Silicon nanowires grown inside nitrogen-doped hollow carbon spheres as anode materials for lithium-ion batteries

Silicon has the highest theoretical lithium storage capacity, low discharge potential, abundance in the Earth's crust and environment-friendly nature, and has been considered as one of the most promising anode materials for lithium-ion batteries. However, high-cost raw materials, complicated processes and harsh reaction conditions are generally used in the preparation of silicon. In addition, the poor intrinsic conductivity, slow diffusion kinetics of lithium ions and large volume expansion in silicon hinder its further application. Here, Si nanowires@nitrogen-doped hollow carbon spheres (Si NWs@N-HCSs) as anode materials for lithium-ion batteries are prepared by the polystyrene template method, with sodium citrate as the reducing agent, high-temperature carbonization of polypyrrole, and the "supercritical fluid-liquid-solid (SFLS)" growth mechanism. The initial coulombic efficiency of the as-obtained Si NWs@N-HCSs is 72% at a current density of 0.5 A g^-1, and it gives a reversible specific capacity of 568 mAh g^-1 after 200 cycles and outstanding rate performance (2709, 2359, 1839, 1566, 1421, and 1028 mAh g^-1 at 0.2, 0.5, 1, 2, 4, and 8 A g^-1). The excellent electrochemical behavior of the composite electrode is attributed to the nitrogen-doped carbon spheres with a hollow structure, gold nanoparticles with high conductivity, and silicon with a nanowire structure. This study not only provides a new synthetic idea for the application of silicon in other fields, but also offers a new design strategy for the improvement of the lithium storage properties of silicon-based anode materials.