Polymerization‐Initiator Engineering Enables High‐Voltage‐Stable Gel Polymer Electrolytes for Lithium Metal Batteries

ABSTRACT In situ polymerized gel polymer electrolytes (GPEs) have attracted much attention due to their high ionic conductivity and excellent interfacial contact. Among this type of GPEs, poly(1,3‐dioxane) (PDOX)‐based composite GPEs have emerged as a promising candidate for high‐energy‐density lithium metal batteries (LMBs) owing to their superior oxidative stability. However, uncontrolled lithium dendrite growth at the PDOX electrolyte/anode interface compromises interfacial stability and long‐term cycling durability. Here, indium trifluoromethanesulfonate (In(OTf) 3 ) is introduced as a bifunctional initiator to trigger the in situ polymerization of 1,3‐dioxane (DOX) to form a GPE. Meanwhile, it also yields a robust solid electrolyte interphase (SEI) enriched with Li x In alloy and LiF, generating a stable lithophilic and highly Li + ‐conductive interfacial layer that effectively suppresses lithium dendrite growth. As a result, the obtained PDOX electrolyte exhibits excellent stability, maintaining stable operation for over 1600 h at 1.0 mA cm −2 in a Li||Li symmetric cell. When paired with high‐voltage cathodes, the Li||NCM523 cells deliver a remarkable capacity retention of 82.5% after 500 cycles at 1 C. Importantly, an Ah‐level Li||NCM712 pouch cell delivers 402 Wh kg −1 energy density and demonstrates 99.3% capacity retention over 100 cycles, showcasing the potential of this GPE for practical applications.