Effect of AlPO4-coating on cathodic behaviour of Li(Ni0.8CO0.2)O2

The cathodic behaviour and thermal stability in the charged state of bare and 1-8 wt.% AlPO4-coated layered oxide Li(Ni0.8Co0.2)O-2 are investigated. The synthesized compounds are characterized by a wide variety of techniques. X-ray diffraction analysis (XRD) shows that the hexagonal a and c lattice parameters of Li(Ni0.8Co0.2)O-2 are not affected by the AlPO4-coating, but there are indications of an increasing number of Ni-ions occupying the Li-sites in the Li-layer with increasing amounts of the coated AlPO4. The O 1 s X-ray photoelectron spectra clearly indicate two different oxygens that correspond to the coated-AlPO4 and the bare compounds. Cyclic voltammetry (2.5-4.3 V) shows that the characteristic structural phase transitions exhibited by the bare compound are suppressed by greater than or equal to5 wt.% AlPO4-coating. Galvanostatic charge-discharge cycling has been carried out at a current density of 30 mA g(-1) in the range of 2.5-4.3 V up to 70 cycles and 2.5-4.5 V up to 40 cycles. Capacity-fading of the 3 and 5 wt.% AlPO4-coated Li(Ni0.8Co0.2)O2 is much less than that shown by the bare compound, which is 21% between 10 and 70 cycles with a 4.3 V cut-off and 48% between 10 and 40 cycles with a 4.5 V cut-off. The coulombic efficiency is >98% in all cases after a few initial cycles. Impedance spectra of cells with 1 and 5 wt.% AlPO4-coated compounds and ex situ XRD of the 3 and 5 wt.% AlPO4-coated charged cathodes are examined and the results interpreted. Differential scanning calorimetry curves of the charged cathodes (4.3 V) reveals that the decomposition temperature of 220degreesC for the bare compound is increased by -10degreesC after 3 and 5 wt.% AlPO4-coating, and more significantly the heat evolution decreases by a factor of 5, which indicates better thermal stability. Nevertheless, this benefit comes at the expense of reversible capacities, which decrease from 202 mAh g(-1) (fifth cycle, 4.3 V) for the bare compound, to 156 and 100 mAh g(-1) for the 3 and 5 wt.% AlPO4-coated Li(Ni0.8Co0.2)02, respectively. (C) 2004 Elsevier B.V. All rights reserved.