Among the lead-free compositions identified as potential capacitor materials, BiScO3-BaTiO3 (BS-BT) relaxor dielectrics exhibit good energy storage performance. In this research, 0.4BS-0.6BT is considered as the parent composition, with NaNbO3 (NN) addition intended to substitute the A and B site cations. The NN modified BS-BT ceramics exhibit excellent temperature stability in terms of their dielectric properties, with the room-temperature dielectric constant on the order of 500–1 000 and variation less than 10% up to 400 °C. In addition, NN has a high band-gap energy leading to increased breakdown strength and energy storage properties in modified compositions. The highest breakdown strength was achieved for 0.4BS-0.55BT-0.05NN, being on the order of 430 kV/cm, and a high energy density of 4.6 J/cm3 with high energy efficiency of 90% was simultaneously achieved. Of particular importance is that the variation of the energy density was below 5% due to the temperature-insensitive dielectric constant, while ∼90% energy efficiency was retained over the temperature range of 25–160 °C. The improved temperature stability with NN addition makes this composition promising for high temperature capacitor and dielectric energy storage applications.