to the generation\nof in the sample of x=0.3, y=0, which is confirmed in Fig. R2. With an increasing\n∙∙\nNa+\n𝑉\n/𝑜Bi3+ ratio at x=0.3, y=0.025, the peaks related to V\nO\n∙∙ decrease from 23.4% to\n17.2%, which also aligns well with the trend observed in the TSDC measurements (Fig.\n2). Meanwhile, the defect complexes concentrations increase (as demonstrated by\nTSDC, Fig. 2d), The variation in the Bi/Na ratio may induce Na defects formation.\nBi\nˊˊ\nThese defects with opposite charges could form defect complexes (we speculate\npossible defect complexes include ′′ ), which are more thermodynamically\n∙∙\nstable and locate deeper into the bandN gaaBpi\n−\nco\\𝑉\nm𝑜pared with isolated point defects1-3, thus\nhindering the long-distance migration of oxygen vacancies and thereby significantly\nimproving the breakdown strength and overall energy storage properties. However,\nthere is for now no direct/effective technique to adequately characterize such point\ndefects. We have updated the relevant figures (Supplementary Fig. S6) and discussion\nin the manuscript and supplementary materials (Please see page 12, line 277-280 and\nSupplementary Page 7, line 47-58).\nFig. R2 High-resolution O 1s XPS spectra of BTO-based ceramics with different y. (a)\nx=0.3, y=0, (b) x=0.3, y=0.025.\nWe acknowledge the reviewer for raising this important point regarding the\npotential influence of sample geometry on breakdown strength of different Na/Bi ratio\nceramics. To ensure a fair comparison, the samples had actually been tested with\nidentical sample geometry (i.e., same thickness of 50-60 μm and electrode area of\n0.00785 cm2). The corresponding Weibull statistical analysis of the breakdown strength\nhas been added to the supplementary materials (Supplementary Fig. S13b, also shown\nbelow as Fig. R3). The consistent results clearly demonstrate that the observed\ndifferences in breakdown strength are directly attributable to variations in material\ncomposition, not to variations in geometry.\nFig. R3 Weibull distribution analysis of the breakdown strength of the ceramics with\ndifferent compositions (and the same geometry).\nThe references are as follows:\n1. Saremi, S. et al. Enhanced electrical resistivity and properties via ion bombardment of\nferroelectric thin films. Adv. Mater. 28, 10750 (2016).\n2. Kim, J. et al. Ultrahigh capacitive energy density in ion-bombarded relaxor ferroelectric\nfilms. Science, 369, 81 (2020).\n3. Pan, H. et al. Enhanced electric resistivity and dielectric energy storage by vacancy\ndefect complex. Energy Storage Mater. 42, 836 (2021).\n4. Structural interpretation raises further concerns. The use of multi-phase Rietveld\nrefinement to claim coexistence of multiple crystallographic phases is not well justified.\nPure BTO is well known to be tetragonal at room temperature, yet is fitted with a mixed\ncubic and tetragonal models. The other three compositions exhibit pseudocubic XRD\npatterns without clear peak splitting. Since XRD cannot resolve nanometer-scale local\nsymmetry variations typical of relaxor systems, multi-phase fitting in this context\nconstitutes overfitting rather than proof of phase coexistence. And no clear lattice-\nparameter or unit-cell-volume trends are presented to correlate with composition.