<i>In Situ</i> Synchrotron Characterization of Layered Oxide Cathodes for Lithium-Ion Batteries: Bridging Synthesis, Operation, and Thermal Stability

This review examines synchrotron-based in situ characterization studies of layered oxide cathodes for lithium-ion batteries (LIBs) and shows that kinetic phenomena, rather than thermodynamic equilibrium, govern their behavior during synthesis, operation, and degradation. Advanced X-ray techniques reveal features including nanoscale structural domains that form substantially earlier than indicated by conventional detection methods, pronounced chemical heterogeneity within individual particles that contributes to mechanical failure, and electronic destabilization that precedes thermal decomposition. Surface–bulk divergence and intraparticle heterogeneity─often regarded as undesirable artifacts─emerge as intrinsic characteristics that strongly influence electrochemical performance. Correlative analyses of diffraction, spectroscopy, and microscopy data indicate that these materials frequently exist in nonequilibrium states in which local kinetics dominate over global thermodynamics. This review suggests a shift from equilibrium-based models toward kinetically controlled frameworks for understanding battery materials, indicating that future cathode designs might benefit from accommodating rather than eliminating these inherent heterogeneities. The mechanistic insights derived from synchrotron radiation provide guidance for developing stabilization strategies to address fundamental challenges in energy storage materials that are relevant to sustainable energy systems.