Structure of aluminum fluoride coated Li[Li1/9Ni1/3Mn5/9]O2 cathodes for secondary lithium-ion batteries

Abstract

The structural properties of layered Li[Li_1/9Ni_1/3Mn_5/9]O₂ 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 °C and 400 °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 ⁶Li and ²⁷Al magic angle spinning NMR spectroscopy. The TEM/EDS and ²⁷Al NMR data provide support for an aluminum-rich amorphous coating that, following drying at 120 °C, comprises six coordinated, partially hydrated aluminum environments. Heat treatment at 400 °C results in a phase that resembles partially fluorinated γ- or γ′-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[Li_1/9Ni_1/3Mn_5/9]O₂ after drying. This phase disappears on heating to 400 °C, suggesting some reorganization of bulk Li[Li_1/9Ni_1/3Mn_5/9]O₂ and possibly some incorporation of Al into the structure. This is in agreement with the ⁶Li 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)Ni_xMn_(2/3−x/3)]O₂ (i.e. Li₂MnO₃) are enhanced on sintering.