Controlling Lewis Acid Effect in the Pores of Metal-Organic Framework Host for Promoting Lithium-Ion Conduction

Metal-organic frameworks (MOFs) have emerged as promising solid-state electrolytes; however, an insufficient ion transference number and low ionic conductivity critically hinder their further application. Herein, a breakthrough in lithium-ion conduction has been realized through an innovative strategy of Lewis acidity controlling engineering, implemented by anion substitution to construct an isostructural MIL-101-X (X = NO₃^-, F^-, Cl^-, and Br^-) series featuring the adjustable Lewis acidity of MOF framework, which enables to establish a correlation between Lewis acid strength and Li⁺ transport performances. It demonstrated that the Lewis acidity strength follows the order MIL-101-Cl > MIL-101-Br > MIL-101-F > MIL-101-NO₃. Experiments combining theoretical analyses reveal that the MIL-101-Cl with the strongest Lewis acidity achieves the most effective anchoring for PF₆^- anions from LiPF₆, which facilitates efficient Li⁺ transport, resulting in a remarkable ionic conductivity (1.65 × 10^-3 S cm^-1), an impressive Li⁺ transference number (0.90), and a wide electrochemical window (5.6 V) for LiPF₆@MIL-101-Cl at room temperature. This study emphasizes the critical role of Lewis acidity in the porous MOF host impacting Li⁺ transport performances and provides valuable insights for the development of high-performance solid-state electrolytes.