Cathodic behaviour of NiO-coated Li(Ni_½Mn_½)O₂

Abstract

The compound, Li(Ni_½Mn_½)O₂ was prepared by the mixed hydroxide method at 950 °C in air. It was coated with 3 and 6 wt.% NiO by the solution method and heat treated at 450 °C in air. The compounds were characterized by X-ray diffraction, SEM, TEM, BET surface area and density. Cyclic voltammmetry of the bare and NiO-coated compounds showed redox processes at 3.69-3.95 V corresponding to the couple Ni^2+/3+/4+. Galvanostatic charge-discharge cycling was carried out at 30 mA g⁻¹ up to 70 cycles in the voltage ranges, 2.5-4.3 and 2.5-4.5 V versus Li at room temperature. The 10th cycle discharge capacities for the bare and NiO-coated compounds are 140 ± 3 mAh g⁻¹ which are stable up to 20-30 cycles after which capacity-fading occurs. The 6% NiO-coated compound performs better with only 11% capacity loss at the 70th cycle. In the range 2.5-4.5 V, the third cycle capacities are 175 ± 5 mAh g⁻¹ for the bare and 3% NiO-coated compounds, but capacity-fading starts from fifth cycle onwards. The 6% NiO-coated Li(Ni_½Mn_½)O₂ showed a stable discharge capacity of 165 ± 3 mAh g⁻¹ during 3-30 cycles after which it degraded, by ~ 9%, at the 70th cycle. Thus, NiO-coating suppresses capacity-fading but not eliminates it in Li(Ni_½Mn_½)O₂. The coulombic efficiency in all cases was >98%. DSC on the cathodes in the charged-state (4.3 V) show that the exothermic decomposition for the bare and 6% NiO-compound occurs at 300 and 307 (±2) °C, respectively.