Controllable Synthesis of a Monophase Nickel Phosphide/Carbon (Ni5P4/C) Composite Electrode via Wet-Chemistry and a Solid-State Reaction for the Anode in Lithium Secondary Batteries

A monophase nickel phosphide/carbon (Ni5P4/C) composite with a thin carbon shell is controllably synthesized via the two-step strategy of a wet-chemistry reaction and a solid-state reaction. In this fabrication, the further diffusion of phosphorus atoms in the carbon shell during the solid-state reaction can be responsible for a chemical transformation from a binary phase of Ni5P4-Ni2P to monophase Ni5P4. Galvanostatic charge-discharge measurements indicate that the Ni5P4/C composite exhibits a superior, high rate capacibility and good cycling stability. About 76.6% of the second capacity (644.1 mA h g-1) can be retained after 50 cycles at a 0.1 C rate. At a high rate of 3 C, the specific capacity of Ni5P4/C is still as high as 357.1 mA h g-1. Importantly, the amorphous carbon shell can enhance the conductivity of the composite and suppress the aggregation of the active particles, leading to their structure stability and reversibility during cycling. As is confirmed from X-ray-diffraction analysis, no evident microstructural changes occur upon cycling. These results reveal that highly crystalline Ni5P4/C is one of the most promising anode materials for lithium-ion batteries.