Spectroelectrochemical Studies of Redox Shuttle Overcharge Additive for LiFePO[sub 4]-Based Li-Ion Batteries

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Abstract
2,5-Di-tert-butyl-1,4-dimethoxybenzene has recently been shown to behave as an electrochemical shuttle for overcharge and overdischarge protection of LiFePO4∕graphiteLiFePO4∕graphite and LiFePO4∕Li4∕3Ti5∕3O4LiFePO4∕Li4∕3Ti5∕3O4 lithium-ion batteries. The redox chemistry of this molecule is investigated here using a spectroelectrochemical method inside modified UV-vis cells as well as voltammetry inside a custom-made cell using an Ag∕AgClAg∕AgCl reference electrode. The redox potential of 2,5-di-tert-butyl-1,4-dimethoxybenzene is about 3.92V3.92V vs. Li∕Li+Li∕Li+ . When clear solutions of 2,5-di-tert-butyl-1,4-dimethoxybenzene are oxidized, the radical cation is yellow in low concentrations (about 0.001M0.001M ) and yellow-green in solutions of higher concentration, making UV-vis studies of the stability of the molecule straightforward. In this paper, we show that reversible oxidation and reduction of this molecule is possible, provided it is not exposed to potentials above 4.2V4.2V vs. Li∕Li+Li∕Li+ . If the potential is raised above 4.2V4.2V , further oxidation followed by complete decomposition to another species occurs.