Unleashed Remarkable Energy Storage Performance in Bi _0.5 K _0.5 TiO ₃ ‐based Relaxor Ferroelectrics by Local Structural Fluctuation

Abstract Dielectric capacitors harvest energy through an electrostatic storage process, which enables an ultrafast charging‐discharging rate and ultrahigh power density. However, achieving high energy density ( W rec ) and efficiency ( η ) simultaneously, especially when preserving them across a wide frequency/temperature range or cycling numbers, remains challenging. In this work, by especially introducing NaTaO 3 into the representative ferroelectric relaxor of Bi 0.5 K 0.5 TiO 3 ‐Bi 0.5 Na 0.5 TiO 3 and leveraging the mismatch between B‐site atoms, we proposed a method of enhancing local structural fluctuation to refine the polar configuration and to effectively improve its overall energy‐storage performances. As a consequence, the ceramic exhibits an ultrahigh W rec of 15.0 J/cm 3 and high η up to 80 %, along with a very wide frequency stability of 10–200 Hz and extensive cycling number up to 10 8 . In‐depth local structure and chemical environment investigations, consisting of atom‐scale electron microscopy, neutron total scattering, and solid‐state nuclear magnetic resonance, reveal that the randomly distributed A/B‐site atom pairs emerge in the system, leading to the evident local structural fluctuations and concomitant polymorphic polar nanodomains. These key ingredients contribute to the large polarization, minimal hysteresis, and high breakdown strength, thereby promoting energy‐storage performances. This work opens a new path for designing high‐performance dielectric capacitors via manipulating local structural fluctuations.