Ni-rich layered transition metal oxides are promising cathodes for Li-ion batteries due to their low cost and high theoretical capacity. However, their practical applications are hindered by the capacity fading caused by intrinsic lattice structure variations, such as changes in the atom arrangement and valence. In situ neutron powder diffraction is a powerful technique for studying the structure of battery materials and is expected to provide more information than other techniques due to its nondestructive, high-resolution, and light-element probing capability. Herein, we employed neutron powder diffraction to probe the structural evolution during the synthesis of LiNi 0.5 Co 0.2 Mn 0.3 O 2 in real time, including the transition metal−oxygen/lithium−oxygen (TM−O/Li−O) bond, phase formation, and lattice parameters as a function of temperature and isothermal dwelling time. The results revealed that the lattice structure of cathode materials is a function of temperature and isothermal dwelling time. Variations of the TM−O/Li−O bond and lattice parameters with the dwelling time at an annealing temperature of 850 °C indicated that the instability of the structure of the layered transition metal oxides may be a possible mechanism for the changes in the discharging capacity and cycle stability of the battery performance. Our findings provide insights into the correlation between the annealing time of NCM cathodes and the electrochemical performance and can be very helpful for synthesizing high-performance transition metal layered oxide materials.

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