YOSHIFUMI HIROTSU

<jats:title>ABSTRACT</jats:title> <jats:p> A series of ionic liquids (ILs) based on tetrahydrothiophenium (Tht) cations bearing alkyl or ether side chains (containing one to four oxygen atoms), paired with bis(fluorosulfonyl)amide (FSA) or bis(trifluoromethanesulfonyl)amide (TFSA) anions, was synthesized to examine relationships among ether side‐chain length, anion structure, and physicochemical properties. All samples showed glass transitions, with glass transition temperature increasing as the ether chain lengthened. Crystallization and melting behavior depended strongly on side‐chain length, anion species, and cooling rate. ILs with short ether side chains exhibited low viscosity and ionic conductivity exceeding 10 <jats:sup>−3</jats:sup>  S cm <jats:sup>−1</jats:sup> at 25°C, while longer ether chains led to higher viscosity and lower conductivity, reflecting a trade‐off between enhanced conformational flexibility and reduced ionic mobility due to larger cation size. FSA‐based ILs generally showed lower viscosity and higher conductivity than TFSA‐based analogues. Electrochemical stability also depended on both side‐chain length and anion species, though trends differed from those governing ion transport. These results demonstrate that the Tht framework can achieve ionic conductivities on the order of 10 <jats:sup>−3</jats:sup>  S cm <jats:sup>−1</jats:sup> , comparable to well‐studied pyrrolidinium‐based ILs, with short‐ether‐chain Tht ILs offering favorable transport properties and tunable physicochemical characteristics—useful guidelines for designing high‐performance ILs. </jats:p>