Binding energy referencing for XPS in alkali metal-based battery materials research (I): Basic model investigations

For the investigation of chemical changes in Li- and Na-ion battery electrode systems, X-ray photoelectron spectroscopy (XPS) is a well-accepted method. Charge compensation and referencing of the binding energy (BE) scale is necessary to account for the involved mostly non-conducting species. Motivated by a conflict in energy scale referencing of Li-metal samples discussed earlier by several authors, further clarifying experimental results on several Li containing reference materials are presented and extended by similar experiments for Na. When correlating the peak positions of characteristic chemical species in all the different prepared model sample states, there seems to be a systematic deviation in characteristic binding energies of several eV if lithium is present in its metallic state. Similar results were found for sodium. The observations are furthermore confirmed by the implementation of inert artificial energy reference material, such as implanted argon or deposited gold. The behavior is associated with the high reactivity of metallic lithium and a phenomenological explanation is proposed for the understanding of the observations. Consequences for data interpretation in Li-ion battery research will be discussed for various applications in part (II).