High-performance ε-VOPO4 cathode materials for sodium ion battery applications

Abstract

The nanoscale ε-VOPO4 materials were successfully prepared by hydrothermal synthesis of H2VOPO4 precursors and further calcination. XRD and AFM characterization showed that the prepared nano-ε-VOPO4 belonged to monoclinic crystalline system with high purity of physical phase and an average particle size of 150 nm. FT-IR spectroscopy showed that the characteristic peak of the H2VOPO4 precursor (at 797 cm−1) disappeared after calcination and a characteristic peak representing ε-VOPO4 was captured at 914 cm−1. Furthermore, the XPS results showed that the V in the prepared ε-VOPO4 was mainly +5 valence. In order to further evaluate the potential of ε-VOPO4 as an energy storage material, it was applied as cathode material in sodium ion battery system. Electrochemical tests have demonstrated that ε-VOPO4 has satisfactory electrochemical performance as cathode material for sodium ion batteries. The initial capacities of ε-VOPO4 electrode materials reached 162.3, 135.1, 86.7, 51.4, and 29.7 mAh g−1 at 0.05, 0.2, 1, 2, and 5 C, respectively, and the long-cycle performance results showed that the electrochemical stability was still superior after 200 cycles at a current density of 0.5 C. Meanwhile, the density functional theory calculation shows that the optimized lattice constants (a = 7.279 Å, b = 6.896 Å and c = 7.268 Å) are in good agreement with the experimental data. Notably, the experimental ε-VOPO4 material has a higher voltage plateau than the theoretical calculation, which shows the potential advantage of ε-VOPO4 as a new generation electrode material for high energy storage systems.