Triethylamine- (TEA-) enabled prequaternization of a polymer for nonalkaline anion-exchange membrane (AEM) fabrication is more facile and environmentally benign than the conventional trimethylamine-based postquaternization route. It is also more compatible with forming a microphase-separated membrane morphology that can facilitate ion transport. However, most studies of TEA-quaternized AEMs have reported unsatisfactory conductivities, and no examples of their application in all-vanadium flow batteries (VFBs) have been reported. In this work, we aim to address this issue and demonstrate that, by employing a favorable solvent, polysulfone can be prequaternized to a high level with TEA and further fabricated into an AEM showing good conductivity (18.2 mS cm(-1) at room temperature) and impressive VFB performance (Coulombic efficiency above 98% and energy efficiency above 80% at 80 mA cm(-2)). In contrast, when an unfavorable solvent is used, membrane quaternization does not occur significantly. This contrast is shown to result from the absence or presence of solvent-TEA competition during quaternization, which is related to the the electron-donating or -withdrawing properties of the solvents used. Our study adds new understanding to the quaternization chemistry of AEMs and also represents the first example, to our knowledge, of a TEA-quaternized AEM yielding high VFB efficiencies.