The redox reaction of 2, 2, 6, 6-tetramethylpiperidine-l-oxyl (TEMPO) has been investigated in a solvate ionic liquid composed of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) and tetraglyme (G4). The one-electron transfer and quasi-reversible redox reaction of TEMPO+/TEMPO was observed in the stable electrochemical potential window of the solvate ionic liquid. The formal potential of TEMPO'S/TEMPO was 0.19 V vs. Ferrocene/ferrocenium. The diffusion coefficients of TEMPO and TEMPO+at 298 K were 1.8 × 10-7 and 7.6 × 10-8cm2 s-1, respectively, reflecting the coulombic interaction between the charged species with the anion of the solvate ionic liquid. The standard rate constant for the redox reaction of TEMPO+/TEMPO at 298 K was 1.9 × 10-4 cm s-1. Charge-discharge of a lithium secondary cell using a lithium-ion conductive glass-ceramics (LICGC) as a diaphragm and [Li(G4)]TFSA containing TEMPO as a catholyte was found to be possible with the average discharge voltage of 3.4 V.