General Observation of Fe~(3+)/Fe~(2+) Redox Couple Close to 4 V in Partially Substituted Li2FeP2O7 Pyrophosphate Solid-Solution Cathodes

摘要

Exploring the newly unveiled Li2iWP2O7 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 Li2M_xFe_(1-x)P2O7 (M = Mn, Co, Mg) phases with a monoclinic structure (space group P21/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 SO4~(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, cither 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.