Superior Energy Storage Properties and Optical Transparency in K0.5 Na0.5 NbO3 -Based Dielectric Ceramics via Multiple Synergistic Strategies.

Eco-friendly transparent dielectric ceramics with superior energy storage properties are highly desirable in various transparent energy-storage electronic devices, ranging from advanced transparent pulse capacitors to electro-optical multifunctional devices. However, the collaborative improvement of energy storage properties and optical transparency in KNN-based ceramics still remains challenging. To address this issue, multiple synergistic strategies are proposed, such as refining the grain size, introducing polar nanoregions, and inducing a high-symmetry phase structure. Accordingly, outstanding energy storage density (Wtotal  ≈7.5 J cm-3 , Wrec  ≈5.3 J cm-3 ) and optical transmittance (≈76% at 1600 nm, ≈62% at 780 nm) are simultaneously realized in the 0.94(K0.5 Na0.5 )NbO3 -0.06Sr0.7 La0.2 ZrO3 ceramic, together with satisfactory charge-discharge performances (discharge energy density: ≈2.7 J cm-3 , power density: ≈243 MW cm-3 , discharge rate: ≈76 ns), surpassing previously reported KNN-based transparent ceramics. Piezoresponse force microscopy and transmission electron microscopy revealed that this excellent performance can be attributed to the nanoscale domain and submicron-scale grain size. The significant improvement in the optical transparency and energy storage properties of the materials resulted in the widening of the application prospects of the materials.

First-principles calculations of electrical properties, structure, and phase transition of K1-xNaxNbO3 solid solutions.

The structure, total energy and orthorhombic as well as tetragonal electronic properties of K1-xNaxNbO3 (KNN) as a function of Na concentration were studied with first principles calculations. When the Na content of KNN was gradually increased the orthogonal phase transformation occurred, which produced an enhanced piezoelectric response of the tetragonal KNN. This result proved that the high d33 originated from the phase transition. The corresponding calculations reveal that the change of Nb-O bond length is the origin of distortion of Nb-O octahedral and phase transition. In addition, the calculations observed an unusual high peak of the KNN piezoelectric parameter, which showed the same trend as the experimental results.