In situ synthesis and in operando NMR studies of a high-performance Ni5P4-nanosheet anode

Abstract

Nickel phosphide (Ni₅P₄) nanosheets are synthesized using in situ chemical vapor deposition of P on Ni foam. The thickness of the as-synthesized Ni₅P₄ film is determined to be ∼5 nm, using atomic force microscopy (AFM). The small thickness shortens the diffusion path of Li ions and results in fast ion transport. In addition, the 2D Ni₅P₄ nanosheets seamlessly connect to the Ni foam, which facilitates electron transfer between Ni₅P₄ and the Ni current collector. Therefore, the binder/carbon free-nickel supported Ni₅P₄ shows fast rate performance as an anode for lithium-ion batteries (LIBs). The specific capacity of 2D Ni₅P₄ is obtained as 600 mA h g⁻¹ at a cycling rate of 0.1C, approaching the theoretical capacity of 768 mA h g⁻¹. Even at a rate of 0.5C, the capacity remains as 450 mA h g⁻¹ over 100 cycles. A capacity >100 mA h g⁻¹ is retained at a very high rate of 20C. Ni₅P₄ also exhibits a low voltage of ∼0.5 V with respect to Li metal, which makes it a suitable negative electrode for LIBs. In operando³¹P NMR and ⁷Li NMR are employed to probe the lithiation and de-lithiation mechanisms upon electrochemical cycling.