Effect of AlF3Coating on Thermal Behavior of Chemically Delithiated Li0.35[Ni1/3Co1/3Mn1/3]O2

Abstract

The thermal behavior of chemically delithiated Li0.35[Ni1/3Co1/3Mn1/3]O2 and the AlF3-coated material was studied in the temperature range from room temperature to 600 °C. Thermogravimetric analysis results showed that the uncoated and the AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 powders experienced distinct weight loss with increasing temperature, of which the weight loss was ascribed to oxygen release from the active materials. The released oxygen amount was less for the AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 than the uncoated material, probably due to the blocking of the oxygen evolution by the AlF3 coating. The weight loss was associated with the irreversible phase transformation from a rhombohedral layer (R3m̅) structure to a cubic spinel (Fd3m) structure, as confirmed by in situ high-temperature X-ray diffraction. The reduced oxygen release brought about by the AlF3 coating delayed the phase transformation to the cubic spinel structure. This entailed shifts of the main exothermic reactions to higher temperatures for the active material in the presence of an electrolyte. The AlF3 coating remained on the surface of the active material to 300 °C. Thereafter, the layer changed to Li−Al−O with increasing temperature, as observed by the time-of-flight secondary ion mass spectroscopy. The improved thermal properties of the chemically delithiated AlF3-coated Li0.35[Ni1/3Co1/3Mn1/3]O2 were ascribed to the suppression of oxygen release from the active material, and this, in turn, retarded the formation of the cubic spinel phase.