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A series of flexible and highly conductive quasi-solid single-ion polymer electrolytes (abbreviated as PU-TFMSI) based on lithium sulfonimide are presented. Owing to the crosslinked structure with three types of segments (hard segment, soft segment and Li rich segment), PU-TFMSI displays a stretchability of 1350% and toughness of 13.6 MJ m⁻³. These PU-TFMSI single-ion electrolytes deliver high ionic conductivity over 1.3 × 10⁻⁴ S cm⁻¹ at ambient temperature, a wide electrochemical stability window (up to 5.0 V vs Li⁺/Li), a high lithium transfer number (LTN ∼ 0.9), and good compatibility with electrodes. The LiFePO₄/PU-TFMSI-2/Li coin cell exhibits stable cycling with an average Coulombic efficiency of 99.58% and superior rate performance. Furthermore, the LiFePO₄//Li soft pouch cell with PU-TFMSI-2 single-ion electrolyte maintains functionality under bending. The flexible single-ion polymer electrolytes …