General Observation of Fe³⁺/Fe²⁺ Redox Couple Close to 4 V in Partially Substituted Li₂FeP₂O₇ Pyrophosphate Solid-Solution Cathodes

Exploring the newly unveiled Li 2 M P 2 O 7 pyrophosphate cathode materials for lithium-ion batteries, the current study reports the general observation of an unusually high Fe 3+ /Fe 2+ redox potential close to 4.0 V vs Li/Li + in mixed-metal Li 2 M x Fe 1– x P 2 O 7 ( M = Mn, Co, Mg) phases with a monoclinic structure (space group P 2 1 / c ). Such a high voltage Fe 3+ /Fe 2+ 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 SO 4 2– 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 Fe 3+ /Fe 2+ couple was stabilized by all dopants, either by larger Mn 2+ or smaller Co 2+ and Mg 2+ ions, where Mg 2+ 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 Fe 3+ /Fe 2+ redox potential is argued and rooted in the stabilized edge-sharing local structural arrangement and the associated larger Gibbs free energy in the charged state.