The use of 2,2,6,6-tetramethylpiperinyl-oxides and derivatives for redox shuttle additives in Li-ion cells

The stable radical, 2,2,6,6-tetramethylpiperinyl-oxide (TEMPO), is shown to be a stable redox shuttle in Li4/3Ti5/3O4/LiFePO4 Li-ion coin cells providing over 120 cycles of shuttle-protected overcharge. Derivatives of TEMPO, such as 4-methoxy-TEMPO and 4-cyano-TEMPO are also stable. Relatives of TEMPO, having a five-membered ring, such as 3-cyano-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (3-cyano-PROXYL) show similar stability. One disadvantage of these molecules is their relatively low oxidation potentials, which are too close to that of LiFePO4 for commercial applications. Ab initio calculations show that the redox potential of these molecules can be tailored by substitutions of fluorine for the hydrogen atoms in the methyl groups. (c) 2006 The Electrochemical Society.