Three Electron Reversible Redox Reaction in Sodium Vanadium Chromium Phosphate as a High-Energy-Density Cathode for Sodium-Ion Batteries

A sodium-ion battery operating at room temperature is of great interest for large-scale stationary energy storage because of its intrinsic cost advantage. However, the development of a high capacity cathode with high energy density remains a great challenge. In this work, sodium super ionic conductor-structured Na3V2-xCrx(PO4)(3) is achieved through the sol-gel method; Na3V1.5Cr0.5(PO4)(3) is demonstrated to have a capacity of 150 mAh g(-1) with reversible three-electron redox reactions after insertion of a Na+, consistent with the redox couples of V2+/(3+), V3+/(4+), and V4+/(5+). Moreover, a symmetric sodium-ion full cell utilizing Na3V1.5Cr0.5(PO4)(3) as both the cathode and anode exhibits an excellent rate capability and cyclability with a capacity of 70 mAh g(-1) at 1 A g(-1). Ex situ X-ray diffraction analysis and in situ impedance measurements are performed to reveal the sodium storage mechanism and the structural evolution during cycling.