Exploring the newly unveiled Li2MP2O7 pyrophosphate cathode materials for lithium-ion batteries, the current study reports the general observation of an unusually high Fe3+/Fe2+ redox potential close to 4.0 V vs Li/Li+ in mixed-metal Li2MxFe1–xP2O7 (M = Mn, Co, Mg) phases with a monoclinic structure (space group P21/c). Such a high voltage Fe3+/Fe2+ operation over 3.5 V has long been believed to be possible only by the existence of much more electronegative but hygroscopic anions such as SO42– or F–. Thereby, this is the first universal confirmation of >3.5 V operation by stable, simple phosphate material. High voltage (close to 4 V) operation of the Fe3+/Fe2+ couple was stabilized by all dopants, either by larger Mn2+ or smaller Co2+ and Mg2+ ions, where Mg2+ is redox inactive, revealing that the high voltage is induced neither by reduced Fe–O bond covalency nor by contamination by the redox couple of other transition metals. The cause of higher Fe3+/Fe2+ redox potential is argued and rooted in the stabi...