Probing Jahn-Teller Distortions and Antisite Defects in LiNiO₂ with ⁷Li NMR Spectroscopy and Density Functional Theory

The long- and local-range structure and electronic properties of the high-voltage lithium-ion cathode material for Li-ion batteries, LiNiO₂, remain widely debated, as are the degradation phenomena at high states of delithiation, limiting the more widespread use of this material. In particular, the local structural environment and the role of Jahn-Teller distortions are unclear, as are the interplay of distortions and point defects and their influence on cycling behavior. Here, we use ex situ ⁷Li NMR measurements in combination with density functional theory (DFT) calculations to examine Jahn-Teller distortions and antisite defects in LiNiO₂. We calculate the ⁷Li Fermi contact shifts for the Jahn-Teller distorted and undistorted structures, the experimental ⁷Li room-temperature spectrum being ascribed to an appropriately weighted time average of the rapidly fluctuating structure comprising collinear, zigzag, and undistorted domains. The ⁷Li NMR spectra are sensitive to the nature and distribution of antisite defects, and in combination with DFT calculations of different configurations, we show that the ⁷Li resonance at approximately −87 ppm is characteristic of a subset of Li-Ni antisite defects, and more specifically, a Li⁺ ion in the Ni layer that does not have an associated Ni ion in the Li layer in its 2nd cation coordination shell. Via ex situ ⁷Li MAS NMR, X-ray diffraction, and electrochemical experiments, we identify the ⁷Li spectral signatures of the different crystallographic phases on delithiation. The results imply fast Li-ion dynamics in the monoclinic phase and indicate that the hexagonal H3 phase near the end of charge is largely devoid of Li.