Nanostructured silicon for lithium ion batteries: from porous silicon to silicon nanowires
Creators: name: Fulton, Alison J.
Publisher: name: Science
Year: 2020
Subjects: subject: Lithium ion battery
subject: silicon nanowires
subject: porous silicon
subject: nanotechnology
subject: pulsed laser-induced dewetting
subject: nanoparticles
subject: Chemistry--General

description: In this work, nanostructured Si materials have been fabricated for lithium ion battery (LIB) anode applications. These include Si nanowires (NWs) by chemical vapor deposition (CVD) and porous Si (PS) via electrochemical anodization. First, PS was fabricated by electrochemical anodization of n-type Si. The effect of the experimental parameters, including anodization time, applied potential and electrolyte concentration, on the morphology were systematically explored. A mesoporous transition layer was formed on top of a macroporous Si layer, with well-defined and separated pores. A mechanistic study of pore formation using chronoamperometry and dissolution valence studies demonstrated a competition between direct and indirect dissolution.Pulsed laser-induced dewetting (PLiD) of Au thin films was used to form Au nanoparticles (NPs) on both polished Si and PS substrates for use as the catalyst in the CVD growth of Si NWs. On polished Si, monodispersed, uniform NP distributions were obtained, with the NP diameter being linearly related to the initial thin film thickness. On PS substrates, the presence of pores and a unique ripple topography resulted in the formation of three distinct NP size distributions. It was demonstrated that the ripples on the PS substrates was of sufficient magnitude to influence dewetting only when the concentration was &lt; 9.4 wt% HF. The laser irradiation time was found to significantly influence the formation of large (~ 200 nm) NPs, which formed in the valleys of the ripples, producing ordered arrays.The NP arrays prepared by PLiD on both polished Si and PS substrates were used for the first time as catalytic arrays in the vapor-liquid-solid synthesis of Si NWs by CVD using SiCl4. This proof-of-principle study followed the systematic exploration of the experimental parameters and how they influence Si NW morphology. By tuning the growth parameters, both verticallyarrayed and dense, entangled Si NWs could be produced. A diameter-dependent growth rate and activation energy were demonstrated, with direct proof of this phenomenon being obtained through use of PS substrates. In addition, it was discovered that Si NWs could be synthesized at 700 °C, which is 100 °C lower than previously reported in the literature for the use of SiCl4.Si NWs were also successfully synthesized on stainless steel substrates via CVD using a Au catalyst and SiCl4 for the first time. Si NWs were obtained under specific growth time windows, although Si deposition was confirmed on all samples despite the dominant presence of high-temperature oxidation products. These materials were characterized for LIB anode applications as a binder-free, carbon-free electrode design with promising results. The highest capacity obtained was 3373 mAh·g-1 and a high coulombic efficiency of approximately 99% was observed for most samples tested. Rate capability testing showed good reversibility while a capacity retention of 42% was found after 500 charge and discharge cycles. The electrochemical performance was superior to current graphite-based anode materials and showed the applicability of this novel synthetic approach towards the fabrication of high-performance LIB anode materials.