Concentration–structure dual-gradient architecture in high-Ni/low-Co single-crystal NCM cathodes

Abstract

High-Ni/low-Co single-crystal NCM cathodes are regarded as promising candidates for next-generation lithium-ion batteries owing to their high energy density and mechanical robustness. However, their practical implementation remains limited by poor electronic conductivity, primarily originating from resistive surface residual lithium compounds (RLCs) and the reduced Co content. Here, we reveal that a dry coating of monodisperse Co₃O₄ nanoparticles followed by controlled thermal annealing induces a concentration–structure dual-gradient architecture in high-Ni/low-Co single-crystal NCM cathodes. This process simultaneously removes RLCs and forms an ultrathin (~30 nm) surface Co-rich coating layer, accompanied by Ni–Co cation interdiffusion that generates a subsurface disordered rock salt (DRX)-like layer (~15 nm) with a reduced Ni oxidation state. The Co-rich coating layer significantly enhances electronic conductivity and lowers interfacial resistance, thereby improving rate capability, while the underlying DRX-like layer functions as a structural pillar under highly delithiated states, suppressing Li-layer collapse. The gradient architecture mitigates electrolyte oxidation, structural degradation, transition-metal dissolution, and microcrack formation, leading to improved cycling stability. This work uncovers a previously underappreciated structural role of controlled cation interdiffusion in stabilizing high-Ni/low-Co layered oxides, providing a general design principle for cathodes combining high electronic conductivity and structural robustness with minimal Co content.