Breaking Down a Complex System: Interpreting PES Peak Positions for Cycled Li-Ion Battery Electrodes

Photoelectron spectroscopy (PES) is an important technique for tracing and understanding the side reactions responsible for decreasing performance of Li-ion batteries. Interpretation of different spectral components is dependent on correct binding energy referencing, and for battery electrodes, this is highly complex. In this work, we investigate the effect on binding energy reference points in PES in correlation to solid electrolyte interphase (SEI) formation, changing electrode potentials, and state of charge variations in Li-ion battery electrodes. The results show that components in the SEI have a significantly different binding energy reference point relative to the bulk electrode material (i.e., up to 2 eV). It is also shown that electrode components with electronically insulating/semiconducting nature are shifted as a function of electrode potential relative to highly conducting materials. Further, spectral changes due to lithiation are highly depending on the nature of the active material and its lithiation mechanism. Finally, a strategy for planning and evaluating PES experiments on battery electrodes is proposed where some materials require careful choice of one or more internal reference points while others may be treated essentially without internal calibration.