Mechanism guided two-electron energy storage for redox-flow batteries using nickel bis(diphosphine) complexes

The storage of multiple electrons per molecule can greatly enhance the energy density of redox-flow batteries (RFBs). Here, we show that nickel bis(diphosphine) complexes efficiently store multiple electrons through either sequential 1e- redox waves or a concerted 2e- redox wave, depending on their coordination environment. Mechanistic studies comparing ligand sterics (-Me vs. -Ph) and coordination of monodentate ligands (MeCN vs. Cl-) allow for selective control of the electron transfer pathway, steering electron storage toward the more favorable 2e- wave. Continuous charge-discharge cycling experiments show more negative charge-discharge potentials and improved capacity retention in the presence of Cl-, thus improving the energy storage of nickel bis(diphosphine) complexes as anolytes in RFBs. This work shows how mechanistic understanding of 2e- redox cycles for transition metal complexes can create new opportunities for multi-electron storage in RFBs.