Rohrer, Jochen ; Albe, Karsten (2013)
Insights into Degradation of Si Anodes from First-Principle Calculations.
In: The Journal of Physical Chemistry C, 117 (37)
doi: 10.1021/jp401379d
Artikel, Bibliographie

Kurzbeschreibung (Abstract)

We perform large-scale density functional theory calculations of various amorphous LixSi (a-LixSi)model geometries that may appear during electrochemical (de)lithiation of silicon anodes. Based on associated formation energies, we derive the evolution of the phase content during insertion/extraction of lithium. For insertion of Li into crystalline Si (c-Si), we find that a c-Si/a-Li2Si two-phase region is thermodynamically favored. Insertion into amorphous Si (a-Si) leads to an a-Li xSi one-phase region. For insertion beyond Li 3.75 Si, an a-Li 3.625 Si/a-Li 4.4 Si two-phase region is expected; this two-phase region may also appear during extraction (if a-Li 4.4 Si was reached during insertion). Crystallization of c- Li 3.75 Si at the very end of Li insertion leads to a two-phase region during Li extraction, where c-Li 3.75 Si and a-Li 2Si coexist. Within two-phase regions, one phase is converted into the other, accompanied by inhomogeneous change of the specific volume. In contrast to the commonly adopted view that the overall volume changes lead to cracking of Si anodes, we argue that the inhomogeneous volume changes are key mechanisms that significantly contribute to degradation. Assuming percental volume changes to be indicative for the degree of degradation, our calculations qualitatively agree with experimental findings that (a) a-Si anodes have longer lifetimes than c-Si anodes and (b) nanowires (where a-Li 4.4 Si is reached) have longer lifetimes than for example nanoparticles or thin films (where crystallization of c-Li 3.75 Si is observed).

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