Molten Salt‐Shielded Synthesis (MS 3 ) of MXenes in Air

MXenes are two‐dimensional transition metal carbides and/or nitrides with unique physiochemical properties and have attracted extensive interest in numerous fields. However, current MXene synthesis methods are limited by hazardous synthesis conditions, high production costs, or difficulty in large‐scale production. Therefore, a general, safe, cost‐effective, and scalable synthesis method for MXenes is crucial. Here, we report the fast synthesis of MXenes in the open air using a molten salt‐shielded synthesis (MS 3 ) method, which uses Lewis‐acid salts as etchants and a low‐melting‐point eutectic salt mixture as the reaction medium and shield to prevent MXene oxidation at high temperatures. Carbide and nitride MXenes, including Ti 3 C 2 T x , Ti 2 CT x , Ti 3 CNT x , and Ti 4 N 3 T x , were successfully synthesized using the MS 3 method. We also present the flexibility of the MS 3 method by scaling the etching process to large batches of 20 and 60 g of Ti 3 AlC 2 MAX precursor in one pot. When used as negative electrodes, the prepared MS 3 ‐MXenes delivered excellent electrochemical properties for high‐rate Li‐ion storage.