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

The deposition of volatilized Na⁺ on the surface of the cathode during sintering results in the formation of surface residual alkali (NaOH/Na₂ CO₃ NaHCO₃ ) 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 O₂ (NCMT). In this study, a strategy is proposed to transform waste into treasure by converting residual alkali into a solid electrolyte. Mg(CH₃ COO)₂ and H₃ PO₄ are reacted with surface residual alkali to generate the solid electrolyte NaMgPO₄ on the surface of NCMT, which can be labeled as NaMgPO4@NaNi_0.4 Cu_0.1 Mn_0.4 Ti_0.1 O₂ -X (NMP@NCMT-X, where X indicates the different amounts of Mg²⁺ and PO₄ ^3- ). 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).