<p>Lithium−sulfur (Li−S) batteries are considered promising next-generation energy storage systems due to their high energy density, low-cost S cathodes, and potential for sustainable large-scale applications. However, they face significant challenges, including the lithium polysulfides (LiPS) shuttle effect associated with the S cathodes and interfacial reactions at the Li metal anodes, which occur concurrently and hinder their stable operation. To address these simultaneous issues at both the cathode and anode, we propose the use of complex hydrides as an electrolyte additive. <i>Closo</i>-type complex hydride Li(CB₁₁H₁₂) dissociates in the electrolyte to form mobile lithium cations and rigid polyanionic complex anions [CB₁₁H₁₂]⁻, which form Li⁺−solvent coordination by its weak coordinating ability. The weakened solvation free energy and the reducing ability of this modified electrolyte suppress LiPS dissolution at the S cathode as well as facilitates stable lithium anode reaction, enhancing the cycling stability and coulombic efficiency of Li−S batteries. The findings of this study underscore the importance of designing electrolyte structures using various complex hydrides and highlight their potential to address the limitations of Li−S batteries, paving the way for future advancements in this field.</p>