Abstract
► We studied Li-ion battery materials by Electron energy loss spectroscopy (EELS). ► Determination of the Fe valence state as a function of lithiation. ► Comparison of Li–K edge of surface electrolyte inter-phase (SEI) with known standards.
Electron energy loss spectroscopy (EELS) is a powerful technique for studying Li-ion battery materials because the valence state of the transition metal in the electrode and charge transfer during lithiation and delithiation processes can be analyzed by measuring the relative intensity of the transition metal L
3 and L
2 lines. In addition, the Li distribution in the electrode material can be mapped with nanometer scale resolution. Results obtained for FeO
0.7F
1.3/C nanocomposite positive electrodes are presented. The Fe average valence state as a function of lithiation (discharge) has been measured by EELS and results are compared with average Fe valence obtained from electrochemical data. For the FeO
0.7F
1.3/C electrode discharged to 1.5
V, phase decomposition is observed and valence mapping with sub-nanometer resolution was obtained by STEM/EELS analysis. For the lowest discharge voltage of 0.8
V, a surface electrolyte inter-phase (SEI) layer is observed and STEM/EELS results are compared with the Li–K edges obtained for various Li standard compounds (LiF, Li
2CO
3 and Li
2O).