PNNL

Abstract

It is well known that upon lithiation both crystalline and amorphous Si transform to a armorphous LixSi phase, which subsequently crystallizes to a (Li, Si) crystalline compound, either Li15Si or Li22Si5. Up to date, the detailed atomistic mechanism of this phase transformation, and degradation process in nanostructured Si, are not fully understood. Here, we report the microstructural evolution and phase transformation characteristic of a specially designed amorphous silicon (a-Si) coated carbon nanofiber (CNF) composite during charge/discharge process using in-situ TEM and density function theory molecular dynamic (DFT-MD) calculation. We found that 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.