Fixed-Linkage Enabled Ultra-Stable Ion Transport Channels in Membranes for Long-Life Alkaline Flow Batteries

Alkaline zinc-iron redox flow batteries (Zn─Fe FBs) are promising candidates for developing high-voltage and cost-effective grid-scale energy storage systems. However, the low ionic conductivity and poor alkaline stability of membranes lead to battery performance degradation and shortened lifespan. Here, we develop fixed ethylene linkage-based polybenzimidazole (FE-PBI) membranes to break this dilemma. Introducing fixed ethylene linkages enlarges the ion transport channel and provides inter-support for polymer chains, thereby preserving highly ordered and flux ion transport channels in the alkaline environment. By this unique channel design, this FE-PBI membrane enables alkaline Zn-based asymmetric FBs to deliver excellent rate performance (up to 240 mA cm^-2) with high reversibility of Zn deposition. Furthermore, the Zn─Fe FBs using FE-PBI membranes deliver a highly stable rate performance across the range of 60-180 mA cm^-2 and 120 mAh cm^-2. At a challenging condition of high current density (100 mA cm^-2) and areal capacity (80 mAh cm^-2), the developed Zn─Fe FBs achieve an impressive energy efficiency of >85.56% and remarkable stability over 800 h (500 cycles), surpassing the lifespan performance of current alkaline Zn─Fe FBs. This work offers a new membrane design approach to combine high energy efficiency with an extended lifespan, realizing long-lifetime alkaline FBs.