Synchrotron x-ray studies of the solid electrolyte interface on cycled lithium-ion battery cathodes

Soft x-ray x-ray absorption spectroscopy (XAS) at the O and F K edges and XAS at the P K edge was used to investigate the inorganic components of the surface electrolyte interface (SEI) on electrodes from cycled and abused cells. XAS at the O K edge in both the electron yield (EY) and fluorescence yield (FY) modes indicate that as received nickel based oxides such as LiNiO ₂, LiNi _0.85Co _0.15O ₂ and LiNi _0.80Co _0.15Al _0.05O ₂ are already covered with a thin (50 Å) surface layer that is different from the bulk. High-resolution transmission electron microscopy (TEM) and electron diffraction shows a rock salt structure at the surface and a layered structure in the bulk. XAS studies of cycled and abused cathodes show the presence of LiF and condensed phosphate species on the surface of both anodes and cathodes. These species come from the decomposition of LiPF ₆. Condensed phosphate films and LiF on the electrode surface can contribute to power fade.