Synthesis, Structure, and Electrochemical Behavior of Li[Ni[sub x]Li[sub 1/3−2x/3]Mn[sub 2/3−x/3]]O[sub 2]

Abstract

The synthesis, structure, and electrochemical performance of Li[NixLi1/3−2x/3Mn2/3−x/3]O2Li[NixLi1/3−2x/3Mn2/3−x/3]O2 for 0<x⩽1/20Li[NixLi1/3−2x/3Mn2/3−x/3]O2Li[NixLi1/3−2x/3Mn2/3−x/3]O2 is prepared by substituting Ni2+Ni2+ for Li+Li+ and Mn4+Mn4+ in Li[Li1/3Mn2/3]O2Li[Li1/3Mn2/3]O2 while maintaining all the remaining Mn in the +4 oxidation state. Samples with x=1/6,x=1/6, 1/4, 1/3, 5/12, and 1/2 have been investigated by X-ray diffraction (XRD) and neutron diffraction as well as by electrochemical measurements. The XRD and neutron diffraction patterns of Li[NixLi1/3−2x/3Mn2/3−x/3]O2Li[NixLi1/3−2x/3Mn2/3−x/3]O2 (0 < x ⩽ 1/2) show that these compounds adopt the O3­LiCoO2O3­LiCoO2 -type structure when synthesized at 800°C and higher. XRD and neutron diffraction also suggest a short-range superlattice ordering of Li, Ni, and Mn in the transition-metal layer for many of the samples. When synthesized at 700°C and lower, the compounds (for x = 1/3 and 1/2) appear to adopt a spinel-type structure like LT­LiCoO2.LT­LiCoO2. Electrochemical studies show that Li[NixLi1/3−2x/3Mn2/3−x/3]O2Li[NixLi1/3−2x/3Mn2/3−x/3]O2 (900°C) with x = 5/12 can deliver a stable capacity of about 160 mAh/g between 3.0 and 4.4 V vs. Li. An irreversible plateau is observed at about 4.5 V during the first charge of Li/Li[NixLi1/3−2x/3Mn2/3−x/3]O2Li/Li[NixLi1/3−2x/3Mn2/3−x/3]O2 cells (x = 1/6, 1/4, 1/3, and 5/12), which we believe corresponds to the simultaneous removal of lithium and oxygen from the structure. After the plateau, Li[NixLi1/3−2x/3Mn2/3−x/3]O2Li[NixLi1/3−2x/3Mn2/3−x/3]O2 cells with x = 1/3 and 5/12 can deliver stable reversible capacities of about 230 and 225 mAh/g between 2.0 and 4.6 V. Li/Li[NixLi1/3−2x/3Mn2/3−x/3]O2Li/Li[NixLi1/3−2x/3Mn2/3−x/3]O2 (0 < x ⩽ 1/2) synthesized at low temperatures (i.e., 600 and 700°C) shows dramatically different differential capacity vs. voltage behavior compared to the high temperature samples, which must be related to the structural differences between materials prepared above and below 750°C. © 2002 The Electrochemical Society. All rights reserved.