Intra-layer ordering and inter-layer disordering of the Li_2MnO_3 phase in Li_(1.07)Mn_(1.93)O_(4-δ) cathode materials: electron diffraction investigation and DIFFaX simulation of X-ray diffraction patterns

摘要

A previous transmission electron microscopy (TEM) analysis revealed the existence of monoclinic Li_2MnO_3 in the lithium-rich and oxygen-deficient Li_(1.07)Mn_(1.93)O_(4-δ) powder. Interestingly, the monoclinic phase exhibits different nanoscale lamellar variants involving a rotation of the stacking direction by 120 or 240° along the pseudo-threefold axis, i.e. the [103]_M//[111]C (M and C denote the monoclinic and cubic phases, respectively) zone axis. Here, a theoretical X-ray diffraction (XRD) study of Li_2MnO_3 employing the DIFFaX program is presented. It is found that, with the occurrence of different stacking configurations, the characteristic superstructure reflections with 2θ between 20 and 35° (Cu Kα) in the XRD pattern become more and more broadened with the increasing degree of stacking disorder, indicating that XRD may fall short in detecting the presence of the monoclinic Li_2MnO_3 phase. Moreover, selective peak asymmetry appears when the stacking sequence becomes extremely disordered. Further selected-area electron diffraction and theoretical neutron diffraction investigation may clarify the similar ambiguity concerning the crystal phases of other structurally related compound cathode materials for lithium-ion batteries (e.g. LiNi_(1/2)Mn_(1/2)O_2, LiNi_(1/3)Co_(1/3)Mn_(1/3)O_2).