Redox flow batteries have attracted much attention as the large-scaled energy storage device for such renewable energy sources as solar cells and windmills. Typical redox flow batteries use aqueous electrolytes because of low cost, low toxicity and high safety. However, the electromotive force of the redox flow batteries using aqueous electrolytes is limited by the electrochemical potential window of the aqueous electrolytes. The enlargement of the electromotive force is expected by using non-aqueous aprotic electrolytes. Although aprotic organic electrolytes are typical of non-aqueous electrolytes, flammability of organic solvents is unfavorable for the large-scaled batteries from the point of view of safety. The equimolar mixture of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) and tetraglyme (G4) has been known to form the solvate ionic liquid consisting of a stable complex cation, [Li(G4)]⁺ and an anion, TFSA^– [1]. The solvate ionic liquid has several favorable properties, such as high thermal stability, high Li ion concentration and low vapor pressure. In the present study, the redox reaction of tris(2,2'-bipyridine)iron(III/II) complexes, [Fe(bpy)₃]^3+/2+, has been investigated in the solvate ionic liquid containing carbon fiber as an electronic conductor aiming at its application to non-aqueous redox flow batteries.

The redox reaction of [Fe(bpy)₃]^3+/2+ was observed at 0.36 V vs. Ag|Ag(I), which was corresponding to about 4 V vs. Li|Li(I), in [Li(G4)]TFSA containing 10 mM [Fe(bpy)₃](TFSA)₂. The diffusion coefficients of [Fe(bpy)₃]²⁺ and [Fe(bpy)₃]³⁺ were estimated to be 4.4 × 10^–8 and 3.1 × 10^–8 cm² s^–1, respectively, at 25 °C. The slow diffusion of the electroactive species was ascribed to the high viscosity of [Li(G4)]TFSA (122 mPa s [2]) and disadvantageous to battery application because the faradaic current is limited by diffusivity of the species. In order to diminish the diffusion limitation of the faradaic current, carbon fiber was added to the solvate ionic liquid as an electronic conductor. The carbon fiber was dispersed homogeneously in the solvate ionic liquid. The electronic conduction through the carbon fiber was confirmed by electric conductivity measurement. The redox reaction of [Fe(bpy)₃]^3+/2+ was found to be possible on the carbon fiber dispersed in the solvate ionic liquid. The reversible charge-discharge of a cell composed of Li anode and the solvate ionic liquid containing [Fe(bpy)₃]^3+/2+ and the carbon fiber using a solid-state Li ion conductor was confirmed to be possible with the average cell voltage of 3.8 V and the coulombic efficiency of 99%.

References

[1] K. Yoshida, M. Tsuchiya, N. Tachikawa, K. Dokko, and M. Watanabe, J. Phys. Chem. C, 115, 18384 (2011).

[2] H. Hirayama, N. Tachikawa, K. Yoshii, M. Watanabe, and Y. Katayama, Electrochemistry, 83, 824 (2015).