It is critical to precisely control atoms ordering, manipulate defect content and engineer morphology in material synthesis, in order to achieve advanced functionality in complex oxides. Understanding of synthesis mechanism accelerates smart material and process design beyond the less-efficient trial-and-error paradigm. Proven as a powerful tool, neutron diffraction is sensitive in detecting light elements (O, C, Li, etc.) and differentiating the neighboring transition-metal-elements. By utilizing sample environments that mimic material synthesis, in-situ neutron diffraction monitors the structure evolution in real time, and particularly, visualizes reversible transitions and intermediate phase involvement. Taking energy storage and conversion materials as examples, the synthesis mechanism and kinetics are revealed from the atomic structure change in material processing, such as solid-state synthesis, powder compacting and ball-milling, correlating to the functional performance. The picture neutron diffraction draws loops back to guide the synthesis and optimization for advanced materials.