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

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Genreith-Schriever, Annalena R  ORCID logo  https://orcid.org/0000-0001-5626-2438
Alexiu, Alexandra 
Phillips, George S 
Coates, Chloe S 
Nagle-Cocco, Liam AV  ORCID logo  https://orcid.org/0000-0001-9265-1588

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 R3�m structure. Through a combination of ab initio molecular dynamics (AIMD) simulations and variable temperature X-ray diffraction (VTXRD), we explore Jahn-Teller distortions in LiNiO2 as a function of temperature. Static JahnTeller distortions are observed at low temperatures (T < 250 K), followed by a broad phase transition occurring between 250 K 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. This transition is seen in 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 amon/bmon and δ angle, the fit quality of an R3�m-based structural refinement, and a peak-sharpening of the diffraction peaks on heating consistent with the loss of distorted domains. Between 250 K 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 being 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.

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Chemistry of Materials
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American Chemical Society (ACS)
Faraday Institution (via University of Oxford) (FIRG063)
Faraday Institution (FIRG016)
Faraday Institution (via University Of Bath) (FIRG016)
Faraday Institution (via University Of Bath) (FIRG016)
Faraday Institution (FIRG016)
Faraday Institution (via University of Oxford) (CATMAT)
Faraday Institution (FIRG024)
Faraday Institution (Unknown)
Faraday Institution (FIRG001)
Faraday Institution (Unknown)
This work was supported by the Faraday Institution degradation project (FIRG011, FIRG020) and CATMAT project (FIRG016). This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 957189 (BIGMAP). The project is part of BATTERY 2030+, the large-scale European research initiative for inventing the sustainable batteries of the future, funded by the European Union's Horizon 2020 research and innovation program under Grant Agreement No. 957213. PXRD measurements were performed at the I11 beamline at Diamond Light Source, for which the authors acknowledge the award of a Block Allocation Grant (CY28349). A.R.G.-S. gratefully acknowledges funding from the German National Academy of Sciences Leopoldina. L.N.C. acknowledges a scholarship EP/R513180/1 to pursue doctoral research from the UK Engineering and Physical Sciences Research Council (EPSRC). We thank Samuel P. Niblett, Euan N. Bassey, Teresa Insinna, Andrey D. Poletayev, Hrishit Banerjee, and Andrew J. Morris for fruitful discussions. Generous computing resources were provided by the Sulis HPC service (EP/T022108/1).