Quantitative atomic scale and molecular level insight into structural (geometrical and electronic) and morphological properties on an absolute scale and under reaction conditions is imperative for understanding mechanisms and phenomena across scales in electrochemical systems. Interpreted in the context of device performance, this knowledge can be used for rational design of and new concepts for improved materials and processes. Following a brief introduction, a general overview of (novel) operando-capable X-ray-based methods and a rationalization of their particular usefulness for studying electrochemical systems, and several examples using model systems, four topics will be discussed in detail.

The first topic concerns charge and mass transport in various electrolytes. Here, we used a combination of correlative operando X-ray photon correlation spectroscopy, small and wide-angle X-ray scattering, and X-ray absorption microscopy to measure electrolyte velocity and concentration fields as well as solvent orientation upon cell polarization. The results were combined with macroscopic and microscopic theory to quantify and rationalize transport numbers, and to provide length-scale bridging insight into ion transport.

The second topic covers the surface-electrochemistry in Li-ion batteries, with a specific focus on the origin of LiF in the solid electrolyte interphase. Towards this end, we sought out a multimodal correlative operando experimental and theoretical approach using model electrodes. Our results reveal that LiF nucleates via the electrocatalytic reduction of HF followed by significant PF₆^- anion reduction. We then applied this understanding to develop a methodologically novel electrochemical approach to selectively remove HF from carbonate-based LiPF₆-containing LIB electrolytes.

The third topic covers desalination batteries. Here, we employed (quasi-high-throughput) operando high-energy X-ray diffraction microscopy to spatially resolve ion intercalation processes and mechanisms in realistic flow-by desalination reactors. We focus on atomic- to electrode-level insight into structural changes and ion selectivity in manganese oxides and iron phosphate electrodes.

The fourth topic concerns some recent methodological developments in surface X-ray scattering. These include surface X-ray scattering measurements using nanoscale X-ray beams, increasing accuracy and precision of geometrical corrections in X-ray reflectivity, and approaches for sub-second surface X-ray scattering.