Fundamental Study of Engineered Nanocrystalline and Amorphous Silicon Based High Capacity, Reversible and Stable Anodes for Lithium-Ion Batteries

Commercial lithium-ion battery (LIB) systems at present employ graphite as the anode having a theoretical capacity of 372 mAh/g. However, for hybrid electric vehicles and electrical grid energy storage, batteries with much higher capacity and cycle life are needed. There is hence a critical need to explore alternative higher capacity alternative systems. Silicon, with a theoretical capacity of 4200 mAh/g is widely considered a promising alternative candidate anode to graphite. However, Si undergoes colossal volume expansion (>300%) during lithium alloying and de-alloying. This leads to pulverization resulting in loss of electrical contact of Si with the current collector thereby causing rapid decrease in capacity and consequent failure. It has been demonstrated that nanostructured (nc-Si) and amorphous (a-Si) forms of Si and Si based nanocomposites provide mechanical integrity preventing pulverization due to the reduced number density of atoms within a nano-sized grain and the ‘free volume’ effects in amorphous Si resulting in better capacity retention and cycle life. In this dissertation, the following simple and cost effective approaches for generating nanostructured composites of silicon are discussed: (1) Si nanoparticles of high specific surface area by high energy mechanical milling (HEMM), (2) Amorphous silicon (a-Si) films by electrodeposition, (3) Heterostructures of vertically aligned carbon nanotubes (VACNTs) and Si by chemical vapor deposition (CVD), and (4) low cost template based high throughput synthesis of hollow silicon nanotubes (h-SiNTs). All of the above amorphous and nanocrystalline Si based composites were thoroughly investigated using material and electrochemical characterizations and accordingly, a structure-property relationship was established. Among the aforementioned structures, the electrodeposited a-Si films exhibited excellent cyclability (0.016% loss per cycle), while CNT/Si heterostructures showed a very low first cycle irreversible loss of only 10%. The hollow silicon nanotubes exhibited a reasonable first cycle irreversible loss (25%) but exhibited extraordinary cycling stability with a low capacity fade rate of ~0.06%loss/cycle at the end of 400 cycles. These amorphous and nanocrystalline based silicon anodes prepared by cost effective methods, due to their superior electrochemical properties, show considerable potential to replace the current graphite based anodes for the next generation of high energy density Li-ion batteries

Pulsed Current Electrodeposition of Silicon Thin Films Anodes for Lithium Ion Battery Applications

Electrodeposition of amorphous silicon thin films on Cu substrate from organic ionic electrolyte using pulsed electrodeposition conditions has been studied. Scanning electron microscopy analysis shows a drastic change in the morphology of these electrodeposited silicon thin films at different frequencies of 0, 500, 1000, and 5000 Hz studied due to the change in nucleation and the growth mechanisms. These electrodeposited films, when tested in a lithium ion battery configuration, showed improvement in stability and performance with an increase in pulse current frequency during deposition. XPS analysis showed variation in the content of Si and oxygen with the change in frequency of deposition and with the change in depth of these thin films. The presence of oxygen largely due to electrolyte decomposition during Si electrodeposition and the structural instability of these films during the first discharge–charge cycle are the primary reasons contributing to the first cycle irreversible (FIR) loss observed in the pulse electrodeposited Si–O–C thin films. Nevertheless, the silicon thin films electrodeposited at a pulse current frequency of 5000 Hz show a stable capacity of ~805 mAh·g−1 with a fade in capacity of ~0.056% capacity loss per cycle (a total loss of capacity ~246 mAh·g−1) at the end of 500 cycles

Pulsed Current Electrodeposition of Silicon Thin Films Anodes for Lithium Ion Battery Applications

Electrodeposition of amorphous silicon thin films on Cu substrate from organic ionic electrolyte using pulsed electrodeposition conditions has been studied. Scanning electron microscopy analysis shows a drastic change in the morphology of these electrodeposited silicon thin films at different frequencies of 0, 500, 1000, and 5000 Hz studied due to the change in nucleation and the growth mechanisms. These electrodeposited films, when tested in a lithium ion battery configuration, showed improvement in stability and performance with an increase in pulse current frequency during deposition. XPS analysis showed variation in the content of Si and oxygen with the change in frequency of deposition and with the change in depth of these thin films. The presence of oxygen largely due to electrolyte decomposition during Si electrodeposition and the structural instability of these films during the first discharge–charge cycle are the primary reasons contributing to the first cycle irreversible (FIR) loss observed in the pulse electrodeposited Si–O–C thin films. Nevertheless, the silicon thin films electrodeposited at a pulse current frequency of 5000 Hz show a stable capacity of ~805 mAh·g−1 with a fade in capacity of ~0.056% capacity loss per cycle (a total loss of capacity ~246 mAh·g−1) at the end of 500 cycles

Nickel catalyst-assisted vertical growth of dense carbon nanotube forests on bulk copper

Vertical growth of carbon nanotubes using thermal chemical vapor deposition (CVD) is demonstrated on bulk copper substrates by first sputtering a thin Ni film on the surface of copper. Vertical growth of carbon nanotubes occurred when the nickel film thickness was 20 nm and the carbon nanotubes were grown using a xylene source and additional ferrocene catalyst during CVD. These results show the effectiveness of this method in directly integrating carbon nanotubes with highly conductive substrates for applications where a conductive carbon nanotube network is desirable. © 2011 American Chemical Society