Layered Molybdenum (Oxy) Pyrophosphate (MoO2)2P2O7 as a Cathode Material for Sodium-Ion Batteries

Layer-structured (MoO2)(2)P2O7 was prepared through the decomposition of MoO2HPO4 and further used as a cathode material for sodium-ion batteries. Results reveal that the capacity and cycling stability of the (MoO2)(2)P2O7/Na battery largely depends on the lower cutoff voltage. When cycling between 4.0 -1.5V, (MoO2)(2)P2O7 delivers a capacity of 180mAhg(-1) in the first cycle, but it drops very fast owing to a phase decomposition of the crystal structure to an amorphous one. The cycling stability is still not good when cycling between 4.0 -1.7V. However, when cycling between 4.0 - 2.0V, the performance can be significantly improved, which delivers 93mAhg(-1) in the first cycle and, after 50 cycles, the capacity can maintain 55mAhg(-1). This is mainly attributed to the stable crystal structure of the Na-intercalated (MoO2)(2)P2O7 electrode. Ex situ X-ray diffraction examination demonstrates that the volume change of the (MoO2)(2)P2O7 electrode during the first charging process is only 1%.