Novel 3.9 V Layered Na3V3(PO4)4 Cathode Material for Sodium Ion Batteries

A new compound Na3V3(PO4)4 is successfully synthesized for sodium ion batteries using a sol–gel method. Composition analysis through ICP-OES confirms the stoichiometry of Na3V3(PO4)4. Structural analysis based on XRD reveals that the new material crystallizes in a monoclinic system with a C2/c space group. The new compound exhibits a layered structure containing 3D Na+ ion channels allowing excellent cycling and rate performance. Even at a high current rate of 3C (1C = 45 mA/g), it still delivers 82% of the theoretical capacity. Meanwhile, 92% of its capacity is retained after 100 electrochemical cycles. The voltage profiles of Na3V3(PO4)4 show that it can reversibly uptake nearly one Na+ ion with a 3.9 V voltage plateau, which is the highest value among Na-containing V-based orthophosphates ever reported.