First principles study of silicon-based nanomaterials for lithium ion battery anodes

Silicon (Si)-based materials have recently emerged as a promising candidate for anodes in lithium-ion batteries because they exhibit much higher energy-storage capacities than the conventional graphite anode. However, the practical use of Si is hampered by its poor cycleability; during lithiation, Si forms alloys with Li and undergoes significant structural and volume changes, which can cause severe cracking/pulverization and consequent capacity fading arising from the loss of electrical contacts. To overcome these drawbacks, many innovative approaches have been explored with encouraging results; however, many fundamental aspects of the lithiation behavior remain ambiguous. Hence, the focus of this work is to develop a better understanding of the lithiation process at the atomistic scale using quantum mechanical calculations. In addition, based on the improved understanding, we attempt to address the fundamental mechanisms behind the successful approaches to enhance the anode performance. To lay a foundation for the investigation of alloy-type anodes, in Chapter 3, we first examine how lithiation occurs in Si and the formation of crystalline and amorphous LixSi alloys (0 ≤ x ≤ 4); followed by assessing the lithiation-induced changes in the energetics, atomic structure, electronic and mechanical properties, and Li diffusivity. The same approach is then extended to analyze the lithiation behavior of germanium (Ge) and tin (Sn) for developing a generalized understanding on the Group IV alloy-type anodes. Along this comparative study, we notice a few distinguishing features pertain only to Si (or Ge), such as the facile Li diffusion in Ge and facet-dependent lithiation in Si, which are discussed in Chapter 4. Beyond the fundamental research, we also look into factors that may contribute to the improved anode performance, including (i) finetuning of the oxidation effects in Si-rich oxides, [alpha] -SiO [subscript 1/3] (Chapter 5), (ii) maximizing the surface effects through nano-engineered structures (Chapters 6 & 7), and finally (iii) the role of interface in Si-graphene (carbon) composites (Chapter 8).