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

Abstract

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‐NaNi0.4Cu0.1Mn0.4Ti0.1O2 (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@NaNi0.4Cu0.1Mn0.4Ti0.1O2‐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).