Changes in the Cation Ordering of Layered O3 Li_xNi_0.5Mn_0.5O₂ during Electrochemical Cycling to High Voltages: An Electron Diffraction Study

Selected area electron diffraction patterns were collected from pristine LiNi 0.5 Mn 0.5 O 2 and cycled Li x Ni 0.5 Mn 0.5 O 2 samples (to either 4.5 V or 5.3 V) in the charged and discharged states. Superlattice reflections characteristic of the √3 a Hex. × √3 a Hex. × c Hex. supercell, which are associated with ordering of Li-rich and Li-deficient sites in the transition metal layer of the pristine sample, were weakened considerably or disappeared completely in the charged samples, indicating a reduction of this long-range ordering. Detailed analysis revealed not only a considerable amount of Ni migration from the Li layer to the transition metal layer upon charging to 4.5 V but also that a complete removal of Ni from the Li layer might be possible upon charging to 5.3 V as evidenced by the detection of the O1 phase with a hexagonal-close-packed oxygen array. The Ni migration was in part reversible upon discharge as the fractions of crystals exhibiting the √3 a Hex. × √3 a Hex. × c Hex. superlattice reflections were considerably higher in the discharged samples than in the charged samples. Additional superlattice reflections that could not be indexed to the √3 a Hex. × √3 a Hex. × c Hex. supercell were observed in some crystallites of the cycled samples, the extent of ordering varying from crystal to crystal. This new long-range ordering was attributed to a nonrandom distribution of Li, Ni, and vacancies in the tetrahedral and/or octahedral sites of the Li layer. Although the nature of this long-range ordering is not fully understood, it is proposed that the Li, Ni, and vacancies order on the tetrahedral sites of the Li layer resulting in a 2 a Hex. × 2 a Hex. × c Hex. supercell with space group R 3̄ m, in the charged samples, while they order on the tetrahedral and octahedral sites of the Li layer in an a Mon. × a Mon. × c Mon. cell having space group P 2/ m, in the discharged samples. The appearance of long-range ordering in the Li layer of the cycled samples is likely due to electrostatic repulsion of cations, which might play an important role in the stability of the O3 layered structure and lithium diffusion in the layered structure.