Micro-X-ray diffraction (micro-XRD) is a powerful analytical technique for investigating the structural properties of battery materials at a microscale resolution. Unlike conventional bulk XRD, which averages information over large sample areas, micro-XRD provides spatially resolved data, enabling researchers to study local crystallographic changes, phase distributions, and degradation mechanisms in electrodes and other battery components. This capability is critical for understanding electrode heterogeneity, identifying failure mechanisms, and optimizing battery performance.

A key aspect of micro-XRD is the ability to focus X-ray beams to micrometer or even sub-micrometer dimensions. Several beam focusing methods are employed to achieve high spatial resolution. Capillary optics, such as polycapillary lenses, are commonly used to concentrate X-rays into a small spot by guiding them through multiple curved channels. These lenses can achieve spot sizes ranging from 5 to 50 micrometers, depending on the energy and configuration. Another approach involves Kirkpatrick-Baez (KB) mirrors, which use elliptical mirrors to reflect and focus X-rays down to sub-micrometer levels. Synchrotron facilities often employ KB mirrors due to their ability to handle high-intensity beams while maintaining tight focus. Additionally, Fresnel zone plates, consisting of concentric rings that diffract X-rays to a focal point, are used in some high-resolution setups, particularly for soft X-ray regimes. The choice of focusing method depends on the required resolution, X-ray energy, and available flux.

Mapping applications of micro-XRD are particularly valuable for studying electrode heterogeneity. By raster-scanning a sample under a focused X-ray beam, researchers can generate 2D or 3D maps of phase distributions, crystallographic orientations, and strain fields. For example, in lithium-ion battery cathodes, micro-XRD mapping has revealed non-uniform phase transitions during cycling, such as the formation of localized high-strain regions or incomplete delithiation in layered oxide materials. These heterogeneities can lead to mechanical stress, capacity fade, and reduced cycle life. Similarly, in silicon anodes, micro-XRD has been used to track the evolution of amorphous and crystalline phases during lithiation and delithiation, providing insights into volume expansion and fracture mechanisms.

One of the most critical applications of micro-XRD is the study of hot spots—localized regions where accelerated degradation or abnormal reactions occur. These hot spots often arise due to inhomogeneous current distribution, poor electrode-electrolyte contact, or manufacturing defects. Micro-XRD can identify crystallographic signatures of degradation, such as the formation of secondary phases, lattice distortions, or amorphization. For instance, in nickel-rich cathodes, micro-XRD has detected localized rock-salt phase formation at particle surfaces, which correlates with oxygen release and thermal instability. In graphite anodes, micro-XRD mapping has revealed lithium plating in regions of high current density, a phenomenon that can lead to dendrite growth and safety hazards.

The technique is also instrumental in analyzing composite electrodes, where active materials, binders, and conductive additives are mixed. Micro-XRD can distinguish between different phases within these composites, enabling researchers to study how individual components interact during cycling. For example, in sulfur cathodes for lithium-sulfur batteries, micro-XRD has been used to track the spatial distribution of lithium polysulfides and solid discharge products, which are critical for understanding the shuttle effect and capacity decay. Similarly, in solid-state batteries, micro-XRD can detect interfacial reactions between solid electrolytes and electrodes, such as the formation of interphases that impede ion transport.

Another advantage of micro-XRD is its compatibility with operando and in-situ studies. By integrating micro-XRD with electrochemical cells, researchers can monitor dynamic structural changes in real time under operating conditions. For example, operando micro-XRD has been used to observe phase propagation waves in lithium iron phosphate (LFP) electrodes, where phase boundaries move non-uniformly during charge and discharge. These studies have demonstrated that phase separation kinetics are influenced by particle size, carbon coating, and electrode architecture. In sodium-ion batteries, micro-XRD has provided insights into the structural stability of Prussian blue analogs during cycling, revealing reversible and irreversible phase transitions that affect performance.

The combination of micro-XRD with other complementary techniques further enhances its utility. Pairing micro-XRD with X-ray fluorescence (XRF) allows simultaneous mapping of elemental distributions and crystallographic phases, which is useful for studying dopant effects or contamination. Coupling with Raman spectroscopy or infrared microscopy can provide additional chemical and molecular information, helping to correlate structural changes with electrochemical behavior. In some cases, micro-XRD is integrated with scanning electron microscopy (SEM) or transmission electron microscopy (TEM) for correlative analysis across multiple length scales.

Despite its advantages, micro-XRD also presents challenges. The requirement for high-brilliance X-ray sources, such as synchrotrons, limits accessibility for some researchers. Laboratory-scale micro-XRD systems are available but often sacrifice resolution or flux compared to synchrotron setups. Sample preparation is another consideration, as electrodes must be thin enough to minimize X-ray absorption while maintaining representative electrochemical properties. Additionally, data analysis can be complex due to the large datasets generated by mapping experiments, requiring advanced software for phase identification, peak fitting, and visualization.

Recent advancements in detector technology and computational methods are addressing some of these challenges. Fast, high-resolution detectors enable quicker data acquisition, making high-throughput mapping more feasible. Machine learning algorithms are being developed to automate phase identification and analyze complex diffraction patterns, reducing the burden of manual interpretation. Furthermore, the development of compact, high-intensity X-ray sources may expand the availability of micro-XRD for routine laboratory use.

In summary, micro-XRD is an indispensable tool for spatially resolved battery analysis, offering unique insights into electrode heterogeneity, degradation mechanisms, and dynamic structural changes. Its ability to map crystallographic properties at the microscale complements bulk techniques and provides a deeper understanding of localized phenomena that impact battery performance and safety. As the technique continues to evolve with improved instrumentation and analytical methods, its role in battery research and development is expected to grow, driving innovations in materials design and manufacturing processes.