Minimization of the cation mixing in Li_(1+x)(NMC)_(1-x)O_2 as cathode material

摘要

Li_(1+x)(Ni_(1/3)Mn_(1/3)Co_(1/3))_(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). Rietveld analysis of XRD data, scanning electron microscopy (SEM) and magnetic measurements carried out by superconducting quantum interference devices (SQUID) showed the well-defined ct-NaFeCh 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 [Li_(1-y)Ni_y]_(3b)[Li_(x+y)Ni_((1-x)/3-y)Mn_((1-x)/3)Co_((1-x)/3)]_(3a)(x=0.02,0.04) and was very small. Both Rietveld refinements and magnetic measurements revealed a concentration of Ni~(2+)-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 gal-vanostatic charge-discharge cycling. Data obtained with Li_(1-x)(Ni_(1/3)Mn_(1/3)Co_(1/3))_(1/3)O_2 reflected the high degree of sample optimization. An initial discharge capacity of 150mAhg~(-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 1 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 Li_(1+x)(Ni_(1/3)Mn_(1/3)Co_(1/3))_(1-x)O_2.