Dibenzodioxin-Based\nPolymers of Intrinsic Microporosity\nwith Enhanced Transport Properties for Lithium Ions in Aqueous Media

Boosting the transport\nand selectivity properties of\nmembranes\nbased on polymers of intrinsic microporosity (PIMs) toward one specific\nworking analyte of interest is challenging. In this work, a novel\nfamily of PIM membranes, prepared by casting and exhibiting optima\nmechanical properties and high thermal stability, was synthesized\nfrom 4,4′-(2,2,2-trifluoro-1-phenylethane-1,1-diyl) bis(benzene-1,2-diol)\nand two tetrafluoro-nitrile derivatives. Gas permeability measurements\nevidenced a CO<sub>2</sub>/CH<sub>4</sub> selectivity up to 170% relative\nto the reference polymer, PIM-1, in agreement with their calculated\nfractional free volume and the analysis of the textural properties\nby N<sub>2</sub> and CO<sub>2</sub> gas adsorption. Besides, the chemical\nmodification by acid hydrolysis of the PIM membranes favored the permeability\nfor lithium ions (LiCl 2M, 6 × 10<sup>–9</sup> cm<sup>2</sup>·s<sup>–1</sup>) compared to other alkali metal\nanalogs such as sodium (NaCl 2M, 7.38 × 10<sup>–10</sup> cm<sup>2</sup>·s<sup>–1</sup>) and potassium (KCl 2M,\n1.05 × 10<sup>–9</sup> cm<sup>2</sup>·s<sup>–1</sup>). Moreover, the complete mitigation of the crossover of redox species\nwith higher molecular sizes than the ions from alkali metal salts\nwas confirmed by using <i>in-line</i> benchtop NMR methods.\nAdditionally, the modified PIM membranes were measured in a symmetric\nelectrochemical flow cell using an aqueous electrolyte by combining\nlithium ferro/ferricyanide redox compounds and lithium chloride. The\nelectrochemical tests showed low polarization, high-rate capability,\nand capacity retention values of 99% when cycled at 10 mA·cm<sup>–2</sup> for over 50 cycles. Based on these results, these\npolymers could be used as highly selective and conducting membranes\nin electrodialysis for lithium separation and lithium-based redox\nflow batteries and as a protective layer in high-energy density lithium\nmetal batteries.