Non-Destructive Chemical State Mapping Using Laboratory XANES and EXAFS

Abstract

X-ray absorption spectroscopy (XAS) is a popular technique for determining chemical states, such as oxidation states by XANES (x-ray absorption near edge structure) or interatomic distances and coordination numbers by EXAFS (extended x-ray absorption fine structure). These techniques are widely utilized by electrochemical researchers in synchrotron facilities worldwide, addressing a range of studies: for example, XANES can help to probe the oxidation states of battery electrode materials at various stages in their operational lifetime, EXAFS can elucidate the dynamics of conversion reactions in operando, thus capturing transients that may otherwise be lost in ex situ studies, and XAS experiments encompassing both XANES and EXAFS have shown unique promise for examining the behavior of sulfur in electrodes for Li-S batteries. The popularity of these techniques in tandem with the 3D microstructural information from X-ray tomography (XCT) and the compositional information available through other types of spectroscopy (e.g., XRF, EDS, and Raman) has grown substantially over the past two decades, as more synchrotron beamlines have become available for these experiments and the true power of the techniques have been demonstrated. In spite of the excitement generated by the various XAS experiments documented in the literature and the growing number of synchrotron facilities worldwide, access to suitable instrumentation remains challenging. This is due largely to the general popularity of synchrotron radiation for studying a variety of materials across a range of disciplines, leading to high levels of competition for limited amounts of available beam time. Efforts have been made in the past to migrate the XAS techniques to laboratory instrumentation, but such efforts have been met with many challenges. Often the exposure times are very long, resulting in multi-day measurements for a single experiment, or the energy resolution is too crude to provide sufficient sensitivity for precise quantification. Thus, a need remains for expanded access to XAS techniques within the electrochemical energy storage and conversion community, which has, so far, not been adequately addressed. Here, we present a novel, compact, laboratory-based XAS system operating in the 4.5-12 keV energy range with a powerful range of capabilities. This instrument provides sub-eV energy resolution over a fairly large energy range, short data collection times (seconds to minutes), and high spatial resolutions (~10 micrometers) for small spot analysis or mapping. The performance of the system is achieved in part with a patented high brightness x-ray source, a novel system design, and the use of the latest x-ray crystal analyzers and spatially resolved detector. Through the unique capabilities of this laboratory instrumentation, many experiments traditionally reserved for the synchrotron can now be accomplished in the laboratory. In this presentation, we will briefly review the spectrum of studies where XAS can provide insight into battery electrode materials and proceed to demonstrate how this laboratory XAS setup can address both routine and more specialized needs for electrochemical characterization. We will also discuss the relationship of this technique to other laboratory approaches, providing some perspectives on both the present and future of non-destructive characterization.