On the Origin of the Monoclinic Distortion in Li<i>_x</i>NiO₂

Using first-principles calculation and thermally equilibrated Li-vacancy configurations, we have calculated the monoclinicity, am/bm, as a function of lithium content in LixNiO2. In agreement with experimental data, maxima in am/bm are predicted around Li0.75NiO2 and Li0.4NiO2. We explain the monoclinicity maxima at Li0.75NiO2 in terms of a Jahn−Teller distortion assisted by the presence of lithium-vacancy ordering. The subsequent removal of lithium causes a decrease in the number of Ni3+ ions and therefore in the monoclinicity. However, the strong lithium-vacancy ordering at Li0.4NiO2 enables new maxima of monoclinicity despite the low Ni3+ content.