(94-x)(Na1/2Bi1/2)TiO3-6BaTiO(3)-x(K1/2Na1/2)NbO3 (NBT-6BT-xKNN) piezoelectric ceramics have notable potential for replacing lead containing piezoelectric ceramics in actuator applications due to their exceptionally large strain. However, a high electric field for producing a large strain and a large hysteresis are critical issues that should be resolved for practical actuator applications. In an attempt to address these issues and optimize the piezoelectric performance, we fabricated NBT-6BT-xKNN (x = 0 - 5) piezoelectric single crystals with a size of 8 x 8x 10 mm by the solid-state single crystal growth method and systematically measured their electrical properties. With increased addition of KNN to replace NBT, the ferroelectricity and piezoelectricity of the fabricated NBT-6BT-xKNN single crystals decreased, but their unipolar strain and hysteresis were considerably improved. For NBT-6BT-5KNN single crystals, the largest maximum strain (S-max) was 0.57% at 6 kV/mm, showing a converse piezoelectric constant (S-max/E-max) of 950 pm/V, and their hysteresis in the unipolar S-E curve was 12% at 6kV/mm, which would be appropriate for some actuator applications. Our results demonstrate the applicability of the produced single crystals as lead-free piezoelectric actuator components.