Multifunctional LiClO₄ for Polymerization Regulation, Ionic Conduction, and Interphase Formation in In Situ Poly(tetrahydrofuran) Gel Polymer Electrolytes

In situ polymerized gel polymer electrolytes (GPEs) offer a promising route to improve electrode–electrolyte interfacial compatibility in quasi-solid-state lithium metal batteries. Although LiClO₄ has been employed in such systems, its role beyond a lithium-ion source has not been systematically investigated. Here, we report a two-component electrolyte consisting of 1 M LiClO₄ and 0.3 M BF₃-THF (1-0.3PTHF) that enables the in situ formation of poly(tetrahydrofuran) (PTHF) with substantially reduced initiator loading. The incorporation of LiClO₄ not only provides mobile Li⁺ ions but also facilitates polymerization. Possible ionic interactions between LiClO₄ species and growing polymer chains may contribute to the improvement. The 1-0.3PTHF exhibited a narrow mass distribution (Đ = 1.23) and a low degree of crystallinity (4.7%) compared to commercial PTHF (Đ = 3.25) and the PTHF polymerized without LiClO₄. The in situ 1-0.3PTHF delivered a bulk ionic conductivity of 2.75 × 10⁻³ S cm⁻¹, an electrochemical window of 5.06 V, and stable lithium plating/stripping up to 3 mA cm⁻², suggesting its promise as a GPE. Post-mortem XPS confirms the coexistence of B-containing species and LiCl within the SEI. Full cell tests demonstrated the preliminary feasibility of the 1-0.3PTHF GPE with LFP, LCO, and NCM cathodes. This work highlights the multifunctional role of LiClO₄ in in situ polymerized PTHF. The resulting 1-0.3PTHF electrolyte simultaneously regulates polymerization, enhances ionic conduction, and promotes protective SEI formation. These findings broaden the role of lithium salts beyond lithium-ion sources to polymerization modulators in in situ polymerized GPEs.