The lithiation process and Li diffusion in amorphous SiO2 and Si from first-principles

• Published in: Electrochimica acta Vol. 331; p. 1
• Main Authors: Sivonxay, Eric, Aykol, Muratahan, Persson, Kristin A.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 20-01-2020; Elsevier BV
• Subjects: Ab-initio molecular dynamics; Activity recognition; Alloy anodes; Amorphization; Amorphous materials; Amorphous silicon; Anodes; Chemical evolution; Density functional theory calculations; Diffusivity; Electrode materials; Energy storage; First principles; Impact analysis; Li-Ion batteries; Lithium oxides; Mathematical analysis; Organic chemistry; Silicides; Silicon anode; Silicon dioxide; Surface layers; Alloy anodes; Ab-initio molecular dynamics; Silicon anode; Li-Ion batteries; Density functional theory calculations
• ISSN: 0013-4686; 1873-3859
• DOI: 10.1016/j.electacta.2019.135344

Silicon is considered the next-generation, high-capacity anode for Li-ion energy storage applications, however, despite significant effort, there are still uncertainties regarding the bulk Si and surface SiO2 structural and chemical evolution as it undergoes lithiation and amorphization. In this paper, we present first-principles calculations of the evolution of the amorphous Si anode, including its oxide surface layer, as a function of Li concentration. We benchmark our methodology by comparing the results for the Si bulk to existing experimental evidence of local structure evolution, ionic diffusivity as well as electrochemical activity. Recognizing the important role of the surface Si oxide (either native or artificially grown), we undertake the same calculations for amorphous SiO2, analyzing its potential impact on the activity of Si anode materials. Derived voltage curves for the amorphous phases compare well to experimental results, highlighting that SiO2 lithiates at approximately 0.7 V higher than Si in the low Li concentration regime, which provides an important electrochemical fingerprint. The combined evidence suggests that i) the inherent diffusivity of amorphous Si is high (in the order 10−9cm2s−1 - 10−7cm2s−1), ii) SiO2 is thermodynamically driven to lithiate, such that Li–O local environments are increasingly favored as compared to Si–O bonding, iii) the ionic diffusivity of Li in LiySiO2 is initially two orders of magnitude lower than that of LiySi at low Li concentrations but increases rapidly with increasing Li content and iv) the final lithiation product of SiO2 is Li2O and highly lithiated silicides. Hence, this work suggests that - excluding explicit interactions with the electrolyte - the SiO2 surface layer presents a kinetic impediment for the lithiation of Si and a sink for Li inventory, resulting in non-reversible capacity loss through strong local Li–O bond formation.
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