Electrochemical, 6Li MAS NMR, and X-ray and neutron diffraction study of LiCoxFeyMn2-(x+y)O4 spinel oxides for high-voltage cathode materials

LiCoxFeyMn2-(x+y)O4 compounds (with x = 0.2, 0.4, 0.8, and 1, and y = 0.0, 0.2, and 0.4) have been prepared. Substitution of manganese by iron and/or cobalt decreases the cubic lattice parameter. The structure of LiCo0.2Fe0.2Mn1.6O4 has been studied in detail by Rietveld refinement of the structure by using data obtained from neutron diffraction measurements. The electrochemical behavior has been tested in Li\LiPF6(EC:DEC)\LiCoxFeyMn2-(x+y)O4 cells, where an increase of the extension of the capacity delivered in the 5-V region and a decrease for the 4-V region was observed when manganese was substituted by iron and/or cobalt. The changes observed in the X-ray diffraction patterns upon lithium extraction have been studied for LiCoMnO4, LiCo0.8Fe0.2MnO4, and LiCo0.2Fe0.2Mn1.6O4. For all these samples a single cubic phase in which the lattice parameter decreases with lithium extraction was observed. The lattice contraction is more marked for Li1-zCo0.2Fe0.2Mn1.6O4 (from 8.187(1) Angstrom to ca. 8.06 Angstrom) and occurs mostly in the region near 4 V. The novel compound LiCo0.8Fe0.2-MnO4 is shown to be a good 5-V cathode material with an initial discharge capacity of 100 mAhg(-1).