Energy storage efficiency ≥ 99.5% achieved in weak-coupling ferroelectric relaxor BaTiO3–Bi(Mg0.5Ti0.5)O3 –Bi(Mg0.5Sn0.5)O3 system

High efficiency (η) is urgently desired for electronic energy storage devices. In this work, an extremely high energy storage efficiency (~ 99.5%) and energy storage density of 2.83 J/cm³ are achieved in lead-free relaxor ferroelectric (1–x)(0.9BaTiO₃–0.1BiMg_0.5Ti_0.5O₃)-xBi(Mg_0.5Sn_0.5)O₃[(1–x) (0.9BT–0.1BMT)–xBMS] ceramic (x = 0.09). Excellent temperature stability with a variation of η less than 1.5% is also obtained in a wide temperature range from 30 to 150 °C. Temperature dependence of the dielectric permittivity of (1–x)(0.9BT–0.1BMT)-xBMS exhibits a typical dipolar-glass-like relaxor ferroelectric behavior. As a result, the ultra-high efficiency of the ceramic is attributed to the weak-coupling polar nanoregions (PNRs) which are analyzed using the Vogel-Fulcher formula and phenomenological statistical model. These results not only help to understand the origin of high efficiency in the (1–x)(0.9BT–0.1BMT)-xBMS system but also provide an effective approach to improve the comprehensive properties of other lead-free ceramic systems used in next-generation power capacitors.