Lithium/sulfur (Li/S) battery has been under scrutiny in the past decades owing to its low cost, abundance, nontoxic as well as much higher energy density than conventional lithium ion battery.¹ Lithium/selenium (Li/Se) battery has also attracted an increasing attention in recent years owing to its comparable volumetric capacity with sulfur and much higher electronic conductivity (~ 20X) than sulfur.² It has been intensively reported that the electrolytes play a great role on the electrochemical performance of selenium and sulfur based cathodes.³ Among them, carbonate-based and ether-based electrolytes are two kinds of popular electrolytes for selenium-sulfur based cathodes. Selenium has been found to be well adaptive to carbonate-based electrolytes; the space-confined selenium displays excellent electrochemical performance in Li-Se battery with carbonate-based electrolytes.⁴ While the ether-based electrolytes could facilitate the redox reaction of sulfur-based cathodes and therefore could offer higher reversible capacity than that of carbonate-based electrolytes. It was found that Se is reduced to the polyselenides, Li₂Se_n (n≥4), Li₂Se₂, and Li₂Se sequentially during the lithiation process, and Li₂Se is oxidized to Se through Li₂Se_n (n≥4) during the de-lithiation process in the 1st cycle, which undergoes similar reaction process to the sulfur system.⁵ Therefore, it is very important to design rational Se cathodes with conductive host to constrain the polyselenides and avoid the shuttle effect. However, even with a very good encapsulation of selenium or selenium-sulfur in the various carbon host materials, most of the previous reported Se/C or Se-S/C cathodes still show a gradual capacity fading in ether-based electrolytes.^6-8 It has been widely reported that sulfur/carbon composites with sulfur loading ranging from 30 wt. %-75wt. % can maintain a stable reversible capacity above 600 mAh g^-1 within 100 cycles in ether-based electrolytes by optimizing the structure of carbon host materials.⁹ Therefore, the confinement of carbon host materials for polysulfides and polyselenides may be different although they both suffer from dissolution of reaction intermediates in ether-based electrolytes. To the best of our knowledge, such a ubiquitous phenomenon has been paid less attention and not yet well addressed. Moreover, the detailed capacity fading mechanism of Se-based cathodes in ether-based electrolytes is still unknown.

Herein, we are motivated to disclose the lithiation/de-lithiation process and capacity fading mechanism of Se-based cathodes in ether-based electrolytes upon continuous cycling by using in-situ X-ray absorption near edge spectroscopy. Microporous carbon (MPC) was selected as carbon host to enhance the confinement for polysulfides and polyselenides. Se₂S₅/MPC composite was then prepared by a modified vaporization-condensation method to ensure a good encapsulation of Se₂S₅ into the MPC host. The in-situ XANES results clearly illustrate that the reaction reversibility of Se was gradually decreased upon continuous cycling which should be responsible for the common capacity fading of Se-based cathodes in ether-based electrolytes. We expect our current study could enable a better understanding on the lithiation/de-lithiation process of Se-based cathodes and facilitate the development of high performance Li/Se and Li/Se-S battery in the future.

Acknowledgements: Research at the Argonne National Laboratory was funded by U.S. Department of Energy, Vehicle Technologies Office. Argonne National Laboratory is operated for the U.S. Department of Energy by UChicago Argonne, LLC, under contract DE-AC02-06CH11357.

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