Introduction Lead-free sodium niobate (NaNbO3) based ceramics with a superior energy storage density have attracted recent attention in high power dielectric energy storage applications. However, a pure NaNbO3 (NN) ceramic exhibits a square-like square hysteresis loop associating with a large hysteresis and a high remnant polarization at room temperature, leading to a high energy dissipation. In general, stabilizing the antiferroelectricity in NN ceramic is one of the effective measures to solve the problems above. However, despite efforts are made on this measure, the large hysteresis is still kept due to the existence of antiferroelectric- ferroelectric phase transition. The compositions in the phase boundary region usually exhibit abnormally enhanced electrical properties and relaxation characteristics, thus constructing a phase boundary region in NN through composition modulation is another promising method. Moreover, sodium element volatilizes inevitably during the high-temperature and long-time sintering process, giving rise to a poor sintering quality in NN system, which is not beneficial to achieving superior energy storage performance. In this paper, (1-x)NaNbO3-xCaTiO3 (NN-CT100x, 0.08≤x≤0.18) ceramics were prepared by a conventional solid-state method with CuO as a sintering aid. The phase structure and polymorphic phase boundary in NN-CT100x system were investigated, and the energy storage performance of compositions in the phase boundary region was analyzed.Materials and method For the synthesis of NN-CT100x powder, Na2CO3 (≥99.8%, in mass fraction, the same below), Nb2O5 (≥99.9%), CaCO3 (≥99.0%), TiO2 (≥98.0%) and CuO (≥99.0%) were used as raw materials (Shanghai Sinopharm Chemical Reagent Co., Ltd., China). The raw materials were weighed/mixed according to the stoichiometric ratio and ground with anhydrous ethanol in a ball mill with zirconia balls for 12 h. After dried and sieved, the powder was calcined at 900 ℃ for 6 h. The calcined powders were mixed with 1.5% (in mole fraction) of CuO as a sintering aid and further ground for 12 h. The ground powder was hand-pressed into discs with the diameter of 7 mm and the thickness of 1.2 mm and then further densified by cold isostatic pressing at 300 MPa for 30 min. The NN-CT100x discs were sintered at 1 050-1 250 ℃ for 2 h.Results and discussion Based on the results by X-ray diffraction, Raman spectroscopy and dielectric behavior, an ‘antiferroelectric P phase (orthorhombic, Pbma)-ferroelectric INC phase-paraelectric CT phase (orthorhombic, Pbnm)’ polymorphic phase boundary (PPB) region exists as 0.09≤x≤0.16 when CaTiO3 content increases. The polarization exhibits an enhancement in compositions within this PPB region. The optimum comprehensive energy storage performance is obtained as x=0.16, having the energy storage density (Wrec) of 3.04 J/cm3 and efficiency (?) of 84.4% at 300 kV/cm, respectively. The difference between the maximum polarization (Pm) and the remnant polarization (Pr) of the NN-CT16 ceramic increases as the electric field increases, thus obtaining an admirable energy storage performance. This composition is in the PPB region and contains a few antiferroelectric phases. These antiferroelectric phases are easily transformed into ferroelectric phases at high electric fields, which ? reduces from 92.4% to 77.0%. The NN-CT16 composition has a high Pm of 42.2 μC/cm2, a high Wrec of 6.1 J/cm3 and ? of 77% at 590 kV/cm. For the NN-CT16 ceramic, Wrec and ? both exhibit a frequency stability at 1-300 Hz, with minimal variations of <10% and <8%, respectively. Furthermore, the Wrec and ? also demonstrate temperature-insensitive characteristics at 25-160 ℃, with small variations of <10% and <5%, respectively. The discharge energy density (Wdis) of the NN-CT16 ceramic increases and reaches a plateau in a short time. The t0.9 (time at which 90% of the stored energy is released) is less than 8 μs. Moreover, the t0.9 decreases with the increasing electric field or temperature, indicating the discharge rate accelerates, which is probably ascribed to domains respond quickly and switch more easily at high electric fields or high temperatures.Conclusions Based on the phase boundary modulation strategy, the NN-CT100x (0.08≤x≤0.18) ceramics were designed and prepared via introducing CaTiO3 into NN with CuO as a sintering aid. The polarization of the compositions in the phase boundary region was enhanced. The NN-CT16 ceramic exhibited a high maximum polarization of 42.2 μC/cm2, a high energy storage density of 6.1 J/cm3 and an energy efficiency of 77% at 590 kV/cm. It also exhibited a good frequency/ temperature stability and an excellent discharge performance. This ceramic could be used as a promising dielectric energy storage material.