Mechanically and Chemically Co‐Robust Ni‐Rich Cathodes with Ultrahigh Capacity and Prolonged Cycle Life

Ni-rich layered oxide (NRLO) materials are considered highly promising cathode for lithium-ion batteries. However, their practical application is limited by capacity loss and interface instability caused by chemical and mechanical failure during cycling. Doping has been identified as a direct and effective method to address these challenges. However, mechanistic understanding of doping enhanced electrochemical performance is still unclear. In this study, the introduction of high-valent Nb ions was employed to achieve mechanical-chemical coupling regulation, thereby concurrently improving the capacity and cycle life of NRLO. First, Nb5+ doping was conducted to refine secondary grains, achieving a "grain refinement" effect similar to that in ceramics and alloys, while further stabilizing the grain boundaries. The intergrain fusion structure of NCM811-0.5Nb effectively dissipates lattice strain under highly delithiated state, suppresses oxygen loss, and prevents cracks that lead to fracture during cycling. Moreover, Nb doping stabilizes the monoclinic phase during phase transitions and promotes the formation of highly stable spinel twin boundaries after cycling. This effectively reduces the Li diffusion barrier, leading to improved reversible specific capacity and rate capability. Lastly, the strong Nb─O bonding restrains oxygen release and transition metal/Li antisite mixing, thus mitigate rock-salt phase formation. This study demonstrates a comprehensive understanding of the concurrent capacity and stability enhancement mechanisms attributed to Nb-doping and highlights the significant potential of the synergistic regulation of mechanical and chemical coupling in improving the capacity and lifespan of NRLOs by Nb-doping.