Metal coordination complexes in nonaqueous redox flow batteries

The ongoing search for new electroactive materials for next-generation redox flow batteries has within the last decade encompassed metal-ligand coordination chemistry. Here, we review the handful of metal coordination complexes proposed as redox flow battery electrolytes. We highlight examples with careful ligand design, driving research towards higher energy density storage materials. Metal coordination complexes designed to be highly soluble not only in the initial redox state but also in all charged states accessed during the battery cycle give particularly impressive performances. Optimisation of flow cell conditions for metal coordination complexes remains largely unexplored, with most complexes screened in nonideal H-cell experiments with little investigation into membrane and electrode materials.