Na4Fe3(PO4)2(P2O7)@NaFePO4@C core-double-shell architectures on carbon cloth: A high-rate, ultrastable, and flexible cathode for sodium ion batteries

Designing and fabricating structurally optimized cathodes are critical to realize excellent electrochemical performance for sodium ion batteries. Herein, a facile and versatile sol-gel fabrication strategy followed by heat treatment is developed to synthesize Na4Fe3(PO4)2(P2O7)@NaFePO4@C core-double-shell architectures grown on flexible carbon cloth, which can be directly used as a binder-free cathode for high-performance sodium ion batteries. In the composite structure, the thickness of the maricite NaFePO4 shell can be stably confined to approximately 15 nm, which effectively promotes its electrochemical activation to achieve a high reversible capacity. The Na4Fe3(PO4)2(P2O7) core possesses a high working voltage and fast ion transport capability, while the carbon cloth and outermost carbon shell jointly build a fast three-dimensional electronic conducting network. In combination with structural and compositional advantages, the Na4Fe3(PO4)2(P2O7)@NaFePO4@C-carbon cloth cathode exhibits a high discharge capacity (136 mAh g−1 at 0.1C), excellent cycling stability (without capacity decay over 3000 cycles at 10C), and good rate capability (68 mAh g−1 at 100C). This work provides a new and effective strategy to achieve synergism between Na4Fe3(PO4)2(P2O7) and NaFePO4 for realizing superior electrochemical performance on flexible substrates.