A scalable approach to Na4Fe3(PO4)2P2O7@carbon/expanded graphite as cathode for ultralong-lifespan and low-temperature sodium-ion batteries

Mixed polyanion phosphate Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP) is regarded as the most promising cathode material for sodium-ion batteries (SIBs), due to its high structural stability and low-cost environmental friendliness. However, its intrinsic low conductivity and sluggish Na + diffusion restricted the fast-charge and low-temperature sodium storage. Herein, an NFPP composite encapsulated by in-situ pyrolytic carbon and coupled with expanded graphite (NFPP@C/EG) was constructed via a sol-gel method followed by a ball-mill procedure. Due to the dual-carbon modified strategy, this NFPP@C/EG only enhanced the electronic conductivity, but also endowed more channels for Na + diffusion. As cathode for SIBs, the optimized NFPP (M-NFPP@C/EG) delivers excellent rate capability (capacity of ∼80.5 mAh/g at 50 C) and outstanding cycling stability (11000 cycles at 50 C with capacity retention of 89.85%). Additionally, cyclic voltammetry (CV) confirmed that its sodium storage behavior is pseudocapacitance-controlled, with in-situ electrochemical impedance spectroscopy (EIS) further elucidating improvements in electrode reaction kinetics. At lower temperatures (0 °C), M-NFPP@C/EG demonstrated exceptional cycling performance (8800 cycles at 10 C with capacity retention of 95.81%). Moreover, pouch cells also exhibited excellent stability. This research demonstrates the feasibility of a dual carbon modification strategy in enhancing NFPP and proposes a low-cost, high-rate, and ultra-stable cathode material for SIBs.