Conformational Control as a Design Strategy to Tune the Redox Behavior of Benzotriazole Negolytes for Nonaqueous Flow Batteries

Here, we present a molecular engineering strategy to tune the reduction potentials of benzotriazole derivatives as high-energy-density negolytes in nonaqueous redox flow batteries. Within nonaqueous electrolytes, these derivatives, notably 2-(<i>o</i>-tolyl)-2<i>H</i>-benzo[<i>d</i>][1,2,3]triazole (<b>1</b>), demonstrate a theoretical capacity of up to 93.8 Ah L^-1 and a reduction potential of -2.35 V vs ferrocene/ferrocenium (Fc/Fc⁺). Introducing dimethyl substitution (i.e., 2-(2,6-dimethylphenyl)-2<i>H</i>-benzo[<i>d</i>][1,2,3]triazole (<b>4</b>)) shifts the reduction potential even more negatively to -2.55 V vs Fc/Fc⁺. We ascribe the nonlinear effect of dimethyl substitution on reduction potential to ground-state conformational effects. Flow battery tests with negolyte <b>1</b> and ferrocene posolyte demonstrated >90% Coulombic efficiency at 6.7 mA cm^-2 with improved cyclability in the presence of lithium bis(trifluoromethylsuylfonyl)imide supporting salt.