Professor Yu Seung-ho’s group in the Department of Chemical and Biological Engineering (first authors: Doctor Um Ji-hyun and Doctor Jin Ai-hua) conducted a joint study with the research team of Professor Héctor D. Abruña (co-corresponding author) at Cornell University and showed a real-time structural change of the selenium cathode and the competitive nucleation and growth behavior of the electrodeposited selenium.
The results presented approaches to achieving nucleation-favored electrodeposition conditions when designing cathode materials based on various chalcogens (oxygen group elements) including electrolyte-soluble intermediates.
The results of the study were published online in Energy & Environmental Science (IF=38.532), an acclaimed international journal, on February 15.
In an effort to overcome the low energy densities of current commercial lithium-ion batteries, the potential cathode materials being considered include not only the conventional layered oxides but also the chalcogens (oxygen (O), sulfur (S), selenium (Se), tellurium (Te), polonium (Po) and livermorium (Lv)). Among them, selenium, featuring a theoretical capacity 2.5 times higher than layered oxide-based cathode materials and high electric conductivity, is emerging as an alternative to sulfur, which, although it has a higher theoretical capacity than selenium, is difficult to develop as a cathode material because its electrical conductivity is as low as an insulator.
The deep-seated problem of selenium cathodes is the shuttling caused by the formation of electrolyte-soluble polyselenide (Li2Sen (n≥4)) intermediates during charge/discharge. This shuttling refers to the diffusion of electrolyte-dissolved polyselenide between the cathode and the anode, resulting in the loss of cathode active materials and poor battery stability. Many recent studies on selenium have focused on designs confining the electrolyte-dissolved polyselenide within a porous carbon structure. However, the essential reaction mechanism of selenium cathodes been insufficiently studied, with no previous studies including an imaging analysis of the lithium selenide (Li2Se) formed during discharging or the selenium formed from lithium selenide during charging.
Professor Yu’s group performed a real-time observation of the actual reactions of the selenium cathode during the charge/discharge process through operando transmission X-ray imaging, in which the characteristic analysis is carried out simultaneously with cathode operation. To analyze the chemical state distribution, X-ray absorption near-edge structure (XANES) imaging was performed on the images obtained from Transmission X-ray microscopy (TXM) with phase contrast enhancement and X-ray absorption spectroscopy (XAS). In other words, the research group performed the world’s first real-time monitoring of liquid polyselenide, electrodeposited selenium, and lithium selenide in the entire process from nucleation to growth during the charge/discharge process. In addition, the research group tracked individual nucleation reactions, thereby revealing the competitive relationship between the growth of existing nuclei and additional nucleation depending on polyselenide concentration. Furthermore, they were able to induce nucleation-favorable conditions to achieve high selenium active material reversibility.
Professor Yu said, “We investigated the reaction mechanism of the selenium cathode and presented the nucleation-favoring battery operation conditions. We expect that our results can make great contributions to the improvement of the performance and stability of next-generation batteries based on various chalcogen elements.”
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