Fluoride-regulated phase evolution in Fe-based metal–organic frameworks enables enhanced lithium adsorption

Efficient lithium extraction from aqueous media remains challenging due to the intrinsically low concentration of Li + and severe competition from coexisting ions, which places stringent requirements on the coordination environment and topology of adsorption sites in adsorbents. In this context, fluoride-mediated crystal phase engineering offers an effective route to regulate metal cluster assembly in metal–organic frameworks (MOFs). Here, this research demonstrates that fluoride incorporation regulates the phase evolution of Fe-based MOFs, promoting the formation of chain-structured TJU-21 from MIL-100(Fe)-type frameworks under appropriate synthesis conditions. This phase evolution led to significant reconstruction of the iron coordination environment, thereby increasing surface electronegativity and the density of accessible Fe F active sites. The TJU-21 exhibits a lithium adsorption capacity of 44.6 mg·g −1 , representing a 45.7% improvement compared with MIL-100(Fe). Mechanistic analysis indicates that inorganic chain-based nodes intrinsically favor Li + binding, while fluorine coordination contributes to the stabilization of open metal sites and enhanced host guest interactions. This work demonstrates that fluoride-mediated phase engineering is a promising strategy for regulating metal cluster topology and functionality in Fe-based MOFs, with potential implications for lithium extraction.