Structural Analysis by X-Ray Diffraction Technique of Transition Metal Doped Zinc Oxide and Its Applications in Energy Storage Systems: A Critical Review

X-ray diffraction (XRD) is a versatile, nondestructive technique widely used for the analysis of crystalline materials. It provides detailed insights into structural characteristics, including phase identification, crystal orientation, grain size, crystallinity, strain, and defects. Zinc oxide (ZnO) demonstrates exceptional chemical and thermal stability as an n-type semiconductor, facilitating its use in applications such as supercapacitors, batteries, biosensors, photocatalysis, and medical devices. This review systematically examines recent advancements in the study of transition metals (TMs; e.g., Co, Fe, Mn, and Ni) doped ZnO materials for supercapacitors and lithium-ion batteries (LIBs). Emphasis is placed on employing XRD to explore how different dopants influence key structural parameters, such as lattice constants, grain size, defect concentration, and defect types. Through in situ XRD, real-time monitoring of structural changes is used to analyze the energy storage mechanisms, electrode optimization, and cycling performance of doped ZnO materials. The potential applications of co-doped ZnO and zinc-based high-entropy materials in energy storage are highlighted, providing valuable guidance for future research.