Lead‐Free Relaxor Ferroelectric Ceramics with Ultrahigh Energy Storage Densities via Polymorphic Polar Nanoregions Design

Abstract One of the long‐standing challenges of current lead‐free energy storage ceramics for capacitors is how to improve their comprehensive energy storage properties effectively, that is, to achieve a synergistic improvement in the breakdown strength ( E b ) and the difference between maximum polarization ( P max ) and remnant polarization ( P r ), making them comparable to those of lead‐based capacitor materials. Here, a polymorphic polar nanoregions (PNRs) structural design by first introducing 0.06 mol BaTiO 3 into Bi 0.5 Na 0.5 TiO 3 is proposed to construct the morphotropic phase boundary with coexisting structures of micrometer‐size domains and polymorphic nanodomains, enhance the electric field‐induced polarization response (increase P max ). Then Sr(Al 0.5 Ta 0.5 )O 3 (SAT)‐doped 0.94 Bi 0.5 Na 0.5 TiO 3 ‐0.06BaTiO 3 (BNBT) energy storage ceramics with polymorphic PNRs structures are synthesized following the guidance of phase‐field simulation and rational composition design (decrease P r ). Finally, a large recoverable energy density ( W rec ) of 8.33 J cm −3 and a high energy efficiency (η) of 90.8% under 555 kV cm −1 are obtained in the 0.85BNBT‐0.15SAT ceramic prepared by repeated rolling process method (enhance E b ), superior to most practical lead‐free competitors increased consideration of the stability of temperature (a variation <±6.2%) and frequency ( W rec > 5.0 cm −3 , η > 90%) at 400 kV cm −1 . This strategy provides a new conception for the design of other‐based multifunctional energy storage dielectrics.