Surface coating effect on thermal properties of delithiated lithium nickel manganese layer oxide

The thermal stability of electrochemically delithiated bare, silica-coated and silicon phosphate-coated Li-0.3[Ni0.7Mn0.3]O-2 is studied with in-situ high temperature X-ray diffraction (HT-XRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and time of flight-secondary ion spectroscopy (ToF-SIMS). For the three delithiated materials, gradual phase transformation appears in the crystal structure in the temperature range of 25-600 degrees C: rhombohedral to salt structure via cubic spinel phase, which results from oxygen evolution from the active materials as noticed in TGA. Coating evidently retards the above phase transition toward a high temperature approximately over 40 degrees C owing to less amount of oxygen release from the crystal structure. This effect appears more prominent in the presence of the silicon phosphate coating layer relative to the silica, presumably due to presence of the Si-P-O covalent character. Also, the surface layer remains up to 600 degrees C, showing original smooth edges. Therefore, thermal degradation of the active materials is delayed when their surfaces are modified by nanoscale coating layers. (C) 2015 Elsevier B.V. All rights reserved.