Unique electrochemical behavior of heterocyclic selenium-sulfur cathode materials in ether-based electrolytes for rechargeable lithium batteries

Mixed chalcogenide systems represent a new class of promising cathode materials for high performance rechargeable lithium batteries. Among them, heterocyclic selenium-sulfur (SexSy) cathodes, coupling the good conductivity of Se and high capacity of S, have attracted great attention in recent years. However, further research is still needed to better understand the lithiation/delithiation process of SexSy cathodes in ether-based electrolytes. Herein, SexSy-based composites with covalent Se-S bonds were prepared by infiltrating various proportions of S and Se powders into the mesoporous carbon microsphere (MCM) host at 500 °C in vacuum. As cathodes for rechargeable lithium batteries, SexSy/MCM composites exhibit unique electrochemical behavior rather than a simple hybrid of Li-S and Li-Se batteries in ether-based electrolytes. The Se-S bonds in SexSy/MCM could anchor S during cycling, and effectively reduce the formation of long-chain polysulfides. As a result, the SexSy/MCM cathodes demonstrate superior overall electrochemical performance than Se/MCM or S/MCM. Moreover, Se/S ratio could also affect the electrochemical behavior of SexSy/MCM composites. The optimal Se2S5/MCM cathode delivers the best performance with a high reversible capacity of 796.4 mAh g−1 at 0.5 C over 100 cycles, and good rate capability of 688.8 mAh g−1 at 5 C. Especially, a superior coulombic efficiency of ~100% is achieved without addition of LiNO3. This work could help us to better understand the inherent synergistic mechanism behind SexSy cathodes for high performance rechargeable lithium batteries in ether-based electrolytes.