Gradient‐Distributed Metal–Organic Framework–Based Porous Membranes for Nonaqueous Redox Flow Batteries

Nonaqueous redox flow batteries (RFBs) have received significant research interest, but the lack of promising separators with advanced performance seriously hinders the development of nonaqueous RFBs. Here, a robust yet flexible membrane with enhanced selectivity for nonaqueous RFBs is designed via in situ synthesis of metal–organic frameworks (MOFs) in a porous polymeric membrane (Celgard) with a gradient density. The crossover of active species is mitigated by the reduced effective pore size while high ionic conductivity is maintained, which is attributed to the 3D channel structure of MOFs and their gradient distribution in the membrane. A Li/ferrocene RFB with the MOF‐imbedded membrane delivers an excellent high‐rate capability and enhanced cycling stability. The discharge capacity reaches as high as ≈94% of theoretical value at a current density of 4 mA cm−2, and maintains 76% even at 12 mA cm−2. Moreover, a much slower capacity decay rate is achieved (0.09% per cycle over 300 cycles) by using the composite membrane compared with the pristine Celgard membrane (0.24% per cycle). The demonstrated strategy provides new insight into rational design and fabrication of size‐sieving separators for RFBs and can promote further research of MOFs' capability in energy storage. A gradient‐distributed metal–organic framework (MOF)–based porous membrane is designed for nonaqueous redox flow batteries (RFBs). Crossover is largely suppressed by reducing pore size while high ionic conductivity is maintained, thanks to the 3D channel structure of MOFs and unique gradient distribution in the membrane. A Li/ferrocene RFB with a designed membrane is demonstrated with excellent high‐rate capability and long‐term stability.