Lead-free relaxor ceramics (1 − x)K0.5Na0.5NbO₃ − xBi(Mn0.5Ni0.5)O₃ ((1 − x )KNN-xBMN) 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 densityWrec of 1.826 J/cm³ and energy storage efficiencyη 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 densityPD∼ 98.90 MW/cm³, discharge timet0.9 < 70 ns and temperature stability (30–180^∘C) was obtained in 0.96KNN–0.04BMN ceramic. The small grain size (∼150 nm) and the high polarizability of Bi3+ 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)K0.5Na0.5NbO₃ − xBi(Mn0.5Ni0.5)O₃ ((1 − x )KNN-xBMN) 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 densityWrec of 1.826 J/cm³ and energy storage efficiencyη 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 densityPD∼ 98.90 MW/cm³, discharge timet0.9 < 70 ns and temperature stability (30–180^∘C) was obtained in 0.96KNN–0.04BMN ceramic. The small grain size (∼150 nm) and the high polarizability of Bi3+ 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.