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 [ Ni x Li 1 / 3 − 2 x / 3 Mn 2 / 3 − x / 3 ] O 2 for 0 < x ⩽ 1 / 2 is presented. Li [ Ni x Li 1 / 3 − 2 x / 3 Mn 2 / 3 − x / 3 ] O 2 is prepared by substituting Ni 2 + for Li + and Mn 4 + in Li [ Li 1 / 3 Mn 2 / 3 ] O 2 while maintaining all the remaining Mn in the +4 oxidation state. Samples with 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 [ Ni x Li 1 / 3 − 2 x / 3 Mn 2 / 3 − x / 3 ] O 2 (0 < x ⩽ 1/2) show that these compounds adopt the O 3 ­ LiCoO 2 -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 LT ­ LiCoO 2 . Electrochemical studies show that Li [ Ni x Li 1 / 3 − 2 x / 3 Mn 2 / 3 − x / 3 ] O 2 (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 [ Ni x Li 1 / 3 − 2 x / 3 Mn 2 / 3 − x / 3 ] O 2 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 [ Ni x Li 1 / 3 − 2 x / 3 Mn 2 / 3 − x / 3 ] O 2 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 [ Ni x Li 1 / 3 − 2 x / 3 Mn 2 / 3 − x / 3 ] O 2 (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.