Layered-to-Spinel Phase Transformation in Li$_{0.5}$NiO$_2$ from First Principles

The phase transition layered Li$_{0.5}$NiO$_2$ to spinel Li(NiO$_2$)$_2$ is a potential degradation pathway in LiNiO$_2$-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 Li$_{0.5}$NiO$_2$, a qualitative difference from the chemically similar Li$_{0.5}$MnO$_2$, 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 LiNiO$_2$-NiO$_2$ composition line, finding a pronounced impact of the symmetry and space group on the layered-to-spinel transition in Li$_{0.5}$NiO$_2$. 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(NiO$_2$)$_2$ spinel.