Photoemission spectroscopy of battery materials

Recognized by the 2019 Nobel Prize in Chemistry, rechargeable lithium-ion battery (LIB) has become a world-revolutionary technology. Further developments of LIB-based and “beyond LIBs” regarding capacity, cycle life, and safety are intimately associated with the fundamental understanding of chemical compositions, structures, physical properties of electrodes and electrolytes, and other related components. The time-evolving snapshots of the dynamical processes occurring during the battery operation can help design better strategies to prevent the formation of uncontrolled interphase layers, dendrites, electrode/electrolyte decompositions, and generation of gases. Photoemission spectroscopy (PES) has become one of the important techniques for understanding the aforementioned aspects. However, many potential pitfalls and cautions need to be considered from sample preparation, during PES measurements, to data analyses. Although the primary focus of this article is not to evaluate the PES technique itself, we first introduce a minimal set of fundamental concepts to minimize misinterpretation arising from the physics of PES. Subsequently, we examine studies that utilize PES techniques to determine chemical compositions of solid- and liquid-state battery materials, energy level diagrams that bridge different terminologies between PES and electrochemistry, along with the theoretical aspects of PES evolving from first-principle calculations to machine learning. Toward the end of this review, we outline potential future research directions.