\nResponse: We appreciate the reviewer’s insightful comments. We fully agree with the\nreviewer that “XRD cannot resolve nanometer-scale local symmetry variations typical\nof relaxor systems”. Our observations are best described as coexisting polymorphic\nnanodomains. The materials exhibit a macroscopically pseudo-cubic symmetry but\npossess different local polar nanodomains, which are typical for relaxor ferroelectric\nmaterials with nanoscale local structural heterogeneity and have been widely\ncharacterized by STEM, consistent with prior reports1-3. We have thoroughly revised\nthe manuscript to replace “four-phase coexistence” by the more precise description\n“polymorphic nanodomain coexistence”. In addition, we also apologize for the\nincorrect use of “mixed cubic and tetragonal models” for the pure BTO. We have made\ncomprehensive corrections in the manuscript and supplementary information. We also\npresent the correlation between composition and trends in lattice parameter and unit-\ncell-volume in the manuscript and supplementary materials (Please see page 6, line\n127-133 and Supplementary Fig. S2, also shown below as Fig. R4). To further elucidate\nthe crystal structures of the ceramics, we have calculated the lattice parameters and\nunit-cell volumes of the samples. The pure BTO sample exhibits a tetragonal phase with\na = 3.994 Å and c = 4.038 Å. With the introduction of KNN (x=0, y=0), the structure\ntransforms into a cubic phase, where the lattice parameter slightly changes to 4.002 Å,\nand the cell volume decreases from 64.414 Å3 to 64.096 Å3. Upon further incorporation\nof BNTZ (x=0.3, y=0), the lattice parameter continues decreases to 3.972 Å and the\nvolume is markedly reduced to 62.665 Å3. Notably, adjusting the Bi3+/Na+ ratio within\nBNTZ leads to a slight increase of lattice parameter to 3.975 Å and a corresponding\nincrease in cell volume to 62.807 Å3. This evolution in lattice parameters is primarily\nattributed to the lattice shrinkage (expansion) caused by the substitution of A-site and\nB-site ions with smaller (larger) ionic radius.\nFig. R4 Lattice parameter and unit-cell volume of BTO-based ceramics.\nThe references are as follows:\n1. Chen, L. et al. Giant energy-storage density with ultrahigh efficiency in lead-free\nrelaxors via high-entropy design. Nat. Commun. 13, 3089 (2022).\n2. Zeng, X. et al. Polymorphic relaxor phase and defect dipole polarization co-reinforced\ncapacitor energy storage in temperature-monitorable high-entropy ferroelectrics. Nat. Commun.\n16, 1870 (2025).\n3. Chen, L. et al. Near‐zero energy consumption capacitors by controlling inhomogeneous\npolarization configuration. Adv. Mater. 36, 2313285 (2024).\n5.several mechanistic explanations are inconsistent with the data. Grain refinement is\nused to explain enhanced breakdown strength, yet the smallest grain size occurs at x =\n0, y = 0, not in the optimized composition. Similarly, slimmer P–E loops are cited as\nmitigating Joule heating and enhancing breakdown strength, but all modified\ncompositions (except pure BTO) exhibit slim loops while showing very different\nbreakdown strengths.\nResponse: We sincerely thank the reviewer for the insightful comment. Here we would\nlike to emphasize that the enhancement of E is attributed to the synergistic strategy of\nb\ndefect engineering, grain refinement and P-E hysteresis reduction.\nFirst, we used the 0.45BTO-0.55KNN to introduces polymorphic nanodomains,\nthereby realizing lower energy loss and higher efficiency (from 47.0% to 95.9%)\ncompared with the pristine BTO, which reduces heat dissipation and thermal\nbreakdown probability. Concurrently, it achieves substantial grain refinement (from\n1.86 μm for BTO to 0.35 μm for 0.45BTO-0.55KNN). The smaller grain size\naccompanied by high-density grain boundaries introduce high barriers for the\ndevelopment of breakdown path. Subsequently, the addition of BNTZ (by adjusting x)\nsignificantly reduces oxygen vacancy concentration (as demonstrated by TSDC and\nXPS, Fig. 2c and Supplementary Fig. S6), leading to a drop in leakage current density\nby more than two orders of magnitude. Furthermore, fine-tuning the Bi/Na ratio (y) at\nthe optimal x enabled further effective defect engineering through the increase the\nconcentration of defect complexes (TSDC, Fig. 2d), which are more\nthermodynamically stable and locate deeper into the band gap compared with isolated\npoint defects1-3, thereby further increasing electrical resistivity and E . Therefore, the\nb\nhighest E is not determined by any single factor, but is achieved through the synergistic\nb\nstrategy of defect engineering, grain refinement and slimmer P-E loops.