To investigate the effects of various sintering aids on energy storage performance, unipolar polarization-electric field (P-E) hysteresis loops were measured for all samples at their respective E_b, as presented in Figure 1A. All loops exhibit similar trends under the applied electric field and consistently show low P_r, an intrinsic characteristic of SBN ceramics. Figure 1B illustrates the dependence of P_m, P_r, and ΔP on the sintering aids. The incorporation of sintering aids systematically enhances P_m while maintaining an almost constant P_r. As a result, a significant improvement in ΔP is achieved, underscoring the effectiveness of sintering aids in optimizing polarization behavior. Specifically, the measured P_m values are 21.89, 24.47, 29.38, and 24.59 µC/cm², with the S2 sample achieving the highest ΔP of 26.59 µC/cm². Consequently, the S2 sample demonstrates the optimal energy storage performance [Figure 1C], delivering a W_rec of 4.41 J/cm³ and an η of 93.4%. Figure 1D presents the P-E hysteresis loops of the S2 ceramic under varying electric fields. As the electric field strength increases, the P_m rises gradually without reaching clear saturation, indicating favorable potential for further optimization of energy storage performance. As summarized in Supplementary Table 1, the S2 ceramic exhibits superior W_rec and η compared to recently reported TTB-structured ceramics, highlighting its potential for high-performance energy storage applications. The underlying mechanisms responsible for the superior performance of the S2 sample will be discussed in detail in the following section.

Figure 2A visualizes a schematic of the SBN lattice structure projected along the [001] axis. In this structure, the C site remains vacant, while the A and B sites are partially occupied: only 5/6 of the A positions are filled, with the remaining 1/6 left empty^[28]. Notably, due to its larger ionic radius (1.61 Å), Ba²⁺ tends to occupy the larger A2 site, whereas the smaller Sr²⁺ (1.44 Å) occupies both the A1 site and the remaining A2 positions. Raman spectroscopy was performed in the range of 150-1,000 cm^-1, with the spectra shown in Figure 2B. The observed features match those previously reported for TTB-type ceramics, confirming the characteristic vibrational modes of the TTB structure. The v₅ mode near 250 cm^-1 is assigned to O-B-O bending vibrations, and the v₂ mode around 630 cm^-1 is corresponding to B-O stretching vibrations^[29]. No discernible changes in these modes were observed following the addition of sintering aids, suggesting that the sintering aids themselves do not alter the local structure. XRD patterns of all samples are presented in Figure 2C. They show good agreement with the standard reference (JCPDS No. 88-0785) and no detectable secondary phases, indicating that the addition of sintering aids does not induce phase separation. Rietveld refinement of the XRD data was performed, with the results presented in Supplementary Figure 1 and the detailed lattice parameters summarized in Supplementary Table 2. All samples were indexed to the pure P4bm space group. Notably, the introduction of sintering aids results in varying degrees of change in the lattice parameters, indicating that these additives can partially enter the crystal lattice of the matrix. To evaluate the effect of sintering aids on densification, the measured and relative density of the ceramics are summarized in Figure 2D. Ceramics modified with sintering aids exhibit relative densities exceeding 98%, higher than the 96.7% achieved for the unmodified SBN ceramic.

The phase structures of the S2 ceramic were further characterized using transmission electron microscopy (TEM), as shown in Figure 2E-F. Along the [110] and [001] zone axes, the basic SAED patterns can be indexed to a standard TTB structure. Notably, superlattice spots are observed along the [110] zone axis, which can be attributed to A-site cation distortion, consistent with previous reports^[30]. Specifically, the A-site cation distortion, as evidenced by these superlattice spots, introduces local random fields that disrupt the long-range ferroelectric order. This disruption promotes the formation of PNRs, which are widely recognized as the hallmark of relaxor ferroelectrics.

Figure 3A-D present the surface SEM images of all SBN ceramics, with their corresponding grain size distributions displayed in Figure 3E. All compositions exhibit a microstructure consisting of a mixture of equiaxed and pillar-type grains. Notably, the unmodified S0 ceramic exhibits a few residual pores, whereas the addition of sintering aids reduced porosity and resulted in a marked improvement in densification. This enhanced densification directly contributes to superior breakdown resistance. On one hand, fewer pores reduce local electric field concentration, delaying breakdown initiation. On the other hand, a more compact microstructure effectively hinders the propagation of breakdown pathways. Based on Gaussian fitting, the average grain sizes are determined to be 1.47, 2.09, 1.92, and 1.74 µm, indicating that the sintering aids promote grain growth to a certain degree. This enhanced densification and grain coarsening can be attributed to the effects of the MnO₂/CuO/Li₂CO₃ sintering additives. During the sintering process, a liquid phase forms, facilitating mass transport and thereby promoting grain growth and densification^[27,31,32]. As illustrated in Supplementary Figures 2-5, the homogeneous distribution of Mn and Cu with no observable segregation confirms that these ions have been incorporated uniformly into the SBN lattice. The improved densification and uniform elemental distribution are favorable for the optimization of the electrical properties.

In this work, the introduction of sintering aids (MnO₂, CuO, and Li₂CO₃) enables the incorporation of Mn⁴⁺, Li⁺, and Cu²⁺ ions into B sites of the lattice, where they substitute for Nb⁵⁺. This aliovalent substitution necessitates charge compensation, leading to the formation of oxygen vacancies ($$ V_{\mathrm{o}}^{\prime \prime} $$)^[33]. These oxygen vacancies tend to form defect dipoles with lattice defects, such as Mn_Nb’. Subsequently, these dipoles locally aggregate and couple to form defect clusters^[34]. As a result, the concentration of mobile charge carriers decreases, enhancing the insulation properties of the ceramics. To further investigate the existence of oxygen vacancies, XPS analysis was performed, and the results are displayed in Supplementary Figure 6. The O 1s spectra of all ceramics could be deconvoluted into three distinct peaks via Avantage software, corresponding to lattice oxygen (O_L), oxygen vacancies (O_V), and adsorbed oxygen (O_H)^[35].

