Ni-rich transition metal oxides are regarded as appealing candidate cathodes for next-generation lithium batteries because of their high discharge capacity. Single-crystal is an effective means to restrain the formation of microcracks to improve the cyclic property of LiNi_0.8Co_0.15Al_0.05O₂ (NCA) materials. Nonetheless, critical structural and interfacial instabilities make single-crystal NCA cathodes hard to achieve ideal electrochemical performance at high voltage. Herein, the fast ion conductor Li₅La₃Nb₂O₁₂ with higher conductivity and wider electrochemical window was selected as the material to enhance the high voltage performances for single-crystal NCA. Interestingly, density functional theory calculations found that La and Nb have different distribution characteristics in NCA. Therefore, we take advantage of oxidation reaction and element diffusion during high temperature sintering to achieve dual-functional modification of La-rich conductor network constructing and near-surface Nb doping for single-crystal NCA material. Impressively, the modified electrodes exhibit excellent cycling performances with a retention of 82.9% after 500 cycles at 2.7–4.3 V and even outstanding stability under extended cutoff voltage as well as improved rate capabilities. Detailed study reveals that the doped Nb with stronger Nb-O bond can suppress the migration of transition metal ion, thus impeding the distortion of material structure. Meanwhile, surface ion-conductor network layer provides fast Li⁺ diffusion channels as well as protecting interface layer from electrolyte decomposition products. The facile approach of Li₅La₃Nb₂O₁₂ modification proposed in this manuscript is suitable to solve interfacial instability and structure stability for cathodes but not only for single-crystal NCA, which provides application hopes for high energy lithium-ion batteries.