With the increasing demand of high-power and pulsed power electronic devices, environmental-friendly potassium sodium niobate ((Na${}_{0.5}$K${}_{0.5}$)NbO₃, KNN) ceramic-based capacitors have attracted much attention in recent years owning to the boosted energy storage density ($W_{\mathrm{\text{rec}}}$). Nevertheless, the dielectric loss also increases as the external electric field increases, which will generate much dissipated energy and raise the temperature of ceramic capacitors. Thus, an effective strategy is proposed to enhance the energy storage efficiency ($\eta$) via tailoring relaxor behavior and bad gap energy in the ferroelectric 0.9(Na${}_{0.5}$K${}_{0.5}$)-NbO₃–0.1Bi(Zn${}_{2/3}$(Nb${}_x$Ta${}_{1-x}$)${}_{1/3}$)O₃ ceramics. On the one hand, the more diverse ions in the B-sites owing to introducing the Ta could further disturb the long-range ferroelectric polar order to form the short–range polar nanoregions (PNRs), resulting in the high$\eta$. On the other hand, the introduction of Ta ions could boost the intrinsic band energy gap and thus improve the$E_{\mathrm{\text{b}}}$. As a result, high$W_{\mathrm{\text{rec}}}$ of 3.29 J/cm³ and ultrahigh$\eta$ of 90.1% at the high external electric field of 310 kV/cm are achieved in$x$ = 0.5 sample. These results reveal that the KNN-based ceramics are promising lead-free candidate for high-power electronic devices.With the increasing demand of high-power and pulsed power electronic devices, environmental-friendly potassium sodium niobate ((Na${}_{0.5}$K${}_{0.5}$)NbO₃, KNN) ceramic-based capacitors have attracted much attention in recent years owning to the boosted energy storage density ($W_{\mathrm{\text{rec}}}$). Nevertheless, the dielectric loss also increases as the external electric field increases, which will generate much dissipated energy and raise the temperature of ceramic capacitors. Thus, an effective strategy is proposed to enhance the energy storage efficiency ($\eta$) via tailoring relaxor behavior and bad gap energy in the ferroelectric 0.9(Na${}_{0.5}$K${}_{0.5}$)-NbO₃–0.1Bi(Zn${}_{2/3}$(Nb${}_x$Ta${}_{1-x}$)${}_{1/3}$)O₃ ceramics. On the one hand, the more diverse ions in the B-sites owing to introducing the Ta could further disturb the long-range ferroelectric polar order to form the short–range polar nanoregions (PNRs), resulting in the high$\eta$. On the other hand, the introduction of Ta ions could boost the intrinsic band energy gap and thus improve the$E_{\mathrm{\text{b}}}$. As a result, high$W_{\mathrm{\text{rec}}}$ of 3.29 J/cm³ and ultrahigh$\eta$ of 90.1% at the high external electric field of 310 kV/cm are achieved in$x$ = 0.5 sample. These results reveal that the KNN-based ceramics are promising lead-free candidate for high-power electronic devices.