An ionic liquid tetraethylammonium bis(trifluoromethylsulfonyl)imide (TEATFSI) is synthesized as the supporting electrolyte in an all-organic redox flow battery system with acetonitrile as the solvent. Benzophenone (BP) and 1,4-di-tert-butyl-2,5-dimethoxybenzene (DBB) are used as the redox active species. The battery exhibits a high open voltage of 2.95 V. The electrode reactions are controlled by diffusion, which is significantly influenced by the cations of the supporting electrolytes. With TEATFSI as the supporting electrolyte, the diffusion coefficients are 1.21-1.43×10-5 cm2 s-1 and 0.89-1.16×10-5 cm2 s-1 at anode and cathode, respectively. The coulombic, voltage and energy efficiencies of the battery are 97%, 46% and 44%, respectively. The battery shows a stable performance during 50 charge-discharge cycles.