The influence of charge ordering on the phase stability of spinel LiNi2O4
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 Li1−xNi2O4, a delithiated form of lithium secondary battery cathode material LiNiO2.