Spatial-Compatibility-Assisted Molecular Intercalation in MXenes

MXenes have emerged as versatile platforms for electrochemical energy storage, stimuli-responsive systems, and nanofluidic technologies, owing to their compositionally tunable surfaces and diverse intercalation chemistries. Although ion intercalation in aqueous or organic media is widely employed to modulate the interlayer structure and electronic state of MXenes, the identification of compatible intercalants remains largely empirical. A generally accepted quantitative framework for evaluating the feasibility of guest incorporation into MXene hosts has not yet been established. Here, we propose a general steric-compatibility criterion, in which intercalation becomes geometrically feasible when the effective 3D size of an intercalant molecule, as defined from its molecular crystal structure (i.e., considering a single molecule in the crystalline state), is comparable to or smaller than the interlayer spacing (Δd) created by Al extraction in the parent MAX phase. Guided by this principle, a series of sterically compatible small molecules can be screened, and their intercalation/etching behavior can be rationalized. This study establishes a universal geometry-based guideline for evaluating guest-host compatibility in 2D MXene systems and lays the foundation for predictive design of MXene intercalation chemistries.