The molecules 10-methylphenothiazine, 10-ethylphenothiazine, 3-chloro-10-methylphenothiazine, 10-isopropylphenothiazine, and 10-acetylphenothiazine are shown to be stable redox shuttle additives in LiFePO4/graphite and LiFePO4/Li4/3Ti5/3O4 Li-ion coin cells to protect against overcharge and overdischarge. The diffusion constant of 10-methylphenothiazine was measured using cyclic voltammetry to be 1.5 x 10(-6) cm(2)/s, which translates to maximum shuttle-protected overcharge current densities near 2 mA/cm(2) in practical cells. Although the redox potentials of these molecules (near 3.5 V) are somewhat low for LiFePO4, their stability over repeated overcharge and overdischarge cycles is about equal to that of 2,5-di-tert-butyl-1,4-dimethoxybenzene that has been shown to provide protection for over 200 cycles of 100% overcharge at C/10. Given the stability of these oxidized molecules, we believe that phenothiazine represents an attractive core for ligand substitution to adjust the redox potential to more practical values. (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.2140615] All rights reserved.