As one of the most promising cathodes for sodium-ion batteries (SIBs), the layered transition metal oxides have attracted great attentions due to their high specific capacities and facile synthesis. However, their applications are still hindered by the problems of poor moisture stability and sluggish Na⁺ diffusion caused by intrinsic structural Jahn–Teller distortion. Herein, we demonstrate a new approach to settle the above issues through introducing K⁺ into the structures of Ni/Mn-based materials. The physicochemical characterizations reveal that K⁺ induces atomic surface reorganization to form the birnessite-type K₂Mn₄O₈. Combining with the phosphate, the mixed coating layer protects the cathodes from moisture and hinders metal dissolution into the electrolyte effectively. Simultaneously, K⁺ substitution at Na site in the bulk structure can not only widen the lattice-spacing for favoring Na⁺ diffusion, but also work as the rivet to restrain the grain crack upon cycling. The as achieved K⁺-decorated P2-Na_0.67Mn_0.75Ni_0.25O₂ (NKMNO@KM/KP) cathodes are tested in both coin cell and pouch cell configurations using Na metal or hard carbon (HC) as anodes. Impressively, the NKMNO@KM/KP||Na half-cell demonstrates a high rate performance of 50 C and outstanding cycling performance of 90.1% capacity retention after 100 cycles at 5 C. Furthermore, the NKMNO@KM/KP||HC full-cell performed a promising energy density of 213.9 Wh·kg⁻¹. This performance significantly outperforms most reported state-of-the-art values. Additionally, by adopting this strategy on O3-NaMn_0.5Ni_0.5O₂, we further proved the universality of this method on layered cathodes for SIBs.