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cathode materials and improve their stability and energy density. [1,2] In particular, Ni-rich NCM (LiNi x Co y Mn 1−x−y O 2 , x > 0.5) is a promising cathode material with high reversible capacity and has been successfully implemented in commercial energy storage systems, such as mobile devices and electric vehicles. [1] For Ni-rich layered oxides, the whole reaction pathway has been described as proceeding through a series of isostructural hexagonal phases, conventionally labeled as H1, H2, and H3 phases depending on the depth of charge. [3,4] The fundamental difference between H1 and H2 is the Li-content-dependent in-plane structure in the Li layer. [3,5] Because of the structural similarities of these evolving phases during cycling, the solid solution reaction has been predicted to be a thermodynamically favorable reaction pathway by first-principle studies [6] and was also observed by operando X-ray diffraction (XRD) experiments at moderate cycling rates in Ni-rich layered oxides, such as LiNi 0.6 Co 0.2 Mn 0.2 O 2 (NCM622) [7] and LiNi 0.8 Co 0.1 Mn 0.1 O 2 . [3,4] Homogeneous Li transport, characterized by solid solution behaviors, is considered to be advantageous for obtaining a long cycle lifetime. [8] However, in dynamic situations such as fast cycling, limited Li diffusivity can induce a heterogeneous Li distribution within Understanding the cycling rate-dependent kinetics is crucial for managing the performance of batteries in high-power applications. Although high cycling rates may induce reaction heterogeneity and affect battery lifetime and capacity utilization, such phase transformation dynamics are poorly understood and uncontrollable. In this study, synchrotron-based operando X-ray diffraction is performed to monitor the high-current-induced phase transformation kinetics of LiNi 0.6 Co 0.2 Mn 0.2 O 2 . The sluggish Li diffusion at high Li content induces different phase transformations during charging and discharging, with strong phase separation and homogeneous phase transformation during charging and discharging, respectively. Moreover, by exploiting the dependence of Li diffusivity on the Li content and electrochemically tuning the initial Li content and distribution, phase separation pathway can be redirected to solid solution kinetics at a high charging rate of 7 C. Finite element analysis further elucidates the effect of the Li-content-dependent diffusion kinetics on the phase transformation pathway. The findings suggest a new direction for optimizing fast-cycling protocols based on the intrinsic properties of the materials.

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