Introduction Energy storage technology plays a vital role in advanced electronic and power systems. Ceramic dielectric capacitors show a great potential in electronics, pulse power, and defibrillators due to their ultrahigh power density, fast charging/discharging speed, and superior reliability. Increasing the energy storage density of dielectric capacitors will boost the development of powder devices towards miniaturization, lightweight, and low cost. It is thus crucial to improve the energy storage density of dielectric capacitor. In general, high energy storage properties (ESP) are closely related to a larger maximum polarization (Pm), a smaller remnant polarization (Pr), and a larger breakdown strength (Eb). The recoverable energy storage density (Wrec) and energy storage efficiency (η) are the crucial parameters to evaluate their ESP. This paper designed and prepared a series of (Bi0.5Na0.5)0.65(Sr1-xCax)0.35Ti0.96(Al0.5Nb0.5)0.04O3 (BNST-xC) ceramics (x=0.05, 0.10, 0.15, 0.20). The effect of relaxation characteristics on the energy storage characteristics of BNT ferroelectric ceramics was investigated. Methods The BNST-xC ceramics were fabricated via conventional solid-state reaction and subsequent tape-casting. The oxides and carbonates as raw materials were weighed according to a stoichiometric ratio and ground in a planetary mill with a nylon grinding chamber for 12 h. After drying, the powders were calcined at 850 ℃ for 6 h and then ground for 12 h. Subsequently, the synthesized BNST-xC powders were mixed with glycerol trioleate, ethanol absolute, butanone, dibutyl phthalate, polyvinyl butyral, and polyethylene glycol to obtain a slurry. The uniformly mixed slurry was tape-casted on the surface of vacuum adsorbed PET release film using a model MSK-AFA-IV automatic thick film coater at a casting speed of 0.6 cm/s. After stacking and pressing, the films were placed in a model PTC LT08001 isostatic pressing machine (EASEN) and pressed at 75 ℃. Finally, the green samples were firstly heated at 600 ℃ for 8 h to remove the organics, and then sintered at 1 180-1 220 ℃for 2 h.The phase composition of ceramic was characterized by a model D/max 2550V X-ray diffractometer (XRD, Rigaku Co., Japan). The grain size of the sample was determined by a model TM4000 desktop scanning electron microscope (SEM, Hitachi Co., Japan). The Raman spectra were collected by a model Jobin Yvon HR800 Raman scattering spectrometer (Horiba Co., France) under 532 nm laser excitation. The hysteresis loop was determined by a model Premier II and radiant ferroelectric analyzer (Precision Co., USA) at 10 Hz; The dielectric performance and frequency dependence were analyzed by a model Agilent E4980A LCR meter and heating equipment (Tongguo Technology Co., China). A specially designed RLC circuit was used to test the charging and discharging performance of the capacitor.Results and discussion From the analysis of phase composition, Sr and Ca ions at a position A, and Al and Nb ions at a position B completely enter the interior of the lattice, forming a solid solution. The improvement of grain size by B-site (Al0.5Nb0.5)4+ ion doping is beneficial to enhancing the breakdown field strength. The average grain size of the ceramic gradually increases from 1.36 μm to 1.09 μm as the Ca doping content increases. From the Raman spectroscopy analysis, the substitution of A-site ions increases an overall local structural disorder. Furthermore, the transformation of their internal phase structure can be investigated based on the dielectric temperature spectra of BNST-xC ceramics. The substitution of Ca causes Ts and Tm moving towards a high-temperature region. Also, the dispersion of the peak shape in the dielectric temperature spectrum further proves that the relaxation degree of the overall material system continuously increases with the increase of Ca ion doping content. To more accurately demonstrate the law of material relaxation degree, the Curie-Weiss law is used to calculate the relaxation coefficient of the dielectric temperature spectrum. The flipping of the internal domain structure at the electric field was tested by PFM, further showing the contribution of Ca ions to the enhancement of relaxation properties.Conclusions A series of BNST-xC ceramics were prepared by a solid-state reaction method. The relationship between the relaxation characteristics and energy storage characteristics of BNT based ceramics was analyzed via adjusting the ratio of Sr and Ca ions. The optimal energy storage performance (i.e., Eb of 630 kV/cm, and Wrec of 6.4 J/cm3) was achieved as x=0.15. The energy storage efficiency reached 85%. At Eb of 300 kV/cm, the ceramic exhibited a good stability at different temperatures and frequencies as well as a high power density and an operating current density. It was indicated that BNT based relaxor ferroelectrics could have a promising application potential.