On the X-ray photoelectron spectroscopy analysis of LiNixMnyCozO2 material and electrodes

Highlights: • Ni2p and fluorine Auger spectra are commonly confused when Al Kα X-ray source used. • Deconvolution of Ni2p XPS spectrum into Ni oxidation states is ambiguous. • Intensity of the satellite in Ni2p spectrum is proportional to the Ni²⁺ content. • Ni²⁺/Ni³⁺ ratio derived from the intensity ratio Ni2p_3/2 main peak/satellite. • Ni²⁺/Ni³⁺ ratios obtained for NMC333, NMC532, NMC622 and NMC811 powder materials. X-ray photoelectron spectroscopy (XPS) is a widely used technique to study surface chemistry and ageing mechanisms of LiNi_xMn_yCo_zO₂ (NMC) based cathodes of lithium ion batteries. Analysis of the literature, however, reveals common problems in interpretation of their XPS spectra: (1) confusion of Ni2p spectra with Auger electron spectra (F_KLL) of fluorine from commonly used PVdF binder and from decomposition of the fluorinated electrolyte components, when the XPS spectra are acquired with Al Kα X-ray source; (2) ambiguous fitting of the XPS spectral lines with several Lorentzian-Gaussian shapes leads to doubtful assignments of the oxidation states and questionable conclusions on the ageing mechanisms of the electrode materials. Revisiting the XPS data from aged NMC electrodes emphasizes the contribution of LiF and downplays the effect of nickel and manganese fluorides in the formation of cathode-electrolyte interface. To quantify Ni²⁺/Ni³⁺ ratio we introduce a novel method based on the ratio between Ni2p_3/2 main peak intensity and intensity of the shake-up satellite. Applying this approach for commercial NMC333, NMC532, NMC622 and NMC811 powder materials we find good correlation with theoretically predicted values for freshly made materials.