Atomic-Resolution Visualization of Distinctive Chemical Mixing Behavior of Ni, Co, and Mn with Li in Layered Lithium Transition-Metal Oxide Cathode Materials

Capacity and voltage fading of layered structured cathode based on lithium transition-metal oxide is closely related to the lattice position and migration behavior of the transition-metal ions. However, it is scarcely clear about the behavior of each of these transition-metal ions in this category of cathode material. We report direct atomic resolution visualization of interatomic layer mixing of transition metals (Ni, Co, Mn) and lithium ions in layered structured oxide cathodes for lithium-ion batteries. Using chemical imaging with an aberration-corrected scanning transmission electron microscope (STEM) and density function theory calculations, we discovered that, in the layered cathodes, Mn and Co tend to reside almost exclusively at the lattice site of transition-metal (TM) layer in the structure or little interlayer mixing with Li. In contrast, Ni shows a high degree of interlayer mixing with Li. The fraction of Ni ions resides in the Li layer followed by a near linear dependence on total Ni concentration before reaching saturation. The observed distinctively different behavior of Ni with respect to Co and Mn provides new insights on both capacity and voltage fade in this class of cathode materials based on lithium and TM oxides, therefore providing scientific basis for selective tailoring of oxide cathode materials for enhanced performance.