Preparation and Characterization of LiNi0.5Co0.5O2 and LiNi0.5Co0.4Al0.1O2 by Molten Salt Synthesis for Li Ion Batteries

The layer compounds LiNi0.5Co0.5O2 and LiNi0.5Co0.4Al0.1O2 were prepared by using the molten salt of the eutectic mixture LiNO3−LiCl at temperatures of 750 and 850 °C in air. They were characterized by X-ray diffraction, Rietveld refinement, SEM, EDAX, chemical analysis, surface area, and density techniques. Cathodic behavior was examined at ambient temperature by cyclic voltammetry, charge−discharge cycling, and impedance spectra in cells with Li−metal as the counter electrode at a current rate of 30 mAg-1 with a lower cutoff voltage of 2.5 V up to 120 cycles. The upper cutoff voltages ranged from 4.2 to 4.5 V. The 10th cycle reversible capacities for LiNi0.5Co0.5O2 varied from 131 to 186(±3) mAhg-1 depending on the upper cutoff voltage, but capacity fading occurred ranging from 5 to 18% at the end of 50 or 80 cycles. The Al-doped compound showed a 20th cycle reversible capacity of 146 (±3) mAhg-1 (4.3 V cutoff) which was stable up to 120 cycles. Good rate capability was also observed. Thus, LiNi0.5Co0.4Al0.1O2 with only 0.4 mol of cobalt is a cheaper and viable alternative to the presently used cathode, LiCoO2. Impedance spectra on cells cycled up to 60 cycles with 4.3 V cutoff have been analyzed and interpreted.