Probing Domain-Boundary-Induced Structural Degradation in Single-Crystalline LiCoO₂ by Nanoscale Imaging

Developing high-capacity cathode materials is pivotal for advancing lithium-ion battery technology. While single-crystalline materials are widely regarded as structurally superior to polycrystalline counterparts, their presumed "perfect" crystallinity has recently been challenged by observations of intrinsic lattice defects and strain heterogeneity. Critically, the lack of direct experimental evidence for these defects and their role in degradation has hindered deeper understanding of single-crystalline cathode failure mechanisms. Here, by employing super-resolved nanoscale X-ray computed tomography (Nano-CT), scanning probe nanodiffraction imaging (SPNDI), and advanced data-driven statistical analysis, we unveil the ubiquitous presence of nanoscale domain boundaries within micrometer-sized LiCoO₂ single crystals, which act as primary hotspots for strain accumulation and microcrack formation during cycling. These boundaries, invisible to conventional characterization techniques, are shown to govern the mechanical and electrochemical degradation of cathode particles. By correlating nanoscale imaging with electrochemical performance, we demonstrate that residual lattice strain at domain boundaries accelerates irreversible phase transitions, while targeted doping element within intragranular can stabilize these critical interfaces. Our findings emphasize that intragranular domain regulation for single-crystalline cathodes, rather than mere morphology control, is essential for designing next-generation high-energy-density batteries.