The utilization of relaxor ferroelectrics is thought to be a feasible approach to enhance energy storage performance due to the low remnant polarizations and slim hysteresis. Herein, environment-friendly (1-x)(Bi0.5Na0.5)TiO3-xSr(Ti0.5Zr0.5)O3 bulk ceramics have been developed, where the synergistic effect of enhanced relaxor-ferroelectricity and grain refinement with increasing x value boosts excellent energy storage performance, with impressive energy density (Wrec) of 4.2 J/cm3 accompanied by satisfactory energy efficiency (η) of 75.1%. Together with high thermal stability of the energy storage density of 2.0–2.1 J/cm3 (with minimal variation of ≤ ±5%) over 25–150 °C at 150.0 kV/cm due to the enhancement of thermal insensitivity, which is evidenced by simulation based on thermal conduction model. Additionally, an extraordinary fast discharge rate (t0.9) of 54.6 ns and a high power density (Pd) of 35.4 MW/cm3 can be achieved in the x = 0.20 ceramic sample. This discovery provides new opportunities to lead-free dielectric materials with a high energy density and efficiency for capacitive electrical energy storage.