Preparation and characterization of LiNi_0.5Co_0.5O₂ and LiNi_0.5Co_0.4Al_0.1O₂ by molten salt synthesis for Li ion batteries

Abstract

The layer compounds LiNi_0.5Co_0.5O₂ and LiNi_0.5Co_0.4Al_0.1O₂ 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 LiNi_0.5Co_0.5O₂ 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, LiNi_0.5Co_0.4Al_0.1O₂ 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.