Dielectric capacitors with high capacitive energy storage are urgently needed to meet the growing demand for high-performance energy storage devices. Herein, a novel lead-free Sr₅BiTi₃Nb₇O${}_{30}$ (SBTN) tungsten bronze relaxor ferroelectric ceramic is prepared and explored for potential energy storage applications. A high recoverable energy density$W_{\mathrm{\text{rec}}}$ ($\sim$ 3.72 J/cm³) and ultrahigh efficiency$\eta$ ($\sim$ 94.2%) at 380 kV/cm are achieved simultaneously. Both$W_{\mathrm{\text{rec}}}$ and$\eta$ exhibit superior stabilities against temperature (30–140${}^{\circ }$C), cycles (10⁰ –10⁵) and frequency (1–500 Hz). In addition, a high current density of 796 A/cm² and a large power density of 71.7 MW/cm³ are achieved, together with good thermal endurance and fatigue resistance. These results demonstrate that the obtained SBTN ceramic can be deemed as the promising candidates for dielectric capacitor applications.Dielectric capacitors with high capacitive energy storage are urgently needed to meet the growing demand for high-performance energy storage devices. Herein, a novel lead-free Sr₅BiTi₃Nb₇O${}_{30}$ (SBTN) tungsten bronze relaxor ferroelectric ceramic is prepared and explored for potential energy storage applications. A high recoverable energy density$W_{\mathrm{\text{rec}}}$ ($\sim$ 3.72 J/cm³) and ultrahigh efficiency$\eta$ ($\sim$ 94.2%) at 380 kV/cm are achieved simultaneously. Both$W_{\mathrm{\text{rec}}}$ and$\eta$ exhibit superior stabilities against temperature (30–140${}^{\circ }$C), cycles (10⁰ –10⁵) and frequency (1–500 Hz). In addition, a high current density of 796 A/cm² and a large power density of 71.7 MW/cm³ are achieved, together with good thermal endurance and fatigue resistance. These results demonstrate that the obtained SBTN ceramic can be deemed as the promising candidates for dielectric capacitor applications.