Lithium Insertion/Extraction into/from LiMX₂O₇ Compositions (M = Fe, V; X = P, As) Prepared via a Solution Method

LiFeP 2 O 7, LiFeAs 2 O 7, and LiVP 2 O 7 were prepared as pure microcrystalline powders via an 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 as positive electrodes in lithium rechargeable batteries through galvanostatic, PITT, and GITT cycling modes and in situ X-ray diffraction. We show that the electrochemical activity of these materials is drastically enhanced by an intimate mixing of the pristine materials with conductive carbon, through ball-milling. This mandatory step resulted in producing amorphous Li−M−X−O domains at the surface of crystalline LiMX 2 O 7 crystallites. A continuous voltage decrease is observed as lithium insertion proceeds into the amorphous part of the composite electrodes. Crystalline LiMX 2 O 7 particles react with lithium through two-phase mechanisms, characterized by insertion plateaus located at 2.90 V for LiFeP 2 O 7 (Fe 3+ /Fe 2+ couple), 1.99 V for LiVP 2 O 7 (V 3+ /V 2+ couple), and 2.5 V for LiFeAs 2 O 7 . The Li-driven second phase is crystalline, with a larger unit cell in the case of isostructural Li 1+ x MP 2 O 7 (M = Fe, V). It is amorphous in the case of Li 1+ x FeAs 2 O 7, which shows, interestingly, a recrystallization process upon charging (when Li is extracted from Li 2 FeAs 2 O 7 ). Finally, lithium extraction from LiVP 2 O 7 occurs at 4.26 V vs Li + /Li (V 3+ /V 4+ couple) and leads through a two phase process to the new diphosphate VP 2 O 7 .