Selenium, with similar chemical properties as sulfur, has recently attracted much research attention as potential electrodes for high energy density lithium-ion batteries. Unlike sulfur cathode with poor cycling performance due to the instability of Li-S compounds formed in lithiation and delithiation, Se is promising in regards to improvement in the cycling stability. We have used in situ transmission electron microscopy (TEM) to investigate the reaction mechanism and dynamics in the lithiation and delithiation cycles. We observe that single crystalline structure of pristine selenium nanowire gradually transformed to a polycrystalline structure as lithium continue insertion without intermediate amorphous phase formed during the whole lithiation process. And Li₂Se phase with a polycrystalline structure finally forms after full lithiation. Li-Se alloying reaction accompanies with dramatic volume expansion, including both elongation along axil direction and expansion in radial direction. High density of stress and strain occurs at reaction front region, and then these stresses and strains rapidly glide to unreacted region, and move out of the nanowire from the surface and edge eventually. This process efficiently drives large amount of lithium ions to move forward. For comparison, we studied also the structural evolutions in the sodiation and desodiation which may have great implications for the development of sodium-ion batteries. These observations and experiments not only can help to deeply understand the reaction mechanism and reaction process, but favor to design novel nanostructure of electrodes with excellent electrochemical performance.

This work was supported as part of the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DEAC02-06CH11357, and the Initiative for Sustainability and Energy at Northwestern (ISEN). This work was also supported by the NUANCE Center, and made use of the EPIC facility (NUANCE Center-Northwestern University), which has received support from the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); and the State of Illinois, through the IIN.