Abstract: Single-crystal Ni-rich cathode materials are the core cathode materials for lithium-ion batteries with high energy density and long cycle life. This paper systematically reviews the scientific principles of micron-sized single-crystal preparation based on the "hydroxide co-precipitation–high-temperature solid-state sintering" route. It focuses on the structure-directing effect of precursor morphology and elemental homogeneity on single-crystal growth, as well as the regulatory mechanisms of key processing parameters (sintering temperature, Li-to-transition-metal ratio, atmosphere, etc.) on grain coarsening and lattice ordering. On this basis, the essential differences between solid-state-dominated and liquid-assisted single-crystal growth mechanisms are clarified from the perspectives of thermodynamic driving force and kinetic constraints. A four-stage non-classical evolution pathway is proposed, including nanocrystal nucleation, mesocrystal assembly, competitive Ostwald ripening, and lattice fusion. Finally, the current bottlenecks and future development directions of single-crystal cathodes are analyzed.