While high-energy-density Ni-rich layered oxides, LiNi_1-x-yCo_xMn_yO₂ (Ni > 0.8), are of significant attention as next-generation cathode materials, their practical application is limited by intergranular cracking and mechanical degradation arising from anisotropic lattice strain. The single-particle cathode strategy has emerged as a promising solution, effectively suppressing intergranular cracking by eliminating boundaries between primary particles. However, the fundamental relationships between single-particle size, internal microstructure, and electrochemical performance remain poorly understood. Here, we present a molten-salt synthesis strategy to produce LiNi_0.92Co_0.03Mn_0.05O₂ single-particle cathodes with tunable particle and crystallite sizes via precursor morphology control. Systematic analysis reveals clear correlations between precursor shape, final particle microstructure, and rate capability. This work aims to establish the causal link between precursor morphology, final particle microstructure, and electrochemical performance, thereby providing a core design principle for developing high-stability and -power single-particle cathode materials.