<p>Lithium-rich manganese-based oxides (LMOs) are pivotal cathode candidates for next-generation lithium-ion batteries, owing to their ultrahigh capacity (&gt; 250 mAh·g⁻¹) enabled by anionic redox activity. However, their practical deployment is impeded by structural degradation from oxygen loss, voltage decay, and sluggish Li⁺ kinetics. Herein, we propose a synergistic dual-modification strategy that integrates Nb⁵⁺ bulk doping via heterogeneous nucleation with in-situ Li₃NbO₄ surface coating to comprehensively address these challenges. The Nb⁵⁺ doping expands Li⁺ diffusion channels while suppressing transition metal migration and oxygen evolution through strengthened metal–oxygen bonds. Concurrently, the Li₃NbO₄ coating, acting as a fast ion conductor, stabilizes the cathode-electrolyte interface by mitigating parasitic reactions and accelerating interfacial Li⁺ transport. The optimized LNO@LMNO-NO cathode achieves a remarkable initial discharge capacity of 327.76 mAh·g⁻¹ at 0.1 C and retains 95.98% capacity after 200 cycles at 1 C, demonstrating enhanced lithium-ion diffusion kinetics. This performance outperforms most reported LMOs, which can be attributed to the synergistic interplay between structural and interfacial engineering.</p>