\nThe references are as follows:\n1. Saremi, S. et al. Enhanced electrical resistivity and properties via ion bombardment of\nferroelectric thin films. Adv. Mater. 28, 10750 (2016).\n2. Kim, J. et al. Ultrahigh capacitive energy density in ion-bombarded relaxor ferroelectric\nfilms. Science, 369, 81 (2020).\n3. Pan, H. et al. Enhanced electric resistivity and dielectric energy storage by vacancy\ndefect complex. Energy Storage Mater. 42, 836 (2021).\nReviewer #2 (Remarks to the Author):\nThe manuscript proposes the design of BaTiO -(K Na )TiO -(Bi Na )Ti Zr O\n3 0.5 0.5 3 0.5 0.5 0.9 0.1 3\nsolid-solution system for high-energy-density applications. The article presents an\ninteresting and current topic on high-performance dielectric capacitors. The motivation\nfor the present study is well-argued, and the current state of research is adequately\nrepresented in the introduction. The work can be of significance to the field and related\nfields.\nThe manuscript was improved, and the answers and corrections fully addressed the\nquestions raised in the previous review.\nIn conclusion, the reviewer considers the revised manuscript suitable for publication in\nNature Communications.\nResponse: We are truly grateful for the time and helpful advice which helped us to raise\nthe quality of our work. We are honored that the manuscript was accepted for\npublication and thank you again.\nBest regards!\nProf. Yuan-Hua Lin\nSchool of Materials Science and Engineering,\nTsinghua University, Beijing, 100084, P. R. China\nPhone: +86-10-62773741\nFax: +86-10-62771160\nE-mail: linyh@tsinghua.edu.cn"}, {"round": 3, "reviewer_comments": {"reviewer_1": "Authors have adequately addressed most of my concerns, and I recommend publication after the following minor revision.\n‘To further elucidate the crystal structures of the ceramics, we have calculated the lattice parameter and unit-cell-volume of the samples (Supplementary Fig. S2). It can be seen that the cell volume decreases significantly from 64.414 Å3 for BTO to 62.665 Å3 with the incorporation of KNN and BNTZ, primarily due to the substitution at the A-site and B-site with smaller ionic radius.’ The authors should quantitatively support this explanation by reporting the calculated average A-site and B-site ionic radii for the relevant compositions and citing the reference."}, "author_response": "A Point-by-point Response Letter for Manuscript #NCOMMS-25-71549B\nSubject: Manuscript #NCOMMS-25-71549B entitled “Ultrahigh energy-storage\ndielectric ceramics via synergistic polymorphic nanodomain and defect design” by M.\nZhang et al.\nDear Editor and Reviewers,\nWe greatly appreciate the constructive comments and suggestions from all\nreviewers. The insightful and constructive comments have helped us improve the\nManuscript. The following are our point-by-point responses to the reviewers’ comments,\nwith the revisions highlighted in the revised manuscript.\nReviewer #1 (Remarks to the Author):\nAuthors have adequately addressed most of my concerns, and I recommend publication\nafter the following minor revision.\nResponse: We thank the reviewer for carefully reading the manuscript and providing\ncomments and suggestions, which help us to improve the manuscript.\n‘To further elucidate the crystal structures of the ceramics, we have calculated the lattice\nparameter and unit-cell-volume of the samples (Supplementary Fig. S2). It can be seen\nthat the cell volume decreases significantly from 64.414 Å3 for BTO to 62.665 Å3 with\nthe incorporation of KNN and BNTZ, primarily due to the substitution at the A-site and\nB-site with smaller ionic radius.’ The authors should quantitatively support this\nexplanation by reporting the calculated average A-site and B-site ionic radii for the\nrelevant compositions and citing the reference.\nResponse: Thank you for the insightful comment. We have quantitatively supported\nour explanation by calculating the average A-site and B-site ionic radii for the relevant\ncompositions based on Shannon effective ionic radii1, which account for the\ncoordination environments in the perovskite structure (CN=12 for A-site, CN=6 for B-\nsite). The ionic radii used are: Ba2+ (1.610 Å), K+ (1.640 Å), Na+ (1.390 Å), Bi3+ (1.360\nÅ) for the A-site, and Ti4+ (0.605 Å), Nb5+ (0.640 Å), Zr4+ (0.720 Å) for the B-site2-5.\nFor pure BTO, the average A-site and B-site radii are 1.610 Å and 0.605 Å, respectively.\nThe addition of KNN, whose average A-site and B-site radii are 1.515 Å and 0.640 Å,\nleads to the initial cell volume shrinkage from 64.414 Å3 to 64.096 Å3 with a reduction\nof 0.49%. With the subsequent incorporation of 30 mol% BNTZ (average radii: 1.375\nÅ for A-site, 0.617 Å for B-site), compared to the 0.45BTO-0.55KNN composition with\naverage radii of 1.558 Å (A-site) and 0.624 Å (B-site), this reduction at the A-site and\nB-site with smaller ionic radius drives the more pronounced cell volume shrinkage to\n62.665 Å3, representing a total volume decrease of 2.72% from pure BTO. Therefore,\nthe calculated reduction in average ionic radii provides direct quantitative support for\nthe conclusion that the observed cell volume shrinkage is primarily due to ion\nsubstitution with smaller cations. We have updated the discussion in the manuscript\n(Please see page 6, line 128-136).\nThe references are as follows:\n1. Shannon R.D. Revised effective ionic radii and systematic studies of interatomic\ndistances in halides and chalcogenides. Acta Cryst. 32, 751 (1976).\n2. Zhang M. et al. Ultrahigh energy storage in high-entropy ceramic capacitors\nwith polymorphic relaxor phase. Science, 384, 185 (2024).\n3. Zha J. et al. High energy storage performance of KNN-based relaxor\nferroelectrics in multiphase-coexisted superparaelectric state. J. Appl. Phys. 136,\n074101 (2024).\n4. Kong X. et al. High-entropy engineered BaTiO -based ceramic capacitors with\n3\ngreatly enhanced high-temperature energy storage performance. Nat. Commun. 2025,\n16, 885 (2025).\n5. Xiong X. et al. Ultrahigh Energy‐Storage in Dual‐Phase Relaxor Ferroelectric\nCeramics. Adv. Mater. 36, 2410088 (2024).\nBest regards!\nProf. Yuan-Hua Lin\nSchool of Materials Science and Engineering,\nTsinghua University, Beijing, 100084, P. R. China\nPhone: +86-10-62773741\nFax: +86-10-62771160\nE-mail: linyh@tsinghua.edu.cn"}, {"round": 4, "reviewer_comments": {"reviewer_1": "The authors have addressed all of my comments, and I am happy to recommend the manuscript for publication."}, "author_response": "A Point-by-point Response Letter for Manuscript #NCOMMS-25-71549C\nSubject: Manuscript #NCOMMS-25-71549C entitled “Ultrahigh energy-storage\ndielectric ceramics via synergistic polymorphic nanodomain and defect design” by M.\nZhang et al.\nReviewer #1 (Remarks to the Author):\nThe authors have addressed all of my comments, and I am happy to recommend the\nmanuscript for publication.\nResponse: We thank the reviewer for carefully reading the manuscript and providing\ncomments and suggestions, which help us to improve the manuscript. We are honored\nthat the manuscript was accepted for publication and thank you again.\nBest regards!\nProf. Yuan-Hua Lin\nSchool of Materials Science and Engineering,\nTsinghua University, Beijing, 100084, P. R. China\nPhone: +86-10-62773741\nFax: +86-10-62771160\nE-mail: linyh@tsinghua.edu.cn"}]

Decision letter