High Recoverable Energy Storage Density in Nominal (1- x )(0.75 Bi 0.5 Na 0.5 TiO 3 –0.02BiFeO 3 –0.23BaTiO 3 )– x Bi 2 SrNb 2 O 9 Lead-Free Composite Ceramics

Bi2SrNb2O9 with a bismuth layer structure was added to a Bi0.5Na0.5TiO3–BiFeO3–BaTiO3 ceramic with a perovskite structure as an additive to improve its energy storage performance and temperature stability. A conventional solid-state reaction route was employed to fabricate a series of lead-free (1-x)(0.75 Bi0.5Na0.5TiO3–0.02BiFeO3–0.23BaTiO3)–xBi2SrNb2O9 (BNT-BF-BT-xBSN, x = 0–0.025) energy storage ceramics. The relaxation behavior and breakdown strength of BNT-BF-BT ceramics were improved, and remnant polarization was reduced after the addition of Bi2SrNb2O9, which originates from the formation of polar nanoregions (PNRs). Finally, the energy storage performance was improved. When x = 0.02, the recoverable energy density (Wrec) is as high as 4.2 J/cm3 under an applied electric field of 160 kV/cm, with an energy storage efficiency (η) of 85%. More importantly, the ceramic has excellent thermal stability (ΔWrec/Wrec < 4%, 25–150 °C). The material also shows a favorable charge–discharge performance, characterized by a power density of ∼36.5 MW/cm3 and a rapid discharge time t0.9 of ∼90 ns. This study demonstrates that the BNT-BF-BT-0.02BSN ceramics exhibit outstanding energy storage properties, making them a highly promising material for advanced dielectric capacitors.