Flexible self-supporting Ni2P@N-doped carbon anode for superior rate and durable sodium-ion storage

Abstract

Transition-metal phosphides are considered as candidates for sodium-ion batteries (SIBs) due to the high theoretical specific capacity. However, serious volume variation and agglomeration during sodiation-desodiation hamper their development. Hence, a microporous structure of Ni2P@C–N polyhedrons embedding in highly rough carbon fiber (Ni2P@C–N⊂CF) is designed as self-supporting anode material. The synergistic effect of ultrasmall Ni2P particles (∼5 nm), graphitic carbon shell and adequate free space of carbon fibers buffer the volume change, the uniform distribution of Ni2P particles shorten Na+ diffusion path, the microporous structure endow adequate electrolyte penetration, the graphitic carbon shell and carbon fibers provide high conductive channel, and thus boosting a remarkable sodium storage performance. The Ni2P@C–N⊂CF delivers long cycling stability (196.8 mAh g−1 at 1000 mA g−1 over 1000 cycles with a capacity dropping of 0.04% cycle−1), superior rate capability (197.1 mAh g−1 at 2000 mA g−1, and returned to 752.5 mAh g−1 at 100 mA g−1). Moreover, the flexible half-cell based on the Ni2P@C–N⊂CF electrode still maintains 81.7% capacity retention after repeated bending states. The full cell based on Ni2P@C–N⊂CF and Na3V2(PO4)3 manifests a capacity of 185.9 mAh g−1 over 100 cycles and energy density of ∼217.4 Wh kg−1 for SIBs. Furthermore, the hybrid energy storage mechanism can also contribute to the outstanding electrochemical performance. The Ni2P@C–N⊂CF shows a potential in flexible electrode materials for Na-storage device.