Ni-rich layered LiNi_1-x-yMn_xCo_yO₂ (NMC, 1-x-y ≥ 0.5) are among the most promising candidates for next-generation high-energy-density lithium-ion batteries for electric vehicle applications, primarily due to their high energy density and good rate capability. Their poor cycle stability and thermal instability, especially at high operating voltages (> 4.3 V), however, could cause shortened life, induce thermal runaway, and thus largely hinder their usages. To overcome these drawbacks, the intrinsic failure mechanisms of Ni-rich NMC need to be clarified. In this work, in-situ X-ray diffraction was performed on LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622) and LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) cycled at different voltage windows (2.8-4.2 V, 2.8-4.4 V, and 2.8-4.6 V), and the phase evolutions and failure mechanisms were systematically investigated. NMC622 shows reversible structural evolutions and an excellent cycle stability in 2.8-4.2 V and 2.8-4.4 V, while a new and inactive phase (presumably the spinel) was clearly observed when charged beyond 4.4 V. For NMC811, the inactive phase emerges at lower voltages ~ 4.3 V, and grows quickly with prolonged cycles in 2.8-4.4 V and 2.8 - 4.6 V. The appearance of the inactive phase seems to be triggered by the transition from hexagonal H2 to H3 phase, along with a large lattice shrinkage in the c-axis. The continuous growth of the electrochemically inactive phase mainly contributes to the poor cycle stability of Ni-rich NMCs at high voltages ≥ 4.4 V, which is further corroborated by our electrochemical impedance spectroscopy analysis.