Abstract: With the rapid development of the new energy industry, the development of cathode materials possessing high energy density, low cost, and environmental friendliness has become a key research focus in the lithium-ion battery sector. Consequently, the high-nickel, cobalt-free layered cathode material LiNi_0.9Mn_0.1O₂ (NM91) has garnered significant attention. However, its practical application is hindered by severe Li⁺/Ni²⁺ cation disordering and poor cycling stability. To address these issues, Mg/Nb co-doped single-crystal LiNi_0.9Mn_0.1O₂ cathode material was successfully synthesized via solid-state sintering. Systematic optimization of the doping ratio revealed that the sample doped with 1 mol% Mg and 0.5 mol% Nb exhibited superior cycling stability under high-voltage operation. Specifically, it demonstrated capacity retentions of 83.1% and 67.3% at cut-off voltages of 4.3 V and 4.4 V, respectively.Mechanistic analysis indicates that Mg²⁺ doping effectively mitigates cation disordering and enhances structural stability. Concurrently, Nb⁵⁺doping, leveraging its high valence state and strong electronegativity, stabilizes lattice oxygen, suppresses oxygen release at high voltages, and reinforces structural integrity under high-voltage conditions. The synergistic effect of Mg/Nb co-doping further widens Li⁺ diffusion pathways, reduces interfacial resistance, and improves charge transfer efficiency. Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV) results corroborate the superior interfacial stability and enhanced electrochemical kinetics of the co-doped material. Therefore, Mg/Nb co-doping represents an effective strategy for improving the high-voltage compatibility and cycling stability of single-crystal SC-NM91 cathodes, offering new insights for the design of high-nickel, cobalt-free cathode materials.