Layered-to-Spinel Phase Transformation in Li0.5NiO₂ from First Principles

The phase transition 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 migrate concertedly towards their respective spinel sites for the layered-to-spinel transformation to occur. 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 Kelvin ground-state structure of Li(NiO₂)₂ spinel.