LiFSI-Initiated In Situ Polymerization of Sulfur-Containing Allyl Thiirane: High-Conductivity, High-Voltage, and Interface-Stabilizing Electrolytes for Lithium-Ion Batteries

The development of safe, high-performance polymer electrolytes with fast lithium-ion transport, wide electrochemical stability, and durable interfaces remains a critical challenge for next−generation lithium-ion batteries. Herein, we report a sulfur-containing allyl thiirane monomer that undergoes LiFSI-initiated in situ cationic ring-opening polymerization to form an amorphous poly(allyl-mercapto-thiirane) (PAMT) gel polymer electrolyte. By integrating a sulfur-rich backbone with pendant allyl groups, the PAMT network simultaneously promotes weak Li + coordination for rapid ion migration and built-in interphase-forming functionality. The optimized PAMT electrolyte exhibits a low glass transition temperature (around −44 °C) and delivers high ionic conductivity up to 8.5 × 10 −3 S cm −1 at 80 °C, a low activation energy of 0.24 eV, and a high lithium-ion transference number of 0.80. Linear sweep voltammetry demonstrates oxidative stability above 5 V vs Li/Li +, enabling compatibility with high-voltage cathodes. The in situ polymerization process generates intimate and conformal electrode−electrolyte interfaces, producing uniform cathode−electrolyte interphases on LiFePO 4 and dense, LiF-rich solid−electrolyte interphases on lithium metal. As a result, LiFePO 4 ∥Li cells deliver an initial discharge capacity of ∼146 mAh g −1 at 0.1 C, retain ∼102 mAh g −1 at 1 C, and maintain stable cycling for over 200 cycles with Coulombic efficiencies exceeding 98%. Compared with representative gel and solid polymer electrolytes, the PAMT system achieves a rare combination of high conductivity, wide voltage tolerance, and interfacial durability. This work establishes LiFSI-triggered in situ polymerization of sulfur-containing thiirane monomers as an effective strategy for designing advanced polymer electrolytes for high-voltage and high-safety lithium batteries.