Improved Surface Stability of C+M_xO_y@Na₃V₂(PO₄)₃ Prepared by Ultrasonic Method as Cathode for Sodium-Ion Batteries

Coated C+M_xO_y@Na₃V₂(PO₄)₃ samples containing 1.5% or 3.5% wt. of M_xO_y (Al₂O₃, MgO or ZnO) have been synthesized by a two-step method including first a citric based sol-gel method for preparing the active material and second an ultrasonic stirring technique to deposit M_xO_y. The presence of the metal oxides properly coating the surface of the active material is evidenced by XPS and electron microscopy. Galvanostatic cycling of sodium half-cells reveals a significant capacity enhancement for samples coated with 1.5% of metal oxides and an exceptional cycling stability as evidenced by Coulombic efficiencies as high as 95.9% for ZnO@ Na₃V₂(PO₄)₃. It is correlated to their low surface layer and charge transfer resistance values. The formation of metal fluorides that remove traces of corrosive HF from the electrolyte is checked by XPS spectroscopy. The feasibility of sodium-ion batteries assembled with C+M_xO_y@Na₃V₂(PO₄)₃ is further verified by evaluating the electrochemical performance of full cells. Particularly, a Graphite//Al₂O₃@ Na₃V₂(PO₄)₃ battery delivers an energy density as high as 260 W h kg^-1 and exhibits a Coulombic efficiency of 89.3% after 115 cycles.