Monoclinic Na2VOP2O7: A 4V-class cost-effective cathode for sodium-ion batteries

It is definitely not enough to realize the large-scale application of sodium-ion batteries (SIBs) only depending on the earth abundance and low cost of Na resources. Low-cost synthesis methods and high performance electrode materials are also imperative. In this work, a simple and low-cost synthesis method is introduced to synthesize both monoclinic and tetragonal Na2VOP2O7, which are the cathode candidates for SIBs. The results of our calculation based on bond valence sum show that monoclinic Na2VOP2O7 has high Na + conductivity. More importantly, monoclinic Na2VOP2O7 presents a high operation voltage about 4.0 V (vs Na+/Na) and a moderate discharge capacity of ∼90 mAh g−1, resulting in a remarkable energy density of 360 W h kg−1.