LiFePO4/Li4/3Ti5/3O4 Li-ion cells have been investigated by many groups and their behavior in standard electrolytes such as 1 M LiPF6 ethylene carbonate: diethyl carbonate (EC: DEC) is well known. Here we report on the behavior of these cells with 2,5-ditertbutyl-1,4-dimethoxybenzene added to the electrolyte as a redox shuttle additive to prevent overcharge and overdischarge. We explore methods to increase the current-carrying capacity of the shuttle and explore the heating of practical cells during extended overcharge. The solubility of 2,5-ditertbutyl-1,4-dimethoxybenzene was determined as a function of salt concentration in lithium bis-oxolatoborate-(LiBOB) and LiPF6-containing electrolytes based on propylene carbonate (PC), EC, DEC, and dimethyl carbonate (DMC) solvents. Concentrations of 2,5-ditertbutyl-1,4-dimethoxybenzene up to 0.4 M can be obtained in 0.5 M LiBOB PC:DEC (1: 2 by volume). Coin-type test cells were tested for extended overcharge protection using an electrolyte containing 0.2 M 2,5-ditertbutyl-1,4-dimethoxybenzene in 0.5 M LiBOB PC:DEC. Sustained overcharge protection at a current density of 2.3 mA/cm(2) was possible and hundreds of 100% shuttle-protected overcharge cycles were achieved at current densities of about 1 mA/cm(2). The diffusion coefficient of the shuttle molecule in this electrolyte was determined to be 1.6 x 10(-6) cm(2)/s from cyclic voltammetry and also from measurements of the shuttle potential vs. current density. The power produced during overcharge was measured using isothermal microcalorimetry and found to be IV as expected, where I is the charging current and V is the cell terminal voltage during shuttle-protected overcharge. Calculations of the temperature of 18650-sized Li-ion cells as a function of time during extended shuttle-protected overcharge at various C-rates are presented. These show that Li-ion cells need external cooling during extended shuttle-protected overcharge if currents exceed about C/5 rates. © 2005 The Electrochemical Society. All rights reserved.