Phase constitution, microstructure, dielectric performance, polarization, breakdown strength as well as energy storage behaviors for the (1 - x)(0.45SrTiO(3)-0.2Na(0.5)Bi(0.5)TiO(3)-0.35BaTiO(3))-xSnO(2) (STNBTBT-Snx) were systematically investigated. The dielectric measurements exhibit a relaxor behavior, and the dielectric loss is very low (< 0.6%) at room temperature for all compositions. Significant refinement of grain size and low dielectric loss were observed with the addition of Sn, accounting for effectively enhanced dielectric breakdown strength (17.0-25.2 kV/mm when x = 0 similar to 0.09), beneficial for the energy storage applications. The sample with x = 0.07 exhibits the highest energy storage density of 2.25 J/cm(3) and an energy storage efficiency of 79.51% at 24 kV/mm. Particularly, its energy storage properties were found to depend weakly on frequency (1-100 Hz). Our results suggest that this system can be a potential lead-free candidate for high electric energy storage and discharge efficiency.