Breakthrough Energy Storage Density in Bi _0.5 Na _0.5 TiO ₃ -Based Films via Synergistic Enhancement of Polarization and Breakdown Strength at Moderate Electric Fields

Dielectric capacitors are indispensable for high-power energy storage systems due to their rapid charge–discharge capabilities, environmental sustainability, and exceptional power density. As a prototypical lead-free relaxor ferroelectric material, bismuth sodium titanate (Bi 0.5 Na 0.5 TiO 3, NBT) is considered a promising candidate for dielectric capacitors owing to its large polarization. However, the inherent contradiction between high polarization and high breakdown strength ( E b ) limits the energy storage performance of NBT-based film capacitors, severely restricting their application in high-pulsed-power systems. In this work, a synergistic strategy is proposed to optimize the polarization performance and achieve outstanding energy storage capabilities. From the introduction of Bi(Mg 0.5 Zr 0.5 )O 3 (BMZ) into NBT-based films, a structural transition is realized from large-scale ferroelectric domains to small-sized, highly dynamic polar nanoregions (PNRs), accompanied by significant grain densification and reduced grain size. Consequently, this approach effectively reduces remnant polarization ( P r ) and minimizes the leakage current. In the optimized 0.7NBT–0.3BMZ films, simultaneous enhancements in polarization behavior and E b are achieved, yielding an ultrahigh W rec of 74.0 J cm –3 and maximum polarization ( P max ) of 110 μC cm –2 at a high E b of 2273 kV cm –1 . Furthermore, the excellent temperature stability (20–200 °C), frequency stability (50–5000 Hz), and cycling stability (1–10 5 cycles) with the variation of W rec < ±4% and efficiency (η) < ±3% are also achieved in the 0.7NBT–0.3BMZ films. This work achieves simultaneous enhancement of polarization and E b in lead-free film capacitors, offering a breakthrough strategy to advance dielectric energy storage devices with superior performance.