Lead-free relaxor ceramics (1 − $x$)K${}_{0.5}$Na${}_{0.5}$NbO₃ − $x$Bi(Mn${}_{0.5}$Ni${}_{0.5}$)O₃ ((1 − $x$ )KNN-$x$BMN) with considerable charge–discharge characteristics and energy storage properties were prepared by a solid state method. Remarkable, a BMN doping level of 0.04, 0.96KNN–0.04BMN ceramic obtained good energy storage performance with acceptable energy storage density$W$${}_{\mathrm{rec}}$ of 1.826 J/cm³ and energy storage efficiency$\eta$ of 77.4%, as well as good frequency stability (1–500 Hz) and fatigue resistance (1–5000 cycles). Meanwhile, a satisfactory charge–discharge performance with power density$P$${}_D$$\sim$ 98.90 MW/cm³, discharge time$t$${}_{0.9}$ < 70 ns and temperature stability (30–180^∘C) was obtained in 0.96KNN–0.04BMN ceramic. The small grain size ($\sim$150 nm) and the high polarizability of Bi${}^{3+}$ are directly related to its good energy storage capacity. This work proposes a feasible approach for lead-free KNN-based ceramics to achieve high-energy storage and ultra-fast charge–discharge performance as well as candidate materials for the application of advanced high-temperature pulse capacitors.Lead-free relaxor ceramics (1 − $x$)K${}_{0.5}$Na${}_{0.5}$NbO₃ − $x$Bi(Mn${}_{0.5}$Ni${}_{0.5}$)O₃ ((1 − $x$ )KNN-$x$BMN) with considerable charge–discharge characteristics and energy storage properties were prepared by a solid state method. Remarkable, a BMN doping level of 0.04, 0.96KNN–0.04BMN ceramic obtained good energy storage performance with acceptable energy storage density$W$${}_{\mathrm{rec}}$ of 1.826 J/cm³ and energy storage efficiency$\eta$ of 77.4%, as well as good frequency stability (1–500 Hz) and fatigue resistance (1–5000 cycles). Meanwhile, a satisfactory charge–discharge performance with power density$P$${}_D$$\sim$ 98.90 MW/cm³, discharge time$t$${}_{0.9}$ < 70 ns and temperature stability (30–180^∘C) was obtained in 0.96KNN–0.04BMN ceramic. The small grain size ($\sim$150 nm) and the high polarizability of Bi${}^{3+}$ are directly related to its good energy storage capacity. This work proposes a feasible approach for lead-free KNN-based ceramics to achieve high-energy storage and ultra-fast charge–discharge performance as well as candidate materials for the application of advanced high-temperature pulse capacitors.