A 3.4 V Layered\nVOPO<sub>4</sub> Cathode for Na-Ion\nBatteries

Vanadyl phostate (VOPO₄) with a layered structure was\nsuccessfully prepared by chemical delithiation of the tetragonal α_I-LiVOPO₄ at room temperature. X-ray diffraction\n(XRD) studies show the resulting VOPO₄ has a crystal structure\nsimilar to α_I-LiVOPO₄. The V⁴⁺/V⁵⁺ redox couple exhibits a discharge plateau around\n3.5 V vs Na/Na⁺ in sodium cells. The sample offers a capacity\nof ∼110 mAh g^–1 (0.67 Na⁺) without\nany conductive coating and ∼150 mAh g^–1 (0.90\nNa⁺) with a preliminary optimization with reduced graphene\noxide (rGO) nanosheets. The electrochemically sodiated VOPO₄ electrodes show XRD patterns similar to the chemically sodiated\nVOPO₄, indicating the formation of the same product by\nthe two sodiation processes. Bond valence sum (BVS) map calculations\nsuggest that sodium ions likely take the sites between two adjacent\nVOPO₄ sheets. Rietveld refinement of the chemically sodiated\nNa_0.8VOPO₄ indicate that the sodium ions could\ntake the 8<i>j</i> sites similar to the lithium ions in\nα_I-LiVOPO₄. This new layered VOPO₄ is a competitive cathode candidate among the polyanion cathodes\nfor sodium-ion batteries.