title: In situ TEM investigation of congruent phase transition and structural evolution of nanostructured silicon/carbon anode for lithium ion batteries.
abstract: It is well-known that upon lithiation, both crystalline and amorphous Si transform to an armorphous Li(x)Si phase, which subsequently crystallizes to a (Li, Si) crystalline compound, either Li(15)Si(4) or Li(22)Si(5). 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 Li(15)Si(4) from amorphous Li(x)Si 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.
authors.affiliation: Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, USA. Chongmin.Wang@pnnl.gov
authors.forename: Chong-Min
authors.initials: CM
authors.lastname: Wang
authors.forename: Xiaolin
authors.initials: X
authors.lastname: Li
authors.forename: Zhiguo
authors.initials: Z
authors.lastname: Wang
authors.forename: Wu
authors.initials: W
authors.lastname: Xu
authors.forename: Jun
authors.initials: J
authors.lastname: Liu
authors.forename: Fei
authors.initials: F
authors.lastname: Gao
authors.forename: Libor
authors.initials: L
authors.lastname: Kovarik
authors.forename: Ji-Guang
authors.initials: JG
authors.lastname: Zhang
authors.forename: Jane
authors.initials: J
authors.lastname: Howe
authors.forename: David J
authors.initials: DJ
authors.lastname: Burton
authors.forename: Zhongyi
authors.initials: Z
authors.lastname: Liu
authors.forename: Xingcheng
authors.initials: X
authors.lastname: Xiao
authors.forename: Suntharampillai
authors.initials: S
authors.lastname: Thevuthasan
authors.forename: Donald R
authors.initials: DR
authors.lastname: Baer
mesh_terms: D002244:Carbon / Q000737:chemistry*; D003198:Computer Simulation; D011211:Electric Power Supplies; D004566:Electrodes; D004867:Equipment Design; D019544:Equipment Failure Analysis; D007477:Ions; D008094:Lithium / Q000737:chemistry*; D046529:Microscopy, Electron, Transmission; D008956:Models, Chemical; D049329:Nanostructures / Q000737:chemistry* / Q000648:ultrastructure; D010316:Particle Size; D044367:Phase Transition; D012825:Silicon / Q000737:chemistry*
chemical_list: D007477:Ions; D002244:Carbon; D008094:Lithium; D012825:Silicon