Introduction With the increasingly severe environmental pollution and global warming, developing green and sustainable energy storage devices with high power density, energy storage density, and good stability becomes a research hotspot. Compared with batteries and electrochemical capacitors, lead-free ceramic dielectric capacitors exhibit eco-friendly, high power density, fast charge/discharge speed and excellent reliability, which are promising candidates for advanced pulse power systems. In general, the energy storage density of ceramic dielectric capacitors can be evaluated via measuring the polarization versus electric field hysteresis loops (i.e., P-E loops). Large maximum polarization (Pmax), small remnant polarization (Pr) and high breakdown strength (Eb) are critical parameters to achieve a high recoverable energy storage density (Wrec) and an energy storage efficiency (η). Although some previous work reported a higher energy storage density at higher electric field (>200 kV/cm), the high electric field could restrict the application in some fields. It is thus crucial to develop ceramic dielectric capacitors with a superior energy storage performance at a lower electric field (<200 kV/cm). In this paper, we designed and prepared a series of (1-x)(0.94Bi0.55Na0.45TiO3-0.06BaTiO3)- xSr0.7Bi0.2TiO3 (x=0, 0.10, 0.20, 0.30, 0.40) lead-free relaxor ferroelectric ceramics to achieve a high energy storage performance and a superior stability at a low electric filed. In addition, the energy storage properties were also investigated.Methods TiO2 (98%), SrCO3 (99%), Bi2O3 (99%), Na2CO3 (99.8%) and BaCO3 (99%) were used as raw materials for the synthesis of (1-x)BNBT-xSBT lead-free ceramics. According to the stoichiometric ratio, these raw materials were weighed. The materials were mixed with alcohol and ground in nylon jars for 12 h. After drying, the powders were calcined at 950 ℃ for 4 h. Subsequently, the calcined powders with alcohol were further ground in nylon jars for 12 h.Afterwards, the obtained powders were mixed with binders and pressed uniaxially into disk-shaped samples with 1 mm in thickness and 10 mm in diameter. Finally, the samples were sintered at 1 150-1 250 ℃ for 2 h after removing the binders at 600 ℃ for 8 h.The phase structure of the ceramics was analyzed by a model D8-Advance X-ray diffractometer (XRD, Germany). The Raman spectra were collected by a model Jobin-Yvon HR800 Raman spectroscope (Horiba Co., France). The microstructure of the ceramics was determined by a model APERO HIVAC scanning electron microscope (SEM, USA). In order to test the electrical properties of the (1-x)BNBT-xSBT ceramics, the silver paste was coated on the surface of the samples and fired at 580 ℃ for 10 min to form silver electrodes. The temperature-dependent dielectric constant and loss were measured by a model Agilent E4980A impedance analyzer. The ferroelectric performance was determined by a model Precision Premier II ferroelectric analyzer (Radiant Co., USA) with 2 mm in electrode diameter.Results and discussion The microstructure analysis indicates that all the ceramics samples have a slight porosity. Although some secondary phases appear in the ceramics due to the non-stoichiometric ratio of Bi and Na elements, the ceramics maintain a main perovskite structure. The difference of peak intensity between v2, v3 and v4, v5 reduces gradually with increasing the SBT content due to the enhancement of structural symmetry. Meanwhile, the peaks of dielectric constant shift towards a lower temperature along with the improvement of temperature stability of dielectric constant as the SBT content increases. At 25-343.9 ℃, the variation of dielectric constant is less than ±10% for (1-x)BNBT-xSBT ceramics when x=0.40. At 10 Hz and 120 kV/cm, the Pmax decreases with increasing the SBT content, but the P-E loops become slimmer, which is beneficial to achieving a higher energy storage efficiency. Meanwhile, four peaks appear in current versus electric field curves (I-E curves) and the current peak shifts towards zero electric field along with the decrement of current intensity when the SBT content increases, indicating the enhancement of response speed for domains after removing the external voltage. The ceramic with the composition of x=0.30 exhibits slim P-E loops at different electric fields and a recoverable energy storage density of 2.16 J/cm3 along with a high energy storage efficiency of 90% at a low electric field of 192 kV/cm. In addition, the energy storage performance shows superior frequency and cycle stability. The variation of recoverable energy storage density and energy storage efficiency is less than ±8% within 1-200 Hz and 1-105 cycles.Conclusions (1-x)BNBT-xSBT lead-free ceramics were prepared by a solid-state reaction method. The effect of SBT content on the microstructure, dielectric, ferroelectric, energy storage properties and stability of (1-x)BNBT-xSBT ceramics were investigated. The temperature stability of dielectric constant was improved and the variation of dielectric constant was less than ±10% at 25-343.9 ℃ for the composition of 0.60BNBT-0.40SBT. The superior energy storage performance with a recoverable energy storage density of 2.16 J/cm3 and an energy storage efficiency of 90% was achieved at a low electric field (<200 kV/cm) and an optimized SBT content. In addition, the energy storage performance of (1-x)BNBT-xSBT ceramics showed a superior frequency and cycle stability. This study demonstrated that the addition of SBT could be beneficial to optimizing the energy storage performance of BNT-based lead-free ceramics and the (1-x)BNBT-xSBT ceramics could be promising candidates for advanced pulse power systems application.