A research group led by Prof. HUANG Qing at the Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences (CAS) has developed a general Lewis acidic etching route for preparing MXenes with enhanced electrochemical performance in non-aqueous electrolyte, in cooperation with Université de Toulouse, Sichuan University and LiU. The study was published in Nature Materials. 

Two-dimensional (2D) transition metal carbides/nitrides (MXenes) are one of the newest members of 2D material family, which have attracted great attention as energy storage materials. MXenes are prepared by selective etching of the A-layer elements in MAX-phase precursors. Since the first report of Ti₃C₂ synthesis in 2011, the preparation of MXenes have mainly involved the selective etching of the A-layer of MAX phases by fluoride ion solution, such as HF (hydrogen fluoride). It is hazardous and the -F group affects electrochemical performance, which has limited the large-scale production and applications of MXenes. 

To achieve a fluorine-free route preparation of MXenes, the researchers proposed a general strategy to synthesize 2D MXenes through high-temperature molten salt Lewis acid route (experiment demonstration).

By constructing the Gibbs free energy mapping of the redox potential of cations with A-site elements and the stripping reaction of A-site element under high-temperature molten salt environment, an A-site etching method of MAX phases in Lewis acidic melts was proposed.  

In addition, they also carried out a systematic electrochemical energy storage study of Ti₃C₂T_x MXenes obtained through Lewis acid molten salt in LiPF₆ organic electrolyte. The synthesized MXene showed excellent cyclic stability, high capacity, high rate charge and discharge, and large potential window.  

Different from the HF etching, the high-temperature molten salt lewis acid stripping method facilitates the safe operation in the laboratory and the surface chemistry tunning. 

The study may promote the fabrication of Mxenes, promising MXenes a bright future for application in electrochemical energy storage devices, such as batteries and lithium-ion capacitors.