Improved Phase Purity and Sodium-Ion Diffusion in Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 Cathodes via Carbon Source Optimization

Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP), possessing high theoretical capacity, cost-effectiveness, and environmental benignity, is an attractive sodium-ion battery (SIBs) cathode candidate. Nevertheless, the corresponding widespread application remains limited by intrinsically low electronic conductivity and a tendency to form impurity phases, particularly NaFePO 4, during synthesis. In this study, NFPP was coated with carbon using two different carbon sources of glucose and citric acid to systematically compare the effects of carbon source on the material’s electrochemical properties. Compared with the glucose-derived composite, the NFPP sample synthesized with citric acid (NFPP-C) effectively suppressed the formation of the NaFePO 4 impurity phase. As a result, NFPP-C with improved phase purity showed remarkable electrochemical improvement, achieving a high capacity of 92.7 mAh g –1 at 0.1C and preserving 74.3 mAh g –1 at 10C. Furthermore, after 200 cycles at 1C, the discharge capacity decreased only slightly from 80.7 to 79.7 mAh g –1, corresponding to 98.8% capacity retention. The reduced impurity guarantees facilitated Na + diffusion and boosted the electrochemical behavior of the material, thereby contributing to the enhanced electrochemical property.