Coupling Grain Architecture Control With Oxygen‐Vacancy Suppression in Ultra‐Large Single‐ and Quasi‐Single‐Crystal Ni‐Rich Cathodes

ABSTRACT Single‐crystalline Ni‐rich layered oxide cathodes exhibit superior cycling and thermal stability over conventional polycrystalline counterparts by eliminating grain‐boundary‐associated degradation. However, the extended Li + diffusion length in single crystals limits practical particle sizes to only a few micrometers, creating a fundamental trade‐off between electrode density and electrochemical performance. Here, we report a practical synthesis strategy for ultra‐large (≈15 µm) single‐crystalline and quasi‐single‐crystalline LiNi 0.95 Co 0.04 Mn 0.01 O 2 cathodes that deliver an exceptional rate capacity of 170 mA h g −1 at 5 C despite their unprecedented particle size. Using these grain‐engineered architectures, we identify oxygen transport during high‐temperature synthesis as a previously overlooked factor governing defect formation in ultra‐large Ni‐rich cathodes. In single‐crystal particles, sluggish oxygen diffusion through the ordered bulk lattice produces a pronounced oxygen‐deficient structural state, accompanied by severe cation disorder. In contrast, internal grain boundaries in quasi‐single‐crystal particles act as rapid oxygen‐transport channels, enabling efficient oxygen redistribution during sintering and thereby suppressing oxygen vacancy formation and cation mixing while preserving structural integrity. Consequently, ultra‐large quasi‐single‐crystal cathodes simultaneously achieve high electrode density, fast reaction kinetics, excellent thermal stability, and long‐term cycling durability. This work identifies grain‐boundary‐assisted oxygen transport as a key design consideration for defect control in ultra‐large, highly Ni‐rich layered oxide cathodes.