Journal
NANO LETTERS
Volume 12, Issue 3, Pages 1624-1632Publisher
AMER CHEMICAL SOC
DOI: 10.1021/nl204559u
Keywords
Si-coated carbon nanofiber anode; Li-ion battery; in situ TEM; DFT-MD; congruent phase transition
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Funding
- Pacific Northwest National Laboratory (PNNL)
- DOE's Office of Biological and Environmental Research
- DOE [DE-AC05-76RLO1830]
- Vehicle Technologies for the Office of Energy Efficiency and Renewable Energy [DE-AC05-00OR22725]
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It is well-known that upon lithiation, both crystalline and amorphous Si transform to an armorphous LixSi phase, which subsequently crystallizes to a (Li, Si) crystalline compound, either Li15Si4 or Li22Si5. Presently, the detailed atornistic mechanism of this phase transformation and the degradation process in nanostructured Si are not fully understood. Here, we report the phase transformation characteristic and microstructural evolution of a specially designed amorphous silicon (a-Si) coated carbon nanofiber (CNF) composite during the charge/discharge process using in situ transmission electron microscopy and density function theory,molecular dynamic calculation. We found the crystallization of Li15Si4 from amorphous LixSi is a spontaneous, congruent phase transition process without phase separation or large-scale atomic motion, which is drastically different from what is expected from a classic nucleation and growth process. The a-Si layer is strongly bonded to the CNF and no spallation or cracking is observed during the early stages of cyclic charge/discharge. Reversible volume expansion/contraction upon charge/discharge is fully accommodated along the radial direction. However, with progressive cycling, damage in the form of surface roughness was gradually accumulated on the coating layer, which is believed to be the mechanism for the eventual capacity fade of the composite anode during long-term charge/discharge cycling.
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