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.