Ni-based cathode materials for lithium-ion batteries (LIBs) have long been in the spotlight because of their high energy density. However, conventional Ni-based cathode materials are generally composed of polycrystalline ceramic powders, the secondary particle morphology of which can lead to several issues requiring mitigation to further improve the cell performance. Namely, electrode compression should be limited to prevent particle rupture and capacity loss caused by inter/intragranular cracking-induced particle pulverization that leads to isolation of the diffusion path among primary particles. A recently proposed successful approach has been the use of single-crystalline particles, which results in improved mechanical properties such as high tap density and high specific surface area, thereby increasing the volumetric energy density of Ni-based cathode cells to levels similar or higher than those of LiCoO2 cathode cells. Despite these efforts, intragranular cracking and cation disordering of single-crystalline particles is inevitable during electrochemical cycling. These difficulties can be resolved through advanced synthetic methods and understanding of the corresponding improvement mechanisms via doping or surface modification. Herein, we discuss technical challenges regarding the synthesis, morphology control, physical and electrochemical stabilities, reaction mechanism, and surface modification of high-energy-density single-crystalline particle Ni-based cathode materials for LIBs.