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

• Published in: Nano letters Vol. 12; no. 3; pp. 1624 - 1632
• Main Authors: Wang, Chong-Min, Li, Xiaolin, Wang, Zhiguo, Xu, Wu, Liu, Jun, Gao, Fei, Kovarik, Libor, Zhang, Ji-Guang, Howe, Jane, Burton, David J, Liu, Zhongyi, Xiao, Xingcheng, Thevuthasan, Suntharampillai, Baer, Donald R
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 14.03.2012
• Subjects: Amorphous silicon; Applied sciences; Carbon - chemistry; Charge; Computer Simulation; Condensed matter: structure, mechanical and thermal properties; Crystal structure; Density; Direct energy conversion and energy accumulation; Discharge; Electric Power Supplies; Electrical engineering. Electrical power engineering; Electrical power engineering; Electrochemical conversion: primary and secondary batteries, fuel cells; Electrodes; Equations of state, phase equilibria, and phase transitions; Equipment Design; Equipment Failure Analysis; Exact sciences and technology; Ions; Lithium - chemistry; Lithium-ion batteries; Low-dimensional structures (superlattices, quantum well structures, multilayers): structure, and nonelectronic properties; Microscopy, Electron, Transmission; Models, Chemical; Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals; Nanostructure; Nanostructures - chemistry; Nanostructures - ultrastructure; Particle Size; Particulate composites; Phase transformations; Phase Transition; Physics; Silicon; Silicon - chemistry; Specific phase transitions; Structural transitions in nanoscale materials; Structure of solids and liquids; crystallography; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); DFT-MD; Si-coated carbon nanofiber anode; Li-ion battery; congruent phase transition; in situ TEM; Nucleation; Phase transformations; Molecular dynamics method; Roughness; Carbon electrode; Cracking; Mechanical properties; Nanostructures; Coatings; Carbon; Phase transitions; Lithium compounds; Lithium battery; Composite materials; Transmission electron microscopy; Nanocomposites; Phase separation; Growth mechanism; Silicon; Damage; Microstructure; Strength functions; Crystal structure
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/nl204559u

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 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 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.