Lithium - Redox Battery Using a Solvate Ionic Liquid

Abstract

Redox flow batteries have been expected as the large scale storage battery. Redox flow batteries utilize the oxidation and reduction of two soluble redox couples for charging and discharging without the solid phase reaction. Therefore, the cycle lives of the redox flow batteries are expected to be much longer than those of rechargeable lithium batteries using insertion-type active materials. Currently, vanadium redox flow batteries have been put into practical use to stabilize the supply of renewable energy. Vanadium redox flow batteries have low energy density compared to other batteries. Since aqueous solution is used for the electrolyte, the electrochemical window of the water limits the cell voltage. Although there has been various studies using an organic electrolyte in order to enlarge the cell voltage, the flammability and volatility of the organic solvents can be disadvantageous from the point of view of safety. 1) Recently, aprotic ionic liquids have attracted attention as nonaqueous electrolytes. Especially, aprotic ionic liquids containing bis(trifluoromethylsulfonyl)amide (TFSA-) have been studied extensively in the electrochemical field because they show favorable features such as incombustibility, involatility and wide electrochemical potential window. We have already reported the redox reactions of various metal complexes in the ionic liquids and their applicability to redox flow batteries. 2) However, there are a few reports regarding the redox battery using the ionic liquids probably due to lack of the suitable ion exchange membrane that can move the career ion in the ionic liquids. In the present study, a new redox battery, namely “lithium-redox battery”, was proposed using a redox reaction of an iron complex for a cathode reaction, dissolution and deposition of lithium for an anode reaction, and LiTFSA – tetraglyme (G4) solvate ionic liquid for an electrolyte. In addition, lithium ion conductive solid electrolyte (LICGCTM) was used as the diaphragm, which prevents penetration of the redox species and enables transfer of lithium ions. Tris(2,2'-bipyridine)iron(II) complex ([Fe(bpy)2]2+) was found to be soluble and stable in the solvate ionic liquid. The redox reaction between [Fe(bpy)3]2+ and [Fe(bpy)3]3+ was confirmed to be reversible by cyclic voltammetry. The formal potential of [Fe(bpy)3]3+/2+ was 4.1 V vs. Li|Li(I). Reversible charge-discharge cycling of the lithium redox battery having an average cell voltage of 3.9 V was possible for 30 cycles with a coulombic efficiency of more than 95%. References 1) Y. Matsuda, K. Tanaka, M. Okada, Y. Takasu, and M. Morita, J. Appl. Electrochem., 18, 909 (1988). 2) Y. Katayama, M. Yoshihara, and T. Miura, J. Electrochem. Soc., 162, H501 (2015).