Mechanism of the Layered-to-Spinel Phase Transformation in Li0.5NiO2

The phase transition of layered Li0.5NiO2 to spinel Li(NiO2)(2) is a potential degradation pathway in LiNiO2-based lithium-ion battery cathodes. We investigated the mechanism of this phase transformation from first principles. Consistent with experimental observations reported in the literature, our results indicate a high energy barrier for the transformation due to high defect formation energies, a complex charge-transfer mechanism, and electronic frustration. Our results suggest that partially inverse spinel phases are unlikely to form for Li0.5NiO2, a qualitative difference from the chemically similar Li0.5MnO2, in which the transformation occurs at room temperature. We show that Ni and Li atoms do not migrate gradually to their respective spinel sites for the layered-to-spinel transformation to occur due to high defect formation energies. We investigated the charge ordering in layered phases along the LiNiO2-NiO2 composition line, finding a pronounced impact of the symmetry and space group on the layered-to-spinel transition in Li0.5NiO2. Finally, we evaluated the relative stability of different spinel space groups, finding that previously reported experimental observations are consistent with a temperature-averaged structure rather than the 0 K ground-state structure of the Li(NiO2)(2) spinel.