High‐Entropy Design Toward Ultrahigh Energy Storage Density Under Moderate Electric Field in Bulk Lead‐Free Ceramics

Abstract Electrostatic capacitors with ultrahigh energy‐storage density are crucial for the miniaturization of pulsed power devices. A long‐standing challenge is developing dielectric materials that achieve ultrahigh recoverable energy density W_rec ≥ 10 J cm⁻³ under moderate electric fields (30 ≤ E ≤ 50 kV mm⁻¹). Herein, a specific high‐entropy strategy is proposed to modulate the phase structure and interfacial polarization of medium‐entropy base materials using linear dielectrics. This strategy ensures a sufficient polar phase and a high enough electric field for complete polarization, thereby achieving ultrahigh W_rec by enhancing polarization strength. The validity of this strategy is demonstrated in the (Na_0.282Bi_0.282Ba_0.036Sr_0.28Nd_0.08)TiO_3‐xCa_0.7Bi_0.2TiO₃ (NBBSNT‐xCBT) (x = 0–0.15) system. The CBT‐modulated samples exhibit a polyphase structure of R3c, P4bm, and Pm‐3m with reduced remnant polarization (Pr). Additionally, the addition of CBT effectively suppresses interfacial polarization, enhancing the maximum polarization (P_max). These factors significantly improve the value of ∆P = P_max − P_r. As a result, an ultrahigh W_rec of 10.5 J cm⁻³ with a high‐efficiency η of 80.3% is obtained in the x = 0.1 sample under a moderate electric field of 45 kV mm⁻¹ for the first time. This work paves the way for achieving superior energy‐storage performance under moderate electric fields.