Quantitative Analysis of Ni 2+ /Ni 3+ in Li[Ni x Mn y Co z ]O 2 Cathode Materials: Non-Linear Least-Squares Fitting of XPS Spectra

Abstract Quantitative analysis of Ni 2+ /Ni 3+ using X-ray photoelectron spectroscopy (XPS) is important for evaluating the crystal structure and electrochemical performance of Lithium-nickel–cobalt-manganese oxide (Li[Ni x Mn y Co z ]O 2 , NMC). However, quantitative analysis based on Gaussian/Lorentzian (G/L) peak fitting suffers from the challenges of reproducibility and effectiveness. In this study, the Ni 2+ and Ni 3+ standard samples and a series of NMC samples with different Ni doping levels were synthesized. The Ni 2+ /Ni 3+ ratios in NMC were quantitatively analyzed by non-linear least-squares fitting (NLLSF). Two Ni 2p overall spectra of synthesized Li [Ni 0.33 Mn 0.33 Co 0.33 ]O 2 (NMC111) and bulk LiNiO 2 were used as the Ni 2+ and Ni 3+ reference standards. Compared to G/L peak fitting, the fitting parameters required no adjustment, meaning that the spectral fitting process was free from operator dependence and the reproducibility was improved. Comparison of residual standard deviation (STD) showed that the fitting quality of NLLSF was superior to that of G/L peaks fitting. Overall, these findings confirmed the reproducibility and effectiveness of the NLLSF method in XPS quantitative analysis of Ni 2+ /Ni 3+ ratio in Li[Ni x Mn y Co z ]O 2 cathode materials.