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

Annalena R Genreith-Schriever; Alexandra Alexiu; George S Phillips; Chloe S Coates; Liam A V Nagle-Cocco; Joshua D Bocarsly; Farheen N Sayed; Siân E Dutton; Clare P Grey

doi:10.1021/acs.chemmater.3c02413

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

Chem Mater. 2024 Feb 20;36(5):2289-2303. doi: 10.1021/acs.chemmater.3c02413. eCollection 2024 Mar 12.

Authors

Annalena R Genreith-Schriever^{1

2}, Alexandra Alexiu¹, George S Phillips^{1

2}, Chloe S Coates^{1

2}, Liam A V Nagle-Cocco^{3

2}, Joshua D Bocarsly^{1

4

2}, Farheen N Sayed^{1

2}, Siân E Dutton^{3

2}, Clare P Grey^{1

2}

Affiliations

¹ Yusuf Hamied Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, U.K.
² The Faraday Institution, Harwell Science and Innovation Campus, Didcot OX11 0RA, U.K.
³ Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, U.K.
⁴ Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States.

Abstract

The atomistic structure of lithium nickelate (LiNiO₂), 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 R3̅m structure. Through a combination of ab initio molecular dynamics (AIMD) simulations and variable-temperature X-ray diffraction (VT-XRD), we explore Jahn-Teller distortions in LiNiO₂ 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 R3̅m-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.