| Abstract
| - LiFeP2O7, LiFeAs2O7, and LiVP2O7 were prepared as pure microcrystalline powders viaan aqueous solution route that allowed the formation of finely dispersed small particles.These three compounds, in which Fe or V is in the oxidation state +3, were evaluated aspositive electrodes in lithium rechargeable batteries through galvanostatic, PITT, and GITTcycling modes and in situ X-ray diffraction. We show that the electrochemical activity ofthese materials is drastically enhanced by an intimate mixing of the pristine materials withconductive carbon, through ball-milling. This mandatory step resulted in producingamorphous Li−M−X−O domains at the surface of crystalline LiMX2O7 crystallites. Acontinuous voltage decrease is observed as lithium insertion proceeds into the amorphouspart of the composite electrodes. Crystalline LiMX2O7 particles react with lithium throughtwo-phase mechanisms, characterized by insertion plateaus located at 2.90 V for LiFeP2O7(Fe3+/Fe2+ couple), 1.99 V for LiVP2O7 (V3+/V2+ couple), and 2.5 V for LiFeAs2O7. The Li-driven second phase is crystalline, with a larger unit cell in the case of isostructuralLi1+xMP2O7 (M = Fe, V). It is amorphous in the case of Li1+xFeAs2O7, which shows,interestingly, a recrystallization process upon charging (when Li is extracted fromLi2FeAs2O7). Finally, lithium extraction from LiVP2O7 occurs at 4.26 V vs Li+/Li (V3+/V4+couple) and leads through a two phase process to the new diphosphate VP2O7.
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