A novel redox shuttle additive, 1,4-bis(trimethylsilyl)-2,5-dimethoxybenzene (BTMSDB), is shown to deliver superb overcharge protection of LiFePO₄ electrode in Li-ion batteries. Using this molecule as a chemical probe, we trace the cause of the eventual failure of this additive to the gradual loss of steric protection in the corresponding radical cation, providing the much needed mechanistic insight in the factors controlling the long-term efficiency of overcharge protection.