Long-range ordering during delithiation of LiMn2O4 cathode material

Abstract

A single crystal of the cubic spinel LiMn₂O₄ was delithiated electrochemically at 4.06 V vs. Li/Li⁺ to probe the structural source of the inflection point at ca. 4.1 V vs. Li/Li⁺ in the discharge curve. The phase thus obtained was investigated by single-crystal X-ray diffraction and the structure refined. The structure is closely related to the original cubic structure, but the new space group is rhombohedral Rm and shows a one-dimensional modulation along the <001>_r direction with wave vector q ≈ 0.75c^*. The cell parameters are: a = 5.762(1) Å and c = 14.080(4), and the (3 + 1)-dimensional super space group is Rm(00γ). The modulated phase involves two unique Mn atoms with coordination polyhedra which differ significantly: one has an average Mn–O distance of 2.24(1) Å, indicative of a lower oxidation state for the Mn ion; the other has an average Mn–O distance of 1.82(2) Å, indicative of a higher oxidation state. Lithium ions appear to be extracted from the structure in a long-range-ordered two-dimensional arrangement between the cubic-close-packed oxygen layers; the manganese atoms are also arranged in charge-ordered Mn³⁺/Mn⁴⁺ sheets. These two features are the source of the modulation in the structure.