<p>Ferroelectric relaxors have been extensively investigated for energy storage applications in pulsed-power electronics owing to their low remnant polarization. Herein, a lead-free Ba(Zr_0.3Ti_0.7)O₃ (BZT) ferroelectric relaxor thin film was synthesized by a sol-gel method for energy storage application. It was found that the annealing temperature has a profound effect on the microstructure and the energy storage performance of the BZT thin film. Increasing annealing temperature promotes the burn-out of the organic residues as well as the densification of the thin films. However, excessive abnormal grain growth occurs at high annealing temperatures and leads to a reduced breakdown strength. Consequently, an optimized energy storage density of 51.4 J/cm³ and energy storage efficiency of 73.4% are achieved in Ba(Zr_0.3Ti_0.7)O₃ thin film annealed at 850 °C with a thickness of 210 nm, which also shows high frequency stability (0.2–20 kHz), high temperature stability (25–140 °C) and long-term anti-fatigue stability up to 10⁷ switching cycles. These results provide guidance to improve the energy storage performance of ferroelectric relaxor film capacitors for applications in advanced high power electronics.</p><p></p>