Unraveling the Role of Resting Period in Gel Polymer Electrolyte Evolution and LiF‐Rich SEI Formation for Stable Lithium Metal Batteries

In situ polymerized gel polymer electrolytes (GPEs) offer a promising strategy for lithium metal batteries, as they combine high ionic conductivity of liquid electrolytes with good mechanical stability of solid electrolytes. However, the composition evolution of GPEs with the resting period and the subsequent formation of solid electrolyte interphase (SEI) remains poorly understood. Herein, the influence of the resting time on the mechanical properties, lithium‐ion transport kinetics, and SEI stability of GPEs is systematically investigated. By optimizing the resting time, the premature decomposition of fluorinated additives is revealed to be suppressed, and interfacial reactions are steered toward the formation of a robust SEI rich in LiF. The as‐formed GPEs exhibit a high ionic conductivity of 1.96 × 10 −3 S cm −1 and a Li + transfer number of 0.67 at 30°C. As a result, Li||Li symmetric cells achieve stable cycling for over 2600 h, while LiFePO 4 ||Li full cells retain 94.68% of initial capacity after 1000 cycles at 1 C. This work elucidates the role of the resting time in modulating electrolyte chemical composition and SEI formation, providing a practical approach from a new perspective for developing long‐life quasi‐solid‐state lithium metal batteries.