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

Coated C+MxOy@Na3V2(PO4)3 samples containing 1.5% or 3.5% wt. of MxOy (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 MxOy. 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+MxOy@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.