Not fade away
Transmission electron microscopy transforms how we see lithium-ion batteries
Generating longer-life lithium-ion (Li-ion) batteries could be part of the remedy for overdependence on fossil fuels, affecting everything from vehicles to manufacturing. Using EMSL’s in situ transmission electron microscopy (TEM) capabilities, scientists are edging closer to pinpointing the atomic-level changes that lead to anode failure in Li-ion batteries. Looking for methods to counter anode capacity fade, a team of EMSL users from national laboratories and industry examined anodes composed of hollow carbon nanofibers (CNFs) coated with a thin layer of amorphous silicon (Si)—a candidate electrochemical electrode material. As the anodes were charged via Li ion injection, TEM imaging showed—and theoretical calculations supported—that crystallization of Li15Si4 happened spontaneously without long range diffusion and phase separation. Upon discharge, it returned to its prior non-crystalline state. The formation of the Li15Si4 corresponds to a Li-holding capacity that is different from what was believed in the past, and appears to be related to phase stability/reversibility of the Si-Li system. The team repeated the charging/discharging cycles and only after this progressive cycling did surface roughness accumulate on the Si layer, a likely cause of anode storage-capacity fade. The work also showed the thin amorphous Si layer strongly adhered to the CNF, which helps account for the observed good battery cyclability using this material. Understanding the structural and phase transformation characteristics of CNFs with Si coatings provides Li-ion battery designers information needed for optimizing Si’s high capacity while maximizing the Li-ion battery’s reliability by manipulating details such as coating layer thickness, CNF diameter, and bonds between the coating layer and CNFs.
Reference: Wang C-M, X Lin, Z Wang, W Xu, J Liu, F Gao, L Kovarik, J-G Zhang, J Howell, DJ Burton, Z Liu, X Xiao, S Thevuthasan, and DR Baer. 2012. “In Situ TEM Investigation of Congruent Phase Transition and Structural Evolution of Nanostructured Silicon/Carbon Anode for Lithium Ion Batteries.” Nano Letters 12(3):1624-1632. DOI: 10.1021/nl204559u.
Participants: EMSL staff and users from Pacific Northwest National Laboratory (PNNL), Oak Ridge National Laboratory (ORNL), Applied Sciences Inc., and General Motors Global Research & Development Center.
Acknowledgment: This work was supported in part by PNNL’s Laboratory Directed Research and Development program. The work was performed using EMSL, a national scientific user facility located at PNNL. EMSL’s TEM was funded as part of the American Recovery and Reinvestment Act of 2009. Work conducted at ORNL was sponsored by the Vehicle Technologies program for the Office of Energy Efficiency and Renewable Energy.
Released: March 27, 2012