Hollow Core-Shelled Na₄Fe_2.4Ni_0.6(PO₄)₂P₂O₇ with Tiny-Void Space Capable Fast-Charge and Low-Temperature Sodium...

Iron-based mixed polyanion phosphate Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) is recognized as a promising cathode for Sodium-ion Batteries (SIBs) due to its low cost and environmental friendliness. However, its inherent low conductivity and sluggish Na⁺ diffusion limit fast charge and low-temperature sodium storage. This study pioneers a scalable synthesis of hollow core-shelled Na₄Fe_2.4Ni_0.6(PO₄)₂P₂O₇ with tiny-void space (THoCS-0.6Ni) via a one-step spray-drying combined with calcination process due to the different viscosity, coordination ability, molar ratios, and shrinkage rates between citric acid and polyvinylpyrrolidone. This unique structure with interconnected carbon networks ensures rapid electron transport and fast Na⁺ diffusion, as well as efficient space utilization for relieving volume expansion. Incorporating regulation of lattice structure by doping Ni heteroatom to effectively improve intrinsic electron conductivity and optimize Na⁺ diffusion path and energy barrier, which achieves fast charge and low-temperature sodium storage. As a result, THoCS-0.6Ni exhibits superior rate capability (86.4 mAh g^-1 at 25 C). Notably, THoCS-0.6Ni demonstrates exceptional cycling stability at -20 °C with a capacity of 43.6 mAh g^-1 after 2500 cycles at 5 C. This work provides a universal strategy to design the hollow core-shelled structure with tiny-void space cathode materials for reversible batteries with fast-charge and low-temperature Na-storage features.