Mechanism of the\nLayered-to-Spinel Phase Transformation\nin Li<sub>0.5</sub>NiO<sub>2</sub>

The phase transition of layered Li<sub>0.5</sub>NiO<sub>2</sub> to spinel Li(NiO<sub>2</sub>)<sub>2</sub> is a potential\ndegradation\npathway in LiNiO<sub>2</sub>-based lithium-ion battery cathodes. We\ninvestigated the mechanism of this phase transformation from first\nprinciples. Consistent with experimental observations reported in\nthe literature, our results indicate a high energy barrier for the\ntransformation due to high defect formation energies, a complex charge-transfer\nmechanism, and electronic frustration. Our results suggest that partially\ninverse spinel phases are unlikely to form for Li<sub>0.5</sub>NiO<sub>2</sub>, a qualitative difference from the chemically similar Li<sub>0.5</sub>MnO<sub>2</sub>, in which the transformation occurs at room\ntemperature. We show that Ni and Li atoms do not migrate gradually\nto their respective spinel sites for the layered-to-spinel transformation\nto occur due to high defect formation energies. We investigated the\ncharge ordering in layered phases along the LiNiO<sub>2</sub>–NiO<sub>2</sub> composition line, finding a pronounced impact of the symmetry\nand space group on the layered-to-spinel transition in Li<sub>0.5</sub>NiO<sub>2</sub>. Finally, we evaluated the relative stability of\ndifferent spinel space groups, finding that previously reported experimental\nobservations are consistent with a temperature-averaged structure\nrather than the 0 K ground-state structure of the Li(NiO<sub>2</sub>)<sub>2</sub> spinel.