Jahn–Teller Distortions and Phase Transitions in LiNiO₂: Insights from Ab Initio Molecular Dynamics and Variable-Temperature X-ray Diffraction

Abstract

The atomistic structure of lithium nickelate (LiNiO 2 ), the parent compound of Ni-rich layered oxide cathodes for Li-ion batteries, continues to elude a comprehensive understanding. The common consensus is that the material exhibits local Jahn−Teller distortions that dynamically reorient, resulting in a time-averaged undistorted R3̅ m structure. Through a combination of ab initio molecular dynamics (AIMD) simulations and variabletemperature X-ray diffraction (VT-XRD), we explore Jahn−Teller distortions in LiNiO 2 as a function of temperature. Static Jahn−Teller distortions are observed at low temperatures (T < 250 K) via AIMD simulations, followed by a broad phase transition that occurs between 250 and 350 K, leading to a highly dynamic, displacive phase at high temperatures (T > 350 K), which does not show the four short and two long bonds characteristic of local Jahn−Teller distortions. These transitions are followed in the AIMD simulations via abrupt changes in the calculated pair distribution function and the bond-length distortion index and in X-ray diffraction via the monoclinic lattice parameter ratio, a mon /b mon , and δ angle, the fit quality of an R3̅ m-based structural refinement, and a peak sharpening of the diffraction peaks on heating, consistent with the loss of distorted domains. Between 250 and 350 K, a mixed-phase regime is found via the AIMD simulations where distorted and undistorted domains coexist. The repeated change between the distorted and undistorted states in this mixed-phase regime allows the Jahn−Teller long axes to change direction. These pseudorotations of the Ni−O long axes are a side effect of the onset of the displacive phase transition. Antisite defects, involving Li ions in the Ni layer and Ni ions in the Li layer, are found to pin the undistorted domains at low temperatures, impeding cooperative ordering at a longer length scale.

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

“…Static DFT calculations can therefore serve to predict the NMR shifts of dynamically distorted LiNiO 2 . Having established this, we can turn to the second part of our question and explore whether static DFT calculations can serve to predict the shifts of experimental samples of LiNiO 2 at room temperature, where we expect coexistent Jahn–Teller undistorted and distorted domains (presumably fluctuating between different orderings) . The local Li environment is very similar in all cases, but the slightly different lattice parameters of the collinear, zigzag, and undistorted structures result in different degrees of orbital overlap, causing slight variations in the Fermi contact shift.…”

“…The previous Fermi contact shift calculations of a collinear cooperatively distorted cell with C 2/ m symmetry were first reproduced before calculating the expected shifts for the more stable (at 0 K) zigzag arrangement of Jahn–Teller distortions (although both configurations are expected to be accessible at room temperature as well as undistorted domains). , The zigzag structure was then used as a starting point to explore the effect of Li Ni and Ni Li antisite defects on the NMR spectrum. The expected 7 Li shifts for the zigzag ( P 2 1 / c ), collinear ( C 2/ m ), and undistorted LNO ( R 3̅ m ) structures are shown in Figure , alongside the local Li environment for the zigzag cell.…”

“…The question then arises if we can predict the NMR shifts of Li in the dynamically distorted/undistorted material at room temperature based on static DFT calculations. To address this, we will first consider a (likely hypothetical) scenario of dynamic distortions without undistorted domains to assess whether static DFT calculations can capture the NMR shifts of the dynamic distortions. Building on this, we will then explore whether the DFT calculations can predict the shifts of experimental samples where the dynamic distortions not only fluctuate between different ordering types but also between Jahn–Teller distorted and undistorted states .…”

“…To address this, we will first consider a (likely hypothetical) scenario of dynamic distortions without undistorted domains to assess whether static DFT calculations can capture the NMR shifts of the dynamic distortions. Building on this, we will then explore whether the DFT calculations can predict the shifts of experimental samples where the dynamic distortions not only fluctuate between different ordering types but also between Jahn–Teller distorted and undistorted states . When a system undergoes structural changes on a time scale shorter than the NMR time scale, the experimentally observed shift is typically a weighted average of the different configurations.…”

“…The recurring formation of Jahn–Teller distorted and undistorted domains in the biphasic regime allows the Jahn–Teller long axes to reorient at room temperature with frequencies that approach a THz. Above 350 K, the material assumes a highly dynamic, displacive phase in which the lattice vibrations are so strong that the NiO 6 octahedra spend most time in or near undistorted configurations − The time scale of Jahn–Teller pseudorotations reported for LNO in the biphasic regime (250 < T < 350 K) is faster than the time scale probed by NMR (1–10 9 Hz), and thus a time-averaged and appropriately weighted signal is likely to be observed in the NMR spectrum for the defect-free material, as explored further below. , …”