Dithiolene Complexes of First-Row Transition Metals for Symmetric Nonaqueous Redox Flow Batteries.

Five metal complexes of the dithiolene ligand maleonitriledithiolate (mnt2- ) with M=V, Fe, Co, Ni, Cu were studied as redox-active materials for nonaqueous redox flow batteries (RFBs). All five complexes exhibit at least two redox processes, making them applicable to symmetric RFBs as single-species electrolytes, that is, as both negolyte and posolyte. Charge-discharge cycling in a small-scale RFB gave modest performances for [(tea)2 Vmnt ], [(tea)2 Comnt ], and [(tea)2 Cumnt ] whereas [(tea)Femnt ] and [(tea)2 Nimnt ] (tea=tetraethylammonium) failed to hold any significant capacity, indicating poor stability. Independent negolyte- and posolyte-only battery cycling of a single redox couple, as well as UV/Vis spectroscopy, showed that for [(tea)2 Vmnt ] the negolyte is stable whereas the posolyte is unstable over multiple charge-discharge cycles; for [(tea)2 Comnt ], [(tea)2 Nimnt ], and [(tea)2 Cumnt ], the negolyte suffers rapid capacity fading although the posolyte is more robust. Identifying a means to stabilize Vmnt 3-/2- as a negolyte, and Comnt 2-/1- , Nimnt 2-/1- , and Cumnt 2-/1- as posolytes could lead to their use in asymmetric RFBs.