Amorphous Cu Intermediate Modification During Lewis Acid Etching of High-Entropy MAX to MXene Using CuCl₂

High-entropy MXenes (HE-MXenes) represent a highly promising frontier in 2D materials, but their safe, fluorine-free synthesis remains a critical challenge. Recently, Lewis acidic molten salt etching has been emerged as a promising alternative due to its high operational safety and precise regulation of MXene surface terminal groups. This work reports a strategy utilizing anhydrous CuCl₂ to etch the high-entropy MAX (HE-MAX) phase, (TiVNbMoW)₃AlC₂. The investigation reveals that the reaction is hindered by the formation of a previously unreported amorphous intermediate structure (M₃C₂-ClCu_x). However, this intermediate phase, trapped at a specific etchant concentration, degrades the material's electrochemical performance. By optimizing the etchant ratio, the adverse influence of the M₃C₂-ClCu_x on the electrochemical performance is effectively mitigated, enabling the successful synthesis of an accordion-like HE-MXene. The electrochemical energy storage performance of this HE-MXene is systematically evaluated in acidic and alkaline electrolytes. More importantly, this study not only presents a viable F-free synthetic route for HE-MXene but also reveals a novel reaction mechanism that is crucial for future process optimization and rational material design.