X-ray photoelectron spectroscopy is based on the long-established phenomenon of photoemission, but it owes its development to advances in vacuum technology and electronics, as well as to the talent and tenacity of Kai Siegbahn (winner of the Nobel Prize in Physics in 1981) and his team at Uppsala.
This spectroscopy consists in analyzing the energy of electrons emitted by a system irradiated with X-ray electromagnetic radiation. It gives direct access to the binding energy of core and valence electrons, providing unique information on the chemical and electronic structure of the systems studied. The binding energy of a core electron is specific to the atom concerned, enabling it to be identified and measured for all elements in the periodic table except hydrogen and helium. The energy distribution of valence electrons provides information on the nature of chemical bonds and electronic structures. In the case of solids, its surface selectivity makes it a key player in the field of surface science.
These different facets, and the variety of scientific and technological problems that spectroscopy helps to solve, are at the root of its popularity. Several thousand works are published every year, and there is no reason to believe that its interest and use will diminish in the future, as the fields it covers (catalysis, polymers, metallurgy, adhesion, energy, biology...) have continued to expand in recent decades. It is therefore well placed to contribute to meeting the many challenges of the future in terms of developing sustainable chemistry.
The foundations, characteristics and various operating modes of this spectroscopy were presented. Its contribution was illustrated in the energy materials sector, and more specifically in that of lithium batteries, where the problems linked to electrode materials and electrode/electrolyte interfaces were addressed within the framework of " multi-probe " approaches. The links between lithium insertion/extraction/electron transfer, including the role of anions, surface chemistry/reactivity aspects, and chemical understanding of interfacial layers/interfaces " buried " were all addressed with a view to illustrating the wealth of information accessible through this spectroscopy.
