Layered Molybdenum (Oxy)Pyrophosphate as Cathode for Lithium-Ion Batteries

Abstract

The layered structure of molybdenum (oxy)pyrophosphate (δ-(MoO2)2P2O7) was synthesized by heating MoO2HPO4·H2O precursor at 560 °C. The synthesis temperature was selected using in situ high-temperature X-ray diffraction (XRD) depicting phase transformations of the precursor from room temperature up to 800 °C. Electrochemical evaluation reveals that up to four Li ions per formula unit can be intercalated into δ-(MoO2)2P2O7 upon discharge to 2 V. Three voltage plateaus are observed at 3.2, 2.6, and 2.1 V, lower than the theoretical predictions. The first plateau corresponds to the intercalation of 1.2 Li forming δ-Li1.2(MoO2)2P2O7, the same structure formed upon chemical lithiation with LiI. In-situ XRD indicates two-phase reaction upon the first lithium insertion and expansion of the lithiated phase unit cell in the a direction. Intercalation of the second lithium results in a different lithiated structure, which is also reversible, giving the capacity of about 110 mAh/g between 2.3 and 4 V. More lithium-ion intercalation leads to loss of crystallinity and structural reversibility. The Mo reduction upon lithiation is consistent with the amount of Li intercalated as confirmed by the X-ray absorption fine structure.