Reduced graphene oxide wrapped alluaudite Na2+2xFe2-x(SO4)3 with high rate sodium ion storage properties

Na2+2xFe2-x(SO4)3 (NFS) is recently reported as a promising cathode material for Na-ion batteries with high voltage generation and high energy density, making it competitive to sodium battery cathodes. Nevertheless, owing to its insulating nature, a formidable challenge remains as to realize high rate capability. Herein, Na2+2xFe2-x(SO4)3/reduced graphene oxide (rGO) has been designed and realized by a facile one-stepped synthesis. Its structural, morphological and electrochemical properties have been characterized and compared with those of the bare NFS material. As a result, NFS/rGO cathode demonstrates enhanced electrochemical performance, as reflected by the initial specific capacity of 90 mAh g−1 and a high specific capacity of 80 mAh g−1 over 100 cycles at a current density of 0.1 C (1 C = 120 mA g−1), an excellent rate capability with specific capacity of 60 mAh g−1 at high current density of 20 C (2.4 A g−1) and long-term cycle life with capacity retention of 84% over 500 cycles at 20 C rate (2.4 A g−1). This improvement may be attributed to the high electronic conductivity of rGO, which facilitates the charge transfer reactions between the active materials and electrolyte interface, and promotes the sodium ion diffusion in the electrode. Therefore, Na2+2xFe2-x(SO4)3/rGO is favorable to realizing superior electrochemical performance for the sulfate, which presents a significant step forward in the development of low-cost large-scale batteries.