Overcharge protection for rechargeable lithium polymer electrolyte cells by addition of redox shuttle additives to the polymer electrolyte was examined. Shuttle onset potentials and effective diffusion coefficients were determined for 12 redox shuttle species in polyethylene oxide-based electrolytes at 85 degrees C. The four most promising additives were tested in Li/PEO-LiN(SO2CF3)(2)/Li2+xMn4O9 cells under normal and severe overcharging conditions. In addition to tricyanobenzene and tetracyanoquinodimethane, two anionic redox shuttle additives, salts of 1,2,4-triazole and imidazole, demonstrated effectiveness in extending cycle life and good compatibility with cell components.