High-performance LiCoO₂ by molten salt (LiNO₃:LiCl) synthesis for Li-ion batteries

Abstract

In an effort to increase and sustain the reversible capacity of LiCoO₂ on cycling, LiCoO₂ is prepared by using the molten-salt of the eutectic LiNO₃-LiCl at temperatures 650-850 °C with or without KOH as an oxidizing flux. The compounds are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), chemical analysis, surface area and density techniques. Cathodic behaviour was examined by cyclic voltammetry (CV) and charge-discharge cycling. The 850 °C-synthesized LiCoO₂, which has excess lithium incorporated in to it, shows a reversible capacity, with ~98% coulombic efficiency, of 167 (±2) mAh g⁻¹at a specific current of 30 mAg⁻¹ in the range 2.5-4.4 V up to 80 cycles with no capacity-fading. When cycled to a higher cut-off voltage (4.5 V), a capacity of 192 (±2) mAh g⁻¹ versus Li is obtained at the fifth cycle, but capacity-fading is observed, viz., ~ 6% after 60 cycles. On the basis of the CV and capacity-voltage profiles, this is attributed to the non-suppression of the hexagonal (H1) ↔ (H1-3) structural transition. A similar capacity-fading, i.e., ~ 5-6%, during 5-40 cycles, is also observed in the LiCoO₂ prepared at 650 and 750 °C when cycled up to only 4.3 V and this is ascribed to the non-suppression of the H1 ↔ M ↔ H1 phase transitions (M = monoclinic).