Polymorphic Superparaelectrics Enable High Energy Density, > 90% Efficiency, and Wide‐Temperature Stability in High‐Entropy KNN‐Based Ceramics

ABSTRACT

Simultaneously achieving high recoverable energy‐storage density ( W _rec ), high efficiency ( η ), and excellent temperature stability remains a major challenge for dielectric energy storage ceramics. Here, a polymorphic superparaelectrics high‐entropy strategy (SPE‐HE strategy) is proposed in K _0.5 Na _0.5 NbO ₃ ‐based ceramics through adding a linear dielectric component, Sr(Zr _0.2 Sn _0.2 Hf _0.2 Ti _0.2 Nb _0.2 )O ₃ . By promoting cation disorder and constructing coexisting orthorhombic‐tetragonal‐cubic (O‐T‐C) phases, the high‐entropy effect induces lattice strain, improving resistivity and electric‐potential homogeneity. Meanwhile, the formation of nanoscale domains lowers the energy barrier for domain reversal, establishing a macroscopically linear‐like polarization response with minimized hysteresis and simultaneously enhanced polarization and breakdown tolerance. The optimized 0.15HE ceramic broadens the superparaelectric‐relaxor ferroelectric transition state (SPE‐RFE) to room temperature, delivering an outstanding W _rec of 7.97 J cm ⁻³ and a high η of 91.3% under 500 kV cm ⁻¹ , as well as a stable energy‐storage performance over a wide temperature range of 25°C–145°C. This work provides an effective route for developing lead‐free dielectric ceramics with high energy storage performance and temperature stability.