Characterization of Structures and Properties of Liquid and Amorphous LixSi Alloys using First Principles Calculations and Molecular Dynamic Simulations
Date Issued
2011
Date
2011
Author(s)
Chiang, Han-Hsin
Abstract
This thesis consists of two main focuses. In the first part, we investigated the structures and properties of the liquid Li-Si alloys, while in the second part we studied those of their amorphous structures. We employed first-principles calculations and molecular dynamic simulations to generate realistic structural models of the liquid and amorphous alloys within various compositions. Based on the generated structural models, we subsequently examined the structures, dynamics, and thermodynamics, as well as the electronic properties of the Li-Si alloy system.
Our results showed that Si atoms usually tend to form covalent bonding in the liquid alloys even when the Si content is very low. Unlike the crystalline systems, Si atoms do not form any specific polyanions in the liquid alloys but form various types of segments containing a wide range of local configurations. Moreover, the average size of Si clusters was found to increase steadily with increasing the Si content, and eventually they turn into a continuous bond network as the Si concentration exceeds a critical value. As the temperature increases, there is no significant change in the average distances between different atom pairs though the size of the Si cluster may drop significantly. The change in the local structures was also manifested in the vibrational spectra in which the high frequency vibrational modes of Si were significantly reduced as the system was at very high temperature.
The calculated dynamic property of the liquid alloys showed that the diffusivities of Li and Si atoms at 1050 K both decrease with increasing the Si content, but the diffusivities of Si atoms appear to be independent of the alloy composition as the temperature is raised above 1500K. Furthermore, the Bader charge analysis showed that there are residual metallic Li atoms in the highly-lithiated liquid alloys, resulting in higher conductivity for those systems. As the Si concentration increases, all the valence electrons of Li in the liquid alloys are ionized by the Si atoms, leading to lower conductivity due to the depletion of free charge carriers in the system.
The structures and electronic properties of the amorphous Li-Si alloys at various compositions were also examined in this study. Our results showed that the short-range order in the amorphous structures is in close resemblance to the liquid alloys at 1050K, but the densities of the amorphous alloys are about 10~15% higher than their liquid phases. Moreover, the configurations of polyanions in the amorphous alloys are similar to those in the liquid states but their average cluster sizes are slightly larger than those in the liquids. On the other hand, we also observed a resistivity plateau at a specific range of composition for the amorphous alloys, which is similar to that found in the liquid phases but the resistivity of the amorphous alloys is slightly higher.
Subjects
Li-Si alloys
First-principles calculations
Molecular dynamics simnulations
Type
thesis
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