Thickness estimation of interface films formed on Li1−xCoO2 electrodes by hard X-ray photoelectron spectroscopy

Solid electrolyte interface (SEI) films formed on Li1−xCoO2 electrodes were observed with hard X-ray photoelectron spectroscopy (HX-PES). This paper particularly focuses on film thickness estimation using HX-PES with theoretical calculation. The validity of the calculation was proven by experiments using model SEI films. The native film formed on a LiCoO2 composite electrode was estimated to be LiF with its thickness of 5 nm. Formation of Co (II) species on top of LiCoO2 was also indicated. Storage of the electrode at 60 °C brought about considerable film growth (30–40 nm) with carbonate compounds formation. SEI film changes during charging of the LiCoO2 electrode were also examined. The main component in the film was deduced to be LiF or a kind of fluorite, with its thickness decreased during charging. The SEI formation mechanisms are also elucidated.

► The solid electrolyte interface films study hard X-ray photoelectron spectroscopy.
► The film thickness estimation use HX-PES with theoretical calculation.
► The LiCoO2 composite electrode is immersed in LiPF6, the SEI film is estimated LiF.
► The SEI film grows to 30–40 nm when such electrodes are stored at 60 °C.
► The content of fluoride-related component in the SEI film decreases during charging.