Carbon coated hollow Na2FePO4F spheres for Na-ion battery cathodes

Carbon coated porous hollow Na2FePO4F (C/Na2FePO4F) spheres with 500 nm diameter and 80 nm wall thickness are synthesized by a one-step template-free ultrasonic spray pyrolysis process using sucrose as the carbon source. Nano-sized porous hollow C/Na2FePO4F spheres allow electrolyte to penetrate into the hollow structure, and thus the electrochemical reaction can take place on both the outside and inside surface and in the pores. Also, the carbon coating on Na2FePO4F hollow spheres enhances the electronic conductivity and charge transfer reaction kinetics. The unique carbon coated porous hollow C/Na2FePO4F cathodes deliver 89 mAh g−1 capacity at a low rate of 0.1C, and 75 mAh g−1 at 1C, and maintains 33% of capacity at 9C. After 750 cycles at 1C, C/Na2FePO4F cathodes can still provide 60 mAh g−1, which is 80% of initial capacity. The significant advance in electrochemical performance of hollow C/Na2FePO4F spheres combined with mature aerosol spray synthesis technology make these carbon coated porous hollow C/Na2FePO4F spheres very promising as cathode materials for practical applications in Na-ion batteries.

► C/Na2FePO4F particles were synthesized using ultrasonic spray pyrolysis.
► C/Na2FePO4F spheres can maintain 80% of initial capacity after 750 cycles.
► Electrochemical performance is comparable with cathodes for Li-ion batteries.
► Enhanced electrochemical performance is attributed to the unique structure.