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Deciphering the sophisticated interplay between thermodynamics and kinetics of high‐temperature lithiation reaction is fundamentally significant for designing and preparing cathode materials. Here, the formation pathway of Ni‐rich layered ordered LiNi0.6Co0.2Mn0.2O2 (O‐LNCM622O) is carefully characterized using in situ synchrotron radiation diffraction. A fast nonequilibrium phase transition from the reactants to a metastable disordered Li1−x(Ni0.6Co0.2Mn0.2)1+xO2 (D‐LNCM622O, 0 < x < 0.95) takes place while lithium/oxygen is incorporated during heating before the generation of the equilibrium phase (O‐LNCM622O). The time evolution of the lattice parameters for layered nonstoichiometric D‐LNCM622O is well‐fitted to a model of first‐order disorder‐to‐order transition. The long‐range cation disordering parameter, Li/TM (TM = Ni, Co, Mn) ion exchange, decreases exponentially and finally reaches a steady‐state as a function of heating time at selected temperatures. The dominant kinetic pathways revealed here will be instrumental in achieving high‐performance cathode materials. Importantly, the O‐LNCM622O tends to form the D‐LNCM622O with Li/O loss above 850 °C. In situ XRD results exhibit that the long‐range cationic (dis)ordering in the Ni‐rich cathodes could affect the structural evolution during cycling and thus their electrochemical properties. These insights may open a new avenue for the kinetic control of the synthesis of advanced electrode materials.

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