Giorgio Nava1 Joseph Schwan1 Lorenzo Mangolini1

1, University of California, Riverside, Riverside, California, United States

In recent times, the research community engaged in significant efforts to explore the use of silicon-carbon nanoarchitectures as Li-ion battery anode active materials with the goal of overcoming the limited energy storage capacity of the state-of-the-art graphite-based devices. [1] The small size of the silicon -Si- structures tackles the large volume expansion undergone by the semiconductor upon lithiation, which causes pulverization of bulk Si electrodes, and promotes a robust cycling. The carbonaceous coatings, on the other hand, improves the electrical conductivity of the composite and prevent the direct interaction of Si with the electrolyte, which in turn assists the formation of a stable SEI. Although a wide range of different Si nanomaterial morphologies and their composites have been investigated, the use of commercial silicon nanoparticles -NPs- with a simple and high-quality carbon surface coating would be a highly-desirable solution for an immediate introduction into actual manufacturing. In this contribution, we describe a facile approach based on a chemical-vapor-deposition (CVD) process to address the problem. Commercial Si nanocrystals with an average size of 100 nm and low oxygen content (below 3% in weight) are introduced into a furnace with an alumina combustion boat. The particles are wrapped with a conformal coating of amorphous carbon resulting from the dissociation of acetylene - C2H2 - at 650 °C. After removing C2H2 from the reaction zone, the furnace is ramped up to 1000°C in Argon yielding a controlled graphitization of the carbon -C- shell. Correspondingly, the Raman analysis of the synthetized composite displays an increase in the ratio of intensity of the D an G peak from 0.7 to 1.4, while the onset of well-defined layered graphitic planes with no detectable silicon-carbide signature is observed from TEM and XRD analysis.[2] Notably, the presented approach does not deploy oxidizing agents during the thermal process, which are instead required for the formation of ordered graphitic shells in the case of a high-temperature methane CVD, hence preventing the detrimental formation of silicon oxide species. [2] The produced nanomaterials were introduced in slurry with no addition of conductive additives, coated onto a copper substrate and studied as anode material in a Li-ion battery half-cell assembly. The amorphous-carbon-coated Si particles, fabricated with the 650°C C2H2 CVD process, shows a first cycle coulombic efficiency – CE - of 85% and capacity of 1600 mAh g-1. The graphitization of the NP carbon shell, achieved through the high-temperature step in Argon, further boosts the electrode performance, reaching a first cycle CE and capacity values of 88% and 2100 mAh g-1 respectively, on a par with some of the best silicon-carbon architectures reported in the literature. [1]

[1] F. Luo et al, Journal of The Electrochemical Society, 162 (14) A2509-A2528, 2015
[2] I. H. Son et al., Small, 12 (5), 658–667, 2016