In Situ Gel Polymer Electrolyte Integrated With Polyethylene Separator for Reduced Tortuosity and Enhanced Li ⁺ Transference in Lithium‐Ion Batteries

ABSTRACT

Liquid electrolytes in lithium‐ion batteries face persistent challenges arising from solvent volatility, separator‐induced transport limitations, and low lithium‐ion transference numbers, especially under elevated‐temperature operation. To address these limitations, this study investigates in situ–formed gel polymer electrolytes (GPEs) integrated into widely used polyethylene separator and coated electrodes, where the electrolyte precursor polymerizes within the separator microstructure to form a partially immobilized yet ion‐permissive medium. When integrated into polyethylene separator, the GPE delivers a bulk ionic conductivity of 2.16 mS cm ⁻¹ , comparable to that of the liquid electrolyte, indicating that polymer integration does not introduce significant transport penalties. The effective lithium‐ion transference number increases markedly from 0.28 to 0.55, consistent with moderated anion mobility within the polymer‐rich domains. The GPE also significantly enhances ion transport and interfacial stability at elevated temperatures, resulting in improved capacity retention, reduced impedance growth, and more stable cycling with sustained Coulombic efficiency. Cross‐sectional FE‐SEM further reveals more intact cathode interphases with mitigated active‐material detachment, consistent with reduced solvent‐volatility‐driven degradation in the GPE system. Collectively, these results demonstrate that separator‐integrated, in situ GPEs offer a compelling route to simultaneously enhance ion‐transport properties and improve high‐temperature durability in practical lithium‐ion batteries.