Ni-rich layered oxides (LiNi_1-xM_xO₂, M=Co, Mn, Al, etc., x<0.5) have been demonstrated as practical cathode materials for high-energy and high-power lithium ion batteries. However, various synthetic factors affect the layered structural ordering of LiNi_1-xM_xO₂ and related electrochemical lithium storage performances. Herein, we report on developing a high-capacity Ni-rich LiNi_0.8Co_0.1Mn_0.1O₂ oxide with enhanced cycleability via the synthetic control. Systemic investigation is made to the phase evolution of LiNi_0.8Co_0.1Mn_0.1O₂ oxide under different annealing temperatures by ex-situ and in-situ synchrotron X-ray diffraction (XRD) measurements, coupled with quantitative structural analysis through refinements. Structural characterization indicates an intriguing phase transition of LiNi_0.8Co_0.1Mn_0.1O₂ from a rock-salt structure at low annealing temperatures directly to a layered α-NaFeO₂-type structure with a R-3m space group at high temperatures. The in-situ XRD studies gain us access to the phase diagram in the confined Ni-rich region of the Ni-Mn-Co space, thereby enabling the design of synthetic protocols for preparing high-capacity LiNi_1-x-yMn_xCo_yO₂ oxides with stabilized structure and reasonable cycling stability. Furthermore, a preheating process and Li₂TiO₃ coating layer have been introduced during the synthesis of LiNi_0.8Co_0.1Mn_0.1O₂ oxide, in order to manipulate the lithium source reaction by fabricating the outer shell in advance. As a result, the resulting LiNi_0.8Co_0.1Mn_0.1O₂ cathode material reveals the higher degree of structural ordering and considerably enhanced lithium storage performances. This work offers a feasible route to optimize the layered structure of Ni-rich cathode materials via the synthetic control of the lithium source reaction, and further sheds light on the fundamental relationship between crystal structure and electrochemical performance for high-energy lithium ion batteries.