Enhanced energy-storage properties of (1−x)[(1−y)(Bi0.5Na0.5)TiO3–y(Bi0.5K0.5)TiO3]–x(K0.5Na0.5)NbO3 lead-free ceramics

• KNN substitution into BNKT100y tends to enhance the energy storage density of the ceramics.
• A large energy storage density value of 1.20 J/cm3 at 100 kV/cm is achieved in BNKT100y–xKNN (y=0, x=0.16) samples.
• BNKT100y–xKNN (y=0, x=0.16) samples are attractive for their fatigue-free behavior and good temperature stability.

In this work, we designed a series of compositions within peudocubic region based on ternary (1−x)[(1−y)(Bi0.5Na0.5)TiO3–y(Bi0.5K0.5)TiO3]–x(K0.5Na0.5)NbO3 (BNKT100y–xKNN) system for energy storage applications. Results showed that the KNN substitution into BNKT100y induced a significant disruption of the ferroelectric order, and tended to enhance the energy storage density of the ceramics. With the external electric field, the energy storage density increased drastically, and a maximum value of 1.20 J/cm3 at 100 kV/cm was achieved in BNKT100y–xKNN (y=0, x=0.16) samples. Furthermore, BNKT100y–xKNN (y=0, x=0.16) ceramics not only exhibited high energy density but also possessed fatigue-free behavior and temperature-independent characteristic. Temperature-dependent structural analysis suggested that the good energy-storage properties insensitive to temperature can be ascribed to the stable relaxor pseudocubic (antiferroelectric-like) phase over a wide temperature range. These results indicate that BNKT100y–xKNN system should be a promising lead-free material for energy-storage capacitor application.