Eco-friendly lead-free polycrystalline Na0.5−x Kx Bi0.5TiO3 (x = 0.30,0.35,0.40,0.45) series were synthesized by the solid-state reaction method. The site-specific substitution of K-ion has been performed by varying its concentration. The processing parameters were optimized with increasing K+1-ion percentage. The polarization (P), current (I) and strain (S) behavior with an electric field (E) support the enhanced relaxor nature which is further strengthened by the temperature-dependent dielectric studies. The maximum value of the dielectric constant was found to be 4726 in x = 0.30 composition. The value of Tm is increased from 320°C to 350°C by the substitution of the higher percentage of potassium. The shape of the P-E and I-E loops transforms from pinched type to open type slanted loop. The increased breakdown strength as well as the coercive field with the potassium concentration were confirmed through P-E and I-E hysteresis. The dielectric breakdown has reached close to ∼102 kV/cm for x = 0.45. The optimum estimated recoverable energy storage density and efficiency are found to be ∼0.5 J/cm3 and 58%, respectively, for x = 0.30 among studied compounds. Thus, x = 0.30 composition is found to be suitable for a dielectric capacitor as well as energy storage applications. © 2022 Elsevier B.V.