Twin Boundaries Contribute to The First Cycle Irreversibility of LiNiO2

LiNiO2 remains a target for layered oxide Li‐ion cathode development as it can theoretically deliver the highest energy density of this materials class. In practice, LiNiO2 suffers from poor capacity retention due to electrochemically‐induced structural changes. While the impact of Ni off‐stoichiometry on the electrochemical performance has been extensively studied, that of planar defects present in the as‐synthesized cathode is not well understood. Advanced ex situ and operando structure probes are used to identify and quantify point and planar defects present in as‐synthesized Li1‐yNi1+yO2 cathodes and monitor their evolution during the first cycle. Specifically, a 7Li nuclear magnetic resonance (NMR) signature characteristic of Li environments near twin boundaries is identified; an assignment supported by first‐principles calculations and scanning transmission electron microscopy (STEM) images of twin boundary defects. The NMR results suggest that the concentration of twin boundaries depends on the amount of Ni excess. Moreover, operando magnetometry and ex situ synchrotron X‐ray diffraction and NMR demonstrate that these planar defects impede Li reinsertion into the bulk cathode at reasonable discharge rates and contribute to the first cycle irreversible capacity. These findings provide new design rules for Li1‐yNi1+yO2 cathodes, whereby a reduced concentration of twin boundaries in the pristine material leads to reduced kinetic limitations and improved cathode utilization. Twin boundaries create kinetic barriers for lithium reinsertion into Li1‐yNi1+yO2, contributing to its significant first cycle irreversible capacity. Twin defects can be identified and quantified with solid‐state nuclear magnetic resonance, and their concentration is reduced with increasing nickel excess in quasi‐stoichiometric Li1‐yNi1+yO2 (y ≤ 0.03) cathodes.