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In this work, lead-free (1- x)Ba 0.85 Ca 0.15 Zr 0.1 Ti 0.9 O 3 - x Bi(Mg 2/3 Ta 1/3)O 3 (BCZT-BMT x) relaxor ferroelectrics (RFEs) were prepared via a traditional solid-state reaction. BMT was introduced into ferroelectric (FE) BCZT ceramics by a strategy of composition modification, which remarkably inhibited the grain growth and triggered a FE-to-RFE phase transition. The breakdown strength (E b) of BCZT ceramics was greatly improved by enhancing the band gap with Ta 2 O 5 (∼4 eV) doping into B-site. BCZT-BMT8 ceramic possesses both high energy efficiency (η) of 93.87% and recoverable energy density (W rec) of 3.4 J/cm3 at a relatively high E b of 440 kV/cm, which are superior to most of other BaTiO 3 -based energy storage ceramics. Especially, excellent energy storage stability with respect to frequency (2–500 Hz, a variation of W rec <4.27% and η < 2.65%) and temperature (30–130 °C, W rec <13.5% and η < 2.4%) was obtained for BCZT-BMT8 ceramic. In brief, the enhanced W rec and E b with significantly high η indicated that BCZT-BMT8 ceramic is a promising candidate for energy storage capacitors. • Novel energy storage ceramics were developed by introducing "relaxor end-member". • An ultrahigh η ∼ 93.8% and high W rec ∼ 3.4 J/cm3 have been achieved in BCZT-BMT8. • BCZT-BMT8 ceramic exhibits an excellent frequency/temperature stability for ESP. • EPR signal of V O • • could be revealed in BCZT-based ceramics. [ABSTRACT FROM AUTHOR]