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Pulsed power and power electronics systems used in electric vehicles (EVs) demand high-speed charging and discharging capabilities, as well as a long lifespan for energy storage. To meet these requirements, ferroelectric dielectric capacitors are essential. We prepared lead-free ferroelectric ceramics with varying compositions of (1 − x)Ba0.7Sr0.3TiO3–(x)BiMg0.5Ti0.5O3 (BST–BMT) (x = 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1) using a solid-state-reaction method. To analyze the crystallinity and structural parameters, we examined the X-ray diffraction (XRD) patterns using the pseudo-Voigt function in the FullProf software. Additionally, Raman spectrum analysis confirmed the presence of ceramic structural distortion caused by microstrain and doping. Microstructure images of the ceramic samples showed an increase in grain size from 1 to 2.4 μm and an improved distribution of grain sizes with increasing doping levels. We investigated the dielectric properties of the BST–BMT ceramic capacitors across a wide range of frequencies and temperatures. Interestingly, as the BMT content increased, the previously saturated ferroelectric (FE) curve for x = 0.01 gradually shifted towards a narrower relaxor ferroelectric (RFE) curve for x = 0.1. The most favorable effective energy storage density was observed with a BMT doping concentration of x = 0.04, which coincided with exceptionally high-energy efficiency (η ~ 91%) under a field strength of 50 kV/cm and a relatively high dielectric normalized energy storage density of 3.71 µJV−1 cm−2 due to structural modifications that causes relaxor ferroelectric behavior. More interestingly, the energy storage performance of 0.96BST–0.04BMT displays a fatigue free characteristic enduring through numerous switching cycles. We also calculated the optical bandgap (Eg) values from UV–Vis spectra and compared them with the increase in BMT concentration. The Eg value for all ceramics was approximately 3.2 eV, similar to the pure BST ceramic sample. Additionally, the resistive switching behavior demonstrated by our bulk ceramic capacitors is not commonly observed in other bulk ceramics. [ABSTRACT FROM AUTHOR]