Hierarchical polar-nonpolar phase architecture enabling excellent lead-free capacitive energy storage

Dielectric capacitors are highly attractive for advanced power electronics owing to their ultrafast charge-discharge rate, high power density, and excellent reliability. Yet their application is hindered by the persistent trade-off between high recoverable energy density (W _rec) and high efficiency (η) owing to the inherent coupling in single-phase dielectrics, where stronger polarization generally comes at the expense of higher hysteresis and limited breakdown strength. Here, we present a polar-nonpolar hierarchical phase architecture design in BaTiO₃-BiMg_0.5Ti_0.5O₃-based ceramics to overcome this limitation by modulating thermodynamic spinodal decomposition. The polar Ba-rich phase provides large maximum polarization, while the non-polar Cd-rich precipitation with a large bandgap acts as a high-resistivity barrier that isolates polar regions and enhances the breakdown field. Atomic-scale electron microscopy analysis reveals that the nanoscale polar regions (∼1-3 nm) with locally disordered configurations emerge in the ceramic, which lowers the energy barrier for domain switching and enables near-zero hysteresis losses. As a result, the optimized hierarchical composition achieves an ultrahigh efficiency of 92.8%, a high recoverable energy density of 9.7 J cm^-3, an outstanding high figure of merit W _F of 135 at 460 kV cm^-1, along with excellent stability against temperature, frequency, and cycling, and fast discharge with a power density up to 185 MW L^-1. This work demonstrates a robust design paradigm based on complementary dual-phase coexistence, offering fundamental insights and a practical pathway toward high-performance, lead-free dielectric capacitors.