Layered Molybdenum (Oxy) Pyrophosphate (MoO 2 ) 2 P 2 O 7 as a Cathode Material for Sodium‐Ion Batteries

Layer‐structured (MoO 2 ) 2 P 2 O 7 was prepared through the decomposition of MoO 2 HPO 4 and further used as a cathode material for sodium‐ion batteries. Results reveal that the capacity and cycling stability of the (MoO 2 ) 2 P 2 O 7 /Na battery largely depends on the lower cutoff voltage. When cycling between 4.0 ‐1.5 V, (MoO 2 ) 2 P 2 O 7 delivers a capacity of 180 mAh g −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 ‐1.7 V. However, when cycling between 4.0 ‐ 2.0 V, the performance can be significantly improved, which delivers 93 mAh g −1 in the first cycle and, after 50 cycles, the capacity can maintain 55 mAh g −1 . This is mainly attributed to the stable crystal structure of the Na‐intercalated (MoO 2 ) 2 P 2 O 7 electrode. Ex situ X‐ray diffraction examination demonstrates that the volume change of the (MoO 2 ) 2 P 2 O 7 electrode during the first charging process is only 1 %.