Molecular engineering, supporting electrolyte, and membrane selections for enhanced cycling stability of non-aqueous organic redox flow batteries: A review

Redox flow batteries (RFBs) have attracted researchers due to their decoupled nature of energy and power modulations, suitability for large-scale stationary energy storage, and integration of renewable intermittent energy sources such as solar and wind power. Water's narrow electrochemical stability window limits the energy density of aqueous redox flow batteries. Thus, a shift to non-aqueous organic redox flow batteries (NAORFBs) is necessary to achieve high energy density while benefiting from organic solvents' expansive electrochemical stability windows. Nonetheless, the degradation and crossover of organic electroactive materials cause rapid capacity loss in NAORFBs. To improve the cycling stability of NAORFBs, molecular engineering is required to enhance the stability of redox-active species, particularly charged species, and the solubility of redox-active species. An appropriate ion-selective membrane that mitigates crossover by selectively allowing the passage of ions of supporting salts needs to be developed. This review discusses molecular design strategies that may improve radical ion stability, increase the solubility of redox-active species, and reduce redox-active species crossover and the selection of appropriate supporting electrolytes and separators/membranes for the overall enhancement of the cycle life and performance.