Simultaneously Achieved Ultrastable Dielectric and Energy Storage Properties in Lead-Free Bi_0.5Na_0.5TiO₃-Based Ceramics

High-temperature dielectric ceramics are required for various emerging applications. However, controlling the dielectric loss at high temperatures remains a significant challenge due to the substantial leakage current at high temperatures. In this work, 0.76Bi0.5Na0.5TiO3–0.2NaNbO3–0.04Sr(ZrMgx)O3+x (BNT–NN–SZMx) lead-free ceramics were constructed by introducing trace MgO to suppress the space charge migration, reduce the leakage current, and enhance the breakdown strength of the ceramic. The BNT–NN–SZM0.05 ceramic showed excellent stability in dielectric permittivity with temperature (Δε/ε25°C ≤ ±15% from −63 to 363 °C) along with low dielectric loss (tan δ ≤ 0.02 from −41 to 330 °C), with a high room-temperature dielectric permittivity of 1150. Besides, the BNT–NN–SZM0.25 ceramic achieved a high discharge energy density (Wdis) of 5.2 J/cm3 under an applied electric field of 370 kV/cm, with a high energy efficiency of 85%. In addition, BNT–NN–SZM0.25 exhibited outstanding stability in energy density with temperature, with Wdis varying less than 7% over a wide temperature range from 25 to 200 °C. The design strategy demonstrated in this work may be applied to design high-temperature dielectric ceramics for energy storage and dielectric applications.