Unveiling Long‐Term Storage Failure Mechanisms of Single‐Crystal High‐Nickel Cathodes During Air Exposure

ABSTRACT Single‐crystal high‐nickel cathode (SC‐HN) materials are promising candidates for advanced lithium‐ion batteries due to their exceptional volumetric and gravimetric energy densities. However, SC‐HN materials face air instability, causing distinct storage failure mechanisms compared to polycrystalline high‐nickel cathode (PC‐HN) materials. The characteristics of SC‐HN, such as their lower specific surface area and reduced grain boundaries, make their failure mechanisms distinct and not directly applicable to PC‐HN materials. To address these unique degradation pathways, this study systematically investigated the storage failure mechanisms of SC‐HN material under ambient air exposure. Using advanced characterization techniques including soft X‐ray absorption spectra (sXAS), wide‐angle X‐ray scattering (WAXS), aberration‐corrected scanning transmission electron microscopy (STEM), and etching‐based X‐ray photoelectron spectroscopy (XPS), we conducted comprehensive multi‐dimensional analyses over 6 months to track the evolution of chemical and structural changes. The results reveal that SC‐HN materials experience a nonlinear progression of structural and surface composition degradation, and surface structural transformations are found to be the main cause of performance decline. The findings deepen understanding of SC‐HN air instability and provide a basis for targeted strategies to enhance storage stability.