The role of nickel content on the structure and electrochemical properties of Lix(NiyCo1-y)O2

Samples have, been prepared with nominal composition Li-x(NiyCo1-y)O-2 (x approximate to 1.25, y = 0.7-1.0) with the structures being examined using Rietveld analysis of X-ray diffraction data. Rietveld analysis shows that the samples prepared with y = 0.70 and 0.75 have no Ni present on the 3a Li site, with all other samples showing a small degree of nonstoichiometry. Structural analysis of the 3b Ni + Co occupancy also suggests the possibility of 3b site disorder, which is in agreement with the results of previous studies. Charge/discharge measurements show an increase in discharge capacity with increasing Ni content, which suggests cobalt plays a very small part in the electrochemical capacities of these phases using the current voltage Limits. Samples with y = 0.75, 0.8 and 0.85 were found to have Very high efficiencies over a number of cycles, typically > 99% over 15 cycles. Samples prepared with y = 0.85 and 0.90 show evidence of a phase change which resembles that seen for LiNiO2. This would suggest that the stabilizing effects of Co in Li-x(NiyCo1-y)O-2 is lost when y greater than or equal to 0.85. Therefore, these particular compositions are probably not suitable for use in Li ion cells. (C) 2001 Elsevier Science B.V. All rights reserved.