X-ray diffraction study on phase transition of orthorhombic LiMnO2 in electrochemical conversions

摘要

Orthorhombic LiMnO2 was synthesized by hydrothermal reaction. Phase transition during electrochemical process has been investigated using the high-resolution X-ray diffraction. Contrary to numerous earlier reports, phase analysis of the orthorhombic LiMnO2 electrode cycled for three times evidences the irreversible structure transition from orthorhombic LiMnO2 (Pmnm) to spinel LiMn2O4 (Fd3m) and rock salt Li0.5Mn0.5O (Fm (3) over bar m). Here, the spinel structure with a cell parameter a= 8.241 ( 1) angstrom has a large cationic disorder on lithium and manganese sites, i.e.,about 9% of the Li positions are occupied by Mn and vice versa. For Li0.5Mn0.5O, the cell parameter is a= 4.121 (3) angstrom, and both Li+ and Mn3+ cations occupy the octahedral 4a sites with mole ratio 1:1. The quantity of Li0.5Mn0.5O phase is greatly dependent on cycling rate, namely, the higher the current density is, the larger the quantity of formed-rock salt structure is.