Relaxor Ferroelectric Ca‐Doped (K,Na,Li)NbO ₃ Single‐Crystal Microcubes with Suppressed Defects and Tunable Symmetry for Flexible High‐Power Capacitors

Abstract Single‐crystal relaxor ferroelectrics (RFEs) offer high polarization, slim hysteresis, and superior breakdown strength, making them ideal for advanced dielectric energy storage. However, scalable synthesis of lead‐free single crystals remains challenging. Here, a molten‐salt strategy is employed to synthesize Ca‐doped (K _0.432 Na _0.528 Li _0.04 ) _1‐x Ca _4x/3 Nb _1‐x/3 O ₃ (KNLN‐xCa, x = 0, 0.01, and 0.03) single‐crystal microcubes with tunable crystal symmetry and defect concentration. The KNLN‐0.01Ca single‐crystal exhibits relaxor behavior with a recoverable energy density of 2.66 J cm ⁻³ and an efficiency of 78.7% at 100 kV cm ⁻¹ . In contrast, the ceramic counterpart shows classical ferroelectric features, highlighting the critical role of the crystallization pathway. Dislocation‐driven nanodomain formation during oriented attachment is identified as the primary mechanism inducing relaxor behavior, independent of chemical disorder. Incorporation of the KNLN‐0.01Ca microcubes into a polydimethylsiloxane (PDMS) matrix produces a flexible composite capacitor with a breakdown strength of 350 kV cm ⁻¹ , a recoverable energy density of 5.76 J cm ⁻³ , and an efficiency of 88%. Under pulsed discharge conditions, the device delivers a discharge energy density of 1.4 J cm ⁻³ with a fast discharge time (≈20 ns) and high‐power density (70 MW cm ⁻³ ). These findings demonstrate a crystallographically engineered, defect‐modulated, and process‐scalable route to high‐performance, lead‐free RFEs for next‐generation flexible energy storage devices.