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

Abstract

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_0.93Co_0.05Al_0.02O₂ (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 Co₃O₄ and Li_xCoO₂, 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.