Bifunctional Role of Surface Cobalt Doping as a Lithium‐Residue Scavenger and Surface Stabilizer in Single‐Crystalline Ultrahigh‐Ni Cathodes

Ultrahigh‐Ni layered cathodes are among the most promising materials for next‐generation lithium‐ion batteries due to their high energy density, but their practical use is often limited by severe surface instability and structural degradation. In this work, surface Co doping of LiNi 0.93 Co 0.05 Al 0.02 O 2 (NCA) is demonstrated to enhance both electrochemical performance and structural stability. The introduction of Co at the surface promotes the formation of stable spinel phases, such as Co 3 O 4 and Li x CoO 2 , which simultaneously remove residual lithium species and suppress the evolution of detrimental NiO‐like rock‐salt phases and resistive interfacial layers. Comprehensive analyses, including electrochemical testing, synchrotron‐based X‐ray techniques, and high‐resolution electron microscopy, confirm that Co‐doped NCA exhibits improved interfacial stability and lithium‐ion transport compared to undoped NCA. These findings highlight surface Co doping as a surface reconstruction stabilizer to mitigate degradation pathways, thereby enabling ultrahigh‐Ni cathodes to achieve greater durability and performance in advanced lithium‐ion battery applications.