Highly Soluble Tris(2,2’-bipyridine) metal bis(trifluoromethanesulfonyl)imide complexes for high energy organic redox flow batteries

Nonaqueous redox flow batteries (RFBs) with an organic electrolyte can work at voltages higher than 2.1 V, thanks to the excellent electrochemical stability of such electrolytes. However, the biggest challenge related to nonaqueous RFBs is that the electrochemically active salts used in these batteries exhibit lower solubilities in organic electrolytes compared to those in aqueous electrolytes. In this study, bis(trifluoromethanesulfonyl)imide (TFSI) is investigated as the counter anions for the complexes of iron and nickel tris(2,2’-bipyridine) ((Bpy)3) for use as electrolytes in high-energy RFBs. The synthesized salts were characterized systematically through ab-initio calculations, nuclear magnetic resonance (NMR), inductively coupled plasma mass spectrometry (ICP-MS) and electrochemical analyses. Ni(Bpy)3(TFSI)₂ and Fe(Bpy)3(TFSI)2 containing bulky imide anions exhibit the improved solubility by weakening inter-anion Coulombic interactions, via charge delocalization.