Ion Selective Bifunctional Metal–Organic Framework-Based Membrane for Lithium Metal-Based Nonaqueous Redox Flow Battery

Nonaqueous redox flow batteries (NARFBs) hold potential application as an electricity energy storage for intermittent renewable energy and can operate with high voltage and energy density. However, their further development is impeded by lack of a proper membrane. Herein, a flexible freestanding anionic metal–organic framework (MOF)-based membrane is prepared through a solution casting method, with an anionic MOF (UiO-66-SO3Li) and polyvinylidene fluoride. The prepared membrane shows an impressive ionic selectivity (37.6) of Li+ to N-(ferrocenylmethyl)-N,N-dimethyl-N-ethylammonium (Fc1N112+) ions and high ionic conductivity. Benefiting from the unique micropore structure of MOF and the anionic transport channels of Li+ across the membrane, the resultant Li-based hybrid NARFB delivers a high-capacity retention (99.95% per cycle) over 500 cycles with a well-assembled stable interphase after long time lithium plating/stripping, which decreases the voltage efficiency during the cycling. Calculations reveal that the membrane easily desolvates Li+ in the unique micropore structure of MOF before Li deposition occurs, which suppresses parasitic reaction and accelerates Li uniform deposition. This work provides a viable method to design bifunctional NARFB membranes which achieve ion sieving and ion exchange functions.