Redox Mediator as an Overcharge Protection Agent for 4 V Class Lithium-Ion Rechargeable Cells

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Abstract
It is well-known that an aromatic compound such as biphenyl is added into electrolyte solutions to prevent lithium-ion cells from overcharging, where it generates hydrogen gas under overcharging conditions. We have examined the oxidative behaviors of one-benzene-ring aromatic compounds including benzene, toluene, ethylbenzene, cumene, tert-butylbenzene, and cyclohexylbenzene under the overcharging conditions. We have found that aromatic compounds without hydrogen atom at the benzylic position such as tert-butylbenzene generated mainly carbon dioxide gas, whereas those with hydrogen atom at the benzylic position showed polymerization accompanied by hydrogen gas evolution. It was considered that tert-butylbenzene works as a redox mediator, which mediates the oxidative decomposition of carbonate solvents evolving the carbon dioxide gas.