Redox Reaction of 2,2,6,6-Tetramethylpiperidine-1-Oxyl (TEMPO) in Lithium Bis(trifluoromethylsulfonyl)Amide-Tetraglyme Solvate Ionic Liquid

Abstract

Electrode reactions of the redox couples dissolved in liquid electrolytes are able to be utilized for energy storage devices. Vanadium redox flow battery is one of the typical large-scaled energy storage devices using aqueous solutions for both anolye and catholyte.[1] The cell voltages of such aqueous redox flow batteries are often limited by the electrochemical potential window of water. In order to enlarge the cell voltage and increase energy density, it is necessary to use aprotic electrolytes having wider electrochemical potential windows. Typical aprotic electrolytes used for non-aqueous batteries like lithium-ion batteries are composed of organic solvents, which are volatile and flammable. However, such organic electrolytes are unfavorable especially for the large-scaled batteries requiring large amount of electrolytes because of safety in case of accident. Therefore, it is necessary to select less flammable electrolytes for non-aqueous redox flow batteries. The equimolar mixture of lithium bis(trifluoromethylsulfonly)amide (LiTFSA) and tetraglyme (G4) has been known to form the solvate ionic liquid, which is composed of a lithium-G4 complex, [Li(G4)]+, and TFSA–, and exhibits such properties as less volatility, less flammability, and wide electrochemical potential window, similar to those of ionic liquids.[2,3] The high concentration of lithium species in the solvate ionic liquid is also advantageous to the batteries using lithium ion as a charge carrier. Since the cathodic limit of the solvate ionic liquid is determined by deposition and dissolution of lithium, it is possible to construct the batteries combining a lithium anode with the redox reactions dissolved in the solvate ionic liquid using a lithium-ion conductive solid-state electrolyte as a separator between the anolyte and catholyte. We have already proposed the battery using the redox reaction of tris(2,2'-bipyridine)iron(III/II) complexes in the solvate ionic liquid with the cell voltage of about 4 V.[4] In the present study, the redox reaction of a stable free radical, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), was investigated in the solvate ionic liquid, [Li(G4)]TFSA, for application to the redox flow battery. Both TEMPO and its oxidized form, TEMPO+, were found to be soluble in the solvate ionic liquid at ambient temperature. The redox reaction of TEMPO+/TEMPO was observed in the cyclic voltammetry of a glassy carbon electrode in [Li(G4)]TFSA containing TEMPO. The diffusion of TEMPO was found to be faster than that of TEMPO+, probably due to coulombic interaction of TEMPO+ with the ions composing the solvate ionic liquid. The standard rate constant was determined by electrochemical impedance spectroscopy. The temperature dependence of the standard rate constant suggested the redox reaction of TEMPO+/TEMPO can be regarded as an outer-sphere electron transfer reaction with a small reorganization energy. The redox battery was constructed using a lithium anode and the catholyte of the solvate ionic liquid containing TEMPO with a thin lithium-ion conductive solid-state electrolyte membrane. The redox battery could be operated successfully with the average discharge voltage of 3.4 V.