In Situ TEM Investigation of Congruent Phase Transition and Structural Evolution of Nanostructured Silicon/Carbon Anode for Lithium Ion Batteries

It is well-known that upon lithiation, both crystalline and amorphous Si transform to an armorphous Li_xSi phase, which subsequently crystallizes to a (Li, Si) crystalline compound, either Li₁₅Si₄ or Li₂₂Si₅. Presently, the detailed atomistic 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 Li_xSi 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.