Lead-free (1 − x)BaTiO₃–xBi(Zn_2/3Nb_1/3)O₃ (x = 0.05–0.20) materials were fabricated via solid-state reactions. A pure perovskite pseudocubic structure is obtained for all compositions. Dielectric measurements reveal an intensified diffusion and relaxor-like characteristics from 5 mol% to 20 mol% Bi(Zn_2/3Nb_1/3)O₃. Weakly coupled relaxor behavior is concluded from the exceptionally high activation energies of ∼0.20–0.22 eV from the Vogel–Fulcher model for x ≥ 0.10, which possibly results in the extremely low dielectric nonlinearity and extra slim polarization–electric field loops. An optimal discharged energy density of 0.79 J cm⁻³ with a high energy efficiency of 93.5% is achieved at 131 kV cm⁻¹ for x = 0.15, which proves that the BaTiO₃–Bi(Zn_2/3Nb_1/3)O₃ material is a promising candidate for high energy storage applications.