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

• Published in: Advanced materials (Weinheim) 2023-10, Vol.35 (42), p.e2301314-n/a
• Main Authors: Xu, Weiliang, Dang, Rongbin, Zhou, Lin, Yang, Yang, Lin, Ting, Guo, Qiubo, Xie, Fei, Hu, Zilin, Ding, Feixiang, Liu, Yunpeng, Liu, Yuan, Mao, Huican, Hong, Juan, Zuo, Zhanchun, Wang, Xiaoqi, Yang, Rui, Jin, Xu, Hou, Xueyan, Lu, Yaxiang, Rong, Xiaohui, Xu, Ning, Hu, Yong‐Sheng
• Format: Article
• Language: English
• Subjects: Cathodes; Electrode materials; Electrolytes; Electrolytic cells; Interface reactions; Ion currents; Materials science; Na‐ion batteries; Performance degradation; Performance enhancement; Phase transitions; Rechargeable batteries; residual alkali; reversible phase transitions; Sodium-ion batteries; Solid electrolytes
• ISSN: 0935-9648; 1521-4095
• DOI: 10.1002/adma.202301314

The 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). NaMgPO4 is used to modify the interface of the O3‐NaNi0.4Cu0.1Mn0.4Ti0.1O2 (NCMT) to reduce surface residual alkali and improve cycle performance. The excellent interface facilitates the transport of Na+ and reduces the dissolution of transition metals (TMs). A high discharge capacity of 157.3 mAh g−1 and a superior stability under the 4.3 V cutoff voltage are obtained successfully in assembled full cells.