First-principles study of the structural and dynamic properties of the liquid and amorphous Li–Si alloys

Abstract

We have performed density functional theory calculations and ab initio molecular dynamics to investigate the structures and dynamic properties of the liquid and amorphous LixSi alloys over a range of composition from x = 1.0 - 4.8. Our results show that Si atoms can form a variety of covalently bonded polyanions with diverse local bonding structures in the liquid alloys. Like in c-LiSi, Si atoms can form a continuous bond network in liquid Li1.0Si at 1050 K, while it gradually disintegrates into many smaller Si polyanions as the Li content increases in the alloys. The average sizes of Si polyanions in these liquid alloys were found to be relatively larger than those in their crystalline counterparts, which can even persist in the highly lithiated Li4.81Si alloy at 1500 K. Our results also show that amorphous LixSi alloys have similar local bonding structures but a largely increased short-range order as compared to their liquid counterparts. The differences between the average coordination number of each atomic pair in amorphous solids and that in the liquids are less than 1.1. Furthermore, our calculations reveal that Li and Si atoms can exhibit very distinct dynamic behaviors in the liquids and their diffusivities appear to be largely dependent on the chemical composition of the alloys. The diffusivity of Li was found to increase with the Li content in the alloys primarily because of the reduced interactions between Li and Si atoms, while the Si diffusivity also increases due to the gradual disintegration of the strongly interconnected Si bond network. The diffusivity of Li in amorphous LixSi was predicted to lie in the range between 10(-7) and 10(-9) cm(2)/s at 300 K, which is more than 20-fold larger than that of Si over the composition range considered. Our calculations further show that the diffusivities of both Li and Si can increase by two orders of magnitude as x increases from 1.0 to 3.57 in amorphous LixSi, indicating a more profound dependence on the alloy composition than those in the liquid state.

Lithium Diffusion in Ion-Beam Sputter-Deposited Lithium–Silicon Layers

“…Note also that in other theoretical work (not shown), diffusivities were calculated which are significantly higher by orders of magnitude. 39,40 As a result of this discussion, it can be stated that Li diffusion in Li x Si samples with a high Li concentration shows a lower activation energy and is faster than in samples with a low Li concentration.…”

Lithiation Products of a Silicon Anode Based on Soft X-ray Emission Spectroscopy: A Theoretical Study

“…However, even at the high Li concentration in amorphous a-Li x Si alloys, Si tends to form Si−Si covalent bonds in a good agreement with the results of previous theoretical calculations. 25,34 In Figure 2, we summarize the average coordination number at the cutoff distance of 2.65 Å as a function of the lithium content calculated for the crystalline and amorphous structures of Li x Si considered in the present work.…”

Interface Structure in Li-Metal/[Pyr14][TFSI]-Ionic Liquid System from ab Initio Molecular Dynamics Simulations

“…Note that the Li-metal part in our simulations should rather be considered as amorphous Li than crystalline. The calculated Li−Li pair radial distribution function (RDF) ( Figure S1 in Supporting Information) has a first peak at ∼2.8 Å which is a typical value for the Li−Li distance in amorphous Li, 32,33 which is shorter than that in the crystalline Li-metal structure (3.02 Å). 34 Reactive DFT-MD on the smaller 83Li/2[TFSI] system.…”