Formation of SEI on Cycled Lithium-Ion Battery Cathodes: Soft X-Ray Absorption Study

Abstract

The formation of a solid electrolyte interface (SEI) on LiNi 0 . 8 5 Co 0 . 1 5 O 2 cathodes from lithium-ion cells cycled at 40 and 70°C was observed and characterized using soft X-ray absorption spectroscopy (XAS). XAS measurements were made in the energy region between 500 and 950 eV, encompassing the Ni and Co L 3 - and L 2 -edges and at the K-edges of O and F. Measurements, obtained in the total electron yield mode, are surface sensitive, probing to a depth of ∼5 nm. XAS at the F K-edge demonstrates the presence of poly(vinylidene fluoride) (PVdF) in addition to LiF on the surface of cycled electrodes. The results indicate that the PVdF in the cycled electrodes is largely intact and that the LiF comes from decomposition of LiPF 6 from the electrolyte. XAS also suggests Fe contamination of cycled cathodes.