(De)lithiation mechanism of Li/SeS_x (x = 0-7) batteries determined by in situ synchrotron x-ray diffraction and X-ray absorption spectroscopy

Electrical energy storage for transportation has gone beyond the limit of converntional lithium ion batteries currently. New material or new battery system development is an alternative approach to achieve the goal of new high-energy storage system with energy densities 5 times or more greater. A series of SeS_x-carbon (x = 0-7) composite materials has been prepared and evaluated as the positive electrodes in secondary lithium cells with ether-based electrolyte. In situ synchrotron high-energy X-ray diffraction was utilized to investigate the crystalline phase transition during cell cycling. Complementary, in situ Se K-edge X-ray absorption near edge structure analysis was used to track the evolution of the Se valence state for both crystalline and noncrystalline phases, including amorphous and electrolyte-dissolved phases in the (de)lithiation process. On the basis of these results, a mechanism for the (de)lithiation process is proposed, where Se is reduced to the polyselenides, Li₂Se_n (n ≥ 4), Li₂Se₂, and Li₂Se sequentially during the lithiation and Li₂Se is oxidized to Se through Li₂Se_n (n ≥ 4) during the delithiation. In addition, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy demonstrated the reversibility of the Li/Se system in ether-based electrolyte and the presence of side products in the carbonate-based electrolytes. For Li/SeS₂ and Li/SeS₇ cells, Li ₂Se and Li₂S are the discharged products with the presence of Se only as the crystalline phase in the end of charge. © 2013 American Chemical Society.