Introduction The common energy storage devices mainly include supercapacitors, lithium-ion batteries, fuel cells, and ceramic capacitors. Compared to other energy storage devices, ceramic capacitors have attracted much attention due to their ultra-high charge-discharge rates and power density. Antiferroelectric ceramics exhibit a large maximum polarization and close-to-zero residual polarization, displaying characteristics of a double hysteresis loop. Also, antiferroelectric ceramics have an advantage of low dielectric loss, making them one of the optimal energy storage materials. Lead-free NaNbO3-based antiferroelectric ceramics become popular due to the environmental concern and cost reduction. However, NaNbO3-based ceramics can transform from an antiferroelectric phase to a ferroelectric phase at a certain applied electric field. Even after removing the applied electric field, some ferroelectric phase remains, increasing the residual polarization strength and affecting the formation of the double hysteresis loop at room temperature. Furthermore, a transition between antiferroelectric phase and ferroelectric phase caused by the complex phase structure results in a loss of most energy in the form of heat, reducing the energy storage efficiency (η). To further regulate the energy storage characteristics of NN-based antiferroelectric ceramics, this paper introduced Bi3+ and Fe3+ into the NaNbO3 matrix to enhance the stability of the antiferroelectric phase and the relaxor behavior of the ferroelectric phase, resulting in a relaxor antiferroelectric ceramic. Different molar fractions of Sm2O3 were doped into the NN-BF ceramics with Sm3+ solid solution into the NaNbO3 lattice. In addition, the influence of doping amount of Sm3+ on the phase structure, microstructure, dielectric properties, and energy storage performance of NN-BF ceramics was also investigated.Methods Different Sm3+ doping amounts of (1-x)[0.9NaNbO3-0.1BiFeO3]-xSm2O3 (x=0.01, 0.02, 0.03, 0.04) relaxor antiferroelectric ceramics were prepared via conventional solid-state reaction. The ceramic green bodies with a diameter of 10 mm and a thickness of 1 mm were prepared. They were sintered in a furnace at a rate of 3 ℃/min at 1,150 ℃ for 3 h, and then cooled naturally to form ceramics. The ceramic samples were polished to a thickness of 0.1 mm, and silver electrodes with a radius of 2 mm were screen-printed on the upper and lower surfaces of the ceramic samples. The samples were heated at 500 ℃ for 1 h to connect the ceramics and electrodes tightly. The phase structures of different ceramic samples were analyzed by an X-ray diffractometer. The microstructure and elemental distribution of the ceramic samples were analyzed by a field emission scanning electron microscope. The dielectric loss and dielectric constant of the ceramic samples at different temperatures (i.e., 25~300 ℃) and frequencies (i.e., 1, 10, 100, 500, 1 000 kHz) were tested by an LCR meter. The electric hysteresis loops of the ceramic samples were measured at room temperature and 10 Hz by a ferroelectric tester, and the energy density and energy storage efficiency of the ceramic samples were calculated based on these measurements. The XPS spectra of Fe 2p in ceramic samples with different Sm3+ doping amounts (x=0.01, 0.02, and 0.03) were determined by an X-ray photoelectron spectrometer.Results and discussion The XRD patterns show that the ceramic with the Sm3+ doping amount (x) of 0.01 or 0.02 has a homogeneous perovskite structure without the second phase. Bi3+, Fe3+, and Sm3+ are all solid-solutions into the NaNbO3 lattice. When the Sm3+ doping amount (x) is 0.02, Sm3+ occupies the position of Na+ in the NaNbO3 lattice. The lattice energy increases after solid solution since the ionic radius of Sm3+ is greater than that of Na+, inhibiting the growth of grains. Consequently, the microstructure of the ceramic becomes denser, with smaller and more uniformly distributed grains. The larger dielectric constant reduces the breakdown field strength of the ceramic, while the smaller dielectric constant decreases the polarization of the ceramic. Therefore, a moderate dielectric constant is conducive to achieving a larger energy storage density for the ceramic composition when the Sm3+ doping amount (x) is 0.02. The hysteresis loop of the NN-BF ceramic is relatively slender, resembling a “waist-like” shape typical of relaxor antiferroelectric. This is because the doping of Bi3+ and Fe3+ enhances the stability of the antiferroelectric phase and the relaxor behavior of the ferroelectric phase. The XPS spectra of Fe 2p indicate that an appropriate doping of Sm3+ can stabilize the valence state of Fe3+, further stabilizing the antiferroelectric phase of NN-BF and reducing the generation of oxygen vacancies in the ceramic. This leads to an increase in the breakdown field strength.Bi3+ in the A-site of the 0.9NaNbO3-0.1BiFeO3 lattice is gradually replaced by Sm3+, resulting in the appearance of the second phase Bi2O3. The low-melting-point Bi2O3 plays a role in promoting sintering during the firing process, thereby facilitating the grain growth. However, the second phase Bi2O3 is not resistant to breakdown, leading to a decrease in the breakdown field strength of the ceramic.Conclusions The stability of the antiferroelectric phase and the relaxor behavior of the ferroelectric phase in NN-BF ceramics with the Sm3+doping amount of 0.02 were enhanced. An appropriate doping of Sm3+ could stabilize the valence state of Fe3+, further stabilizing the antiferroelectric phase of NN-BF ceramics. In an applied electric field of 300 kV/cm, the maximum polarization (Pmax) of NN-BF ceramics (i.e.,17.83 μC/cm2), the minimum residual polarization (Pr) (i.e., 2.87 μC/cm2), and the maximized Pmax-Pr (i.e.,14.96 μC/cm2) were obtained. The energy density and energy storage efficiency of NN-BF ceramics both reached their maximum values (i.e., 3.67 J/cm3 and 71.63%), respectively. The breakdown field strength of the ceramic was calculated to be 438 kV/cm through the Weibull distribution. These results indicated that NN-BF relaxor antiferroelectric ceramics with an appropriate Sm3+ doping could have promising potential applications in energy storage.