Ultrahigh capacitive energy storage in BNT-based polymorphic relaxor ceramics with dense microstructure and core-shell structure

The exploration of eco-friendly dielectric ceramics for electric energy storage has drawn increasing interest due to their wide applications in high/pulsed power electronic systems. However, achieving ultrahigh recoverable energy storage density (W_rec ≥ 8 J/cm3) with ultrahigh efficiency (η ≥ 90%) is still a huge challenge for them, restricting the development of ceramic-based energy storage capacitors. Here, comprehensive outstanding energy storage performance is realized in lead-free Bi_0.5Na_0.5TiO₃ (BNT)-based ceramics due to collaborative optimization of complex ion doping and viscous polymer process (VPP). Highly dynamic polar nanoregions (PNRs) with the coexistence of rhombohedral (R) + tetragonal (T) phases are formed by Nd3+/Hf4+/Mg2+ co-doping at both A and B sites of (Bi_0.05Na_0.05)_0.94Ba_0.06TiO₃. This, together with the construction of a core–shell structure, ensures a large polarization difference under moderate external electric fields. Furthermore, the optimum composition prepared by VPP exhibits a significant enhancement in dielectric breakdown strength due to its dense microstructure with ultrafine grains and low-concentration defects (e.g., oxygen vacancies). As a result, excellent energy storage performance with ultrahigh W_rec ≈ 9.38 J/cm3 and ƞ ≈ 94.4% is realized in highly dense polymorphic relaxor ceramics under a large electric field of 480 kV/cm. This work provides a two-step cooperative optimization strategy to design advanced ceramic-based dielectric capacitors with great potential for practical energy storage applications.