Amorphous Cu Intermediate Modification During Lewis Acid Etching of High‐Entropy MAX to MXene Using CuCl ₂

Abstract 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 CuCl ₂ to etch the high‐entropy MAX (HE‐MAX) phase, (TiVNbMoW) ₃ AlC ₂ . The investigation reveals that the reaction is hindered by the formation of a previously unreported amorphous intermediate structure (M ₃ C ₂ –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 M ₃ C ₂ –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.