Operando x-ray absorption spectroscopy on battery materials: a review of recent developments

Although batteries represent a key tool for sustainable development, their working processes, in terms of reaction mechanisms, side reactions, ion transport and formation of a solid–electrolyte interface, are not yet fully understood. In this respect, operando experiments are of enormous importance for providing hints on the relevant chemical species that form ‘while a battery is working’. X-ray absorption spectroscopy (XAS) has for a long time been the standard in the investigation of local structures of materials. In this regard, applied operando can provide invaluable information on the working mechanisms of batteries. In this review, after introductory paragraphs concerning battery chemistry and the principles of XAS, some of the most important developments in operando XAS applied to battery science are considered. Emphasis is given to Li-metal, Na-ion, Li/sulfur and all solid-state batteries. Related and advanced techniques, such as resonant inelastic x-ray scattering and high-resolution fluorescence-detected x-ray absorption spectroscopy are discussed as well. Suggestions are offered for planning an XAS experiment at the synchrotron radiation source, and finally, some considerations concerning future developments are presented.