The non-aqueous redox flow battery (NARFB) is a promising device for grid-scale energy storage. However, its electrochemical performance and long-term stability still need to be improved. This work shows that the electrochemical performance of NARFB can be remarkably enhanced by optimizing the supporting electrolyte and separator. The battery with 2,1,3-benzothiadiazole (BzNSN) as anolyte, 2,5-di-tert-butyl-1-methoxy-4-[2′-methoxyethoxy]benzene (DBMMB) as catholyte, tetraethylammonium bis(trifluoromethylsulfonyl)imide (TEATFSI) as supporting electrolyte, and porous Daramic 250 separator delivers an average discharge capacity of 1.7 Ah L−1, voltage efficiency of 87.8%, Coulombic efficiency of 89.1%, and energy efficiency of 78.3% over 100 cycles at the current density of 40 mA cm−2 with 0.1 M active materials. The reasons for the capacity decay over cycling in terms of electrolyte leakage, crossover, and chemical decomposition are discussed.