Scalable synthesis of Na3V2(PO4)3/C porous hollow spheres as a cathode for Na-ion batteries

Abstract

Na₃V₂(PO₄)₃ (NVP) has been considered as a very promising cathode material for sodium-ion batteries (SIBs) due to its typical NASICON structure, which provides an open and three dimensional (3D) framework for Na⁺ migration. However, the low electronic conductivity of NVP limits its rate capability and cycling ability. In this study, carbon coated hollow structured NVP/C composites are synthesized via a template-free and scalable ultrasonic spray pyrolysis process, where the carbon coated NVP particles are uniformly decorated on the inner and outer surfaces of the porous hollow carbon spheres. When evaluated as a cathode material for SIBs, the unique NVP/C porous hollow sphere cathode delivers an initial discharge capacity of 99.2 mA h g⁻¹ and retains 89.3 mA h g⁻¹ after 300 charge/discharge cycles with a very low degradation rate of 0.035% per cycle. For comparison, the NVP/C composite, prepared by the traditional sol–gel method, delivers a lower initial discharge capacity of 97.4 mA h g⁻¹ and decreases significantly to 71.5 mA h g⁻¹ after 300 cycles. The superior electrochemical performance of NVP/C porous hollow spheres is attributed to their unique porous, hollow and spherical structures, as well as the carbon-coating layer, which provides a high contact area between electrode/electrolyte, high electronic conductivity, and high mechanical strength.