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

Samples have been prepared with nominal composition Lix(NiyCo1−y)O2 (x≈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 withy=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 Lix(NiyCo1−y)O2 is lost when y≥0.85. Therefore, these particular compositions are probably not suitable for use in Li ion cells.