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

The layered structure of molybdenum (oxy)pyrophosphate (delta-(MoO2)(2)P2O7) was synthesized by heating MoO2HPO4 center dot H2O precursor at 560 degrees C. The synthesis temperature was selected using in situ high-temperature Xray diffraction (XRD) depicting phase transformations of the precursor from room temperature up to 800 degrees C. Electrochemical evaluation reveals that up to four Li ions per formula unit can be intercalated into delta-(MoO2)(2)P2O7 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 delta-Li-1.2(MoO2)(2)P2O7, 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.