Influence of Cathode Origin on the Crystallization of LiNbO$_3$ Coatings on Ni‐Rich Layered Oxides

Layered transition-metal oxides are widely used as cathode materials for lithium-ion batteries, and surface coating with lithium niobates is a common strategy to improve their electrochemical performance. In many studies, the formation of crystalline Li$_3$NbO$_4$ is reported as the final product of Li–Nb–O coating processes after thermal treatment. However, experimental observations obtained for different cathode morphologies and processing conditions indicate that the structural evolution of Li–Nb–O coatings is more complex. A systematic analysis of temperature-dependent diffraction data, combined with comparison to literature results for structurally related systems, shows that Li$_3$NbO$_4$ is not the only crystalline phase that can form during high-temperature treatment. Depending on coating optimization, cathode morphology, and thermal history, the formation of an Nb-stabilized rock-salt–type Li–Nb–M–O phase, described as xLi$_3$NbO$_4$−(1−x)LiMO$_2$ (M = Ni, Co, Mn), is observed. These findings explain the absence of LiNbO$_3$ and/or Li$_3$NbO$_4$ reflections in certain coated materials and demonstrate that the frequent assignment of Li–Nb–O coating products solely to Li$_3$NbO$_4$ is incomplete. The results were obtained from samples containing increased Li–Nb coating amounts to facilitate detection of crystalline reaction products by laboratory X-ray diffraction (XRD).