The influence of charge ordering on the phase stability of spinel LiNi2O4

Abstract

The transition metal ions in spinel LiNi₂O₄ are commonly assumed to be in a single charge state and uniformly distributed throughout the crystal lattice. However, multivalent Ni can exist in a number of different charge states simultaneously within the crystal, whilst still maintaining overall charge neutrality. By carrying out a systematic study of possible charge states of Ni and their distribution in LiNi₂O₄ using first-principles calculations within the framework of density functional theory, we show that two charge-ordered phases are more stable than the charge-disordered spinel phase: (a) an orthorhombic phase containing Ni³⁺ and Ni⁴⁺ ions in a 1 : 1 ratio resulting in Imma symmetry, and (b) a cubic phase with Ni²⁺ and Ni⁴⁺ in a 1 : 3 ratio with P4₃32 symmetry. Our results indicate that charge ordering is an important process in stabilizing the spinel phase of LiNi₂O₄, and should not be ignored in any quantitative study. Our findings are of particular importance for elucidating the relative stabilities of the spinel phase and layered phase of Li_1−xNi₂O₄, a delithiated form of lithium secondary battery cathode material LiNiO₂.