Operando Synchrotron Radiation X-Ray Diffraction Study on Phase Evolution of Nb2GaC MAX during Molten Salt Etching

Lewis acidic molten salt method is a promising synthesis strategy to achieve MXenes with controllable surface terminations from numerous MAX materials. However, the molten salt etching strategy is not totally controllable and most experimental parameters are usually set empirically due to the unclear reaction process. Understanding the phase evolution chemistry and time-dependency during etching and post-processing is highly desirable, but still a key challenge due to the lack of operando characterizations and the complexity of the reaction process. Herein, we introduce an operando synchrotron radiation X-ray diffraction (SRXRD) technique to unveil the phase evolution process of Nb₂GaC MAX under molten-salts ambient. Several critical steps are demonstrated, including ultra-fast evolution from Nb₂GaC to Nb₂CuC, quick extraction of Cu atoms to form Nb₂CT_x MXene, and interlayer expansion caused by Cl grafting. Accordingly, we propose a controllable synthesis of Nb₂CT_x through precise temperature-controlling and time-adjusting. Afterward, the phase structure of Nb₂CT_x is also successfully tailored from hexagonal to amorphous by the time-dependent subsequent persulfate oxidation. The amorphous Nb₂CT_x with well-patterned morphology and numerous chloride terminations exhibits highly improved specific capacity, rate, and long cycling capability in lithium-ion batteries (LIBs). In conclusion, the operando SRXRD with online devices is promising for the dynamic study of the Lewis acid molten salt etching process such as phase evolution, lattice change, and etching time scale, which could not be provided by ex-situ methods.