AbstractThe deposition of volatilized Na⁺ on the surface of the cathode during sintering results in the formation of surface residual alkali (NaOH/Na₂CO₃NaHCO₃) in layered cathode materials, leading to serious interfacial reactions and performance degradation. This phenomenon is particularly evident in O3‐NaNi_0.4Cu_0.1Mn_0.4Ti_0.1O₂ (NCMT). In this study, a strategy is proposed to transform waste into treasure by converting residual alkali into a solid electrolyte. Mg(CH₃COO)₂ and H₃PO₄ are reacted with surface residual alkali to generate the solid electrolyte NaMgPO₄ on the surface of NCMT, which can be labeled as NaMgPO4@NaNi_0.4Cu_0.1Mn_0.4Ti_0.1O₂‐X (NMP@NCMT‐X, where X indicates the different amounts of Mg²⁺ and PO₄³⁻). NaMgPO₄ acts as a special ionic conductivity channel on the surface to improve the kinetics of the electrode reactions, remarkably improving the rate capability of the modified cathode at a high current density in the half‐cell. Additionally, NMP@NCMT‐2 enables a reversible phase transition from the P3 to OP2 phase in the charge–discharge process above 4.2 V and achieves a high specific capacity of 157.3 mAh g⁻¹ and outstanding capacity retention in the full cell. The strategy can effectively and reliably stabilize the interface and improve the performance of layered cathodes for Na‐ion batteries (NIBs).