<i>Operando</i> SWAXS for batteries

Abstract Advanced characterisation tools are needed to visualise and analyse the structural changes occurring in battery materials during cycling. Linking the electrochemical performance of the device to materials evolution is challenging as it requires to access microscopic information inside the system while it is working. As battery components are often multi-phase and multi-scale, the holistic analysis relies on combining or coupling techniques capable to reveal parameters of interest, such as local chemical properties, electronic structures, lattice parameters, morphology or chemomechanics. In this context, small angle and wide-angle x-ray scattering (SWAXS) techniques are complementary as they enable accessing the crystalline and amorphous/nanostructured phases. They are particularly suitable for probing composite or hierarchical materials at relevant length scales, including in real-time operando mode. In this tutorial article, we describe the principle of SWAXS, its applicability to battery research, and provide guidelines for conceiving, realizing and analysing SWAXS experiments. Beyond established set-ups where the averaged battery cell is probed in transmission or grazing incidence geometry, novel modalities using scanning and tomographic options are now available at 4th generation synchrotrons. These open new horizons to spatially-resolve reaction dynamics and associated (nano)structural heterogeneities, which is believed to be critical for in-depth mechanistic understanding and development of advanced materials. Here we illustrate the experimental approaches and workflows with two examples: silicon-graphite composites and hard carbons, both used as negative electrodes for Li-ion and Na-ion batteries, respectively. Archetypal data acquisition, reduction, processing and analysis methods are introduced, highlighting their advantages and pitfalls, while reporting a summary of key results and discoveries reported so far on the chosen examples. Finally, the future challenges for advanced SWAXS studies are discussed, encompassing novel beamtime access modes, data analysis strategies and development of AI-semi-automatised pipelines for high-throughput data correlations.