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Redox flow batteries (RFBs) are among the most promising grid‐scale energy storage technologies. However, the development of RFBs with high round‐trip efficiency, high rate capability, and long cycle life for practical applications is highly restricted by the lack of appropriate ion‐conducting membranes. Promising RFB membranes should separate positive and negative species completely and conduct balancing ions smoothly. Specific systems must meet additional requirements, such as high chemical stability in corrosive electrolytes, good resistance to organic solvents in nonaqueous systems, and excellent mechanical strength and flexibility. These rigorous requirements put high demands on the membrane design, essentially the chemistry and microstructure associated with ion transport channels. In this Review, we summarize the design rationale of recently reported RFB membranes at the molecular level, with an emphasis on new chemistry, novel microstructures, and innovative fabrication strategies. Future challenges and potential research opportunities within this field are also discussed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Widespread implementation of redox flow batteries (RFBs) is restricted by inefficient ion conducting membranes. It is especially challenging for membranes to achieve high ionic conductivity, high ion selectivity, and excellent chemical stability, simultaneously. Herein, we demonstrate a versatile solution to this toughest problem via designing multi-functional sidechain topology and manipulating their arrangement in ion conducting membranes. A series of comb-shaped ion conducting membranes with variable sidechains, along which the number of hydrophilic sites is different, are constructed, and their structure-performance relationships are comprehensively explored. Specifically, in the case of the same number of hydrophilic sites, the membranes with densely grafted short chains (the "stone") outperform those with loosely grafted long chains in terms of proton conductivity and ion selectivity (the first two "birds"), which is attributed primarily to the interconnected and size-constrained hydrophilic channels formed in the former, as demonstrated by an integrated experimental and simulation study. Furthermore, the former arrangement improves chemical stability of the membranes (the third "bird") since the size-constrained channels hamper the offensive active species from entering. These benefits result in higher energy efficiencies (EE: 92.7–71.7% at 40–200 mA cm−2) and slower capacity decay rate (CDR: 0.09% per cycle) of vanadium RFBs compared with the membranes with loosely grafted long chains (EE: 90.8–66.7% at 40–200 mA cm−2, CDR: 0.15% per cycle). This work sheds light on the path to comprehensively outstanding ion conducting membranes via molecular engineering, for energy-related devices and beyond. [Display omitted] • Sidechain topology & graft density have great impact on VRFB membrane. • Densely grafted short chain (DGSC) outperforms loosely grafted long chain (LGLC). • DGSC induced interconnected and size-constrained hydrophilic channels. • DGSC is advantageous for proton conductivity, ion selectivity & chemical stability. • DGSC enabled VRFB with high efficiencies and a slow capacity decay of 0.09%/cycle. [ABSTRACT FROM AUTHOR]