Conjugation effect of amine molecules in non-aqueous Mg redox flow batteries

Abstract

Nonaqueous magnesium redox flow batteries (Mg RFBs) are attractive for low-cost, high-energy-density and long-cycle-life stationary energy storage applications. However, state-of-the-art cathode redox-active molecules suffer from low solubility and low redox potential. Herein, we screened a range of cathode redox-active molecules and identified amine molecules as optimal to couple with Mg anode. The properties of amine derivatives and their performances were collected to establish the correlation between molecular structures and electrochemical performances. The results confirm that the redox potential and solubility of these amine molecules are influenced by π-conjugated and non-conjugated structures of amine derivatives. Density functional theory (DFT) simulations and inverse aromatic fluctuation index (FLU^-1) verified that the conjugation plays an important role in stabilizing the molecule and increasing its redox potential. Notably, tris[4-(diethylamino)phenyl]amine (TDPA) achieves the highest theoretical energy density (~120 Wh/L) due to its high solubility (~0.9 M) and voltage (~2.5 V vs. Mg/Mg²⁺). The study also demonstrates that ether solvents are crucial for stable, high-solubility catholytes, while bulk anions do not affect the redox potential of these p-type molecules. In a Mg-amine RFB configuration, the battery delivered 2.50 V, 106.5 mAh/g specific discharge capacity, 90.74% initial Coulombic efficiency, and 93.88% capacity retention after 150 cycles.