Fluorine-induced dual defects in NiP₂ anode with robust sodium storage performance

Metal phosphides have shown great application potential as anode for sodium-ion batteries (NIBs) owing to high theoretical capacity, suitable operation voltage and abundant resource. Unfortunately, the application of NiP₂ anode is severely impeded by low practical capacity and fast capacity decay due to the huge volume variation and low reactivity of internal phosphorus (P) component towards Na⁺. Herein, electronic structure modulation of NiP₂ via heteroatoms doping and introducing vacancies defects to enhance Na⁺ adsorption sites and diffusion kinetics is successfully attempted. The as-synthesized three-dimensional (3D) bicontinuous carbon matrix decorated with well-dispersed fluorine (F)-doped NiP₂ nanoparticles (F-NiP₂@carbon nanosheets) delivers a high reversible capacity (585 mAh·g⁻¹ at 0.1 A·g⁻¹) and excellent long cycling stability (244 mAh·g⁻¹ over 1,000 cycles at 2 A·g⁻¹) when tested as anode in NIBs. Density functional theory (DFT) calculations reveal that F doping in NiP₂ induces the formation of P vacancies with increased Na⁺ adsorption energy and accelerates the alloying of internal P component. The F-NiP₂@carbon nanosheets//Na₃V₂(PO₄)₃ full cell is evaluated showing stable long cycling life. The heteroatoms doping-induced dual defects strategy opens up a new way of metal phosphides for sodium storage.