Relaxor Ferroelectric Ca-Doped (K,Na,Li)NbO₃ Single-Crystal Microcubes with Suppressed Defects and Tunable Symmetry for Flexible High-Power Capacitors

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_0.432Na_0.528Li_0.04)_1-xCa_4x/3Nb_1-x/3O₃ (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.