In Situ TEM Investigationof Congruent Phase Transitionand Structural Evolution of Nanostructured Silicon/Carbon Anode forLithium Ion Batteries.

It is well-known that upon lithiation, both crystallineand amorphousSi transform to an armorphous LixSi phase,which subsequently crystallizes to a (Li, Si) crystalline compound,either Li15Si4or Li22Si5. Presently, the detailed atomistic mechanism of this phase transformationand the degradation process in nanostructured Si are not fully understood.Here, we report the phase transformation characteristic and microstructuralevolution of a specially designed amorphous silicon (a-Si) coatedcarbon nanofiber (CNF) composite during the charge/discharge processusing in situ transmission electron microscopy and density functiontheory molecular dynamic calculation. We found the crystallizationof Li15Si4from amorphous LixSi is a spontaneous, congruent phase transition process withoutphase separation or large-scale atomic motion, which is drasticallydifferent from what is expected from a classic nucleation and growthprocess. The a-Si layer is strongly bonded to the CNF and no spallationor cracking is observed during the early stages of cyclic charge/discharge.Reversible volume expansion/contraction upon charge/discharge is fullyaccommodated along the radial direction. However, with progressivecycling, damage in the form of surface roughness was gradually accumulatedon the coating layer, which is believed to be the mechanism for theeventual capacity fade of the composite anode during long-term charge/dischargecycling. [ABSTRACT FROM AUTHOR]