Comparing calendar and cycle life stability of redox active organic molecules for nonaqueous redox flow batteries

Abstract

High stability of energy-rich redox active organic molecules (ROMs) in all states of charge is required for reliable operation of nonaqueous redox flow batteries (NRFBs), in which charged ROMs are used to store electric energy in external reservoirs. Calendar life stability and cycle life stability characterize capacity fade during storage of charged ROMs in the reservoirs vs. continuous cycling of the electrochemical cell. For insufficiently understood reasons, these two metrics of cell performance can be at odds with each other. In this study, we examine ROM systems consisting of dialkoxybenzene and 2,1,3-benzothiadiazole derivatives. By varying ROM structure and electrolyte composition, chemical stability of the charged states is varied over a wide range, and calendar life and cycle life stabilities are compared. For ROM systems that exhibit the highest chemical stability, the cycle life is largely (but not exclusively) limited by parasitic reactions involving the crossover of reaction products between the cell compartments. It appears that in many instances the cycling performance is strongly affected by poor membrane selectivity.