Operando Electrochemical XPS at the Electrode-Electrolyte Interface

A major challenge in the development of reversible batteries is the huge irreversible capacity loss incurred during the first cycle, which is primarily due to electrolyte decomposition on the anode surface to form a solid electrolyte interface (SEI) layer. Thus, an in situ study on the behaviour of species at the electrode-electrolyte interface, in addition to proper characterisation of the SEI layer, is of great interest. One such emerging area with great prospect is that of in situ electrochemical X-ray photoelectron spectroscopy (XPS). A wealth of information for the top ~5 nm of a sample can be obtained using XPS, including identification and quantification of elemental surface composition, determination of oxidation state and local chemical environment. Additionally, the high-vacuum requirement of traditional lab-based XPS studies can be overcome using ionic liquid (IL) electrolytes, due to their negligible vapor pressure. Further beneficial properties of IL electrolytes, including a large electrochemical window, superior thermal stability and potential for greener chemistry make them exciting alternatives to organic electrolytes.<br/> <br/>In this contribution, we demonstrate the unique information that can be obtained from operando electrochemical XPS. Cyclic voltammetry (CV) measurements were performed in tandem with XPS analysis, with the X-ray beam focussed on the electrode-electrolyte interface of the working electrode (WE) in a three-electrode electrochemical cell, for a variety of different systems containing IL electrolytes. We report on the binding energy shifts of characteristic signals from the electrode and electrolyte species with the applied electrochemical potential, and the evidence of electrochemical decomposition products as evidenced by the formation of new species. The techniques presented here may be applied to an extensive range of systems containing IL electrolytes and may bring new insights into complex electrochemical reactions at the electrode-electrolyte interface, ultimately helping us to design and develop reversible batteries for the future.