Taken together, these structural analyses demonstrate that the incorporation of sintering aids effectively promotes densification while preserving the single-phase TTB structure of SBN. These microstructural advantages underpin the enhanced energy storage performance observed in the corresponding samples.

To explore the origin of the electrical properties and the enhancement in energy storage performance, temperature-dependent dielectric permittivity (ε_r) and dielectric loss (tanδ) for all SBN ceramics are depicted in Figure 4A-D. All samples exhibit frequency dispersion with a broad phase transition temperature (T_m), indicative of typical relaxor behavior. Figure 4E compares the temperature-dependent ε_r across compositions, while the maximum permittivity (ε_m) and the corresponding T_m measured at 1 kHz are summarized in Figure 4F. The incorporation of sintering additives moderately influences the dielectric properties. Notably, the ε_r values of S1, S2, and S3 at room temperature are higher than those of S0. This increase in ε_r, accompanied by consistently low tanδ, is attributed to improved domain wall mobility and facilitated polarization rotation. Moreover, the addition of sintering aids leads to a decrease in T_m, which can be explained by the enhanced random fields introduced by the additives.

As a typical relaxor ferroelectric, SBN exhibits characteristic relaxor dispersion. Figure 5A presents the relaxation temperature span ΔT_relax (defined as T_m at 100 kHz minus T_m at 100 Hz) for each sample. ΔT_relax progressively increases from 7.4 °C for S0 to 9.1 °C for S1, 11.6 °C for S2, and finally to 12.4 °C for S3. Furthermore, a modified Curie-Weiss law is used to evaluate the relaxor degree^[36,37]:

(4) $$ \frac{1}{\varepsilon}-\frac{1}{\varepsilon_{\mathrm{m}}}=\frac{\left(T-T_{\mathrm{m}}\right)^{\gamma}}{\mathrm{C}} $$

A value of γ = 1 corresponds to normal ferroelectric behavior, whereas γ = 2 indicates ideal relaxor characteristics. The fitted γ values for S0-S3 are 1.71, 1.79, 1.80, and 1.81, respectively [Figure 5B]. These results provide powerful evidence that the addition of sintering additives effectively enhance the relaxor nature of the SBN ceramics, which originates from the formation of random fields and compositional disorder within the lattice.

To evaluate the influence of the additive on the band gap, UV-vis absorption spectra [Figure 6A] were measured and analyzed using the Tauc plot method, expressed as^[38,39]:

(5) $$ (\alpha h v)^{\frac{1}{\mathrm{n}}}= A\left(h v-E_{\mathrm{g}}\right) $$

where α, h, v, E_g denote the absorption coefficient, Planck’s constant, frequency, and band gap. A is a constant, and n is an index taken as 1/2 for dielectric ceramics. The calculated E_g values for all samples are 3.31, 3.28, 3.29, and 3.26 eV [Figure 6B], indicating good insulating properties. While the band gap width is generally considered to influence the intrinsic breakdown strength of dielectric materials, the variation in E_g among the samples is relatively small and cannot solely account for the significant differences observed in E_b. The substantial enhancement of E_b following the introduction of sintering aids is therefore primarily attributed to enhanced densification. The breakdown behavior was further assessed using Weibull statistics. Figure 6C displays the Weibull distributions fitted with the equation^[40,41]:

(6) $$ P\left(E_{\mathrm{i}}\right)=1-\exp \left[-\left(\frac{E_{\mathrm{i}}}{E_{\mathrm{b}}}\right)^{\beta}\right] $$

where E is the applied electric field, P(E) presents the cumulative probability at E, E_b is the characteristic breakdown strength [P(E) = 63.2%], and β is the shape parameter that reflects data reliability (higher indicates better reliability). The fitted E_b values are 255.9, 342.6, 442.3, and 343.7 kV/cm, as shown in Figure 6D. The highest E_b achieved in S2 is consistent with its superior energy storage performance shown in Figure 1C.

In conclusion, the addition of sintering aids plays a crucial dual role in optimizing the ceramic’s energy storage performance. Firstly, it enhances densification, which effectively suppresses premature failure under high electric fields, thereby improving the breakdown strength. Secondly, it strengthens the relaxor characteristics, which minimizes hysteresis losses during charge-discharge cycles. The combination of a higher breakdown strength and lower polarization loss ultimately yields a markedly improved overall energy storage performance.

Furthermore, the thermal and frequency stability of the S2 ceramic was evaluated under an applied electric field of 320 kV/cm, as shown in Figure 7. Within the temperature range of 25-125 °C, the W_rec consistently exceeds 2.75 J/cm³, with variation below 10%, while the η remains above 91%. Notably, both W_rec and η also exhibit outstanding frequency stability, with variations of less than 2% across the measured frequency range. The excellent thermal stability can be attributed to the relaxor behavior and uniform microstructure promoted by the CuO sintering aid, which suppresses polarization degradation and loss increase at elevated temperatures. Overall, the CuO-modified SBN ceramics show remarkable stability in both temperature and frequency, underscoring their great potential for use in advanced pulse power devices.