One Step Core-Shell Silicon/Carbon Nanoparticle Synthesis By Laser Pyrolysis: Application to Anode Material in Lithium-Ion Batteries

Graphitic carbon, the most used anode in lithium-ion batteries (LIB), will not meet the ever increasing energy density requirements due to a low theoretical specific capacity (372 mA.h.g^-1 for LiC₆). To further increase the energy density of LIB, high-capacity anode materials have been intensely studied. Silicon appears as an ideal and abundant material for carbon replacement due to its high theoretical specific capacity (3579 mAh.g^-1 for the lithiated phase Li_3,75Si) and its low discharging potential against Li/Li+ reference (approximately 0,4V). However, huge volume change of Li_xSi_yalloys upon cycling together with SEI formation induces poor cycling stability and rapid capacity fading. The volume change can be partially counteracted by decreasing silicon particles to the nanosize where mechanical effects appear less severe and/or by limiting the expansion using a protecting shell.

In this context, we consider, as active material, nanoparticles covered by a carbon shell. Silicon coating techniques often imply two or more steps and the manipulation of nanopowders. Moreover, the low quantity of final product in most Si@C synthesis cannot meet mass production requirement. In the context to overcome these limitations, we developed a reactor where core-shell silicon carbon nanoparticles (Si@C Nps) are synthesized using the laser pyrolysis technique in a configuration with two reaction stages. The reactor is composed of two successive reaction zones: silicon cores are synthetized at the first stage and the carbon coating is achieved at the second. Silane is used as silicon precursor of the core and ethylene as carbon precursor of the shell. The Si core is not exposed to air before shell deposition preventing from SiO₂formation around silicon. Moreover the formation of silicon carbide, which is very detrimental to electrochemical properties, is avoided by complete decomposition of silane in the first stage. In this configuration, we can control the core diameter in the range 20 to 200 nm, the core organization (amorphous or polycrystalline), the shell thickness and its organization (turbostratic or graphitic).

We investigated both the effect of size reduction and the effect of carbon shell. Using pure 30 nm diameter Si Nps, we confirm that reducing the size (by comparison with a commercial grade of Si, 200 nm diameter) leads to a better stability of the electrode. In order to investigate the effect of the carbon shell we synthetized a control sample of 30 nm diameter Si Nps without insertion of ethylene. The same silicon core was covered with a 3 nm thick carbon shell by addition of ethylene in the second stage. We confirmed by Auger electron spectroscopy and high resolution transmission electron microscopy that Si@C nPs are well covered with carbon. XRD shows that the carbon shell is mainly turbostratic. Within this small size range, the beneficial effect of the carbon shell is observed compared to pure Si Nps. Without the carbon shell, half of the capacity is lost after 40 cycles while a good stability and high capacity (higher than 2000 mA.h.g^-1) is observed in the batteries elaborated from Si@C material. The origin of this beneficial effect will be discussed by comparison with literature.