High‐Performance All‐Vanadium Redox Flow Batteries Enabled by BPA ‐Based Rigid‐Flexible Synergistic SPEEK Membrane

ABSTRACT The development of high‐performance, low‐cost ion exchange membranes is crucial for advancing vanadium redox flow battery (VRFB) technology. While sulfonated poly(ether ether ketone) (SPEEK) is a promising alternative to costly Nafion, its performance is limited by excessive swelling at high sulfonation degrees and poor chemical stability. Herein, we report the rational design of a SPEEK‐based copolymer incorporating a rigid‐flexible synergistic backbone by employing Bisphenol A. Molecular dynamics (MD) simulations revealed that the introduction of BPA significantly increases the bond energy of the ether linkages, which are the most vulnerable sites in the polymer chain. This molecular‐level reinforcement, combined with enhanced interchain interactions from the flexible BPA segments and controlled sulfonation, effectively suppresses water‐induced swelling and improves dimensional stability. Consequently, the optimized membrane exhibits remarkably low vanadium permeability and superior chemical durability. The assembled VRFB achieves outstanding performance, delivering high energy efficiencies of 80.27% at 200 mA cm −2 and maintaining 81.25% over 500 cycles (~450 h) at 80 mA cm −2 . This work provides a simple yet effective strategy for developing high‐performance VRFB membranes and offers deep molecular‐level insights into the design of robust polymer electrolytes.