Tailoring KNN ceramics through metal oxide doping: a comprehensive review on structure-property relationships and...

Potassium sodium niobate (K_0.5Na_0.5NbO₃, KNN) is one of the most outstanding lead-free piezoceramics due to its high Curie temperature (∼420 °C), eco-friendliness, and suitability in sensing, actuating, energy-harvesting, etc. However, its widespread application is restricted by alkali volatilization, unstable phase coexistence, defect creation and electromechanical properties inferior to that of lead-based piezoceramics. The present review provides a comprehensive and mechanistic discussion on metal-oxide-doped KNN ceramics, focusing on the structure-performance relationships that connect dopant chemistry, structure evolution, and defect engineering to energy harvesting device application and performance. A comparative assessment demonstrates that A-site dopants, like Li⁺, Bi³⁺ and Sm³⁺, promote polymorphic phase boundary formation and polarization rotation, leading to maximum piezoelectric responses, while B-site dopants like Fe³⁺ achieve the highest dielectric permittivity through oxygen-vacancy-mediated polarization and Zr⁴⁺ provides the highest thermal stability by stabilizing the perovskite framework. The highest piezoelectric coefficient of 814 pm V^-1 has been achieved in optimally doped KNN samples through phase boundary engineering and controlled defect chemistry. This review examines the structure-performance relationships of KNN ceramics, provides a detailed comparison of KNN ceramics with PZT and other lead-free piezoceramics, and discusses ceramic synthesis methods, nanogenerator device designs, realistic performance parameters and the applications of KNN ceramics. The major scientific findings reported in this review, such as synergistic phase coexistence, lattice deformation, and domain wall motion, combined with futuristic AI/ML-assisted materials discovery and DFT-guided dopant selection, would scale up the manufacturing of hybrid systems and accelerate the commercial application of KNN ceramics.