Mechanism-Based Development of a Low-Potential, Soluble, and Cyclable Multielectron Anolyte for Nonaqueous Redox Flow Batteries

The development of nonaqueous redox flow batteries (NRFBs) has been impeded by a lack of electroactive compounds (anolytes and catholytes) with the necessary combination of (1) redox potentials that exceed the potential limits of water, (2) high solubility in nonaqueous media, and (3) high stability toward electrochemical cycling. In addition, ideal materials would maintain all three of these properties over multiple electron transfer events, thereby providing a proportional increase in storage capacity. This paper describes the mechanism-based design of a new class of metal-coordination complexes (MCCs) as anolytes for NRFBs. The tridentate bipyridylimino isoindoline (BPI) ligands of these complexes were designed to enable multielectron redox events. These molecules were optimized using a combination of systematic variation of the BPI ligand and the metal center along with mechanistic investigations of the decomposition pathways that occur during electrochemical cycling. Ultimately, these studies led to the identification of nickel BPI complexes that could undergo stable charge-discharge cycling (<5% capacity loss over 200 cycles) as well as a derivative that possesses the previously unprecedented combination of high solubility (>700 mM in CH₃CN), multiple electron transfers at low redox potentials (–1.7 and –1.9 V versus Ag/Ag⁺), and high stability in the charged state for days at high concentration. Overall, the studies described herein have enabled the identification of a promising anolyte candidate for NRFBs and have also provided key insights into chemical design principles for future classes of MCC-based anolytes.