Structure of aluminum fluoride coated Li[Li1/9Ni1/3Mn5/9]O-2 cathodes for secondary lithium-ion batteries

The structural properties of layered Li[Li1/9Ni1/3Mn5/9]O-2 positive electrodes nominally coated with aluminum fluoride are studied. Coatings were prepared by using aqueous solutions with various concentrations of aluminum and fluorine and are compared with samples treated under similar conditions but with aqueous HCl solutions. Samples were investigated following heat treatment at 120 degrees C and 400 degrees C with powder X-ray diffraction, transmission electron microscopy including energy dispersive X-ray spectroscopy (TEM/EDS), elemental analysis via inductively coupled plasma-optical emission spectroscopy (ICP-EA), and both Li-6 and Al-27 magic angle spinning NMR spectroscopy. The TEM/EDS and Al-27 NMR data provide support for an aluminum-rich amorphous coating that, following drying at 120 degrees C, comprises six coordinated, partially hydrated aluminum environments. Heat treatment at 400 degrees C results in a phase that resembles partially fluorinated gamma- or gamma'-Al2O3, at least locally. An Al : F ratio of 2 : 1 is obtained in stark contrast to the ratio used in the original solution (1 : 3). No AlF3 is detected by PXRD and instead some evidence for a protonated phase (formed by ion exchanging protons for lithium) is detected along with Li[Li1/9Ni1/3Mn5/9]O-2 after drying. This phase disappears on heating to 400 degrees C, suggesting some reorganization of bulk Li[Li1/9Ni1/3Mn5/9]O-2 and possibly some incorporation of Al into the structure. This is in agreement with the 6 Li NMR spectra, which indicate that the local environments that are found in the Ni-free end member of the series Li[Li(1/3-2x/3)NixMn(2/3-x/3)]O-2 (i.e. Li2MnO3) are enhanced on sintering.