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

摘要

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. udIn 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). ududAll 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.ud