Two-dimensional Nb-based M4C3Tx MXenes and their sodium storage performances

Abstract The MAX phases and MXenes are attractive materials for diverse applications. In this work, the two-dimensional Nb3.5Ta0.5C3Tx and Nb3.9W0.1C3Tx MXenes and the based phase Nb4C3Tx (where Tx is a surface termination) were synthesized by selective etching off aluminum, Al, from their corresponding MAX phase ceramics Nb3.5Ta0.5AlC3, Nb3.9W0.1AlC3 and Nb4AlC3. The XRD results indicate that the intermediate Al layers have been removed as the intensity of the major peaks of all MAX ceramics were reduced. The typical multilayered structure also confirms the successful synthesis of MXenes. X-ray photoelectron spectroscopy (XPS) shows that Nb0 exists in the three MAX ceramics owing to the metallic Nb-Nb bonding, which is inherent in the MAX ceramics. Since oxidation in the etching process is inevitable, there are some high-valent oxidation states exhibiting in the M-site elements. The electrochemical performance of the as-prepared MXenes towards sodium-ion batteries (SIBs) was further examined by cyclic voltammetry (CV), galvanostatic charge-discharge and rate performance tests. The results exhibit that the based phase Nb4C3Tx shows better electrochemical performance than Nb3.5Ta0.5C3Tx and Nb3.9W0.1C3Tx.