Electrochemical properties of aryladamantanes as new overcharge protection compounds for lithium cells

The development of new overcharge protection compounds for 4 V-class lithium cells with LiCoO2 cathodes was attempted in order to protect the cells from the hazards of ignition and/or explosion when overcharged. At least two requirements must be met as regards overcharge protection compounds. They must have an oxidation potential (Eox) in the 4.6–4.8 V range versus Li/Li+ and they must not adversely influence the lithium cycling efficiency (Eff). The overcharge protection compounds were used by adding them to the electrolyte solution, which was LiClO4/propylene carbonate. First, this study involved a basic examination of the relationships among the Eox values of arenes including polyacenes, naphthalene derivatives and methylbenzenes, their chemical structure, the aromatic ring size and the effects of substituent functional groups. Next, the Eox and Eff values of cycloalkanes including cyclohexane, decaline, adamantane and norbornane were examined. Based on these examination results, various adamantane derivatives were synthesized and their electrochemical properties were investigated. Finally, four aryladamantanes (1-(p-tolyl)adamantane, 1-(m-tolyl)adamantane, 1-(o-tolyl)adamantane, 1-(4-ethylphenyl)adamantane) were found to exhibit better Eox and Eff values than those of biphenyl, which is already used in commercial cells as an overcharge protection additive.