Journal of Power Sources, Vol.195, No.5, 1292-1301, 2010
Minimization of the cation mixing in Li1-x(NMC)(1-x)O-2 as cathode material
Li1-x(Ni1/3Mn1/3Co1/3)(1-x)O-2 layered materials were synthesized by the co-precipitation method with different Li/M molar ratios (M = Ni + Mn + Co). Elemental titration evaluated by inductively coupled plasma spectrometry (ICP), structural properties studied by X-ray diffraction (XRD), Rielveld analysis of XRD data, scanning electron microscopy (SEM) and magnetic measurements carried out by superconducting quantum interference devices (SQUID) showed the well-defined alpha-NaFeO2 structure with cationic distribution close to the nominal formula. The Li/Ni cation mixing on the 3b Wyckoff site of the interlayer space was consistent with the structural model [Li1-yNiy](3b)[Lix+yNi(1-x)/3-yMn(1-x)/3Co(1-x)/3](3a)O-2 (x=0.02, 0.04) and was very small. Both Rietveld refinements and magnetic measurements revealed a concentration of Ni2+-3b ions lower than 2%; moreover, for the optimized sample synthesized at Li/M = 1.10, only 1.43% of nickel ions were located into the Li sublattice. Electrochemical properties were investigated by galvanostatic charge-discharge cycling. Data obtained with Li1+x(Ni1/3Mn1/3Co1/3)(1-x)O-2 reflected the high degree of sample optimization. An initial discharge capacity of 150 mAh g(-1) was delivered at 1 C-rate in the cut-off voltage of 3.0-4.3 V. More than 95% of its initial capacity was retained after 30 cycles at I C-rate. Finally, it is demonstrated that a cation mixing below 2% is considered as the threshold for which the electrochemical performance does not change for Li1+x(Ni1/3Mn1/3Co1/3)(1-x)O-2. (C) 2009 Elsevier B.V. All rights reserved.
Keywords:LiNi1/3Mn1/3Co1/3O2;Co-precipitation method;Rietveld refinement;Magnetic properties;Lithium-ion batteries