Conversion of Surface Residual Alkali to Solid Electrolyte to Enable Na‐Ion Full Cells with Robust Interfaces

Abstract The deposition of volatilized Na + on the surface of the cathode during sintering results in the formation of surface residual alkali (NaOH/Na 2 CO 3 NaHCO 3 ) in layered cathode materials, leading to serious interfacial reactions and performance degradation. This phenomenon is particularly evident in O3‐NaNi 0.4 Cu 0.1 Mn 0.4 Ti 0.1 O 2 (NCMT). In this study, a strategy is proposed to transform waste into treasure by converting residual alkali into a solid electrolyte. Mg(CH 3 COO) 2 and H 3 PO 4 are reacted with surface residual alkali to generate the solid electrolyte NaMgPO 4 on the surface of NCMT, which can be labeled as NaMgPO4@NaNi 0.4 Cu 0.1 Mn 0.4 Ti 0.1 O 2 ‐ X (NMP@NCMT‐ X , where X indicates the different amounts of Mg 2+ and PO 4 3− ). NaMgPO 4 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).