Highly Conductive Ultrafine N-Doped Silicon Powders Prepared by High-Frequency Thermal Plasma and Their Application as Anodes for Lithium-Ion Batteries

Silicon materials are widely regarded as highly promising candidate anodes for the next generation of lithium-ion batteries. However, the violent volume expansion and low intrinsic conductivity hinder their practical application. In this study, ultrafine N-doped silicon powders (N-doped Si) were prepared by using high-frequency thermal plasma (HF-plasma) technology, in which nanocrystallization and N doping were conducted in a single step without the formation of the Si3N4 phase. Through characterization of X-ray photoelectron spectroscopy, X-ray diffraction, and Raman analysis, it is ascertained that N is doped in silicon after HF-plasma treatment. According to the UV–vis and conductivity tests, N-doped Si has a notably narrower bandgap and a higher conductivity than those of undoped Si. N-doped Si with a submicrosphere (N–Si-0.5) delivered a reversible capacity of 974.1 mA h g–1 at 0.2 A g–1 after 50 cycles and an initial Coulombic efficiency (ICE) of 88.72%. Even at 6 A g–1, N–Si-0.5 can still exhibit a high reversible capacity of 200.5 mA h g–1, while Si without doping (N–Si-0.0) only gives a reversible capacity of 526.8 mA h g–1 at 0.2 A g–1 after 50 cycles with an ICE of 85.81% and an unnoticeable capacity at 6 A g–1. It is clear that Si shows higher ICE, better cycle stability, and rate performance. For further enhancement of the electrochemical performances of N-doped Si, the Si nanowires (NW-Si) were prepared. Experimental results showed that the initial capacity, ICE, and rate performance all gradually improved as the N2 flow rate increased. NW-Si-1.0 has an initial capacity of 2725.7 mA h g–1 and an ICE of 80.18%. Even at 6 A g–1, it can provide a reversible capacity of 584.7 mA h g–1. The enhanced electrochemical performances of N-doped Si can be ascribed to the introduction of the N dopant and nanowire, which raised carrier concentration, accelerated electron transfer, and alleviated volume expansion.