Extensive overcharge protection via redox shuttling in lithium-ion batteries has been limited by the solubility of stable electrolyte additives. Only a few stable compounds dissolve at concentrations above 0.1 M in carbonate-based electrolytes. Here we report the performance of the highly soluble redox shuttle 3,7-bis(trifluoromethyl)-N-ethylphenothiazine at concentrations as high as 1.0 M. The compound enables overcharge protection for hundreds of hours at rates as high as 1C, and even at 1.0 M, normal battery function is not affected by the presence of the redox shuttle at low charging rates. The results on these performance metrics constitute the best yet observed for LiFePO4 cathodes. (C) 2015 The Electrochemical Society.