Effect of AlPO₄-coating on cathodic behaviour of Li(Ni_0.8Co_0.2)O₂

Abstract

The cathodic behaviour and thermal stability in the charged state of bare and 1-8 wt.% AlPO₄-coated layered oxide Li(Ni_0.8Co_0.2)O₂ 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(Ni_0.8Co_0.2)O₂ 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 1s 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 ≥5 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(Ni_0.8Co_0.2)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 220 °C for the bare compound is increased by ~ 10 °C 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(Ni_0.8Co_0.2)O₂, respectively.