Enhanced Lithium-Ion Transportability of Poly(ether-thioether)s and Their Oxidized Products under Polymer-in-Salt Conditions

Solid polymer electrolytes (SPEs) under polymer-in-salt conditions have attracted considerable attention due to their potential to achieve high ionic conductivity overcoming conventional SPEs under salt-in-polymer conditions. Although the efficiency of moderate interaction between polymers and lithium salts for high ionic conductivity has been reported, a design strategy for highly conductive polymers, especially for sulfur-containing polymers, was still unclear. In this research, we report that sulfonyl groups in the backbone of aliphatic polyethers give rise to high ionic conductivity, derived from the combination of machine learning predictions and experiments. We also revealed that LiFTFSI, a lithium salt with an asymmetric structure, behaved as a supercooled liquid near room temperature in the composite with the sulfur-containing polymer. The moderate Coulombic interaction between the sulfonyl units in the polymer and the lithium salt appeared to help maintain the metastable state, which contributed to the mobility of the ions due to the thermal motion of the lithium salt, allowing ionic conduction under the polymer-in-salt conditions.