Preparation and application of the high-performance sodium-ion battery cathode material Na3.6Fe2.6(PO4)1.6P2O7@CNT with dual-carbon coating

Abstract

Iron-based mixed phosphate Na_3.6Fe_2.6(PO₄)_1.6P₂O₇ (1.38-NFPP) is considered as an ideal cathode material for sodium-ion batteries. Because of its good economic efficiency and environmental sustainability, it is expected to be widely used in future large-scale energy storage. However, the inherent low electronic conductivity of 1.38-NFPP limits its feasibility for practical commercialization. In this study, the in situ carbon layer is modified to enhance the electrochemical properties of the material through a dual-carbon coating strategy using dot-like SP and line-like CNT, respectively. The results show that the optimal Na_3.6Fe_2.6(PO₄)_1.6P₂O₇@CNT (1.38-NFPP@CNT) has excellent rate performance (89.1 mA h g⁻¹ capacity at 50C) and ultra-long cycling capability (89.3% capacity retention at 50C for 10 000 cycles). The single-phase solid solution sodium-storage mechanism of 1.38-NFPP@CNT is demonstrated by non-in situ XRD. In addition, a batch of 12 Ah pouch cells is assembled with 1.38-NFPP@CNT as the positive electrode and commercial hard carbon as the negative electrode. The pouch cells exhibit a capacity retention rate of 85.7% after 3000 cycles at 1C and still deliver a capacity of 11.8 Ah at 30C (equivalent to 94.4% of 1C), indicating their good overall electrochemical performance. Therefore, the rational dual-carbon coating strategy improves the feasibility of 1.38-NFPP for practical use, thus revealing the great application prospects of iron-based hybrid phosphate materials.