A 3.4 V Layered VOPO4 Cathode for Na-Ion Batteries

Vanadyl phostate (VOPO4) with a layered structure was successfully prepared by chemical delithiation of the tetragonal alpha(I)-LiVOPO4 at room temperature. X-ray diffraction (XRD) studies show the resulting VOPO4 has a crystal structure similar to alpha(I)-LiVOPO4. The V4+/V5+ redox couple exhibits a discharge plateau around 3.5 V vs Na/Na+ in sodium cells. The sample offers a capacity of similar to 110 mAh g(-1) (0.67 Na+) without any conductive coating and similar to 150 mAh g(-1) (0.90 Na+) with a preliminary optimization with reduced graphene oxide (rGO) nanosheets. The electrochemically sodiated VOPO4 electrodes show XRD patterns similar to the chemically sodiated VOPO4, indicating the formation of the same product by the two sodiation processes. Bond valence sum (BVS) map calculations suggest that sodium ions likely take the sites between two adjacent VOPO4 sheets. Rietveld refinement of the chemically sodiated Na0.8VOPO4 indicate that the sodium ions could take the 8j sites similar to the lithium ions in alpha(I)-LiVOPO4. This new layered VOPO4 is a competitive cathode candidate among the polyanion cathodes for sodium-ion batteries.