The role of nickel content on the structure and electrochemical propertiesof Li-x(NiyCo1-y)O-2

Samples have, been prepared with nominal composition Li-x(NiyCo1-y)O-2 (x a pproximate to 1.25, y = 0.7-1.0) with the structures being examined using R ietveld analysis of X-ray diffraction data. Rietveld analysis shows that th e samples prepared with y = 0.70 and 0.75 have no Ni present on the 3a Li s ite, with all other samples showing a small degree of nonstoichiometry. Str uctural analysis of the 3b Ni + Co occupancy also suggests the possibility of 3b site disorder, which is in agreement with the results of previous stu dies. Charge/discharge measurements show an increase in discharge capacity with i ncreasing 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 efficien cies over a number of cycles, typically > 99% over 15 cycles. Samples prepa red 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. Therefo re, these particular compositions are probably not suitable for use in Li i on cells. (C) 2001 Elsevier Science B.V. All rights reserved.